Varying the rate of intravenous cocaine infusion influences the temporal dynamics of both drug and dopamine concentrations in the striatum

Abstract

The faster drugs of abuse reach the brain, the greater is the risk of addiction. Even small differences in the rate of drug delivery can influence outcome. Infusing cocaine intravenously over 5 vs. 90â 100Â s promotes sensitization to the psychomotor and incentive motivational effects of the drug and preferentially recruits mesocorticolimbic regions. It remains unclear whether these effects are due to differences in how fast and/or how much drug reaches the brain. Here, we predicted that varying the rate of intravenous cocaine infusion between 5 and 90Â s produces different rates of rise of brain drug concentrations, while producing similar peak concentrations. Freely moving male Wistar rats received acute intravenous cocaine infusions (2.0Â mg/kg/infusion) over 5, 45 and 90Â s. We measured cocaine concentrations in the dorsal striatum using rapidâ sampling microdialysis (1Â sample/min) and highâ performance liquid chromatographyâ tandem mass spectrometry. We also measured extracellular concentrations of dopamine and other neurochemicals. Regardless of infusion rate, acute cocaine did not change concentrations of nonâ dopaminergic neurochemicals. Infusion rate did not significantly influence peak concentrations of cocaine or dopamine, but concentrations increased faster following 5â s infusions. We also assessed psychomotor activity as a function of cocaine infusion rate. Infusion rate did not significantly influence total locomotion, but locomotion increased earlier following 5â s infusions. Thus, small differences in the rate of cocaine delivery influence both the rate of rise of drug and dopamine concentrations, and psychomotor activity. A faster rate of rise of drug and dopamine concentrations might be an important issue in making rapidly delivered cocaine more addictive.Varying the rate of i.v. cocaine delivery between 5 and 90Â s determines the drug’s effects on brain and behaviour. We show that injecting cocaine between 5 and 90Â s in rats alters the rates of rise of cocaine and dopamine in the dorsal striatum, without significantly changing peak concentrations. Faster injections also increase locomotor behaviour earlier than slower injections. Thus, beyond achieved dose, differences in the rates of rise of cocaine and dopamine can determine outcome.