AJP - GI Information on EB 2010
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Gastrointest Liver Physiol 292: G492-G500, 2007. First published October 5, 2006; doi:10.1152/ajpgi.00257.2006
0193-1857/07 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
292/2/G492    most recent
00257.2006v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Gao, N.
Right arrow Articles by Wood, J. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Gao, N.
Right arrow Articles by Wood, J. D.

NEUROREGULATION AND MOTILITY

Stimulation of adenosine A1 and A2A receptors by AMP in the submucosal plexus of guinea pig small intestine

Na Gao, Hong-Zhen Hu, Sumei Liu, Chuanyun Gao, Yun Xia, and Jackie D. Wood

Departments of Physiology and Cell Biology and Internal Medicine, College of Medicine, The Ohio State University, Columbus, Ohio

Submitted 13 June 2006 ; accepted in final form 27 September 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Actions of adenosine 5'-monophosphate (AMP) on electrical and synaptic behavior of submucosal neurons in guinea pig small intestine were studied with "sharp" intracellular microelectrodes. Application of AMP (0.3–100 µM) evoked slowly activating depolarizing responses associated with increased excitability in 80.5% of the neurons. The responses were concentration dependent with an EC50 of 3.5 ± 0.5 µM. They were abolished by the adenosine A2A receptor antagonist ZM-241385 but not by pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid, trinitrophenyl-ATP, 8-cyclopentyl-1,3-dimethylxanthine, suramin, or MRS-12201220. The AMP-evoked responses were insensitive to AACOCF3 or ryanodine. They were reduced significantly by 1) U-73122, which is a phospholipase C inhibitor; 2) cyclopiazonic acid, which blocks the Ca2+ pump in intraneuronal membranes; and 3) 2-aminoethoxy-diphenylborane, which is an inositol (1,4,5)-trisphosphate receptor antagonist. Inhibitors of PKC or calmodulin-dependent protein kinase also suppressed the AMP-evoked excitatory responses. Exposure to AMP suppressed fast nicotinic ionotropic postsynaptic potentials, slow metabotropic excitatory postsynaptic potentials, and slow noradrenergic inhibitory postsynaptic potentials in the submucosal plexus. Inhibition of each form of synaptic transmission reflected action at presynaptic inhibitory adenosine A1 receptors. Slow excitatory postsynaptic potentials, which were mediated by the release of ATP and stimulation of P2Y1 purinergic receptors in the submucosal plexus, were not suppressed by AMP. The results suggest an excitatory action of AMP at adenosine A2A receptors on neuronal cell bodies and presynaptic inhibitory actions mediated by adenosine A1 receptors for most forms of neurotransmission in the submucosal plexus, with the exception of slow excitatory purinergic transmission mediated by the P2Y1 receptor subtype.

gastrointestinal tract; neurogastroenterology; enteric nervous system; synaptic transmission; purinergic receptors

THE CELLULAR NEUROPHYSIOLOGY of purinergic receptors in the enteric nervous system (ENS), which encompasses both P1- and P2-type receptors, continues to be a focus of attention for neurogastroenterology. Adenosine is generally accepted as the ligand for the P1 receptor; ATP is the ligand for P2 receptors (1).

P1 receptors are further classified as A1, A2A, A2B, and A3 receptors according to their pharmacology, molecular sequences, and signal transduction pathways. Both A2A and A2B adenosine receptors are coupled through a Gs protein to adenylate cyclase, elevation of cAMP, and stimulation of neuronal excitability. On the other hand, A1 and A3 adenosine receptors are negatively coupled to adenylate cyclase through the same Gs protein (35, 43) and inhibit cAMP formation and neuronal excitability.

In the ENS, adenosine acts as a presynaptic neuromodulator to influence the release of the inhibitory neurotransmitters norepinephrine and somatostatin (2, 53) and the excitatory neurotransmitters acetylcholine (ACh) and tachykinins (6, 7, 9, 12, 13, 38, 40). Presynaptic inhibition by adenosine is mediated by adenosine A1 receptors, which act via pertussis toxin (PTX)-sensitive G proteins to suppress voltage-activated calcium currents (7).

Tonic stimulation of postsynaptic A1 adenosine receptors suppresses cAMP formation in dissociated myenteric ganglia (52). Suppression of histamine-activated chloride and calcium conductance (2, 47) by adenosine in myenteric neurons is also mediated by A1 adenosine receptors. Exposure to adenosine increases K+ conductance and clamps the membrane potential at hyperpolarized levels in myenteric neurons in guinea pig small intestine (15, 42). Stimulation of A2A adenosine receptors and activation of protein kinase A in the coupled signal transduction cascade are suggested to account for the excitatory actions of adenosine in the guinea pig small intestinal submucosal plexus (3, 4). Presynaptic inhibitory A1 receptors in the submucosal plexus suppress neurotransmitter release at excitatory synapses and thereby inhibit mucosal secretory reflexes (3, 17).

P2 receptors are subdivided into P2X and P2Y receptor subtypes. P2X receptors are ligand-gated ion channels, which mediate ATP-evoked responses in neurons of the myenteric and submucosal divisions of the ENS (5, 6, 54). P2Y receptors are G protein-coupled metabotropic receptors. The P2Y1 purinergic receptor subtype expressed by enteric neurons in the submucosal division of the ENS is activated by presynaptic release of ATP from other enteric neurons and sympathetic postganglionic fibers (32). It is a metabotropic receptor that is linked to activation of phospholipase C (PLC), synthesis of inositol (1,4,5)-trisphosphate and mobilization of Ca2+ from intracellular stores (28, 32).

Following synaptic release, ATP is hydrolyzed by several families of ectonucleotidases to ADP, AMP, and adenosine in this sequence (55). Actions of both ADP and adenosine on neuronal excitability and neurotransmission in the ENS are known; less is known about the actions of AMP (5, 54). The aim of the present study was to gain better understanding of the actions of AMP, the receptors that mediate its action, and postreceptor mechanisms of signal transduction in the submucosal plexus of guinea pig small intestine. A preliminary report has appeared in abstract form (26).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Tissue preparation. Tissues were obtained from 30 male Hartley-Dawley guinea pigs (300–350 g), which were euthanized by stunning followed by exsanguination from the cervical blood vessels. This procedure was approved by the Ohio State University Institutional Laboratory Animal Care and Use Committee and Veterinary Inspectors from the United States Department of Agriculture. Segments of intestine were removed and placed in ice-cold Krebs solution, which contained 1 µM nicardipine. Composition of the Krebs solution was (in mM) 120.9 NaCl, 5.9 KCl, 1.2 MgCl2, 1.2 NaH2PO4, 14.4 NaHCO3, 2.5 CaCl2, and 11.5 glucose. The segments of ileum were opened along the mesenteric border and the contents were flushed away with Krebs solution. Submucosal plexus preparations were obtained from segments of small intestine removed 10–20 cm proximal to the ileocecal junction. The segments were microdissected for electrophysiological recording as described earlier (24, 53). The preparations were superfused with chilled Krebs solution while the muscular coat and mucosa were dissected away to yield a flat preparation of the submucous plexus embedded in connective tissue. The preparation was transferred to a l-ml recording chamber and superfused at a rate of 5 ml/min with oxygenated Krebs solution at 36°C.

Equimolar N-methyl-D-glucamine or choline chloride was substituted for NaCl to reduce the Na+ concentration from 120 to 26 mM in protocols that required depletion of Na+ in the Krebs solution. All pharmacological agents were applied by addition to the tissue bath.

Intracellular recording. Transmembrane electrical potentials were recorded with conventional intracellular "sharp" microelectrodes filled with 1% biocytin in 2 M KCl containing 0.05 M Tris-(hydroxy-methyl)-aminomethane buffer (pH 7.4) and having resistances of 80–200 M{Omega}. The preamplifier (M767, World Precision Instruments, Sarasota, FL) had bridge circuitry for intraneuronal injection of electrical current. Constant-current rectangular pulses were driven by a Grass SD9 stimulator (Grass Instrument Division, Astro-Med, Warwick, RI). Electrometer output was amplified and observed on an oscilloscope (Tektronics 5113; Tektronics, Beaverton, OR). All data were recorded on digital tape for later analysis. Postsynaptic potentials were evoked by focal electrical stimulation of interganglionic fiber tracts with electrodes made of 20-µm diameter Teflon-coated platinum wire and connected through a stimulus-isolation unit (Grass SIN 5) to a Grass S 48 stimulator. Fast excitatory postsynaptic potentials (EPSPs) were recorded with fast sweep speeds and printed out through a digital real-time oscilloscope (Tektronics TDS 210; Tektronics) on a Laserjet 5L printer (Hewlett Packard, Palo Alto, CA). Chart records were made on an Astro-Med thermal recorder (Astro-Med). The amplitude of spikes, in some of the figures, was blunted by the low-frequency response of the recorder. At the end of the recording session, the neuronal tracer biocytin was injected into the impaled neurons from the recording electrodes by the passage of hyperpolarizing current (0.5 nA for 3–10 min). The anal end of the preparation was marked and the tissue was transferred into a disposable chamber filled with fixative (2% formaldehyde plus 15% of a saturated solution of picric acid) and kept at 4°C overnight.

Morphological identification of neurons. The biocytin-injected neurons were visualized by development of fluorescin streptavidin. The preparations were cleared in three changes of DMSO and three 10-min washes with phosphate-buffered saline. After this, they were reacted with fluorescin streptavidin (1:200) for 30 min at 37°C and observed with a Nikon Eclipse E600 fluorescent microscope that was equipped with a SPOT-2 chilled color and black-and-white digital camera (Diagnostic Instruments, Sterling Heights, MI).

Substance P, serotonin, nicardipine, scopolamine, and hexamethonium were purchased from Research Biochemical (Natick, MA). PTX, U-73122, U-73343, ryanodine, nicardipine, 8-cyclopentyl-1,3-dimethylxanthine (8-CPT), and cyclopiazonic acid (CPA) were purchased from Sigma-Aldrich (St. Louis, MO). AACOCF3, 2-aminoethoxydiphenylborane (2-APB), GF109203X, H89, and W-7 were purchased from Tocris Cookson (Ballwin, MO). KN-62 was obtained from BIOMOL Research Laboratories (Plymouth Meeting, PA). Chemicals were applied by addition to the superfusion solution.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Electrophysiology. Application of AMP (0.3–100 µM) to submucosal preparations evoked either hyperpolarizing or depolarizing neuronal responses depending on the identity of the impaled neuron. Slowly activating hyperpolarizing responses, which were associated with suppressed excitability and decreased input resistance, occurred in each of seven neurons that expressed AH-type electrophysiological behavior and Dogiel type II multipolar morphology (see Ref. 50 for a review of AH-type neurons). The membrane hyperpolarization occurred in a concentration-dependent manner (Fig. 1A). The mean for the hyperpolarizing responses to 30 µM AMP in AH-type neurons was 7.4 ± 3.5 mV for five animals. Presence in the bathing solution of the selective adenosine A1 receptor antagonist 8-CPT (1 µM) suppressed the hyperpolarizing action of AMP in each of 7 AH-type neurons (Fig. 1A). The presence of the selective adenosine A2A receptor antagonist ZM-241385 (1 µM) did not alter the AMP-evoked hyperpolarizing responses in any of the five neurons that were tested (data not shown).


Figure 1
View larger version (34K):
[in this window]
[in a new window]

  		Fig. 1. Exposure to AMP evoked hyperpolarizing responses in neurons with AH-type electrophysiological behavior and Dogiel II multipolar morphology and depolarizing-excitatory responses in neurons with S-type electrophysiological behavior and uniaxonal morphology in the guinea pig submucosal plexus. A: hyperpolarizing responses to AMP in an AH-type neuron were concentration dependent over a range of 3 to 30 µM and were suppressed by the selective adenosine A1 receptor antagonist 8-CPT. Morphology of the neuron appears in the inset. Downward deflections on the voltage traces are electrotonic potentials produced by repetitive injection of 100-ms rectangular hyperpolarizing constant-current pulses. B: depolarizing responses evoked by AMP in an S-type neuron were suppressed by reduction of the concentration of Na+ in the bathing medium and were enhanced by reduction of Ca2+ in the bathing medium. Morphology of the neuron appears in the inset.

 
Application of AMP (0.3–100 µM) to submucosal neurons with S-type electrophysiological behavior and uniaxonal morphology evoked slowly activating depolarizing responses associated with elevated excitability in 80.5% (66/82) of the neurons (Figs. 1–5; see Ref. 50 for a review of S-type neurons). Elevated excitability was reflected by an increase in the number of action potentials evoked by intracellular injection of constant-current depolarizing current pulses and by an increase in the spontaneous discharge of action potentials in some of the neurons (Figs. 1, 2, 4). The depolarizing responses for 15 neurons were associated by either a decrease or no change in input resistance and were concentration dependent with an EC50 of 3.5 ± 0.5 µM. To explore whether the AMP-evoked responses were due to secondary synaptic release of other excitatory neurotransmitters, 1 µM tetrodotoxin or reduction of Ca2+ in the bathing medium was used to suppress neurotransmitter release. The depolarizing actions of AMP persisted in 1 µM tetrodotoxin in each of five neurons tested (data not shown). This was consistent with a direct action of AMP on the neuron from which the recording was made rather than activation of neurons synaptically connected with the impaled neuron. Depletion of Ca2+ in the bathing medium prolonged the duration of the AMP-evoked responses without any significant change in the peak amplitude of the depolarization in five neurons (Figs. 1B and 4).


Figure 5
View larger version (43K):
[in this window]
[in a new window]

  		Fig. 5. Data showing suppression of AMP-evoked depolarizing responses in submucosal neurons with S-type electrophysiological behavior and uniaxonal morphology by agents that interfere with the PLC -> IP3 -> {uparrow}Ca2+ -> PKC postreceptor signal transduction cascade. Numbers of neurons appear in parentheses. *P < 0.001.

 

Figure 2
View larger version (21K):
[in this window]
[in a new window]

  		Fig. 2. Depolarizing responses evoked by AMP in an S-type neuron were suppressed by the selective adenosine A2A receptor antagonist ZM-241385 but not by the selective A1 receptor antagonist 8-CPT nor the selective A3 receptor antagonist MRS-1220. A: response to AMP alone. B: response to AMP in the presence of 8-CPT in the bathing medium. C: responses to AMP in the presence of MRS-1220. D: response to AMP in the presence of ZM-241385. E: response to AMP alone following washout of the last receptor antagonist. F: the neuron from which each of the electrophysiological records was obtained.

 

Figure 4
View larger version (29K):
[in this window]
[in a new window]

  		Fig. 4. Signal transduction for AMP-evoked depolarizing responses in submucosal neurons with S-type electrophysiological behavior and uniaxonal morphology. A: response to AMP alone. B: a high concentration of ryanodine did not suppress AMP-evoked excitatory response. C: the Ca2+-ATPase inhibitor CPA suppressed excitatory responses to AMP. D: the 1,4,5-inositol trisphosphate (IP3) receptor antagonist 2-APB suppressed excitatory responses to AMP. E: response to AMP alone following washout of the last pharmacological agent. F: the neuron from which each of the electrophysiological records was obtained. Downward deflections on the voltage traces are electrotonic potentials produced by repetitive injection of 100-ms rectangular hyperpolarizing constant-current pulses.

 
Because it was known that increased conductance in nonselective cation channels accounts for the membrane depolarization during metabotropic slow EPSPs and slow EPSP-like responses to excitatory agonists in S-type submucosal neurons, we examined the effects of Na+ depletion on the depolarizing responses to AMP (33, 49). Depletion of Na+ in the bathing medium suppressed the depolarizing responses to AMP (Figs. 1B and 4). Responses evoked by 10 µM AMP were suppressed to 6.5 ± 2.5% of control (n = 5 neurons) when the Na+ concentration was reduced from 145 mM to 26.2 mM by substitution of equimolar N-methyl-D-glucamine or choline chloride for NaCl (Fig. 4). The resting membrane potential was unchanged by reduction of the extracellular Na+ concentration from 145 to 26.2 mM.

Pharmacology of excitatory responses. We used selective antagonists for P1 and P2 receptors as pharmacological tools in an attempt to identify the purinergic receptor that might be responsible for the AMP-evoked excitatory responses in the S-type submucosal neurons. The presence of a supramaximal concentration (1 µM) of the specific A1 receptor antagonist 8-CPT in the bathing medium did not suppress the AMP-evoked depolarizing responses (Figs. 2B and 3). Depolarizing responses to 10 µM AMP were 11.0 ± 1.9 mV for seven neurons in the presence of 8-CPT and were not different from responses of 11.6 ± 2.0 mV in eight neurons evoked by 10 µM AMP in the absence of 8-CPT (P > 0.05). Exposure to the selective adenosine A3 receptor antagonist MRS-1220 (100 nM) likewise had no effect on the depolarizing responses evoked by 10 µM AMP (Figs. 2C and 3). Depolarizing responses to 10 µM AMP were 13.1 ± 2.9 mV for seven neurons in the presence of MRS-1220 and were not different from responses of 12.9 ± 3.0 mV in seven neurons evoked by 10 µM AMP in the absence of MRS-12201220 (P > 0.05). On the other hand, coapplication of AMP with the selective A2A receptor antagonist ZM-241385 (300 nM) suppressed responses to 10 µM AMP to 24.5 ± 3.7% of the responses in the absence of ZM-241385 (Figs. 2D and 3). The depolarizing responses to 10 µM AMP alone amounted to 10.5 ± 0.6 mV in 16 neurons and were reduced to 2.4 ± 0.4 mV for 15 neurons when coapplied with ZM-241385 (P < 0.01). A selective A2A receptor agonist CGS-21680, evoked depolarizing responses with amplitudes of 7.6 ± 1.2 mV for five neurons when applied at 300 nM. The AMP-like responses to CGS-21680 were reduced to 1.8 ± 0.8 mV when it was coapplied with 300-nM ZM-241385 in five S-type neurons.


Figure 3
View larger version (50K):
[in this window]
[in a new window]

  		Fig. 3. Data showing suppression of AMP-evoked depolarizing responses in submucosal neurons with S-type electrophysiological behavior and uniaxonal morphology by the selective adenosine A2A receptor antagonist ZM-241385 and lack of effect of other purinergic receptor antagonists. PPADS, pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid; TNP-ATP, trinitrophenyl-ATP. Numbers of neurons appear in parentheses. *P < 0.001.

 
We also tested a number of selective and nonselective selective purinergic receptor antagonists on the depolarizing responses to AMP and found that none altered the AMP-evoked responses (Fig. 3). These included 1) the nonselective P2 receptor antagonist suramin (300 µM), 2) the P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid (30 µM), 3) trinitrophenyl-ATP (1 µM), and 4) the selective P2Y1 receptor antagonist MRS-2179 (3 µM).

Signal transduction for excitatory responses. We used pharmacological agents known to interfere with specific steps in postreceptor signal transduction cascades as tools to investigate signal transduction mechanisms for AMP-evoked depolarizing responses in the S-type neurons. Involvement of PTX-sensitive G proteins in the AMP-evoked excitatory responses was investigated by incubating submucosal plexus preparations with PTX (2 µg/ml) for 12 h before recording of electrophysiological responses of the neurons to AMP. The preparations were microdissected as described in MATERIALS AND METHODS, washed three times with sterile PBS, and incubated in culture media (DMEM+10% fetal bovine serum+1% penicillin-streptomycin) at 37°C in a CO2 incubator. Viability of the tissue after incubation was verified by recording typical excitatory responses to application of bradykinin as reported by Hu et al. (33). Incubation in PTX did not alter the responses to AMP in seven neurons (data not shown).

CPA was another of the tools. CPA is an inhibitor of the endoplasmic reticulum Ca-ATPase and acts to suppress refilling of intracellular calcium stores (46). Preincubation of the submucosal preparations with 50 µM CPA suppressed the AMP-evoked depolarizing responses to 33.2 ± 9.2% of control responses to 10 µM AMP in the absence of CPA for five neurons (Figs. 3C and 5).

We tested a hypothesis that the postreceptor signal transduction cascade for AMP involves activation of PLC, synthesis 1,4,5-inositol trisphosphate (IP3) and elevation of intraneuronal Ca2+ by incubating the preparations with the PLC inhibitor U-73122 before application of AMP. Preincubation with 10 µM U-73122 reduced the depolarizing responses to 10 µM to 6.0 ± 2.2% of the responses to 10 µM AMP in the absence of U-73122 for five neurons (P < 0.01, Fig. 5). U-73343, which is the inactive analog of U-73122, did not affect AMP-evoked depolarizing responses (Fig. 5). Depolarizing responses to 10 µM AMP were 9.8 ± 1.5 mV for eight neurons in the presence of 10 µM U-73343 and were not different from responses of 9.9 ± 1.7 mV evoked by 10 µM AMP in the absence of U-73343 (P > 0.05). Preincubation of the preparations with the phospholipase A2 inhibitor AACOCF3 (30 µM) had no effect on the AMP-evoked depolarizing response in six neurons (Fig. 5).

Preincubation with the IP3 receptor antagonist 2-APB (100 µM) for 3 min reduced the amplitude of the depolarizing responses to 10 µM AMP to 24.6 ± 5.5% of control responses obtained without 2-APB pretreatment (Fig. 5). The mean depolarizing response to 10 µM AMP was 9.4 ± 1.4 mV in the absence of 2-APB in seven neurons, and this was reduced to 2.6 ± 0.9 mV by 2-APB (P < 0.01).

In view of reports by others that stimulation of ryanodine receptors might be a significant factor for excitation of neurons in the myenteric plexus (30, 34), we investigated the effects of ryanodine on AMP-evoked depolarizing responses in neurons of the submucosal plexus. Ryanodine was used as a tool for studying involvement of Ca2+ mobilization from intraneuronal membranes in the AMP signal transduction cascade because it is a high-affinity ligand for the Ca2+-induced Ca2+ release channel (i.e., the ryanodine receptor). Ryanodine promotes activation of the channel at low concentrations and blocks the channels when present in concentrations greater than 10 µM (24). Depolarizing responses to 10 µM AMP were 10.0 ± 2.5 mV for six neurons in the presence of 10 µM ryanodine and were not different from responses of 9.5 ± 2.3 mV evoked by 10 µM AMP in the absence of ryanodine (P > 0.05; Figs. 4B and 5).

Preincubation with the selective protein kinase A inhibitor H89 for 30 min had no effect on AMP-evoked responses. The mean depolarization to 10 µM AMP was 9.7 ± 0.9 mV for seven neurons and was not significantly different from the mean of 9.6 ± 0.7 mV in the presence of 1 µM H89 (P > 0.05). On the contrary, preincubation for 30 min with the selective PKC inhibitor GF109203X (1 µM) reduced the amplitude of responses evoked by 10 µM AMP to 29.7 ± 7.2% of the control responses in the absence of GF109203X for five neurons (P < 0.05, Fig. 5). Furthermore, 30-min pretreatment with 3 µM KN-62, which inhibits calmodulin kinases, reduced responses to 10 µM AMP to 31.5 ± 5.7% (P < 0.01) of the amplitude of the depolarizing responses in the absence of 3 µM KN-62 in five neurons (Fig. 5).

Presynaptic inhibition (fast EPSPs). Nicotinic fast EPSPs were evoked in the submucosal plexus by focal electrical stimulation of interganglionic fiber tracts. Application of AMP reversibly suppressed the fast EPSPs in all of 15 uniaxonal neurons that had S-type electrophysiological behavior (Fig. 6). Suppression of the fast EPSPs by AMP was concentration dependent with an IC50 of 3.9 ± 1.2 µM for five neurons (Fig. 6, A–C). Maximal inhibition was a reduction in amplitude to 34.3 ± 7.9% of control by 100 µM AMP. The inhibitory action of AMP was abolished by the selective adenosine A1 receptor antagonist 8-CPT (1 µM), which was pre- and coapplied with 30 µM AMP for six neurons (Fig. 6D). Micropressure pulses of ACh evoked characteristic EPSP-like depolarizing responses that were suppressed or abolished by 100 µM hexamethonium. The EPSP-like responses to ACh in six neurons were unaltered by the presence of 100 µM AMP in the tissue bath (data not shown).


Figure 6
View larger version (26K):
[in this window]
[in a new window]

  		Fig. 6. AMP suppressed fast excitatory postsynaptic potentials (EPSPs) in a concentration-dependent manner in a submucosal neuron with S-type electrophysiological behavior and uniaxonal morphology. A: suppression of fast EPSPs by 10 µM AMP. Stim., stimulation. B: suppression of fast EPSPs by 30 µM AMP. C: suppression of fast EPSPs by 100 µM AMP. D: presence of the selective adenosine A1 receptor antagonist 8-CPT in the bathing medium suppressed the inhibitory action of AMP on the fast EPSPs. E: the neuron from which each of the electrophysiological records was obtained. Selected EPSPs marked by arrows are shown on an expanded time scale above in the uppermost records.

 
Presynaptic inhibition (slow EPSPs). Slow EPSPs were also evoked in the submucosal plexus by focal electrical stimulation of interganglionic fiber tracts. The submucosal slow EPSPs were subdivided into three categories according to their sensitivity to the P2Y1 receptor antagonist MRS-2179 as follows: 1) "pure" purinergic slow EPSPs were abolished by 10 µM MRS-2179; 2) partially purinergic slow EPSPs were suppressed, but not abolished, by 10 µM MRS-2179; and 3) nonpurinergic slow EPSPs were unaltered by 10 µM MRS-2179. The pure and partially purinergic slow EPSPs were recorded in neurons with S-type electrophysiological behavior and uniaxonal morphology, and the nonpurinergic slow EPSPs occurred in neurons with AH-type electrophysiological behavior and Dogiel type II multipolar morphology as we reported earlier (32).

The effects of AMP on the slow EPSPs in neurons with S-type electrophysiological behavior and uniaxonal morphology were different from the effects on slow EPSPs in neurons with AH-type electrophysiological behavior and Dogiel type II multipolar morphology. In 5 AH/Dogiel II neurons, AMP suppressed the stimulus-evoked nonpurinergic slow EPSPs in a concentration-dependent manner with an EC50 of 2.3 ± 0.2 µM (Fig. 7, A and D). Exposure to 100 µM AMP abolished the slow EPSPs in each of the 7 AH/Dogiel II neurons tested (Fig. 7A). Bath application of either substance P or serotonin evoked slow EPSP-like depolarizing responses in the AH/Dogiel II neurons (Fig. 7, B and C). The presence of 100 µM AMP, in the bathing medium, did not change the amplitude of the depolarizing responses to either substance P or serotonin (Fig. 7, B and C). Presence in the bathing solution of the selective adenosine A1 receptor antagonist 8-CPT reduced suppression of the slow EPSPs by AMP. On the other hand, the selective adenosine A2A receptor antagonist ZM-241385 affected neither suppression on stimulus-evoked slow EPSPs by AMP nor AMP-evoked hyperpolarizing responses (n = 5 neurons; data not shown).


Figure 7
View larger version (33K):
[in this window]
[in a new window]

  		Fig. 7. AMP suppressed slow EPSPs in a concentration-dependent manner in a submucosal neuron with AH-type electrophysiological behavior and multipolar Dogiel II morphology. A: stimulus-evoked slow EPSP was reduced reversibly by 10 µM AMP and abolished by 100 µM AMP. B: substance P-evoked slow EPSP-like response was unaffected by AMP. C: serotonin-evoked slow EPSP-like response was unaffected by AMP. D: concentration-response relation for suppression of slow EPSPs in submucosal AH-type neurons by AMP. E: the neuron from which each of the electrophysiological records was obtained.

 
In contrast to the AH/Dogiel II neurons, application of AMP did not change the amplitude of the purinergic slow EPSPs, which were sensitive to MRS-2179 in the neurons with S-type electrophysiological behavior and uniaxonal morphology (Fig. 8A). We applied 100 µM AMP to five S-type submucosal neurons that had stimulus-evoked pure purinergic slow EPSPs and found no effect on the amplitude of the EPSPs. We also applied 100 µM AMP to five S-type neurons that had partially purinergic slow EPSPs (i.e., MRS-2179 suppressed the EPSP amplitude by only 53.3 ± 3.3%). These particular neurons received both a purinergic slow excitatory synaptic input mediated by ATP and a slow excitatory synaptic input mediated by a nonpurinergic transmitter as we reported earlier (32). Application of 100 µM AMP reduced the amplitude of these partially purinergic slow EPSPs by 50.0 ± 4.1% in five neurons. Coapplication of AMP with the selective P2Y1 receptor antagonist MRS-2179 abolished the partially purinergic slow EPSPs (Fig. 8, B and C). The effect of AMP on partially purinergic slow EPSPs was abolished by 1 µM 8-CPT (n = 5, data not shown).


Figure 8
View larger version (25K):
[in this window]
[in a new window]

  		Fig. 8. Slow EPSPs mediated solely by the presynaptic release of ATP and its action at postsynaptic P2Y1 receptors in submucosal S-type neurons, unlike EPSPs mediated by nonpurinergic neurotransmitters, were not suppressed by AMP. A: a "purely" ATP-mediated slow EPSP was abolished by the selective P2Y1 receptor antagonists and was unaffected by AMP in the neuron shown in the inset. B: a slow EPSP mediated partially by the release of ATP was suppressed, but not abolished, by the selective P2Y1 receptor antagonist MRS-2179; coapplication of MRS-2179 and AMP abolished the EPSP. The neuron from which the records were obtained appears in the inset. C: data for AH-type submucosal neurons that received slow synaptic input that was partially suppressed by MRS-2179 or AMP and abolished when both MRS-2179 and AMP were present in the bathing solution.

 
Presynaptic inhibition (slow IPSPs). Focal electrical stimulation of interganglionic fiber tracts in the submucosal plexus evoked characteristic noradrenergic slow inhibitory postsynaptic potentials (IPSPs), in addition to fast and slow EPSPs. Application of AMP suppressed the amplitude and duration of the slow IPSPs in a concentration-dependent manner with an EC50 of 36.3 ± 7.5 µM for seven neurons (Fig. 9, A and C). Maximal inhibition occurred with 300 µM AMP, which suppressed the slow IPSP amplitude by 87.5 ± 1.5% for seven neurons. The presence of 1 µM 8-CPT in the tissue bath abolished the inhibitory effect of 30 µM AMP in each of seven neurons (Fig. 8A). The presence of the selective adenosine A2A receptor antagonist ZM-241385 (300 nM) did not alter the inhibitory action of 30 µM AMP on the slow IPSPs (n = 5, data not shown).


Figure 9
View larger version (32K):
[in this window]
[in a new window]

  		Fig. 9. AMP suppressed noradrenergic slow inhibitory postsynaptic potentials (slow IPSPs) in a concentration-dependent manner in a submucosal neuron with S-type electrophysiological behavior and uniaxonal morphology. A: the amplitude and duration of stimulus-evoked slow IPSPs were progressively reduced as the concentration of AMP in the bathing medium was increased in increments from 3 to 300 µM. Inhibitory action of AMP on the slow IPSPs was suppressed by the presence of the selective adenosine A1 receptor antagonist 8-CPT in the bathing medium. B: norepinephrine-evoked slow EPSP-like responses were unaffected by AMP. C: concentration-response relation for suppression of slow IPSPs in submucosal AH-type neurons by AMP. D: the neuron from which each of the electrophysiological records was obtained.

 
Micropressure pulses of norepinephrine evoked characteristic IPSP-like hyperpolarizing responses that were suppressed or abolished by 1 µM phentolamine. The slow IPSP-like responses evoked by microejection application of norepinephrine to the impaled neurons were unchanged by the presence of 10 µM AMP in the bathing medium (Fig. 9B).


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Exposure to AMP suppressed excitability in neurons with AH-type electrophysiological behavior and Dogiel type II morphology. This action for AH-type neurons was essentially the same as reported for adenosine in AH-type neurons (42). Both adenosine and AMP act to increase K+ conductance and thereby clamp the membrane potential of AH neurons to a hyperpolarized level near the K+ equilibrium potential (i.e., –90 mV) and hold the cell soma in a state of low excitability or inexcitability. The hyperpolarizing action of AMP for AH-type neurons most likely reflected stimulation of the adenosine A1 receptor subtype because, like the action of adenosine (14, 15), it was suppressed by the selective A1 receptor antagonist 8-CPT.

In contrast to AH-type neurons, exposure to AMP evoked depolarization of the membrane potential and enhanced excitability in neurons with S-type electrophysiological behavior and uniaxonal morphology. Adenosine A1 receptors could not be implicated in this action because pretreatment with the selective adenosine A1 receptor antagonist 8-CPT did not alter the excitatory responses to AMP. Likewise, neither the adenosine A3 receptor subtype nor P2Y receptor subtypes were implicated in the excitatory action of AMP because the excitatory responses were unaffected by pretreatment with selective antagonists for adenosine A3 and P2Y receptors. On the other hand, application of the selective adenosine A2A receptor antagonist ZM-241385 suppressed or abolished the AMP-evoked excitatory action. This was consistent with the adenosine A2A subtype as the receptor responsible for the AMP-evoked excitatory responses in the S-type neurons.

The postreceptor signal transduction mechanism for the adenosine A2A receptor has been reported to be Gs protein coupling to adenylate cyclase and elevation of intracellular cAMP (20, 21, 23). Nevertheless, we found that the cAMP-dependent protein kinase inhibitor H89 had no effect on the AMP-evoked excitatory responses in the S-type neurons. This suggests that postreceptor signal transduction for the adenosine A2A receptor in guinea pig submucosal S-type neurons differs from other tissues. PTX-sensitive G proteins were not involved in the AMP-evoked responses because preincubation with PTX had no effect on the actions of AMP. Most of our pharmacological evidence supports the hypothesis that a PLC -> IP3 receptor activation -> intracellular Ca2+ mobilization -> calmodulin-dependent protein kinase and/or PKC cascade was involved in the AMP-evoked excitatory responses. Essentially the same signal transduction pathway for the adenosine A2A receptor was reported for rat neostriatal neurons, in which the adenosine A2A receptor mediates inhibition of NMDA receptor-activated inward ionic current (48).

The A2A receptor can now be added to a growing list of G protein-coupled metabotropic receptors expressed by enteric neurons with S-type electrophysiological behavior and uniaxonal morphology, which share the same postreceptor signal transduction cascade. The list includes the P2Y1 receptor (32) protease activated receptors (27) and the B2 bradykinin receptor (33).

Our findings for the adenosine A2A receptor underscore an important aspect of metabotropic slow excitatory neurotransmission, which distinguishes enteric neurons with AH-type electrophysiological behavior and Dogiel type II multipolar morphology from neurons with S-type electrophysiological behavior and uniaxonal morphology (reviewed in Ref. 49). Slow excitatory responses in neurons with AH-type electrophysiological behavior and multipolar Dogiel type II morphology are characterized by membrane depolarization associated with closure of Ca2+-gated K+ channels, which is reflected by increased neuronal input resistance. Slow excitatory responses in neurons with S-type electrophysiological behavior and uniaxonal morphology are characterized by membrane depolarization associated with opening of cationic channels and either decreased neuronal input resistance or negligible change in the input resistance. Postreceptor signaling, which involves activation of adenylate cyclase, stimulation of cAMP formation, and activation of protein kinase A, generates excitatory responses characterized by increased neuronal input resistance in AH-type neurons. Postreceptor signaling, which involves activation of PLC, release of IP3 and diacylglycerol, and activation of PKC and calmodulin kinases, generates excitatory responses characterized by decreased neuronal input resistance in S-type neurons.

Presynaptic inhibition (fast EPSPs). All actions of AMP on synaptic transmission were at presynaptic inhibitory receptors, which suppressed both excitatory and inhibitory neurotransmission. Exposure to AMP suppressed fast nicotinic EPSPs without any reduction of nicotinic depolarizing responses to exogenously applied ACh. This was evidence for presynaptic suppression of ACh release. Blockade by the selective adenosine A1 receptor antagonist 8-CPT is consistent with identification of the presynaptic inhibitory receptor as the adenosine A1 subtype.

Presynaptic inhibition (slow EPSPs). Metabotropic slow EPSPs in AH- and S-type neurons are mimicked by a variety of signal substances including biogenic amines, neuropeptides, and mast cell proteases (reviewed in Refs. 29 and 50). AMP suppressed the metabotropic slow EPSPs. It suppressed the slow EPSPs in AH-type neurons without affecting slow EPSP-like responses evoked by application of substance P or serotonin, which like the results for fast EPSPs suggest presynaptic inhibition of release of excitatory neurotransmitters. A1 receptors also mediated this presynaptic inhibitory action.

An exception to the suppression of slow metabotropic synaptic excitation by AMP was the finding that slow EPSPs, which were mediated by the release of ATP and its action at purinergic P2Y1 receptors, were not suppressed by AMP. Secretomotor neurons with S-type electrophysiological behavior, uniaxonal morphology, noradrenergic IPSPs, and expression of the neurotransmitter vasoactive intestinal peptide comprise the primary population of neurons in the guinea pig submucosal plexus that receive purinergic P2Y1 excitatory synaptic input. Absence of presynaptic inhibitory receptors at these purinergic synapses is likely to be of significance in the normal and/or pathophysiological minute-to-minute functioning of the ENS; nevertheless, the specific significance is unclear.

Presynaptic inhibition (slow IPSPs). One kind of slow IPSP in S-type neurons of the guinea pig submucosal plexus reflects the intramural release of norepinephrine from sympathetic postganglionic axons (41, 53). Our results suggest that accumulation of AMP would act to suppress norepinephrine release from the sympathetic innervation. Suppression of norepinephrine release appeared to be a direct action at presynaptic inhibitory receptors on the sympathetic nerve terminals because AMP did not suppress the hyperpolarizing action of exogenously applied norepinephrine. Blockade of the presynaptic inhibitory action of AMP on the noradrenergic IPSPs by the selective A1 receptor antagonist 8-CPT suggests that suppression of noradrenergic synaptic transmission by AMP was mediated by the adenosine A1 receptor.

Others have reported that a subset of IPSPs in submucosal neurons is mediated by transmitter(s) other than norepinephrine (31, 37). The nonadrenergic transmitter has not been identified unequivocally; nevertheless, somatostatin emerges as the most likely candidate (36, 45). Unlike the adrenergic IPSP, the nonadrenergic IPSP was seldom observed in our study and actions of AMP were not investigated.

Most of the neurons in the guinea pig submucosal plexus, which receive inhibitory noradrenergic synaptic input, are secretomotor neurons with S-type electrophysiological behavior and uniaxonal morphology. Suppression of norepinephrine release by an accumulation of AMP in the functioning intestine would be expected to remove sympathetic braking action from the neurons. This, together with AMP-evoked elevation of excitability in the neuronal cell bodies, is expected to enhance secretion of H2O, electrolytes and mucus and thereby increase the liquidity of the luminal contents, such as might occur in inflammatory states where ATP release is enhanced (8, 10, 11, 16, 18, 19, 22, 25, 39, 44, 51).


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
National Institute of Diabetes and Digestive and Kidney Diseases Grants RO1 DK-068258, RO1 DK-57075 (to J. D. Wood) and KO8 DK-60468 (to Y. Xia) and a Pharmaceutical Manufacturers of America Foundation Postdoctoral Fellowship (to S. Liu) supported this work.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. D. Wood, Dept. of Physiology and Cell Biology, 304 Hamilton Hall, 1645 Neil Ave., Columbus, OH 43210-1218 (e-mail: wood.13{at}osu.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Abbracchio MP, Burnstock G. Purinoceptors: are there families of P2X and P2Y purinoceptors? Pharmacol Ther 64: 445–475, 1994.[CrossRef][Web of Science][Medline]
  2. Baidan LV, Zholos AV, Wood JD. Modulation of calcium currents by G-proteins and adenosine receptors in myenteric neurones cultured from adult guinea pig small intestine. Br J Pharmacol 116: 1882–1886, 1995.[Web of Science][Medline]
  3. Barajas-Lopez C, Surprenant A, North RA. Adenosine A1 and A2 receptors mediate presynaptic inhibition and postsynaptic excitation in guinea pig submucosal neurons. J Pharmacol Exp Ther 258: 490–495, 1991.[Abstract/Free Full Text]
  4. Barajas-Lopez C. Adenosine reduces the potassium conductance of guinea pig submucosal plexus neurons by activating protein kinase A. Pflügers Arch 424: 410–415, 1993.[CrossRef][Web of Science][Medline]
  5. Barajas-Lopez C, Espinosa-Luna R, Gerzanich V. ATP closes a potassium and opens a cationic conductance through different receptors in neurons of guinea pig submucous plexus. J Pharmacol Exp Ther 268: 1397–1402, 1994.[Abstract]
  6. Barajas-Lopez C, Huizinga JD, Collins SM, Gerzanich V, Espinosa-Luna R, Peres AL. P2x-purinoceptors of myenteric neurones from the guinea-pig ileum and their unusual pharmacological properties. Br J Pharmacol 119: 1541–1548, 1996.[Web of Science][Medline]
  7. Barajas-Lopez C, Peres AL, Espinosa-Luna R. Cellular mechanisms underlying adenosine actions on cholinergic transmission in enteric neurons. Am J Physiol Cell Physiol 271: C264–C275, 1996.[Abstract/Free Full Text]
  8. Bertrand PP, Bornstein JC. ATP as a putative sensory mediator: activation of intrinsic sensory neurons of the myenteric plexus via P2X receptors. J Neurosci 22: 4767–4775, 2002.[Abstract/Free Full Text]
  9. Broad RM, McDonald TJ, Brodin E, Cook MA. Adenosine A1 receptors mediate inhibition of tachykinin release from perifused enteric nerve endings. Am J Physiol Gastrointest Liver Physiol 262: G525–G531, 1992.[Abstract/Free Full Text]
  10. Burnstock G. Release of vasoactive substances from endothelial cells by shear stress and purinergic mechanosensory transduction. J Anat 194: 335–342, 1999.
  11. Burnstock G. Purinergic signalling. Br J Pharmacol 147: S172–S181, 2006.
  12. Christofi FL, Tack J, Wood JD. Suppression of nicotinic synaptic transmission by adenosine in myenteric ganglia of the guinea-pig gastric antrum. Eur J Pharmacol 216: 17–22, 1992.[CrossRef][Web of Science][Medline]
  13. Christofi FL, Wood JD. Presynaptic inhibition by adenosine A1 receptors on guinea pig small intestinal myenteric neurons. Gastroenterology 104: 1420–1429, 1993.[Web of Science][Medline]
  14. Christofi FL, Baidan LV, Fertel RH, Wood JD. Adenosine A2 receptor-mediated excitation of a subset of AH/Type 2 neurons and elevation of camp levels in myenteric ganglia of guinea-pig ileum. Neurogastroenterol Motil 6: 67–78, 1994.
  15. Christofi FL, Wood JD. Electrophysiological subtypes of inhibitory P1 purinoceptors on myenteric neurones of guinea-pig small bowel. Br J Pharmacol 113: 703–710, 1994.[Web of Science][Medline]
  16. Christofi FL, Wunderlich J, Yu JG, Wang YZ, Xue J, Guzman J, Javed N, Cooke H. Mechanically evoked reflex electrogenic chloride secretion in rat distal colon is triggered by endogenous nucleotides acting at P2Y1, P2Y2, and P2Y4 receptors. J Comp Neurol 469: 16–36, 2004.[CrossRef][Web of Science][Medline]
  17. Cooke HJ, Wang Y, Liu CY, Zhang H, Christofi FL. Activation of neuronal adenosine A1 receptors suppresses secretory reflexes in the guinea pig colon. Am J Physiol Gastrointest Liver Physiol 276: G451–G462, 1999.[Abstract/Free Full Text]
  18. Cooke HJ, Xue J, Yu JG, Wunderlich J, Wang YZ, Guzman J, Javed N, Christofi FL. Mechanical stimulation releases nucleotides that activate P2Y1 receptors to trigger neural reflex chloride secretion in guinea pig distal colon. J Comp Neurol 469: 1–15, 2004.[CrossRef][Web of Science][Medline]
  19. Dalziel HH, Sneddon P. The mechanism of action of AMP-induced inhibition of sympathetic neurotransmission in the isolated vas deferens of the rat and guinea-pig. Br J Pharmacol 94: 961–967, 1988.[Web of Science]
  20. Duarte-Araujo M, Timoteo MA, Correia-de-Sa P. Adenosine activating A(2A)-receptors coupled to adenylate cyclase/cyclic AMP pathway downregulates nicotinic autoreceptor function at the rat myenteric nerve terminals. Neurochem Int 45: 641–651, 2004.[CrossRef][Web of Science][Medline]
  21. Duarte-Araujo M, Nascimento C, Alexandrina Timoteo M, Magalhaes-Cardoso T, Correia-de-Sa P. Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A(2A) and extrajunctional inhibitory A(1) receptors. Br J Pharmacol 141: 925–934, 2004.[CrossRef][Web of Science][Medline]
  22. Ehrlich BE, Kaftan E, Bezprozvannaya S, Bezprozvanny I. The pharmacology of intracellular Ca(2+)-release channels. Trends Pharmacol Sci 15: 145–149, 1994.[CrossRef][Medline]
  23. Fredholm BB, Arslan G, Halldner L, Kull B, Schulte G, Wasserman W. Structure and function of adenosine receptors and their genes. Naunyn Schmiedebergs Arch Pharmacol 362: 364–374, 2000.[CrossRef][Web of Science][Medline]
  24. Frieling T, Cooke HJ, Wood JD. Electrophysiological properties of neurons in submucosal ganglia of guinea pig distal colon. Am J Physiol Gastrointest Liver Physiol 260: G835–G841, 1991.[Abstract/Free Full Text]
  25. Galligan JJ. P2X receptors as possible drug targets for treatment of the irritable bowel syndrome. Br J Pharmacol 141: 1294–1302, 2004.[CrossRef][Web of Science][Medline]
  26. Gao N, Hu HZ, Liu S, Gao C, Xia Y, Wood JD. Adenosine 5'-monophosphate activates postsynaptic A2A receptors and presynaptic A1 receptors on S-type neurons in guinea-pig ileal submucosal plexus (Abstract). Gastroenterology 120: A177, 2001.
  27. Gao C, Liu S, Hu HZ, Gao N, Kim GY, Xia Y, Wood JD. Serine proteases excite myenteric neurons through protease-activated receptors in guinea-pig small intestine. Gastroenterology 123: 1554–1564, 2002.[CrossRef][Web of Science][Medline]
  28. Gao N, Hu HZ, Zhu MX, Fang X, Liu S, Gao C, Wood JD. The P2Y1 purinergic receptor expressed by enteric neurones in guinea-pig intestine. Neurogastroenterol Motil 18: 316–323, 2006.[CrossRef][Web of Science][Medline]
  29. Grundy D, Al-Chaer ED, Aziz Q, Collins SM, Ke M, Taché Y, Wood JD. Fundamentals of neurogastroenterology. Gastroenterology 130: 1391–1411, 2006.[CrossRef][Web of Science][Medline]
  30. Hillsley K, Kenyon JL, Smith TK. Ryanodine-sensitive stores regulate the excitability of AH neurons in the myenteric plexus of guinea-pig ileum. J Neurophysiol 84: 2777–2785, 2000.[Abstract/Free Full Text]
  31. Hirst GDS, Silinsky EM. Some effects of 5-hydroxytryptamine, dopamine and noradrenaline on neurons in the submucous plexus of guinea-pig small intestine. J Physiol 251: 817–832, 1975.[Abstract/Free Full Text]
  32. Hu HZ, Gao N, Zhu MX, Liu S, Ren J, Gao C, Xia Y, Wood JD. Slow excitatory synaptic transmission mediated by P2Y1 receptors in the guinea-pig enteric nervous system. J Physiol 550: 493–504, 2003.[Abstract/Free Full Text]
  33. Hu HZ, Gao N, Liu S, Ren J, Wang X, Xia Y, Wood JD. Metabotropic signal transduction for bradykinin in submucosal neurons of guinea-pig small intestine. J Pharmacol Exp Ther 309: 310–319, 2004.[Abstract/Free Full Text]
  34. Kimball BC, Yule DI, Mulholland MW. Caffeine- and ryanodine-sensitive Ca2+ stores in cultured guinea pig myenteric neurons. Am J Physiol Gastrointest Liver Physiol 270: G594–G603, 1996.[Abstract/Free Full Text]
  35. Linden J. Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection. Annu Rev Pharmacol Toxicol 41: 775–787, 2001.
  36. Liu S, Xia Y, Hu HZ, Ren J, Gao C, Wood JD. Histamine H3 receptor-mediated suppression of inhibitory synaptic transmission in the guinea-pig submucous plexus. Eur J Pharmacol 397: 49–54, 2000.[CrossRef][Web of Science][Medline]
  37. Mihara S, Nishi S, North RA, Surprenant A. A non-adrenergic, non-cholinergic slow inhibitory post-synaptic potential in neurons of the guinea-pig submucous plexus. J Physiol 390: 357–365, 1987.[Abstract/Free Full Text]
  38. Moneta NA, McDonald TJ, Cook MA. Endogenous adenosine inhibits evoked substance P release from perifused networks of myenteric ganglia. Am J Physiol Gastrointest Liver Physiol 272: G38–G45, 1997.[Abstract/Free Full Text]
  39. Monro RL, Bertrand PP, Bornstein JC. ATP participates in three excitatory postsynaptic potentials in the submucous plexus of the guinea pig ileum. J Physiol 556: 571–584, 2004.[Abstract/Free Full Text]
  40. Nitahara K, Kittel A, Liang SD, Vizi ES. A1-receptor-mediated effect of adenosine on the release of acetylcholine from the myenteric plexus: role and localization of ecto-ATPase and 5'-nucleotidase. Neuroscience 67: 159–168, 1995.[CrossRef][Web of Science][Medline]
  41. North RA, Surprenant A. Inhibitory synaptic potentials resulting from {alpha}2-adrenoceptor activation in guinea-pig submucous plexus neurons. J Physiol 358: 17–32, 1985.[Abstract/Free Full Text]
  42. Palmer JM, Wood JD, Zafirov DH. Purinergic inhibition in the small intestinal myenteric plexus of the guinea-pig. J Physiol 387: 357–369, 1987.[Abstract/Free Full Text]
  43. Ribeiro JA, Sebastiao AM, de Mendonca A. Adenosine receptors in the nervous system: pathophysiological implications. Prog Neurobiol 68: 377–392, 2002.[CrossRef][Web of Science][Medline]
  44. Ross FM, Brodie MJ, Stone TW. Adenosine monophosphate as a mediator of ATP effects at P1 purinoceptors. Br J Pharmacol 124: 818–824, 1998.[CrossRef][Web of Science]
  45. Shen KZ, Surprenant A. Somatostatin-mediated inhibitory postsynaptic potential in sympathetically denervated guinea-pig submucosal neurons. J Physiol 470: 619–635, 1993.[Abstract/Free Full Text]
  46. Seildler NW, Jona I, Vegh M, Martonosi A. Cyclopiazonic acid is a specific inhibitor of the Ca2+-ATPase of sarcoplasmic reticulum. J Biol Chem 264: 17816–17823, 1989.[Abstract/Free Full Text]
  47. Starodub AM, Wood JD. Histamine H(2) receptor activated chloride conductance in myenteric neurons from guinea pig small intestine. J Neurophysiol 83: 1809–1816, 2000.[Abstract/Free Full Text]
  48. Wirkner K, Assmann H, Koles L, Gerevich Z, Franke H, Norenberg W, Boehm R, Illes P. Inhibition by adenosine A(2A) receptors of NMDA but not AMPA currents in rat neostriatal neurons. Br J Pharmacol 130: 259–269, 2000.[CrossRef][Web of Science][Medline]
  49. Wood JD, Kirchgessner A. Slow excitatory metabotropic signal transmission in the enteric nervous system. Neurogastroenterol Motil 16, Suppl 1: 71–80, 2004.
  50. Wood JD. Cellular neurophysiology of enteric neurons. In: Physiology of the Gastrointestinal Tract (4th ed.), edited by Johnson LR, Barrett KE, Ghishan FK, Merchant JL, Said HM, Wood JD. Burlington, MA: Elsevier, 2006, p. 629–664.
  51. Wu SN, Lin YT, Chen SS. Evidence of direct activation of adenosine A1 receptor by 5'-adenosine monophosphate in isolated guinea pig atrial myocytes. Jpn J Physiol 42: 35–47, 1992.[CrossRef][Web of Science][Medline]
  52. Xia Y, Baidan LV, Fertel RH, Wood JD. Determination of levels of cyclic AMP in the myenteric plexus of guinea-pig small intestine. Eur J Pharmacol 206: 231–236, 1991.[CrossRef][Web of Science][Medline]
  53. Zafirov DH, Cooke HJ, Wood JD. Elevation of cAMP facilitates noradrenergic transmission in submucous neurons of guinea pig ileum. Am J Physiol Gastrointest Liver Physiol 264: G442–G446, 1993.[Abstract/Free Full Text]
  54. Zhou X, Galligan JJ. P2X purinoceptors in cultured myenteric neurons of guinea-pig small intestine. J Physiol 496: 719–729, 1996.[Abstract/Free Full Text]
  55. Zimmermann H. Extracellular metabolism of ATP and other nucleotides. Naunyn Schmiedebergs Arch Pharmacol 362: 299–309, 2000.[CrossRef][Web of Science][Medline]



This article has been cited by other articles:


Home page
 Eur Respir JHome page
C. R. Esther Jr, N. E. Alexis, M. L. Clas, E. R. Lazarowski, S. H. Donaldson, C. M. Pedrosa Ribeiro, C. G. Moore, S. D. Davis, and R. C. Boucher
Extracellular purines are biomarkers of neutrophilic airway inflammation
Eur. Respir. J., May 1, 2008; 31(5): 949 - 956.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
292/2/G492    most recent
00257.2006v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Gao, N.
Right arrow Articles by Wood, J. D.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Gao, N.
Right arrow Articles by Wood, J. D.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2007 by the American Physiological Society.