Suprachiasmatic pacemaker organization analyzed by viral transynaptic transport

The suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circad ian pacemaker, is a paired structure with two subdivisions, a ventral core receiving photic input and a dorsal shell receiving non-photic input. Rhyth micity is thought to be generated by individual SCN neurons which are coupl ed to achieve synchrony [D.K. Welsh, D.E. Logothetis, M. Meister, S.M. Repp ert, Individual neurons dissociated from rat suprachiasmatic nucleus expres s independently phased circadian firing patterns, Neuron, 14 (1995) 697-706 ]. Normally, the core and shell, and the nuclei on each side, act in unison to transmit rhythmicity to effector systems. It is not known how coupling between neurons in the two subdivisions, and between the two SCNs, takes pl ace. In the present study, we analyze the intrinsic, commissural, and effer ent projections of the SCN using the swine herpesvirus (pseudorabies virus, PRV) as a tool for transynaptic analysis of circuits and small iontophoret ic injections of the conventional tracer horseradish peroxidase (HRP) conju gated to fluorescein. We find that the core and shell each project through commissural efferents to homologous contralateral areas. The core projects densely to shell but we find little reciprocal innervation. The two subdivi sions project to different hypothalamic areas, with the core projecting to the lateral subparaventricular zone and shell to the dorsomedial hypothalam ic nucleus and medial subparaventricular zone. These data are the first dem onstration that connections within the SCN, and from the SCN to effector re gions, are topographically organized and lend insight into the flow of info rmation through and out of the pacemaker. (C) 1999 Published by Elsevier Sc ience B.V. All rights reserved.