Synaptic reorganisation of the medial amygdala during puberty.

The medial amygdala (MeA) is an important site for the gonadal hormone control of several socio-sexual behaviours that emerge during puberty, including aggression, mating and parental behaviour. We have previously shown that rising levels of pubertal androgens increase the regional volume and mean soma size of neurones in the posterodorsal subnucleus of the MeA, the MePD. The present study aimed to determine some of the constituents of pubertal volumetric growth. Using computer-guided unbiased stereology, we compared the regional volume, mean somal volume and the overall number of neurones and glia in 45-day-old male Siberian hamsters (Phodopus sungorus). Half of the hamsters had completed puberty, whereas the remainder were prepubertal as a result of photoinhibition of the hypothalamic-pituitary-gonadal axis. Puberty significantly increased MePD regional volume and mean somal volume, as previously observed. We also compared the number of puncta immunoreactive for vesicular glutamate transporter-2 (vGlut2) and post-synaptic density 95 (PSD-95), which are both markers of glutamatergic pre- and post-synaptic specialisations, as well as glutamic acid decarboxylase 65 (GAD-65), which is a marker of GABAergic terminals. Puberty increased the number of vGlut2 and PSD-95 immunoreactive puncta by two- and three-fold, respectively, whereas the number of GAD-65 immunoreactive puncta was unchanged. These results suggest that numerous excitatory synapses are added to the MeA during puberty. More broadly, they show that the pubertal emergence of sexual behaviour is accompanied by synaptic reorganisation of a key network involved in the expression of sexual behaviour.