The Effect of Light on the Prolactin Surges of Pseudopregnant and Ovariectomized, Estrogenized Rats1, 2

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Abstract
The role of light in the regulation of the prolactin surges of the female rat was investigated. Two model systems were utilized: one was the pseudopregnant rat which normally has 2 daily prolactin surges and the other was the ovariectomized (OVX) polyestradiol phosphate (PEP)-treated rat which normally has a daily afternoon prolactin surge. A 6 h phase advance of the 14 h:10 h photoperiod phase advanced the prolactin surges of pseudopregnant and OVX+PEP rats a corresponding 6 h. Blinded pseudopregnant and OVX+PEP rats had prolactin surges which were free running with respect to time of day, but the maximum values were similar to sighted 14 h:10 h controls. Pseudopregnant or OVX+PEP animals maintained on constant light also had free running prolactin rhythms, but the maximum prolactin values were significantly less than 14 h:10 h controls. Pinealectomy had no effect on the prolactin values of pseudopregnant rats maintained on constant light. It was concluded that the prolactin surges of pseudopregnant and OVX+PEP rats are endogenous circadian rhythms which can be entrained to an exogenous light-dark cycle.

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