Effect of morphine on the sensitivity of mouse vas deferens and ileum during chronic morphine treatment and its correlation with analgesic tolerance.

Supersensitivity to norepinephrine (NE) in vas deferens and to acetylcholine (ACh) in ileum was observed during chronic morphine treatment and precipitated withdrawal in mice. Analgesic tolerance and sensitivity changes during chronic morphine exposure followed a biphasic pattern demonstrating a positive correlation between them. Potassium chloride (KCl)-induced response remained unaffected. A high degree of supersensitivity to ACh was evident during naloxone precipitated withdrawal suggesting a greater involvement of ACh in the withdrawal phenomenon. The results support the theory proposed by Collier, to explain tolerance and dependence.