Estrogen-induced autonomic effects are mediated by NMDA and GABAA receptors in the parabrachial nucleus.

The present study was done to determine if estrogen interacts with excitatory and/or inhibitory amino acid neurotransmitters to alter neuronal excitability within the parabrachial nucleus (PBN) and modulate autonomic tone. First, the role of estrogen in modulating autonomic tone was investigated in male rats anesthetized with Inactin (100 mg/kg). Animals were instrumented to record blood pressure, heart rate, vagal parasympathetic and renal sympathetic nerve activities as well as baroreflex sensitivity. Direct, bilateral injection of 17beta-estradiol (0.5 microM; 200 nl/side) into the PBN resulted in a significant decrease in blood pressure (17+/-4 mmHg), sympathetic tone (20+/-5%) and heart rate (22+/-5 beats/min) while increasing parasympathetic tone (34+/-4%) 30 min post-injection. These estrogen-induced effects were completely blocked by the co-injection of estrogen with the estrogen receptor antagonist, ICI 182,780 (20 microM; 200 nl/side). Co-injection of the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP; 10 microM; 200 nl/side), with estradiol resulted in complete blockade of the estrogen-induced decrease in heart rate and increase in parasympathetic tone only. Co-injection of estradiol with the GABA(A) receptor antagonist, (+)-bicuculline (0.1 microM; 200 nl/side), resulted in complete blockade of the estrogen-induced decrease in blood pressure and sympathetic nerve activity only. These results suggest that estrogen acts on estrogen receptors on neurons in the PBN to modulate GABA(A)-receptor mediated inhibitory neurotransmission to alter sympathetic tone and blood pressure and on neurons in a separate, parallel pathway to modulate NMDA-receptor mediated neurotransmission to alter parasympathetic tone and heart rate.