The suprachiasmatic nucleus balances sympathetic and parasympathetic output to peripheral organs through separate preautonomic neurons.

Opposing parasympathetic and sympathetic signals determine the autonomic output of the brain to the body and the change in balance over the sleep-wake cycle. The suprachiasmatic nucleus (SCN) organizes the activity/inactivity cycle and the behaviors that go along with it, but it is unclear how the hypothalamus, in particular the SCN, with its high daytime electrical activity, influences this differentiated autonomic balance. In a first series of experiments, we visualized hypothalamic pre-sympathetic neurons by injecting the retrograde tracer Fluoro-Gold into the thoracic sympathetic nuclei of the spinal cord. Pre-parasympathetic neurons were revealed by injection of the retrograde trans-synaptic tracer pseudorabies virus (PRV) into the liver and by sympathetic liver denervation, forcing the virus to infect via the vagus nerve only. This approach revealed separate pre-sympathetic and pre-parasympathetic neurons in the brainstem and hypothalamus. Next, selective retrograde tracing with two unique reporter PRV strains, one injected into the adrenal and the other into the sympathetic denervated liver, demonstrated that there are two separate populations of pre-sympathetic and pre-parasympathetic neurons within the paraventricular nucleus of the hypothalamus. Interestingly, this segregation persists into the SCN, where, as a result, the day-night balance in autonomic function of the organs is affected by specialized pre-sympathetic or pre-parasympathetic SCN neurons. These separate preautonomic SCN neurons provide the anatomical basis for the circadian-driven regulation of the parasympathetic and sympathetic autonomic output.