Episodic Withdrawal Promotes Psychomotor Sensitization to Morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex-a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization-a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction. Neuropsychopharmacology (2010) 35, 2579-2589; doi:10.1038/npp.2010.134; published online 1 September 2010