SEASONAL-CHANGES IN MEDIAN-EMINENCE DOPAMINE IN MALE SYRIAN-HAMSTERS - ROLE OF THE GONADS AND DURATION OF EXPOSURE TO SHORT DAYS

Median eminence dopamine concentrations are reduced in male hamsters housed in a short photoperiod for 12 weeks, but tyrosine hydroxylase activity and dopamine metabolism are unaffected. The mechanism responsible for this decrease in dopamine is unclear. The purpose of the experiments presented here was to examine the effects of gonadal steroids on this decline in median eminence dopamine concentrations, and to characterize the time course over which this change occurs. Experiment 1 examined the effects of short day exposure on median eminence dopamine concentrations and tuberoinfundibular dopamine neuronal activity (i.e., accumulation of 3,4-dihydroxyphenylanine) in castrated male hamsters. The results of that experiment suggest that the short-day induced decline in median eminence dopamine concentrations is maintained in the absence of gonadal steroids. In Experiments 2 and 3, tuberoinfundibular dopamine neuronal activity (accumulation of 3,4-dihydroxyphenylalanine and 3,4-dihydroxyphenylacetic acid concentrations, respectively), median eminence dopamine concentrations and circulating prolactin levels were measured in animals housed in short days for varying lengths of time. This was done to determine when the decline in median eminence dopamine concentrations occurs and if the decline is preceded by a change in tuberoinfundibular dopamine neuronal activity. The results of these two experiments suggest that median eminence dopamine concentrations and circulating prolactin levels decline after approximately 4 weeks of short-day exposure, and that these photoperiod induced changes are not associated with changes in tuberoinfundibular dopaminergic neuronal activity.