Nicotinic receptor subtypes in rat subfornical organ neurons and glial cells

It is unclear which nicotinic acetylcholine receptor (nAChR) subtypes are involved in the nicotinic activation of cells in the subfornical organ (SFO). We investigated the nAChR subtype using molecular biological, electrophysiological, pharmacological and immunohistochemical techniques. The use of reverse transcription–polymerase chain reaction in rats demonstrated the presence of mRNAs for the α2, α3, α4, α6, α7, β2 and β4 subunits in the SFO. The characteristics and dose-dependency of nicotine-induced inward currents in many dissociated SFO neurons were similar to those induced by acetylcholine in the presence of atropine. The nicotine-induced currents were larger than those induced by cytisine in most responding cells, suggesting the predominance of the β2- rather than the β4-containing nAChR. NIC-induced currents were significantly inhibited by dihydro-β-erythroidine (a relatively selective antagonist for α4-containing nAChRs, and a partial antagonist for α2 or α3) at 300 nM in all responding cells. Additionally, the currents were significantly inhibited by α-conotoxin MII at 10 nM (a selective antagonist for α3- and/or α6-containing nAChRs) in some but not all responding cells. Methyllycaconitine at 10 nM (a selective α7-nAChR antagonist) reduced the nicotine-induced current significantly, but to a lesser extent. Fluorescence-labeled α-bungarotoxin (a homomeric α7 subtype selective binding drug) binding and immunofluorescence for the α7 subunit showed that positive images almost overlapped with those immunopositive for an astrocyte marker. These results suggest that the α4β2 subtype is the main functional receptor in SFO neurons while α2, α3, α6, and β4 subunits have some effect, and homomeric the α7 subtype exists in SFO astrocytes.