Receptor subtypes mediating depressor responses to microinjections of nicotine into medial NTS of the rat

Microinjections (50 nl) of nicotine (0.01-10 mM) into the nucleus of the so litary tract (NTS) of adult, urethan-anesthetized, artificially ventilated, male Wistar rats, elicited decreases in blood pressure and heart rate. Pri or microinjections of alpha-bungarotoxin (alpha-BT) and alpha-conotoxin ImI (specific toxins for nicotinic receptors containing alpha 7 subunits) elic ited a 20-38% reduction in nicotine responses. Similarly, prior microinject ions of hexamethonium, mecamylamine, and alpha-conotoxin AuIB (specific blo ckers or toxin for nicotinic receptors containing alpha 3 beta 4 subunits) elicited a 47-79% reduction in nicotine responses. Nicotine responses were completely blocked by prior sequential microinjections of alpha-BT and meca mylamine into the NTS. Complete blockade of excitatory amino acid receptors (EAARs) in the NTS did not attenuate the responses to nicotine. It was con cluded that 1) the predominant type of nicotinic receptor in the NTS contai ns alpha 3 beta 4 subunits, 2) a smaller proportion contains alpha 7 subuni ts, 3) the presynaptic nicotinic receptors in the NTS do not contribute to nicotine-induced responses, and 4) EAARs in the NTS are not involved in med iating responses to nicotine.