Pharmacological characterization of the α2A-adrenergic receptor inhibiting rat hippocampal CA3 epileptiform activity: comparison of ligand efficacy and potency

The mechanism underlying the antiepileptic actions of norepinephrine (NE) is unclear with conflicting results. Our objectives are to conclusively delineate the specific adrenergic receptor (AR) involved in attenuating hippocampal CA3 epileptiform activity and assess compounds for lead drug development. We utilized the picrotoxin model of seizure generation in rat brain slices using electrophysiological recordings. Epinephrine (EPI) reduced epileptiform burst frequency in a concentration-dependent manner. To identify the specific receptor involved in this response, the equilibrium dissociation constants were determined for a panel of ligands and compared with established binding values for α1, α2, and other receptor subtypes. Correlation and slope of unity were found for the α2A-AR, but not other receptors. Effects of different chemical classes of α-AR agonists at inhibiting epileptiform activity by potency (pEC50) and relative efficacy (RE) were determined. Compared with NE (pEC50, 6.20; RE, 100%), dexmedetomidine, an imidazoline (pEC50, 8.59; RE, 67.1%), and guanabenz, a guanidine (pEC50, 7.94; RE, 37.9%), exhibited the highest potency (pEC50). In contrast, the catecholamines, EPI (pEC50, 6.95; RE, 120%) and α-methyl-NE (pEC50, 6.38; RE, 116%) were the most efficacious. These findings confirm that CA3 epileptiform activity is mediated solely by α2A-ARs without activation of other receptor systems. These findings suggest a pharmacotherapeutic target for treating epilepsy and highlight the need for selective and efficacious α2A-AR agonists that can cross the blood–brain barrier.

去甲肾上腺素（norepinephrine, NE）抗癫痫作用的潜在机制尚不明确，且相关研究结果存在矛盾。本研究旨在明确介导海马CA3区癫痫样活动减弱的特异性肾上腺素能受体（adrenergic receptor, AR）亚型，并筛选可用于先导药物开发的化合物。我们采用印防己毒素诱导癫痫发作的大鼠脑片模型，结合电生理记录开展实验。肾上腺素（epinephrine, EPI）可呈浓度依赖性降低癫痫样爆发频率。为明确该效应所涉及的特异性受体，我们测定了一组配体的平衡解离常数，并与已报道的α1、α2及其他受体亚型的结合参数进行对比。结果发现，仅α2A肾上腺素能受体（α2A-AR）的相关性及斜率符合单位值，其余受体均未观察到该现象。我们进一步测定了不同化学类别的α肾上腺素能受体激动剂对癫痫样活动的抑制效应，包括其效价强度（pEC50）与相对效能（relative efficacy, RE）。与去甲肾上腺素（pEC50=6.20；RE=100%）相比，咪唑啉类激动剂右美托咪定（pEC50=8.59；RE=67.1%）与胍类激动剂胍那苄（pEC50=7.94；RE=37.9%）展现出最高的效价强度。与之相反，儿茶酚胺类激动剂肾上腺素（pEC50=6.95；RE=120%）与α-甲基去甲肾上腺素（pEC50=6.38；RE=116%）则具备最高的相对效能。本研究结果证实，海马CA3区的癫痫样活动仅由α2A-AR介导，未涉及其他受体系统的激活。上述发现为癫痫的药物治疗提供了潜在靶点，并提示亟需研发可穿透血脑屏障的高选择性、高效能α2A-AR激动剂。