An early pineal-induced suppression of pituitary prolactin mRNA levels in light-deprived male hamsters.

Light deprivation by blinding or exposure to short photoperiod results in depressed prolactin (PRL) cell activity and gonadal regression in the male Syrian hamster after at least 42-56 days of treatment, and both of these events can be prevented if pineal removal accompanies light deprivation. In the present study, we investigated changes in PRL cell activity after 11, 21, or 42 days of blinding by measuring changes in: (1) pituitary PRL mRNA levels; (2) newly synthesized pituitary PRL, and (3) pituitary-RIA-PRL. The purpose of this study was to characterize the early (i.e. prior to gonadal regression and loss of reproductive function) effects of light deprivation on PRL production in the pituitary gland of the male hamster, and to determine whether a reduction in PRL mRNA levels precedes the inhibition of PRL synthesis and storage. After 11, 21, and 42 days of blinding, PRL mRNA levels in the male hamster pituitary (relative PRL mRNA/pituitary) were significantly depressed compared to intact controls by 33, 41, and 65%, respectively. Pituitary PRL synthesis (dpm of 3H-PRL/pituitary) and RIA-PRL (micrograms PRL/pituitary) were significantly depressed (by 49 and 53%, respectively) only after 42 days of blinding. All of these statistically significant depressions were prevented when the surgical blinding procedure was accompanied by pineal removal. There were no differences in testicle, pituitary, or body weight as a result of treatment. These results support the hypothesis that light deprivation by blinding causes a pineal-dependent decline in PRL mRNA levels, which leads to a decrease in PRL synthesis and pituitary RIA-PRL.