Evidence of an immune system to brain communication axis that affects central opioid functions: Muramyl peptides attenuate opiate withdrawal

Muramyl peptides are metabolic breakdown products of bacterial cell walls formed in vivo by the reticuloendothelial system. These agents have a variety of immune modulatory and neuropharmacologic effects. It has previously been demonstrated that a variety of immune modifying agents can induce alterations in certain behaviors elicited by opiates. In the present study we investigate possible reciprocal interactions between muramyl dipeptides (MDPs) and central opioid systems using three different experimental models: (1) naloxone-precipitated withdrawal behavior in morphine-dependent rats; (2) the tail immersion assay for determination of morphine-induced antinociception and (3) rectal temperature measurement of the pyrogenic activity of MDP. It is shown that two derivatives of MDP attenuate the severity of naloxone-precipitated withdrawal and morphine-induced antinociception. In addition, it is demonstrated that the pyrogenic activity of a stearoyl derivative of MDP is altered by chronic morphine treatment. These findings suggest both novel neuropharmacologic properties of muramyl dipeptides, as well as demonstrate that yet another immune modifier interacts with centrally mediated opioid phenomena.