Involvement of hypothalamic periventricular GABAergic nerves in the central pressor response to clonidine in freely-moving conscious rats

Microinjection of the α 2-adrenoceptor agonist clonidine into the hypothalamic periventricular nuclei (pvn) induces the pressor response associated with bradycardia in freely-moving conscious rats. this study investigated the involvement of γ-aminobutyric acid nerves (gabaergic nerves) and glutamatergic nerves in the cardiovascular response to microinjection of clonidine in the pvn. male wistar rats were chronically implanted with a microinjection cannula into the pvn and an arterial catheter into the abdominal aorta through the femoral artery. blood pressure and heart rate were measured under a conscious unrestrained state. pvn injection of clonidine induced a dose-dependent pressor response concomitant with bradycardia. pvn pretreatment with gaba,muscimol (gaba a-receptor agonist),or bicuculline (gaba a-receptor antagonist) significantly inhibited the pressor response to pvn-injected clonidine without affecting bradycardia. pvn pretreatment with baclofen (gaba b-receptor agonist),2-hydroxysaclofen (gaba b-receptor antagonist),or kynurenic acid (non-selective nmda-type glutamate-receptor and ionotropic glutamate-receptor antagonist) did not affect the pressor response to pvn-injected clonidine. these results suggest that clonidine induces a pressor response by stimulating the presynaptic α 2-adrenoceptor of gabaergic nerves in the pvn,thereby inhibiting gabaergic nerve activity. © the japanese pharmacological society.