Inhibitory effects of galanin on evoked [Ca2+]i responses in cultured myenteric neurons

Abstract

Galanin modulates gastrointestinal motility by inhibiting the release of ACh from enteric neurons. It is, however, not known whether galanin also inhibits neuronal cholinergic transmission postsynaptically and whether galanin also reduces the action of other excitatory neurotransmitters. The aim of the present study was thus to investigate the effect of galanin on the evoked intracellular Ca2+ concentration ([Ca2+]i) responses in myenteric neurons. Cultured myenteric neurons from small intestine of adult guinea pigs were loaded with the Ca2+ indicator fluo-3 AM, and the [Ca2+]i responses following the application of different stimuli were quantified by confocal microscopy and expressed as a percentage of the response to high-K+ solution (75 mM). Trains of electrical pulses (2 s, 10 Hz) were applied to stimulate the neuronal fibers before and after a 30-s superfusion with galanin (10−6 M). Substance P (SP), 5-HT, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP), and carbachol were used as direct postsynaptic stimuli (10−5 M, 30 s) and were applied alone or after galanin perfusion. Galanin significantly reduced the responses induced by electrical fiber stimulation (43 ± 2 to 35 ± 3%, P = 0.01), SP (15.4 ± 1 to 8.0 ± 0.3%, P < 0.01), and 5-HT (26 ± 2 to 21.4 ± 1.5%, P < 0.05). On the contrary, galanin did not affect the responses induced by local application of DMPP and carbachol. We conclude that in cultured myenteric neurons, galanin inhibits the excitatory responses induced by electrical stimulation, SP, and 5-HT. Finally, the inhibitory effect of galanin on electrical stimulation, but not on DMPP- and carbachol-induced responses, suggests that, at least for the cholinergic component, galanin acts at the presynaptic level.