Adenosine receptors mediating inhibitory electrophysiological responses in rat hippocampus are different from receptors mediating cyclic AMP accumulation

Summary

Electrophysiological and biochemical techniques were used to characterize adenosine receptors in rat hippocampus. The site which mediates the inhibitory action of adenosine on excitatory synaptic transmission and on spontaneous interictal spiking had properties similar to the adenosine A1 receptor. Thus, the relative order of potency for adenosine analogs was l-PIA≥CHA>NECA> 2CA (l-PIA = N⁶-phenylisopropyladenosine; CHA = N⁶-cyclohexyl-adenosine; NECA = adenosine 5′-ethylcarboxamide; 2CA = 2-chloroadenosine), with EC50 values for the most potent analogs between 10–30 nM. The effect of the stable adenosine analog, particularly CHA and l-PIA, was slow in onset and very slowly reversible. This is suggested to be due both to a slow dissociation of these compounds from the receptors but particularly to the slow equilibrium between the concentration of the drug in the medium surrounding the slices and the biophase within the slices. Adenosine analogs bound specifically to membrane preparations of the rat hippocampus with the order of potency ³H-CHA≥³H-l-PIA>³H-NECA. Eadie-Hofstee plots of the binding data were curvilinear for each ligand, but only for ³H-l-PIA could the existence of two binding sites with different apparent K _d-values (0.27 and 11.8 nM) be confirmed by curve-fitting. The estimated K _d-values for CHA and NECA were 1.5 and 20 nM, respectively. The adenosine analogs also enhanced ³H-cyclic AMP accumulation in ³H-adenine-labelled hippocampal slices. The rank order of potency of adenosine analogs in increasing cyclic AMP (NECA>2CA>l-PIA>CHA) suggests that this effect is mediated by adenosine A2 receptors. The EC50 values for the accumulation of cyclic AMP were 10–1000 × higher than the EC50 values derived from electrophysiological experiments and the K _d-values from measurements of radioligand binding. Thus, on the basis of absolute as well as rank order potencies of drugs, the adenosine analog-induced electrophysiological responses appear to be related to actions at an A1 receptor site. By contrast, the adenosine receptor-mediated increases in cyclic AMP appears to involve an A2 receptor, the functional role of which is not clear.