Nicotinic acetylcholine receptors in the rat and primate nucleus tractus solitarius and on rat and human inferior vagal (nodose) ganglia: evidence from in vivo microdialysis and [125I]alpha-bungarotoxin autoradiography.

The nucleus tractus solitarius is a key brain centre involved in the regulation of numerous autonomic functions. The present study has employed in vitro autoradiography and in vivo microdialysis to investigate the presence and function of nicotinic acetylcholine receptors located in the medial nucleus tractus solitarius of the rat. Autoradiography using [125I]alpha-bungarotoxin (0.5 nM) enabled visualization of binding sites on sections of rat and monkey brainstem. Specific binding was highest in the medial nucleus tractus solitarius. The presence of binding sites was also apparent on sections of rat nodose ganglia/vagus nerve and human inferior vagal ganglia. Subsequent to unilateral ligation of the vagus nerve in the rat, an accumulation of binding sites was visualized adjacent to the ligature. Unilateral nodose ganglionectomy in the rat caused an approximate 97% reduction in [125I]alpha-bungarotoxin binding site density in the medial nucleus tractus solitarius from 814 +/- 183 to 27 +/- 2 d.p.m./mm2. Microdialysis results indicated that local administration of nicotine (1 mM) into the nucleus tractus solitarius of the rat resulted in increases of extracellular L-glutamate of 146 +/- 9% of basal levels. This effect was not reproducible following a second stimulation and was also blocked by prior and co-administration of the nicotinic acetylcholine receptor antagonist mecamylamine (100 microM). Extracellular levels of L-aspartate exhibited a similar pattern although results were not significant. Taken together, these results are supportive of the presence of a population of [125I]alpha-bungarotoxin binding sites located presynaptically with respect to vagal afferent terminals in the medial nucleus tractus solitarius of the rat. It is possible that these binding sites are the site of action of locally administered nicotine on extracellular levels of L-glutamate, the favoured neurotransmitter at primary baroreceptor afferent fibres. These data are discussed in relation to the functional pharmacology of acetylcholine and nicotinic acetylcholine receptors in this region of the brain.