Academic journal article The Psychological Record

Repeated Acquisition in the Morris Swim Task: Effects of Methylenedioxymethamphetamine, Methamphetamine, and Methylphenidate

Academic journal article The Psychological Record

Repeated Acquisition in the Morris Swim Task: Effects of Methylenedioxymethamphetamine, Methamphetamine, and Methylphenidate

Article excerpt

Psychomotor stimulants such as methylenedioxymetham-phetamine (MDMA), methamphetamine (MA), and methylphenidate (MPD) are associated with high rates of abuse worldwide, and there is increasing concern about both the acute and long-term effects of these drugs on psychological functioning in users. In particular, use of both MDMA and MA has been associated with neurotoxicity, and clinical studies have raised the possibility of adverse effects on cognition, learning, and memory (e.g., Baicy and London 2007; Homer et al. 2008; McCann et al. 2008; Nulsen et al. 2010; Scott et al. 2007; Stough et al. 2012). In contrast, however, some stimulant drugs are used to improve cognitive functioning in disorders such as ADHD and have been shown to produce cognitive enhancement following acute administration to healthy humans in laboratory settings (Barch and Carter 2005; Hart et al. 2008; Kirkpatrick et al. 2012).

Ethical and practical issues limit determination of the effects of potentially hazardous drugs in humans and point to the importance of research with nonhumans to characterize the effects of such drugs on cognitive function. The problem is the development of reliable and valid animal models of learning and memory that are sensitive to impairment and enhancement by psychoactive drugs. Several studies have assessed acute effects of psychomotor stimulants using various models of learning in nonhumans with somewhat mixed results. Although most studies have found only impairment of learning across dose-response functions for MDMA (e.g., Braida et al. 2002; Byrne et al. 2000), MA (e.g., Mayorga et al. 2000) and MPD (e.g., Chuhan and Taukulis 2006; Mayorga et al. 2000), cognitive enhancements by these drugs have also been reported (e.g., Calhoun and Jones 1974; Handley and Calhoun 1978; Quintero-Munoz et al. 2010; Tian et al. 2009; Zhu et al. 2007).

These studies differ in many aspects of methodology, making it difficult to determine the basis for their differing outcomes. One pervasive problem is in determining the specificity of drug effects on processes involving learning and memory as separated from effects on more general determinants of performance, such as motivation and perceptual or motor processes. Historically, the preparation in behavioral pharmacology that has been most successful in separating learning processes from more general performance processes is the repeated acquisition/performance (RAP) procedure (Thompson and Moerschbaecher 1979a). RAP procedures typically involve a multiple schedule in which two components alternate within a session. In both components, a particular sequence of responses is required to produce reinforcement, but in one component (acquisition) the sequence changes each session, and thus the animal must learn the correct sequence during the session. In the other component (performance), the same sequence is used in every session, and the animal simply executes a previously well-learned pattern of behavior. The inclusion of this performance component thus provides a within-session control for drug effects, which are specific to acquisition (learning) processes.

A few studies have evaluated the acute effects of psychomotor stimulants using such RAP procedures. Thompson and Moerschbaecher (1979b) found that d-amphetamine impaired acquisition of response chains in monkeys at doses that spared performance. Similarly, effects of d-amphetamine and MPD that were selective to learning were also found in pigeons (Moerschbaecher et al. 1979; Thompson 1976). However, MDMA did not produce selective effects on acquisition of repeated chains in monkeys, as learning was impaired only at doses that also produced performance impairments (Thompson et al. 1987). In contrast, Galizio et al. (2009) found selective effects of MDMA, but only nonselective effects of MPD and MA using a RAP procedure in rats. These findings were precisely the opposite of those reviewed in monkeys and pigeons in which MPD and MA both produced effects that were selective to acquisition and MDMA had only nonselective effects. …

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