Localization of nigrostriatal dopamine receptor subtypes and adenylate cyclase

Quantitative autoradiography using [³H]-SCH 23390, [³H]-sulpiride and [³H]-forskolin was used to assess the effects of single and combined neurotoxin lesions of the nigrostriatal pathway in the rat brain on dopamine (DA) receptor subtypes and adenylate cyclase (AC), respectively. Ibotenic acid (IA) lesions of the caudate-putamen (CPu) resulted in near total loss of both [³H]-SCH 23390 and of [³H]-forskolin binding in the ipsilateral CPu and substantia nigra reticulata (SNR). [³H]-sulpiride binding in the CPu was only partially removed by this same lesion, and nigral [³H]-sulpiride binding was virtually unchanged. 6-Hydroxydopamine (6-OHDA) and IA lesions of the substantia nigra compacta (SNC) did not affect [³H]-SCH 23390 or [³H]-forskolin binding, but largely removed [³H]-sulpiride binding in the SNC. A 6-OHDA lesion of the nigrostriatal pathway followed by an ipsilateral IA injection of the CPu failed to further reduce [³H]-sulpiride binding in the CPu. These results demonstrate that postsynaptic DA receptors in the CPu are of both the D1 and D2 variety; however, a portion of D2 receptors in the CPu may be presynaptic on afferent nerve terminals to this structure. D1 receptors in the SNR are presynaptic on striatonigral terminals, whereas the D2 receptors of the SNC are autoreceptors on nigral DA neurons. The existence of presynaptic D2 receptors on nigrostriatal DA-ergic terminals could not be confirmed by this study. Co-localization of D1 receptors and AC occurs in both the CPu and SNR.