Beta-endorphin injected into the nucleus of the raphe magnus facilitates metastatic tumor growth.

Electrical stimulation of the periaqueductal gray of the rat's midbrain analgesia leads to an increase in the number of artificial pulmonary metastases from the Walker 256 tumor. In an effort to investigate the influence of the pain suppression system and its associated peptides on this phenomenon, we activated the pain suppression system directly from the Nucleus of the Raphe Magnus, a non-opioid subsystem. After inducing analgesia by direct injection of beta-endorphin on the Nucleus of the Raphe Magnus, we noted an increase in the number of artificial pulmonary metastases. This result could be blocked by pretreatment with naloxone. If the Nucleus of the Raphe Magnus was activated by electrical stimulation sufficient to induce analgesia, the metastatic effect was still present but markedly attenuated.