Differential distribution of metabotropic glutamate receptor subtype mRNAsin the thalamus of the rat

L-Glutamate (L-Glu) is present in most excitatory synapses of the mammalian brain, acting on several receptor subtypes. Height different genes encodin g metabotropic glutamate receptors (mGluRs) subtypes have been described (m GluR1-8), having a distinct distribution in the brain. In the present study , the distribution of mGluR1, 3, 4, 5 and 7 mRNAs was determined in 20 thal amic nuclei of adult rats by performing in situ hybridisation with subtype- specific S-35-labelled oligonucleotide probes. High expression of mGluR1 mR NA mainly occurred in midline nuclei such as the centromedial/centrolateral (CM/CL) nuclei, parafascicular and submedius nuclei, and in the ventropost eromedial (VPM) and posterior (Po) nuclei. In contrast, mGluR5 mRNA was mor e uniformly distributed at weak to moderate levels, except in the reuniens nucleus where a strong signal was detected. The mGluR3 mRNA was highly expr essed in the reticular thalamic nucleus and almost not detectable in any ot her thalamic region. Additionally, mGluR3 mRNA was found not only in neuron es but also in putative glial cells. The mGluR4 mRNA was abundant in most t halamic nuclei, with prominent expression in the CM/CL, Po and ventrobasal complex (VPM and ventroposterolateral, VPL). Finally, mGluR7 transcripts we re found evenly distributed throughout the thalamus at moderate levels, the highest signal being detected in the paraventricular thalamic nucleus, VPM , VPL and Po. This differential distribution of mGluR subtypes in the rat t halamus may contribute to the heterogeneity of glutamate effects on thalami c neurones. The mGluR1, mGluR4 and mGluR7 receptors may be involved in the processing of somatosensory information because they an expressed in nuclei that receive direct sensory input. (C) 2000 Elsevier Science B.V. All righ ts reserved.