Dopaminergic responses to self-administered cocaine in Rhesus monkeys do not sensitize following high cumulative intake

Sensitization of mesolimbic dopamine (DA) systems by administration of psychostimulants has been observed repeatedly in rodents. This phenomenon has been incorporated into theories of neurobiological adaptation underlying addiction, and is believed to be a mechanism whereby drug-associated cues acquire the ability to control behaviour via a conditioned release of DA. However, we have previously demonstrated in nonhuman primates that drug cues that cause cocaine seeking do not promote a conditioned increase in DA release of sufficient endurance to be measured in 2-min samples. In addition, imaging studies in humans and nonhuman primates that have been chronically exposed to psychostimulants have not demonstrated an increase in DA release upon psychostimulant challenge. Here we report that following 32 weeks of self-administration by rhesus monkeys, no increase over time in the DA response to self-administered cocaine was observed in any striatal subregion or individual animal. These results are consistent with clinical imaging studies showing a lack of DA sensitization, and might provide a mechanism to explain our previous observation that the rodent and primate differ in neurochemical response to drug-associated cues.

Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

The ability of environmental cues associated with cocaine availability to cause relapse may result from conditioned activation of dopamine (DA) release. We examined this hypothesis in macaque monkeys by conducting microdialysis studies in animals during exposure to a cocaine predictive compound cue. In addition to studying DA release in mesolimbic and sensorimotor striatum, both DA and serotonin levels were determined in the prefrontal cortex (medial orbitofrontal and anterior cingulate). The compound cue employed visual, auditory, and olfactory components, and was salient to the animals as demonstrated by anticipatory lever pressing in the absence of cocaine. During a 10-min period of exposure prior to cocaine availability, there was no significant increase in striatal or cortical DA. The addition of a DA uptake inhibitor to the striatal perfusate to reduce the potential interference of neuronal uptake did not alter the results. In contrast to the lack of any change in striatal DA, a significant decrease in extracellular serotonin in the prefrontal cortex during the 10 min of cue exposure was observed.

Local anesthetic effects of cocaethylene and isopropylcocaine on rat peripheral nerves

Cocaethylene is a naturally occurring cocaine derivative that has been used as a tool in both clinical studies of cocaine reward and as a potential model compound for agonist substitution therapy in cocaine dependence. It is equipotent to cocaine at inhibiting dopamine uptake in-vitro and in-vivo. Because it has been reported that local anesthetic properties may influence the reinforcing effects of dopamine uptake inhibitors, we investigated the local anesthetic properties of cocaethylene as well as isopropylcocaine, another potential pharmacological tool in studies of cocaine reward and agonist substitution therapy. We compared the efficacy of nerve impulse blockade by lidocaine, cocaine, cocaethylene and isopropylcocaine using rat sciatic nerves and dorsal roots (DRs). Nerves were placed in a modified sucrose gap chamber and repetitively stimulated at high frequency. The amplitude of compound action potentials (CAPs) at the beginning and end of each stimulus train was measured before and after exposure to each compound. All compounds produced concentration-dependent and use-dependent decrements in CAP amplitude, but cocaethylene and isopropylcocaine at medium to high concentration (0.375-1.875 mM) showed a more prolonged block after washout relative to cocaine or lidocaine. Patch clamp studies on dorsal root ganglion (DRG) neurons indicated a use-dependent blockade of sodium channels. These studies provide a more complete understanding of the pharmaocology of potential agonist treatment candidates, and suggest a mechanism whereby cocaethylene produces a decreased euphoria in humans compared to cocaine.

Dose-dependent effect of ethanol on extracellular dopamine in mesolimbic striatum of awake rhesus monkeys: comparison with cocaine across individuals

RATIONALE

Dependence on both alcohol and cocaine is a widespread example of polydrug abuse/dependence. It has been hypothesized that ethanol reward is mediated via increased dopaminergic neurotransmission in mesolimbic striatum, as is the case for cocaine. However, little is known about the neurobiology of ethanol in primates, or how the effects of ethanol compare to those of cocaine across individual animals.

OBJECTIVES

To determine in animals with a history of cocaine exposure whether there is a dopaminergic impact of ethanol in non-human primates, and if so, whether the magnitude of that effect correlates with the dopaminergic effect of cocaine across individuals.

METHODS

Microdialysis studies were conducted in rhesus monkeys previously trained to self-administer cocaine. The dopaminergic impact of cocaine had been determined in those animals during cocaine self-administration sessions. Probes were placed in the ventral mesolimbic and associational central striatum. Ethanol was administered non-contingently by a slow intravenous infusion at doses of 0.5 g/kg (administered over 10 min) and 1.0 g/kg (administered over 20 min).

RESULTS

The mean dopaminergic response to ethanol in four animals (with 2-4 trials in each animal at each dose) indicated a small but significant increase in extracellular dopamine at each dose (12% above baseline at 0.5 g/kg, 22% above baseline at 1.0 g/kg). Examining the responses across individual animals indicated substantial variability, in that two of the four animals showed no increase at either dose. Across individuals, regression analysis of cocaine-induced changes in dopamine with 1.0 g/kg ethanol-induced changes indicated a positive correlation between the drug effects, with a trend in this direction observed with the 0.5-g/kg dose of ethanol.

CONCLUSIONS

These results provide support for the ability of ethanol to elevate extracellular dopamine in the mesolimbic striatum, though with a modest effect size and variability among individuals. Further, they suggest that some common mechanism influences the effects of ethanol and cocaine on dopaminergic output despite seemingly unrelated pharmacological mechanisms of action.

Dynamics of extracellular dopamine in the acute and chronic actions of cocaine

Cocaine amplifies dopaminergic neurotransmission via blockade of presynaptic neuronal uptake. This action is believed to be a crucial component of cocaine's ability to exert its reinforcing effects. This review will provide a brief overview of extracellular dopamine dynamics associated with cocaine. The acute effects of cocaine reviewed include comparison of intravenous and intraperitoneal routes of administration to better understand how fast and slow routes (e.g., crack and intranasal) differ in their pharmacokinetics and neurochemical effects and how those differences relate to differences in abuse potential. Changes in the acute effects of cocaine within a session have been examined in neurochemical studies of acute tolerance to self-administered cocaine in rhesus monkeys, and the potential impact of that tolerance to patterns of use is discussed. Between-session sensitization of the dopaminergic response to cocaine is reviewed, and data indicating this also occurs in primates have been obtained in self-administering rhesus monkeys, demonstrating neurochemical sensitization in a primate species. The important question of whether cocaine-associated environmental cues elicit conditioned increases in dopamine release has also been examined in the rhesus monkey, with results indicating that, unlike rats, nonhuman primates do not show conditioned increases in dopamine release.

NMDA antagonist effects on striatal dopamine release: microdialysis studies in awake monkeys

Brain imaging studies have suggested that the NMDA antagonist ketamine is as potent a releaser of striatal dopamine as amphetamine. This conclusion contradicts microdialysis findings in the rodent that NMDA antagonists, in contrast to amphetamine, have little or no effect on striatal dopamine release. The present study addressed two mechanisms that could account for this discrepancy: 1) whether there is a species difference, i.e., rodents vs. primates, in the responsivity of striatal dopamine to NMDA antagonists, and 2) whether rapid uptake of dopamine prevents reliable measures of synaptic dopamine release by microdialysis in response to NMDA antagonists. MRI-directed in vivo microdialysis was used to compare the effects of psychotomimetic NMDA antagonists phencyclidine (PCP), ketamine, and amphetamine on extracellular striatal dopamine levels in awake rhesus monkeys. The effect of PCP was also investigated in the presence of intrastriatally applied nomifensine, a dopamine uptake blocker. Amphetamine (0.1 or 0.4 mg/kg) produced robust and dose-dependent increases in dopamine release ranging 2-10-fold above baseline. PCP at 0.1 mg/kg had no effect and at 0.3 mg/kg produced a small 50% increase over baseline. Ketamine, at the relatively high dose of 5 mg/kg, produced only a 30% increase in dopamine release. Intrastriatal application of nomifensine did not influence the effect of PCP, suggesting that rapid uptake of dopamine is not preventing the detection of a PCP-induced increase in dopamine release. These findings suggest that in the primate, ketamine and PCP are not effective dopamine releasers, as has been suggested by previous imaging studies.

Applications of microdialysis methodology in nonhuman primates: practice and rationale

This review provides a comprehensive summary of the use of microdialysis procedures in nonhuman primates. Using these methods in primates is feasible, including both the anesthetized and awake preparations. The latter permit neurochemical sampling during complex information processing and behavioral performance. The close homology in structure and function between human and nonhuman primate brain makes using nonhuman primates particularly appealing, and the literature suggests that their use has provided instances of unique insight into brain structure and function. The continued use of these methods in areas such as the neurobiology of addiction, affective disorders, and psychosis will help in our ever-increasing understanding of the complex pathophysiologies of these disorders.

Acute and chronic dopamine dynamics in a nonhuman primate model of recreational cocaine use

Using a model of recreational cocaine consumption, we have determined in four rhesus monkeys the impact of self-administered cocaine on mesolimbic and sensorimotor striatal dopaminergic neurotransmission. The effects of cocaine repeated within a self-administration session and across multiple sessions over a 6 month period were determined by the use of fixed-ratio self-administration and microdialysis procedures. The exposure to cocaine was modest, with at most two 0.5 mg/kg infusions permitted in each weekly session. Within a cocaine self-administration session, acute tolerance to the ability of cocaine to elevate extracellular striatal dopamine was observed. Over a period of 6 months of repeated self-administration, there was a significant increase in the impact of a fixed dose on extracellular dopamine, indicating that neurochemical sensitization to the effects of self-administered cocaine occurs in primates. A pronounced dopaminergic response to noncontingent cocaine was also observed, with no increases in extracellular dopamine in response to an unexpected saline substitution, indicating that the neurochemical response to self-administered cocaine is primarily caused by direct pharmacological effects of the drug rather than by conditioning to external environmental cues. These results highlight the contrast in time-dependent changes in neurochemical responsiveness to cocaine, depending on whether within-session or between-session comparisons are made. They also demonstrate that recreational levels of cocaine consumption can result in neurochemical sensitization, an enduring change in brain function that may contribute to addiction.

Impact of self-administered cocaine and cocaine cues on extracellular dopamine in mesolimbic and sensorimotor striatum in rhesus monkeys

Studies were conducted to determine the impact of self-administered cocaine on extracellular striatal dopamine in four rhesus monkeys. The extent to which external cue conditioning contributed to the effects of cocaine and whether there is activation of striatal dopaminergic neurotransmission during drug-seeking behavior was also examined. Microdialysis measurements were made at 2 min intervals in sensorimotor (dorsolateral) and mesolimbic (central and ventromedial) striatum. A fixed-ratio schedule of reinforcement was used, with cocaine availability signaled by a visual cue. Studies examined the effects of cocaine or cocaine cues against a drug-free baseline. Large (fivefold to eightfold) increases in extracellular dopamine after a self-administered infusion of 0.5 mg/kg cocaine were quite rapid and matched the time course of reported subjective effects in human laboratory studies. To determine if conditioning to external cues contributed to the cocaine-induced increases, saline was substituted for cocaine in the infusion, leaving all other visual and auditory stimuli unchanged. No increase in extracellular dopamine in either sensorimotor or mesolimbic striatal subdivisions was observed. Extracellular dopamine during extended periods of drug-seeking behavior triggered by a visual cue was determined in both central and ventromedial striatum. This procedure also did not result in any measurable changes in extracellular dopamine. These studies demonstrate rapid and pronounced pharmacological actions of self-administered cocaine. No apparent conditioned component of those actions was associated with external environmental cues, suggesting that cues that trigger drug-seeking behavior in nonhuman primates do not cause conditioned increases in mesolimbic striatal dopamine.

Rapid induction of behavioral and neurochemical tolerance to cocaethylene, a model compound for agonist therapy of cocaine dependence

RATIONALE

Tolerance to abused drugs may impact on patterns of abuse, and in the case of agonist therapies, may be beneficial in that it reduces the reward value of a given dose of abused drug. Cocaethylene, a psychoactive metabolite resulting from concurrent alcohol and cocaine consumption, was examined because of its use in human research studies of drug reward mechanisms, and its potential as a model compound for an agonist based therapy for cocaine dependence.

OBJECTIVE

Comparisons were made between cocaine and cocaethylene in the acute development of tolerance to the neurochemical and behavioral effects of cocaine. With chronic exposure, tolerance to the behavioral effects of cocaine was examined.

METHODS

In awake rats with a microdialysis probe in the nucleus accumbens and a jugular catheter, an IV bolus/3-h infusion of cocaine or cocaethylene and a subsequent cocaine challenge was administered while extracellular dopamine and locomotion were monitored. Chronic IV treatment with cocaine, cocaethylene, and a water control was accomplished for 7 days using osmotic minipumps attached to jugular catheters. Animals were then challenged with an IV bolus of cocaine.

RESULTS

With acute treatment, the IV bolus of cocaethylene at the beginning of the infusion period resulted in an initial behavioral activation equivalent to that caused by cocaine, after which there was a striking difference in that the cocaethylene group displayed a return to predrug levels of activity, while the cocaine group showed high levels of activity throughout the 3-h period. Both cocaethylene and cocaine resulted in an initial increase in the extracellular concentration of dopamine. However, after that initial increase, levels of dopamine dropped in the cocaethylene group while the cocaine group levels remained elevated. A 1-week infusion of cocaine or cocaethylene resulted in tolerance to the behavioral activating effects of a subsequent cocaine challenge.

CONCLUSIONS

These results demonstrate a rapid induction of tolerance to the behavioral and neurochemical properties of cocaethylene, resulting in a diminished behavioral response to a cocaine challenge both acutely, and after 7 days. The relevance of these data for the use of cocaethylene as a model compound for an agonist approach to therapy for cocaine dependence is discussed.

Clozapine preferentially increases dopamine release in the rhesus monkey prefrontal cortex compared with the caudate nucleus

Despite substantial differences between species in the organization and elaboration of the cortical dopamine innervation, little is known about the pharmacological response of cortical or striatal sites to antipsychotic medications in nonhuman primates. To examine this issue, rhesus monkeys were chronically implanted with guide cannulae directed at the principal sulcus, medial prefrontal cortex, premotor cortex, and caudate nucleus. Alterations in dopamine release in these discrete brain regions were measured in response to administration of clozapine or haloperidol. Clozapine produced significant and long-lasting increases in dopamine release in the principal sulcus, and to a lesser extent, in the caudate nucleus. Haloperidol did not produce a consistent effect on dopamine release in the principal sulcus, although it increased dopamine release in the caudate. Clozapine's preferential augmentation of dopamine release in the dorsolateral prefrontal cortex supports the idea that clozapine exerts its therapeutic effects in part by increasing cortical dopamine neurotransmission.

Effects of repeated amphetamine treatment on regional GABAA receptor binding

The present study examined the effects of repeated exposure to amphetamine on GABAA receptor binding in cortical and subcortical areas. The goal of the study was to determine whether changes in specific binding were related to behavioral sensitization. Animals were exposed to either saline (0.3 ml, s.c.; n=12) or d-amphetamine (2.5 mg/kg, s.c.; n=12) for 6 consecutive days and sacrificed after a 14-day withdrawal period. Differences in GABAA receptor binding in these two groups of animals were assessed using the GABAA receptor antagonist [3H]SR 95531. To verify that the preceding treatment regimen led to the development of behavioral sensitization, a separate set of animals (n=8/group) was exposed to the same regimen and challenged with d-amphetamine (2.5 mg/kg, s.c.) after the 14-day withdrawal period. As expected, preexposure to amphetamine led to the development of amphetamine sensitization. There were no differences in GABAA receptor binding in animals preexposed to saline and amphetamine in the prefrontal cortex, caudate-putamen, hypothalamus, or cerebellum. These findings do not provide support for the idea that changes in GABAA receptor binding in the medial prefrontal cortex or various subcortical areas are related to the development of behavioral sensitization.

Microdialysis and SPECT measurements of amphetamine-induced dopamine release in nonhuman primates

The competition between endogenous transmitters and radiolabeled ligands for in vivo binding to neuroreceptors might provide a method to measure endogenous transmitter release in the living human brain with noninvasive techniques such as positron emission tomography (PET) or single photon emission computerized tomography (SPECT). In this study, we validated the measure of amphetamine-induced dopamine release with SPECT in nonhuman primates. Microdialysis experiments were conducted to establish the dose-response curve of amphetamine-induced dopamine release and to document how pretreatment with the dopamine depleter alpha-methyl-para-tyrosine (alpha MPT) affects this response. SPECT experiments were performed with two iodinated benzamides, [123I]IBZM and [123I]IBF, under sustained equilibrium condition. Both radio-tracers are specific D2 antagonists, but the affinity of [123I]IBZM (KD-0.4 nM) is lower than that of [123I]IBF (KD 0.1 nM). With both tracers, we observed a prolonged reduction in binding to D2 receptors following amphetamine injection. [123I]IBZM binding to D2 receptors was more affected than [123I]IBF by high doses of amphetamine, indicating that a lower affinity increases the vulnerability of a tracer to endogenous competition. With [123I]IBZM, we observed an excellent correlation between reduction of D2 receptor binding measured with SPECT and peak dopamine release measured with microdialysis after various doses of amphetamine. Pretreatment with alpha MPT significantly reduced the effect of amphetamine on [123I]IBZM binding to D2 receptors, confirming that this effect was mediated by intrasynaptic dopamine release. Together, these results validate the use of this SPECT paradigm as a noninvasive measurement of intrasynaptic dopamine release in the living brain.

Novelty-associated locomotion: correlation with cortical and sub-cortical GABAA receptor binding

The present study was designed to determine whether variability in GABA (eta-aminobutyric acid)A receptor binding in cortical and subcortical brain regions was correlated with locomotor activity in a novel environment. Twenty four animals were rated for locomotor activity in a novel circular runway. Eight days later, locomotor activity was assessed following 1.5 mg/kg amphetamine sulfate (i.p.). After four to six days, animals were killed and samples were pooled in groups of four animals ranked according to novely locomotor score, and specific binding of the GABAA receptor antagonist [2-(3'-carboxy-2'-propyl)-3-amino-6-p-methoxy phenylpyridazinium bromide] ([3H]SR95531) was determined. Significant negative correlations were seen between specific ([3H]SR95531) binding and novelty induced locomotion in the cingulate and prefrontal cortices, and in the ventral pallidum. A near-significant negative correlation was seen in the striatum. Correlation coefficients between locomotion scores in the novel environment and specific [3H]SR95531 binding were: cingulate cortex, R = -0.91, P = 0.012; prefrontal cortex, R = -0.85, P = 0.032; ventral pallidum, R = -0.85, P = 0.030; striatum, R = -0.73, P = 0.097; and nucleus accumbens, R = -0.09, P = 0.85. The positive correlation between novelty- and amphetamine-induced locomotion was also quite high (R = 0.95, P = 0.004). These results are discussed in terms of their relevance to potential biochemical correlates of drug abuse vulnerability.

Alcohol plus cocaine: the whole is more than the sum of its parts

Cocaethylene, an active metabolite that arises through hepatic transesterification of cocaine when cocaine and ethanol are used together, shares many neurochemical and pharmacological properties with cocaine. Cocaethylene is similar to cocaine in its properties as an indirect dopamine agonist, and human subjects cannot distinguish its effects from those of cocaine. Cocaethylene, and especially its isopropyl analog, are more selective indirect dopamine agonists than cocaine, with relatively weak potency at the serotonin transporter. Cocaethylene may contribute to the manifestations and consequences of combined cocaine and ethanol use, although its relative importance remains unclear.

Effect of ethanol on extracellular 5-HT and glutamate in the nucleus accumbens and prefrontal cortex: comparison between the Lewis and Fischer 344 rat strains

The present study investigated the impact of systemic (i.p.) ethanol administration on extracellular levels of serotonin and glutamate in the prefrontal cortex and the nucleus accumbens in Lewis and Fischer 344 rat strains using in vivo microdialysis. At 1.0 g/kg, ethanol elicited a significant increase in nucleus accumbens-dialysate levels of both 5-HT (44% +/- 16, P = 0.002) and glutmate (90% +/- 43, P = 0.009) in Lewis rats. In Fischer rats, there was no increase in 5-HT (6% +/- 7: P = 0.5), and a trend toward an increase in glutamate (88% +/- 46: P = 0.1). The 0.5 and 2.0 g/kg doses did not result in any significant change in extracellular 5-HT or glutamate in the nucleus accumbens or prefrontal cortex of either strain. The basal levels of glutamate, in both brain regions, were significantly lower in Lewis than in Fischer 344 rats. The basal levels of 5-HT were also lower in the nucleus accumbens of Lewis rats. These findings suggest that enhanced sensitivity of the mesoaccumbens 5-HT or glutamate systems to ethanol and/or inherent low basal levels of 5-HT or glutamate activity may be associated with the predisposition to alcohol-drinking behavior seen in Lewis rats.

Serotonin-mediated increase in prefrontal cortex dopamine release: pharmacological characterization

Interactions between serotonin (5-HT) and dopamine (DA) neuronal systems in the prefrontal cortex (PFC) may be important in the pathophysiology of cognitive disorders such as schizophrenia. We have examined the effect of 5-HT, applied locally through a microdialysis probe, on extracellular DA in the PFC, and compared the response to that observed in the striatum. 5-HT in concentrations of 1 to 10 microM increased extracellular DA dose-dependently to a greater extent in the PFC than in the striatum. The PFC response was pharmacologically characterized to determine the 5-HT receptor subtype mediating the increase in DA levels. The coperfusion of selective 5-HT2A and 5-HT3 antagonists MDL 100,907 ((R-(+)-(2,3-dimethoxyphenyl)-1-[2(4-flourophenylethyl)]-4- piperidine-methanol) and MDL 72222 (3-tropanyl-3,5-dichlorobenzoate), respectively, with 5-HT failed to significantly attenuate the 5-HT induced increase of extracellular DA. Furthermore, the local application of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl aminopropane did not yield an increase in extracellular DA. On the other hand, coperfusion of the selective 5-HT1B/1D antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1 ,2,4-oxadiazol-3-yl)-[1,1-biphenyl]-4-carboxamide)) with 5-HT completely blocked the effect of 5-HT alone. Infusion of the selective 5-HT1B agonists CP 93,129 (3-(1,2,5,6-tetrahydro-4-pyridyl)pyrrolo[3,2-b]pyrid-5-one) and CP 94,253 (3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrolo[3,2-b]pyridine) resulted in a significant increase in extracellular DA and the effect of CP 93,129 was attenuated by coperfusion of GR 127935. The results obtained demonstrate a functional interaction between DA and 5-HT pathways in the PFC, with evidence of potential mediation by the 5-HT1B receptor subtype.

Effect of ethanol on extracellular dopamine in nucleus accumbens: comparison between Lewis and Fischer 344 rat strains

The purpose of this study was to examine the differential effects of intraperitoneal ethanol on the mesoaccumbens dopamine (DA) system in Fischer 344 and Lewis rat strains, utilizing microdialysis in awake animals. At the lowest dose tested (0.5 g/kg), there were no changes in extracellular DA in the nucleus accumbens in either strain. There was a differential response to the intermediate dose of 1 g/kg ethanol, with an 84% increase in extracellular DA in the Fischer, but no change in Lewis rats. The highest dose administered (2 g/kg) did not induce significant increases in DA in either strain. These data demonstrate that the mesoaccumbens DA systems of Fischer and Lewis rat strains differ in their susceptibility to activation by ethanol, and suggest that the higher alcohol preference of Lewis rats is not associated with an enhanced DAergic response to acute experimental administration of ethanol.

The role of serotonin in the actions of psychostimulants: molecular and pharmacological analyses

Cocaine is a highly abused psychostimulant which is a local anesthetic and inhibitor of the reuptake of dopamine (DA), serotonin (5-HT) and norepinephrine (NE). This manuscript details a brief summary and the primary conclusions of several presentations geared to present recent pharmacological analyses of the interaction of cocaine with 5-HT systems. These data illustrate the complexity of actions for cocaine in the brain and emphasize that, to fully understand the mechanisms which underlie its potent behavioural effects, the impact of this drug on 5-HT function as well as the interactions between 5-HT and the function of DA mesolimbic pathways must be considered.

Cocaine and cocaethylene: effects on extracellular dopamine in the primate

Cocaine and cocaethylene (a psychoactive metabolite of concurrent cocaine and ethanol consumption) were studied in the anesthetized vervet monkey. The ability of each to elevate extracellular DA in the caudate nucleus was assessed using microdialysis probes acutely lowered through chronic guide cannulae. Blood samples were also collected to determine plasma levels of the two drugs. Doses of 1.5 mumol/kg cocaine (equivalent to 0.5 mg/kg cocaine-HCl) and cocaethylene were administered intravenously. Microdialysis and blood samples were collected at 5-min intervals immediately following drug administration. Both drugs caused a maximal four-fold increase in extracellular DA during the 5- to 10-min period following drug administration. This is the first report of cocaine (and cocaethylene) induced alterations in extracellular DA in primates. The abilities of cocaine and cocaethylene to produce euphoria are being compared in ongoing clinical research studies. The potential use of these results for interpreting the neurochemical basis of any differences in those studies is discussed.

Excitatory amino acid antagonists attenuate the effects of cocaine on extracellular dopamine in the nucleus accumbens

The effect of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate excitatory amino acid antagonists on systemic cocaine-induced increases in extracellular dopamine was examined. The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor subtype antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovalerate and dizocilpine, were infused via a microdialysis probe placed in the nucleus accumbens. The local infusion of 2-amino-5-phosphonovalerate (500 microM), dizocilpine (50 microM) and CNQX (100 microM), started 80 min before cocaine injection, significantly inhibited the cocaine-induced increase in extracellular dopamine. The CNQX blockade was dose-dependent with respect to both CNQX concentration infused and dose of cocaine administered. Simultaneous infusion of the two antagonists (500 microM 2-amino-5-phosphonovalerate and 100 microM CNQX) did not lead to further reductions in the effects of cocaine when compared to either antagonist alone. Our results suggest that both N-methyl-Daspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate subtypes of excitatory amino acid antagonist receptors may contribute to the stimulatory effect of cocaine on extracellular dopamine in the nucleus accumbens.

Differences in bioavailability between cocaine and cocaethylene and their implications for drug-reward studies

Cocaethylene, a psychoactive metabolite resulting from combined ethanol/cocaine consumption, is of interest because its psychostimulant properties may partially underlie combined cocaine/ethanol use, and because it has the potential for use as a probe of drug reward mechanisms due to its enhanced selectivity at monoamine uptake sites compared to cocaine. To determine the relative systemic bioavailabilities of cocaine and cocaethylene, sequential plasma samples were obtained from awake rats following drug administration. Following intravenous administration of 3 mumol/kg (molar equivalent of 1 mg/kg cocaine-HCl), both drugs achieved similar time courses and areas under the plasma concentration versus time curve. In contrast, intraperitoneal administration of 44 mumol/kg (molar equivalent of 15 mg/kg cocaine HCl) showed peak plasma levels, and the area under the plasma concentration vs time curve for cocaine to be approximately twice that for cocaethylene. Comparison of dose corrected areas under the curve of the two routes of administration for each drug indicated that relative systemic bioavailability of cocaethylene following intraperitoneal administration is only 58% that of cocaine. In addition, the elimination of both cocaine and cocaethylene was found to be slower following intraperitoneal administration compared to the intravenous route. The implications of these results are discussed with respect to the relative potency of these two compounds, as inferred from behavioral, drug reward, and lethality studies. Also, the differences noted will need to be taken into account when making mechanistic interpretations from comparative drug reward studies.