Nitric Oxide in the Dorsal Medulla Modulates Excitatory Somatosympathetic Reflexes

Activation of afferent cutaneous or mixed nerves, such as the sural or the sciatic, results in changes in sympathetic activity and arterial blood pressure by excitatory somatosympathetic reflexes (SSR). The underlying causes and modulation of SSR functions in the dorsal medulla are poorly understood. This review focuses our recent findings incorporated with the publications from other investigators implicating that: 1) The gracile nucleus is an integration center for somatic and visceral information flowing into the thalamus, and sensory stimulation of the hindlimb somatic afferent modifies neuronal activities in the nucleus tractus solitarius (NTS); 2) nitric oxide (NO) in the NTS produces decreases in arterial blood pressure and heart rate, but the effects are independent to baroreflexes; 3) L-arginine-derived NO synthesis in the gracile nucleus inhibits the excitatory cardiovascular responses to stimulus-evoked SSR; and 4) Neuronal NO synthase (nNOS) expression is increased in the dorsal medulla, the gracile nucleus and the NTS by electrical stimulation of the sural nerve. These results suggest that stimulation of somato-sensory afferents induces nNOS expression in the dorsal medulla, and L-arginine-derived NO synthesis in the nuclei produces an inhibitory regulation of excitatory SSR, which plays a role in the feedback autonomic control of the circulation.