Evidence for inhibition of opossum LES through intrinsic gastric nerves

Vagal neurotransmission to the ferret lower oesophageal sphincter: inhibition via GABA(B) receptors

GABA(B) receptors modulate the function of the lower oesophageal sphincter (LOS) in vivo by inhibiting neurotransmitter release in the vagal pathway controlling LOS relaxation. We aimed to determine whether this effect was mediated peripherally on vagal motor outflow to the ferret LOS in vitro. The LOS, with intact vagal innervation, was prepared from adult ferrets and LOS tension measured. Vagal stimulation (0.5 - 10 Hz, 30 V) evoked a tetrodotoxin-sensitive, frequency-dependent relaxation. Both GABA (3x10(-4) M) and (+/-)baclofen (2x10(-4) M) inhibited vagally-stimulated LOS relaxation. The potent GABA(B) receptor-selective agonist 3-APPA dose-dependently inhibited vagally-stimulated LOS relaxation, with an EC(50) value of 0.7 microM Decreased responses following vagal stimulation in the presence of (+/-)baclofen or 3-APPA were reversed with the potent GABA(B) receptor antagonist CGP 62349. Neither CGP 62349 nor muscimol (GABA(A) receptor agonist) alone affected LOS responses following vagal stimulation. Agonists of other G protein-coupled receptors (clonidine (alpha(2)-adrenoceptor) (5x10(-6) M), U50488 (kappa opioid) (10(-5) M), neuropeptide Y (10(-6) M)) did not affect vagally-mediated LOS relaxation. The present study supports a discrete presynaptic inhibitory role for GABA(B) receptors on vagal preganglionic fibres serving inhibitory motorneurones in the ferret LOS.

Brainstem pathways responsible for oesophageal control of gastric motility and tone in the rat

1. Previous anatomical studies indicate that the nucleus of the solitary tract, pars centralis (NSTc) contains the neurones which receive vagal afferent input from the oesophagus. The purpose of the present study was to characterize the NSTc circuits in the medulla that may be responsible for oesophageal control of gastric motility. 2. Moderate balloon distension of the oesophagus of the rat (14-18 mmHg) provoked a significant reduction in gastric motility and tone recorded with strain gauges. This receptive relaxation effect was eliminated by bilateral lesions centred on the NSTc. 3. NSTc cells activated by oesophageal distension were labelled extracellularly and juxtacellularly with neurobiotin. NSTc neurones send axonal projections throughout the entire rostral-caudal extent of the dorsal motor nucleus of the vagus (DMN). These NSTc-DMN connections were confirmed by retrograde transport of neurobiotin from DMN to NSTc. NSTc neurones were observed with dendrites arborizing within the ependymal lining of the fourth ventricles. Thus, NSTc neurones may be in position to monitor blood-borne or ventricular agents and to alter the function of gastric-vago-vagal reflexes in response to these stimuli. 4. Neurophysiological recordings identified two subpopulations of DMN neurones which may be either activated or inhibited by oesophageal distension. Neurones excited by oesophageal distension were located mainly lateral and caudal in the DMN; neurones inhibited by oesophageal stimulation were located in medial and rostral DMN. 5. Our neurobiotin tracing results verified earlier studies showing that the NSTc projects to the intermediate reticular nucleus and the compact division of the nucleus ambiguus. Additionally, we found that the NSTc may be involved in reciprocal connections with the anterior, rostrolateral NST. 6. These results suggest that the gastric relaxation evoked by oesophageal distension is critically dependent on intact brainstem vago-vagal circuits. The NSTc, the recipient of oesophageal afferent projections from the vagus nerve, sends axons to the entire DMN, the source of parasympathetic control of the stomach. DMN neurones respond differentially to oesophageal distension, reinforcing the view that oesophageal afferents may provoke gastric relaxation by activating a vagal inhibitory pathway while simultaneously inhibiting a vagal excitatory pathway.