Dopamine (DA) releasing agents, e.g. d-amphetamine and phentermine, are stimulant drugs of abuse. Although some DA reuptake inhibitors, e.g. cocaine and methylphenidate, are highly abused, most are not, e.g. bupropion and sibutramine. In animals, DA releasers and reuptake inhibitors generalise to d-amphetamine and also maintain self-administration. Hence, further pharmacological testing is needed to identify the abuse risks posed by potentially stimulant CNS drug-candidates. We report how in vivo microdialysis measurements of DA efflux in nucleus accumbens (ACB) of freely-moving rats can be used to identify the stimulant potential of CNS drugs. Microdialysis probes (2mm) were implanted into ACB (AP +2.2mm, ML +/-1.5mm, DV -8.0mm relative to bregma) of anaesthetised male, Sprague-Dawley rats (300±50g). After ≥16hr recovery, 20min samples (1.2µl/min artificial CSF) were taken for 4hr after i.p. administration of d-amphetamine [0.1-3mg/kg], phentermine [1-9mg/kg], methylphenidate [1-10mg/kg], bupropion [10-50mg/kg], sibutramine [2-6mg/kg] or vehicle. DA was measured by hplc-ecd. All results are reported as mean±SEM, n = 6-11. Basal DA efflux was 5.29±0.22fmol/5μl (0-4hr) or 0.69±0.05fmol/20µl. d-Amphetamine and phentermine dose-dependently increased DA efflux with peaks at 40min of 3393±399% (p<0.001; 3mg/kg) and 924±196% (p<0.001; 9mg/kg), respectively. DA efflux declined rapidly thereafter. Methylphenidate produced a peak DA increase of 487±105% (p<0.001; 10mg/kg) at 40min that was followed by a rapid decline. There was no ceiling for the effects of the stimulants, d-amphetamine, phentermine and methylphenidate. Sibutramine (6mg/kg) produced a gradual, sustained increase in DA efflux with a peak of 331±87% (p<0.001) at 60min; it did not significantly increase DA efflux at 2.0mg/kg. Bupropion, increased extracellular DA with maximum effects at 20-40min. Peak increases: 228±37% [10mg/kg], 552±78% [30mg/kg] and 441±104% [50mg/kg] (all p<0.001). There was a clear dose-effect ceiling for bupropion. d-Amphetamine, phentermine and methylphenidate, which are stimulant euphoriants in man (Schoedel et al, 2012, J Clin Psychopharm, 32, 492; Heal et al, 2014, Neuropharmacology 87,19), produce rapid, large and transient increases in DA efflux with no effect ceiling. Sibutramine and bupropion, which are not stimulant euphoriants (Heal et al, 2014), produced smaller and sustained DA increases, and as shown by bupropion, there was a clear ceiling to the effect.
Key words: microdialysis, dopamine, stimulants, reuptake inhibitors
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