Muscarine Activates a Non-selective Cation Current Through an M3 Muscarinic Receptor Subtype in Rat Dorsolateral Septal Nucleus Neurons. HASUO, HIROSHI, TAKASHI AKASU, AND JOEL P. GALLAGHER. Department of Physiology, Kurume University School of Medicine, 67 Asahi- machi, Kurume 830, Japan and Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031 U.S.A.
APStracts 3:0136N, 1996.
1. In the present study, we examined the cellular mechanism and receptor type responsible for a muscarine-induced inward current (I mi ) in neurons of rat dorsolateral septal nucleus (DLSN) using single-microelectrode voltage-clamp and slice patch-clamp techniques. 2. I mi was associated with an increase of membrane conductance in 75% of DLSN neurons. 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) determined 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-dependent; 3 and 100 [mu]M muscarine produced the minimum (22 +/- 4.6 pA, n =8) and maximum (167 +/- 28 pA, n =7) responses, respectively. An EC 50 was determined to be 15 [mu]M ( n =8). Oxotremorine-M (1-100 [mu]M) also produced an inward current with similar potency compared to muscarine. On the other hand, McN-A-343 and pilocarpine (3-100 [mu]M) did not produce any inward current in DLSN neurons. 4. A tropine (1 [mu]M) completely reduced I mi produced by 30 [mu]M muscarine, while 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 competitively 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, respectively. The apparent K d for methoctramine was 420 nM. In contrast, AF-DX 116 (1 [mu]M) did not significantly inhibit I mi. 6. Intracellular dialysis with GTP_S, a non-hydrolyzable analogue of GTP, suppressed irreversibly I mi . Pre- treatment of DLSN neurons with pertussis toxin (PTX) did not prevent I mi ( n =8). 7. We suggest that muscarine causes this inward current by activating an M 3 subtype of muscarinic receptor which is coupled to a PTX-insensitive GTP- protein in rat DLSN neurons.

Received 30 August 1996; accepted in final form 14 June 1996.
APS Manuscript Number J574-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96