Direct projection from the suprachiasmatic nucleus to hypophysiotrophic corticotropin-releasing factor immunoreactive cells in the paraventricular nucleus of the hypothalamus demonstrated by means of Phaseolus vulgaris-leucoagglutinin tract tracing.
The diurnal rhythm of the activity of the hypothalamo-pituitary-adrenal axis is generated by the circadian pacemaker located in the suprachiasmatic nuclei (SCN). However, the neuronal circuit connecting the SCN with the neurosecretory corticotropin-releasing factor (CRF) neurons in the paraventricular nucleus of the hypothalamus is not clear. To investigate the existence of a direct link between the SCN and the CRF neurons in the PVN we combined microiontopheretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into the SCN with immunohistochemical detection of CRF in adrenalectomized male rats. The majority of the PHA-L-ir axons originating from the SCN terminated in the subparaventricular area. A minor contingent of fibers continued into the PVN proper, involving the medial and dorsal parvicellular subnuclei of the PVN. All PHA-L injections involving the entire SCN gave rise to PHA-L positive fibers endowed with boutons en passage and terminal boutons contacting CRF positive cell bodies in the PVN. Notably, varicosities on the PHA-L labelled fibers were present in close proximity to cell bodies and proximal dendrites of a subportion of the CRF neurons located in the periphery of the CRF cell cluster. The present study provides the first evidence to suggest a direct connection between the SCN and the CRF producing neurons of the hypothalamo-pituitary-adrenocortical axis in the PVN. Considering the sparse number of PHA-L-ir varicosities in close proximity to the CRF-ir cells, it seems likely that this direct pathway constitutes but a part of a projection system from the SCN, possibly involving multisynaptic pathways, influencing the hypothalamo-pituitary-adrenocortical axis.
Vrang
N
N
Institute of Medical Anatomy, University of Copenhagen, Denmark.
Larsen
P J
PJ
Mikkelsen
J D
JD
D000818:Animals; D001931:Brain Mapping; D002940:Circadian Rhythm; D003346:Corticotropin-Releasing Hormone / Q000378:metabolism*; D007150:Immunohistochemistry / Q000379:methods; D008297:Male; D009434:Neural Pathways / Q000166:cytology*; D010286:Paraventricular Hypothalamic Nucleus / Q000166:cytology / Q000502:physiology*; D010835:Phytohemagglutinins; D010902:Pituitary Gland / Q000166:cytology* / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D013493:Suprachiasmatic Nucleus / Q000166:cytology* / Q000502:physiology
D010835:Phytohemagglutinins; C100156:leukoagglutinins, plants; D003346:Corticotropin-Releasing Hormone