DESENSITIZATION OF NICOTINE-STIMULATED [H-3] DOPAMINE RELEASE FROM MOUSE STRIATAL SYNAPTOSOMES

Potential desensitization of brain nicotinic receptors was studied using a [H-3]dopamine release assay. Nicotine-stimulated [H-3]dopamine release from mouse striatal synaptosomes was concentration-dependent with an EC50 of 0.33 +/- 0.13 muM and a Hill coefficient of 1.44 +/- 0.18. Desensitization by activating concentrations of nicotine had a similar EC50 and a half-time of 35 s. Concentrations of nicotine that evoked little release also induced a concentration-dependent desensitization (EC50 = 6.9 +/- 3.6 nM, t1/2 = 1.6-2.0 min, n(H) = 1.02 +/- 0.01). Both types of desensitization produced a maximum 75% decrease in [H-3]dopamine release. Recovery from desensitization after exposure to low or activating concentrations of nicotine was time-dependent with half-times of 6.1 min and 12.4 min, respectively. Constants determined for binding of [H-3]nicotine to striatal membrane at 22-degrees-C included a K(D) of 3.7 +/- 0.5 nM, B(max) of 67.5 +/- 2.2 fmol/mg, and Hill coefficient of 1.07 +/- 0.06. Association of nicotine with membrane binding sites was biphasic with half-times of 9 s and 1.8 min. The fast rate process contributed 37% of the total reaction. Dissociation was a uniphasic process with a half-time of 1.6 min. Comparison of constants determined by the release and binding assays indicated that the [H-3]nicotine binding site could be the presynaptic receptor involved in [H-3]dopamine release in mouse striatal synaptosomes.