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 alpha 2 adrenergic receptor (AR) activation on pyramidal cells. In this study, we investigated which alpha 2AR subtype(s) mediates this effect. First, (alpha) 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 alpha(2A) AR transcript; 3 of the 12 cells additionally expressed mRNA for alpha(2C)AR subtype and no cells possessing alpha(2B) AR mRNA. Hippocampal CA3 epileptiform activity was then examined using field potential recordings in brain slices. The selective alpha(AR) agonist 6- fluoronorepinephrine caused a reduction of CA3 epileptiform activity, as measured by decreased frequency of spontaneous epileptiform bursts. In the presence of beta AR blockade, concentration-response curves for AR agonists suggest that an alpha 2AR mediates this response, as the rank order of potency was 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14304) >= epinephrine > 6-fluoronorepinephrine > norepinephrine phenylephrine. Finally, equilibrium dissociation constants (K b) of selective alpha AR antagonists were functionally determined to confirm the specific alpha 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 alpha(2A) AR subtype (r = 0.99, slope = 1.0). These results suggest that, under conditions of impaired GABAergic inhibition, activation of alpha(2A) ARs is primarily responsible for the antiepileptic actions of norepinephrine in the rat hippocampal CA3 region.