Submitted on January 31, 2005
Accepted on May 25, 2005
Secretin activates vagal primary afferent neurons in the rat: evidence from electrophysiological and immunocytochemical studies
Ying Li1, Xiaoyin Wu1, Harry Yao1, and Chung Owyang1*
1 Gastroenterology Research Unit, Department of Internal Medicine, University of Michigan Health System, Ann Arbor, MI, USA
* To whom correspondence should be addressed. E-mail: cowyang{at}umich.edu.
In this study we evaluated the vagal afferent response to secretin at physiological concentrations
and localized the site of secretin's action on vagal afferent pathways in the rat. The discharge of
sensory neurons supplying the gastrointestinal tract was recorded from the nodose ganglia. Of 91
neurons activated by electrical vagal stimulation, 19 neurons showed an increase in firing rate in
response to intestinal perfusion of 5-HT (1.5 ± 0.2 to 25 ± 4 impulses/20 s) but no response to
intestinal distension. Close intra-arterial injection of secretin (2.5 and 5.0 pmol) elicited
responses in 15 of these 19 neurons (1.5 ± 0.2 impulses/20 s at basal to 21 ± 4 and 43 ± 5
impulses/20 s, respectively). Subdiaphragmatic vagotomy and perivagal application of capsaicin,
but not supranodose vagotomy, completely abolished the secretin-elicited vagal nodose neuronal
response. In a separate study, 9 tension receptor afferents among 91 neurons responded positively
to intestinal distension but failed to respond to luminal 5-HT. These 9 neurons also showed no
response to administration of secretin. As expected, immunohistochemical studies showed that
secretin administration significantly increased the number of Fos-positive neurons in the vagal
nodose ganglia. In conclusion, we demonstrated for the first time that vagal sensory neurons are
activated by secretin at physiological concentrations. A subpopulation of secretin-sensitive vagal
afferent fibers is located in the intestinal mucosa, many of which are responsive to luminal 5-HT.
