The viscerosympathetic response in rabbits is mediated by GABAergic and glutamatergic inputs into the sympathetic premotor neurons of the rostral ventrolateral medulla.

Neurons in the rostral ventrolateral medulla (RVLM) receive inputs from various sources, including baroreceptors, and then regulate the activity of sympathetic preganglionic neurons in the spinal cord. Whether RVLM neurons mediate the viscerosympathetic reflex has yet to be clarified. In the present study, we investigated the role of RVLM neurons in the viscerosympathetic reflex in anaesthetized and vagotomized rabbits. Electrical stimulation of the greater splanchnic nerve (SplN) evoked reflex responses in renal sympathetic activity that were composed of inhibitory and/or excitatory components. Bilateral microinjection of muscimol, a GABA(A) receptor agonist, into the RVLM blocked the reflex responses. Bilateral microinjection of bicuculline, a GABA(A) receptor antagonist, largely attenuated the inhibitory component, whereas kynurenic acid, a glutamate receptor antagonist, eliminated the excitatory component. The activities of 21 RVLM barosensitive bulbospinal neurons were recorded. Twenty of the neurons responded to the SplN stimulation. The responses also consisted of inhibitory and/or excitatory components. The excitatory component of these neurons preceded that of the renal sympathetic nerve activity by about 100 ms. This latency difference was almost the same as that of the inhibitory responses evoked by aortic nerve stimulation. Therefore, the renal sympathetic reflex responses evoked by SplN stimulation are mediated by RVLM neurons, and GABAergic and glutamatergic transmission in the RVLM are related to this reflex.