LARGE SOMAL SIZE IS ASSOCIATED WITH THE EXPRESSION OF GALANIN BUT NOTWITH NEURONAL BIRTH-DATE IN THE SEXUALLY DIMORPHIC MALE NUCLEUS OF FERRETS PREOPTIC AREA ANTERIOR HYPOTHALAMUS

Using Nissl and Golgi stains, a sexually dimorphic male nucleus (MN) c omprised of a cluster of large cells with large dendritic arbors has b een identified in the dorsal preoptic area/anterior hypothalamus (POA/ AH) of male ferrets. The MN-POA/AH is formed only in males by the acti on of estradiol derived from the neural aromatization of testosterone during the last quarter of a 41-day gestation. The ferret's dorsal POA /AH is also characterized by a sex difference in the expression of the neuropeptide galanin which first arises in males around embryonic day (e) 34. We asked whether the male-typical phenotype of large somal si ze is related to birthdate and/or the capacity of dorsal POA/AH neuron s to express galanin. In experiment 1 we labeled cohorts of cells born on E20, E24, or E28 by injecting the amniotic sacs of individual fetu ses with the thymidine analogue bromodeoxyuridine (BrdU). On postnatal day 20, BrdU-immunoreactive cells were visualized immunohistochemical ly, counterstained with cresyl violet, and their somal sizes were meas ured. BrdU-immunoreactive cells were significantly larger in the males ' MN-POA/AH than in a comparable region of females, regardless of when they were born between E20 and E28. In experiment 2 galanin-immunorea ctive cells in the dorsal POA/AH of adult ferrets were visualized immu nohistochemically, and their somal sizes were measured. Somal areas of galanin-immunoreactive cells were significantly larger in the MN-POA/ AH of intact, breeding, or castrated and testosterone-treated males th an in the corresponding area of females. Our results suggest that cell s in the males' MN-POA/AH are more likely to be larger than cells in f emales' corresponding region, regardless of birthdate. Finally, in adu lthood the male-typical phenotype of large Nissl-stained somal areas o f MN-POA/AH cells may, in part, reflect their increased galanin expres sion.