Endogenous activation of group-I metabotropic glutamate receptors is required for differentiation and survival of cerebellar Purkinje cells

We have applied subtype-selective antagonists of metabotropic glutamate, (m Glu) receptors mGlu1 or mGlu5 [7-(hydroxymino) cyclopropa[b]chromen-1a-carb oxylate ethyl ester (CPCCOEt) or 2-methyl-6-(phenylethynyl)pyridine (MPEP)] to mixed rat cerebellar cultures, containing both Purkinje and: granule ce lls. The action of these two drugs on neuronal survival was cell specific. Although CPCCOEt (1, 10, 30 mum) reduced the survival of Purkinje cells, MP EP (3, or 30 mum) selectively reduced the survival of granule cells. Both e ffects required an early exposure of cultures ta antagonists [from 3 to 6 d in vitro (DIV) for CPCCOEt, and from 3 to 6 or 6 to 9 DIV for MPEP]. Addit ion of MPEP from 6 to 9, 9 to 13, or 13 to 17 DIV also, induced profound mo rphological changes In the dendritic tree and dendritic spines of Purkinje, cells, suggesting that endogenous activation of mGlu5 receptors is require d for the age-dependent refinement of Purkinje cell phenotype. In in vivo s tudies, an early blockade of mGlu1 receptors induced in rats by local injec tions of LY367385 (20 nmol/2 mul), local injections of mGlu1 antisense olig onucleotides (12 nmol/2 mul), or systemic administration of CPCCOEt (5 mg/k g, s.c.) from postnatal day (P) 3 to P9 reduced the number and dramatically altered the morphology of cerebellar Purkinje cells. In contrast, mGlu5 re ceptor blockade induced by local injections of antisense oligonucleotides r educed the number of granule cells but also produced substantial morphologi cal changes in the dendritic tree of Purkinje cells. These results provide the first evidence that the development of cerebellar neurons is under the control of mGlu1 and mGlu5 receptors, i.e., the two mGlu receptor subtypes coupled to polyphosphoinositide hydrolysis.