Hormonal regulation of the annual reproductive cycle of golden hamsters

Abstract

The seasonally breeding golden hamster, Mesocricetus auratus, is a photoperiodic species. Reproduction is confined to the spring and summer months and is curtailed by the short days of fall and winter. The photoperiodic response is dependent on an endogenous circadian sensitivity to light. Reacquisition of reproductive activity occurs spontaneously, even in the complete absence of light. These animals are refractory to the inhibitory effects of short days. Refractoriness is terminated by exposure to long days for a period of 11 or more weeks, after which the animals are once again sensitive to short days. The short-day response in the male is characterized by testicular regression induced by a decline in pituitary and circulating titers of LH, FSH, and PRL. These animals demonstrate an increased sensitivity to negative-feedback effects of exogenous (and, presumably, endogenous) testosterone that is associated with a decreased castration response. The reproductively inactive female fails to ovulate. Estrous cycles are absent, supplanted by a daily surge of LH and FSH. Spontaneous recovery of testicular function is associated with an increase in circulating titers of LH, FSH, PRL, and testosterone, and a resumption of spermatogenic activity. In the female the daily surge of LH and FSH is replaced by the 4-day estrous cycle. The pineal gland is essential for the occurrence of photoperiodically induced changes in the hamster's reproductive cycle. The pineal produces the indole, melatonin, with peak synthesis and release occurring during the subjective night. Administration of melatonin, by injection, at the appropriate time of day, will induce gonadal regression in both intact and pinealectomized hamsters maintained on normally stimulatory photoperiods. Subcutaneous melatonin implants, however, prevent gonadal regression induced by short photoperiods or melatonin injections. Melatonin implants also prevent photoinduced gonadal recrudescence. Constant release melatonin implants may function by interfering with target tissue sensitivity to endogenous melatonin. The target tissue for melatonin and the mechanism by which the pineal is involved in the photoperiodic response remain unknown.