Differential expression of group I metabotropic glutamate receptors in functionally distinct hippocampal interneurons

Metabotropic glutamate receptors (mGluRs) have been proposed to be involved in oscillatory rhythmic activity in the hippocampus. However, the subtypes of mGluRs involved and their precise distribution in different populations of interneurons is unclear. In this study, we combined functional analysis of mGluR-mediated inward currents in CA1 oriens-alveus interneurons with a natomical and immunocytochemical identification of these interneurons and e xpression analysis of group I mGluR using single-cell reverse transcription -PCR (RT-PCR). Four major interneuron subtypes could be distinguished based on the mGluR-mediated inward current induced by the application of 100 mu M trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) under volt age-clamp conditions and the action potential firing pattern under current- clamp conditions. Type I interneurons responded with a large inward current of similar to 224 pA, were positive for somatostatin, and the majority exp ressed both mGluR1 and mGluR5. Type II interneurons responded with an inwar d current of similar to 80 pA, contained calbindin, and expressed mainly mG luR1. Type III interneurons responded with an inward current of similar to 60 pA. These interneurons were fast-spiking, contained parvalbumin, and exp ressed mainly mGluR5. Type IV interneurons did not respond with an inward c urrent upon application of ACPD, yet they expressed group I mGluRs. Activat ion of group I mGluRs under current-clamp conditions increased spike freque ncy and resulted in rhythmic firing activity in type I and II, but not in t ype III and IV, interneurons. RT-PCR results suggest that activation of mGl uR1 in the subsets of GABAergic interneurons, classified here as type I and II, may play an important role in mediating synchronous activity.