Glutamate receptor subunits GluR5 and KA-2 are coexpressed in rat trigeminal ganglion neurons.

To determine the subunit composition of high-affinity kainate receptors in native neurons is a challenging problem because of the expression of more than one GluR subunit. In the present study the question of whether GluR5 and/or GluR6 subunits combine with KA-1 or KA-2 subunits in vivo is addressed by performing detailed physiological, pharmacological, and molecular characterization of functional kainate receptor channels in acutely dissociated trigeminal ganglion (TG) neurons. The results show that (1) smaller diameter TG neurons (<30 microm) respond to L-glutamate and kainate, and the currents gated by kainate desensitize with prolonged agonist exposure; (2) all kainate receptor subunits are detected to some extent by reverse transcriptase-PCR, whereas glutamate receptor subunits GluR5 and KA-2 are expressed at high levels in the TG; (3) there is an obvious similarity between the features of native kainate receptor channels in TG neurons and of heteromeric recombinant GluR5(R)/KA-2 channels in pharmacological properties, desensitization, rectification, ion permeability, and mean channel conductance; and (4) the age-dependent increase in GluR5 and KA-2 RNA levels in the TG is correlated well with an increased number of kainate-sensitive cells during postnatal development. Our data suggest that the heteromeric GluR5/KA2 combination actually occurs in TG neurons and give a clue as to the subunit composition of native kainate receptor channels.