Synaptic plasticity in the rat supraoptic nucleus during lactation involves GABA innervation and oxytocin neurons: a quantitative immunocytochemical analysis.

The hypothalamic supraoptic nucleus (SON) is known to undergo synaptic remodeling in response to physiological stimuli, such as lactation. We here investigated the involvement of the GABAergic innervation in such plasticity by carrying out comparative ultrastructural analyses after postembedding immunogold labeling for GABA on sections of the SON from virgin and lactating rats. Using random single-section analysis to estimate the numerical density of synapses (from areal measurements of the anterior, mid, and posterior portions of the nucleus), we found that the overall density in either group was about 35 x 10(6) synapses/mm3, of which over one-third were GABA immunoreactive. The GABAergic terminals formed mainly symmetrical synaptic contact on neurosecretory somata and dendrites; the density of axodendritic contacts was twice that of axosomatic contacts. Despite hypertrophy of the neurosecretory neurons in lactating rats, the overall synaptic density did not diminish and was similar to that evaluated in virgin rats. When we estimated synaptic densities in the neuropil (by subtracting the proportion of sampled areas occupied by soma profiles), we found a statistically significant increase in the density of GABAergic synapses, in particular in the mid and posterior portions of the SON in lactating animals. On the other hand, the density of GABA-immunonegative synapses did not differ in the two groups. Terminals contacting more than one postsynaptic element in the same plane of section ("shared" synapses) were visible in the nuclei of virgin and lactating rats and about half in each group were GABAergic. They were comparatively rare but their incidence doubled in lactating animals.(ABSTRACT TRUNCATED AT 250 WORDS)