EFFECTS OF PHOTOPERIOD ON REPRODUCTION AND THE GONADOTROPIN-RELEASINGHORMONE-IMMUNOREACTIVE NEURON SYSTEM IN THE POSTPUBERTAL MALE DJUNGARIAN HAMSTER

The present study tested the hypothesis that photoperiodic control of reproductive function in the postpubertal Djungarian hamster is associ ated with changes in the number, morphology, or distribution of GnRH-i mmunoreactive cell bodies in the brain. To initiate or arrest sexual m aturation, males were reared in long (LD, 16L:8D) or short (SD, 10L:14 D) days from birth. In two other groups that were chronologically past the normal onset of puberty, males were transferred at 30 days of age from LD to SD or from SD to LD to arrest or initiate reproductive fun ction, respectively. At 40, 60, or 90 days of age, 4-6 hamsters in eac h of the four photoperiod treatment groups were killed by intracardiac perfusion. Testes weights were significantly increased in males expos ed to long days (LD and SD-to-LD groups) compared to those treated wit h short days (SD and LD-to-SD groups). Serum FSH concentrations at 40 days of age were also increased in the two groups of males in long day s compared to those in both groups in short days (p < 0.05, ANOVA); LH concentrations were unaffected by photoperiod treatments. Brain secti ons (GO mu m) from the corpus callosum decussation to the suprachiasma tic nucleus in the anterior hypothalamus were processed for GnRH immun ocytochemistry. In brain regions that contained the majority of GnRH n eurons, i.e., the medial preoptic area and diagonal band of Broca, the numbers of GnRH-immunoreactive cell bodies were the same among the fo ur treatment groups. Similar numbers and a comparable ratio of unipola r to bipolar GnRH somata were observed whether reproductive developmen t had been stimulated by long days or blocked by short days. The findi ngs indicate that a relatively stable population of GnRH-immunoreactiv e neurons is present in the postpubertal male hamster and contrast wit h previous observations that increased numbers of unipolar GnRH-immuno reactive cell bodies are associated with sexual maturation.