tokens: Rats treated for 11 days with morphine and withdrawn for 36 - 40 h showed differences in the development of tolerance : about half of the animals showed a rigidity after the test dose of morphine that was not significantly less than in the controls and were akinetic ( A group ). The other rats showed a strong decrease in the rigidity and the occurrence of stereotyped ( S ) licking and / or gnawing in presence of akinetic or hyperkinetic ( K ) behaviour ( AS / KS group ), suggesting signs of dopaminergic activation .