MUSCARINE ACTIVATES A NONSELECTIVE CATION CURRENT THROUGH A M(3) MUSCARINIC RECEPTOR SUBTYPE IN RAT DORSOLATERAL SEPTAL NUCLEUS NEURONS

1. In the present study, we examined the cellular mechanism and recept or type responsible for a muscarine-induced inward current (I-mi) in n eurons of rat dorsolateral septal nucleus (DLSN) using single-microele ctrode voltage-clamp and ''slice'' patch-clamp techniques. 2. I-mi was associated with an increase of membrane conductance in 75% of DLSN ne urons. There was no voltage-dependence of I-mi between -60 and -140 mV ; it exhibited a reversal potential of -17.0 +/- 5.3 mV (n = 14) deter mined by extrapolation of I-mi and voltage relationship recorded using whole cell patch recording. Lowering extracellular sodium (26 mM) or potassium (1.4 mM) ions depressed I-mi. 3. I-mi was concentration depe ndent; 3 and 100 mu M muscarine produced the minimum [22 +/- 4.6 pA, ( mean +/- SE) n = 8] and maximum (167 +/- 28 pA, n = 7) responses, resp ectively. An EC(50) was determined to be 15 mu M (n = 8). Oxotremorine -methiodide (1-100 mu M) also produced an inward current with similar potency compared with muscarine. On the other hand, McN-A-343 and pilo carpine (3-100 mu M) did not produce any inward current in DLSN neuron s. 4. Atropine (1 mu M) completely reduced I-mi produced by 30 mu M mu scarine, whereas pirenzepine (PZP) shifted the concentration-response curve for muscarine in a parallel manner to the right. The EC(50) for muscarine was shifted to 32, 52, and 204 mu M by 0.2, 0.5, and 2 mu M PZP, respectively. The apparent K-d value for PZP estimated by Schild plot analysis was 190 nM (n = 5). 5. Methoctramine (1 mu M) also compe titively depressed I-mi the calculated EC(50) values were 26, 41, and 107 mu M in concentrations of 0.2, 2, and 10 mu M methoctramine, respe ctively. The apparent K-d for methoctramine was 420 nM. In contrast, A F-DX 116 (1 mu M) did not significantly inhibit I-mi. 6. Intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, suppressed irreversibly I-mi. Pretreatment of DLSN ne urons with pertussis toxin (PTX) did not prevent I-mi (n = 8). 7. We s uggest that muscarine causes this inward current by activating a M(3) subtype of muscarinic receptor, which is coupled to a PTX-insensitive GTP-protein in rat DLSN neurons.