Properties of two classes of rat brain acidic amino acid receptors induced by distinct mRNA populations in Xenopus oocytes

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Abstract
The Xenopus laevis oocyte expression system was used to study the molecular composition of mRNAs encoding acidic amino acid (AA) receptors from rat brain. Xenopus oocytes injected with poly(A) mRNA express two general classes of AA receptors. One class consists of AA-gated cation channels. Responses are evoked by N-methyl-D-aspartate (NMDA), by kainate, and to a lesser extent by L-glutamate or quisqualate. The second class of receptor is coupled to an intracellular second messenger pathway activating an oocyte-encoded Ca2+-activated Cl− conductance. This second messenger-coupled AA receptor can be activated by L-glutamate or quisqualate. DL-2-amino-5-phosphonopentanoic acid and D-α-aminohexanedioic acid inhibit the AA-gated cation conductances activated by NMDA or kainate with different potencies but do not inhibit the second messenger-coupled AA receptor. Responses to NMDA are enhanced by micromolar level of glycine and are inhibited by Mg2+, Zn2+, or MK-801. Dose-response analysis reveals that the AA-gated cation conductance activated by kainate requires the binding of two agonist molecules. To study the molecular composition, the mRNAs were size fractionated by denaturing agarose gel electrophoresis. About20-fold purification in specific activity (nA/ng of mRNA injected) of mRNAs encoding the second messenger coupled AA receptor was achieved. In contrast, only a slight enrichment of the mRNAs encoding the AA-gated channel was observed. This suggests that the second messenger coupled AA receptor is encoded by a single size class of mRNA, whereas the AA-gated cation channel(s) is encoded by multiple species of mRNAs or by mRNAs whose size distribution is heterogeneous.