MECHANISMS UNDERLYING DEVELOPMENTAL-CHANGES IN THE EXPRESSION OF METABOTROPIC GLUTAMATE RECEPTORS IN CULTURED CEREBELLAR GRANULE CELLS - HOMOLOGOUS DESENSITIZATION AND INTERACTIVE EFFECTS INVOLVING N-METHYL-D-ASPARTATE RECEPTORS

Glutamate receptors coupled to polyphosphoinositide (PPI) hydrolysis ( metabotropic glutamate receptors, mGluR), are highly efficient during the early stages of postnatal life and are thought to be involved in d evelopmental plasticity. The dramatic decrease with age in mGluR activ ity suggests the existence of mechanisms that down-regulate this recep tor after a certain stage of neuronal maturation. In cultured cerebell ar granule neurons grown under conditions that promote the survival an d maturation of cells (serum-containing medium with 25 mm K+), enzymat ic depletion of extracellular glutamate prevented the age-dependent de crease in mGluR agonist-stimulated PPI hydrolysis that normally occurs after 4 days of maturation in vitro, suggesting that mGluR activity d eclines as a result of developmental changes affecting homologous dese nsitization. This was borne out by the observation that glutamate at l ow concentrations (1-10 muM) readily desensitized mGluR at 7 days but not at 4 days in culture. Furthermore, the critical period during whic h the high sensitivity to agonist-induced desensitization of mGluR dev eloped coincided with the period when phorbol ester-activated protein kinase C acquired the ability to suppress mGluR activity. The developm ental pattern of mGluR agonist-induced PPI hydrolysis was similar in g ranule cells grown under ''trophic'' and ''nontrophic'' conditions (in cultures in 25 mm K+ and in a medium containing ''low'' K+, in this s tudy, 10 mm, respectively). However, the developmental decline in the response to mGluR stimulation after 4 days in vitro was not prevented in cells grown in 10 mm K+ by the removal of extracellular glutamate; rather, it could be counteracted by treatment with N-methyl-D-aspartat e (NMDA) (EC50, approximately 4 muM), which blocked the development of mGluR desensitization. The effect was NMDA receptor mediated and requ ired DNA transcription and protein synthesis. However, NMDA exerted a different effect in cells grown in 25 mm K+, inducing a substantial de crease rather than an increase in mGluR activity. The effect of growth conditions was also examined on mGluR mRNA levels, which were not alw ays correlated with mGluR activity. In general, either increases in th e medium K+ concentrations or NMDA supplementation of the cultures res ulted in a decrease in mGluR mRNA levels. It is noteworthy that NMDA c ould also restore mGluR activity after the metabotropic response had r eached its peak. This implies that NMDA receptor activation may be inv olved in the increase in mGluR activity in adult life under conditions that elicit plastic changes in the nervous system.