The role of A3 adenosine receptors in central regulation of arterial blood pressure

1. Pharmacological studies have suggested that A₃ receptors are present on central neurons. Recently this adenosine receptor subtype has been identified in the rat and its presence in the central nervous system has been confirmed.
2. In this study we investigated the effects of acute intracerebroventricular (i.c.v.) injections of N⁶-2-(4-aminophenyl)-ethyladenosine (APNEA), a non-selective A₃ adenosine receptor agonist, on arterial blood pressure (ABP) and heart rate (HR), after treatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective antagonist of A₁ adenosine receptors.
3. Anaesthetized rats, after DPCPX (12 μg⁻¹ kg i.c.v.), were treated with APNEA (0.4–4 μg kg⁻¹ i.c.v.) resulting in a transitory and dose-dependent decrease in arterial blood pressure without a change in heart rate. APNEA also induced hypotensive responses after i.c.v. pretreatment with aminophylline, at a dose of 20 μg kg⁻¹. In contrast, pretreatment 48 h before, with 4 μg kg⁻¹ i.c.v. of pertussis toxin reduced the hypotensive effect induced by APNEA. Administration of APNEA at a higher dose (20 μg kg⁻¹ i.c.v.), after DPCPX, induced a decrease in ABP of −66±5.4 mmHg and after 3 min a decrease in heart rate of −62±6.0 beats min⁻¹. Transection of the spinal cord abolished this significant fall in ABP, but not the decrease of HR.
4. These results suggest that a population of A₃-receptors is present in the CNS, whose activation induces a decrease in blood pressure with no change of heart rate.