Immunocytochemical localization of GABA(B) receptors in mesencephalic trigeminal nucleus neurons in the rat.

We examined immunoreactivities for gamma-aminobutyric acidB-receptor (GABA(B)R) subtypes, GABA(B)R1 and GABA(B)R2, in the mesencephalic trigeminal nucleus neurons (MTN neurons) of the rat. Immunoreactivity for GABA(B)R1 was prominent in cell bodies of MTN, whereas that for GABA(B)R2 was very weak, if existed. For electron microscopy, the immunogold-silver method for GABA(B)R1 was combined with the immunoperoxidase method for glutamic acid decarboxylase (GAD: the synthetic enzyme of GABA). Immunogold-silver particles indicating GABA(B)R1 immunoreactivity were distributed widely in the cytoplasm of the cell bodies postsynaptic to GAD-immunoreactive axon terminals, but were rarely associated with synaptic membrane specialization or extrasynaptic sites of plasma membrane. It has been indicated that GABA(B)R1 may not be transported to plasma membrane when no GABA(B)R2 exists. Thus, it was presumed that GABA(B)R1 in the cell body of the rat MTN neurons might not be involved in the synaptic transmission.