SUSTAINED HORMONAL RESPONSES OF SIBERIAN HAMSTERS (PHODOPUS-SUNGORUS)TO A SINGLE LONGER DAY AT WEANING

Siberian hamsters undergo gonadal development for several weeks after exposure to a single longer day at weaning. To characterize changes in gonadotropin secretion after a single acute light stimulus, hamsters housed in a long photoperiod (16L:8D) were given a single longer day ( 20L:4D) or maintained in the 16L:8D photoperiod at 19 days of age and transferred to a short photoperiod (8L:16D) on Day 20. Elevated plasma FSH concentrations were detected in male hamsters at 5, 7, and 12 but not at 17 days after the single longer day. Melatonin treatment durin g light exposure and on two succeeding nights blocked the stimulatory effect of light on the reproductive axis; melatonin injections limited to one night were marginally effective. Pinealectomy during the dark phase of the photocycle and the resultant truncation of the melatonin signal for one night did not stimulate a greater degree of gonadal dev elopment than pinealectomy during the light phase. We conclude that th e single extra 4-h light pulse at weaning alters hypothalamic-pituitar y function for approximately 2 wk. Trophic effects of the light pulse appear to be mediated by suppression of melatonin secretion for severa l days; one truncated melatonin signal is not sufficient to simulate t he effects of a single long day on the reproductive axis.