Authors :
Presenting Author: Rahul Prasad, BS – New York Medical College
Judah Hoffman, BS – Medical Student First year, Cell Biology and Anatomy, New York Medical College; LINDA FRIEDMAN, PhD – Associate Professor, Cell Biology and Anatomy, New York Medical College
Rationale:
Chronic exposure of commonly used anticonvulsant drugs (ADs) to rat pups during the second to third postnatal period failed to attenuate behavioral or electrographic seizures and behavioral pathology induced by kainic acid (KA). However, several of the ADs tested exerted some neuroprotective effects. Glutamatergic AMPA and Group I mGluR1α receptor distributions were examined in the hippocampus to evaluate their underlying neuroprotective role.
Methods:
Lamotrigine (LTG), carbamazepine (CBZ), phenytoin (PHT), and valproate (VPA) were systemically administered to rat pups for seven days beginning on postnatal (P) day fourteen (P14). Half of the animals were injected with KA after the last treatment to trigger status epilepticus (SE) on P20, an age when the CA1 subregion becomes preferentially sensitive to seizure-induced cell injury. Cellular expression of GluR1, GluR2, and mGluR1α subunits were immunohistochemically analyzed with quantitative measures.
In control pups, GluR1, GluR2, and mGluR1α subunit expression was robust and uniform throughout the hippocampus. Intense neuropillar staining for GluR1 and mGluR1α was observed; GluR2 predominated in somata of pyramidal and granule cell layers. mGluR1α preferentially labeled interneurons of the stratum oriens (SO) and hilus. Following KA-induced SE, GluR1 levels were significantly reduced in the CA1 and upper blade of the dentate gyrus (DG). GluR2 was decreased or relatively steady. mGluR1α was selectively elevated by over two fold in the CA3. Decreases were observed in the SO and stratum radiatum (SR), subregions involved in synaptic plasticity. mGluR1α subunits were over-expressed in the CA3 AD+KA treated groups, the highest being after VPA. Marked increases were also found in stratum oriens/alveus (SO/A INs) and the indusium gresium (IG). In the absence of seizures, VPA-treated pups had marked increases in GluR1 expression throughout the hippocampus, particularly within the pyramidal cell layers, whereas control GluR1 levels were observed in response to LTG, CBZ, or PHT. After KA, GluR1 was further intensified in VPA pretreated pups. In the LTG pretreated group, both GluR1 and GluR2 levels were stable, but GluR1 reductions persisted in the SR and DG. Except for CBZ treated pups, GluR2 expression was steady in the other AD+KA treated groups.
Conclusions:
Several ADs prevented dysregulation of AMPA receptor composition which could reduce ion permeability and fast synaptic conductance to prevent CA1 injury without fully blocking KA-SE. Increases in mGluR1α expression in principal neurons may facilitate endocytosis of AMPA GluR1 receptors after SE to further reduce fast synaptic neurotransmission causing neuroprotection. In contrast, elevations within interneurons located in the SO/A INs and IG that receive excitatory inputs from CA1 neurons may increase synaptic strength between them and SR interneurons. Enhanced GABAergic output via these mGluR1α positive cells can also reduce glutamate excitotoxicity and may in part explain why performance in learning acquisition in the VPA treated group exceeded that of the other ADs.
Funding: NONE