Immunocytochemical localization of GABAB receptors in mesencephalic trigeminal nucleus neurons in the rat

We examined immunoreactivities for γ-aminobutyric acidB-receptor (GABABR) subtypes, GABABR1 and GABABR2, in the mesencephalic trigeminal nucleus neurons (MTN neurons) of the rat. Immunoreactivity for GABABR1 was prominent in cell bodies of MTN, whereas that for GABABR2 was very weak, if existed. For electron microscopy, the immunogold-silver method for GABABR1 was combined with the immunoperoxidase method for glutamic acid decarboxylase (GAD: the synthetic enzyme of GABA). Immunogold-silver particles indicating GABABR1 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 GABABR1 may not be transported to plasma membrane when no GABABR2 exists. Thus, it was presumed that GABABR1 in the cell body of the rat MTN neurons might not be involved in the synaptic transmission.