THE EFFECT OF SHORT-PHOTOPERIOD EXPOSURE ON TUBEROINFUNDIBULAR DOPAMINE NEURONS IN MALE AND FEMALE SYRIAN-HAMSTERS

In male hamsters, exposure to a short photoperiod results in a signifi cant decrease in median eminence (ME) dopamine (DA) concentrations. Th e mechanism responsible for this decrease in DA is unknown. The experi ments described in this paper were designed to examine the effects of photoperiod on DA metabolism and synthesis in the ME to determine if a change in these processes is responsible for the short-photoperiod-in duced decrease-in ME DA concentrations. In the first experiment, the m etabolism of DA in tuberoinfundibular dopamine (TIDA) neuronal termina ls was determined by measuring ME concentrations of 3,4-dihydroxypheny lacetic acid (DOPAC; a major metabolite of DA) and DA in male and fema le hamsters housed in long and short photoperiods. In both males and f emales, exposure to the short photoperiod induced a collapse of the re productive system and a reduction in circulating prolactin. In males, but not in females, exposure to the short photoperiod reduced ME DA co ncentrations; however, DOPAC concentrations were not affected by photo period. Thus, the decrease in ME DA seen in males is not the result of an increase in DA metabolism. In the second experiment, tyrosine hydr oxylase (TH) activity in the ME of males was determined by injecting a nimals housed in long and short photoperiods with a L-aromatic amino a cid decarboxylase inhibitor (NSD 1015) and measuring 3,4-dihydroxyphen ylalanine (DOPA). Consistent with Experiment 1, ME DA concentrations w ere significantly decreased in gonadally regressed males housed in a s hort photoperiod; however, ME DOPA accumulation was not affected. Thus , the observed decrease in DA is not the result of a decrease in TH ac tivity in the ME, The results of the experiments presented here indica te that (1) in males but not females, the decrease in circulating prol actin seen in animals housed in a short photoperiod for 12 weeks is as sociated with a decrease in ME DA concentrations, and (2) the decrease in ME DA seen in males housed in a short photoperiod is not the resul t of an increase in DA metabolism or a decrease in synthesis by TIDA n eurons.