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Reversal of Hippocampal LTP by Spontaneous Seizure-Like Activity: Role of Group I mGluR and Cell Depolarization

Department of Physiology and Pharmacology, State University of New York, Brooklyn, New York

Submitted 24 February 2004; accepted in final form 23 July 2004

Memory impairment is a common consequence of epileptic seizures. The hippocampal formation is particularly prone to seizure-induced amnesia due to its prominent role in mnemonic processes. We used the isolated CA1 slice preparation to examine effects of seizure-like activity on hippocampal plasticity, long-term potentiation (LTP), and long-term depression (LTD). Repeated spontaneous ictal events, generated in the presence of antagonists of GABA_A receptor function, led to a stepwise erasure of LTP (termed spontaneous depotentiation, SDP). SDP could be initiated at various stages of LTP consolidation (tested 120 min after the induction of LTP). Renewed tetanic stimulation re-established LTP. SDP was remarkably specific: baseline transmission and other forms of hippocampal plasticity, i.e., Ca²⁺-induced LTP and two forms of LTD [(RS)-3,5-dihydroxyphenyglycine (DHPG) mediated and low-frequency stimulation mediated] were not affected by the same type of seizure activity. SDP was blocked in the presence of the group I mGluR antagonist (S)-4-carboxyphenylglycine. The mGluR1 antagonist (S)-(+)--amino-methylbenzeneacetic acid blocked 80%, the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)-pyridine 30% of SDP. Most efficient implementation of SDP was observed during seizures in the combined presence of the group I mGluR agonist DHPG and the GABA_A antagonist bicuculline. However, similar ictal activity generated in the presence of DHPG alone did not lead to SDP in the vast majority of recordings. Complete disinhibition and at least partial activation of group I mGluR were necessary conditions for the induction of SDP. The depotentiating pharmacological conditions were accompanied by tonic membrane depolarization of CA1 pyramidal cells. Since hyperpolarization (by negative current injection) prevented intracellular SDP under depotentiating pharmacological conditions and depolarization (by positive current injection) led to selective intracellular SDP in the non-depotentiating seizure protocol of DHPG, it is concluded that cell depolarization was a sufficient condition for seizure-like activity to reverse hippocampal LTP.

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