Calcitonin gene-related peptide mediates capsaicin-induced neuroendocrine responses in rat antrum.

BACKGROUND: Calcitonin gene-related peptide (CGRP) is a 37-amino acid peptide localized to primary sensory afferent nerves in the rat stomach. The actions of CGRP in regulating antral neuroendocrine function were examined in vitro through the use of capsaicin, an agent capable of evoking neuropeptide release from peripheral sensory nerve endings. These results were compared with the effects of exogenous CGRP and CGRP antagonist, CGRP8-37. METHODS: Rat antral mucosal/submucosal fragments were incubated in either static or dynamic perifusion experiments. Media were assayed for gastrin, somatostatin, CGRP, and acetylcholine. RESULTS: Capsaicin, like exogenous CGRP, stimulated antral somatostatin release and inhibited both gastrin release and acetylcholine discharge. Low dose capsaicin (1 x 10(-5) mol/L) caused significant stimulation of CGRP release: 33 +/- 0.2 vs. 14 +/- 1 pg/mL protein; P < 0.001. Tetrodotoxin blocked capsaicin-induced inhibition of acetylcholine release and prevented partially capsaicin-mediated stimulation of CGRP release. The CGRP receptor antagonist CGRP8-37 prevented capsaicin-induced D-cell stimulation and inhibition of G-cell secretion and cholinergic discharge. CONCLUSIONS: The effects of capsaicin-induced changes in antral D- and G-cell secretion and acetylcholine discharge are due primarily to release of CGRP. Antral CGRP release from primary sensory afferent nerve terminals may act as a local effector substance to regulate antral neuroendocrine function.

Calcitonin gene-related peptide: mechanisms of modulation of antral endocrine cells and cholinergic neurons.

Actions of human calcitonin-gene related peptide (hCGRP) on acetylcholine (ACh) discharge and gastrin and somatostatin release from rat antral mucosal-submucosal fragments were examined in both dynamic perifusion experiments and short-term static incubation studies. The principal findings of the dynamic perifusion experiments were that hCGRP exerted a dual or biphasic effect on ACh discharge and gastrin release. Initial exposure of antral tissues to hCGRP (1 x 10(-8) M) resulted in stimulation of both ACh and gastrin release that was of brief duration. Continued hCGRP perifusion caused subsequent inhibition of ACh and gastrin release that was substantially greater in duration and magnitude than the initial stimulatory responses. Static incubation studies indicated that hCGRP (10(-10) to 10(-7) M) stimulated somatostatin and inhibited gastrin release in a dose-dependent manner. Inhibition of gastrin and ACh release by hCGRP appeared to be an indirect effect that was mediated by somatostatin as suggested by studies with pertussis toxin (200 ng/ml). Furthermore, studies with atropine (1 x 10(-6) M) and tetrodotoxin (1 x 10(-6) M) indicated that CGRP-induced stimulation of somatostatin release and inhibition of ACh discharge occurred independent of muscarinic receptor activation and nerve excitation. In conclusion, results of these studies indicate that CGRP is capable of exerting both stimulatory and inhibitory effects on ACh release from mucosal-submucosal neurons and gastrin release from antral mucosal G cells in in vitro studies. These data suggest that the inhibitory effects of CGRP on cholinergic discharge and gastrin release are due to the paracrine effects of somatostatin released from antral D cells by direct action of CGRP.

gamma-Aminobutyric acid localization and function as modulator of cholinergic neurotransmission in rat antral mucosal/submucosal fragments.

gamma-Aminobutyric acid, a neurotransmitter in the central nervous system, has been shown to be present in and synthesized and secreted by rodent and feline myenteric plexus neurons. The aims of the present studies were to measure gamma-aminobutyric acid concentrations and synthesis and to establish cellular localization and uptake of gamma-aminobutyric acid by immunocytochemistry and autoradiography, respectively, within mucosal and submucosal tissues of the rat antrum. Direct demonstration of [3H]gamma-aminobutyric acid release and the effects of exogenous gamma-aminobutyric acid and muscimol, a GABA alpha agonist, on [3H]acetylcholine release from antral mucosal/submucosal fragments were examined in perifusion experiments. gamma-Aminobutyric acid content and synthesis, as reflected by glutamic acid decarboxylase activity, were present within antral mucosa at levels two to three times that of the body and muscular layers of both the gastric body and antrum. gamma-Aminobutyric acid was identified immunocytochemically, principally in mucosal epithelial cells of the antrum. Exogenous gamma-aminobutyric acid and muscimol were capable of stimulating acetylcholine release through a GABA alpha receptor-mediated mechanism that was abolished by tetrodotoxin. These results indicate that gamma-aminobutyric acid is present in and taken up by epithelial cells of the gastric antrum and that gamma-aminobutyric acid is capable of being synthesized by antral mucosal/submucosal tissues. Furthermore, these studies suggest that a peripheral gamma-aminobutyric acid mechanism that may modulate cholinergic neurotransmission and endocrine cell function exists within the antrum.