Abstract 2009
Neurology Platform, Sunday, 5/2
Glutamate is the main excitatory amino acid in the mammalian central nervous system and among its receptors, NMDA receptor is implicated in brain developmental processes as in excitotoxicity and seizures mediation. Moreover, NMDA receptors are more effective in mediating brain injury in the neonatal period than in adulthood. In a previous study in the developing rat, we showed that NMDA receptor properties are rapidly altered by an acute episode of seizures, especially in the young animals (i.e., 5 days of age). The present study was carried out in order to assess medium- and long-term modifications of brain NMDA receptors following repeated seizures indued at different developmental stages. Generalized seizures lasting for one to two hours were induced in rats by repeated i.p. injections of subconvulsant doses of bicuculline, a GABAA receptor antagonist. Seizures were generated once a day for three consecutive days, either starting from postnatal day 5 (P5) to postnatal day 7 (P7), a very immature developmental cerebral stage, or from P15 to P17, when synaptic connections are well organized. Control animals from the same litter received equivalent doses of saline. NMDA receptors were then analysed at P15, P25 and P60 (adulthood) by measuring the binding of the non-competitive selective antagonist [3H]MK-801 on cerebral membrane preparations (n = 6 for each experimental condition). As a functional approach, the allosteric activation of NMDA receptors by the co-agonists glutamate and glycine (10 µM) was also investigated. P5-P7 seizures led to a 22% increase in the density of [3H]MK-801 binding sites measured at P15, Bmax increasing from 1702 ± 83 to 2068 ± 99 fmol/mg protein (p<0.05), without any modification of their affinity. Seizures did not affect significantly NMDA receptor density or affinity at P25 and P60. P15-P17 seizures led to a 21% decrease in the density of the binding sites, Bmax decreasing from 2129 ± 64 to 1672 ± 33 fmol/mg protein (p<0.01), along with a 33% decrease in their dissociation constant, from 21.9 ± 0.4 to 14.6 ± 0.6 nM (p<0.01) at P25, while seizures were without long-term effect at P60. Also, P5-P7 and P15-P17 seizures were both associated with a suppression of the glutamate/glycine-induced receptor activation at P60, as shown by the lack of difference between Kd values measured in the presence or in the absence of glutamate and glycine. This study showed an age-dependent medium-term modification of NMDA receptors' structural properties and a long-term functional alteration following repeated seizures, whatever the developmental stage at which they were induced. In respect with the physiopathology of NMDA receptors, these modifications may account for long-term alterations in cerebral excitability or plasticity after early convulsive disorders, relevant to cognitive capacities, epileptogenesis and brain sensitivity to recurrent seizures.
