Functional characterization of the β-adrenergic receptor subtypes expressed by CA1 pyramidal cells in the rat hippocampus

Recent studies have demonstrated that activation of the beta-adrenergic receptor ( AR) using the selective beta-AR agonist isoproterenol (ISO) facilitates pyramidal cell long-term potentiation in the cornu ammonis 1 ( CA1) region of the rat hippocampus. We have previously analyzed beta-AR genomic expression patterns of 17 CA1 pyramidal cells using single cell reverse transcription-polymerase chain reaction, demonstrating that all samples expressed the beta 2-AR transcript, with four of the 17 cells additionally expressing mRNA for the beta 1-AR subtype. However, it has not been determined which beta-AR subtypes are functionally expressed in CA1 for these same pyramidal neurons. Using cell-attached recordings, we tested the ability of ISO to increase pyramidal cell action potential (AP) frequency in the presence of subtype-selective beta-AR antagonists. ICI- 118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl) amino]-2-butanol] and butoxamine [alpha-[1-(t-butylamino) ethyl]-2,5-dimethoxybenzyl alcohol) hydrochloride], agents that selectively block the beta 2-AR, produced significant parallel rightward shifts in the concentration-response curves for ISO. From these curves, apparent equilibrium dissociation constant (K-b) values of 0.3 nM for ICI- 118,551 and 355 nM for butoxamine were calculated using Schild regression analysis. Conversely, effective concentrations of the selective beta 1-AR antagonists CGP 20712A [(+/-)-2-hydroxy-5-[2-([2-hydroxy-3(4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl] phenoxy) propyl] amino)ethoxy]-benzamide methanesulfonate] and atenolol [4-[2'-hydroxy-3'-(isopropyl-amino)propoxy]phenylacetamide] did not significantly affect the pyramidal cell response to ISO. However, at higher concentrations, atenolol significantly decreased the potency for ISO-mediated AP frequencies. From these curves, an apparent atenolol Kb value of 3162 nM was calculated. This pharmacological profile for subtype-selective beta-AR antagonists indicates that beta 2-AR activation is mediating the increased AP frequency. Knowledge of functional AR expression in CA1 pyramidal neurons will aid future long-term potentiation studies by allowing selective manipulation of specific beta-AR subtypes.