DISTRIBUTION OF METABOTROPIC GLUTAMATE-RECEPTOR TYPE 1A IN PURKINJE-CELL DENDRITIC SPINES IS INDEPENDENT OF THE PRESENCE OF PRESYNAPTIC PARALLEL FIBERS

The metabotropic glutamate receptor type la (mGluR1a) is expressed at a high level in the molecular layer of the cerebellar cortex, where it is localized mostly in dendritic spines of Purkinje cells, innervated by parallel fibers, Treatment with methylazoxymethanol (MAM) of mouse pups at postnatal days (PND) 0 + 1 or 5 + 6;results in the partial lo ss of granule cells, the extent of which depends on the age of the ani mal at the time of injection, As a consequence of hypogranularity, the number of parallel fibers is decreased to such an amount that many of the postsynaptic Purkinje cell dendritic spines are devoid of axonal input, and only a limited number of spines participate in the formatio n of parallel fiber synapses, or, infrequently, in heterologous or het erotopic synapses with other presynaptic partners, At PND 30, 50% of t he spines in the cerebella of mice treated with MAM at PND 0 + 1 was n ot contacted by any presynaptic element, compared to 5% in controls or 15% in the cerebella of mice treated with MAM at PND 5 + 6, The local ization of mGluR1a was visualized by immunocytochemistry on ultrathin sections: approximately 80% of all Purkinje cell dendritic spines were immunopositive in controls and in both groups of MAM-treated mice, in dicating that mGluR1a was present in Purkinje dendritic spines even wh en the corresponding synaptic input was absent, This observation indic ates that the expression and subcellular distribution of mGluR1a are i nherent, genetically determined properties of Purkinje cells. (C) 1997 Wiley-Liss, Inc.