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.