α2A Adrenergic Receptor Activation Inhibits Epileptiform Activity in the Rat Hippocampal CA3 Region

Norepinephrine has potent antiepileptic properties, the pharmacology of which is unclear. Under conditions in which GABAergic inhibition is blocked, norepinephrine reduces hippocampal cornu ammonis 3 (CA3) epileptiform activity through α 2 adrenergic receptor (AR) activation on pyramidal cells. In this study, we investigated which α 2 AR subtype(s) mediates this effect. First, α 2 AR genomic expression patterns of 25 rat CA3 pyramidal cells were determined using real-time single-cell reverse transcription-polymerase chain reaction, demonstrating that 12 cells expressed α 2A AR transcript; 3 of the 12 cells additionally expressed mRNA for α 2C AR subtype and no cells possessing α 2B AR mRNA. Hippocampal CA3 epileptiform activity was then examined using field potential recordings in brain slices. The selective αAR agonist 6-fluoronorepinephrine caused a reduction of CA3 epileptiform activity, as measured by decreased frequency of spontaneous epileptiform bursts. In the presence of βAR blockade, concentration-response curves for AR agonists suggest that an α 2 AR mediates this response, as the rank order of potency was 5-bromo- N -(4,5-dihydro-1 H -imidazol-2-yl)-6-quinoxalinamine (UK-14304) ≥ epinephrine >6-fluoronorepinephrine > norepinephrine ⋙ phenylephrine. Finally, equilibrium dissociation constants ( K b ) of selective αAR antagonists were functionally determined to confirm the specific α 2 AR subtype inhibiting CA3 epileptiform activity. Apparent K b values calculated for atipamezole (1.7 nM), MK-912 (4.8 nM), BRL-44408 (15 nM), yohimbine (63 nM), ARC-239 (540 nM), prazosin (4900 nM), and terazosin (5000 nM) correlated best with affinities previously determined for the α 2A AR subtype ( r = 0.99, slope = 1.0). These results suggest that, under conditions of impaired GABAergic inhibition, activation of α 2A ARs is primarily responsible for the antiepileptic actions of norepinephrine in the rat hippocampal CA3 region.