Nicotinic ACh receptor subtypes on gastrointestinally projecting neurones in the dorsal motor vagal nucleus of the rat.

To determine the predominant nicotinic ACh receptor (nAChR) located on neurones in the dorsal motor nucleus of the vagus (DMV) that project to the gastrointestinal tract, we used the rat brainstem slice preparation and whole-cell recordings of DMV neurones identified by retrograde DiI tracing to pharmacologically characterize nAChRs. Pressure ejection of acetylcholine (ACh, 250 microM for 200 ms) from a patch pipette placed approximately 10-20 microm from the surface of the recorded cell produced an inward current in most DMV neurones sampled. The average currents for neurones projecting to the fundus, antrum and caecum were 149 +/- 38 (n = 25), 115 +/- 18 (n = 29) and 117 +/- 23 pA (n = 6), respectively. Blockade of the alpha7 subtype of nAChR with either alpha-bungarotoxin (alpha-BGT) or methyllycaconitine (MLA) counteracted 60-75 % of the ACh-evoked current in DMV neurones projecting to the fundus, antrum and caecum. In neurones projecting to the fundus and the antrum, currents resistant to alpha-BGT were significantly blocked by dihydro-beta-erythroidine (10-20 nM), an antagonist of the alpha4beta2 subtype of nAChR. In neurones projecting to the caecum, currents resistant to alpha-BGT were significantly depressed by a low concentration of mecamylamine (1 microM). Cytisine (100 microM), an agonist of nAChRs that contain the alpha7 or the beta4 subunit, evoked significant currents in caecum-projecting neurones that were previously exposed to alpha-BGT. In contrast, cytisine had no effect on DMV neurones previously exposed to alpha-BGT that project to the fundus or antrum. Our data indicate that the prevailing nAChR subtype in DMV neurones projecting to the GI tract is the alpha7 subtype. In addition, we obtained evidence for the co-expression of the alpha4beta2 nAChR subtype on DMV neurones projecting to the fundus and antrum, and the alpha3beta4 nAChR subtype on DMV neurones projecting to the caecum.