Physiological and Molecular Properties of AMPA/Kainate Receptors Expressed by Striatal Medium Spiny Neurons

Abstract

The mechanisms by which glutamate shapes the activity of striatal medium spiny neurons are of fundamental importance to our understanding of normative and pathological striatal physiology. Non-N-Methyl-D-aspartate (non-NMDA) glutamate receptor expression and function were studied in medium spiny neurons with a combination of single cell RT-PCR, immunocytochemistry and whole-cell voltage-clamp techniques. Reverse transcription polymerase chain reaction analysis found that GluR2 mRNA appeared to be the most abundant and widely distributed AMPA receptor mRNA. GluR1 was also commonly detected. However, GluR3 mRNA was preferentially expressed by neurons coexpressing substance P and enkephalin and GluR4 mRNA was not detected in identified medium spiny neurons. All neuronal classes appeared to express GluR5 or GluR6 and/or GluR7 mRNA in addition to kainate (KA) subunit mRNA. Immunocytochemical studies confirmed the mRNA distributions and also revealed that GluR1 protein was largely restricted to dendritic spines. Although the mRNA and protein for both α-amino-3-hydroxy-5-methyl-ioxyzole-4-proprionic acid (AMPA) and KA class subunits was detected, the physiological response to glutamatergic ligands and the benzothiadizine cyclothiazide was characteristic of AMPA, not KA receptors. The AMPA receptor antagonist GYKI 52466 blocked the response to AMPA and all but a small transient component of the response to KA. The current-voltage relationship of the AMPA-evoked currents was relatively linear but Ca²⁺ fluorometry revealed that substantial changes in intracellular Ca²⁺ concentration accompanied exposure to either agonist. These results argue that somatodendritic non-NMDA glutamate receptors in medium spiny neurons are primarily GluR2-containing receptors of the AMPA class but that activation of these receptors as a group nevertheless results in a significant Ca²⁺ influx.