Reverse tolerance to stimulant-induced abnormal behavior

Kava (Piper methysticum) in the United States: the quiet rise of a substance with often subtle effects

Background: Piper methysticum, commonly called kava, has long been consumed in beverage form in the Pacific Islands. Kava use in the US has slowly increased since the 1990s, but is not assessed in major epidemiological surveys.Objectives: To analyze social-media posts about kava from current, past, and prospective users, for motivations, patterns of co-use, and effects.Methods: Text from Reddit posts, and accompanying metadata, were collected and thematically coded by two independent raters.Results: 423 posts were collected, spanning January 2006 through December 2021. Of the 1,211 thematic codes applied, 1,098 (90. 7%) were concordant. Motivations for use bifurcated into self-treatment (for psychiatric or physical health conditions) and recreation; these were not mutually exclusive. Kava was rarely considered strongly euphoriant, but was valued as an anxiolytic. Kava was frequently used with other substances, most commonly kratom. Kava was used at lower doses for self-treatment than for other purposes (pseudo-R² = 0.11). Undesirable effects (gastrointestinal upset, fatigue) were mentioned, though less often than benefits. Hepatotoxicity, reported elsewhere as a rare, non-dose-related risk, was disputed on the basis of its not having been experienced by those posting.Conclusion: Kava appears to be conceptualized among Reddit posters as an anxiolytic with few risks or adverse effects. As it grows in popularity, especially among people who use other drugs that are more liable to misuse or addiction, it should be assessed in probability samples (i.e. in the major national drug surveys) and clinical practice for its risks, potential benefits, and possible drug-drug interactions.

Effects of repeated RU 24969 treatment on the locomotor activity, motoric capacity, and axillary temperatures of male and female preweanling rats

Serotonin (5-HT) _1A and _1B receptors have been implicated in behavioral sensitization, but adult rats appear to develop tolerance to RU 24969 (a 5-HT_1A/1B receptor agonist) rather than a sensitized response. The purpose of the present study was to determine whether a one- or four-day pretreatment regimen of RU 24969 would cause sensitization or tolerance in male and female preweanling rats. Depending on experiment, rats were pretreated with RU 24969 (0, 2.5, or 5 mg/kg) for 1 or 4 days (PD 17-20), while testing with lower or higher doses of RU 24969 occurred on PD 22. Locomotor activity, motoric capacity, and axillary temperatures were recorded. The role of Pavlovian contextual conditioning was assessed by administering RU 24969 to rats in either the home cage or a novel environment. On the first pretreatment day, RU 24969 caused both an increase in forward locomotion and motoric impairment, along with a substantial decrease in axillary temperatures. Repeated treatment with the same dose of RU 24969 caused all three dependent measures to show a tolerance response. When given a higher dose of RU 24969 on the test day, the responses lost due to repeated drug treatment were fully (locomotor activity) or partially (motoric capacity and axillary temperatures) reinstated. There was no evidence of behavioral tolerance. Results are consistent with the hypothesis that a subsensitivity of 5-HT_1A/1B receptors is at least partially responsible for the tolerance caused by RU 24969, but dispositional tolerance cannot be excluded as a contributing factor.

Bipolar Disorder: Its Etiology and How to Model in Rodents

Characterized by the switch of manic and depressive phases, bipolar disorder was described as early as the fifth century BC. Nevertheless up to date, the underlying neurobiology is still largely unclear, assuming a multifactor genesis with both biological-genetic and psychosocial factors. Significant process has been achieved in recent years in researching the causes of bipolar disorder with modern molecular biological (e.g., genetic and epigenetic studies) and imaging techniques (e.g., positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)). In this chapter we will first summarize our recent knowledge on the etiology of bipolar disorder. We then discuss how several factors observed to contribute to bipolar disorder in human patients can be manipulated to generate rodent models for bipolar disorder. Finally, we will give an overview on behavioral test that can be used to assess bipolar-disorder-like behavior in rodents.

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

Adolescent exposure to cocaine, amphetamine, and methylphenidate cross-sensitizes adults to methamphetamine with drug- and sex-specific effects

The increasing availability, over-prescription, and misuse and abuse of ADHD psychostimulant medications in adolescent populations necessitates studies investigating the long-term effects of these drugs persisting into adulthood. Male and female C57Bl/6J mice were exposed to amphetamine (AMPH) (1.0 and 10 mg/kg), methylphenidate (MPD) (1.0 and 10 mg/kg), or cocaine (COC) (5.0 mg/kg) from postnatal day 22 to 31, which represents an early adolescent period. After an extended period of drug abstinence, adult mice were challenged with a subacute methamphetamine (METH) dose (0.5 mg/kg), to test the long-term effects of adolescent drug exposures on behavioral cross-sensitization using an open field chamber. There were no sex- or dose-specific effects on motor activity in adolescent, saline-treated controls. However, AMPH, MPD, and COC adolescent exposures induced cross-sensitization to a subacute METH dose in adulthood, which is a hallmark of addiction and a marker of long-lasting plastic changes in the brain. Of additional clinical importance, AMPH-exposed male mice demonstrated increased cross-sensitization to METH in contrast to the female-specific response observed in MPD-treated animals. There were no sex-specific effects after adolescent COC exposures. This study demonstrates differential drug, dose, and sex-specific alterations induced by early adolescent psychostimulant exposure, which leads to behavioral alterations that persist into adulthood.

Caffeine induces behavioural sensitization and overexpression of cocaine-regulated and amphetamine-regulated transcript peptides in mice

This study examined whether repeated administration of caffeine would induce behavioural sensitization and overexpression of cocaine-regulated and amphetamine-regulated transcript (CART) peptides in mice. The involvement of dopaminergic receptors and adenosine receptors in caffeine-induced behavioural sensitization and CART overexpression was studied. The relevance of D₁R and D₂R, and A₁R and A(2A)R in the overexpression of CART peptides in mouse striatum was also evaluated. Repeated administration of caffeine induced behavioural sensitization in mice. Significant increases in CART mRNA levels were observed on day 3 and peaked at day 5 of caffeine administration, and then decreased gradually. Higher proportions of CART⁺ cells were observed in the dorsolateral and ventrolateral part of the caudate putamen than in the nucleus accumbens shell and core. The behavioural sensitization induced by caffeine was inhibited by dopaminergic receptor antagonists and adenosine receptor agonists. D₁R and D₂R, and cyclic AMP (cAMP)/protein kinase A (PKA)/phospho-cAMP response element-binding protein (pCREB) signalling were activated by caffeine, but A₁R and A(2A)R were inhibited. Overexpression of caffeine-induced CART peptides and pCREB activity were blocked by N-cyclopentyladenosine (CPA, an A₁R agonist) and 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680, an A(2A)R agonist), but not by R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390, a D₁R antagonist) or raclopride (a D₂R antagonist). Caffeine-induced overexpression of CART peptides was associated with the inhibition of A₁R and A(2A)R, and the activation of cAMP/PKA/pCREB signalling. Moreover, the A(2A)R-D₂R heterodimer might be involved in the overexpression of CART peptides induced by caffeine.

Acute and chronic effects of cocaine on the spontaneous behavior of pigeons

The present experiment examined the effects of acute and daily cocaine on spontaneous behavior patterns of pigeons. After determining the acute effects of a range of doses, 9 pigeons were divided into three groups that received one of three doses of cocaine daily, either 1.0, 3.0, or 10.0 mg/kg cocaine. Measures were taken of spontaneous locomotion, pecking, preening, and emesis. Under daily administration, cocaine induced consistent and substantial enhancements of its locomotor effects in all 9 pigeons, consistent with the phenomenon of locomotor sensitization. The maximum locomotor output did not differ according to the size of the daily dose. Locomotion was not elevated following tests of the saline vehicle, suggesting the effect was due to cocaine, not to a change in baseline or reactivity to the injection procedure. Cocaine dose-dependently decreased preening when given acutely, and those effects were not altered by repeated cocaine administration. Pecking occurred at very low rates and was unresponsive to cocaine treatment. Cocaine-induced emesis showed a dose-dependent increase under initial tests with cocaine, and those effects were attenuated following daily exposure. In a final condition, cocaine was replaced with daily saline for 30 days to assess the persistence cocaine-related increases in locomotion. Approximately half of the pigeons continued to show enhanced effects even after 30 days without cocaine, so although persistence was obtained, it showed marked intersubject variability. The data indicate that the effects of repeated cocaine administration on the behavior of pigeons shows parallels with many effects commonly reported with rodents (i.e., increased locomotion following repeated treatment, decrease in preening or grooming, persistence following drug withdrawal).

Neonatal status epilepticus alters prefrontal-striatal circuitry and enhances methamphetamine-induced behavioral sensitization in adolescence

Neonatal seizures may alter the developing neurocircuitry and cause behavioral abnormalities in adulthood. We found that rats previously subjected to lithium-pilocarpine (LiPC)-induced neonatal status epilepticus (NeoSE) exhibited enhanced behavioral sensitization to methamphetamine (MA) in adolescence. Neurochemically, dopamine (DA) and metabolites were markedly decreased in prefrontal cortex (PFC) and insignificantly changed in striatum by NeoSE, but were increased in both PFC and striatum by NeoSE+MA. Glutamate levels were increased in both PFC and striatum in the NeoSE+MA group. DA turnover, an index of utilization and activity, was increased by NeoSE but reversed by MA in PFC. Gene expression of the regulator of G-protein signaling 4 (RGS4) was downregulated in PFC and striatum by NeoSE and further suppressed by MA. These findings suggest NeoSE affects both dopaminergic and glutamatergic systems in the prefrontal-striatal circuitry that manifests as enhanced behavioral sensitization to MA in adolescence.

Stress and Rodent Models of Drug Addiction: Role of VTA-Accumbens-PFC-Amygdala Circuit

Stress can trigger, intensify, and prolong drug consumption, as well as reinstate previously extinguished drug-taking behavior by directly impacting a neural circuit often referred to as a reward pathways. Animal models of drug abuse have been used to understand these neural circuits mediating stress-induced drug intake and relapse through examination of cellular and subcellular molecular mechanisms. Several types of intermittent stressors have been shown to induce cross-sensitization to psychomotor stimulants, enhance conditioned place preference under most conditions, increase self-administration of cocaine and amphetamine and induce reinstatement of heroin and cocaine seeking via activation of the mesocorticolimbic dopamine system.

Bimodal effect of amphetamine on motor behaviors in C57BL/6 mice

Amphetamine-induced motor behaviors, i.e., locomotor and stereotypic activities, were simultaneously characterized in C57BL/6 mice, a strain commonly used for genetic studies. Our findings show relatively high levels of focused activities in drug-naive C57BL/6 mice, confirming the lively nature of this mouse strain. Acute amphetamine induced a dose-dependent, bimodal response: locomotion predominated at lower doses of amphetamine and was gradually displaced by stereotypic behavior as dose and time increased. The sum total of both behavioral activities increased with amphetamine dose, supporting the notion that amphetamine-induced locomotion and stereotypy form a continuum. These data provide a basis for using C57BL/6 mice as a strain to study the molecular and cellular mechanisms underlying psychostimulant effects, drug addiction and psychotic disorders.

Clorgyline-induced modification of behavioral sensitization to quinpirole: effects on local cerebral glucose utilization

Sensitization refers to augmented behavioral responses produced by repeated, intermittent injections of dopaminergic psychostimulants. The locomotor manifestations observed after a sensitizing course of quinpirole, a D(2)/D(3) dopamine agonist, can be modified by the MAO(A) inhibitor clorgyline, by a mechanism apparently unrelated to its actions on MAO(A). Alterations in regional neuronal activity produced by quinpirole in quinpirole-sensitized rats with or without clorgyline pretreatment were assessed based on LCGU using the [(14)C]-2-deoxyglucose (2-DG) method. Adult, male Long-Evans rats (180-200 g, n=9-10/group) were subjected to an injection of either clorgyline (1.0 mg/kg, s.c.) or saline 90 min prior to an injection of quinpirole (0.5 mg/kg, s.c.) or saline, 1 set of injections administered every 3rd day for 10 sets. The 2-DG procedure was initiated 60 min after an 11th set of injections in freely moving rats. LCGU was determined by quantitative autoradiography. LCGU was decreased in a number of limbic (nucleus accumbens and ventral pallidum) and cortical (medial/ventral orbital and infralimbic) regions and in the raphe magnus nucleus in quinpirole-sensitized rats (P<0.05 vs. saline-saline). Quinpirole-sensitized rats pretreated with clorgyline had similar alterations in LCGU, but LCGU was higher in the locus coeruleus compared to quinpirole alone (P<0.05), was not decreased in the raphe magnus nucleus, and was decreased in the piriform cortex and septum. This implicates altered activity of the noradrenergic, serotonergic, olfactory, and limbic systems in the modified behavioral response to quinpirole with clorgyline pretreatment.

Validation of a mechanical apparatus for the measurement of avian walking

Recent research has demonstrated that avian species may show an enhanced locomotor response to repeated drug exposure, a phenomenon called sensitization. Further research in this domain is warranted as such results not only establish the generality of previous findings, but may lend to a better understanding of sensitization in general. At present, there are no well-studied measurement devices for automating the measurement of bird movement. The present research evaluated a mechanical device comprised of moveable floor panels to assess its suitability. Evaluations of the device were made over the course of repeated cocaine administration. The results showed that there was high correspondence between the apparatus' responses and human observers' responses to movement by pigeons. The apparatus, furthermore, provided useful data on spatial orientation that revealed individual differences in the response to cocaine beyond those available from a collapsed measure of overall output. In sum, this recording strategy appears a viable instrument for the measurement of avian locomotion.

Rapid and persistent sensitization to the reinforcing effects of cocaine

The development of drug addiction involves a transition from recreational use to compulsive drug seeking and taking, and this progression can occur rapidly with cocaine use. These data highlight the importance of early drug exposure and the development of drug dependence; however, little experimental attention has been paid to this phenomenon in animal models of drug abuse. The present experiments demonstrate a progressive and rapid sensitization to the reinforcing strength of cocaine assessed using a progressive ratio (PR) schedule in rats. The first experiment found that rats show increased breakpoints over a 2-week period following acquisition. Subsequent experiments examined the role of total cocaine intake during the initial exposure period and found that low intakes (20 mg/kg/day x 5 days) resulted in sensitization, whereas relatively higher intake (60 or 100 mg/kg/day x 5 days) suppressed the development of sensitization. In contrast, this higher level of intake (60 mg/kg/day x 5 days) only transiently suppressed the expression of sensitization. Examination of breakpoints maintained by various doses of cocaine revealed an upward and leftward displacement of the cocaine dose-effect curve, relative to nonsensitized animals. These studies describe a form of sensitization that occurs rapidly to the reinforcing effects of cocaine, and provide a model to study the potential impact of initial experience on the development of drug dependence.

Tolerance and sensitization to the locomotor-activating effects of cocaine are mediated via independent mechanisms

Tolerance or sensitization to the locomotor-activating effects of cocaine occurs depending upon the treatment regimen that is used. When cocaine is injected on a daily basis, sensitization occurs, whereas continuously infused cocaine leads to tolerance. Male Sprague-Dawley rats were treated for 7 days with continuous cocaine (50 mg/kg/day) via subcutaneously implanted osmotic minipumps, after which the pumps were removed. Locomotor activity was measured for 1 h each day. Some rats were challenged with an injection of cocaine (7.5, 15 or 30 mg/kg) either 2 or 9 days after pump removal. Two days after the pumps were removed (Day 10), there were no significant differences between cocaine- or saline-treated rats in the amount of locomotor activity produced by the challenge injections. However, cocaine-treated rats challenged with cocaine 9 days after pumps were removed (Day 17) exhibited significant tolerance, as evidenced by a shift downward of the cocaine curve, as compared to saline controls. When the rats were injected again on the next day (Day 18), the activity levels of both groups increased, as compared to the effects observed on Day 17. Thus, although the cocaine-treated rats were still tolerant compared to the saline-treated rats, they were sensitized compared to their previous response to a challenge injection. These findings indicate that tolerance and sensitization to the locomotor-activating effects of cocaine can exist simultaneously, which suggests that they are mediated by separate mechanisms.

Inhibition of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity as measured in mice, and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of the GABA(A) agonist, muscimol (0.3, 0.5 and 1.0 mg kg(-1) i.p.). However, daily repeated administration of morphine caused the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.). The administration of muscimol inhibited the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.) in mice that had received chronic administration of morphine. Postsynaptic dopamine receptor supersensitivity, as shown by the enhanced ambulatory activity after administration of apomorphine (2 mg kg(-1) s.c.), also developed in reverse-tolerant mice. Muscimol also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited via the activation of GABA(A) receptors.

Behavioral sensitization is greater after repeated versus single chronic cocaine dosing regimens

Cocaine dosing regimens in animals are used to model behavioral and neurochemical changes in human cocaine abusers. Typically, rats are dosed for 5-14 days and assessed at some point during withdrawal. However, human cocaine bingers undergo multiple periods of several days of abuse. Here, we model the human binge pattern by giving rats two separate cocaine dosing regimens which results in greater behavioral sensitization than a single cocaine dosing regimen. This model also allows for the testing of drugs in reversal of a previously established sensitization. Multiple cocaine regimens may thus provide a better model for the human condition.

GABA(B) receptor gene expression is not altered in cocaine-sensitized rats

Recent behavioral and neurobiological data indicate that GABA(B) receptor transmission is involved in cocaine-induced hyperactivity and reinforcement. GABA(B) receptor gene expression in cocaine-sensitized rats was examined in this study. Rats were injected with cocaine (15 mg/kg, daily, i.p.) or saline for 5 consecutive days, and challenged with the same dose of cocaine after a 1-, or 20-day hiatus. The locomotor activities of rats were recorded after challenge, and the rats were killed 24 hr later. GABA(B)R1a, 1b, and GABA(B)R2 mRNA in discrete brain regions was detected by RPA and In Situ Hybridization; GABA(B)R1a protein was measured by Western blotting. Rats pretreated with cocaine developed a hyperactivity to the cocaine challenge after a 1-day or 20-day hiatus, but GABA(B)R subunit mRNA and GABA(B)R1a protein densities in the targeted regions showed no significant difference compared to those in control rats. These data indicate that GABA(B) receptor gene expression is not necessarily relevant to the behavioral sensitization of cocaine.

Inhibition of THIP on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of tetrahydroisoxazolopyridine (THIP), a GABAA agonist, on the morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity in mice. However, the morphine-induced hyperactivity was inhibited dose-dependently by the administration of THIP (0.2, 0.4 and 0.8 mg/kg, i.p.). In contrast, daily administration of morphine resulted in a reverse tolerance to the hyperactivity caused by morphine (10 mg/kg, s.c.). THIP inhibited the development of reverse tolerance in the mice that had received the repeated same morphine (10 mg/kg, s.c.) doses. The postsynaptic dopamine receptor super-sensitivity, which was evidenced by the enhanced ambulatory activity after the administration of apomorphine (2 mg/kg, s.c.), also developed in the reverse tolerant mice. THIP also inhibited the development of the postsynaptic dopamine receptor supersensitivity induced by the chronic morphine administration. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited activating the GABAA receptors.

Inhibition of diazepam on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of diazepam on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of diazepam (1, 2 and 4 mg x kg(-1), i.p.), an agonist for benzodiazepine receptor linked to the GABA(A) receptor. Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of diazepam inhibited the morphine-induced hyperactivity and the diazepam administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg x kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg x kg(-1), s.c.). Diazepam also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of the GABA(A) receptor induced by diazepam.

Differential behavioral responses to cocaine are associated with dynamics of mesolimbic dopamine proteins in Lewis and Fischer 344 rats

Differential behavioral and biochemical responses to drugs of abuse may reflect genetic makeup as suggested by studies of inbred Lewis (LEW) and Fischer 344 (F344) rats. We investigated locomotor activity, stereotypy signs, and levels of specific proteins in the nucleus accumbens (NAc) and ventral tegmental area (VTA) in these strains at baseline and following chronic administration of cocaine (30 mg/kg/day for 14 days). Using Western blot analysis, we replicated our previous findings of baseline strain differences and found lower levels of DeltaFosB immunoreactivity in NAc of F344 vs. LEW rats. F344 rats showed greater baseline locomotor activity, sniffing, and grooming compared to LEW rats. Chronic cocaine increased DeltaFosB levels in NAc in both strains, whereas adaptations in other proteins were induced in F344 rats only. These included reduced levels of tyrosine hydroxylase (TH) in NAc and increased TH and glial fibrillary acidic protein (GFAP) immunoreactivity in VTA. Chronic cocaine led to greater increases in overall stereotypy in F344 vs. LEW rats and decreased exploratory behaviors in LEW rats. Opposing effects by strain were seen in locomotor activity. Whereas F344 rats showed higher initial activity levels that decreased with cocaine exposure (tolerance), LEW rats showed increased activity over days (sensitization) with no strain differences seen at 14 days. Further, conditioned locomotor activation to vehicle injections was greater in F344 vs. LEW rats. These results suggest that behavioral responsiveness to chronic cocaine exposure may reflect dynamics of mesolimbic dopamine protein levels and demonstrate the role of genetic background in responsiveness to cocaine.

Inhibition of Baclofen on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of baclofen on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose dependently by the administration of a GABA(B)receptor agonist, baclofen (1.25, 2.5 and 5 mg kg(-1), i.p.). Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of baclofen inhibited the morphine-induced hyperactivity and the baclofen administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse-tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg kg(-1), s.c.). Baclofen also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of GABA(B)receptors induced by baclofen.

The effect of reserpine on concurrent repeated administration of d-amphetamine

The augmentation of the rat stereotypy response with repeated amphetamine doses, put forward as a model of human amphetamine psychosis, was examined during concurrent reserpinization. The effects of reserpinization, amphetamine treatment, and amphetamine dose on four dependent variables representing the time course of stereotypy ratings after a post-treatment amphetamine dose, were tested by three-way MANOVA. An earlier onset of stereotypy , as occurred in nonreserpinized rats, was not detected in reserpinized rats, but an earlier offset of stereotypy with repeated amphetamine occurred in these rats when high amphetamine doses were used.

Transient amelioration of the sensitization of cocaine-induced behaviors in rats by the induction of tolerance

Intermittent administration of cocaine produced a progressive increase in the stereotypy response of rats to a challenge dose of cocaine (7.5 mg/kg, i.p.). Continuous infusion of cocaine (80 mg/kg per day) via osmotic pumps for 7 days into the sensitized rats produced tolerance to the behavioral responses to the challenge dose of cocaine 1 day after the removal of the pumps. Therefore, tolerance can mask the expression of behavioral sensitization in rats. However, by 10 days after the removal of the pumps, the behavioral tolerance was reversed and the rats again displayed a sensitized response to cocaine. Therefore, the tolerance to cocaine was temporary while the underlying sensitization persisted. The development of tolerance did not alter the underlying sensitization demonstrating that these represent independent phenomena. The relationship between sensitization and tolerance observed in these studies may provide a model relevant to the progress in humans of addiction to psychomotor stimulants.

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.

Influence of novel and habituated testing conditions on cocaine sensitization

Female Swiss-Webster mice were treated daily for 10 days with cocaine (15 mg/kg i.p.) followed by 10 days with saline or ethanol (1.6 g/kg i.p.) or the reverse; following each injection in the experimental conditions locomotion was quantified in photocell cages. In animals given cocaine first, cocaine-induced locomotion was initially high and did not increase further with successive injections. In animals given prior saline or ethanol treatments, cocaine-induced locomotion was initially low but increased with successive cocaine treatments. There was no evidence of sensitization to the locomotor-stimulating effects of ethanol or of cross-sensitization between ethanol and cocaine. With respect to subsequent cocaine sensitization, the essential feature of prior saline or ethanol treatment appeared to be the handling and injection experience itself; a control group receiving prior saline injection in the home cage also showed a low level of cocaine-induced locomotion on the first day of cocaine testing but increasing locomotion with repeated cocaine testing. Thus, cocaine sensitization, rather than a progressive augmentation of motor function, may reflect a progressive reversal of the behavioral suppression caused by habituation to aspects of the testing situation or to some form of situational anxiety that precludes normal exploratory responses.

Dose-dependent characterization of the rewarding and stimulant properties of cocaine following intraperitoneal and intravenous administration in rats

Dose-dependent differences in the rewarding and stimulant properties of cocaine administered intravenously (IV) and intraperitoneally (IP) were compared. Six 2-day conditioning trials were conducted over consecutive days. Rats received cocaine and were placed into a compartment on one day of the trial, and were directly placed into a different compartment without drug on the other day. Rats were exposed to the compartments for either 20 or 40 min. The effects of cocaine on stimulant behaviors, including locomotion and stereotypies, were compared following the first and last injection. After conditioning, three tests were given with 1 rest day intervening each: (1) conditioned place preference (CPP) was measured as an increase in the amount of time animals spent in the injection compartment relative to the noninjection compartment when given access to both, (2) conditioned activity (CA) was measured as an increase in stimulant behaviors in cocaine-treated animals relative to saline controls following an injection of saline in the injection compartment and (3) context-independent sensitization was measured as an increase in stimulant behaviors following an injection of cocaine in the noninjection compartment relative to the animals' behavior following the first injection. Cocaine did not reliably produce sensitization of locomotion under any of the conditions examined. Cocaine produced sensitization of headbobbing that was more robust following IP administration than it was following IV administration. In both cases, sensitization of headbobbing involved a context-independent component. Cocaine produced CPP and CA with both routes of administration. CPP was established more readily with 40-min relative to 20-min exposures following IV administration, whereas CA was more prevalent with 20-min relative to 40-min exposures. This study provides a thorough characterization of the behavioral effects of cocaine administered IV and a new efficient method for assessing the effects of cocaine on conditioned and unconditioned behaviors following repeated administration.

Inhibition by ginseng total saponin of the development of morphine reverse tolerance and dopamine receptor supersensitivity in mice

1. Ginseng total saponin (GTS), 200 mg/kg i.p. 3 hr prior to morphine, inhibited the development of reverse tolerance to the ambulatory-accelerating effect of morphine. 2. GTS, 200 mg/kg, also prevented the development of dopamine receptor supersensitivity induced by the chronic administration of morphine, 10 mg/kg a day for 7 days. 3. These results suggest that GTS may be useful for the prevention and therapy of the adverse action of morphine.

Blockade by ginseng total saponin of the development of cocaine induced reverse tolerance and dopamine receptor supersensitivity in mice

Daily repeated administration of cocaine (15 mg/kg, over a 7-day period) developed reverse tolerance to the ambulation-accelerating effect of cocaine. Intraperitoneal administration of ginseng total saponin (GTS, 100 and 200 mg/kg of body weight) prior to and during chronic administration of cocaine inhibited the development of reverse tolerance. Dopamine receptor supersensitivity was also developed in reverse tolerant mice that had received the same cocaine. The development of dopamine receptor supersensitivity was evidenced by the enhanced hypothermic response to apomorphine (1 mg/kg) and the enhanced ambulatory activity of apomorphine (4 mg/kg). GTS also prevented the development of dopamine receptor supersensitivity induced by the chronic administration of cocaine. These results provide that GTS may be useful for the prevention and therapy of the adverse action of cocaine. It is concluded that the development of reverse tolerance to the ambulation-accelerating effect of cocaine may be associated with the enhanced dopamine receptor sensitivity because both phenomena were blocked by GTS.

Withdrawal from chronic amphetamine elevates baseline intracranial self-stimulation thresholds

Intracranial self-stimulation was assessed before, within, and after a chronic amphetamine treatment regimen. Amphetamine was given twice daily 5 days per week for 6 weeks at dosages escalating from 1 to 10 mg/kg per injection. Lateral hypothalamic self-stimulation rate-frequency functions were taken 36 h after the last injection in each weekly series and weekly for 3 weeks following the last injection. Frequency thresholds increased and maximal response rates decreased progressively as a function of amphetamine withdrawal during treatment; each returned to near normal levels within 2 weeks of the last injection. When subsequently tested under amphetamine, animals previously receiving the 6-week amphetamine treatment regimen had self-stimulation thresholds and maximal response rates that did not differ significantly from those of saline-treated control animals. These data confirm that chronic amphetamine treatment results in a dependence syndrome characterized in part by a phasic depression in the brain mechanism mediating the reinforcing effects of lateral hypothalamic electrical stimulation.

Disposition of cocaine in blood and brain after a single pretreatment

In vivo microdialysis studies reveal that repeated cocaine administration will enhance the extracellular level of dopamine that occurs following subsequent challenge injections of cocaine. Not only are dopamine levels enhanced, but the in vivo concentration of cocaine in the brain and blood is also increased as a result of repeated cocaine exposure. Increases in cocaine levels can, in part, account for the behavioral sensitization that occurs as a result of repeated cocaine treatment. The present study reveals that enhanced cocaine concentrations can be observed after a single exposure to cocaine and that enhanced levels are not observed when the challenge injection of cocaine is given by an intravenous as opposed to an intraperitoneal injection. Enhanced cocaine levels were also not due to a decrease in the rate of cocaine metabolism that leads to the formation of benzoylecgonine. Repeated cocaine exposure produces effects that enhance in vivo levels of cocaine and these effects occur outside of blood and brain environments. Pharmacokinetic effects play a major role in the development of cocaine-induced behavioral sensitization.

Cocaine-induced rausch: overt behaviour and plasma concentrations in rhesus monkeys

This study was designed to characterize the cocaine-induced rausch or hyperarousal syndrome in rhesus monkeys. This syndrome mimics the stage observed in human abusers bingeing on cocaine and is considered crucial in the progression from recreational use to compulsive abuse. However, little research has focused on this important aspect of cocaine use. Cocaine was administered i.v. at doses of 0.0, 0.5, 1.0 and 2.0 mg/kg. Plasma concentrations were determined by gas chromatograph mass spectrometry (GC/MS) using deuterated internal standards d3 cocaine and d3 benzoylecgonine (BE). Mean plasma concentrations of cocaine, were on samples collected 1 min after infusion, 46 +/- 31, 88 +/- 15 and 275 +/- 116 mg/microliters in the 0.5, 1.0 and 2.0 mg/kg dose groups, respectively. There were no detectable concentrations of BE in any of the specimens nor was cocaine detected in the saline controls. Analysis of the behavioural data revealed that the 0.5 and 1.0 mg/kg results were intermediate between the results obtained at doses of 0.0 and 2.0 mg/kg and that the 1.0 mg/kg dose produced a higher response than the 0.5 mg/kg dose up to the 12 min. Regarding individual behavioural signs, those designated escape attempts, checking, feinting, restlessness, searching, vocalizing, chewing, crouching and wide-eyed were noted most frequently. The results showed dose-response relationships for both plasma concentrations of cocaine and for the total number of overt behavioural signs. The plasma concentrations were in the range reported for human cocaine abusers.

Dopamine antagonists can inhibit methamphetamine sensitization, but not cocaine sensitization, when assessed by ambulatory activity in mice

The repeated subcutaneous administration of methamphetamine (2 mg kg-1) and cocaine (10 mg kg-1) at 3-4 day intervals induced sensitization to their ambulation-increasing effects in mice. Subcutaneous administration of SCH 23390 (R-(+)-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; 0.003-0.03 mg kg-1) and YM-09151-2 (cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4- methylaminobenzamide; 0.003-0.03 mg kg-1), the selective dopamine D1 and D2 antagonists, respectively, reduced dose-dependently the acute ambulation-increasing effect of methamphetamine. The development of methamphetamine sensitization was inhibited when it was administered in combination with either SCH 23390 or YM-09151-2 in the repeated administration schedule. Although SCH 23390 (0.01-0.1 mg kg-1) and YM-09151-2 (0.01-0.1 mg kg-1) also reduced the ambulation-increasing effect of cocaine (10 mg kg-1), neither drug inhibited the cocaine sensitization. Mice given cocaine with SCH 23390 (0.03 mg kg-1) or YM-09151-2 (0.03 and 0.1 mg kg-1) showed higher sensitivity than those given cocaine alone. The present results suggest that, although both the dopamine D1 and D2 antagonists reduce the acute stimulant effects of both methamphetamine and cocaine, they are only effective for inhibition of the methamphetamine sensitization. Mechanisms other than the dopaminergic system appear to be involved in the cocaine sensitization.

Serotonin and dopamine sensitization in the nucleus accumbens, ventral tegmental area, and dorsal raphe nucleus following repeated cocaine administration

The present study was designed to examine the effects of chronic cocaine administration on the extracellular response of serotonin (5-HT) and dopamine (DA) to a peripheral cocaine injection using in vivo brain microdialysis in awake rats. Two different dual probe preparations were used: One group of animals had guide cannulae aimed at the ventral tegmental area (VTA) and nucleus accumbens (N ACC) and a second group of animals had guide cannulae aimed at the dorsal raphe nucleus (DRN) and N ACC. Rats from both groups were given daily injections of either cocaine (20 mg/kg i.p.) or saline (0.9%; 0.05 ml/kg i.p.) for 10 consecutive days. On day 11, baseline dialysate levels of DA, 5-HT, dihydroxyphenylacetic acid, and 5-hydroxyindoleacetic acid were obtained from either the N ACC and VTA or the N ACC and DRN, followed by a 10 mg/kg i.p. cocaine injection and an additional 150 min of dialysate sampling. The percent baseline increases of both 5-HT and DA were significantly higher in the N ACC, VTA, and DRN of animals that received daily injections of cocaine compared with saline controls (p < 0.05, in each region). Maximum dialysate 5-HT concentrations after cocaine challenge were significantly higher in the N ACC and VTA (p < 0.05) and DRN (p < 0.01) of chronically treated animals compared with saline controls. Maximum dialysate DA concentrations were significantly higher in the N ACC and DRN (p < 0.05) of chronically treated animals compared with saline controls. There was no significant difference between acute and chronic animals in the maximum dialysate DA concentration from the VTA after cocaine challenge.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug- and behavior-associated changes in dopamine-related electrochemical signals during intravenous heroin self-administration in rats

High-speed chronoamperometry was used to monitor dopamine-related electrochemical signals in the nucleus accumbens of rats allowed to self-administer heroin intravenously and rats that received similar injections passively. Rats self-administered 100 micrograms/kg of heroin at approximately 20-min intervals. Dopamine-related electrochemical signals increased monotonically after the first injection of each day; the effect was weaker on the first than on the second and subsequent days. The second and subsequent injections in each session caused biphasic effects: the initial effect was a decrease in signal--a minor one when compared to the increase caused by the first injection--and this was followed by an increase that brought the signal back to or somewhat higher than the level at the time of the injection. Over the course of each 4-h session, the electrochemical signal reached and fluctuated around an elevated plateau; doubling the injection dose did not elevate this plateau but did cause larger phasic decreases and subsequent increases. Qualitatively similar electrochemical changes were seen in the animals passively receiving the drug, but there were two notable quantitative differences. First, in the passive animals the initial depressions in signal were of shorter duration. Second, in the passive animals (which were injected at intervals determined by the self-administering animals) the electrochemical signal reached a maximum and began to fall prior to the time of the next injection; in the animals that self-administered the drug, the signal was still rising at the time of the next injection. The changes in electrochemical signal are unlikely to represent fluctuations of ascorbate or dopamine metabolites; thus it appears that whereas self-administered heroin injections cause a slow and long-lasting elevation of extracellular dopamine concentration, short-term increases in dopamine concentration are associated with the behavioral activation that precedes the injections and it is short-term decreases that appear to be associated with the period usually thought to be most significant for positive reinforcement.

Suppressive effect of cycloheximide on behavioral sensitization to methamphetamine in mice

The effect of a protein synthesis inhibitor, cycloheximide, on behavioral sensitization to methamphetamine was investigated in mice. As indicated by the sensitization tests, repeated injection of methamphetamine (2 mg/kg i.p.) at intervals of 3 and 4 days resulted in a progressive augmentation of the locomotor-stimulating effect of methamphetamine. This phenomenon, called locomotor sensitization, was attenuated by simultaneous treatment with cycloheximide (120 mg/kg i.p.) at the time of stimulant injection. In contrast, when mice were treated with cycloheximide 4 h after stimulant injection, locomotor activity was progressively augmented in the same way as observed in mice receiving repeated injections of methamphetamine alone. On challenge, it was noted that locomotor activity was significantly higher in mice injected repeatedly with the stimulant alone and in those mice treated with the inhibitor 4 h after the stimulant injection compared to the saline-treated control mice. However, mice that had been simultaneously treated with cycloheximide and methamphetamine showed almost the same locomotor activity as the saline-treated control mice. These observations indicated that the locomotor sensitization to methamphetamine was possibly suppressed by simultaneous treatment with cycloheximide. We then examined the dose- and time-dependent nature of the effect of cycloheximide on locomotor sensitization. The stimulation of locomotion observed after repeated injection of the stimulant at a dose of 1.5 mg/kg was significantly attenuated by simultaneous treatment with 120 or 240 mg/kg of cycloheximide, but not by treatment with 60 mg/kg of the inhibitor. However, all the treatments failed to suppress the development of locomotor sensitization elicited by 3 mg/kg of methamphetamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of sensitization to ambulation-increasing effects of cocaine and methamphetamine after repeated administration in mice

The effects of repeated (5 times) subcutaneous administration of cocaine (10, 20 or 40 mg kg-1) and methamphetamine (1, 2 or 4 mg kg-1) at 3-4 day intervals have been compared in mice placed individually into tilting activity cages. A progressive enhancement of the ambulation-increasing effect was noted for 3-4 h after each administration, indicating that sensitization occurred. This occurrence and the existence of an optimal dose producing sensitization were similar for both drugs. However, enhancement of the effect after cocaine progressed rapidly and maximum sensitization was observed earlier than after methamphetamine administration. Moreover, the higher doses of cocaine (40 mg kg-1) caused stereotypies concurrent with preconvulsive signs of short duration that were enhanced by serial administration. In contrast, methamphetamine caused a more progressive enhancement, but stereotypies with no preconvulsive signs were produced by the higher dose (4 mg kg-1). The respectively, effective doses for the development of enhancement suggested that cocaine was less potent than methamphetamine in producing sensitization. Cross-sensitization occurred between both drugs. Thus, sensitization to cocaine was distinct from that to methamphetamine due to differences in its rapidity, intensity, and the presence or absence of preconvulsive changes.

Cocaine-induced locomotor activity in rats

Rats were injected SC or IP with a dose of cocaine at 20 mg/kg twice daily or saline (2 ml/kg) for 15 consecutive doses. Horizontal (including ambulatory and repetitive activity) and ambulatory locomotor activities were assessed following the first (acute) and the 15th (chronic) injections. Total locomotor activity (area under curve, AUC) following the acute and the chronic administration of cocaine were comparable, regardless of the route of drug administration. However, the temporal patterns of activity were significantly different; the peak of locomotor activity occurred earlier (chronic vs. acute, 20 vs. 40 min after IP; 130 vs. 180 min after SC) following chronic cocaine administration. Furthermore, the peak activity was significantly higher (3-fold after IP and 50% after SC) in chronically than in acutely treated rats, providing evidence for sensitization. In contrast, activity in the late session (240-280 min after SC) was significantly lower following the chronic SC cocaine administration, providing evidence for desensitization. The absolute slope values of the ascending phase and the descending phase were significantly larger following chronic administration of cocaine than that following the acute dosing. The possibility of changes in locomotor activity with alteration of pharmacokinetics on chronic cocaine treatment is discussed.

Cross-sensitization between cocaine and GBR 12909, a dopamine uptake inhibitor

In the present experiment, rats which had received repeated injections of 20 mg/kg GBR 12909, a selective dopamine uptake inhibitor, were subjected to challenge doses of GBR 12909 and cocaine which were near or below the threshold for locomotor activation in drug-naive animals. The behavior measured was general motor activity, monitored in photocell testing cages. Results verify earlier findings which showed that rats treated chronically with GBR 12909 display a potentiated locomotor response to low doses of the drug (behavioral sensitization). In addition, it was found that GBR 12909-sensitized rats show a potentiated locomotor response to 5 mg/kg cocaine (cross-sensitization). These results are discussed with regard to various mechanisms which may underlie the sensitization phenomena observed, including possible neurochemical changes at the presynaptic level.

Differences in the pharmacokinetics of cocaine in naive and cocaine-experienced rats

Enhanced cocaine concentrations in brain and blood observed after an intraperitoneal challenge dose in rats exposed to cocaine for 10 days by subcutaneous administration are traced to a change in the absorption process from the site of an intraperitoneal injection to general circulation. This conclusion is reached by three sets of corroborating results: (a) Adipose tissue of rats treated for 10 days with repeat subcutaneous injections of cocaine did not reveal a buildup of cocaine in sufficient concentrations to account for the twofold increase in brain and blood concentrations seen during intraperitoneal administration; (b) administration of the drug by an intravenous route after 10-day cocaine treatment did not show a significant difference between treatment and control groups; (c) nonlinear regression on the intravenous and intraperitoneal data sets using a two-compartment open model indicated a difference in the absorption process but not in the metabolic and blood-brain transfer processes.

Concomitant sensitization of amphetamine-induced behavioral stimulation and in vivo dopamine release from rat caudate nucleus

Rats were treated twice daily either with saline or d-amphetamine (5 mg/kg) for 5 days. When challenged approximately 15 days later with an injection of 0.5 mg/kg amphetamine, the chronic amphetamine animals showed (1) an augmented release of dopamine in the caudate nucleus in vivo and (2) an increase in stereotyped behavior compared to the chronic saline animals. These results suggest that an increase in dopamine release from the caudate may contribute to amphetamine-induced behavioral sensitization.

Extracellular concentrations of cocaine and dopamine are enhanced during chronic cocaine administration

Chronic cocaine administration produces significant increases in cocaine-induced locomotor activity and stereotypy. In vivo microdialysis procedures were used to monitor extracellular dopamine (DA) and cocaine concentrations in the nucleus accumbens (N ACC) and cocaine concentrations in plasma of animals that received chronic or acute cocaine treatments. Following a cocaine challenge injection, concentrations of both cocaine and DA increased to significantly higher levels over time in animals that had received daily cocaine injections for 10 or 30 days than in control animals that received daily injections of saline. Concentrations of cocaine and DA in the N ACC reached maximum levels in the first 30 min following a challenge injection of cocaine. The maximum cocaine concentrations of 10- and 30-day chronic animals were, respectively, 186% and 156%, whereas the maximum DA concentrations were 264% and 216% above the maximum values observed in acute control animals. The results indicate that reverse tolerance effects observed following chronic cocaine administration may in part be accounted for by increased cocaine concentrations. Furthermore, chronic cocaine administration (over a 10- or 30-day period) increased the concentration of cocaine detected in plasma above control levels following a challenge injection. The increase in brain concentrations of cocaine in chronic animals is apparently due to increased concentrations of cocaine in plasma. A physiological change occurs in the periphery as a result of chronic cocaine administration that increases cocaine concentrations in plasma, increases extracellular cocaine levels in the brain, and increases the extracellular concentration of DA in the N ACC.

Behavioral and biochemical effects of intra-accumbens dopaminergic grafts

6-Hydroxydopamine (6-OHDA) lesions in the ventral tegmental area (VTA) in rats prevented the hyperactivity response to methamphetamine in an open field. Transplantation of mesencephalic dopaminergic cells, obtained from rat embryos, into the nucleus accumbens (NAC) of 6-OHDA-lesioned animals restored the hyperactivity 4 weeks after grafting. By microdialysis of the NAC in freely moving rats no significant differences in baseline concentration of dopamine (DA) among the 3 groups (control, lesioned, grafted) were observed. However, after methamphetamine administration, DA increased significantly during the first 80 min in control animals, during the first 40 min in grafted animals, but did not increase in lesioned animals. On the other hand baseline concentrations of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) decreased to one sixth to one third of those of controls in 6-OHDA-lesioned animals, and they did not respond to methamphetamine. After grafting, however, DOPAC and HVA restored to control levels and responded to methamphetamine with decreases as was observed in control animals. Data suggest that grafts not only restore the ability to release DA but also improve DA metabolism in the NAC. This might be a reason for recovery of locomotor activity.

Effects of 24-hr fasting on methamphetamine- and apomorphine-induced locomotor activities, and on monoamine metabolism in mouse corpus striatum and nucleus accumbens

The effects of 24-hr fasting on the vertical (VMA) and horizontal (HMA) locomotor activities, on cage climbing activity and on brain monoamine-related substances, were examined using male ddY mice. Both the VMA and HMA increased with fasting, but not the cage climbing activity. Methamphetamine (2 mg/kg, SC) increased the VMA and HMA in both the feeding and fasting mice, whereas with apomorphine (0.1 mg/kg, SC) both decreased. Furthermore, pretreatment with haloperidol (0.025 mg/kg, SC) showed no influence on the methamphetamine-induced VMA increase in both the feeding and fasting mice. However, pretreatment with haloperidol inhibited the methamphetamine-induced HMA increase in both the feeding and fasting mice and showed a higher level of HMA in fasting mice than in feeding mice. When measuring brain monoamine-related substances, the DA, NE, 5-HT, and 5-HIAA levels in the corpus striatum increased, whereas the 3-MT levels decreased. The monoamine levels in the nucleus accumbens of fasting mice were the same as those in feeding mice, except for a decrease of the 3-MT level. These results suggest that the locomotor activity in fasting mice may be increased by a change in the sensitivity of dopaminergic neurons in the corpus striatum.

Subchronic cocaine treatment enhances cocaine-induced dopamine efflux, studied by in vivo intracerebral dialysis

Repeated administration of cocaine in animals results in behavioral sensitization. In order to investigate the neurochemical mechanism underlying such behavioral sensitization, we designed the following two experiments. In both experiments, rats were pretreated with cocaine (20 mg/kg i.p.) or saline, once daily for 14 consecutive days. Exp. 1: 7 days after withdrawal from the drug, the stereotyped behavioral response to a challenge of cocaine (20 mg/kg i.p.) was measured. Exp. 2: 7 days after withdrawal from the drug, we measured extracellular dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) after the challenge administration of cocaine using an in vivo intracerebral dialysis technique. The rats pretreated with cocaine (20 mg/kg i.p.) exhibited behavioral augmentation in response to a challenge of cocaine. The challenge administration of cocaine caused an increase in DA and a decrease in DOPAC. The DA level in the striatal perfusates of the cocaine-pretreated rats was significantly greater than that in the saline-pretreated rats. These results suggest that the increased extracellular DA concentration in the striatum plays an important role in the cocaine-induced behavioral sensitization.

Time-dependent changes in the sensitivity of dopamine neurons to low doses of apomorphine following amphetamine infusion

Changes in dopamine autoregulatory mechanisms following a chronic infusion of amphetamine were studied. Rats were infused with D-amphetamine (4.8 mg/day) for 7 days by osmotic minipumps, and were studied at various times after withdrawal. In contrast to no changes in the spontaneous firing rate of single dopamine cells, the potency of apomorphine in the substantia nigra was markedly decreased soon after withdrawal, followed by an increase 7 days later. The ventral tegmental area showed no changes in either spontaneous firing rate or sensitivity to apomorphine following a 7-day withdrawal. Moderate decreases in striatal and tubercle dopamine concentrations were not accompanied by any significant decrease in basal dopamine synthesis. Under intact impulse-flow, the sensitivity of terminal dopamine synthesis to low doses of apomorphine was decreased immediately following withdrawal; by Day 7 supersensitivity was observed. Direct assessment of terminal autoreceptors following a 7-day withdrawal revealed normo- and supersensitivity of these receptors in the striatum and olfactory tubercle, respectively. Possible mediating mechanisms as well as implications of these findings for the development of characteristic behavioral syndromes during and after amphetamine infusion are discussed.

Reduction of rat striatal thyrotropin-releasing hormone receptors produced by repeated methamphetamine administration

It has been reported previously that repeated, but not continuous, administration of methamphetamine (MAP) to animals produces progressive and sustained enhancement of MAP-induced behavior (behavioral sensitization), which may be related to functional changes in central dopamine (DA) systems. To investigate the possible involvement of thyrotropin-releasing hormone (TRH), a neuromodulator of DA, both immunoreactive TRH (IR-TRH) levels and specific TRH binding were examined in rat brain regions after MAP administration either repeatedly (4 mg/kg intraperitoneally once a day for 14 consecutive days) or continuously (about 4 mg/kg/day for 13 consecutive days). Although no significant changes were observed in IR-TRH levels in any regions of the brain following repeated MAP injections, specific TRH binding in the striatum significantly decreased. Scatchard analysis revealed that the decrease was due to a reduction in the maximum number of binding sites (Bmax). Pretreatment with haloperidol prior to each MAP injection prevented this decrease. Continuous MAP administration had no effect on regional specific TRH binding. These results suggest that repeated MAP administration caused lasting dysfunction in the brain TRH system, which may be implicated in the behavioral sensitization.