The effects of D-histiding on the expression of morphine tolerance and physical dependence in mice.

D-Histidine, administered in the 'wthdrawal' phase of morphine addiction, failed to modify the expression of tolerance and physical dependence in mice. L-Histidine, on the contrary, can enhance tolerance and inhibit physical dependence. Whole brain histamine is markedly increased by L-histidine administration, but only minimally by D-histidine. This confirms that the actions of L-histidine on morphine addiction are stereospecific, and so can be more confidently correlated with the increase in brain histamine levels produced by the natural amino acid precursor.