Effects of daily morphine treatment on impulsivity in rats responding under an adjusting stop-signal reaction time task

Opioids can enhance delay discounting and premature responding under attentional tasks that might reflect increased impulsivity; although it is not clear whether repeated opioid administration alters behavioral inhibition. Effects of morphine and amphetamine were determined before, during, and after daily morphine administration in rats responding under a stop-signal reaction time task, measuring behavioral inhibition and motor impulsivity. Rats (n = 5) completed a two-response sequence to earn food. Occasionally, a tone (stop signal) was presented signifying that food would only be presented if the second response was withheld. Responding after the stop signal measured inhibition, and responding before the start of the trial (premature) measured motor impulsivity. Before daily treatment, morphine (0.32-17.8 mg/kg, intraperitoneally) decreased premature responding but did not increase responding on stop trials, whereas amphetamine (0.1-3.2 mg/kg, intraperitoneally) increased premature responding. Daily morphine administration (3.2 mg/kg/day) enhanced its effects on premature responding but did not impact other effects. Daily morphine treatment diminished the effects of amphetamine on premature and timeout responding. Repeated morphine treatment increased motor impulsivity but did not enhance behavioral inhibition. These data add to studies elucidating the relationship between impulsivity and opioid treatment and suggest that opioids differentially impact impulsive behaviors.