A role for central A3-adenosine receptors. Mediation of behavioral depressant effects

Abstract

The behavioral effects of a selective A3 adenosine receptor agonist 3-IB-MECA (N6-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine) in mice and the localization of radioligand binding sites in mouse brain were examined. Low levels of A3 adenosine receptors were detected in various regions of the mouse brain (hippocampus, cortex, cerebellum, striatum), using a radioiodinated, high-affinity A3-agonist radioligand [125I]AB-MECA (N6-(3-iodo-4-aminobenzyl)-5'-N-methylcarboxamidoadenosine). Scatchard analysis in the cerebellum showed that the Kd value for binding to A3 receptors was 1.39 +/- 0.04 nM with a Bmax of 14.8 +/- 2.1 fmol/mg protein. 3-IB-MECA at 0.1 mg/kg i.p. was a locomotor depressant with > 50% reduction in activity. Although selective A1 or A2a antagonists reversed locomotor depression elicited by selective A1 or A2a agonists, respectively, the behavioral depressant effects of 3-IB-MECA were unaffected. 3-IB-MECA also caused scratching in mice, which was prevented by coadministration of the histamine antagonist cyproheptadine. The demonstration of a marked behavioral effect of A3 receptor activation suggests that the A3 receptor represents a potential new therapeutic target.

Fig. 1

Structures of adenosine agonists that have been used to characterize A₃ receptors.

Fig. 2

A representative saturation curve for binding at A₃ receptors in mouse cerebellar membranes using [¹²⁵l]AB-MECA in the presence of 100 nM CPX. Conditions are described in section 2 (specific binding shown). The K_d value for binding to A₃ receptors was 1.39 ± 0.04 nM with a B_max of 14.8 ± 2.1 fmol/mg protein.

Fig. 3

Locomotor activity in male NIH Swiss mice. Effect of the A₃-selective adenosine agonist 3-IB-MECA. *P value is < 0.05 vs. vehicle control (n = 6–19).

Fig. 4

Locomotor activity in mice following injection of an A₁- (CPA, 100 μg/kg), A_2a- (APEC, 16 μg/kg), or A₃- (3-IB-MECA, 100 μg/kg) selective agonist and the effects of coadministration of selective antagonists (n = 6–19). Locomotor activity as a percent of control is shown for no antagonist (unshaded bars) or for coadministration of selective xanthine antagonists: CPX (shaded bars, AI, 0.25 mg/kg) or CSC (hatched bars, A_2a, 1.0 mg/kg). * P value < 0.05 vs. adenosine agonist alone.