High levels of histamine H3 receptors (H3Rs) are found in the globus pallidus (GP), a neuronal nucleus in the basal ganglia involved in the control of motor behavior. By using rat GP isolated nerve terminals (synaptosomes), we studied whether H3R activation modified the previously reported enhancing action of adenosine A2A receptor (A2AR) stimulation on depolarization-evoked [(3)H]-GABA release. At 3 and 10 nM, the A2AR agonist CGS-21680 enhanced [(3)H]-GABA release induced by high K(+) (20 mM) and the effect of 3 nM CGS-21680 was prevented by the A2AR antagonist ZM-241385 (100 nM). The presence of presynaptic H3Rs was confirmed by the specific binding of N-α-[methyl-(3)H]-histamine to membranes from GP synaptosomes (maximum binding, Bmax, 1327 ± 79 fmol/mg protein; dissociation constant, Kd, 0.74 nM), which was inhibited by the H3R ligands immepip, clobenpropit, and A-331440 (inhibition constants, Ki, 0.28, 8.53, and 316 nM, respectively). Perfusion of synaptosomes with the H3R agonist immepip (100 nM) had no effect on K(+)-evoked [(3)H]-GABA release, but inhibited the stimulatory action of A2AR activation. In turn, the effect of immepip was blocked by the H3R antagonist clobenpropit, which had no significant effect of its own on K(+)-induced [(3)H]-GABA release. These data indicate that H3R activation selectively counteracts the facilitatory action of A2AR stimulation on GABA release from striato-pallidal projections.

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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/H 3 R interaction, (b) opposite actions of A 2A Rs and H 3 Rs on depolarization-induced Ca 2+ entry, and (c) an A 2A R/H 3 R interaction at the level of adenosine 3′,5′-cyclic monophosphate (cAMP) formation. In GP synaptosomal membranes, H 3 R activation with immepip reduced A 2A R affinity for the agonist 2-p-(2-carboxyethyl)phenethylamino-5′-Nethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680) (K i control 4.53 nM; + immepip 9.32 nM), whereas A 2A R activation increased H 3 R affinity for immepip (K i control 0.63 nM; + CGS-21680 0.26 nM). Neither A 2A R activation nor H 3 R stimulation modified calcium entry through voltage-gated calcium channels in GP synaptosomes, as evaluated by microfluorometry. A 2A Rmediated facilitation of depolarization-evoked [2,3-3 H]-γ-aminobutyric acid ([ 3 H]-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). H 3 R activation failed to reduce the facilitation of [ 3 H]-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 H 3 R 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.