Morphine‐induced kinetic alterations of choline acetyltransferase of the rat caudate nucleus

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Abstract
1 In order to explain the decrease of choline acetyltransferase (2.3.1.6.) activity observed in the caudate nucleus of morphine-treated rats, partially purified preparations of the enzyme were used in kinetic studies, with choline as substrate. 2 The apparent Michaelis constant for the enzyme obtained from normal rats was found to be 0·9 mm choline; this value doubled when the animals were killed one hour after a single injection of morphine (30 mg/kg). When the rats were injected daily for 4 or 15 days, and killed one hour after the last injection, the apparent Km value was 2·1 mm in each case. Prolonged daily treatment with morphine, followed by 48 h withdrawal, or by administration of 4 mg/kg of naloxone (given half an hour after the last injection of morphine) resulted in apparent Km values of 1·3–1·5 mm of choline, suggesting a gradual return to the lower, normal substrate requirement. Vmax changes were insignificant. 3 The effect of morphine added in vitro to different enzyme preparations was also studied. The Km values of 0·9 mm, in the enzyme isolated from normal rats, increased to 2·0 after incubation in vitro with 12·5 mm morphine. Similar increases were found in enzymes obtained from rats 48 h after the withdrawal of morphine or from rats injected with naloxone after prolonged morphine treatment. The high apparent Km values, found in enzyme obtained from animals killed one hour after the last dose of morphine, did not change upon incubation with 12·5 mm morphine. A similar pattern of Km changes was noticed after incubation with 25 mm acetylcholine. 4 An increase of 32% in acetylcholine (ACh) level was found in the caudate nucleus one hour after subcutaneous injection of 30 mg/kg of morphine. Return to normal values was observed when morphine was administered daily. After two to three weeks of daily treatment and subsequent withdrawal from morphine for 48 h, the levels of ACh were normal. If the daily treated rats were given naloxone within half an hour of the last injection of morphine, and killed 30 min later, the levels of ACh remained normal. 5 Fifty per cent inhibition of enzyme activity was observed upon in vitro incubation with 75 mm acetylcholine, or with 25 mm morphine. The same degree of inhibition was noticed when the enzyme was obtained from normal or from morphine-treated rats.