Changes in photoperiod alter the daily rhythms of pineal melatonin content and hypothalamic beta-endorphin content and the luteinizing hormone response to naloxone in the male Syrian hamster.

This study examines the possible involvement of beta-endorphin in the photoperiodic control of reproduction in the Syrian hamster. beta-Endorphin and LHRH concentrations in the medial basal hypothalamus (MBH), anterior hypothalamus (AHA), and the preoptic area (POA) as well as pineal melatonin content were determined by RIA in male Syrian hamsters exposed to either a long day [(LD) 16-h light; 8-h dark; lights on 0700-2300] or short day [(SD) 8-h light, 16-h dark; lights on 0700-1500] for 8 weeks. Groups of eight animals from each photoperiod were killed by decapitation at 4-h intervals over 24 h. Twenty minutes before death half the animals from each photoperiod were given naloxone (5 mg/kg, sc), the other half saline. Exposure to a long photoperiod maintained testicular activity while a short photoperiod induced testicular regression. Pineal melatonin content in both photoperiods was maximal at 0500 h, i.e. 2 h before the onset of light (SD, 435.58 +/- 82.7 pg/pineal; LD, 276.78 +/- 56.8 pg/pineal). However, the duration of the nighttime rise in pineal melatonin content was increased in SD animals with elevated melatonin levels at 2100 h (157.10 +/- 41.8 pg/pineal) and 0100 h (199.11 +/- 58.9 pg/pineal). In contrast pineal melatonin content in LD animals was only higher than daytime values at 0500 h. A daily rhythm of beta-endorphin content within both the AHA and MBH of animals exposed to a short photoperiod coincided with this prolonged nighttime rise in pineal melatonin content, although a causal relationship between the two was not established. Peak levels of beta-endorphin occurred at 2100 h (AHA, 6.569 +/- 1.2 pmol/mg protein; MBH, 4.877 +/- 0.45 pmol/mg protein) and at 0100 h (AHA, 6.107 +/- 0.66 pmol/mg protein; MBH, 4.49 +/- 00.79 pmol/mg protein) which was 6 h and 10 h into the dark phase, respectively, with lowest levels in the middle of the light phase (AHA, 3.561 +/- 0.56 pmol/mg protein; MBH, 2.688 +/- 0.3 pmol/mg protein). This rhythm was absent in animals exposed to a long photoperiod. There was no effect of photoperiod or time of day on the content of beta-endorphin in the POA. LHRH levels were not altered by changes in photoperiod in all three brain regions studied. In the AHA and MBH, concentrations of LHRH were similar at all times of day whereas, in the POA, LHRH levels varied with time in both photoperiods. Peak levels occurred in the middle of the dark phase at 0100 h (LD, 2.774 +/- 0.24 pmol LHRH/mg protein; SD, 3.206 +/- 0.48 pmol LHRH/mg protein) with lowest levels during the light phase (LD, 1.664 pmol LHRH/mg protein; SD, 1.775 pmol LHRH/mg protein).(ABSTRACT TRUNCATED AT 400 WORDS)