Xylazine and opioid coadministration enhances stereotyped movements in planarians.

Xylazine is a veterinary drug and α2-adrenoceptor agonist that has been increasingly misused as an adulterant in illicit opioids; however, only a few preclinical studies have investigated xylazine's pharmacological profile and impact on opioid-derived behaviors. We investigated the behavioral effects of xylazine alone and in combination with morphine in planarians, which are the simplest living animals having a central nervous system with cephalization. Planarians also express mammalian-like behaviors and neurotransmitters. Our specific experiments investigated the effects of xylazine, morphine, and combinations thereof on stereotyped movements (C-shapes, corkscrews, scrunches, head swings, and head bops) and motility. Clonidine, a xylazine analog and Food and Drug Administration-approved α2-adrenoceptor agonist, was tested for comparison. Both xylazine (1-1000 µM) and clonidine (1-1000 µM), at concentrations greater than or equal to 100 µM, increased stereotypies and reduced motility. Xylazine produced greater maximal effects, and clonidine was more potent. Morphine (1-1000 pM) elicited stereotypies and reduced motility. For combination experiments, morphine (0, 10, and 100 pM) was tested with different concentrations (1, 10, 100, or 1000 µM) of xylazine or clonidine. In the presence of morphine, stereotypies elicited by xylazine or clonidine were further increased, with a particularly robust enhancement of head swings. A notable distinction was that C-shapes and corkscrews were further increased by cotreatment of morphine with xylazine but not with clonidine. Our results identified xylazine-opioid interactions in planarians and showed that xylazine and clonidine elicited stereotyped movements that were enhanced further by cotreatment with morphine.