Adenosine A(2A) and histamine H-3 receptors interact at the cAMP/PKA pathway to modulate depolarization-evoked [H-3]-GABA release from rat striato-pallidal terminals

摘要

We previously reported that the activation of histamine H-3 receptors (H(3)Rs) selectively counteracts the facilitatory action of adenosine A(2A) receptors (A(2A)Rs) on GABA release from rat globus pallidus (GP) isolated nerve terminals (synaptosomes). In this work, we examined the mechanisms likely to underlie this functional interaction. Three possibilities were explored: (a) changes in receptor affinity for agonists induced by physical A(2A)R/H3R interaction, (b) opposite actions of A(2A)Rs and H(3)Rs on depolarization-induced Ca2+ entry, and (c) an A(2A)R/H3R interaction at the level of adenosine 3,5-cyclic monophosphate (cAMP) formation. In GP synaptosomal membranes, H3R activation with immepip reduced A(2A)R affinity for the agonist 2-p-(2-carboxyethyl)phenethylamino-5-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680) (K-i control 4.53nM;+immepip 9.32nM), whereas A(2A)R activation increased H3R affinity for immepip (K-i control 0.63nM;+CGS-21680 0.26nM). Neither A(2A)R activation nor H3R stimulation modified calcium entry through voltage-gated calcium channels in GP synaptosomes, as evaluated by microfluorometry. A(2A)R-mediated facilitation of depolarization-evoked [2,3-H-3]--aminobutyric acid ([H-3]-GABA) release from GP synaptosomes (130.4 +/- 3.6% of control values) was prevented by the PKA inhibitor H-89 and mimicked by the adenylyl cyclase activator forskolin or by 8-Bromo-cAMP, a membrane permeant cAMP analogue (169.5 +/- 17.3 and 149.5 +/- 14.5% of controls). H3R activation failed to reduce the facilitation of [H-3]-GABA release induced by 8-Bromo-cAMP. In GP slices, A(2A)R activation stimulated cAMP accumulation (290% of basal) and this effect was reduced (-75%) by H3R activation. These results indicate that in striato-pallidal nerve terminals, A(2A)Rs and H(3)Rs interact at the level of cAMP formation to modulate PKA activity and thus GABA release.