Developmental expression of the group III metabotropic glutamate receptor mGluR4a in the medial nucleus of the trapezoid body of the rat

A preembedding immunocytochemical method for light microscopy was used to s tudy the postnatal development of expression of the group III metabotropic glutamate receptor mGluR4a in the medial nucleus of the trapezoid body (MNT B) 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 pro gressively from postnatal day 3 (PND3), showing the highest density of labe led calyces by PND9. From this postnatal age on, a gradual reduction in the number of mGluR4a-immunopositive calyces of Held was observed, reaching th e lowest level of labeled profiles in adult tissue. The developmental expre ssion of mGluR4a in calyces of Held correlates well with previous studies i n young animals showing a modulation of synaptic neurotransmission by group III mGluRs in these giant excitatory synapses made on MNTB principal neuro ns. All these observations together suggest that the expression of mGluR4a mainly between PND7 and PND12 might be relevant to the maturation and modul ation of synaptic transmission at the calyces of Held. (C) 1999 Wiley-Liss, Inc.