Functional downregulation of GluR2 in piriform cortex of kindled animals.

We have previously shown kindling-induced downregulation of the AMPA receptor GluR2 subunit in piriform cortex, as measured by Western blotting. In the present studies, we performed whole-cell patch clamp analysis of AMPA receptor-mediated currents from kindled and control animals to determine if the downregulation observed previously had any functional significance. These experiments were done in the absence and presence of N-hydroxyphenylpropanoyl spermine (HPPS), a polyamine that blocks currents through AMPA receptors lacking GluR2. We report that AMPA receptor-mediated currents recorded from piriform cortex layer II pyramidal cells in slices from animals kindled to 10 fully generalized seizures were blocked by HPPS. In contrast, application of HPPS had no effect on current amplitude in control animals, or in animals that had not been fully kindled. Western blotting revealed that decreases in GluR2 were seen in animals that had experienced at least one fully generalized seizure, but were not observed at earlier stages of kindling development. The increased polyamine sensitivity of AMPA receptor-mediated currents in kindled animals is consistent with the hypothesis that kindling induces formation of AMPA receptors that lack GluR2 in piriform cortex pyramidal cells. It has been demonstrated that polyamine sensitivity is directly correlated with the calcium permeability of the AMPA receptor, suggesting that kindling results in the formation of AMPA receptors that are calcium-permeable. Increases in intracellular calcium through these receptors could act as a second messenger and play a role in the initiation of long-term changes that contribute to the pathogenesis of kindling-induced epilepsy.