Behavioral analogues of anxiety. Animal models.

In the absence of fully characterized biological indexes, anxiety is at present measured as unpleasant effects reported verbally by patients. Because of the subjective nature of the syndrome, animal analogues have been difficult to design, but quests for new anxiolytics and a deeper understanding of the neurobiology of anxiety have fostered the development of several animal models. Usually, animals are exposed to exteroceptive or interoceptive stimuli which can be interpreted as capable of causing anxiety in humans. Then, the animals are observed for responses or behavioral deficits resulting from those stimuli in order to provide an index of anxiety. Behavioral responses that are reliably produced by those stimuli and that are also antagonized by anxiolytic drugs are accepted as analogues of anxiety. Exteroceptive stimuli, useful in this respect, consist of a variety of noxious treatments such as exposure to conflict-situations or unavoidable electric shock, whereas interoceptive stimuli consist of treatment with anxiogenic drugs or electrical stimulation of selected brain areas. Elicitation of unconditioned behavior or changes in the rate of conditioned (learned) responding have been employed as measures of anxiety responses following application of either exteroceptive or interoceptive stimuli. These measures, although useful in detecting anxiolytic drugs, possess several weaknesses. They suffer from difficulties in obtaining quantitative and objective data, they do not differentiate between anxiety and stress or fear, they are unable to measure further deterioration of behavior expected to occur when more potent anxiogenic stimuli are tested and they often present difficulty in differentiating direct motor effects of a number of stimuli are not related to anxiety. More recently, interest in the development of other analogues of anxiety has led to the use of drug-discrimination paradigms. In this approach, interoceptive discriminative stimuli, produced by anxiogenic drugs, are used as analogues of anxiety in animals. As an example of this approach, data are reviewed showing that pentylenetetrazol, an anxiogenic drug in humans, produces interoceptive stimuli which can be readily discriminated by rats. Further, these stimuli can be easily quantified through dose-response analysis. All known anxiogenic drugs generalize to pentylenetetrazol-induced interoceptive discriminative stimuli. Similarly, other anxiety-provoking situations in humans, such as withdrawal from dependence on benzodiazepines, also generalize to the pentylenetetrazol-induced stimuli. Alternatively, all known anxiolytic drugs antagonize these stimuli with a relative potency similar to

Modulation of the discriminative stimulus produced by pentylenetetrazol by centrally administered drugs.

Pentylenetetrazol is anxiogenic in humans and produces an interoceptive discriminative stimulus in rats which is mimicked by anxiogenic drugs and other treatments and antagonized by anxiolytic drugs. It was proposed that the discriminative stimulus of pentylenetetrazol originates centrally. This hypothesis was tested by injecting small amounts of anxiogenic or anxiolytic drugs into the brain and comparing their ability to mimic or block, respectively, the response to pentylenetetrazol, observed after systemic injection. Food-restricted rats were trained in a two-lever operant task to discriminate the interoceptive discriminative stimulus produced by pentylenetetrazol. Intraperitoneal or intracerebroventricular injection of Ro 5-3663 was substituted in a dose-dependent manner for the stimulus produced by systemically administered pentylenetetrazol. Diazepam injected systemically, blocked the pentylenetetrazol-like stimulus associated with Ro 5-3663 administered systemically or centrally. Midazolam injected intracerebroventricularly and in a dose-dependent manner, antagonized the discriminative stimulus produced by systemic injection of pentylenetetrazol. When injected into the amygdala, midazolam also antagonized in a dose-dependent manner the pentylenetetrazol-induced stimulus. Thus, these data suggest that there are sites in the CNS for both the initiation of a pentylenetetrazol-like stimulus by Ro 5-3663 and the antagonism of the stimulus produced by pentylenetetrazol by midazolam.

Discriminative stimulus properties of a small dose of cocaine.

This study characterized the interoceptive discriminative stimulus (IDS) produced by a small dose of cocaine. Rats were trained to use a dose of cocaine of 1.25 mg/kg vs saline as the basis for choosing one of two levers for food reinforcement on a fixed ratio 10 schedule. The discrimination was acquired over approx. 60 training sessions. d-Amphetamine generalized to cocaine with approximately equal potency (ED50's for cocaine and d-amphetamine were 0.07 and 0.06 mg/kg, respectively); 20 mg/kg cocaine and 10 mg/kg methylphenidate also generalized to the cocaine lever. Pentylenetetrazol, 20 mg/kg, did not generalize to the cocaine lever, and diazepam, 10 mg/kg, did not block the 1.25 mg/kg cocaine discrimination. These data indicate that when a small dose of cocaine is used as the basis of discrimination training, the discriminative stimulus that it produces is qualitatively and quantitatively similar to that produced by small doses of amphetamine, is still discriminated with a large dose of cocaine, and is dissimilar to the discriminative stimulus produced by pentylenetetrazol.

Tolerance to N6-(L-phenylisopropyl) adenosine. Contribution of behavioral mechanisms and cross-tolerance profile.

The contribution of behavioral mechanisms to tolerance to N6-(L-phenylisopropyl)adenosine (L-PIA) was studied, along with the degree of cross-tolerance to other drugs active in the CNS. Rats were stabilized on a fixed-ratio of a 20 lever-pressing schedule for food reward and were then assigned to three daily-treatment groups. One group (saline-behavior associated) was injected with saline 15 min before the session, another (L-PIA-behavior associated) was injected with L-PIA (0.08 mg/kg) 15 min before the session and the last (L-PIA-behavior dissociated) was injected with L-PIA (0.08 mg/kg) immediately after the session. Tolerance developed to the decreasing effects of L-PIA on response rate in both groups, L-PIA-behavior associated and L-PIA-behavior dissociated. Behavioral mechanisms were thus not important in tolerance to L-PIA. In subsequent cross-tolerance tests, L-PIA-tolerant rats were cross-tolerant to the adenosine Al receptor agonist, N6-cyclohexyladenosine. The drugs 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), diazepam, pentobarbital, ketamine, clonidine, d-amphetamine and caffeine did not produce differential effects in L-PIA-tolerant and non-tolerant subjects; however, L-PIA-tolerant subjects were more sensitive to the suppressive effects of chlorpromazine on the response-rate.

Task-specific tolerance to d-amphetamine.

Rats were trained concurrently on sweetened-milk drinking and bar-press-responding behavior, which alternated on a daily basis. Dose-response functions for d-amphetamine were determined before and after conditions of chronic treatment. When given before chronic treatment, d-amphetamine decreased both milk consumption and reinforcement received for lever-pressing in a dose-dependent manner. Subsequently, three conditions of chronic injection were established in which one group received saline, prior to both tasks, another group received d-amphetamine prior to drinking milk and saline prior to lever-pressing and the third group received d-amphetamine prior to lever-pressing and saline before drinking milk. The rats became tolerant to d-amphetamine in the task in which the drug had been administered chronically; however, the same rats showed no tolerance in the other task in which saline had been administered chronically. Tolerance to d-amphetamine was thus shown to be behaviorally specific.

Opioid modulation of the discriminative stimulus produced by pentylenetetrazol.

Rats were trained to detect the stimulus properties of pentylenetetrazol (PTZ), 16 mg/kg, and prototypic drugs for mu, kappa and sigma opioid receptors were tested for their ability to block or substitute for PTZ. Only the sigma agonist, phencyclidine, showed any capacity for blocking the PTZ stimulus. Drugs with selective kappa or sigma actions did not substitute for PTZ. However, morphine, fentanyl and Mr 2034 did substitute for the PTZ stimulus. This substitution was found to be centrally mediated in that quaternary morphine did not produce a PTZ-like stimulus. In contrast to the substitution of these drugs for PTZ, in rats trained to detect the stimulus properties of fentanyl, no substitution of PTZ for the fentanyl stimulus occurred. In tests of the capacity of various drugs to block the PTZ-like stimulus of mu agonists, the stimulus produced by morphine or fentanyl was blocked by naloxone, diazepam and haloperidol, but not by scopolamine. These results demonstrate that drugs with mu agonist properties show a one-way substitution for the discriminative stimulus produced by PTZ. The observation that haloperidol blocked the PTZ-like stimulus of mu agonists suggests the possible involvement of dopaminergic mechanisms in the mediation of the effect.

Substitution and cross-tolerance profiles of anorectic drugs in rats trained to detect the discriminative stimulus properties of cocaine.

Rats were trained to discriminate cocaine, 10.0 mg/kg, using a two-lever operant procedure. Dose-effect data were determined for the substitution of cocaine, diethylpropion, methylphenidate, phenmetrazine, phentermine, and fenfluramine for the cocaine stimulus. All of these drugs, except fenfluramine, substituted fully for the cocaine stimulus. Subsequently, training was halted and cocaine, 20 mg/kg/8 h, was administered for 9 days, and dose-effect data were redetermined for all of these drugs on days 7-9 of chronic administration. Chronic administration of cocaine produced tolerance to the stimulus properties of cocaine, and cross-tolerance to the stimulus properties of methylphenidate, phenmetrazine, and phentermine, such that doses approximately two-fold higher than those used acutely were necessary to reproduce the original effect; the potency for the substitution of diethylpropion for the cocaine stimulus was decreased greater than four-fold; and fenfluramine still failed to substitute for the cocaine stimulus. These data suggest that 1) a common mechanism may mediate tolerance to the discriminative stimulus properties of cocaine, methylphenidate, phenmetrazine, and phentermine, and 2) tolerance in the drug discrimination procedure may have potential for establishing a comprehensive evaluation of dependence liability of CNS stimulants.

Sensitivity of pentylenetetrazol discrimination increased by a stimulus fading technique.

The interoceptive stimulus produced by pentylenetetrazol (PTZ) is pharmacologically similar to anxiety and is used in a behavioral assay for anxiety-related stimuli (the PTZ model of anxiety). The stimulus fading technique was tested as a method to increase the sensitivity of this assay. Rats were trained with food-reward to press one lever after injection of PTZ and an alternate lever after saline. Rats initially learned the discrimination at a PTZ dose of 20 mg/kg. They were then trained with sequentially lower doses until they reliably discriminated a PTZ dose of 10 mg/kg. Substitution tests with other doses and drugs showed that, after the fading procedure, dose-response curves were shifted to lower doses for PTZ, Ro 5-3663, and nicotine Similarly, the dose of diazepam required to block the low dose of PTZ was lower than that required to block the higher dose of PTZ. These results indicated that the sensitivity of the discrimination was enhanced in rats trained to discriminate a lower dose of PTZ. Doses of nikethamide, cocaine, and yohimbine that did not substitute for the higher dose of PTZ also did not substitute for the lower dose. These data suggest that rats can be trained to discriminate a low dose of PTZ by the stimulus fading technique. Moreover, they suggest that this training method does not compromise the specificity of the discrimination.

Discriminative stimulus effects of midazolam and abecarnil in rats treated chronically with diazepam or abecarnil.

Abecarnil (ABC) is a beta-carboline that acts as an agonist at benzodiazepine (BZD) receptors. It possesses anxiolytic and anticonvulsant properties, but produces little sedation and is without muscle relaxant effects. To explain this unusual profile of activity, two hypotheses have been advanced: either 1) ABC acts as a partial agonist or 2) ABC acts as a full agonist, but only at a sub-population of BZD receptors. The present experiment used cross-tolerance profiles between BZDs and ABC to differentiate these hypotheses based upon predictions of receptor theory: tolerance produced to a full agonist should confer even greater cross-tolerance to a partial agonsit. Rats were trained in a three-choice drug discrimination procedure to detect the benzodiazepine, midazolam (MDZ, 1.0 mg/kg) from pentylenetetrazole (PTZ, 20 mg/kg) from saline. Tested acutely, MDZ and ABC substituted for MDZ with similar potencies. Following chronic treatment with the BZD-agonist diazepam (DZP; 20 mg/kg per 8 h for 7 days), both the MDZ and ABC dose-effect curves were significantly shifted to the right, and both drugs showed a comparable three-fold decrease in potency. The chronic administration of ABC (4.0 mg/kg per 8 h for 7 days) produced a different spectrum of results. No significant shift occurred in the MDZ dose-effect curve, but there was a significant seven-fold shift to the right of the ABC dose-effect curve. Throughout all test, PTZ-lever responding rarely occurred and did not account for more than 20% of lever selections for any individual test. These data support the hypothesis that ABC acts as a full agonist at a sub-population of BZD receptors, which mediate its substitution for MDZ.

Tolerance to the reinforcing effects of cocaine in a progressive ratio paradigm.

This experiment used rats to test whether a regimen of chronic cocaine would produce tolerance to cocaine i.v. self-administration under a progressive ratio (PR) schedule of reinforcement. Under this PR schedule, an increasing number of responses was required to complete the ratio for each subsequent cocaine injection, and failure to complete the required ratio for the next injection within 1 h of the previous cocaine injection terminated the session. The number of injections taken in the session was termed the breaking point and used as the dependent variable. Rats were trained under this schedule until breaking point values were stable, after which cocaine dose-effect data were obtained: the breaking point increased as the dose of cocaine increased. Subsequently, rats were assigned to one of two groups for 7 days of chronic treatment: one group was infused with cocaine (18 mg/kg, given over 20 min once every 8 h) and the other group received 0.9% saline. Following termination of chronic treatment, cocaine dose-effect data were redetermined in both groups. Chronic cocaine treatment significantly decreased breaking point values across the entire dose-effect curve, although the effect was observed in only four of seven subjects. In contrast, chronic saline treatment produced no significant effect on the breaking point measures. Following a further 5 days of recovery from chronic treatment, cocaine dose-effect data were redetermined in both groups; these curves were essentially identical to those obtained before chronic treatments. These data support the hypothesis that tolerance occurs to the reinforcing effects of cocaine, as measured by a decrease in the breaking point, at least for a subset of animals.

Tolerance and selective cross-tolerance to the motivational effects of opioids.

The issue of whether tolerance develops to the motivational effects of opioids was addressed by use of an unbiased place preference conditioning procedure. Administration of the mu-opioid agonists morphine or fentanyl produced dose-related preferences for the drug-associated place in control rats. In contrast, the kappa-opioid agonist, U-69593 produced conditioned place aversions. Non-contingent administration of morphine (5.0 mg/kg/12 h) for 4 days prior to conditioning resulted in tolerance to its reinforcing effects, and cross-tolerance to the effects of fentanyl. No cross-tolerance to the motivational effects of the psychostimulant d-amphetamine or the kappa-opioid agonist U-69593 was observed. Chronic administration of U-69593 prior to conditioning produced tolerance to its aversive effects. This treatment did not, however, modify the reinforcement produced by morphine. These data demonstrate that tolerance develops to both the reinforcing and aversive properties of opioids and suggest that differential cross-tolerance may provide a useful method for determining the pharmacological basis underlying drug-induced motivational effects.

Quantal detection and homogeneous sensitivity in a pentylenetetrazol discrimination.

Rats were trained to discriminate pentylenetetrazol (PTZ, 20 mg/kg) from saline in a two-lever operant task. Correct lever presses were reinforced with food under the control of a fixed ratio 10 schedule. In tests of the effect of PTZ dose on lever selection, rats selected the PTZ lever in a dose-dependent manner, with peak latency at the approximate ED50 dose (10 mg/kg). Rats usually pressed only the selected lever, regardless of dose, indicating that lever selection was a quantal (or bimodal) function of stimulus intensity. Lever biases observed during training sessions did not predict the performance of individual rats in tests with the ED50 dose. In three independent trials with this intermediate dosage, the rats selecting the PTZ lever varied from trial to trial, suggesting that rats detecting this dose did not form a stable subgroup. The pattern of lever selections across these three trials was not significantly different from that predicted by a model in which all subjects shared the same probability for detecting the drug stimulus. These results demonstrate that lever selection in a two-lever drug-discrimination task can be quantal in nature, and suggest that rats trained with PTZ, 20 mg/kg, are homogeneous in sensitivity to this stimulus.

Animal models of drug withdrawal symptoms.

There have been few attempts to model subjective symptoms of drug withdrawal using animals as subjects. Two approaches for developing such models are reviewed. First, using drug discrimination methodology, it may be possible to train animals to detect the effects of withdrawal. This method has two difficulties: 1) the only discriminations trained to date involve precipitated withdrawal, and 2) the stimulus controlling behavior is difficult to specify. Second, withdrawal from many drugs of abuse produces the symptom of anxiety, and it seems likely that animal models of anxiety could be useful for studying drug withdrawal. This hypothesis has been explored most fully using subjects trained to detect the discriminative stimulus properties of the putative anxiogenic drug pentylenetetrazole (PTZ). Withdrawal from benzodiazepines or ethanol substitutes fully for PTZ, and withdrawal from cocaine, morphine, and nicotine substitutes partially for PTZ. Emerging data suggest that other animal models of anxiety may also be useful for detecting drug withdrawal. The final portion of this review examines a behavioral test that is very sensitive for detecting physical signs of withdrawal in animals. In subjects maintained on an operant baseline using food as a reinforcer, withdrawal from a drug of dependence frequently is associated with disruption of that operant behavior. For example, tetrahydrocannabinol and cocaine, drugs that are not traditionally seen as having significant withdrawal signs, produce disruption of operant responding when high-dose administration is terminated, and their readministration reverses this behavioral disruption. Based on the observation that withdrawal is associated with anxiogenic stimuli, we suggest a method to determine if disruption of operant behavior may be related to these stimuli.

Variations in cocaine self-administration by inbred rat strains under a progressive-ratio schedule.

This study investigated the influence of genetics on extent of cocaine taking in rats that were self-administering cocaine under a progressive-ratio schedule. Fischer 344, ACI and Brown Norway rats were subjects because previous genetic studies on dopamine receptor loci have indicated that these are genetically divergent strains. All subjects were assessed for acquisition and stability of cocaine self-administration under a progressive ratio schedule. Subsequently, a dose-effect curve for cocaine self-administration was determined for each strain. Fischer 344 rats maintained a higher average breaking point than did the ACI or Brown Norway strains. In addition, dopamine receptor antagonists differentially reduced the ability of cocaine to serve as a reinforcer across the three strains. The D1-selective dopamine receptor antagonist, SCH 23390, and the D2/D3-selective dopamine receptor antagonist, eticlopride were significantly more effective in reducing the self-administration of cocaine in Brown Norway rats than for the other two strains. The results of this study demonstrate that genetic differences may play an important role in determining responding under progressive-ratio schedules for cocaine, possibly due to differences in the reinforcing efficacy of cocaine.

Withdrawal from chronic nicotine substitutes partially for the interoceptive stimulus produced by pentylenetetrazol (PTZ).

Rats were trained on an FR10 schedule of food reinforcement to press one lever after pentylenetetrazol (PTZ), 20 mg/kg, IP, and an alternate lever after saline. After acute nicotine, 0.64 mg/kg, SC, 35% of the rats pressed the PTZ-lever. Diazepam, 5 mg/kg, IP, blocked the stimulus produced by PTZ, and mecamylamine, 5 mg/kg, IP, blocked the stimulus produced by nicotine. Training was then suspended and rats were treated with nicotine, at 8-h intervals, 0.64 mg/kg on the 1st day, and 1.25 mg/kg on subsequent days, for 21 days. To determine whether nicotine withdrawal substitutes for the stimulus produced by PTZ, rats were tested with saline at various times after chronic nicotine injections. Data from this part of the study were replicated in another group given nicotine for 15 days. Saline at 8 h after nicotine (five determinations each group) produced a small but stable degree of PTZ lever selection (35 +/- 4%). At 48 h after termination of nicotine treatment, the percentage of rats selecting the PTZ lever (50%) was greater than that in a control group tested after an equivalent period without training. The PTZ-like stimulus detected after chronic nicotine was not altered by mecamylamine, was additive with PTZ, and was blocked by diazepam. These data suggest that withdrawal from chronic nicotine produces a weak PTZ-like stimulus, which can be antagonized by an anxiolytic drug.

A method to shorten the training phase of drug discrimination.

Rats were trained to discriminate "drug" from "no drug" in a two-lever, food-reinforced task. One group was trained with cocaine (10 mg/kg) and a second group was trained with pentylenetetrazol (20 mg/kg). A method designed to shorten the time required for the training phase of drug discrimination experiments was assessed in subgroups for each drug. In one subgroup, single training sessions were conducted daily. In the other subgroup, a second session (either drug or saline) was conducted on days for which the first condition was saline. The training conditions were presented in an irregular sequence, with the same condition occurring in no more than two consecutive sessions. Rats trained by the accelerated method learned the discrimination in fewer days, with no decrement in acquisition per session, suggesting that drug discrimination training can be accomplished more rapidly by reducing inter-session interval.

Withdrawal from morphine generalizes to a pentylenetetrazol stimulus.

Rats trained to discriminate pentylenetetrazol (PTZ) from saline in a two-lever food-reinforced operant task were given a three-day course of morphine, 15 to 45 mg/kg tid, ip. On the third day naloxone produced dose-dependent generalization to the PTZ stimulus, with 66% of subjects selecting the PTZ lever after the highest dose (0.32 mg/kg). Following termination of morphine injections, generalization of spontaneous withdrawal was tested. Approximately 50% of subjects selected the PTZ lever at 24 and 48 hrs after the last morphine, and by 96 hrs the percentage of subjects selecting the PTZ lever had dropped to 11%. Rats that chose the PTZ lever at 48 hrs were given diazepam, 5.0 mg/kg, which blocked the PTZ-like stimulus. These data demonstrate that morphine withdrawal produces a stimulus with PTZ-like characteristics which can be blocked by an anxiolytic, and they suggest that the PTZ discrimination may have general utility for investigating drug dependence and withdrawal in animals.

Quantal vs. graded generalization in drug discrimination: measuring a graded response.

In drug discrimination research, detection of drug stimuli by animals is used for investigating various properties of psychoactive drugs. The major issue addressed by this paper is whether detection of drug stimuli by animals is a quantal or graded event. Some data suggest that detection of a drug stimulus by animals is quantal in nature. Thus, variations in drug stimulus substitution may only reflect variations in threshold for detecting the training stimulus rather than the current concept of these data reflecting graded responding to stimulus intensity. Therefore, drug discrimination procedures may have limited utility for detecting quantitative differences in the subjective effects of varying drug doses. In order to examine this problem, a method for measuring continuous response gradients in individual animals is needed. Tests for quantal responding generally use the distribution of responses on two manipulanda as the dependent measure. However, this variable may be inadequate for detecting a graded response, and may actually reflect loss of stimulus control or a deterioration in performance, rather than changes in response magnitude. Most alternative measures utilize response rate. Unfortunately, these measures are influenced by the direct rate-altering properties of some drugs. One possible alternative method is conditioned taste aversion as the discriminative task. This paradigm provides a means for not only ascertaining if graded discriminative responses occur in individual animals, but also more rapidly training a drug discrimination. Thus, using conditioned taste aversion techniques for measuring a drug discrimination may provide better indices for detecting response gradations.

Neural mechanisms of tolerance to the effects of cocaine.

Chronic use of cocaine in high doses can produce tolerance as assessed by various behavioral, neurochemical, cellular and molecular measures in specific brain regions. Tolerance to cocaine is indicated by drug discrimination and intracranial self-stimulation models, which show the development of tolerance after approximately 1 week of frequent cocaine treatment, with recovery after a similar period of cocaine abstinence. Tolerance to the reinforcing properties of cocaine depends on dose, duration and frequency of cocaine self-administered by experimental animal or human subjects. The mechanism underlying this effect may involve an absolute or relative attenuation of dopamine response to cocaine challenge after frequent or repeated treatment in the nucleus accumbens (NAc). Similarly, afferent and efferent NAc circuits exhibit reduced metabolic activity, which lasts throughout the early period of withdrawal following repeated treatment. Attenuation of immediate early gene response also occurs, which might be related to a functional desensitization of dopamine D1-like receptors. Furthermore, intracellular adaptive responses to chronic cocaine exposure induce striatal dynorphin expression decreasing the behavioral potency of subsequent drug treatment. Thus, a combination of various pharmacodynamic mechanisms and the attenuation of dopamine response induced by sufficient dose, duration and frequency of cocaine exposure ultimately invoke the transient development of tolerance to the reinforcing effects of cocaine.

Dopamine release during cocaine self-administration in rats: effect of SCH23390.

This experiment tested the hypotheses that: (1) self-administration of cocaine would produce an increase in dopamine (DA) oxidation current in the nucleus accumbens (n. acc.); and (2) a faster rate of cocaine intake in the presence of a D1 receptor antagonist would produce a greater increase in DA levels. Rats trained to self-administer cocaine under a fixed-ratio 2 schedule were implanted with stearate-modified graphite paste electrodes bilaterally in the n. acc. The effect of pretreatment with the D1 receptor antagonist, SCH23390, on the DA oxidation current associated with self-administration of cocaine (1 mg/inj.) or saline was investigated using chronoamperometry. Pretreatment with SCH23390 produced a 2-fold increase in the amount of cocaine intake. This in turn resulted in a 2-fold increase in the DA oxidation current recorded in the n. acc. Pretreatment with SCH23390 did not, however, produce any significant change in either the number of saline injections received or the DA oxidation current recorded during saline self-administration. These findings show that cocaine increases DA oxidation currents in the n. acc., and that both the rate of cocaine self-administration and the magnitude of these currents increase even further following SCH23390. The results also imply that the baseline rate of cocaine self-administration does not result in the occupation of all possible DA transporter sites.