Abstracts

RATIONALE: The excitatory neurotoxin kainate induces seizure activity but whether this occurs through its actions on AMPA or kainate receptors is uncertain. We used ATPA, a selective GluR5 kainate receptor agonist, to determine if activation of kainate receptors alone can induce epileptiform activity. Our experiments were carried out with field potential recording in slices of the rat basolateral amygdala (BLA). We have previously shown that GluR5 kainate receptors, a member of the ionotropic glutamate receptor family, mediate a portion of synaptic excitation in the BLA. Moreover, [italic]in situ[/italic] hybridization and immunocytochemical studies have confirmed that GluR5 kainate receptors are localized to BLA synapses.
METHODS: Coronal amygdala slices (500 [mu]m) were vibratome cut from brains of 50-100 g male Sprague-Dawley rats and placed in a submerged recording chamber with oxygenated ACSF flowing at 2-3 ml/min (31 [degree]C, pH 7.4). After equilibration, an insulated bipolar stimulating electrode was positioned in the external capsule (EC) and a glass recording micropipette (5-10 M[Omega]) placed in the BLA. The specific GluR5 agonist ([italic]RS[/italic])-[alpha]-amino-3-hydroxy-5-[italic]tert[/italic]-butyl-4-isoxazolepropionic acid (ATPA) was bath applied at concentrations from 1-10 [mu]M. In control experiments, molar equivalent concentrations of AMPA were used. In some experiments, the GluR5 specific antagonist LY 293558 was also added to the superfusion solution. The EC-evoked population and spontaneous extracellular field responses were amplified and stored in digital form.
RESULTS: In 9 of 10 slices, bath application of ATPA (10 [mu]M) elicited spontaneous epileptiform bursts within 5-10 min of the onset of drug perfusion. Ordinarily, the bursts consisted of a fast negative going wave followed by a slower positive wave with an overall amplitude [lt]0.5 mV. The frequency varied greatly by slice, but generally was [lt1 Hz. Occasionally, slices manifested prolonged seizure-like discharges composed of multiple bursts in rapid succession. An increase in the number of waves in the EC-evoked field response was also sometimes seen with ATPA. Bursting terminated within 5 min after removal of ATPA from the perfusion medium. Coadministration of LY 293558 at concentrations of 100-250 nM markedly reduced the frequency of ATPA-induced bursting and terminated bursting at 500 nM. ATPA concentrations [lt]10[mu]M generally did not cause large amplitude bursts although an increase in noise was apparent. Spectral analysis revealed a concentration-dependent increase in power at frequencies of 5-100 Hz with ATPA (threshold concentration, 2.5 [mu]M). Bath application of AMPA at similar concentrations usually led to immediate termination of all activity including the [dsquote]population spike[dsquote] in the EC-evoked response, but occasionally did induce some bursting at concentrations of 1[mu]M or less.
CONCLUSIONS: Selective activation of the GluR5 receptors in the BLA leads to spontaneous epileptiform activity. In contrast, activation of AMPA receptors generally suppress epileptiform activity, presumably because of neuronal overdepolarization. These results provide strong support for the concept that the seizure inducing properties of kainate, at least in the amygdala, result from its ability to activate kainate receptors and not AMPA receptors.
[Supported by: NINDS]