SCN efferents to peripheral tissues: Implications for biological rhythms

The suprachiasmatic nucleus (SCN) is the principal generator of circadian r hythms and is part of an entrainment system that synchronizes the animal wi th its environment. Here, we review the possible communication of timing in formation from the SCN to peripheral tissues involved in regulating fundame ntal physiological functions as revealed using a viral, transneuronal tract tracer, the pseudorabies virus (PRV). The sympathetic nervous system inner vation of the pineal gland and the sympathetic outflow from brain to white adipose tissue were the first demonstrations of SCN-peripheral tissue conne ctions. The inclusion of the SCN as part of these and other circuits was th e result of lengthened postviral injection times compared with those used p reviously. Subsequently, the SCN has been found to be part of the sympathet ic outflow from the brain to brown adipose tissue, thyroid gland, kidney, b ladder, spleen, adrenal medulla, and perhaps the adrenal cortex. The SCN al so is involved in the parasympathetic nervous system innervation of the thy roid, liver, pancreas, and submandibular gland. Individual SCN neurons appe ar connected to more than one autonomic circuit involving both sympathetic and parasympathetic innervation of a single tissue, or sympathetic innervat ion of two different peripheral tissues. Collectively, the results of these PRV studies require an expansion of the traditional roles of the SCN to in clude the autonomic innervation of peripheral tissues and perhaps the modul ation of neuroendocrine systems traditionally thought to be controlled sole ly by hypothalamic stimulating/inhibiting factors.