Male-female differences in the intra-amygdaloid input to the medial amygdala

Following lesion of the posterior cortical nucleus of the amygdala (PCAN), the number of degenerating axon terminals and alterations of synaptic pattern were studied in the molecular layer (ML) of the medial nucleus of the amygdala (MAN) of male and female rats. Semiquantitative analyses by electron microscopy indicated that, 1 and 2 days after the lesion, the number of degenerating terminals in the ventral ML of male rats was statistically greater than that of female rats. Ten days after the operation, intact synapses remaining on dendritic shafts of the medial ML and those on dendritic spines of the ventral ML of male rats significantly decreased in number, compared with unoperated controls. On the other hand, no significant reduction was noted in synapses of the lesioned female rats killed 10 days after the operation. Thus, the number of axon terminals in the male ML originating from the lesioned area was greater than that of the female ML. The number of synapses in the ML of unoperated male rats was statistically greater than that of unoperated females. However, these sex differences in synaptic number became undetectable 10 days after the operation. These findings provide morphological evidence indicating that the fibers from and/or through the PCAN participate in emergence of synaptic sexual dimorphism in the ML of the MAN.