α1A-adrenergic receptors are functionally expressed by a subpopulation of cornu Ammonis 1 Interneurons in rat hippocampus

The importance of adrenergic receptors (ARs) in the hippocampus has generally focused on beta ARs; however, interest is growing in hippocampal alpha ARs given their purported neuroprotective role. We have previously reported alpha(1)AR transcripts in a subpopulation of cornu ammonis 1 (CA1) interneurons. The goal of this study was to identify the specific alpha(1)AR subtype (alpha(1A), alpha(1B), alpha(1D)) functionally expressed by these cells. Using cell-attached recordings to measure action potential frequency changes, concentration-response curves for the selective alpha(1)AR agonist phenylephrine ( PE) were generated in the presence of competitive subtype-selective alpha(1)AR antagonists. Schild regression analysis was then used to estimate equilibrium dissociation constants (K-b) for each receptor antagonist in our system. The selective alpha(1A)AR antagonists, 5-methylurapidil and WB-4101 [2-[(2,6-dimethoxyphenoxyethyl) aminomethyl]-1,4-benzodioxane hydrochloride], produced consecutive rightward shifts in the concentration-response curve for PE when used at discriminating, nanomolar concentrations. Calculated K-b values for 5-methylurapidil (10 nM) and WB-4101 ( 5 nM) correlate to previously published affinity values for these antagonists at the alpha(1A)AR. The selective alpha(1B)AR antagonist L-765,314 [(2S)-4-( 4-amino-6,7-dimethoxy-2-quinazolinyl)-2-[[(1,1-dimethylethyl)amino] carbonyl]-1-piperazinecarboxylic acid], as well as the selective alpha(1D)AR antagonist BMY7378 [8-[2-[4-(2-methoxyphenyl)-1- piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride], produced significant rightward shifts in the concentration-response curve for PE only when used at nondistinguishing, micromolar concentrations. Calculated K-b values for L-765,314 ( 794 nM) and BMY7378 ( 316 nM) do not agree with affinity values for these antagonists at the alpha(1B) or alpha(1D)AR, respectively. Rather, these K-b values more closely match equilibrium dissociation constants estimated for these compounds when used to identify alpha(1A)AR subtypes. Together, our results provide strong evidence to support functional expression of alpha(1A)ARs in a subpopulation of CA1 interneurons.