Short-photoperiod exposure reduces l-aromatic-amino-acid decarboxylase immunostaining in the arcuate nucleus and median eminence of male Syrian hamsters

In male hamsters, exposure to short photoperiod (SD) results in a decrease in median eminence and posterior pituitary dopamine (DA) concentrations. To determine if the SD-induced decline in DA is due to a decrease in the number of neurons synthesizing DA, immunocytochemistry was used to identify cells containing tyrosine hydroxylase (TH) and l-aromatic amino acid decarboxylase (AADC) in animals housed under long photoperiod (LD) or SD. Immunopositive cells were counted in the arcuate and caudal periventricular nuclei, regions where the DA neurons that project to the median eminence and posterior pituitary are located. AADC immunopositive (AADC + ) cells were also counted in the median eminence. Photoperiod did not affect the number of TH immunopositive (TH + ) or AADC + cells in the caudal periventricular nuclei. In the arcuate nuclei, SD exposure did not affect the number of TH + neurons, but produced a significant decline in the number of AADC + cells. The number of AADC + cells was also reduced in the median eminence of SD-exposed animals. This decline in AADC + cells may play a role in SD-induced changes in hypothalamic regulation of anterior pituitary hormone release, including the decline in median eminence DA concentrations.