Repeated NMDA receptor activation induces distinct intracellular calcium changes in subpopulations of striatal neurons in vitro

Abstract

The mechanisms underlying long-term calcium changes evoked by excitatory amino acids have not been previously examined in striatal neurons. Fura-2 fluorescence measurements were used to examine intracellular calcium concentration ([Ca 2+] i) changes due to repeated N-methyl- D-aspartate (NMDA) receptor activation, in primary cultures of murine striatal neurons. Three applications of 200 μM NMDA (for 2 min, each application separated by 7 min), in 0 magnesium-containing artificial cerebral spinal fluid, elicited three distinct responses. In 50 ± 8% of the NMDA-responsive neurons, no persistent increases in [Ca 2+] i (final[Ca 2+] i ≦ 150%baseline) were observed, while in 33 ± 7%and17 ± 3% of the cells, sustained (peak response > final [Ca 2+] i > 150% baseline) and uncontrolled increases (final[Ca 2+] i ≧peak response) were observed, respectively. NMDA-responsive neurons that were intensely immunoreactive for the calcium binding protein calbindin-D 28k never exhibited uncontrolled increases in [Ca 2+] i. Removal of extracellular Ca 2+ significantly attenuated sustained, but not uncontrolled, increases in [Ca 2+] i; sustained increases in some neurons were also attenuated by application of verapamil (100 μM) or MK-801 (1 μM). Pre-treatment of striatal neurons with the protein kinase C blocker sphingosine (20 μM), virtually eliminated the development of sustained or uncontrolled increases in [Ca 2+] i. These findings suggest that specific intracellular mechanisms regulate the distinct [Ca 2+] i responses of subpopulations of striatal neurons to repeated NMDA receptor activation.