Adenosine reduces opioid withdrawal symptoms by activating A(1) adenosine receptors, probably by inhibiting excitatory amino acid release. Since blockade of A(2A) adenosine receptors seems to enhance dopaminergic striatopallidal transmission, we evaluated the role of the purinergic system in the opiate withdrawal syndrome by using two A(1) receptor agonists [ N(6)-cyclohexyladenosine, CHA and 2-chloro- N(6)-cyclopentyladenosine, CCPA], and two A(2A) receptor antagonists (SCH 58261 and 8-(3-chlorostyryl)caffeine, CSC). Male adult rats received increasing doses of morphine sulphate suspended in 5 ml/kg of a sustained release preparation (40-100 mg/kg s.c.) daily for 4 days and 20 h after the last administration, the withdrawal syndrome was evoked by naloxone (5 mg/kg i.p.). Animals were observed for 30 min for signs of opiate withdrawal. Other groups of rats were implanted with concentric probes for microdialysis and dopamine levels were measured in the nucleus accumbens. CHA and CCPA (0.05, 0.1 or 0.5 mg/kg i.p.) significantly reduced "wet-dog" shakes, diarrhoea, teeth chattering, jumping and writhing. SCH 58261 and CSC (0.1, 0.5 or 1 mg/kg i.p.), given 10 min before naloxone, also reduced signs of opiate withdrawal. CHA plus SCH 58261 and CCPA plus CSC greatly enhanced the reduction of withdrawal signs observed with CHA and CCPA or CSC and SCH 58261 alone. In vivo microdialysis showed that naloxone significantly decreased DA release; this effect was prevented by pretreatment with systemic SCH 58261 and CSC, but not with CHA and CCPA. Our results demonstrate that A(1) and A(2A) adenosine receptors mediate the effect induced by adenosine in opiate withdrawal syndrome and suggest that adenosine A(1) agonists and adenosine A(2A) antagonists may be beneficial in the treatment of this syndrome.
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