Circadian variations in substance P, luliberin (LH-RH) and thyroliberin (TRH) contents in hypothalamic and extrahypothalamic brain nuclei of adult male rats.

Using both the 'punch' microdissection and radioimmunological techniques, circadian variations in substance P, luliberin (LH-RH) and thyroliberin (TRH) concentrations can be statistically validated in some discrete brain regions of the (Wistar CFY) male rat investigated in May. Animals were synchronized with light from 06.00 to 18.00 h and darkness. Water and food were available ad libitum. Very well marked circadian rhythms were in evidence in brain areas where the neuronal peptides investigated are mostly present in nerve terminals in high concentrations: medial basal hypothalamus for luliberin and thyroliberin and substantia nigra for substance P. On the contrary, no significant rhythms were detected in a number of areas where luliberin (preoptic area) or substance P (preoptic area, central gray matter, amygdala, globus pallidus) synthetizing perikarya are present. This suggests that at least luliberin and substance P are not secreted according to a circadian rhythmicity. The most striking finding was that crest time locations were situated at only two different times of the day: 14.00 h for thyroliberin and 20.00 h for substance P and luliberin. This suggests that circadian variations are not solely dependent on the activity of the endocrine system.