Effect of short photoperiod on hypothalamic methionine-enkephalin and LHRH content and serum beta-endorphin-like immunoreactivity (beta-end LI) levels in golden hamsters.

Adult female golden hamsters were used to study the effect of short photoperiod on the endogenous opioid system and the effect of pinealectomy on the serum beta-endorphin-like immunoreactivity (beta-end LI) levels. Hamsters were housed under either long photoperiod (14L:10D) or short photoperiod (2L:22D) and the regularity of the estrous cycles was determined by daily vaginal exfoliative cytology. Hamsters under short photoperiod became acyclic after about 7 wk. At the end of 8 wk, all the hamsters were decapitated and medial basal hypothalamic (MBH) content of LHRH and methionine-enkephalin (met-enkephalin) were measured by specific radioimmunoassays (RIA). Both LHRH and met-enkephalin levels of the MBH were significantly elevated in the short-photoperiod hamsters as compared to the normally cycling control animals under long photoperiod. In a second experiment, the effect of pinealectomy (PNX) on the serum levels of beta-end LI in the short-photoperiod hamsters was determined. The serum beta-end LI levels were increased approximately threefold in the noncyclic hamsters housed under 8 wk of short-photoperiod conditions. Pinealectomized hamsters kept under 8 wk of short-photoperiod exhibited lower serum beta-end LI levels similar to those of normally cycling hamsters kept under long photoperiod. These results indicate a possible functional relationship between increased pineal activity (as a result of short photoperiod) and increased MBH met-enkephalin, LHRH, and serum beta-end LI levels.