Abstract
Striatonigral substance P pathways have been postulated to be the excitatory component of a feedback loop which assists in modulating the activity of the nigrostriatal dopaminergic system. Data presented in this paper supports this hypothesis by demonstrating that alterations in dopamine receptor activity resulted in significant changes in the Striatonigral substance P system as measured by nigral levels of substance P-like immunoreactivity. Thus, both activation and blockade of dopamine receptors by apomorphine and haloperidol, respectively, resulted in specific, dose-dependent and reversible decreases in the concentration of nigral substance P-like immunoreactivity. The mechanisms mediating apomorphine- and haloperidol-induced changes in nigral substance P levels appear to be different because of the dissimilar time-response pattern and an antagonism of the apomorphine effect by haloperidol. These results suggest that dopamine receptor activity, mediated by the nigrostriatal dopaminergic system, exerts significant influence on the substance P Striatonigral feedback loop.
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