Attenuation of morphine withdrawal signs by a D1 receptor agonist in the locus coeruleus of rats.

In the present study, the effects of intra-locus coeruleus injection of a dopamine D(1) receptor agonist (SKF38393) on naloxone-induced withdrawal signs of morphine-dependent rats were examined. Twenty different withdrawal signs were assessed. The total withdrawal score was calculated and used as an index of withdrawal intensity for comparison. The D(1) agonist and antagonist were injected 15 and 30 min prior to expression of naloxone-induced withdrawal signs, respectively. SKF38393 (2 and 4 microg/site) decreased while SCH23390 (a D(1) antagonist) had no effect on the total withdrawal score. On the other hand, SCH23390 (25 ng/site) reversed the SKF38393 effect. It may be concluded that activation of dopamine D(1) receptors in the locus coeruleus attenuates naloxone-induced withdrawal.