Abstracts

In hippocampal kindling, the molecular mechanisms by which repetitive stimuli evoke long-lasting changes in the synaptic strength, initially in local circuits and eventually in cortical neurons, are incompletely understood. Glycogen synthase kinase-3 (GSK-3[beta]), when inhibited by phosphorylation at Ser 9, plays a critical role in neurite extension and branching. Moreover, lithium and valproate, agents which modulate seizure threshold, also inhibit GSK-3[beta] activity. Therefore, we examined kindling-induced alterations in GSK-3[beta] (Ser 9) phosphorylation in the neocortex. Accumulation of the transcriptional co-activator [beta]-catenin- an event downstream of GSK-3[beta] inhibition- was also examined along with phosphorylation of PKC and Akt, kinases that are capable of directly phosphorylating GSK-3[beta] at Ser 9. Adult, male Wistar rats were stereotaxically implanted in the right dorsal hippocampus with stimulatory and recording electrodes and underwent a rapid kindling protocol. The progression of kindling was verfied behaviorally, by scoring seizures according to Racine[apos]s scale and electrophysiologically, by recording after-discharges. Immunoblot analysis of Ser 9-phosphorylated GSK3[beta] was employed as an indication of GSK3[beta] inhibition and was performed on ipsilateral and contralateral neocortical tissues (whole-cell lysates) isolated from naive and kindled animals, 15 minutes following the administration of one stimulus. These tissues were also analyzed for total [beta]-catenin levels, as well as for phosphorylated- (Thr 514, pan) protein kinase C (PKC) and -(Ser 473) Akt1. As expected, four consecutive days of kindling led to increased after-discharges as measured by EEG and stage-V seizures. Stimulus-induced GSK-3[beta] phosphorylation increased significantly in the ipsilateral neocortex of kindled animals compared to their na[iuml]ve counterparts. Accumulation of the transcriptional co-activator, [beta]-catenin, was not associated with GSK-3[beta] phosphorylation, suggesting that aberrant synaptic activity induces activation of [beta]-catenin-independent signaling pathways in the neocortex. Phosphorylation of the upstream activating kinases, PKC and Akt1, was not consistently increased in the ipsilateral neocortex of kindled animals (PKC: 2 out of 5 animals; Akt: 3 out of 5 animals). Augmented kindling-induced GSK-3[beta] (Ser 9) phosphorylation responses in the neocortex may indicate a role for this kinase in counteracting/ promoting aberrant synaptic remodeling associated with the progression of kindling to cortical regions. (Supported by NIH NS 23002.)