Projections of the suprachiasmatic nucleus to stress‐related areas in the rat hypothalamus: A light and electron microscopic study

Abstract

The purpose of the present study was to investigate the sites in the hypothalamus where the suprachiasmatic nucleus (SCN) may influence corticosteroid secretion. In spite of the well established, SCN-mediated, daily rhythms in adrenocorticotrophic hormone (ACTH) and corticosteroid secretion, previous studies determining the projections of the suprachiasmatic nucleus failed to illustrate direct connections with corticotrophin-releasing hormone neurons (CRH). In order to identify where in the central nervous system the SCN may influence corticosteroid secretion, areas were selected that contained SCN efferents contacting neurons involved in the stress response. To achieve this in the present study, SCN efferents were visualized by Pha-L tract-tracing, together with the neurons involved in the stress response by immunocytochemical staining for c-fos protein. The sites where these efferents contacted c-fos-positive neurons were established by light microscopic double staining and electron microscopic immunocytochemical studies. It appeared that apart from the medial parvocellular area of the paraventricular nucleus (PVN) of the hypothalamus, many more regions showed fos-positive neurons. Sites where SCN efferents contacted such neurons are limited only to areas immediately adjacent to these putative CRH neurons but are not concentrated on these neurons themselves. These areas consist of the periventricular and rostral PVN together with the dorsomedial hypothalamus: all three regions are known to project into the PVN. Therefore, it is concluded that the SCN transmits its information related to corticosteroid secretion via interneurons in and around the PVN to the CRH-containing neurons, rather than by a direct interaction with these neurons themselves.

A Diurnal Rhythm of Stimulatory Input to the Hypothalamo–Pituitary–Adrenal System as Revealed by Timed Intrahypothalamic Administration of the Vasopressin V1Antagonist

“…The early onset of the stimulatory SCN input (i.e., between CT2 and CT6), as revealed by our timed VP antagonist infusions, would agree with the often-reported phaseadvanced rhythms of CRH and ACTH compared with that of corticosterone (Szafarczyk et al, 1980;Graf et al, 1988;Carnes et al, 1989;Owens et al, 1990;Kwak et al, 1992;Cai and Wise, 1996) and the elevated daytime levels of GRP mRNA in the SCN (Zoeller et al, 1992;Inouye et al, 1993;Inouye and Shibata, 1994). Because VP-, VIP-, and GRP-containing SCN fibers display clearly separated innervation patterns of the PVN/DMH subdivisions (Buijs et al, 1993b;Kalsbeek et al, 1993), a differentiated control of ACTH and corticosterone release by SCN efferences is certainly possible. However, as yet no conclusive anatomical data are available to back up the dual control pattern of ACTH and corticosterone release as proposed in the foregoing.…”

Interaction between hypothalamic dorsomedial nucleus and the suprachiasmatic nucleus determines intensity of food anticipatory behavior

“…Early tracing studies examining the output of the DMH have shown that its projections are mainly limited to hypothalamic structures (34). This observation, together with data showing that the DMH receives an important input from the nucleus of the solitary tract, arcuate, and SCN (35)(36)(37), supports the view that the DMH is essential for the distribution of metabolic and circadian signals within the hypothalamus. The present results emphasize the important role of the DMH because they show that, under normal conditions, the DMH has the capacity to "silence" the SCN to allow locomotor activity at a moment of the day when such behavior is normally prevented by the activity of the SCN.…”

Suprachiasmatic GABAergic Inputs to the Paraventricular Nucleus Control Plasma Glucose Concentrations in the Rat via Sympathetic Innervation of the Liver

“…In addition, there is also evidence for GABAergic projections from the DMH to the PVN (Boudaba et al, 1996). Because the DMH is also a target area for SCN projections (Watts et al, 1987;Buijs et al, 1993), the SCN might use both direct and indirect GABAergic projections to the preautonomic neurons in the PVN to control basal plasma glucose concentrations, implying that SCN projections to the GABAergic DMH neurons are stimulatory. In addition to the SCN and DMH, GABAergic inputs from other sources may restrain the activity of preautonomic PVN neurons controlling HGP.…”