The effect of different photoperiods on plasma concentrations of melatonin, prolactin, and cortisol in the domestic cat.

These studies investigated the effects of photoperiod on plasma melatonin, PRL, and cortisol concentrations in cats. Animals were placed in one of three different photoperiod regimens [short (SP), 8 h of light, 16 h of darkness (8L:16D); normal (NP), 14L:10D; long (LP), 24L:OD] 140 days before experimentation (n = 4/group). In the first experiment, melatonin, PRL, and cortisol concentrations were measured in plasma obtained at 2-h intervals for 24 h. Peak melatonin concentrations were significantly different (P less than 0.05) in the groups with SP greater than NP greater than LP (9226 +/- 1052 vs. 3890 +/- 556 vs. 590 +/- 198 pg/ml, respectively). Melatonin concentrations declined significantly (P less than 0.01) during the last 2 h of dark in the SP animals, but not in NP animals. Acrophases for melatonin biorhythms occurred at 0030, 0430, and 0215 h for SP, NP, and LP, respectively. Significant regression coefficients were found for 8-, 12-, and 24-h cycles in the SP and for 8- and 24-h cycles in the NP (none in the LP). PRL concentrations were significantly higher during darkness in cats under a longer duration of dark with SP greater than NP greater than LP (164 +/- 5 vs. 57 +/- 3 vs. 26 +/- 7 ng/ml, respectively; P less than 0.05). Acrophases for SP and NP PRL biorhythms were similar (0145 vs. 0200 h, respectively), while exposure to a LP resulted in a major change in the acrophase (1200 h). Cortisol secretion was not affected by photoperiod. In the second experiment, animals entrained to SP for about 160 days were exposed to light at 2000 h for 12 h (6 h after lights off). Significant decreases (P less than 0.05) in PRL concentrations were noted within 2 h and in melatonin concentrations by 4 h (first sample analyzed). In a third experiment, animals entrained to a LP released significantly more PRL in response to TRH administration (P less than 0.001) than did those in NP and SP. The data indicate that melatonin and PRL secretion, but not cortisol secretion, are very responsive to changes in photoperiod in the cat.