Rationale: Both l- and d-methamphetamine (l- and d-MA) are more potent to release norpepinephrine (NE) than dopamine, and the selectivity is greater for l-MA than d-MA. Little is known of the in vivo pharmacology of l-MA.
Objective: This study compared the effects of l-MA and d-MA in assays of nociception, behavioral disruption, and impulsivity.
Methods: Antinociceptive effects of d- and l-MA were examined in two pain assays: the warm water tail withdrawal test for acute nociception and the von Frey test in complete Freund's adjuvant (CFA)-treated rats for chronic inflammatory pain. Food-maintained operant responding and locomotion tests were used to assess generalized behavioral disruption. The 5-choice serial reaction time test (5-CSRTT) was used to assess drug-induced effects on impulse control. A delay discounting procedure was used to determine drug-induced changes in sensitivity to reinforcer delay (impulsive choice).
Results: l-MA (3.2-10 mg/kg) produced dose-dependent antinociception in both pain assays, decreased the rate of food-maintained operant responding, and decreased locomotor activity at a higher dose (17.8 mg/kg). In contrast, d-MA (0.32-3.2 mg/kg) did not produce antinociception in either assay, produced biphasic effects on response rate, and increased locomotor activity. In the 5-CSRTT, d-MA but not l-MA produced significant increase in premature responses. In the delay discounting procedure, both drugs did not affect the delay function at doses that did not increase omissions.
Conclusions: These data suggest that d- and l-MA have different behavioral profiles. Consideration should be given to these differences in future studies when l-MA is proposed for potential therapies.
Keywords: 5-CSRTT; Antinociception; Delay discounting; Locomotion; Operant responding; Rats; l-Methamphetamine.