Effect of GABA(A) Receptors in the Rostral Ventrolateral Medulla on Cardiovascular Response to the Activation of the Bed Nucleus of the Stria Terminalis in Female Ovariectomized Rats

Background: The areas of the bed nucleus of the stria terminalis (BST) with a high density of estrogen receptors are involved in cardiovascular regulation and send axonal projections to the rostroventrolateral medulla (RVLM). We aimed to find the contribution of the RVLM to cardiovascular responses elicited by glutamate microinjection into the BST. Methods: Experiments were done in α-chloralose anesthetized ovariectomized (OVX) or OVX estrogen treated (OVX+E) female Wistar rats. Drugs were microinjected into the BST and RVLM. The average changes in mean arterial pressure (MAP) and heart rate (HR) were compared between the case and control groups using t test and with the pre-injection values using paired t test. Results: Unilateral microinjection of glutamate (0.25 M/50 nl) into the BST decreased MAP and HR, in the OVX+E and OVX rats. These cardiovascular responses were reversibly attenuated 10 minutes after microinjection of synaptic blocker cobalt chloride (CoCl(2), 5 mM/50 nl) into the ipsilateral RVLM. Re-stimulation of the BST 60 min after CoCl(2) injection elicited cardiovascular responses that were not different from the control values. Ipsilateral microinjection of GABA(A) antagonist bicuculline (1.0 mM/50 nl) into the RVLM caused a 50% attenuation of glutamate induced depressor and bradycardic responses in both groups. Ipsilateral microinjection of GABA(B) antagonist, phaclophen (5.0 mM/50 nl), into the RVLM did not affect the depressor and bradycardic responses due to re-stimulation of the BST by glutamate. Conclusion: The RVLM sympathetic premotor neurons contain GABA(A) receptors that mediate in part the sympathoinhibitory responses to stimulation of the BST in the OVX animals.