Extrasynaptic localization of glycine receptors in the rat supraoptic nucleus: Further evidence for their involvement in glia-to-neuron communication

Neurons of the rat supraoptic nucleus (SON) express glycine receptors (GlyRs), which are implicated in the osmoregulation of neuronal activity. The endogenous agonist of the receptors has been postulated to be taurine, shown to be released from astrocytes. We here provide additional pieces of evidence supporting the absence of functional glycinergic synapses in the SON. First, we show that blockade of GlyRs with strychnine has no effect on either the amplitude or frequency of miniature inhibitory postsynaptic currents recorded in SON neurons, whereas they were all suppressed by the GABAA antagonist gabazine. Then, double immunostaining of sections with presynaptic markers and either GlyR or GABAA receptor (GABAAR) antibodies indicates that, in contrast with GABAARs, most GlyR membrane clusters are not localized facing presynaptic terminals, indicative of their extrasynaptic localization. Moreover, we found a striking anatomical association between SON GlyR clusters and glial fibrillary acidic protein (GFAP)-positive astroglial processes, which contain high levels of taurine. This type of correlation is specific to GlyRs, since GABAAR clusters show no association with GFAP-positive structures. These results substantiate and strengthen the concept of extrasynaptic GlyRs mediating a paracrine communication between astrocytes and neurons in the SON.