Metabotropic glutamate receptor 2/3 immunoreactivity in the developing rat cerebellar cortex

In adult rat cerebellar cortex, the metabotropic glutamate receptors (mGluRs) 2 and 3 (mGluR2/3) are present in somata, dendrites, and terminals of Golgi cells as well as in presumed glial processes (Ohishi et al. [1994], Neuron 13:55-66). In the present study, spatiotemporal changes in immunostaining for mGluR2/3 were examined in postnatal rat cerebellar cortex. mGluR2/3-immunoreactive Golgi cell somata appeared first in the internal granular layer at postnatal day 3 (P3) and were restricted to lobules IX and X; however, by P5, they were present in all lobules. Immunoreactive Golgi cell axons were adult-like, appearing as tortuous fibers with clusters of varicosities. They were observed first in the internal granular layer at P7 and increased in number and complexity with time. It was confirmed that mGluR2/3-immunoreactive Golgi cell axon terminals belong to the synaptic glomerulus by P10. Immunoreactive Golgi cell dendrites extending into the molecular layer became prominent after P15. By that time, the immunostaining pattern was characteristic of Golgi cells, as seen typically in adults. Many intensely immunoreactive radial processes existed at birth (P0). These traversed the molecular and external granular layers, reaching the pial surface in every cerebellar lobule. Because they showed coimmunoreactivity for glial fibrillary acidic protein, they were confirmed to be Bergmann glial fibers. After P9, they began to lose immunoreactivity at the portion corresponding to the molecular layer, while an immunostained granular pattern appeared in that layer. Immunoreactive radial processes, however, remained in the external granular layer, and finally, at P21, they disappeared together along with the external granular layer. Granular staining in the molecular layer reached background levels at this time. These spatiotemporal changes in mGluR2/3 distribution suggested that there may be distinct roles for mGluR2/3 in Golgi cells and Bergmann glial cells during the early postnatal period. mGluR2/3 in Golgi cells might be associated closely with systemic maturation, whereas mGluR2/3 in Bergmann glia might be needed for neuron-glia interactions related to granule cell development.