Changes in dopamine-dependent electrochemical signal in the nucleus accumbens associated with repeated cocaine injections in rats

Dopamine signaling in the nucleus accumbens of animals self-administering drugs of abuse

Abuse of psychoactive substances can lead to drug addiction. In animals, addiction is best modeled by drug self-administration paradigms. It has been proposed that the crucial common denominator for the development of drug addiction is the ability of drugs of abuse to increase extracellular concentrations of dopamine in the nucleus accumbens (NAcc). Studies using in vivo microdialysis and chronoamperometry in the behaving animal have demonstrated that drugs of abuse increase tonic dopamine concentrations in the NAcc. However, it is known that dopamine neurons respond to reward-related stimuli on a subsecond timescale. Thus, it is necessary to collect neurochemical information with this level of temporal resolution, as achieved with in vivo fast-scan cyclic voltammetry (FSCV), to fully understand the role of phasic dopamine release in normal behavior and drug addiction. We review studies that investigated the effects of drugs of abuse on NAcc dopamine levels in freely moving animals using in vivo microdialysis, chronoamperometry, and FSCV. After a brief introduction of dopamine signal transduction and anatomy and a section on current theories on the role of dopamine in natural goal-directed behavior, a discussion of techniques for the in vivo assessment of extracellular dopamine in behaving animals is presented. Then, we review studies using these techniques to investigate changes in phasic and tonic dopamine signaling in the NAcc during (1) response-dependent and -independent administration of abused drugs, (2) the presentation of drug-conditioned stimuli and operant behavior in self-administration paradigms, (3) drug withdrawal, and (4) cue-induced reinstatement of drug seeking. These results are then integrated with current ideas on the role of dopamine in addiction with an emphasis on a model illustrating phasic and tonic NAcc dopamine signaling during different stages of drug addiction. This model predicts that phasic dopamine release in response to drug-related stimuli will be enhanced over stimuli associated with natural reinforcers, which may result in aberrant goal-directed behaviors contributing to drug addiction.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Dopamine fluctuations in the nucleus accumbens during maintenance, extinction, and reinstatement of intravenous D-amphetamine self-administration

Moment-to-moment fluctuations of nucleus accumbens dopamine (DA) were determined in rats self-administering or passively receiving "yoked" intravenous infusions of D-amphetamine. The initial lever presses of each session caused elevations in DA concentration, usually to an initial peak that was not maintained throughout the rest of the session. As the initial ("loading") injections were metabolized, DA levels dropped toward baseline but were sustained at elevated plateaus by subsequent lever pressing that was spaced throughout the remainder of the 3 hr sessions. During this period, DA levels fluctuated phasically, time-locked to the cycle of periodic lever pressing. Consistent with the known pharmacological actions and dynamics of amphetamine, peak DA elevations were seen approximately 10-15 min after each injection, and the mean DA level was at a low point in the phasic cycle at the time of each new lever press. During extinction periods when saline was substituted for amphetamine, DA levels dropped steadily toward baseline levels despite a dramatic increase in (now-unrewarded) lever pressing. Noncontingent injections during extinction reinstated lever-pressing behavior and increased nucleus accumbens DA concentrations. These data are consistent with the hypothesis that under the conditions of this experiment-during periods of amphetamine intoxication in well-trained animals-the timing of amphetamine self-administration comes primarily under the control of extracellular DA concentrations. The probability of lever pressing during the maintenance phase is highest when DA concentrations fall near a characteristic trigger point, a trigger point that is significantly elevated above baseline, and falls as DA concentrations fall below or increase above that trigger point.

Intraaccumbens administration of NMDA receptor antagonist (+/-)-CPP prevents locomotor activation conditioned by morphine and amphetamine in rats

In the present experiments the influence of NMDA receptor antagonist (+/-)-CPP on morphine- and amphetamine-conditioned activation of locomotor activity was studied in rats chronically implanted with bilateral cannulas in the nucleus accumbens septi. Animals were conditioned by pairing subcutaneous injections of morphine (3.0 mg/kg), amphetamine (1.5 mg/kg), or saline with a distinctive environment. Following the five drug-environment pairings, rats displayed significant increase in locomotion when exposed to the drug-paired environment. The expression of this conditioned response was completely prevented by the bilateral intraaccumbens pretreatment with (+/-)-CPP (1.0, but not 0.1 or 0.3 microgram/ microliter/side). These findings suggest that the locomotor hyperactivity conditioned by morphine and amphetamine involves the activation of NMDA receptors within the nucleus accumbens.

Conditioned changes in nucleus accumbens dopamine signal established by intravenous cocaine in rats

To explore the role of cocaine-induced conditioning in the regulation of mesolimbic dopamine (DA) activity, high-speed chronoamperometry with Nafion-coated carbon fiber electrodes was used to measure changes in nucleus accumbens (NAc) DA-dependent signal induced by a light stimulus previously paired and not paired with intravenous cocaine (0.85 mg/kg per injection), and cocaine injections paired and not paired with light. A small phasic DA signal increase was found after light presentation in drug-naive rats, whereas greater and longer signal increases were induced by the same light after it had been repeatedly paired with cocaine. The DA signal changes associated with repeated cocaine injections paired with light differed from those induced by cocaine alone and occurring during drug self-administrations. These data suggest that cocaine-related sensory cues, acting alone, may induce a conditioned mesolimbic DA activation, and, acting in the presence of the drug, may significantly modify the DA response to the drug.

Enhanced locomotor reactivity to apomorphine following repeated cocaine treatment

To study the involvement of postsynaptic dopamine (DA) receptors in cocaine-induced behavioral sensitization, locomotor responses to apomorphine (APO, 360 micrograms/kg, IP), a direct DA agonist, were compared in rats repeatedly treated with cocaine (15 mg/kg, IP x 5 and one challenge injection 4 days later) and saline under different environmental conditions. When cocaine was injected under activated conditions, immediately before animal placement in activity chamber, more powerful locomotor response to the initial drug and a significant effect of repeated treatment (sensitization) were found compared to drug administration under quiet conditions, 2 h after animal placement in activity chamber. In this case, locomotor response to the initial drug was similar to that of saline, and the effect of repeated injections on locomotion was absent (no sensitization). Locomotor stimulation induced by APO was significantly enhanced in both groups of cocaine-treated animals compared to saline controls. This effect, however, was more powerful in animals treated with cocaine under activated conditions. Present data suggest that enhanced responsiveness of postsynaptic DA receptors developed due to chronic cocaine treatment may contribute to stable alteration of DA transmission thought to mediate cocaine sensitization.