Abstract
Photoperiod-synchronized rhythms in non-CSN tissues persist in total darkness. Clock
genes involved in maintaining regular biorhythms within the suprachiasmatic nucleus
(SCN) of the hypothalamus are expressed in extra-CNS tissues and continue periodic
expression in vitro. Understanding the details of how the SCN clock is coupled with
peripheral clocks is only incompletely understood and may involve a multiplicity of
feedback systems. The present study is an extension of our previous work showing that
brain levels of TRH (pGlu-His-Pro-NH2) and TRH-like peptides (X-TRH: pGlu-X-Pro-NH2, where “X” can be any amino acid residue) fluctuate throughout the day-night cycle.
Male rats were maintained in a stable environment, lights on 6–18 h. TRH and TRH-like
peptides in liver, pancreas, testis, prostate, epididymis, and heart were measured
at 3, 10, 16, and 22 h. The greatest change in peptide level was a 12-fold increase
for TRH in prostate at 16 h relative to the corresponding value at 3 h. The TRH, Tyr-TRH
and Phe-TRH levels in liver declined steadily to about 40% of the 3-h values by 22 h.
Changes, in the order of decreasing number of significant increases (↑) and/or decreases
(↓), were: testis (5↑, 1↓), liver (3↓), epididymis (2↑), prostate (1↑, 1↓) and heart
(1↑). Peptide levels in liver and testis correlated with serum leptin and serum corticosterone,
respectively, which are potent releasers of these peptides. Testosterone and glucose
were also highly correlated. These tripeptides may participate in the regulation of
metabolic and reproductive functions, which change during the day-night cycle.
Key words
thyrotropin-releasing hormone - reproduction - metabolism
