Modulation of dopaminergic terminal excitability by D1 selective agents

The effect of the active R (+) enantiomer of the dopaminergic selective D1 agonist 1-Phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7,8-diol HCL (R-SKF 38393) was examined on the excitability of antidromically identified nigro-striatal dopaminergic neurons. Striatal infusions of R-SKF 38393 produced a decrease in terminal excitability, which was reversed by subsequent infusion of the Dopaminergic D1 selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-o l-HCL (SCH 23390). The actions of these D1 specific agents are localized to the terminal fields of the dopaminergic neurons and are not nonspecific effects on the axonal membrane since application along the axons of these cells in the medial forebrain bundle produced no change (R-SKF 38393) or only a slight decrease (SCH 23390) in excitability. The results suggest that the terminal excitability of antidromically identified nigro-striatal dopaminergic neurons can be modified with specific Dopamine D1 receptor agents via a receptor mediated mechanism. Previous studies from this laboratory have demonstrated that the electrical excitability of nigro-striatal dopaminergic terminals is reduced by the dopaminomimetics apomorphine and amphetamine and is increased by the dopamine antagonists haloperidol, fluphenazine and sulpiride (Groves, Fenster, Tepper, Nakamura, and Young 1981; Tepper, Nakamura, Young and Groves 1984). Since, with the exception of sulpiride, these compounds affect both the D1 and D2 subclasses of dopamine receptors, it can not be concluded from these reports, which, if only one, of these receptors mediates the effect on terminal excitability.(ABSTRACT TRUNCATED AT 250 WORDS)