Immunocytochemical investigation of luteinizing hormone-releasing hormone neurons in Syrian hamsters maintained under long or short days.

Light-microscope immunocytochemistry was used to investigate the LHRH system of adult male Syrian hamsters. Half of the animals were transferred from long to short photoperiods (14L:10D to 6L:18D) for 10 wk, causing plasma gonadotropin levels and the testes to revert to a prepubertal condition. In spite of the marked differences in the reproductive axis between the two groups of hamsters, the number of immunopositive LHRH neurons observed in the preoptic-medial septal area and diagonal band of Broca was approximately 400 in both cases; of these, 87-91% were monopolar and 9-13% were bipolar, regardless of whether the brains were sectioned in a coronal or sagittal plane. These results, therefore, fail to support the hypothesis that photoperiodic changes in the number of LHRH neurons play a major role in controlling the seasonal regression and recrudescence of the reproductive system in the hamster. However, morphometric analysis of the perikarya using an IBAS 2000 automatic image analyzer revealed a photoperiod-related difference. Surprisingly, the perikarya of both monopolar and bipolar LHRH neurons were significantly larger in hamsters that had been maintained on short days, as opposed to long days. These findings, therefore, are in harmony with the view that the inhibitory effect of short days on the reproductive axis is mediated through a suppression of LHRH secretion, which in turn is reflected as an increase in the net content of LHRH within the brain.