Group II metabotropic glutamate receptors are differentially expressed in the medial nucleus of the trapezoid body in the developing and adult rat

The existence of a neuronal-glial signalling through the activation of neur otransmitter receptors expressed in glia is well-documented. In excitatory synapses, glutamate released from presynaptic terminals activates not only postsynaptic receptors, but also ionotropic and metabotropic glutamate rece ptors localized in the glia ensheathing the synapses. The medial nucleus of the trapezoid body of the auditory system is involved in the localization of sounds in the space. In this nucleus, the large excitatory synaptic term inals formed by the calyces of Held on the principal globular cell bodies a re wrapped by astrocytic processes. Since these synapses are functional fro m early postnatal days, glia receiving excitatory synaptic signals from the calyces may participate in modulating the maturation and development of th e system. Groups I and II of metabotropic glutamate receptors (mGluRs) have been loca lized in glial cells in different brain regions. To investigate whether gro up II mGluRs are present in the medial nucleus of the trapezoid body, we ha ve studied the pattern of expression of mGluR2/3 in the developing and matu re nucleus by means of immunocytochemical methods. The most remarkable find ing was the switch in the occurrence of mGluR2/3 from glial to neuronal com partments. Thus, a preferential localization of mGluR2/3 immunoreactivity w as observed in astrocytic processes surrounding the calyces of Held during the early postnatal development. In contrast, the main feature in adult rat s was the presence of the group II mGluRs in presynaptic calyces of Held an d postsynaptic principal globular cells. From these observations we suggest a role for group II mGluRs in neuronal-g lial signalling in the calyx of Held-principal globular neuron synapses. Ac tivation of these receptors might be relevant to the maturation and modulat ion of synaptic transmission in the medial nucleus of the trapezoid body. ( C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.