Xylazine, veterinary anesthetic and recreational drug adulterant, reduces cephalic regeneration and alters defensive responding in planarians.

Xylazine is a veterinary anesthetic and α2-adrenoceptor agonist found in illicit drug supplies. Injection of opioids adulterated with xylazine is correlated with necrotic tissue injury and a withdrawal syndrome including anxiety. Planarians are flatworms with a simple nervous system and no circulatory system that exhibit several characteristics, including regenerative capacity and negative phototaxis (light avoidance). Because regeneration reflects tissue repair and negative phototaxis constitutes defensive behavior, planarians provide a simple in vivo system to explore possible effects of xylazine on planarian wound repair and anxiety-like behavior. Using planarians, we investigated the effects of xylazine on cephalic regeneration and defensive responding (i.e., determined by time spent on light side of a dish divided into light and dark compartments). In decapitated planarians, chronic xylazine exposure (0.001-0.1µM, over 2 weeks) reduced head regeneration rate. For intact worms, acute xylazine (1µM, 5min) caused a reduction in defensive responding (i.e., increased time in light) that was abolished by yohimbine (0.001µM), an α2-adrenoceptor antagonist. Clonidine (0.01-1µM), another α2-adrenoceptor agonist, mimicked xylazine's defensive responding effects. We next assessed effects of xylazine withdrawal on defensive responding. Early xylazine abstinence (0.01-1µM, 5 and 30min), like acute xylazine, reduced defensive responding. In contrast, during later xylazine abstinence (0.001-1µM, 60min), defensive responding robustly increased with no significant changes in motility. Our data show that xylazine impairs planarian regeneration and bidirectionally impacts planarian defensive behaviors with distinct acute and withdrawal-dependent effects. These results suggest that some effects of xylazine on tissue injury and anxiety-like behavior are conserved in planarians.