Key Role of Mucosal Primary Afferents in Mediating the Inhibitory Influence of Capsaicin on Vagally Mediated Contractions in the Mouse Esophagus

Abstract

ABSTRACT. Transient receptor potential ion channel of the vanilloid type 1 (TRPV1)-dependent pathway, consisting of capsaicin-sensitive tachykininergic primary afferent and myenteric nitrergic neurons, was suggested to mediate the inhibitory effect of capsaicin on the vagally mediated striated muscle contractions in the rat esophagus. These primary afferent neurons upon entering into the esophagus are distributed through the myenteric plexus, terminating either in the myenteric ganglia or en route to the mucosa where they branch into a delicate net of fine varicose fibers. Therefore, this study aimed to investigate whether the mucosal primary afferents are a main mediator for the capsaicin inhibitory influence on vagally mediated contractions in the mouse esophagus. For this purpose, the vagally induced contractile activity of a thoracic esophageal segment was measured in the circular direction with a force transducer. Vagal stimulation (30 µsec, 25 V, 1-50 Hz for 1 sec) produced monophasic contractile responses, whose amplitudes were frequency-dependent. These contractions were completely abolished by d-tubocurarine (5 µM) while resistant to atropine (1 µM) and hexamethonium (100 µM). Capsaicin (30 µM) significantly inhibited the vagally induced contractions in esophagi with intact mucosa while its effect on preparations without mucosa was insignificant. Additionally, immunocytochemistry revealed the presence of TRPV1-positive nerve fibers in the tunica mucosa. Taken together, we conclude that in the mouse esophagus, capsaicin inhibits the vagally mediated striated muscle contractions mainly through its action on mucosal primary afferents, which in turn activate the presumed inhibitory local reflex arc. KEY WORDS: capsaicin-sensitive neurons, enteric co-innervation, local effector function, TRPV1, vagus nerve.J. Vet. Med. Sci. 69(4): 365-372, 2007 A peripheral reflex arc composed of capsaicin-sensitive tachykininergic primary afferent and myenteric nitrergic neurons have been shown to negatively influence the vagally mediated striated muscle contractions in the rat esophagus [27]. Esophageal neuromuscular junctions receive a dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus, the so-called enteric co-innervation [for review see 21, 34]. Enteric neurons in the esophagus are contacted by primary afferents of spinal and vagal origin [19,20,25]. A substantial number of these primary afferents, mainly of spinal origin, were shown to be immunoreactive to the transient receptor potential ion channel of the vanilloid type 1 (TRPV1) and called capsaicin-sensitive sensory neurons [12,14,15,23,30,31,33]. These neurons can modulate intestinal motility by transferring signals from the gastrointestinal tract to the central nervous system and simultaneously releasing transmitters, in particular substance P (SP) and calcitonin generelated peptide (CGRP) that can influence the activity of intrinsic neurons [14,16].Primary affe...

CO2 chemosensing in rat oesophagus

“…All ASIC paralogues were observed in the prickle cell layer in oesophageal mucosa (fig 6A,D,G). Furthermore, ASIC3 was expressed in nerve fibres in the muscularis mucosa (fig 6G), and TRPV1 was localised in the muscularis mucosa as previously reported, 33 consistent with its functional involvement in oesophageal chemosensitivity and mechanosensitivity. 3435 Interestingly, TRPV1 was clearly recognised in the granular cells in the stratum granulosum (fig 6J), between the prickle cell layer and stratum corneum, consistent with the expression of TRPV1 in the human epidermis, where TRPV1 is localised in granular cells (most intense), prickle cells and basal cells.…”

Galanin modulates vagally induced contractions in the mouse oesophagus

“…In this context it would be useful to employ tetrodotoxin, a selective blocker of action potentials, to further support the neuronal origin of galanin evoked response. However, in our previous studies, 6,7 tetrodotoxin also inhibited electrical field stimulation induced contractions that were unaffected by d ‐tubocurarine, indicating that tetrodotoxin at this concentration not only blocks neural transmission but also directly inhibits striated muscle contraction. This may be attributed to tetrodotoxin‐sensitive voltage‐dependent sodium channels distributed, although at lower densities, over the entire muscle cell membrane 38 and renders this drug useless for our purpose.…”

“…One pin was fixed to the bottom of the organ bath and the other was connected to the bar of an isometric force transducer (T7–30–240; Orientec, Tokyo, Japan). Contractile activity was measured in the circular direction so as to avoid contamination from contractions of the longitudinally oriented smooth muscularis mucosae 7 . An initial resting tension of 0.5 g was applied to the preparations, which were subsequently allowed to equilibrate for 30–45 min.…”

“…2,3 An inhibitory function of co-innervating enteric neurons as suggested by anatomical studies 2 was recently supported by experiments using in vitro vagus nerve-oesophagus preparations of hamster, rat and mouse. [4][5][6][7] In these models, the capsaicin-triggered inhibition of vagally induced striated muscle contractions was significantly, though not completely, reduced by the NOS blocker NG-nitro-L-arginine methyl ester (L-NAME) and in preparations pretreated with tachykinin receptor antagonists, and was abrogated in neonatally capsaicin-treated animals. [5][6][7] This indicated that capsaicin stimulated transient receptor potential vanilloid type 1 (TRPV1) expressing tachykininergic neurons, most likely primary afferents, that in turn excited nitric oxide (NO) releasing co-innervating neurons via neurokinin-1 (NK-1) receptors.…”

“…[4][5][6][7] In these models, the capsaicin-triggered inhibition of vagally induced striated muscle contractions was significantly, though not completely, reduced by the NOS blocker NG-nitro-L-arginine methyl ester (L-NAME) and in preparations pretreated with tachykinin receptor antagonists, and was abrogated in neonatally capsaicin-treated animals. [5][6][7] This indicated that capsaicin stimulated transient receptor potential vanilloid type 1 (TRPV1) expressing tachykininergic neurons, most likely primary afferents, that in turn excited nitric oxide (NO) releasing co-innervating neurons via neurokinin-1 (NK-1) receptors. 8 The incomplete reduction of the capsaicintriggered inhibition by L-NAME suggested involvement of synergistic co-transmitters, e.g.…”