On the speed of cocaine

Pharmacokinetics trumps pharmacodynamics during cocaine choice: a reconciliation with the dopamine hypothesis of addiction

Cocaine is known to increase brain dopamine at supranormal levels in comparison to alternative nondrug rewards. According to the dopamine hypothesis of addiction, this abnormally large dopamine response would explain why cocaine use is initially highly rewarding and addictive. Though resting on solid neuroscientific foundations, this hypothesis has nevertheless proven difficult to reconcile with research on cocaine choice in experimental animals. When facing a choice between an intravenous bolus of cocaine and a nondrug alternative (e.g., sweet water), both delivered immediately after choice, rats do not choose the drug, as would be predicted, but instead develop a strong preference for the nondrug alternative. Here we report evidence that reconciles this finding with the dopamine hypothesis of addiction. First, a systematic literature analysis revealed that the delays of effects of intravenous cocaine on nucleus accumbens dopamine are of the order of tens of seconds and are considerably longer than those of nondrug reward. Second, this was confirmed by measuring response times to cocaine omission during self-administration as a behavioral proxy of these delays. Finally, when the influence of the drug delays was reduced during choice by adding an increasing delay to both the drug and nondrug rewards, rats shifted their choice to cocaine. Overall, this study suggests that cocaine is indeed supranormal in reward magnitude, as postulated by the dopamine hypothesis of addiction, but is less preferred during choice because its pharmacokinetics makes it an inherently more delayed reward than the alternative. Reframing previous drug choice studies in rats as intertemporal choice studies reveals that the discounting effects of delays spare no rewards, including supranormal ones, and that during choice, pharmacokinetics trumps pharmacodynamics.

Reciprocal Inhibitory Interactions Between the Reward-Related Effects of Leptin and Cocaine

Cocaine is habit-forming because of its ability to enhance dopaminergic neurotransmission in the forebrain. In addition to neuronal inputs, forebrain dopamine circuits are modulated by hormonal influences; one of these is leptin, an adipose-derived hormone that attenuates the rewarding effects of food- and hunger-associated brain stimulation reward. Here we report reciprocal inhibition between the reward-related effects of leptin and the reward-related effects of cocaine in rats. First, we report that cocaine and the expectancy of cocaine each depresses plasma leptin levels. Second, we report that exogenous leptin, given systemically or directly into the ventral tegmental area, attenuates the ability of cocaine to elevate dopamine levels in the nucleus accumbens, the ability of cocaine to establish a conditioned place preference, and the ability of cocaine-predictive stimuli to prolong responding in extinction of cocaine-seeking. Thus, whereas leptin represents an endogenous antagonist of the habit-forming and habit-sustaining effects of cocaine, this antagonism is attenuated by cocaine and comes to be attenuated by the expectancy of cocaine.

Conditioned contribution of peripheral cocaine actions to cocaine reward and cocaine-seeking

Cocaine has actions in the peripheral nervous system that reliably precede--and thus predict--its soon-to-follow central rewarding effects. In cocaine-experienced animals, the peripheral cocaine signal is relayed to the central nervous system, triggering excitatory input to the ventral tegmental origin of the mesocorticolimbic dopamine system, the system that mediates the rewarding effects of the drug. We used cocaine methiodide, a cocaine analog that does not cross the blood-brain barrier, to isolate the peripheral actions of cocaine and determine their central and behavioral effects in animals first trained to lever-press for cocaine hydrochloride (the centrally acting and abused form of the drug). We first confirmed with fast-scan cyclic voltammetry that cocaine methiodide causes rapid dopamine release from dopamine terminals in cocaine hydrochloride-trained rats. We then compared the ability of cocaine hydrochloride and cocaine methiodide to establish conditioned place preferences in rats with self-administration experience. While cocaine hydrochloride established stronger place preferences, cocaine methiodide was also effective and its effectiveness increased (incubated) over weeks of cocaine abstinence. Cocaine self-administration was extinguished when cocaine methiodide or saline was substituted for cocaine hydrochloride in the intravenous self-administration paradigm, but cocaine hydrochloride and cocaine methiodide each reinstated non-rewarded lever-pressing after extinction. Rats extinguished by cocaine methiodide substitution showed weaker cocaine-induced reinstatement than rats extinguished by saline substitution. These findings suggest that the conditioned peripheral effects of cocaine can contribute significantly to cocaine-induced (but not stress-induced) cocaine craving, and also suggest the cocaine cue as an important target for cue-exposure therapies for cocaine addiction.