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Evidence that [3H]Dopamine Is Taken Up and Released from Nondopaminergic Nerve Terminals in the Rat Substantia Nigra In Vitro

Abstract: Potassium chloride (25 mM) and (+)‐amphet‐amine (100 μM) both stimulated the release of radioactivity from slices of substantia nigra preincubated with [3H]3,4‐dihydroxyphenylethylamine ([3H]dopamine). Potassium chloride (25 mM) released radioactivity from slices of both zona compacta and zona reticulata. Prior 6‐hydroxydopamine (6‐OHDA) lesions of one nigro‐striatal pathway did not reduce the spontaneous release of radioactivity, or the potassium chloride‐ or amphetamine‐induced release of radioactivity from slices of nigra ipsilateral to the lesion after preincubation with [3H]dopamine. The accumulation of radioactivity following incubation of nigral slices from 6‐OHDA‐lesioned animals with [3H]dopamine was increased when compared to uptake into slices from intact tissue. In synap‐tosomal preparations of striatum, nomifensine but not de‐sipramine or fluoxetine inhibited [3H]dopamine uptake. In contrast, nomifensine, desipramine, and fluoxetine all inhibited [3H]dopamine uptake in nigral synaptosomal preparations. Following 6‐OHDA lesions of one nigro‐striatal pathway the uptake of [3H]dopamine into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was substantially decreased. In contrast, bilateral electrolesions of the dorsal and medial raphe nuclei reduced [3H]dopamine uptake into nigral preparations but not into striatal synaptosomes. The uptake of [3H]5‐hydroxytryptamine ([3H]5‐HT) into synaptosomal preparations of substantia nigra was abolished by fluoxetine and reduced by desipramine, but was unaffected by nomifensine. In contrast, fluoxetine, desipramine, and nomifensine all inhibited [3H]5‐HT uptake into striatal synaptosomal reparations. Following 6‐OHDA lesions of one nigro‐striatal pathway the uptake of [3H]5‐HT into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was reduced. In contrast, bilateral electrolesions of the dorsal and medial raphe nuclei reduced [3H]5‐HT uptake into both nigral and striatal synaptosomal preparations. These results suggest that in tissue preparations from substantia nigra, the in vitro uptake and release of [3H]dopamine may not reflect dendritic dopamine function alone, but may involve other neuronal processes, particularly 5‐HT neurones.