Immunocytochemical localization of the metabotropic glutamate receptor mGluR4a in the piriform cortex of the rat

This study evaluates the localization of the metabotropic glutamate recepto r mGluR4a in the piriform cortex of rats using preembedding immunocytochemi cal methods. At the light microscopic level, punctate labeling was evident in layers Ia and Ib of the piriform cortex, and immunolabeled fibers were p resent in layers II and III. Following bilateral destruction of the olfacto ry bulb, the density of labeled puncta in layer Ia decreased. These results suggest that the receptor is present on the terminals of the lateral olfac tory tract (LOT). Electron microscopic evaluation of layers Ia and Ib revea led that mGluR4a was localized in synaptic terminals in layers Ia and Ib. T he terminals had clear, round synaptic vesicles and terminated on asymmetri c synapses on dendritic spines and shafts. There was also immunolabeling of some dendritic profiles in layers Ia and Ib that were postsynaptic to unla beled presynaptic terminals. These observations suggest that mGluR4a is pre sent on presynaptic terminals in the layers of the piriform cortex that rec eive LOT and associational synapses. This is the same area in which previou s studies have revealed the presence of mGluR7 and mGluR8, suggesting that all three receptors may be colocalized. (C) 2000 Wiley-Liss, Inc.