Expression of GABAB receptors in magnocellular neurosecretory cells of male, virgin female and lactating rats.

GABA is one of the key neurotransmitters that regulate the firing activity of neurones in the supraoptic (SON) and paraventricular (PVN) nuclei. In the present study, we used immunohistochemical techniques to study the distribution and subcellular localisation of metabotropic GABA(B) receptors in magnocellular neurones in the SON and PVN. Robust GABA(B) receptor immunoreactivity (GABA(B)R; both subunit 1 and subunit 2 of the heterodimer), was observed in the SON and PVN. At the light microcope level, GABA(B)R immonoreactivity displayed a clustered pattern localised both intracytoplasmically and at the plasma membrane. Densitometry analysis indicated that GABA(B)R immunoreactivity was significantly more intense in vasopressin cells than in oxytocin cells, both in male, virgin female and lactating rats, and was denser in males than in virgin females. Light and electron microscope studies indicated that cytoplasmic GABA(B)R was localised in various organelles, including the Golgi, early endosomes and lysosomes, suggesting the cycling of the receptor within the endocytic and trafficking pathways. Some smaller clusters at the level of the cell plasma membrane were apposed to glutamic acid decarboxylase 67 immunoreactive boutons, and appeared to be colocalised with gephyrin, a constituent protein of the postsynaptic density at inhibitory synapses. The presence of GABA(B)R immunoreactivity at synaptic and extrasynaptic sites was supported by electron microscopy. These results provide anatomical evidence for the expression of postsynaptic GABA(B) receptors in magnocellular neurosecretory cells.