Contribution of galanin to non-cholinergic, non-adrenergic transmission in rat ileum

Abstract

1 We examined the possibility that the neuropeptide, galanin, may act as a transmitter in longitudinal muscle isolated from the rat ileum. 2 Galanin at nanomolar concentrations produced a phasic contraction with a concomitant increase in rhythmic activity. At concentrations in excess of 3 × 10⁻⁸ m, the contraction was followed by a rapid desensitization; hence, with the cumulative re-addition of galanin, there was no response. This desensitization was probably selective for galanin because there was no attenuation of the contractile responses to substance P, neurokinin A and B, bradykinin or carbachol. 3 The phasic contraction induced by galanin was not inhibited by atropine, guanethidine, hexamethonium, naloxone, tetrodotoxin or [D-Pro², D-Trp^7,9]-substance P. 4 Electrical stimulation of intramural nerves at low frequencies (1–5 Hz) led to an augmentation of spontaneous rhythmic contractions, which were completely or partially inhibited by atropine. However, guanethidine, hexamethonium, naloxone, [D-Pro², D-Trp^7,9]-substance P and desensitization to galanin were without effect on the response to such electrical stimulation. 5 In contrast, transmural electrical stimulation at higher frequencies in the presence of atropine and guanethidine produced biphasic contractile responses with transient and slow components. The slow component was selectively attenuated by galanin desensitization. 6 The slow component induced by high frequency stimulation was markedly attenuated by repeated electrical stimulation at short intervals (2.5 min between 30 s trains). Following repeated stimulation, the contractile response to galanin was also attenuated. Thus, a cross-desensitization between the mediator of the slow component and galanin had to be considered. In contrast, responses to tachykinins and the transient component induced by electrical stimulation were without effect. 7 Somatostatin, vasoactive intestinal polypeptide and α,β-methylene ATP were without effect on the tone of the muscle. Calcitonin gene-related peptide, neurotensin, gastrin-releasing peptide, neuropeptide Y and capsaicin produced either a transient arrest of the spontaneous rhythmic activity or a transient relaxation. 8 These results suggest that the slow component of the non-cholinergic non-adrenergic contraction, as induced by intramural nerve stimulation is apparently due to the endogenous release of galanin, presumably released from galanin-containing nerves in the rat ileum.