GABA(B) RECEPTORS IN THE DORSOMEDIAL HYPOTHALAMUS AND HEART-RATE IN ANESTHETIZED RATS

Previous studies have shown that: (I) activation of neurons in the dor somedial hypothalamus (DMH) of the rat by blockade of local GABA, rece ptors with bicuculline methiodide (BMI) elicits cardiovascular changes resembling those seen in experimental stress, including marked sympat hetically-mediated tachycardia, and (2) inhibition of neurons in the s ame region by local microinjection of the GAB(A) receptor agonist musc imol can virtually abolish stress-induced tachycardia. This study exam ined the possibility that GAB(B) receptors exist in the neural circuit ry of the DMH, and that stimulation of these receptors might suppress the cardiovascular response to local disinhibition with BMI. Microinje ction of BMI 10 pmol into the DMH in urethane-anesthetized rats result ed in marked tachycardia with little or no effect on arterial pressure . Simultaneous injection of the GABA, receptor agonist baclofen at dos es of 2.5, 5.0 and 10 pmol produced dose-related suppression of BMI in duced tachycardia. Coinjection of the GABA, receptor antagonist 2-hydr oxysaclofen 100 or 200 pmol had no significant effect on the heart rat e response to BMI, but reversed the suppression elicited in the presen ce of baclofen. These findings indicate that (1) functional GABA(B) re ceptors exist in the DMH, and (2) stimulation of these receptors inhib its the tachycardia resulting from blockade of local GABA(A) receptors . (C) 1998 Elsevier Science B.V.