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.

GABAergic projections of the suprachiasmatic nucleus (SCN) were demonstrated in a double-labelling ultrastructural study which visualised the efferents of the SCN by PHA-L tracing, diaminobenzidine (DAB) immunocytochemistry, and GABA with immunogold postembedding staining. The results show a strong contralateral projection of the SCN that is partly GABA-containing. In addition, ipsilateral SCN projections to the dorsomedial hypothalamus and periventricular part of the paraventricular nucleus and sub-paraventricular nucleus were shown to contain GABA. The present results indicate that the SCN may utilize this inhibitory neurotransmitter to regulate and organize its own circadian rhythm as well as using GABA to transmit its diurnal information to other regions of the brain.

The diurnal rhythm of corticosteroid secretion is controlled by the suprachiasmatic nucleus (SCN). In rats, plasma corticosteroid levels rise just before the onset of the activity period during the dark phase. Our previous results indicated that vasopressin as a neurotransmitter from the SCN inhibited corticosteroid secretion in the area of the paraventricular/dorsomedial nucleus of the hypothalamus. We hypothesized that during the day the SCN may serve as an inhibitory system for corticosteroid secretion. To investigate this possibility, intact and SCN-lesioned animals were exposed to mild stress in the morning and evening and their plasma corticosteroid levels were monitored. The results indicate that SCN-lesioned animals have higher morning corticosteroid levels and respond both in the morning and evening with higher corticosteroid levels after stress than do intact control animals. We conclude, therefore, that these results indicate an inhibitory role of the SCN on corticosteroid secretion. The apparent discrepancy with the reported stimulatory role of the SCN on adrenocorticotropic hormone secretion is discussed.