[(3)H]Minaprine binding to membrane preparation from rat brain.

The specific binding site of minaprine [3-(2-morpholino-ethylamino)-4-methyl-6-phenyl-pyridazine] in the rat brain was studied. The specific binding site of [(3)H]minaprine was found to be mostly evenly distributed in rat brain. The affinity of [(3)H]minaprine was highest in the hippocampus, with a dissociation constant (K(d)) of 56.3 nM, compared with the affinity in the cerebral cortex and the striatum. The specific [(3)H]minaprine binding on a mg-protein basis was largest in the microsomal fraction. The affinity of [(3)H]minaprine was significantly higher in the synaptosomal fraction compared with the affinity in the mitochondrial and microsomal fractions in the brain. The affinity of [(3)H]minaprine binding was slightly increased by increasing the reaction temperature. A net increase of entropy in [(3)H]minaprine binding was observed, accompanied by a decrease in free energy and a slight increase in enthalpy. Minaprine and an active metabolite of minaprine, 3-(2-morpholino)-4-methyl-6-(4-hydroxyphenyl)-pyridazine (M-3), inhibited [(3)H]minaprine binding. Neurotransmitter-related compounds all failed to inhibit [(3)H]minaprine binding, except phencyclidine, tetraethylammonium (TEA) and 4-aminopyridine. These results suggest that minaprine binds with high affinity to a specific binding site on the synaptic membrane, and that the site is related to a voltage-dependent K(+) channel.