DESENSITIZATION OF NICOTINE-STIMULATED RB-86(-BRAIN SYNAPTOSOMES() EFFLUX FROM MOUSE)

The desensitization of nicotine-stimulated Rb-86(+) efflux from synapt osomes prepared from C57BL/6 mouse brain was investigated. Nicotine st imulated a saturable, concentration-dependent efflux of Rb-86(+) from synaptosomes (EC(50) = 0.60 mu M), but the response decreased with tim e of exposure to nicotine. The rate of decrease of the response (desen sitization) increased as the nicotine concentration was increased (EC( 50) = 0.35 mu M; maximal rate of desensitization = 1.1 min(-1)). Desen sitization of nicotine-stimulated Rb-86(+) efflux was also observed wh en synaptosomes were exposed to low (1-200 nM) concentrations of nicot ine that caused little or no stimulation of efflux (EC(50) = 13 nM). T he rate of desensitization observed with low nicotine concentrations ( 0.30 min(-1)) was less than that measured at stimulating concentration s. Desensitization was not fully reversible for synaptosomes exposed t o nicotine concentrations between 10 nM and 10 mu M: Only 60-40% of th e control response was regained after a 10-min washout period. The kin etics of functional desensitization were compared with the kinetics of [H-3]nicotine binding. [H-3]Nicotine binding to midbrain particulate fractions displayed both a fast and a slow phase. The EC(50) values fo r these two phases were 2.6 and 14 nM, respectively. Data obtained fro m functional desensitization and ligand binding experiments were analy zed using a two-state model. The kinetic constants obtained from the a nalyses of these two processes were very similar. Overall, the results suggest that nicotinic receptor function measured with ion flux desen sitizes when exposed to either stimulating or nonstimulating concentra tions of nicotine. In addition, the kinetic properties calculated for the functional desensitization are comparable to those for [H-3]nicoti ne binding.