Direct, real-time assessment of dopamine release autoinhibition in the rat caudate-putamen.

Inhibition of endogenous dopamine release by photo-released dopamine (i.e., autoinhibition) was characterized in the rat caudate-putamen using combined caged-dopamine photolysis and fast-scan cyclic voltammetry. Coronal brain slices (400 microm thick) were perfused with caged-dopamine (150-200 microM in artificial cerebrospinal fluid). Ultraviolet illumination of increasing duration (25-250 ms, approximately 100 microm beam diameter) was focused at the tip of the recording electrode to uncage increasing amounts of exogenous dopamine at the recording sites (0.5-5 microM); a single biphasic electrical stimulus was delivered 0.1-10 s later to induce endogenous dopamine release. The concentrations of both endogenous and exogenous dopamine were determined using voltammetry, thus enabling determination of concentration-dependent inhibition of the endogenous release by the latter. While unaffected by control ultraviolet illumination, endogenous dopamine release was rapidly inhibited by photo-released dopamine in a concentration-dependent manner. Photo-application of 3-5 microM exogenous dopamine inhibited the endogenous release by 90-100% (electrical stimulus applied 1 s after photolysis initiation), an effect prevented by 2 microM sulpiride. The autoinhibition was dependent on the time between photolysis onset and electrical stimulation. Terminal dopamine autoreceptor stimulation led to robust inhibition of endogenous dopamine release with a latency of approximately 200 ms and effective duration of less than 5 s. The percent autoinhibition was a skewed, U-shaped function of photolysis/electrical stimulation intervals with the peak inhibition at 1 s. This study directly demonstrates that autoreceptor-mediated inhibition of terminal dopamine release in caudate-putamen is designed to provide a rapid, robust, yet short-lasting modulation of terminal dopamine release.

Differential time-course profiles of dopamine release and uptake changes induced by three dopamine uptake inhibitors.

Using real-time voltammetry, we compared the effects of cocaine (1.0, 3.0, or 10 microM), WIN 35428 (0.1, 0.5, or 2.0 microM), and nomifensine (0.2, 1.0, or 5.0 microM) on electrically evoked dopamine release and uptake in the rat accumbens slice. The time course for onset and offset of the drug effects were determined by perfusing single drug concentration for 30 min, followed by a 60-min washout. Cocaine elicited a rapid, concentration-independent increase in dopamine release and a more gradual, concentration-dependent inhibition of uptake. During washout, uptake inhibition rapidly abated to near baseline values. During the same period, the potentiation of dopamine release exhibited a slower offset for all concentrations and, for 10 microM cocaine, was even greater than that observed during drug perfusion ("rebound" increase). The release rebound was not observed during continuous 90-min perfusion, verifying that cocaine washout per se was a sufficient condition. Selective D1 or D2 antagonists (0.5 microM SCH 39166 or 2 microM sulpiride, respectively) were without effect on cocaine-induced release alterations. WIN 35428 and nomifensine induced similar changes in dopamine kinetics during perfusion. However, in contrast to cocaine, no consistent release rebound was observed during their washout. For 2 microM WIN 38425, washout and continuous perfusion groups exhibited similar changes in dopamine release and uptake. The time-course mismatch between uptake inhibition and DA release potentiation as well as release rebound during washout suggests that altered dopamine release might play a role in behavioral effects of cocaine.

Methamphetamine neurotoxicity: necrotic and apoptotic mechanisms and relevance to human abuse and treatment.

Research into methamphetamine-induced neurotoxicity has experienced a resurgence in recent years. This is due to (1) greater understanding of the mechanisms underlying methamphetamine neurotoxicity, (2) its usefulness as a model for Parkinson's disease and (3) an increased abuse of the substance, especially in the American Mid-West and Japan. It is suggested that the commonly used experimental one-day methamphetamine dosing regimen better models the acute overdose pathologies seen in humans, whereas chronic models are needed to accurately model human long-term abuse. Further, we suggest that these two dosing regimens will result in quite different neurochemical, neuropathological and behavioral outcomes. The relative importance of the dopamine transporter and vesicular monoamine transporter knockout is discussed and insights into oxidative mechanisms are described from observations of nNOS knockout and SOD overexpression. This review not only describes the neuropathologies associated with methamphetamine in rodents, non-human primates and human abusers, but also focuses on the more recent literature associated with reactive oxygen and nitrogen species and their contribution to neuronal death via necrosis and/or apoptosis. The effect of methamphetamine on the mitochondrial membrane potential and electron transport chain and subsequent apoptotic cascades are also emphasized. Finally, we describe potential treatments for methamphetamine abusers with reference to the time after withdrawal. We suggest that potential treatments can be divided into three categories; (1) the prevention of neurotoxicity if recidivism occurs, (2) amelioration of apoptotic cascades that may occur even in the withdrawal period and (3) treatment of the atypical depression associated with withdrawal.

The dopamine D2/D3 antagonist DS121 potentiates the effect of cocaine on locomotion and reduces tolerance in cocaine tolerant rats.

To explore the significance of dopamine (DA) autoreceptors in cocaine tolerance and cocaine induced locomotor activity rats were treated with saline and cocaine (40 mg/kg per day via osmotic minipump; normal and cocaine tolerant rats, respectively). Injections of DS121 (0-7 mg/kg, i.p.; S(-)-3-(3-(cyanophenyl)-N-n-propylpiperidine), a DA D2/3 and autoreceptor preferring antagonist, either alone (i.e. DS121 + saline injection) or in combination with cocaine (7.5 mg/kg, i.p.) were also given. DS121 (+ saline) increased locomotor activity in both saline and cocaine pump (CP) treated animals. DS121 also potentiated the effect of cocaine on locomotor activity; this effect was greatest in CP (tolerant) animals. It is concluded that DS121 can increase locomotor activity and that this effect is greatest when the DA tone is high, that is when cocaine is present, suggestive of a presynaptic mechanism. Furthermore, because DS121 potentiation of cocaine induced locomotor activity is greatest in tolerant animals it is concluded that supersensitive DA autoreceptors underlie this effect. These data further support our previous data, which show that DA autoreceptors are sensitized after continuous cocaine (minipump) treatment.

Effect of cocaine, nomifensine, GBR 12909 and WIN 35428 on carbon fiber microelectrode sensitivity for voltammetric recording of dopamine.

Electrochemical measurements using voltammetry or amperometry at carbon-fiber microelectrodes have been used in vitro and in vivo to examine regulatory mechanisms for the central dopamine system. In many of these experiments, dopamine efflux concentrations under control conditions are determined followed by their alterations in response to a drug treatment. The present study demonstrates that some drugs can affect dopamine measurements, not only by their expected pharmacological action but also by directly altering the microelectrode responsivity. The commonly used reuptake inhibitors GBR 12909 (10 microM) and nomifensine (5 microM) drastically reduce electrode sensitivity and, in the case of nomifensine, increase the time to reach a plateau in response to dopamine boluses (i.e. reduced 'frequency response'). Cocaine (10 microM) and WIN 35428 (2 microM) have negligible effect on these indices. This decrease in sensitivity was found in both nafion and non-nafion coated electrodes. Further, the reduction in sensitivity seen in non-nafion coated electrodes was not prevented by increasing the reversal potential (from +1.0 to +1.3 V) and voltage scan rate (from 350 to 450 V/s). These data suggest that care must be taken when interpreting data from voltammetric or amporometric experiments using carbon electrodes where GBR 12909 or nomifensine are used, especially at high concentrations. Furthermore, wherever possible, direct effects of a drug on electrode sensitivity and frequency response should be determined.

Long-term blockade of the expression of cocaine sensitization by ondansetron, a 5-HT(3) receptor antagonist.

Intermittent cocaine administration induces sensitization (reverse tolerance) to its behavioral effects. The mechanism(s) mediating sensitization is not clear, however, previous research has implicated 5-HT(3) receptors in the expression of sensitization. The present experiment evaluated the ability of the 5-HT(3) receptor antagonist, ondansetron, administered during withdrawal from chronic intermittent cocaine administration, to block the expression of sensitization. Rats were pretreated for 14 days by daily subcutaneous injections of either 40 mg/kg cocaine or 0.9% saline. During the first 5 days of withdrawal from this pretreatment regimen, all rats received a daily subcutaneous injection of 0-1.0 mg/kg ondansetron. On days 7, 14 or 28 of withdrawal from the cocaine pretreatment, the rats received a 15.0-mg/kg cocaine challenge. Ambulatory behavior was automatically recorded for 60 min. Ondansetron had no significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron blocked the expression of sensitization at all withdrawal times. We thus report that it is possible to permanently block the expression of sensitization once it has developed by administering a 5-HT(3) receptor antagonist.

Altered sensitivity of dopamine autoreceptors in rat accumbens 1 and 7 days after intermittent or continuous cocaine withdrawal.

Using slice preparations, we investigated the effects of chronic cocaine treatment on dopamine autoreceptor sensitivity in the nucleus accumbens core. Cocaine (40 mg/kg/day) was given for 14 days, either by continuous subcutaneous infusion (osmotic minipumps) or single daily injections. One or 7 days after cocaine withdrawal, we used fast scan cyclic voltammetry (10 Hz sampling rate) to measure inhibition of electrically evoked dopamine release by quinpirole (3-300 nM). Continuous cocaine infusion increased quinpirole sensitivity on day 1 of withdrawal, particularly at low concentrations of quinpirole, but this effect was no longer evident by day 7. Intermittent cocaine injections had no effect on day 1 of withdrawal but by day 7 there was a quinpirole subsensitivity. On either withdrawal day, the baseline peak dopamine release or uptake half-life exhibited no treatment group differences. It is suggested that these cocaine dosing regimes cause differential and dynamic changes in dopamine autoreceptor sensitivity during the early withdrawal phase.

The effects of continuous cocaine dose on the induction of behavioral tolerance and dopamine autoreceptor function.

The current experiment evaluated the dose-dependent nature of the induction of behavioral tolerance, and changes in dopamine autoreceptor function, by continuously administering different doses of cocaine. For all experiments, rats were exposed to a 14-day pretreatment regimen involving the continuous administration of either 0, 5, 10, 20, or 40 mg/kg/day cocaine. All subjects were then withdrawn from the pretreatment regimen for 7 days. The subjects were placed in activity monitors, and ambulation measured. In experiment 1, the subjects were challenged with 0.0, 7.5, or 15.0 mg/kg i.p. cocaine on day 7 of withdrawal from the continuous cocaine administration regimen. The results indicated that all continuous cocaine doses induced significant tolerance to the 15.0 mg/kg cocaine challenge, relative to the control group. Furthermore, the 5.0 mg/kg/day group exhibited significantly less tolerance than the 40.0 mg/kg/day group. In experiment 2, the subjects were challenged with 0.0, 0.063, or 0.125 mg/kg quinpirole. The results indicated that the 0.063-mg/kg quinpirole challenge inhibited activity, while the 0.125 mg/kg quinpirole challenge enhanced behavior. The results further suggested that the inhibition of behavior was greater in the cocaine-pretreated subjects than in the saline control group. In experiment 3, the subjects were challenged with the same doses of quinpirole in combination with 15 mg/kg i.p. cocaine. The low quinpirole challenge dose inhibited cocaine-induced hyperactivity, while the higher challenge dose enhanced cocaine-induced hyperactivity. The results suggest that the induction of tolerance by continuous cocaine administration is dose-dependent. Continuous cocaine administration did induce dopamine autoreceptor supersensitivity. However, different continuous cocaine doses did not induce differential degrees of dopamine autoreceptor supersensitivity.

Altered activity of midbrain dopamine neurons following 7-day withdrawal from chronic cocaine abuse is normalized by D2 receptor stimulation during the early withdrawal phase.

Using in vivo single-unit recording in rats, we compared the effects of continuous cocaine infusion via minipump or single daily injections (both 40 mg/kg/d x 14 days, S.C.) on the activity of putative dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA). On days 1-5 after cocaine withdrawal, animals were further treated with single daily injections of DA agonists. On withdrawal day 7 continuous cocaine caused a reduction in spontaneously active neurons in the SNC and reduced bursting in the VTA. In contrast, intermittent cocaine resulted in an increase in the number of active neurons in the VTA. These changes were all reversed by apomorphine or quinpirole given during the first 5 withdrawal days. The D1 antagonist SCH 39166 did not antagonize the effects of apomorphine in either region. The role of D2 receptors in modulating baseline DA activity during intermediate cocaine withdrawal is discussed.

Withdrawal from continuous cocaine administration: time dependent changes in accumbens 5-HT3 receptor function and behavioral tolerance.

We have previously reported that continuous cocaine administration functionally down regulates 5-HT3 receptors in the nucleus accumbens. The current experiments evaluated the duration of behavioral tolerance to cocaine and whether the duration of behavioral tolerance and 5-HT3 receptor down-regulation co-varied. Rats were withdrawn from a pretreatment regimen (40 mg/kg/per day cocaine or 0.9% saline for 14 days) for 1, 7 or 14 days. The rats were either sacrificed, and slices from the nucleus accumbens obtained, or were exposed to behavioral rating procedures. The results indicated that continuous cocaine administration significantly attenuated the ability of mCPBG to facilitate K+ -stimulated DA release on days 1 and 7, but not day 14, of withdrawal. Furthermore, continuous cocaine administration induced behavioral tolerance to a cocaine challenge on days 1 and 7, but not day 14, of withdrawal. These results suggest that continuous cocaine administration functionally down-regulates 5-HT3 receptors in the nucleus accumbens, and this functional down-regulation co-varies with the behavioral tolerance induced by continuous cocaine administration. Hence, a functional down-regulation of accumbens 5-HT3 receptors may represent a partial mechanism for the tolerance following continuous cocaine administration.

Blockade of accumbens 5-HT3 receptor down-regulation by ondansetron administered during continuous cocaine administration.

The present experiment examined whether ondansetron, co-administered with continuous cocaine, would block the down regulation of accumbens 5-HT3 receptors. Rats were exposed to a 14-day pretreatment regimen that involved the continuous infusion of 40 mg kg(-1) day(-1) cocaine or 0.9% saline via a subcutaneously implanted osmotic minipump. In addition to the continuous cocaine or saline administration, all subjects received daily subcutaneous (s.c.) injections of either vehicle or 0.1 mg kg(-1) ondansetron for the entire 14-day pretreatment regimen. The rats were then withdrawn from this pretreatment regimen for seven days, and slices from the nucleus accumbens obtained. The slices were perfused with 25 mM K+ in the absence and presence of 0, 12.5, 25, or 50 microM m-Chlorophenyl-biguanide HCl (mCPBG). The efflux samples were assayed for dopamine content by high pressure liquid chromatography (HPLC) with electrochemical detection. Continuous cocaine administration significantly attenuated the ability of mCPBG to facilitate K+-induce dopamine overflow compared to saline control rats. In addition, the rats that received ondansetron and cocaine during the 14-day pretreatment period, the ability of mCPBG to enhance K+ stimulated dopamine release was not significantly different from the saline control subjects. For all groups except the cocaine alone group, the effects of mCPBG on K+ stimulated dopamine release were Ca2+ dependent, suggesting that these effects are receptor mediated. These results suggest that continuous cocaine administration functionally down-regulates 5-HT3 receptors in the nucleus accumbens, and that this down-regulation can be blocked by chronic ondansetron administration. Hence, a functional down regulation of accumbens 5-HT3 receptors represents a significant contribution to the tolerance induced by continuous cocaine administration.

Acute deleterious effects of cocaine on cardiac conduction, hemodynamics, and ventricular fibrillation threshold: effects of interaction with a selective dopamine D1 antagonist SCH 39166.

Cocaine has demonstrated cardiotoxicity that has led to sudden death by unknown mechanisms. SCH 39166, a selective dopaminergic D1-receptor antagonist, suppresses the compulsive drug-intake actions of cocaine in primates. This study examined the cumulative toxic effects of cocaine after the long-term administration of SCH 39166. After pretreatment with oral placebo/SCH 39166 for 5 days, an i.v. infusion of 0.25 mg/kg/min of cocaine HCl was delivered to 14 anesthetized dogs, and cardiac conduction, arterial blood pressure, ventricular refractoriness, and arrhythmogenesis were examined. The cocaine infusion was stopped when QRS width increased by 20% from baseline (QRS20). In Coc + Placebo regimen, the QRS and His-Ventricular (HV) intervals showed a dose-dependent lengthening. Initially, the mean blood pressure (MBP) increased followed by a precipitate decrease at a mean dose of 2.03 +/- 0.5 mg/kg of cocaine. At QRS20, the ventricular effective refractory period (ERP) increased significantly, whereas the ventricular fibrillation threshold (VFT) showed a significant reduction from the baseline. In Coc + SCH, the QRS, HV intervals, and ERP increased similarly, but the decrease in MBP was attenuated, and the VFT was increased. A relatively small infusion of cocaine causes a hemodynamic compromise. The His-ventricular conduction delay and lengthened ERP suggest a predominant direct local anesthetic effect. Cocaine additionally decreased the VFT, suggesting an increased susceptibility to VF. SCH 39166 did not potentiate the cardiotoxic effects of cocaine. It displayed a protective trend by suppressing the arrhythmogenic effects and the hemodynamic compromise caused by cocaine.

Altered cocaine potency in the nucleus accumbens following 7-day withdrawal from intermittent but not continuous treatment: voltammetric assessment of dopamine uptake in the rat.

Using in vitro fast scan cyclic voltammetry, we measured cocaine potency for inhibiting dopamine uptake/clearance in accumbens slices 7 days after withdrawal from chronic cocaine pretreatments. Rats were pretreated with 40 mg/kg per day for 14 days, either via continuous osmotic minipumps or by once-daily injections. The cocaine potency was subsequently assessed for endogenous and exogenous dopamine applied via single-pulse electrical stimulation and caged-dopamine photolysis, respectively. Under baseline conditions, no differences in either endogenous or exogenous dopamine kinetics were observed in the two cocaine pretreatment groups. In contrast, the potency of bath-applied cocaine for inhibiting endogenous dopamine uptake was enhanced in the intermittent injection group with no change in the continuous infusion group. The selective increase in the cocaine potency following injections was also demonstrable for clearance of photo-applied DA. The enhanced cocaine potency in the accumbens slices following 7 days of withdrawal is consistent with the residual sensitization to cocaine-induced locomotion following daily cocaine injections. Behavioral tolerance following continuous infusion, on the other hand, may be mediated via a mechanism distinct from altered dopamine uptake.

Blockade of the expression of sensitization and tolerance by ondansetron, a 5-HT3 receptor antagonist, administered during withdrawal from intermittent and continuous cocaine.

The present experiment evaluated the ability of the 5-HT3 antagonist, ondansetron, administered during withdrawal from chronic cocaine administration, to block the expression of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg/per day cocaine for 14 days by either s.c. injections or osmotic minipumps, or 0.9% saline, administered via osmotic minipump. During the first 5 days of withdrawal from this pretreatment regimen, all rats received a daily s.c. injection of 0-1.0 mg/kg ondansetron. On day seven of withdrawal from the cocaine pretreatment (2 days after the final ondansetron injection) all subjects received a 15.0 mg/kg i.p. cocaine challenge. Their behavior was then rated according to the Ellinwood and Balster (1974) scale for 60 min. The results indicated that daily injections of ondansetron, on days 1-5 of withdrawal from the pretreatment regimen, had no significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron, on days 1-5 of withdrawal from intermittent cocaine administration, significantly blocked the expression of sensitization. In the continuous cocaine group, ondansetron injections, on days 1-5 of withdrawal from continuous cocaine administration, also blocked the expression of behavioral tolerance. The results therefore indicate that changes in 5-HT3 receptor function are associated with the expression of tolerance and sensitization, respectively.

Alterations in baseline activity and quinpirole sensitivity in putative dopamine neurons in the substantia nigra and ventral tegmental area after withdrawal from cocaine pretreatment.

Using in vivo single-unit recording, we compared in rats the-effects of continuous infusion and once-a-day injections of cocaine on the activity of single putative dopamine neurons in the substantia nigra and ventral tegmental area. After a 7-day withdrawal, we determined: (1) the number of spontaneously active neurons and their bursting patterns and (2) sensitivity of these neurons to intravenous quinpirole. In the substantia nigra, continuous cocaine infusion reduced the number of neurons without affecting the bursting patterns; daily injections were without effects. In the ventral tegmental area, continuous infusion reduced the bursting activity without affecting the number of neurons, whereas injections increased number of neurons without changes in the bursting pattern. Acute sulpiride normalized all the changes in both cell body areas. The quinpirole sensitivity was selectively increased in the nigral neurons following withdrawal from continuous infusion. Possible role of D2/autoreceptor mechanisms in these changes is discussed.

Psychomotor effects of the anxiolytic abecarnil: a comparison with lorazepam.

Abecarnil is a metabolically stable beta-carboline that binds with high affinity and selectivity to central benzodiazepine receptors. The effects on cognitive and psychomotor skills of abecarnil (ZK 112-119), 2.5 mg and 5.0 mg, were compared with lorazepam 2.0 mg and placebo. Twenty-four healthy, young males participated in a double-blind, four-way Latin square design and performed batteries of behavioral tests at predrug and at 20, 40, 60, 80, 100, 120, 180, 240, 360 and 480 min after drug administration. Abecarnil 5.0 mg and lorazepam 2.0 mg displayed similar impairment profiles in tests of cognitive functions including memory encoding. Abecarnil 2.5 mg was substantially less impairing than lorazepam. Impairment levels of the abecarnil and lorazepam treatments peaked at 2-3 h after oral administration. The two abecarnil doses showed dose-dependent effects on the cognitive and psychomotor tasks. All three drug treatments were well tolerated by the subjects, with no one terminating early due to adverse events. The incidence of reported adverse events for abecarnil was dose-dependent. The most frequent, statistically significant adverse effects were drowsiness, lack of concentration and visual disturbance for abecarnil 5.0 mg; and lack of concentration and dizziness for lorazepam 2.0 mg. There were no significant differences in adverse incidence rates between abecarnil 2.5 mg and placebo.

Blockade of cocaine sensitization and tolerance by the co-administration of ondansetron, a 5-HT3 receptor antagonist, and cocaine.

The present experiment evaluated the ability of the 5-HT3 antagonist, ondansetron, administered during chronic cocaine administration, to block the development of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg per day cocaine for 14 days by either SC injections or osmotic minipumps, or 0.9% saline, administered by SC injection. During this chronic (cocaine) treatment, all rats received a daily SC injection of 0-1.0 mg/kg ondansetron. The rats were then withdrawn from the pretreatment regimen for 7 days. On day 7 of withdrawal from the cocaine pretreatment all subjects received a 15.0 mg/kg IP cocaine challenge, and their behavior was then rated according to the modified Ellinwood and Balster scale for 60 min. The results indicated that daily injections of ondansetron had no consistent or significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron with cocaine significantly blocked the development of sensitization with an inverted U-shape dose-response curve. In the continuous cocaine group ondansetron injections also attenuated the development of behavioral tolerance. The results therefore indicate that 5-HT3 receptor stimulation during continuous and intermittent cocaine administration is an important link in the development of behavioral tolerance and sensitization.

Cocaine and amphetamine elicit differential effects in rats with a unilateral injection of dopamine transporter antisense oligodeoxynucleotides.

We have developed an antisense oligodeoxynucleotide to the dopamine transporter and used it to discriminate the behavioral properties of amphetamine and cocaine. In SK-N-MC cells permanently transfected with the dopamine transporter complementary DNA, treatment with 5 mM antisense oligodeoxynucleotide reduced dopamine uptake by 25% when compared to sense control. Unilateral intranigral administration of dopamine transporter antisense (50 microM) twice daily in freely moving rats for 2.5 days was sufficient to reduce dopamine transporter messenger RNA by 70% as measured by in situ hybridization, but not protein levels as measured by [3H]mazindol binding. However, intranigral treatment via implanted osmotic minipump over a period of seven days produced reductions in both dopamine transporter messenger RNA and protein levels (32%) at a dose of 500 pmol/day. These results indicate a longer half-life for the dopamine transporter than expected. Potassium chloride depolarization of ipsilateral striatal slices showed a greater than 200% increase in dopamine overflow on the antisense-treated side compared to the control side. Since imbalance of dopamine tone is known to induce rotational activity, we tested this behavioral paradigm in rats treated with various oligodeoxynucleotides at different doses and time-points. We have found that antisense-treated animals did not rotate spontaneously under any experimental conditions. Using various psychostimulants that target the dopamine transporter and increase dopamine levels, we found that the antisense-treated animals consistently rotated contralaterally in response to amphetamine (2 mg/kg), but not to cocaine (10 mg/kg) or nomifensine (10 mg/kg). These results bring in vivo evidence for a different mode of action of amphetamine and cocaine on the dopamine transporter and lend direct support to the view that amphetamine acts as a dopamine releaser, whereas cocaine acts by blocking dopamine transport.

Differential effects of SCH 23390 on the apomorphine subsensitivity in the substantia nigra and ventral tegmental area 1 day following withdrawal from continuous or intermittent cocaine pretreatment.

Using extracellular single-unit recordings in rats, the effects of chronic intermittent injections and continuous infusion of cocaine on single dopamine neurons were directly compared in the substantia nigra and ventral tegmental area. After 1-day withdrawal we determined: (1) the neuronal sensitivity to the mixed D1/D2 agonist apomorphine and (2) its modulation by the D1 antagonist SCH 23390. The nigral dopamine neurons exhibited subsensitivity to the impulse-inhibiting effects of apomorphine following both intermittent and continuous regimens. SCH 23390 selectively reversed the apomorphine subsensitivity in the intermittent group, while having minimal effects in the other group. Dopamine neurons in the ventral tegmental area, on the other hand, were sub- and normosensitive to apomorphine following intermittent and continuous dosing regimens, respectively. In contrast to the substantia nigra, SCH 23390 failed to alter the apomorphine sensitivity in either of the pretreatment groups. Possible mechanisms underlying these distinctive changes in the substantia nigra and ventral tegmental area following intermittent and continuous cocaine pretreatment regimens are discussed.