• circadian rhythms;
  • hypothalamus;
  • neuroendocrine system;
  • paraventricular nucleus;
  • ventral lateral geniculate nucleus


The efferent projections of the suprachiasmatic nucleus (SCh) in the rat hypothalamus have been reexamined with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L), which displays labeled axons with the clarity of a Golgi impregnation. Fibers from the SCh can be divided into six pathways for descriptive purposes. By far the densest terminal field arising from cells in the SCh ends in a roughly comma-shaped zone between the SCh and paravontricular nucleus on the one hand and the periventricular nucleus and anterior hypothalamic area on the other. A few axons continue dorsally from this “subparaventricular zone” to pass through parvicellular parts of the paraventricular nucleus and the overlying midline thalamic nuclei to end in midrostrocaudal parts of the paraventricular nucleus of the thalamus, and a larger number continue caudally to end in the dorsomedial nucleus, dorsal parts of the cell-sparse zone surrounding the ventromedial nucleus, and the posterior hypothalamic area. The other five pathways all consist of relatively small numbers of fibers and give rise to relatively sparse terminal fields. The second pathway consists of rostrally directed fibers that end in ventral parts of the medial preoptic area and anteroventral periventricular nucleus. The third consists of anterodorsally oriented fibers that pass through the medial preoptic nucleus and adjacent regions to end ventrally in the intermediate lateral septal nucleus. The fourth consists of fibers just caudal to the third group that end in the preoptic continuation of the bed nucleus of the stria terminalis, as well as in the parataenial nucleus and rostral part of the paraventricular nucleus of the thalamus. The fifth consists of laterally directed fibers that course over the optic tract to end in the ventral lateral geniculate nucleus. And the sixth consists of fibers that course posteriorally through the anterior hypothalamic and retrochiasmatic areas to end in the cell-sparse zone between the arcuate nucleus and ventral parts of the ventromedial nucleus, as well as in adjacent parts of the lateral hypothalamic area.

The distribution of projections labeled following PHA-L injections centered in the subparaventricular zone was also examined and was confirmed with retrograde tracer experiments (Watts and Swanson: J. Comp. Neurol. 258:230–252, '87). The results indicate that the subparaventricular zone projects to essentially the same regions as the SCh, only much more densely, and also sends fibers back to the SCh. In addition, the subparaventricular zone innervates a much larger extent of the lateral septal nucleus than the SCh and also sends fibers throughout the length of the periaqueductal gray.

The results of this study indicate that the output of the SCh may be viewed best as a two-stage process in concert with the subparaventricular zone. Unfortunately, it is still not entirely clear how the SCh effects all manner of circadian and diurnal rhythms since it does not appear to innervate in a significant way cell groups that are known to play a direct role in somatomotor, autonomic, and neuroendocrine responses. Nevertheless, the possible functional role of cell groups innervated directly by the SCh and subparaventricular zone is discussed.