A functional interaction between the mesolimbic dopamine system and vasopressin release in the regulation of blood pressure in conscious rats

The aim of the present study was to further characterize the involvement of the mesolimbic dopamine system in central blood pressure regulation, with particular emphasis on the interaction of this system with the effects of circulating vasopressin. In conscious rats we stimulated the release of endogenous dopamine from mesolimbic/mesocortical terminals by administration of the substance P analogue DiMe-C7 ([pGlu⁵, MePhe⁸, Sar⁹]-Substance P_{5 11}; 10 nmol) into the ventral tegmental area. Chemical stimulation of the ventral tegmental area resulted in a significant increase in blood pressure and heart rate. These effects were prevented by either bilateral electrolytic lesions of the hypothalamic supraoptic nucleus or by systemic pretreatment with the dopamine D₂ receptor antagonist raclopride (0.5 mg/kg). Stimulation of the ventral tegmental area also produced a marked increase in the expression of the proto-oncogene c-fos in the supraoptic nucleus and a significant increase in plasma vasopressin levels, suggesting activation of vasopressinergic neurons in this nucleus. However, this effect of stimulation of the ventral tegmental area was not significantly inhibited by pretreatment with raclopride. We suggest that the effects on blood pressure and heart rate of stimulation of the ventral midbrain by micro-injection of DiMe-C7 are the result of combined activation of both dopaminergic and nondopaminergic cell bodies in this region. Stimulation of non-dopaminergic cells in the ventral midbrain may induce a moderate increase in plasma vasopressin levels by activation of the supraoptic nucleus. An additional stimulation of dopaminergic cells in the ventral midbrain allows the increase in circulating vasopressin levels to become manifest as a pressor response, possibly by inhibition of vasopressin-induced facilitation of baroreflex responses.