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This study evaluates the localization of the metabotropic glutamate receptor mGluR4a in the piriform cortex of rats using preembedding immunocytochemical methods. At the light microscopic level, punctate labeling was evident in layers Ia and Ib of the piriform cortex, and immunolabeled fibers were present in layers II and III. Following bilateral destruction of the olfactory bulb, the density of labeled puncta in layer Ia decreased. These results suggest that the receptor is present on the terminals of the lateral olfactory tract (LOT). Electron microscopic evaluation of layers Ia and Ib revealed that mGluR4a was localized in synaptic terminals in layers Ia and Ib. The terminals had clear, round synaptic vesicles and terminated on asymmetric synapses on dendritic spines and shafts. There was also immunolabeling of some dendritic profiles in layers Ia and Ib that were postsynaptic to unlabeled presynaptic terminals. These observations suggest that mGluR4a is present on presynaptic terminals in the layers of the piriform cortex that receive LOT and associational synapses. This is the same area in which previous studies have revealed the presence of mGluR7 and mGluR8, suggesting that all three receptors may be colocalized., (Copyright 2000 Wiley-Liss, Inc.)

A preembedding immunocytochemical method for light microscopy was used to study the postnatal development of expression of the group III metabotropic glutamate receptor mGluR4a in the medial nucleus of the trapezoid body (MNTB) of the rat. Immunoreactivity for mGluR4a was localized in axonal endings wrapping the principal globular neurons in MNTB, known as calyces of Held. The percentage of calyces of Held immunoreactive for mGluR4a increased progressively from postnatal day 3 (PND3), showing the highest density of labeled calyces by PND9. From this postnatal age on, a gradual reduction in the number of mGluR4a-immunopositive calyces of Held was observed, reaching the lowest level of labeled profiles in adult tissue. The developmental expression of mGluR4a in calyces of Held correlates well with previous studies in young animals showing a modulation of synaptic neurotransmission by group III mGluRs in these giant excitatory synapses made on MNTB principal neurons. All these observations together suggest that the expression of mGluR4a mainly between PND7 and PND12 might be relevant to the maturation and modulation of synaptic transmission at the calyces of Held., (Copyright 1999 Wiley-Liss, Inc.)