Extrinsic afferent nerve sensitivity and enteric neurotransmission in murine jejunum in vitro

Abstract

Enteric and extrinsic sensory neurons respond to similar stimuli. Thus they may be activated in series or in parallel. Because signal transmission via synapses or mediator release would depend on calcium, we investigated its role for extrinsic afferent sensitivity to chemical and mechanical stimulation. Extracellular multiunit afferent recordings were made in vitro from paravascular nerve bundles supplying the mouse jejunum. Intraluminal pressure and afferent nerve responses were recorded under control conditions and under four conditions designed to interfere with enteric neurotransmission. We found that phasic intestinal contractions ceased after switching perfusion to Ca2+-free buffer with or without a purinergic P2 receptor antagonist, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) or cadmium (blocking all Ca2+-channels) but not following ω-conotoxin GVIA (N-type Ca2+-channel blocker). Luminal HCl (pH 2) and 5-HT (500 μM) evoked peak firing of 17 ± 4 impulses per second (imp/s) ( n = 10) and 21 ± 4 imp/s ( n = 13) under control conditions. These responses were reduced to 4 ± 2 imp/s and 5 ± 2 imp/s by cadmium ( n = 7, P < 0.05), to 7 ± 2 imp/s and 6 ± 1 imp/s by Ca2+-free perfusion ( n = 6, P < 0.05), and to 3 ± 1 imp/s and 4 ± 1 imp/s by Ca2+-free perfusion with PPADS ( n = 6, P < 0.05). Responses were unchanged by ω-conotoxin GVIA. Mechanical ramp distension of the intestinal segment to 60 cmH2O was not altered by any of the experimental conditions. We concluded that HCl and 5-HT activate extrinsic afferents via a calcium-dependent mechanism, which is unlikely to involve enteric neurons carrying N-type calcium channels. Extrinsic mechanosensitivity is independent of enteric neurotransmission. It appears that cross talk from the enteric to the extrinsic nervous system does not mediate extrinsic afferent sensitivity.