Bradycardic effects mediated by activation of G protein-coupled estrogen receptor in rat nucleus ambiguus

New Findings center dot What is the central question of this study? Clinical and experimental studies indicate that estrogen increases cardiac vagal tone. The role of the G protein-coupled estrogen receptor in autonomic cardiac control is not known. center dot What is the main finding and its importance? Using calcium imaging, electrophysiological techniques and in vivo studies of microinjection into the nucleus ambiguus and measurement of cardiovascular responses, we show that activation of the G protein-coupled estrogen receptor in cardiac vagal neurons of the nucleus ambiguus increases cytosolic Ca2+ and depolarizes these neurons, leading to a decrease in heart rate. Our findings suggest a novel role for the G protein-coupled estrogen receptor in modulation of cardiac vagal tone. The G protein-coupled estrogen receptor (GPER) has been identified in several brain regions, including cholinergic neurons of the nucleus ambiguus, which are critical for parasympathetic cardiac regulation. Using calcium imaging and electrophysiological techniques, microinjection into the nucleus ambiguus and blood pressure measurement, we examined the in vitro and in vivo effects of GPER activation in nucleus ambiguus neurons. A GPER selective agonist, G-1, produced a sustained increase in cytosolic Ca2+ concentration in a concentration-dependent manner in retrogradely labelled cardiac vagal neurons of nucleus ambiguus. The increase in cytosolic Ca2+ produced by G-1 was abolished by pretreatment with G36, a GPER antagonist. G-1 depolarized cultured cardiac vagal neurons of the nucleus ambiguus. The excitatory effect of G-1 was also identified by whole-cell visual patch-clamp recordings in nucleus ambiguus neurons, in medullary slices. To validate the physiological relevance of our in vitro studies, we carried out in vivo experiments. Microinjection of G-1 into the nucleus ambiguus elicited a decrease in heart rate; the effect was blocked by prior microinjection of G36. Systemic injection of G-1, in addition to a previously reported decrease in blood pressure, also reduced the heart rate. The G-1-induced bradycardia was prevented by systemic injection of atropine, a muscarinic antagonist, or by bilateral microinjection of G36 into the nucleus ambiguus. Our results indicate that GPER-mediated bradycardia occurs via activation of cardiac parasympathetic neurons of the nucleus ambiguus and support the involvement of the GPER in the modulation of cardiac vagal tone.