Capsaicin-sensitive vasodilatatory mechanisms in the rat substantia nigra and striatum.

Thermoregulatory and neurochemical effects of capsaicin microinjection into the substantia nigra (SN) or caudatus putamen (CPu) were studied in rats. Administration of capsaicin into these brain structures induced a peripheral vasodilatation which was associated with a decrease in body temperature. Pretreatment of the rats with capsaicin either as adults or neonates abolished the thermolytic response to the drug, indicating that the effect is executed specifically upon capsaicin sensitive structures. Analyses of the levels of monoamines and their metabolites in the striatum following injection of capsaicin into the SN or CPu revealed that dopaminergic neurons are not primarily involved in this effect. This view is also supported by our findings that neurochemical lesion of unilateral nigrostriatal dopaminergic neurons did not influence the vasodilatatory response. Since the pharmacological effect of intranigral capsaicin was not abolished by unilateral axotomy (hemisection) we presume a capsaicin-sensitive, non-dopaminergic descending vasodilatatory pathway from the SN.