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 decr ease in median eminence and posterior pituitary dopamine (DA) concentr ations. To determine if the SD-induced decline in DA is due to a decre ase in the number of neurons synthesizing DA, immunocytochemistry was used to identify cells containing tyrosine hydroxylase (TH) and L-arom atic amino acid decarboxylase (AADC) in animals housed under long phot operiod (LD) or SD. Immunopositive cells were counted in the arcuate a nd caudal periventricular nuclei, regions where the DA neurons that pr oject to the median eminence and posterior pituitary are located. AADC immunopositive (AADC +) cells were also counted in the median eminenc e. Photoperiod did not affect the number of TH immunopositive (TH +) o r AADC + cells in the caudal periventricular nuclei. In the arcuate nu clei, SD exposure did not affect the number of TH + neurons, but produ ced a significant decline in the number of AADC + cells. The number of AADC + cells was also reduced in the median eminence of SD-exposed an imals. This decline in AADC + cells may play a role in SD-induced chan ges in hypothalamic regulation of anterior pituitary hormone release, including the decline in median eminence DA concentrations.