INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

ISOLATED SEGMENTS OF BOWEL can perform complex and coordinated contractile activity, which is dependent on interactions between myogenic and local neural mechanisms. The polarized reflex responses to intestinal stimuli, first described by Bayliss and Starling (3), are now recognized to involve activation of ascending and descending enteric pathways leading to contraction and relaxation of smooth muscle (6). A variety of different transmitters and neuromodulators in these enteric pathways contributes to the coordination of this peristaltic activity. Somatostatin [somatotropin release-inhibitory factor (SRIF)] is present in a subpopulation of descending interneurones that project caudally within the myenteric plexus but not into either the longitudinal or circular muscle layers of the intestine (10, 31, 35). SRIF is also found in submucous plexus neurones, around submucosal blood vessels, and is present in mucosal endocrine cells in human (31), guinea pig (10), and rodent intestine (12, 13). SRIF is released by intestinal distension (11) and participates in the coordination of descending relaxation (21). The effects of exogenous SRIF on intestinal motility are complex. In the guinea pig ileum, both excitatory and inhibitory effects, operating through prejunctional mechanisms (14, 18, 24, 36, 45), have been described. The contractile response of the rat colon to SRIF also appears to involve activation of noncholinergic nerves (33), possibly by a process of disinhibition of vasoactive intestinal peptide (VIP) ergic/nitrergic interneurones supplying longitudinal muscle motoneurones (22). In contrast, the activity of VIPergic/nitregic neurons supplying the circular muscle layer is augmented by SRIF leading to relaxation (21). Thus SRIF plays a neuromodulatory role by regulating transmitter release (45), although postjunctional effects on enteric neuronal excitability have also been described (30, 34).

The diverse effects of SRIF are mediated by specific, high-affinity, membrane-bound receptors termed sst_1-5 (28). Expression of all five of these receptors has been described in the wall of the gastrointestinal tract (32). A number of synthetic peptides have been identified that displays selectivity for recombinant SRIF receptors. Octreotide and BIM-23027 are agonists with some selectivity for the sst₂ receptor, but each has additional agonist activity at sst₅ receptors, whereas Cyanamid154806 (Cyn) and BIM-23056 are antagonists for sst₂ and sst₅ receptors, respectively (2, 42). In this respect, BIM-23027 has been shown to inhibit neurogenically mediated contraction in the guinea pig ileum (15), whereas octreotide modulates gastrointestinal motility in both animals and human tissue (29, 40). The aims of this study were therefore to characterize the spontaneous peristaltic activity observed in isolated preparations of rat and mouse small intestine and to examine its modulation by SRIF and selective sst-receptor ligands. The availability of an sst₂-receptor knockout mouse provided an opportunity to further characterise the role of sst₂ receptors and also to compare the effects of SRIF in rat and mouse tissues.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Experiments were performed on in vitro segments of jejunum from Sheffield-strain female hooded Lister rats (250-350 g) and 10- to 12-wk-old male C57BL/6 mice (25-37 g). The sst₂-receptor knockout mice (Sstr₂^/) were generated at the Babraham Institute (Cambridge, UK) by gene targeting (1). Briefly, the Sstr₂ coding sequence was replaced by homologous recombination in HM-1 embryonic stem cells (129/OlaHsd) with a cassette comprising a neomycin selectable marker and a lacZ reporter gene. Chimeras were produced by blastocyst injection and then mated to C57BL/6 to achieve germline transmission. No sst₂-receptor expression was detectable in Sstr₂^/ by RT-PCR. The mutation was back-crossed onto C57BL/6 for three generations. Heterozygous intercrossing was then employed to generate the wild-type and sst₂-receptor knockout animals examined in the current study.

Animals were stunned by a blow to the head and killed by cervical dislocation. A midline laparotomy was performed, and a segment of proximal jejunum was rapidly excised beginning 2-3 cm from the ligament of Treitz. The excised segment was placed in gassed (95% O₂ and 5% CO₂) Krebs bicarbonate buffer solution (composition in mM: 117 NaC1, 4.7 KC1, 25 NaHCO₃, 2.5 CaC1₂, 1.2 MgCl₂, 2 NaH₂PO₄, 1.2 H₂O, and 11 D-glucose), cleared of any mesenteric connective tissue, and the lumen was flushed with Krebs solution. Two jejunal segments ~5 cm in length were prepared from each animal, and four in total were mounted horizontally in separate 20-ml perfusion chambers. The oral and aboral ends of each segment were secured to two metal catheters fixed at either end of the chamber and adjusted to maintain the segments at their resting length. For each segment, the oral end was connected to a perfusion pump for intrajejunal infusion of Krebs solution at a rate of 0.16 ml/min, and the aboral end was attached to a pressure transducer (Elcomatic EM 760, Elcomatic Ltd, Glasgow, UK) to record contractile activity as changes in intraluminal pressure under isovolumetric conditions. Tissues were maintained at 37°C, perfused with Krebs solution at a rate of 5 ml/min, and allowed to equilibrate for at least 30 min before experiments started. In some intestinal segments, spontaneous contractile activity developed during this equilibration period, but in others it did not. However, in preliminary experiments, it was found that activity developed more readily when the segment was distended. We, therefore, standardized the experimental setup by routinely infusing Krebs buffer into the closed segment to an initial intraluminal pressure of 10-11 cmH₂O in the rat and 2.5-3.5 cmH₂O in the mouse. Regular aborally propagating waves of contraction [migrating motor complexes (MMCs)] developed under these conditions and could be maintained for several hours. The output from the pressure transducers was relayed to a data-acquisition system (CED 1401+, Cambridge Electronic Design, Cambridge, UK) and from there to a computer running Spike 2 software (CED), which displayed the four-channel pressure recordings online and also stored the data for subsequent offline analysis.

Experimental protocol. Only preparations in which regular migrating motor complexes were maintained were used for subsequent experiments. Drugs or the appropriate vehicle was added to the chambers 20 min after stopping perfusion and recording continued for a further 10 min before washing out the drugs and reinstating perfusion.

Spatiotemporal maps. To determine the nature of the contractile activity generated by these isolated jejunal segments, we used an imaging analysis system as described previously (25). Briefly, a digital video camera (Sony, DCR-PC100E) was mounted above the preparation, and brief sequences of contractile activity were recorded for subsequent analysis and construction of spatiotemporal maps as described in detail elsewhere (25). Movement of the intestinal wall are mapped as changes in diameter along the entire length of the segment and plotted over time. The widest diameter is coded black, and the narrowest is coded white, enabling contractions to appear as dynamic changes in shading in the spatiotemporal maps.

Data analysis. MMCs were quantified in terms of their peak amplitude above baseline (cmH₂O), duration (s), and interval between them (s; Fig. 1). Baseline values were taken during the 10 min before drug application and the response effect in the 10 min following application. In preliminary studies, it was established that SRIF-receptor ligands influenced the interval between contraction complexes without significantly affecting the contraction amplitude. This effect was quantified by calculating the maximum interval between MMCs in the 10-min period before and after agonist administration. If contractions were abolished, then an interval of 600 s was recorded and used in subsequent statistical analysis. Responses are expressed as absolute values ± SE, with n being the number of animals. Paired data were compared using Student's t-test or Wilcoxon's rank sum test as appropriate. Grouped data from wild-type and sst₂-receptor knockout animals were compared using repeated-measures ANOVA. In all cases, a probability of P < 0.05 was considered as significant.

View larger version (68K): [in this window] [in a new window]   Fig. 1.   Spatiotemporal maps. Spatiotemporal maps illustrating motor activity in an isolated segment of mouse jejunum. On the right running top to bottom is a representative trace of intraluminal pressure showing periodic increases in intraluminal pressure separated by periods of relative quiescence. The parameters used to quantify this activity are illustrated. An expanded recording of 1 contraction complex is shown together with the associated spatiotemporal map obtained from the video sequence, a single frame of which is shown above. The maps are generated by measuring the diameter of the segment at each point along its length and converting this value into a grayscale (calibration bar) ranging from white (the minimum diameter) to black (the maximum). The grayscale coded pixels from each frame of the video sequence were used to construct a single row. Sequential rows were stacked 1 below the previous image to produce the spatiotemporal map of intestinal diameters. An expanded map showing activity during the contractile period is shown on the far left. Note that light bands represent waves of contractions that propagate from the oral end of the segment toward the aboral end at an interval of ~2 s.

Drugs. SRIF, L-NAME, D-NAME, atropine sulphate, nifedipine, -conotoxin GVIA, and TTX were purchased from Sigma Chemical. Octreotide acetate "Sandostatin" [D-Phe-c (Cys-Phe-D-Trp-Lys-Thr-Cys) Thr-ol] was obtained from a pharmaceutical supplier. BIM-23027 [c(N-Me-Ala-Tyr-D-Trp-Lys-Abu-Phel)], Cyn [AcNH-4-NO₂-Phe-c(D-Cys-Tyr-D-Trp-Lys-Thr-Cys)-Tyr-NH₂], and BIM-23056 [(D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH₂)] were custom synthesized by Neosystem Laboratoire (Strasbourg). All the peptides were dissolved in distilled water with the exception of SRIF, which was dissolved in 1% bovine serum albumen in distilled water, and BIM-23056, which was initially dissolve in 10% dimethylsulfoxide. Atropine sulfate and -conotoxin were dissolved in saline (0.9% NaCl). All drugs were stored at 20°C. Freshly diluted aliquots were maintained on ice during the course of the experiments and added to the bath in microlitre volumes.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Migrating motor complexes. Luminal distension of isolated segments of rat and mouse jejunum evoked a regular pattern of contractile activity. The activity consisted of periodic increases in intraluminal pressure separated by periods of relative quiescent (Fig. 1). Spatiotemporal maps of both mouse and rat jejunal contractile activity revealed a similar pattern of activity. The increase in intraluminal pressure coincided with waves of contraction, seen as parallel lines on the maps that originated at the oral end of the segment and propagated aborally (Fig. 1). These contractions occurred at ~2-s intervals and traveled at about 5 mm/s. The contraction region itself migrated more slowly (~1 cm/min) and coincided with the maintained rise in intraluminal pressure. The pressure returned to baseline as the burst of peristaltic activity came to an end.

MMCs in the rat jejunum. Baseline activity in a sample of 16 control tissues consisted of periodic increases in intraluminal pressure of 31 ± 2.1-s duration and 10.5 ± 0.7-cmH₂O amplitude separated by periods of relative inactivity. The mean interval between such MMCs was 104.4 ± 9.6 s. MMCs were completely abolished (Fig. 2) by TTX (0.6 µM, n = 3),  conotoxin (0.1 µM, n = 5), atropine (1 µM, n = 3), and nifedipine (1 µM, n = 7). In contrast, the nitric oxide synthase (NOS) inhibitor L-NAME (100 µM) produced an increase in both MMC frequency and amplitude (Fig. 5A), such that maximum intervals decreased from 194.4 ± 27 to 75.1 ± 7 s and amplitude increased from 12.2 ± 1.5 to 17.8 ± 1.7 cmH₂O (n = 9, P < 0. 01). The inactive isomer D-NAME was without effect (100 µM, n = 4, data not shown).

View larger version (14K): [in this window] [in a new window]   Fig. 2.   Contractile activity in the rat jejunum. Migrating motor complexes (MMCs) in distended segments of rat jejunum consisted of periodic increases in intraluminal pressure separated by periods of relative quiescence. Representative traces showing the abolition of MMCs by TTX (0.6 µM), -conotoxin (0.1 µM), atropine (1 µM), and nifedipine (1 µM).

SRIF inhibits contractions via activation of sst₂ receptors in rat jejunum. SRIF (1-1,000 nM, n = 8-12) produced a concentration-dependent reduction in the frequency of MMCs by increasing the interval between them (Fig. 3, A and B). This inhibition appeared as a period of contractile quiescence followed by a return of activity at the rate observed before the addition of drug rather than a long-term reduction in contraction frequency. Octreotide and BIM-23027 mimicked the effect of SRIF, producing an increase in the interval between MMCs. When tested at a concentration of 10 nM, the increase in interval produced by both BIM-23027 (n = 8, P < 0.01) and octreotide (n = 4, P < 0.05) was greater than that produced by SRIF at the same concentration (Fig. 4A), suggestive of an action at sst₂ receptors. In the presence of the selective sst₂-receptor antagonist Cyn (1 µM, n = 7), which had no effect on MMCs itself, the inhibitory action of SRIF (300 nM) was abolished (Fig. 4B). Indeed, after Cyn, there was a trend (P = 0.1) toward a decreased interval between MMCs following SRIF.

View larger version (26K): [in this window] [in a new window]   Fig. 3.   Effect of somatotropin release-inhibitory factor (SRIF) on MMC intervals in the rat jejunum. A: representative trace showing the transient increase in the interval between MMCs produced by SRIF (300 nM). B: SRIF (1-1,000 nM) evoked a concentration-dependent increase in MMC interval. Means ± SE for 8-12 preparations before () and after () administration of SRIF. ** P < 0.01, *** P < 0.001 compared with predrug control.

View larger version (24K): [in this window] [in a new window]   Fig. 4.   Effect of sst2-receptor agonists and antagonists on MMC intervals in the rat jejunum. A: histograms showing the maximum interval between MMCs before () and after () addition of SRIF (10 nM, n = 8), octreotide (10 nM, n = 4), and BIM-23027 (10 nM, n = 8); * P < 0.05, ** P < 0.01 compared with predrug control. The effects of BIM-23027 and octreotide were significantly greater than that of SRIF. #P < 0.05, ##P < 0.01 compared with SRIF (10 nM). B: histograms showing the effect of Cyanamid154806 (Cyn) on MMCs and on the inhibitory effect of SRIF. The left histogram shows the MMC interval before () and after () SRIF (300 nM, n = 9). On the right are data showing the prevention of the inhibitory effect of SRIF (300 nM, n = 7) by Cyn (1 µM, n = 7). Note that the antagonist alone had no effect on MMC intervals. ** P < 0.01 compared with predrug control.

Inhibition of contractions in rat jejunum by sst₂-receptor activation involves an NOS pathway. As described above, 100 µM L-NAME produced a decrease in the MMC interval, an effect that developed within 1 min of its application and that was sustained in its continued presence for up to 25 min. SRIF (300 nM, n = 6) had no effect on MMCs in the presence of L-NAME (Fig. 5B).

View larger version (24K): [in this window] [in a new window]   Fig. 5.   Effect of nitro-L-arginine methyl ester (L-NAME) on the response to SRIF in the rat jejunum. A: representative trace showing the absence of the inhibitory effect of SRIF on contraction complexes in the presence of the nitric oxide synthase inhibitor L-NAME (100 µM). L-NAME augmented both the amplitude and frequency of MMCs and blocked the effect of SRIF (300 nM) on MMC interval. B: histograms representing group data from 6 experiments. ** P < 0.01 compared with predrug control.

MMC in the mouse jejunum. Contractile activity in isolated mouse jejunum followed a similar pattern to that observed in the rat, although lower intraluminal pressures were required for their initiation (2.5-3.5 cmH₂0). Contractions in mouse tissue (based on a sample of 16) had a mean duration of 71.5 ± 4 s and an amplitude of 1.6 ± 0.1 cmH₂O and were separated by a mean interval of 131.2 ± 8 s.

Inhibition of contractions by SRIF in mouse tissue involves more than one receptor subtype. As observed in the rat, SRIF produced a concentration-dependent increase in the MMC interval (0.01-100 nM, n = 4; Fig. 6). Although we were unable to determine maximum effective concentrations, and thus EC50 values, in either mouse or rat tissue (due to the 600-s ceiling imposed by experimental protocol) the effect of SRIF in mouse tissue appeared to be more potent than that in rat, because equivalent concentrations produced a greater increase in interval in the former tissue. Indeed, with 100 nM SRIF, there was a complete absence of contractile activity in the majority of mouse jejunal segments. In contrast to observations in rat tissue, in which the selective agonists produced a greater effect than SRIF at the equivalent concentration, a higher concentration of BIM-23027 was required to inhibit MMCs in mouse tissue (Fig. 7). Furthermore, although the inhibitory action of BIM-23027 (30 nM, n = 7) in mouse tissue was abolished by prior administration of Cyn (1 µM), the antagonist only partially prevented the inhibition of MMCs produced by the nonselective agonist SRIF (10 nM, n = 5; Fig. 7). The sst₅-receptor antagonist BIM-23056 did not prevent the inhibition of contractions by SRIF (10 nM, n = 4; data not shown).

View larger version (31K): [in this window] [in a new window]   Fig. 6.   Concentration-dependent effect of SRIF in the mouse jejunum. Histograms showing the MMC interval before () and after () administration of SRIF (0.01-100 nM, n = 4). Note that equivalent concentrations of SRIF had a greater magnitude of effect in the mouse jejunum compared with the rat. * P < 0.05, *** P < 0.001 compared with predrug control.

View larger version (27K): [in this window] [in a new window]   Fig. 7.   The effect of the sst2-receptor antagonist Cyn in the mouse. Histograms showing increase in MMC intervals in the mouse by SRIF (10 nM, n = 4) and BIM-23027 (30 nM, n = 5). Note that in this species, the magnitude of the response to the selective sst2-receptor agonist is less than that to SRIF itself, in contrast to findings in the rat (see Fig. 3A). The sst2-receptor antagonist Cyn abolished the inhibitory action of the sst2-selective agonist BIM-23027 (30 nM, n = 7) but only attenuated the inhibition produced by SRIF (10 nM, n = 5). * P < 0.05 compared with predrug control; #P < 0.05 compared with Cyn untreated tissues.

Studies in sst₂-receptor knockout mice. A comparison of the actions of SRIF and BIM-23027 on MMCs was made in tissue taken from mice lacking the sst₂ receptor (Sstr₂^/) and their wild type littermates. There was no difference in the parameters of the contractile activity between the tissues taken from sst₂-receptor knockout and wild-type animals (P > 0.05 n = 5; Fig. 8). MMCs in knockout mouse tissue (n = 5) had a mean duration of 72.3 ± 11 s, mean amplitude of 2.2 ± 0.5 cmH₂O, and were separated by a mean interval of 83.5 ± 18 s. Those in wild-type tissue had a mean duration of 85.8 ± 18 s, mean amplitude of 1.8 ± 0.8 cmH₂O, and were separated by a mean interval of 113.8 ± 24 s. Due to restricted tissue supply, single doses of SRIF (10 nM) and BIM-23027 (30 nM) were chosen that would be expected to produce an inhibitory effect based on the data above. In wild-type jejunum, both agonists SRIF (10 nM) and BIM-23027 (30 nM) produced a significant increase in the MMC interval (139.2 ± 30 vs. 526 ± 58 s, n = 5, P < 0.01 and 116 ± 24 vs. 256.2 ± 62 s, n = 5, P < 0.05, respectively), although the response to BIM-23027 was significantly less than that to SRIF (P < 0.05). In contrast, BIM-23027 was without effect in the knockout mouse jejunum, whereas SRIF produced a significant increase in the MMC interval, the magnitude of which was significantly less than that in the wild-type animal (526 ± 58 compared with 320 ± 84 s, P < 0.05, n = 5; Fig. 9).

View larger version (28K): [in this window] [in a new window]   Fig. 8.   MMC in the wild-type and sst2-receptor knockout mouse. Representative traces showing MMCs evoked by distension (intraluminal pressure 2.5-3.5 cmH2O) in the knockout (/) mice (A) and wild-type littermates +/+ (B). There is no difference in the properties of the MMCs between the 2 groups.

View larger version (12K): [in this window] [in a new window]   Fig. 9.   Effect of SRIF and BIM-23027 on MMC intervals in the mouse jejunum. Individual data points for 5 wild-type (A) and 5 knockout animals (B) showing MMC interval before and after addition of SRIF (10 nM) and BIM-23027 (30 nM). Both BIM-23027 and SRIF significantly increased the interval between MMCs in the wild-type tissue, but only SRIF was effective in the knockout tissue. * P < 0.05 compared with predrug control.

Inhibition of contractions in mouse jejunum by SRIF-receptor activation does not involve an NOS pathway. L-NAME (100 µM) decreased the interval between MMCs from 136.6 ± 25 to 103.1 ± 13 s (n = 8, P < 0.05) and increased the mean amplitude from 2.6 ± 0.5 to 3.6 ± 0.6 cmH₂O (n = 8, P < 0.05). The effect of SRIF (10 nM, n = 4) and BIM-23027 (30 nM, n = 4) on the MMC interval was not influenced by prior treatment with L-NAME, both agonists still producing a significant increase in the MMC interval (Fig. 10, A and B).

View larger version (19K): [in this window] [in a new window]   Fig. 10.   Effect of L-NAME on the inhibition of MMCs produced by SRIF and BIM-23027 in the mouse jejunum. A: L-NAME (100 µM) augmented the amplitude of MMCs and decreased the intervals (data in text). In contrast to observations in rat tissue (see Fig. 4), the effect of BIM-23027 (30 nM) and SRIF (10 nM) on MMC interval was maintained in the presence of L-NAME. B: histograms showing the group data illustrating the magnitude of the effect of BIM-23027 (30 nM, n = 4) and SRIF (10 nM, n = 4) on MMC interval in jejunal segments pretreated with L-NAME (n = 4). * P < 0.05 compared with predrug control.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The ability of SRIF to inhibit intestinal peristalsis peripherally is well documented. However, the site of action of SRIF and the receptor subtype(s) responsible have not been fully characterized. In this study, we have demonstrated that SRIF caused a concentration-dependent inhibition of peristaltic activity evoked by luminal distension in isolated segments of rat and mouse jejunum, with a major role for sst₂ receptors in this effect. However, whereas in rat jejunum, SRIF mediated its inhibitory action via NOS-dependent pathways, this was not the case in mouse jejunum. Moreover, experiments with receptor-selective ligands and in the sst₂ knockout mouse revealed a non-sst₂ receptor-mediated inhibition of intestinal peristalsis in mouse tissue that was not apparent in the rat.

Mechanisms underlying the inhibitory effect of SRIF in the rat and mouse. Somatostatin is widely distributed in the gastrointestinal tract and can be visualized immunocytochemically in mucosal D cells, in some subpopulations of sympathetic nerve terminals, and in the cell bodies and nerve fibers of submucosal and myenteric neurones (10, 13, 31, 38). Colocalization and fiber tracing studies have shown that somatostatin is present in interneurones that form descending chains of neurones that connect to muscle motor neurones (10, 31, 35). This topography is consistent with transmitter release studies by Grider and co-workers (20-23) implicating somatostatin in a complex interaction with GABAergic and enkephalinergic neuronal mechanisms that ultimately regulate NO and VIP release during descending relaxation in the rat colon. A similar neuronal interaction is suggested to regulate tachykinin release from longitudinal muscle motoneurones (22). Thus somatostatin is likely to act within the enteric circuits controling peristalsis rather than at the level of the neuromuscular junction, and this would explain the ability of SRIF to attenuate neurogenically mediated contraction of the guinea pig ileum (14, 18, 24, 36, 45).

Receptor characterization in the rat jejunum. SRIF mediates its actions through a family of G protein-coupled receptors that have been recently cloned (28). All five receptors (sst 1-5) have been shown to be expressed in the gastrointestinal tract (32, 44), with high levels of sst₂ receptor in the rat jejunum. Our attempts to pharmacologically characterize the receptor(s) involved in the inhibition of jejunal peristalsis centered on the use of ligands that have selectivity for the sst₂ receptor. Both octreotide and BIM-23027 are agonists that show selectivity for sst₂ receptors but also have some affinity for the sst₅-receptor subtype, and both mimicked the effect of SRIF in inhibiting contraction complex generation. Although EC₅₀ values could not be calculated, lower concentrations of octreotide and BIM-23027 than of SRIF were necessary to inhibit MMCs, consistent with an action at sst₂ receptors. BIM-23027 was about three times more potent than SRIF at inhibiting neurogenic contractions of the guinea pig ileum (15), and a similar potency order was observed for sst₂-mediated inhibition of rat parietal cell secretion (43) and rat colon contractions (33).

Receptor characterization in the mouse jejunum. Although SRIF exerted an inhibitory effect on MMC generation in the mouse jejunum, there were some notable differences from the observations in the rat. Firstly, whereas octreotide and BIM-23027 were more effective at lower concentrations than SRIF in the rat, the reverse was true in the mouse jejunum, in which SRIF was the most effective of the agonists. One explanation for this is that the different sst receptors are expressed in the rat and mouse. In this report, although Cyn abolished the response to BIM-23027 in the mouse jejunum, it only attenuated the response to SRIF. Thus, in the mouse, there is an additional non-sst₂ receptor that is functionally linked to inhibition of intestinal peristalsis. The lack of effect of BIM-23056, which has been shown to have antagonist activity at human sst₅ receptors, on either MMC intervals themselves or on the response to SRIF, would argue against the involvement of this receptor subtype.

Responses in the sst₂ knockout mouse. MMCs were not different in the sst₂ receptor knockout mouse compared with its wild-type littermate. Similarly, the sst₂ antagonist had no effect on baseline MMC activity in the rat jejunum. Thus, although exogenous SRIF can exert a profound inhibitory effect via sst₂ receptors, there is little evidence that endogenous SRIF is involved in the regulation of MMC generation under the current experimental circumstances. This may reflect redundancy in the sst-receptor mechanisms that influence intestinal contractile activity. Indeed, the observation that SRIF but not BIM-23027 was able to evoke an inhibitory effect on MMC generation (despite the absence of functional sst₂ receptors) confirms that an additional receptor subtype is involved in the regulation of intestinal peristalsis in the mouse.