Vesicular glutamate transporters (VGLUTs) are responsible for glutamate trafficking and for the subsequent regulated release of this excitatory neurotransmitter at the synapse. Three isoforms of the VGLUT have been identified, now known as VGLUT1, VGLUT2, and VGLUT3. Both VGLUT1 and VGLUT2 have been considered definitive markers of glutamatergic neurons, whereas VGLUT3 is expressed in nonglutamatergic neurons such as cholinergic striatal interneurons. It is widely believed that VGLUT1 and VGLUT2 are expressed in a complementary manner at the cortical and thalamic levels, suggesting that these glutamatergic neurons fulfill different physiological functions. In the present work, we analyzed the pattern of VGLUT1 and VGLUT2 mRNA expression at the thalamic level by using single and dual in situ hybridization. In accordance with current beliefs, we found significant expression of VGLUT2 mRNA in all the thalamic nuclei, while moderate expression of VGLUT1 mRNA was consistently found in both the principal relay and the association thalamic nuclei. Interestingly, individual neurons within these nuclei coexpressed both VGLUT1 and VGLUT2 mRNAs, suggesting that these individual thalamic neurons may have different ways of trafficking glutamate. These results call for a reappraisal of the previously held concept regarding the mutually exclusive distribution of VGLUT transporters in the central nervous system.
(c) 2007 Wiley-Liss, Inc.