Abstracts

Norepinephrine (NE) has been shown to have anti-epileptic effects both [italic]in vitro[/italic] and [italic]in vivo[/italic]. One area of the brain where this has been demonstrated is the hippocampus. We hypothesize that NE exerts its anti-epileptic effect in the hippocampus in part through activation of an [alpha][sub]1[/sub] adrenergic receptor (AR) located on a specific subset of CA1 GABAergic interneurons., Hippocampal slices prepared from young rats were used for all studies. [italic]Functional Studies[/italic]: To determine the specific AR subtype expressed by CA1 interneurons, a panel of subtype specific AR agonists and antagonists was utilized. Cell-attached recordings were conducted under IR/DIC optics, with aCSF perfusion enabling drug delivery in timed increments. Action potential frequency of the interneuron was recorded, plotted as a concentration response curve, and analyzed by Schild regression. To determine if NE promotes GABA release in CA1, cell-attached and whole-cell recordings were conducted on pyramidal cells. NE challenges were made in normal aCSF and then aCSF containing GABA receptor antagonists. [italic]Molecular Studies[/italic]: Cytoplasm was collected from NE-responsive CA1 interneurons and used for single-cell RT PCR. The resulting cDNA template was profiled in terms of AR subtype, neuronal cell type, and neuropeptide expression using real time PCR., 6-fluoronorepinephrine (6FNE), an [alpha] AR agonist, produced a dose-dependent increase in action potential frequency in [sim]50% of CA1 interneurons. The response to 6FNE could be blocked by the [alpha][sub]1[/sub] AR antagonist prazosin, but not by the [alpha][sub]2[/sub] AR antagonist rauwolscine, suggesting that NE-responsive interneurons express an [alpha][sub]1[/sub] AR. Subsequent studies performed with the [alpha][sub]1[/sub] AR subtype specific antagonists 5-methylurapidil, WB 4101, L-765,314, and BMY7378 produced Schild regressions that suggested predominate functional expression of the [alpha][sub]1A[/sub] AR. Molecular analysis of NE-responsive CA1 interneurons revealed transcripts for the [alpha][sub]1A[/sub] and [alpha][sub]1B[/sub] AR, the neuropeptide somatostatin and the GABAergic enzyme glutamate decarboxylase. Initial recordings from pyramidal cells suggest that [alpha][sub]1[/sub] AR activation decreases the activity of these cells, an effect that is attenuated by GABA blockade., We have identified a distinct subset of somatostatin expressing, GABAergic CA1 interneurons that respond to NE via an [alpha][sub]1A[/sub] AR. Adrenergic activation of these interneurons appears to increase the amount of GABA released to pyramidal cells. These findings elucidate one mechanism by which NE may be exerting an anti-epileptic effect in the hippocampus, and furthermore identify a specific receptor subtype that may serve as a potential therapeutic target in the treatment of epilepsy., (Supported by a pre-doctoral fellowship from the American Epilepsy Society.)