Abstracts

Neuropeptide galanin has been implicated in inhibiting seizures and protecting hippocampal neurons from excitotoxic injury. In the hippocampus galanin acts through two receptor subtypes, GalR1, expressed in CA1, and GalR2, abundant in dentate gyrus. We examined the role of hippocampal GalR2 in regulating seizures, neuronal survival and plasticity in the rat model of self-sustaining status epilepticus (SSSE), using a selective semi-chronic knockdown of GalR2 in the hippocampus. The experiments were done in adult male Wistar rats. Local down-regulation of GalR2 was achieved by seven-day infusion of peptide nucleic acid antisense (PNA, 25 [mu]M) targeted at GalR2. The effectiveness of GalR2 knockdown was measured using radioligand binding assay with selective GalR2 agonist galanin (2-11). SSSE was induced by 30-min electrical stimulation of perforant path (PPS) through chronically implanted electrode. Electrographic activity was recorded from ipsilateral dentate gyrus and analyzed off-line using Harmonie software (Stellate Systems). Mixed GalR1/GalR2 agonist galanin (1-29), or selective GalR2 agonist Ala²-galanin (both 5 nmole) were injected into hippocampus ipsi-, or contralateral to PNA. Neuronal injury was assessed 3 days after PPS using FluoroJade B staining. Neural progenitor division and neuronal profile of proliferating cells in the dentate gyrus was studied using bromodeoxyuridine, neuron-specific enolase, NeuN and PROX1 immunohistochemistry. Administration of GalR2 PNA resulted in fifty-percent reduction of GalR2 binding in the hippocampus.GalR2 knockdown led to the augmentation of SSSE, evident as the increase of the cumulative seizure time and of the average duration of seizure events. PNA treatment weakened anticonvulsant effects of galanin (1-29) and abolished anticonvulsant action of Ala²-galanin, when the peptides were injected into the site of PNA injection. PNA delivery did not affect seizure-protecting effects of peptides, when the latter were administered contralateral to PNA. GalR2 knockdown alone led to mild neuronal injury in the hilus of dentate gyrus. Furthermore, GalR2 knockdown potentiated SSSE-induced hilar injury. Down-regulation of GalR2 inhibited both SSSE-induced increase of neural progenitor proliferation in subgranular zone of dentate gyrus, and differentiation of neuronal progenitor into neurons, without affecting glial proliferation associated with SSSE. GalR2 in the dentate gyrus plays important role in counteracting seizure activity, promoting neuronal survival and plasticity in response to seizures. The data are useful for understanding brain defensive mechanisms involved in seizure control and neuroprotection. (Supported by NIH grant NS 43409, and Swedish Research Council of Natural Sciences.)