Distinct localization of GABAB receptors relative to synaptic sites in the rat cerebellum and ventrobasal thalamus

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) are involved in modulation of synaptic transmission and activity of cerebellar and thalamic neurons. We used subtype-specific antibodies in pre- and postembedding immunohistochemistry combined with three-dimensional reconstruction of labelled profiles and quantification of immunoparticles to reveal the subcellular distribution of pre- and postsynaptic GABA(B)R1a/b and GABA(B)R2 in the rat cerebellum and ventrobasal thalamus. GABA(B)R1a/b and R2 were extensively colocalized in most brain regions including the cerebellum and thalamus. In the cerebellum, immunoreactivity for both subtypes was prevalent in the molecular layer. The most intense immunoreactivity was found in Purkinje cell spines with a high density of immunoparticles; at extrasynaptic sites peaking at around 240 nm from glutamatergic synapses between spines and parallel fibre varicosities. This is in contrast to dendrites at sites around GABAergic synapses where sparse and random distribution was found for both subtypes. In addition, more than one-tenth of the synaptic membrane specialization of spine-parallel fibre synapses were labelled at pre- or postsynaptic sites. Weak immunolabelling for both subtypes was also seen in parallel fibres but only rarely in GABAergic axons. In the ventrobasal thalamus, immunolabelling for both receptor subtypes was intense over the dendritic field of thalamocortical cells. Electron microscopy demonstrated an extrasynaptic localization of GABA(B)R1a/b and R2 exclusively in postsynaptic elements. Quantitative analysis further revealed the density of GABA(B)R1a/b around GABAergic synapses was higher than glutamatergic synapses on thalamocortical cell dendrites. The distinct localization of GABA(B)Rs relative to synaptic sites in the cerebellum and ventrobasal thalamus suggests that GABA(B)Rs differentially regulate activity of different neuronal populations.