24-hour variation in content and release of hypothalamic neuropeptides in the rat.
The tissue content of up to eight neuropeptides, viz bombesin (BOM), cholecystokinin (CCK-8), neurotensin (NT), neuropeptide Y (NPY), peptide histidine isoleucine amide (PHI), somatostatin (SRIF), substance P (SP) and vasoactive intestinal polypeptide (VIP), in rat hypothalami removed at various times of the day, was measured using specific radioimmunoassays. There was significant variation in the content of BOM, CCK-8, NT, PHI, SP and VIP across a 24-h period. The levels of BOM, CCK-8 and NT were lowest around the onset of darkness (1900 h) and rose throughout the night to reach a peak around the time of lights on. Hypothalamic content of all eight peptides fell between 0700 h and 1300 h by an average of 45 +/- 4%. Basal release of these peptides, as well as that in the presence of 48 mM potassium (K+), was measured from hypothalami removed between 0700 and 1900 h and incubated in vitro in a CSF-like medium. Basal secretion of NT significantly increased, whilst that of CCK-8 significantly decreased over the same period. There was no significant change in the basal release of the other neuropeptides. The release in the presence of 48 mM K+ of SP decreased significantly during the day, whilst that of VIP significantly increased. There was also a significant change in the stimulated release of BOM, levels falling during the morning and rising again at 1900 h. 48 mM K+ caused a significant increase in the release of SRIF and SP at all times tested. Whilst 48 mM K+ induced a significantly higher release of CCK-8 and NT in the morning, this stimulus was ineffective in the evening. The contrary was true in the case of BOM, NPY and VIP, where a significant stimulation was induced only at 1900 h. The possible implications of these findings are discussed.
D000818:Animals; D001839:Bombesin / Q000378:metabolism; D002766:Cholecystokinin / Q000378:metabolism; D002940:Circadian Rhythm; D013007:Growth Hormone-Releasing Hormone / Q000378:metabolism; D007031:Hypothalamus / Q000378:metabolism*; D008297:Male; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009478:Neuropeptide Y; D009496:Neurotensin / Q000378:metabolism; D010451:Peptide PHI; D010455:Peptides / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013373:Substance P / Q000378:metabolism
D009419:Nerve Tissue Proteins; D009478:Neuropeptide Y; D010451:Peptide PHI; D010455:Peptides; D013373:Substance P; D009496:Neurotensin; D002766:Cholecystokinin; D013007:Growth Hormone-Releasing Hormone; D001839:Bombesin