Enhanced NMDA receptor-dependent LTP in the epileptic CA1 area via upregulation of NR2B.

Impairment of synaptic plasticity such as long-term potentiation (LTP) is a common finding in various animal models of a number of neurodegenerative disorders. While cognitive deficits associated with these models are plausibly attributed to impaired plasticity, it is an intriguing question whether learning impairment correlates in general with compromised synaptic plasticity. In the present study, we have addressed this issue and discovered an enhancement of theta-burst stimulation-induced LTP at Schaffer collateral-CA1 synapses from chronically epileptic animals. The LTP enhancement was abolished by the NMDA receptor 2B (NR2B) blocker Ro 25-6981 (1μM) while it was preserved following application of the NR2A blocker NVP-AAM077 (50nM). Moreover, pharmacological characterization of intracellularly recorded excitatory postsynaptic potentials (EPSP) from CA1 pyramidal neurons indicated an increased NR2B/NR2A ratio in epileptic tissue, and NMDA receptor mediated excitatory postsynaptic currents showed significantly longer decay times. Quantitative reverse-transcriptase PCR confirmed the transcriptional up-regulation of NR2B-mRNA in chronically epileptic animals. To test the significance for epileptiform activity, recurrent epileptiform discharges (REDs) in the CA1 area induced by bath application of either high K(+) (8mM) plus gabazine (5μM) or 4-aminopyridine (50μM), were also characterized pharmacologically. While in control slices the presence of Ro 25-6981 had no effect on the RED frequency, NR2B inhibition significantly increased epileptic activity in tissue from epileptic animals. Our results demonstrate that CA1 synapses in chronically epileptic tissue can undergo an LTP enhancement due to an NR2B up-regulation in CA1 pyramidal neurons. On the network level, this up-regulation appears to be a compensatory process, since blockade of these receptors leaves the tissue more susceptible to hyperexcitability.