Female rats raised from birth under a light/dark (LD) or constant light (LL) regimen were given injections at either 24, 29, 33 or 35 days of age of either 5 IU PMSG or 0.1 ml saline. Two days following PMSG or saline injection, the rats were given injections of either 1,000 ng synthetic gonadotropin releasing hormone (GnRH) or 0.1 ml saline. Blood samples were taken before and 20 min after injection for LH determinations. PMSG caused a rise in basal LH levels, which was greater in LL than in LD animals. GnRH administration resulted in an LH surge in both LD and LL rats at all ages tested. The LL animals released greater amounts of LH in response to GnRH than did LD animals. In PMSG-primed animals there were no differences in the GnRH-induced LH surge due to either age or photoperiod. It is concluded that PMSG and LL enhance the pituitary's response to GnRH.
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