Dopaminergic involvement in the cocaine-induced up-regulation of benzodiazepine receptors in the rat caudate nucleus

The combination of metyrapone and oxazepam for the treatment of cocaine and other drug addictions

Although scientists have been investigating the neurobiology of psychomotor stimulant reward for many decades, there is still no FDA-approved treatment for cocaine or methamphetamine abuse. Research in our laboratory has focused on the relationship between stress, the subsequent activation of the hypothalamic-pituitary-adrenal (HPA) axis, and psychomotor stimulant reinforcement for almost 30 years. This research has led to the development of a combination of low doses of the cortisol synthesis inhibitor, metyrapone, and the benzodiazepine, oxazepam, as a potential pharmacological treatment for cocaine and other substance use disorders. In fact, we have conducted a pilot clinical trial that demonstrated that this combination can reduce cocaine craving and cocaine use. Our initial hypothesis underlying this effect was that the combination of metyrapone and oxazepam reduced cocaine seeking and taking by decreasing activity within the HPA axis. Even so, doses of the metyrapone and oxazepam combination that consistently reduced cocaine taking and seeking did not reliably alter plasma corticosterone (or cortisol in the pilot clinical trial). Furthermore, subsequent research has demonstrated that this drug combination is effective in adrenalectomized rats, suggesting that these effects must be mediated above the level of the adrenal gland. Our evolving hypothesis is that the combination of metyrapone and oxazepam produces its effects by increasing the levels of neuroactive steroids, most notably tetrahydrodeoxycorticosterone, in the medial prefrontal cortex and amygdala. Additional research will be necessary to confirm this hypothesis and may lead to the development of improved and specific pharmacotherapies for the treatment of psychomotor stimulant use.

Alprazolam and oxazepam block the cue-induced reinstatement of extinguished cocaine seeking in rats

RATIONALE

We have previously reported that pretreatment with benzodiazepines reduces intravenous cocaine self-administration in rats.

OBJECTIVE

This experiment was designed to investigate whether or not benzodiazepines would also inhibit the reinstatement of cocaine seeking induced by the presentation of a conditioned reinforcer.

MATERIALS AND METHODS

Adult male rats were implanted with jugular catheters and trained to self-administer cocaine (0.25 mg/kg/infusion) during daily 2-h sessions. During training, cocaine delivery was paired with the presentation of a tone and the illumination of a houselight. Once a stable baseline of cocaine self-administration was observed, lever pressing was extinguished to less than 20% of baseline rates. During reinstatement testing, responding resulted in the presentation of the conditioned reinforcer (i.e., the houselight and tone previously paired with self-administered cocaine).

RESULTS

The response-contingent presentation of the conditioned reinforcer reliably reinstated cocaine seeking. Pretreatment with alprazolam (2 or 4 mg/kg, ip) or oxazepam (20 or 40 mg/kg, ip) reversed the conditioned reinforcer-induced reinstatement of extinguished cocaine-seeking behavior at doses that did not reliably affect the conditioned reinforcer-induced reinstatement of extinguished food seeking, suggesting that benzodiazepines may not have reduced reinstatement responding due to non-specific reductions in behavior.

CONCLUSIONS

These data suggest that benzodiazepines may be useful in blocking the ability of environmental cues to stimulate cocaine seeking.

Effects of the combination of metyrapone and oxazepam on cocaine and food self-administration in rats

For several years, our laboratory has investigated the role for the HPA axis in cocaine reinforcement. Two classes of drugs that we have studied include corticosterone synthesis inhibitors (e.g., metyrapone) and benzodiazepine receptor agonists (e.g., oxazepam). In the experiments described in this manuscript, we tested the effects of various doses of metyrapone and oxazepam against several doses of self-administered cocaine. Behavioral, endocrine and pharmacokinetic measures of the effects of the combination of metyrapone and oxazepam on cocaine reward are presented. Combinations of metyrapone and oxazepam at doses that produced no observable effects when administered separately significantly reduced cocaine self-administration without affecting food-maintained responding during the same sessions. Changes in pharmacokinetics or endocrine function do not appear to mediate these effects, suggesting a central mechanism of action. Therefore, although these drugs produce their effects through distinct mechanisms, an additive effect on cocaine self-administration is obtained when these drugs are administered together, suggesting that combinations of low doses of metyrapone and oxazepam may be useful in reducing cocaine seeking with a reduced incidence of unwanted side effects and a decreased potential for abuse.

Effects of oxazepam on methamphetamine-induced conditioned place preference

Our laboratory has been investigating the role for the hypothalamo-pituitary-adrenal (HPA) axis and benzodiazepines in the behavioral effects of cocaine for several years now. The following represents our initial investigation of the influence of benzodiazepines on methamphetamine reward using conditioned place preference. In these experiments, methamphetamine (0.5 mg/kg ip) resulted in a robust conditioned place preference that was attenuated when the rats were pretreated with oxazepam (10 mg/kg ip) on the day of preference testing. These data suggest a potential role for benzodiazepines in the behavioral effects of methamphetamine. Additional research will be necessary to determine if the nature of these effects is similar with what has been observed with cocaine.

Die neuropsychologischen Effekte von Kokain

Zusammenfassung: Kokainkonsumenten sind in der Regel entweder Gelegenheitskonsumenten mit geringem und relativ gut kontrollierbarem Kokainkonsum oder Kokainabhängige, die exzessiv große Mengen Kokain einnehmen (Binge-Taking). Neuropsychologische Untersuchungen bei Gelegenheitskonsumenten mit kleinen bis mittleren Kokaindosen, die geeignet sind, “Euphorie” und “Hochgefühle” auszulösen, ergaben bisher keine Hinweise auf neuropsychologische Funktionsbeeinträchtigungen in der akuten Phase. Vielmehr wurden in der akuten Kokainphase verbesserte Aufmerksamkeitsleistungen und schnellere Reaktionszeiten gemessen, die bis zum Beginn der postakuten Phase anhielten. Explizite Untersuchungen zur postakuten Phase oder Studien über die Effekte von Kokain in einem hohen Dosisbereich liegen derzeit nicht vor. Kokainabhängige weisen in einem Abstinenzzeitraum von bis zu mindestens 3 Monaten neuropsychologische, neurologische und unter Umständen auch psychiatrische Defizite auf. Deutliche Defizite abstinenter Kokainabhängiger wurden bei der Aufmerksamkeitsleistung, der Konzentration, der inhibitorischen Kontrolle und im Gedächtnis gefunden, wobei die verschiedenen Gedächtnistypen unterschiedlich betroffen sind. Kommt es bei Kokainabhängigen zu einer erneuten Kokaineinnahme, d.h. zu einer Binge-Phase, so ist unmittelbar danach in der so genannten Crash-Phase noch zusätzlich mit zum Teil schweren emotional-motivationalen Beeinträchtigungen zu rechnen.

Differential effects of prenatal cocaine exposure on selected subunit mRNAs of the GABA(A) receptor in rabbit anterior cingulate cortex

We have previously shown that in the dopamine-rich anterior cingulate cortex (ACC), significant changes in gamma-aminobutyric acid (GABA) immunoreactivity occur in the offspring of rabbits given intravenous injections of cocaine (3 mg/kg) twice daily during pregnancy. In the present study, the effects of prenatal cocaine exposure on the developmental expression of specific GABA(A) receptor subunit mRNAs were investigated. We compared the distribution of the alpha1, beta2, and gamma2 subunit mRNAs in cocaine- and saline-treated offspring aged postnatal days 20 and 60 (P20, P60). At P20, prenatal cocaine exposure resulted in a significant increase in alpha1 subunit mRNA in ACC lamina III and a significant reduction in the amounts of the beta2 subunit mRNA in ACC lamina II. No differences between cocaine- and saline-treated controls were detected for gamma2 subunit mRNA levels in ACC. Although the pattern of labeling was altered in cocaine-exposed animals, Nissl sections revealed no differences in lamination, indicating that the changes in GABA(A) subunit mRNAs could not be attributed to abnormal cytoarchitectonics. In P60 brains, no significant differences were observed between cocaine- and saline-treated material, indicating that the observed differences were transient. Collectively, our data show that prenatal cocaine exposure elicits differential, lamina-specific changes in mRNA levels encoding selected subunits of the GABA(A) receptor. Since these changes occur during a critical period when fine tuning of synaptic organization is achieved by processes of selective elimination or stabilization of synapses, we suggest that specific subunit mRNAs of the GABA(A) receptor play a role in cortical development.

Time-course effects of a single administration of cocaine on receptor binding and subunit mRNAs of GABA(A) receptors

We investigated the time-course effects of a single administration of cocaine (20 mg/kg) on GABA(A) receptor binding labeled by t-[(35)S]butylbicyclophophorothionate (TBPS) and on several types of GABA(A) receptor subunit mRNAs in the rat brain by in vitro quantitative receptor autoradiography and in situ hybridization. The levels of alpha 1, beta 2, and beta 3 subunit mRNAs in several brain regions such as the cortex, cerebellum, and striatum were significantly decreased within 1 h, while beta 3 subunit mRNA was increased in the dentate gyrus. All of these changes were transient, occurring within 1 h after the injection of cocaine. In the cortex and cerebellum, the reduction in alpha1 subunit mRNA was followed by a significant decrease in [(35)S]TBPS receptor binding, which occurred 4 h after cocaine injection. These findings suggest that acute cocaine administration discretely regulates GABA(A) receptor subunit mRNA levels in several brain regions through a change in transcription or turnover rates of subunit mRNAs, which may be closely related to cocaine-induced behavioral abnormalities.

Tolerance and sensitization to the behavioral effects of cocaine in rats: relationship to benzodiazepine receptors

Tolerance and sensitization to the behavioral effects of cocaine were investigated in rats responding under a fixed-consecutive-number eight schedule of food reinforcement. The development of tolerance or sensitization was induced by delivering the drug either immediately before or after each behavioral session during chronic administration. Chronic cocaine administered before each session resulted in tolerance, as indicated by the shift to the right in the cocaine dose response curve. This tolerance was more likely to develop in the presence of an external discriminative stimulus. On the other hand, when cocaine was delivered after each session, the injections did not disrupt responding and sensitization or increased sensitivity rather than tolerance developed. This sensitization was more likely to occur when the external discriminative stimulus was not present. These data suggest that either tolerance or sensitization to the behavioral effects of cocaine can occur following the same number of chronic injections, with the effect dependent on the context under which the drug is delivered. Significant differences in benzodiazepine receptor binding measured autoradiographically using [3H]flumazenil were observed between rats that received cocaine before or after each session, suggesting that the development of tolerance and sensitization may be mediated through changes in benzodiazepine receptors in discrete brain regions.

A neuroendocrine role in cocaine reinforcement

Cocaine stimulates the secretion of corticosterone and ACTH, probably through a CRF-related mechanism, indicating that the drug activates the HPA axis. Indeed, cocaine has been reported to produce anxiety and to precipitate episodes of panic attack during chronic use and withdrawal in humans and to induce anxiogenic behavior in animals. Cocaine also alters benzodiazepine receptor binding in discrete regions of the rat brain. Some of these changes in binding are obviously related to the convulsions and seizures which are often observed in an acute cocaine overdose. However, data from behavioral studies have suggested that some of these effects may be related directly to cocaine reinforcement since receptor changes also were observed when binding in the brains of rats that self-administered cocaine was compared with that from animals that had received identical yoked, but non-contingent infusions of the drug. In this regard, pretreatment with the benzodiazepine receptor agonists chlordiazepoxide and alprazolam decreased cocaine self-administration without decreasing food-reinforced responding, suggesting that these effects were specific for cocaine. Since this attenuation of self-administration was reversed by increasing the unit dose of cocaine, it is likely that these drugs were decreasing cocaine reinforcement. In contrast, exposure to stress increases vulnerability to self-administer psychostimulants. In these experiments, low-dose cocaine self-administration was related directly to stress-induced increases in plasma corticosterone, such that plasma corticosterone was always greater than 150 ng/ml for rats which subsequently self-administered cocaine at doses of 0.125 mg/kg/infusion or lower, suggesting a threshold for the hormone in cocaine reinforcement. In other experiments, bilateral adrenalectomy completely abolished the acquisition of intravenous cocaine self-administration in naive rats, while metyrapone decreased ongoing self-administration. In addition, ketoconazole pretreatment resulted in patterns of self-administration that were virtually indistinguishable from that observed during saline extinction, suggesting that plasma corticosterone is not only important, but may even be necessary for cocaine reinforcement. The mechanisms through which adrenocorticosteroids alter cocaine reinforcement remain to be determined, but there is increasing evidence that the mesocorticolimbic dopaminergic system is involved. In particular, the medial prefrontal cortex appears to be at least one brain region where dopamine and adrenocorticosteroids may interact to affect cocaine reinforcement.

Intrauterine cocaine exposure of rabbits: persistent elevation of GABA-immunoreactive neurons in anterior cingulate cortex but not visual cortex

The effects of prenatal cocaine exposure on the development of the rabbit cerebral cortex were studied. Two cortical areas were compared: primary visual cortex (VC) and anterior cingulate cortex (ACC). ACC was selected because behavioral deficits observed in cocaine-exposed infants suggest the involvement of ACC. In addition, ACC receives dense dopaminergic innervation and cocaine's action in inhibiting the re-uptake of dopamine is believed to underly the rewarding properties of cocaine. VC was selected as a control area because there is no evidence of behavioral deficits associated with visual perception in cocaine-exposed infants, and because VC receives minimal dopaminergic innervation. Two aspects of cortical development were studied: (i) cortical morphology, growth and cytoarchitectonic organization; and (ii) the development of the GABAergic neurotransmitter system. Measures of postnatal cortical growth, including cortical lamination, cell number and soma size, were compared in cocaine-exposed or control (saline) rabbits aged P5-P60. There was no difference between cocaine and saline animals in any of these parameters, and cortical cytoarchitecture appeared normal. However, despite the absence of major abnormalities in cortical development, we found that the number of GABA-immunoreactive neurons in cocaine-exposed animals was significantly higher than normal in ACC. This effect was highly consistent, was present in all laminae and at all ages studied, and persisted into maturity (P60). In contrast, in VC, the number of GABA-immunoreactive neurons in cocaine-exposed animals did not differ from normal. We suggest that increased GABA immunoreactivity may reflect a compensatory response to excessive excitatory input to ACC. A change in the balance of excitation and inhibition in ACC, reflecting 'noisy' or dysfunctional intracortical circuitry, may underly the emotional lability and attentional deficits characteristically described in infants exposed in utero to cocaine.

Length of continuous cocaine exposure determines the persistence of muscarinic and benzodiazepine receptor alterations

The effects of varied durations of cocaine (1, 3 or 5 days) on muscarinic (MSC) and benzodiazepine (BZD) binding sites in striatum and hippocampus were investigated using homogenate receptor binding. The progressive alterations in these receptor sites from a 5 day cocaine administration were also examined 12 h, 2 days or 21 days after drug exposure. Neither a one nor a three day exposure to cocaine produced any long-term alteration in BZD binding in either structure whereas a 5 day administration produced significant increases in binding. Decreases in MSC receptor binding were apparent in striatum from either a 3 or 5 day cocaine exposure and in hippocampus from a 5 day exposure. The 5 day cocaine exposure produced immediate increases in striatal and hippocampal BZD binding which persisted for 21 days. Conversely, 5 days of cocaine produced a short-term increase in MSC receptor binding in both structures which then became significantly decreased 21 days later. Based on the divergent pattern of changes in BZD and MSC receptor types over time in these structures, it appears that cocaine may induce such changes via separate mechanisms. In addition, it is apparent that changes in the numbers of these receptor sites after cocaine exposure can be quite dynamic, changing rapidly over time.

Peripheral benzodiazepine receptors are decreased during cocaine withdrawal in humans

In the present study, homovanillic acid in plasma (pHVA) and benzodiazepine receptors (3H-PK11195 binding) in neutrophil membranes were determined in blood obtained from cocaine-dependent (DSM-III-R) adult male inpatients at baseline-(within 72 hr of last cocaine use) and after 3 weeks of cocaine abstinence, and normal controls. The mean (+/- SEM) pHVA at baseline (10.3 ng/ml +/- 1.1) was similar to normals and did not change after 3 weeks of cocaine abstinence. Similarly, the binding indices of benzodiazepine receptors in cocaine-dependent subjects as a group were not significantly different than in normal controls. In 10 cocaine-dependent subjects, however, where both blood samples were available, the number of 3H-PK11195 binding sites was significantly (p < 0.05) decreased after 3 weeks of cocaine abstinence (mean +/- sem: Bmax = 6371 +/- 657 fmol/mg protein) compared with baseline (Bmax = 7553 +/- 925 fmol/mg protein), although there were no differences in the binding affinity (mean +/- sem: KD = 8.6 +/- 1.2 nmol/L after 3 weeks of abstinence compared with 8.1 +/- 1.0 nmol/L at baseline). These preliminary results suggest that peripheral benzodiazepine receptors may play an important role in the pathophysiology of cocaine withdrawal in cocaine-dependent human subjects.

Non-contingent electric footshock facilitates the acquisition of intravenous cocaine self-administration in rats

The experiments described below were designed to investigate whether contingent versus non-contingent electric footshock would affect the acquisition of intravenous cocaine self-administration in rats. During the first component of a multiple schedule, triads of rats were trained to respond under a discrete-trial, fixed-ratio 10 schedule of food reinforcement. Random footshock presentation (0.6 mA) for the first and second rats from each triad was yoked to food lever responding by the rats in the first group only, while the third group of rats was never shocked. When stable baselines of food-reinforced responding were obtained, all three rats from each triad were allowed to self-administer increasing doses of cocaine (0.031-0.5 mg/kg per infusion) during the second component. Rats from the second group, receiving non-contingent footshock presentation, self-administered cocaine (0.125 mg/kg per infusion) at higher rates and at one-half the dose which maintained responding in rats from the other two treatment groups. Plasma corticosterone, measured before the acquisition of cocaine self-administration, was highly correlated with drug intake at this concentration for all three groups of rats. These data demonstrate that non-contingent experimental stress facilities the acquisition of intravenous cocaine self-administration in rats.

Cocaine binding sites in mouse striatum, dopamine autoreceptors, and cocaine-induced locomotion

BALB/cByJ mice received cocaine (25 mg/kg IP) once a day for 3 days, resulting in a greater locomotor response to cocaine on day 3 than on day 1. On day 4, a dose (0.03 mg/kg SC) of apomorphine, targeted at dopamine autoreceptors, caused the same degree of locomotor depression in cocaine- as in saline-pretreated mice. In addition, no change was found in either the affinity or density of cocaine binding sites in their striatum as measured by the binding of [3H]CFT. C57BL/6ByJ, mice displayed a greater locomotor response to cocaine than BALB/cByJ mice, but had the same number of striatal [3H]CFT binding sites with the same affinity. Factors other than striatal cocaine binding sites, or dopamine autoreceptors as measured by apomorphine-induced depression of locomotion, should be considered for the explanation of the enhancement of the locomotor response upon daily cocaine administration in BALB/cByJ mice, or for the different locomotor response to cocaine of this strain compared with the C57BL/6ByJ strain of mice.

Continuous cocaine administration produces persisting changes in brain neurochemistry and behavior

Rats were administered either continuous cocaine, daily injections of cocaine, continuous amphetamine, or no drug for 5 days and then given a 30 day drug-free recovery period. When subsequently tested in open field, the daily cocaine injection animals were the most hyperactive whereas the cocaine pellet animals were the most fearful. In vitro autoradiography was then utilized to examine persisting changes in receptor binding for D2 ([3H]spiperone), D1 ([3H]SCH23390), benzodiazepine ([3H]flunitrazepam), 5-HT1 ([3H]5-HT), 5-HT2 ([3H]ketanserin), and muscarinic acetylcholine (ACh) receptors ([3H]QNB; quinuclidinyl benzilate). In the amphetamine pellet animals, there were large increases in [3H]spiperone binding in several dopamine (DA)-rich regions; these were accompanied by conversely decreased [3H]SCH23390 binding. Cocaine pellet animals showed a completely different pattern, with appreciable increases in [3H]flunitrazepam binding in DA-rich areas, cortex, and amygdala but decreased [3H]QNB binding in DA-rich areas, hippocampus, and amygdala. While cocaine injection animals showed elevated [3H]spiperone binding in caudate and substantia nigra, they had generally smaller changes in most brain regions than the other drug groups. These findings replicate and extend previous reports that continuous drug administration induces long-lasting alterations in brain chemistry, but indicate that continuous cocaine has enduring effects on different neurochemical systems from continuous amphetamine.

Chronic cocaine administration alters corticotropin-releasing factor receptors in the rat brain

Two groups of 12 rats received daily injections of cocaine (20 mg/kg, i.p.) or saline for 15 days following bilateral infusions of 6-hydroxydopamine or vehicle into the lateral ventricles. Cocaine administration resulted in significant decreases in CRF receptor labeling primarily in brain areas associated with the mesolimbic/mesocortical dopaminergic system. All of the cocaine-induced changes in CRF binding were attenuated in the lesioned animals, suggesting that these effects may be mediated, in part, through the actions of the drug on dopaminergic neuronal activity. Dopamine may also be involved in the release of CRF in the rat brain since the purported loss of dopaminergic innervations resulted in increased CRF binding in the saline-treated animals.