Pharmacologic Reversal of Xylazine-Induced Unconsciousness in Rats

Xylazine is an alpha-2 adrenergic agonist approved for veterinary use as a sedative and analgesic for animals. Unfortunately, xylazine has recently become a common adulterant of street drugs in the United States with xylazine-related overdoses and deaths increasing each year. Although the alpha-2 adrenergic antagonist, atipamezole, is an efficacious reversal agent for xylazine that is approved for use in animals, it is not approved for humans. In this study, we aimed to test alternative reversal agents for xylazine, and compare them with atipamezole in a rat model of xylazine-induced unconsciousness.

In adult Sprague-Dawley rats, we induced loss of righting reflex (LORR, a surrogate end point for loss of consciousness) with xylazine (5 mg/kg, intravenous [IV]) and attempted to restore consciousness by administering agents with distinct molecular mechanisms of action: atipamezole (alpha-2 adrenergic antagonist, 200 µg/kg IV); d-amphetamine (norepinephrine and dopamine reuptake inhibitor and releasing agent, 1 mg/kg); chloro-APB (dopamine D1 receptor agonist, 3 mg/kg IV); and atomoxetine (norepinephrine reuptake inhibitor, 3 mg/kg IV). Pulse oximetry and heart rate were monitored continuously. After administration of the reversal agents, time to return of righting reflex (RORR) was recorded (n = 12) and animals were assessed with a novel object recognition test (n = 17). One subset of animals underwent surgery to have electroencephalogram (EEG) leads implanted (n = 4). EEG data were recorded after xylazine injection and ensuing administration of a reversal agent and spectral analysis was performed.

After xylazine-induced unconsciousness, the median time to RORR in atipamezole-, d-amphetamine-, and chloro-APB-treated rats was 1.5 minutes (Interquartile Range [1.0-2.0]), 2 minutes (interquartile range [IQR] [1.0-3.0]), and 2 minutes (IQR [1.0-2.0]) post drug injection, respectively, compared to 56 minutes (IQR [39.5-70.5]) after saline control ( F [4,40] = 41.62, P < .0001). Atomoxetine did not significantly accelerate time to RORR. During the novel object recognition test, all animals spent the same amount of time with the familiar and novel object (range 0-143.5 sec), indicating that no reversal agents restored recognition memory. Xylazine induced an EEG pattern dominated by slow-delta oscillations. Atipamezole, d-amphetamine, and chloro-APB restored EEG oscillations similar to the awake state.

Atipamezole, d-amphetamine, and chloro-APB accelerate emergence from xylazine-induced unconsciousness and restore EEG oscillation patterns consistent with wakefulness. However, none of these reversal agents restore recognition memory.