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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-NH) and TRH-like peptides (X-TRH: pGlu-X-Pro-NH, 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.

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