During stress, the activity of the sympathetic nervous system is changed in a global fashion, leading to an increase in cardiovascular function and a release of adrenal catecholamines. This response is thought to be regulated by a common set of brain neurons that provide a dual input to the sympathetic preganglionic neurons regulating cardiac and adrenal medullary functions. By using a double-virus transneuronal labeling technique, the existence of such a set of central autonomic neurons in the hypothalamus and brainstem was demonstrated. These neurons innervate both of the sympathetic outflow systems and likely function in circumstances where parallel sympathetic processing occurs, such as in the fight-or-flight response.

Circadian rhythms are daily changes in behavior and physiology produced by the suprachiasmatic nucleus (SCN) even in the absence of external stimuli 1 , although photic input from the retina to the SCN entrains these changes to a 24-hour cycle. The SCN modulates autonomic and neuroendocrine function to prepare for diurnal or nocturnal changes in behavior, but its precise connections to the autonomic nervous system are unknown. We used viral transneuronal labeling 2 to demonstrate extensive connections of the SCN with diverse types of sympathetic as well as parasympathetic motor systems. Double-virus transneuronal tracing showed connections of single SCN neurons to multiple autonomic systems. However, targets of SCN modulation seem limited to those that operate continuously under tonic, rather than phasic, control.To demonstrate that the SCN is connected to all the major sympathetic outflow systems via a multisynaptic pathway 3-5 , we first used an attenuated pig herpesvirus, the Bartha strain of pseudorabies virus (PRV), as a retrograde transneuronal tracer ( Fig. 1). In 4 separate experiments using 49 rats, the superior cervical ganglion, stellate ganglion, adrenal gland or celiac ganglion was injected with PRV. In the celiac experiments, the vagus was sectioned below the diaphragm before PRV injections to prevent labeling of the central vagal motor system. After five to eight days, the rats were anesthetized and perfused with fixative; their brains were subsequently processed for two-color immunohistochemistry to demonstrate PRV and arginine vasopressin (AVP) in single neurons 6 . PRV infection was seen first in the dorsomedial SCN region, generally within five to six days of injection, but only after seven days in the celiac ganglion experiments. About 20% of infected neurons were AVP positive ( Fig. 1). One day later, infection spread to the ventrolateral SCN. Control intravenous PRV injections with comparable viral dosage and postinjection times did not result in SCN labeling (n = 33). Overall, these results indicate that the SCN connects with extensive and diverse types of sympathetic motor systems.Second, SCN cytochemistry was studied to establish participation of specific neuronal phenotypes in circadian cardiosympathetic regulation. Each rat (n = 12), received a PRV injection into the stellate ganglion, the main sympathetic ganglion innervating the heart; 5 days later, its lateral cerebral ventricle was injected with 100 µg of colchicine in 10 µl of saline to enhance neuropeptide accumulation in central neurons for better immunohistochemical detection. One day later, each rat was perfused. Subsequently, brain sections through the SCN were immunostained for PRV and one of four neuropeptides, AVP, vasoactive intestinal polypeptide (VIP), somatostatin or gastrin releasing peptide (GRP). AVP neurons were labeled during the early stage of SCN infection. When the infection spread into the ventrolateral SCN, ∼3% of the VIP-, ∼10% of the somatostatinand ∼4% of the GRP-labeled neuronal populations were labeled. B...

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