Abstract
Subcutaneous implantation of morphine tablets in rats with a unilateral 6-hydroxydopamine lesion of nigro-striatal dopamine neurones caused a reduction, after 4 days, in the rates of contralateral/ ipsilateral circling produced by apomorphine and d-amphetamine. Naloxone-precipitated morphine withdrawal on day 5 led to faster d-amphetamine-induced circling together with, in the majority of rats, a corresponding increase in the apomorphine-induced circling rate. A minority of the naloxone-withdrawn rats failed to circle when given apomorphine and they remained unresponsive for 2 weeks after morphine withdrawal. The findings, although consistent with the hypothesis that chronic morphine treatment induces a degree of supersensitivity in dopamine receptors, suggest that other changes take place in the striatum. No change in the d-amphetamine-induced circling rate of 6-hydroxydopaminelesioned mice was recorded after 4 days of morphine treatment. Naloxone-precipitated morphine withdrawal revealed a subsensitivity to d-amphetamine which subsequently changed to an increased sensitivity and then returned to normal 14 days after morphine withdrawal. The results emphasize that chronic morphine administration does not have the same effect in the striatum of rats and mice.
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