REGULATION OF F-ACTIN STABILITY IN DENDRITIC SPINES BY GLUTAMATE RECEPTORS AND CALCINEURIN

Neuronal degeneration and cell death can result from excessive activat ion of receptors for the excitatory neurotransmitter glutamate; howeve r, the very earliest changes in cytoskeletal organization have not bee n well documented:We have used an in vitro model system to examine the early consequences of intense glutamate receptor activation on dendri tic spine synapses. Cultured hippocampal neurons exposed to NMDA for a s little as 5 min exhibited a rapid and extensive loss of dendritic sp ines. Staining for the presynaptic marker synapsin 1 and the postsynap tic density proteins PSD-95 and the NR1 subunit of NMDA receptors rema ined intact. The disappearance of spines was accompanied by a selectiv e loss of filamentous actin staining at synapses. The NMDA-induced los s of spine actin was time- and concentration-dependent and blocked by NMDA receptor antagonists. The effect was mimicked by L-glutamate, AMP A, and ionomycin but not by agonists of L-type calcium channels or of metabotropic glutamate receptors. The effect of NMDA on local actin as sembly was strongly attenuated by pretreatment with an actin stabilizi ng compound or by an antagonist of the calcium-dependent protein phosp hatase calcineurin. Immunoreactivity for calcineurin was enriched at s ynapses together with F-actin. These results indicate that the actin-m ediated stability of synaptic structure is disrupted by intense glutam ate receptor activity and that calcineurin blockers may be useful in p reventing such destabilization.