The effects of ethanol, acetaldehyde, morphine and naloxone on histamine methyltransferase activity

• 1.

  1. The microassay method of Taylor and Snyder for determining the activity of histamine N-methyltransferase (E.C.2.1.1.8) (HMT), the histamine metabolizing enzyme, was modifies to permit the simultaneous measurement of histamine and HMT activity in the same tissue sample under our experimental conditions.

• 2.

  2. The synthesis and purification of τ-methyl-[¹⁴C]-histamine was performed to assess the efficiency of extraction of methylhistamine which in this modified methodology is approximately 80 percent.

• 3.

  3. The influence of ethanol, acetaldehyde, morphine and naloxone alone and in combination on hypothalamic, thalamus-midbrain and cerebral cortical HMT activity was investigated at concentrations of 0.1 μM to 500 μM.

• 4.

  4. Ethanol, acetaldehyde and ethanol in the presence of 1 μM morphine failed to markedly influence HMT activity.

• 5.

  5. At high concentrations morphine strongly inhibited while naloxone enhanced enzyme activity. Morphine's effects on HMT remained unaltered by 1 μM naloxone.

• 6.

  6. In the present study low concentrations of ethanol and morphine have little effect on HMT activity ‘in vitro’. These observations suggest that changes in HMT activity may play a minor role in the alcohol and morphine-induced central histamine changes observed in our previous ‘in vivo’ studies.