Alterations in brain endogenous histamine levels in rats after chronic morphine treatment and morphine withdrawal

Chronic but not acute treatment with morphine resulted in a significant decrease in the histamine concentration in the rat hypothalamus while a slight decrease was noted in the brain stem and cortex. Naloxone precipitated morphine withdrawal caused a significant decrease in histamine concentration in all three brain regions investigated. Withdrawal of morphine resulted in a further significant decrease in histamine level in the hypothalamus, brain stem and cortex. Substitution of morphine by methadone induced changes similar to these seen in rats chronically treated with morphine alone. The present data suggest that in addition to the other biogenic amines histamine may be involved in the pharmacological effects of morphine.