DAILY EXPOSURE TO A NONPHOTIC STIMULUS CAN ALTER PHOTOPERIODIC RESPONSE TO SHORT DAYS IN HAMSTERS

The ability of mammals to measure seasonal changes in daylength depend s upon a circadian clock and the phase-relationship between this clock and the light: dark cycle. Recently, a number of pharmacological and nonpharmacological stimuli have been shown to have pronounced effects on the phase of the circadian clock of rodents. The objective of the p resent study was to determine if a drug-induced change in the phase-re lationship between a measurable circadian rhythm (i.e., wheel running behavior) and the light:dark cycle would alter the effects of the ligh t cycle on the neuroendocrine-gonadal axis. Adult male hamsters with r egressed testes due to exposure to an inhibitory 10:14-hr light:dark c ycle were daily injected with vehicle or the short-acting benzodiazepi ne, triazolam, while remaining on short days, while a control group of hamsters was transferred to a photostimulatory 14:10-hr light:dark cy cle. Two other groups of hamsters with regressed testes were blinded a nd daily injected with vehicle or triazolam. The injections were timed to occur about 4 hr before activity onset because previous studies ha d demonstrated that injections of triazolam at this time can lead to a phase advance in the activity rhythm. The circadian rhythm of wheel r unning behavior was measured in all the animals maintained on the 10:1 4-hr light: dark cycle in order to monitor circadian phase. While no t esticular growth was observed after 25 days of vehicle injections, gro wth was observed in the triazolam-treated animals that was comparable to that observed in control animals transferred to long days. Testicul ar growth in triazolam-treated animals was associated with an earlier onset of locomotor activity, when compared with the vehicle-treated an imals. Importantly, triazolam had no effect on the testicular size of blind animals. These results indicate that daily injections of triazol am can stimulate neuroendocrine-gonadal activity by altering the phase -relationship between the cycle and the circadian clock involved in ph otoperiodic time measurement, and that agents which can affect the clo ck may be useful in altering seasonal cycles.