Sex differences in the magnocellular subdivision of the medial preoptic nucleus in Syrian hamsters

Exposure to pheromonal cues initiates male mating behavior. Pheromones are processed within a pathway that converges on the magnocellular subdivision of the medial preoptic nucleus. Lesions of this area eliminate male copulatory behavior, but do not affect anogenital investigation. Exposure to pheromones stimulates cells of the magnocellular subdivision of the medial preoptic nucleus in a sex-specific manner. In this study, we hypothesize that sex differences in cell number may underlie sex differences in pheromone-induced neural stimulation. The current study used unbiased stereological methods to identify sexual dimorphisms in the magnocellular subdivision of the medial preoptic nucleus. Sex differences were found in the number and density of neurons, but not in overall volume or neuron volume. Consequently, the total volume is not sexually differentiated because neurons are more densely packed within the male magnocellular subdivision of the medial preoptic nucleus. These results support the hypothesis that additional neurons in the magnocellular subdivision of the medial preoptic nucleus are critical for the expression of male copulatory behaviors in adulthood. Furthermore, they suggest that sexual differentiation of the magnocellular subdivision of the medial preoptic nucleus is an important process that forms the anatomical basis for sex-specific behavioral responses to pheromonal stimulation.