Differential effects of N-methyl-d-aspartate on Ca homeostasis in developing and adult rat striatum: in vivo microdialysis approach

This in vivo study concerns developmental differences in the sensitivity of striatal neurons to N-methyl-d-aspartate (NMDA). Changes in calcium homeostatis in adult vs immature rats at postnatal days 8–10, evoked by NMDA, were evaluated by measurements of ⁴⁵Ca efflux and of Ca²⁺ taurine and phosphoethanolamine concentrations in striatal microdialysates. The efflux of ¹⁴C]sucrose was employed in order to measure changes in extracellular space volume. In adult rats the addition of 5 mM NMDA for 20 min to the perfusion medium resulted in a 30–40% increase in ⁴⁵Ca efflux, and in a 15% increase in ¹⁴C]sucrose efflux. Ten minutes after NMDA perfusion, ⁴⁵Ca and ¹⁴C]sucrose efflux returned to the baseline. No significant changes in Ca²⁺ or amino acid concentrations were observed in the dialysate of the adult rat striatum. NMDA perfusion in the striatum of immature rats initially induced a transient (5 min) increase in the efflux of ⁴⁵Ca (by 13%) and ¹⁴C]sucrose (by 9%). This was followed by a prolonged (lasting 45–50 min) 45% decrease in ⁴⁵Ca efflux, an 80% reduction of Ca²⁺ concentration, and increases in taurine and phosphoethanolamine concentrations in the dialysate, whereas ¹⁴C]sucrose efflux recovered within 10 min.These data illustrate differences in the NMDA response between developing and adult rat striatum. Only in developing rats did NMDA induce a large and prolonged influx of extracellular calcium to neurons that may explain the enhanced NMDA neurotoxicity in immature rats.