Sex differences in the number of synaptic junctions in the binocular area of the rat visual cortex.

We had found that the binocular area of the visual cortex is larger in volume and has more neurons in male than in female rats. The present study examined whether the number of synaptic junctions in this area is sexually dimorphic. Ten littermate pairs of 90-day-old (socially housed) Long-Evans hooded rats were used. Synaptic junctions were counted and their lengths were measured on electron micrographs taken from layers II-III of the binocular visual cortex. There were no sex differences in the numerical density of synaptic junctions, the number of synaptic junctions per neuron, or the length of synaptic junctions within any synaptic category or of all synapses combined. Sex differences were found in the total number of synaptic junctions and in several categories (asymmetric synapses, spine synapses, asymmetric spine synapses): male rats had more synaptic junctions than female rats because of the larger volume of layers II-III in the binocular area of male rats. These data indicate that neurons in the binocular visual cortex of both male and female rats receive a characteristic number of synaptic junctions, but the greater number of neurons in the binocular area of male rats results in more synaptic junctions in the area.