Rationale: Although widely prescribed to treat opioid addiction, little is known about the possible side effects of methadone on memory functions.
Objectives: The aim of this study is to compare the effects of acute and chronic methadone on memory retrieval in rats and to explore the selectivity of possible deficits.
Methods: Administration of acute (0, 1.25, 2.5, and 5 mg/kg SC) and chronic steady state methadone (0, 10, 30, and 55 mg/kg/day SC by osmotic mini-pump) was tested on recall of three different types of information: stimulus-reward (10-arm parallel maze), stimulus-response (8-arm radial maze), and stimulus-stimulus (Barnes maze). Acute and steady state methadone doses were also compared on tests of locomotor activity and reactivity to aversive stimuli (i.e., swimming and acoustic startle).
Results: In the stimulus-reward task, acute methadone impaired performance as a result of severe depression of locomotion. This motor deficit, however, was modulated by the motivational valence of environmental stimulation. In fact, acute methadone did not eliminate forced swimming behavior. In the stimulus-response and stimulus-stimulus tasks, accuracy was impaired independently of direct motor deficits, but rats were hyper-reactive to aversive stimulation and, in fact, 5 mg/kg enhanced acoustic startle. Importantly, chronic steady state methadone did not affect accuracy of memory retrieval, did not depress motor or swimming activity, and did not change startle reactivity.
Conclusion: Only acute methadone impaired accuracy and/or performance on three tests of memory retrieval. These findings in rats suggest that memory deficits reported in methadone-maintained individuals may not be directly attributable to methadone.