Effects of acute and chronic morphine on regional rat brain acetylcholine turnover rate

A simplified method to study the acetylcholine (ACh) turnover rate (TR_ACh) in brain parts of drug treated rats has been presented. In striatum and occipital cortex of rats receiving a large dose of morphine (140 μ moles/kg i.p.) or implanted chronically with morphine pellets, the TR_ACh is influenced in a different manner. The single injection of morphine reduced the synthesis of ACh in cortex but not in striatum. Morphine pellets decreased striatal TR_ACh but failed to alter the TR_ACh in occipital cortex. Naloxone reversed both changes of TR_ACh elicited by morphine although it was devoid of any effect of the synthesis of ACh in rat brain parts. We suggest that morphine may prevent the ACh release from neurons as proposed by others, however, this effect in striatum of rats receiving a single dose of morphine is masked by the simultaneous action of morphine on the dopaminergic nigrostriatal pathway which regulates the turnover rate of striatal ACh.