Withdrawal and bidirectional cross-withdrawal responses in rats treated with adenosine agonists and morphine.

The aim of this study was to investigate whether the A1/A2 receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), and the selective A1 agonist, N6-cyclopentyladenosine (CPA), induced physical dependence by quantifying specific antagonist-precipitated withdrawal syndromes in conscious rats. In addition, the presence of bidirectional cross-withdrawal was also investigated. The agonists were administered s.c. to groups of rats at 12 h intervals. Antagonists were administered s.c., 12 hours after the last dose, followed by observation and measurement of faecal output for 20 min. NECA (4 x 0.03 mg kg(-1), s.c) and CPA (4 x 0.03, 0.1 and 0.3 mg kg(-1), s.c.) induced physical dependence, as shown by the expression of a significant withdrawal syndrome when challenged with the adenosine A1/A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX, 0.1 mg kg(-1), s.c.) and the A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPDPX, 0.1 mg kg(-1), s.c.) respectively. The syndromes consisted of teeth chattering and shaking behaviours shown to occur in morphine-dependent animals withdrawn with naloxone viz, paw, body and 'wet-dog' shakes, but with the additional behaviours of head shaking and yawning. In further contrast to the opiate withdrawal syndrome, no diarrhoea occurred in the groups of animals treated with adenosine agonists and withdrawn with their respective antagonists. Bidirectional cross-withdrawal syndromes were also revealed when naloxone (3 mg kg(-1), s.c.) was administered to adenosine agonist pre-treated rats and adenosine antagonists were given to morphine pre-treated rats. This study provides further information illustrating that close links exist between the adenosine and opiate systems.