Arginine vasopressin enhances GABAergic inhibition of cardiac parasympathetic neurons in the nucleus ambiguus.
Previous studies have shown that arginine vasopressin is an important neuropeptide that can modulate the reflex control of blood pressure and heart rate. The nucleus ambiguus, where cardiac parasympathetic neurons are located, receives dense arginine vasopressin projections. However the mechanisms by which arginine vasopressin alters cardiac parasympathetic activity are unknown. We tested the hypothesis that arginine vasopressin can alter the activity of cardiac parasympathetic neurons by altering the spontaneous GABAergic input to these neurons. Experiments were conducted using whole cell patch clamp recordings of cardiac parasympathetic neurons in an in vitro slice preparation in rats. The results of this study demonstrate that arginine vasopressin increases the frequency and amplitude of GABAergic inhibitory post-synaptic currents in cardiac parasympathetic neurons. Arginine vasopressin did not alter the GABAergic currents evoked by exogenous application of GABA. Similarly, in the presence of tetrodotoxin, arginine vasopressin did not alter the frequency, amplitude or decay time of GABAergic miniature synaptic events evoked by high osmolarity. These results indicate that arginine vasopressin likely acts on neurons precedent to cardiac parasympathetic neurons and that arginine vasopressin likely acts not at the synaptic terminal but at the soma or dendrites of the precedent neuron. Oxytocin and agonists for the V(2)-arginine vasopressin and V(1b)-arginine vasopressin receptors had no effect. By contrast, the arginine vasopressin-evoked responses were completely abolished by a selective V(1a)-arginine vasopressin receptor antagonist indicating arginine vasopressin responses are mediated by V(1a)-arginine vasopressin receptors. We conclude that the V(1a)-arginine vasopressin receptor-mediated increase in frequency and amplitude of inhibitory GABAergic activity to cardiac parasympathetic neurons may be at least one mechanism by which central arginine vasopressin may increase heart rate and inhibit reflex bradycardia.