Metabotropic Glutamate Receptor Subtypes in Axon Terminals of Projection Fibers from the Main and Accessory Olfactory Bulbs: a Light and Electron Microscopic Immunohistochemical Study in the Rat

Localization of metabotropic glutamate receptor subtypes, mGluR1, mGluR1alpha, mGluR2/3, mGluR4a, mGluR5, mGluR7a, mGluR7b, and mGluR8, was examined in some of the target areas of projection fibers from the main and accessory olfactory bulbs (MOB and AOB) by using subtype-specific antibodies. The superficial layer of the olfactory tubercle and layer Ia of the piriform cortex, the target areas of MOB, showed marked mGluR1-, mGluR5-, mGluR7a-, and mGluR8-like immunoreactivities (-LI), and rather weak mGluR2/3-LI. The periamygdaloid cortical region including the target areas of both MOB and AOB showed intense mGluR2/3-LI as well as marked mGluR1-, mGluR5-, mGluR7a-, and mGluR8-LI. No significant mGluR1alpha-, mGluR4a-, or mGluR7b-LI was seen in these regions. After transection of the lateral olfactory tract, mGluR2/3-, mGluR7a-, and mGluR8-LI were reduced markedly in the target regions on the side ipsilateral to the transection; no significant changes were detected in mGluR1- or mGluR5-LI. Double labeling experiments indicated light and electron microscopically colocalization of mGluR7a- and mGluR8-LI in axon terminals on dendritic shafts of presumed interneurons in the superficial layer of the olfactory tubercle and layer Ia of the piriform cortex. Electron microscopically mGluR2/3-LI was seen in preterminal and terminal portions of axons, whereas mGluR7a- and mGluR8-LI were associated with presynaptic membrane specialization. Immunolabeled axon terminals were filled with round synaptic vesicles and constituted asymmetric synapses with dendritic profiles. The results suggest that glutamate release from axon terminals of projection fibers from MOB and AOB is regulated presynaptically and differentially through mGluR2/3, mGluR7a, and/or mGluR8.