SELECTIVE ALTERATIONS IN BINDING KINETIC-PARAMETERS AND ALLOSTERIC REGULATION OF N-METHYL-D-ASPARTATE RECEPTORS AFTER PROLONGED SEIZURES INTHE DEVELOPING RAT-BRAIN

Among glutamate receptor subtypes, the N-methyl-D-aspartate (NMDA) rec eptor plays a key role in brain development and cognitive processes, a nd mediates excitotoxic injury. To test the hypothesis that prolonged seizures may affect NMDA receptor characteristics in the developing br ain, a 30-min episode of generalized seizures was induced in rats at 5 , 10, 15 and 25 d of age by i.p. administrations of bicuculline. NMDA receptors were analyzed using specific binding of [H-3]-labeled 1-dihy dro-5H-dibenzo-[a,d]-cycloheptane-5,10-imine maleate (MK-801) in brain membrane preparations, and allosteric regulation was studied by addit ion of glutamate (10 mu M) and glycine (10 mu M). In control pups, tot al number of binding sites increased between 5 and 25 d, B-max,, value s varying from 1032 +/- 93 to 2311 +/- 449 fmol/mg protein, whereas re ceptor affinity decreased with age, the affinity constant (K-d,) chang ing from 20.9 +/- 2.0 to 29.1 +/- 2.0 nM. Activation of NMDA receptors by glutamate and glycine led to age-dependent decreases in K-d values , from 30% at 5 d to 72% at 25 d. Seizures altered receptor density on ly at 5 d (by 40%). Receptor affinity was increased after seizures at 5, 15 and 25 d (from 12 to 60%). The capacity of receptor activation b y glutamate and glycine was significantly reduced by seizures at 5 d. There was no change either in density nor affinity of receptors at 10 d. Therefore, as previously shown for central adenosine and benzodiaze pine receptors, sustained seizures are able to alter the characteristi cs of NMDA receptors in a specific way depending on the maturational s tage, suggesting developmental changes in the mechanisms of brain resp onse to seizures.