Cocaine-induced hypophagia and hyperlocomotion in rats are attenuated by prazosin

Evaluation of multitarget drugs on the expression of cocaine-induced locomotor sensitization in male rats: A comparative study

Purpose

- Cocaine use disorder (CUD) is a complex disease. Several studies have shown the efficacy of multitarget drugs used to treat CUD. Here we compare the efficacy of mirtazapine (MIR), pindolol (PIN), fluoxetine (FLX), risperidone (RIS), trazodone (TRZ), ziprasidone (ZPR), ondansetron (OND), yohimbine (YOH), or prazosin (PRZ), to reduce long-term cocaine-induced locomotor activity and the expression of cocaine-induced locomotor sensitization in rats.

Methods

- The study consists of four experiments, which were divided into four experimental phases. Induction (10 days), cocaine withdrawal (30 days), expression (10 days), and post-expression phase (10 days). Male Wistar rats were daily dosed with cocaine (10 mg/kg; i.p.) during the induction and post-expression phases. During drug withdrawal, the MIR, PIN, FLX, RIS, TRZ, ZPR, OND, YOH, or PRZ were administered 30 min before saline. In the expression, the multitarget drugs were administered 30 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min.During the agonism phase, in experiment four, 8-OH-DPAT, DOI, CP-809-101, SR-57227A, or clonidine (CLO) was administered 30 min before MIR and 60 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min.

Results

-MIR, FLX, RIS, ZPR, OND, or PRZ attenuated the cocaine-induced locomotor activity and cocaine locomotor sensitization. PIN, TRZ, and YOH failed to decrease cocaine locomotor sensitization. At the optimal doses used, PIN, FLX, RIS, TRZ, ZPR, OND, YOH, or PRZ failed to attenuate long-term cocaine locomotor activation. MIR generated a decrease in cocaine-induced locomotor activity of greater magnitude and duration than the other multitarget drugs evaluated.

Conclusion

- At the optimal doses of multitarget drugs evaluated, MIR was the multitarget drug that showed the greatest long-term cocaine-induced behavior effects compared to other multitarget drugs.

Noradrenergic circuits and signaling in substance use disorders

The central noradrenergic system innervates almost all regions of the brain and, as such, is well positioned to modulate many neural circuits implicated in behaviors and physiology underlying substance use disorders. Ample pharmacological evidence demonstrates that α1, α2, and β adrenergic receptors may serve as therapeutic targets to reduce drug -seeking behavior and drug withdrawal symptoms. Further, norepinephrine is a key modulator of the stress response, and stress has been heavily implicated in reinstatement of drug taking. In this review, we discuss recent advances in our understanding of noradrenergic circuitry and noradrenergic receptor signaling in the context of opioid, alcohol, and psychostimulant use disorders.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

The Alpha-1 Adrenergic Receptor Antagonist Prazosin Reduces Binge-Like Eating in Rats

Background: Binge-eating disorder is a pervasive addiction-like disorder that is defined by excessive and uncontrollable consumption of food within brief periods of time. The aim of the current study was to examine the role of the brain noradrenergic system in binge-like eating through the use of the alpha-1 adrenergic receptor antagonist prazosin. Methods: For this purpose, we employed a limited access model whereby male Wistar rats were allowed to nosepoke for either chow (Chow rats) or a sugary, highly palatable food (Palatable rats) for 1 h/day. The effects of prazosin (0, 0.5, 1 and 2 mg/kg, i.p.) were tested in a fixed ratio 1 (FR1) and progressive ratio (PR) schedule of reinforcement. Results: The results show that prazosin preferentially reduced the responses for palatable food in a FR1 reinforcement schedule; when tested in a PR schedule of reinforcement, prazosin increased breakpoint in both Chow and Palatable rats, but more potently and more efficaciously in the latter. Our results suggest that prazosin treatment preferentially increased the motivational properties of the palatable diet. Conclusions: The current findings provide the characterization of the effects of prazosin on binge-like eating and offer support to the existing literature showing the important role of the noradrenergic system in addiction-like behavior.

Phasic Stimulation of Midbrain Dopamine Neuron Activity Reduces Salt Consumption

Salt intake is an essential dietary requirement, but excessive consumption is implicated in hypertension and associated conditions. Little is known about the neural circuit mechanisms that control motivation to consume salt, although the midbrain dopamine system, which plays a key role in other reward-related behaviors, has been implicated. We, therefore, examined the effects on salt consumption of either optogenetic excitation or chemogenetic inhibition of ventral tegmental area (VTA) dopamine neurons in male mice. Strikingly, optogenetic excitation of dopamine neurons decreased salt intake in a rapid and reversible manner, despite a strong salt appetite. Importantly, optogenetic excitation was not aversive, did not induce hyperactivity, and did not alter salt concentration preferences in a need-free state. In addition, we found that chemogenetic inhibition of dopamine neurons had no effect on salt intake. Lastly, optogenetic excitation of dopamine neurons reduced consumption of sucrose following an overnight fast, suggesting a more general role of VTA dopamine neuron excitation in organizing motivated behaviors.

Caffeine Modulates Food Intake Depending on the Context That Gives Access to Food: Comparison With Dopamine Depletion

Caffeine is a methylxanthine consumed in different contexts to potentiate alertness and reduce fatigue. However, caffeine can induce anxiety at high doses. Caffeine is also a minor psychostimulant that seems to act as an appetite suppressant, but there are also reports indicating that it could stimulate appetite. Dopamine also is involved in food motivation and in behavioral activation. In the present series of experiments, we evaluated the effects of acute administration of caffeine on food consumption under different access conditions. CD1 male adult mice had access to highly palatable food (50% sucrose) in a restricted but habitual context, under continuous or intermittent access as well as under anxiogenic, or effortful conditions. Caffeine (2.5-20.0 mg/kg) increased intake at the highest dose under familiar continuous and intermittent access. However, this high dose reduced food intake in the dark-light paradigm. In contrast, a dopamine-depleting agent, tetrabenazine (TBZ; 1.0-8.0 mg/kg) did not affect food intake in any of those experimental conditions. In the T-maze-barrier task that evaluates seeking and taking of food under effortful conditions, caffeine (10.0 mg/kg) decreased latency to reach the food, but did not affect selection of the high-food density arm that required more effort, or the total amount of food consumed. In contrast, TBZ (4.0 mg/kg) reduced selection of the high food density arm with the barrier, thus affecting amount of food consumed. Interestingly, a small dose of caffeine (5.0 mg/kg) was able to reverse the anergia-inducing effects produced by TBZ in the T-maze. These results suggest that caffeine can potentiate or suppress food consumption depending on the context. Moreover, caffeine did not change appetite, and did not impair orientation toward food under effortful conditions, but it rather helped to achieve the goal by improving speed and by reversing performance to normal levels when fatigue was induced by dopamine depletion.

A single dose of cocaine potentiates glutamatergic synaptic transmission onto locus coeruleus neurons

The brainstem locus coeruleus (LC), the primary norepinephrinergic (NE) nucleus in the brain, has been implicated in the abuse of drugs such as opioids. However, whether and how the LC-NE system is involved in cocaine addiction remains elusive. Here, we demonstrated cocaine-evoked synaptic plasticity of glutamatergic transmission onto LC neurons as one of the earliest traces occurring after a single injection of cocaine. Twenty-four hours after mice were injected intraperitoneally with cocaine, the evoked α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) mediated synaptic transmission onto LC neurons were strongly potentiated without major effect on N-methyl-d-aspartate receptor (NMDAR) mediated synaptic transmission. Compared with saline-pretreated mice, AMPAR-mediated excitatory postsynaptic currents (EPSCs) of cocaine-pretreated mice showed a marked inward rectification, demonstrating the insertion of GluR2-lacking AMPARs to plasma membrane. In addition, the single injection of cocaine did not affect presynaptic glutamate release probability measured by paired pulse ratio. Furthermore, we found that the cocaine-induced potentiation of AMPAR EPSCs could be blocked by prazosin, an inhibitor of α1-adrenoreceptor (AR), indicating that cocaine increases AMPAR transmission via α1-ARs. These results reveal that LC-NE serves as an initial target of drug intake.

The Involvement of Norepinephrine in Behaviors Related to Psychostimulant Addiction

Although it is generally accepted that the abuse-related effects of amphetamines and cocaine result from the activation of the brain dopaminergic (DA) system, the psychostimulants also alter other neurotransmitter systems. In particular, they increase extracellular levels of norepinephrine (NE) and serotonin by inhibiting respective plasma membrane transporters and/or inducing release. The present review will discuss the preclinical findings on the effects of the NE system modulation (lesions, pharmacological and genetic approaches) on behaviors (locomotor hyperactivity, behavioral sensitization, modification of intracranial self-stimulation, conditioned place preference, drug self-administration, extinction/reinstatement of drug seeking behavior) related to the psychostimulant addiction.

The Roles of Dopamine and α1-Adrenergic Receptors in Cocaine Preferences in Female and Male Rats

Cocaine dependence is characterized by compulsive drug taking and reduced involvement in social, occupational, or recreational activities. Unraveling the diverse mechanisms contributing to the loss-of-interest in these 'non-drug' pursuits is essential for understanding the neurobiology of addiction and could provide additional targets for treating addiction. The study objectives were to examine changes in cocaine-induced dopamine (DA) overflow in the nucleus accumbens (NAc) over the course of self-administration and determine the roles of α1- and β-adrenergic receptors (AR) in the loss-of-interest in food rewards following the development of an addicted phenotype in male and female rats. Subjects were given access to cocaine and palatable food pellets in a choice self-administration paradigm to identify 'addicted' cocaine-preferring (CP) individuals and resistant pellet-preferring (PP) individuals based on their patterns of self-administration over 7 weeks. Cocaine-induced DA overflow in the NAc was examined with microdialysis early and late during self-administration (weeks 2 and 7). Subjects were treated in counter-balanced order with propranolol (β-AR antagonist), terazosin (α1-AR antagonist), or vehicle for an additional 3 weeks of self-administration. CP rats displayed increased motivation for cocaine and attenuated motivation for pellets following the development of cocaine preferences. In females, the estrous cycle affected pellet, but not cocaine, self-administration. CP rats displayed attenuated cocaine-induced DA overflow in the NAc. Propranolol enhanced cocaine reinforcement and reduced pellet intake, whereas terazosin enhanced motivation for pellets and reversed preferences in a subset of CP rats. The implications of these results for the treatment of addiction are discussed.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

Blocking alpha2A adrenoceptors, but not dopamine receptors, augments bupropion-induced hypophagia in rats

OBJECTIVE

Anti-obesity drugs have adverse effects which limit their use, creating a need for novel anti-obesity compounds. We studied effects of dopamine (DA) and norepinephrine (NE) reuptake inhibitor bupropion (BUP), alone and after blocking α1- or α2-adrenoceptors (AR), D1/5, D2/3, or D4 receptors, to determine which receptors act downstream of BUP.

DESIGN AND METHODS

Effects on caloric intake, meal patterning and locomotion were assessed, using an automated weighing system and telemetry in male rats with 18-h access to Western Human style diet.

RESULTS

BUP (30 mg/kg) induced hypophagia by reducing meal size and postponing meal initiation. WB4101 (α1-AR; 2 mg/kg) and imiloxan (α2B-AR; 5 mg/kg) attenuated BUP's effect on meal size, while WB4101 and BRL 44408 (α2A/D-AR; 2 mg/kg) counteracted effect on meal initiation. Atipamezole (α2-AR; 1 mg/kg) and imiloxan further postponed initiation of meals. SKF 83566 (D1/5; 0.3 mg/kg), raclopride (D2/3; 0.5 mg/kg) and to a lesser extent FAUC 213 (D4; 0.5 mg/kg), attenuated BUP-induced hypophagia. BUP stimulated locomotion, which was blocked by all antagonists, except FAUC 213 or BRL 44408.

CONCLUSIONS

Alpha1-, α2A/D- and α2B-ARs, and DA receptors underlie BUP's effects on size and initiation of meals, while blocking pre-synaptic α2-ARs enhanced BUP-induced hypophagia. An inverse agonist of (pre-synaptic) α2A-ARs could enhance BUP-induced anorexia and treat eating disorders and obesity.

α₁-Adrenergic receptors contribute to the acute effects of 3,4-methylenedioxymethamphetamine in humans

Preclinical studies implicate a role for α₁-noradrenergic receptors in the effects of psychostimulants, including 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy"). The present study evaluated the effects of the α₁-noradrenergic receptor antagonist doxazosin on the acute pharmacodynamic and pharmacokinetic response to MDMA in 16 healthy subjects. Doxazosin (8 mg/d) or placebo was administered for 3 days before MDMA (125 mg) or placebo using a randomized, double-blind, placebo-controlled, 4-session, crossover design. Doxazosin reduced MDMA-induced elevations in blood pressure, body temperature, and moderately attenuated positive mood but enhanced tachycardia associated with MDMA. The results indicate that α₁-adrenergic receptors contribute to the acute cardiostimulant and to a minor extent possibly also to the thermogenic and euphoric effects of MDMA in humans.

The expression of MC4Rs in D1R neurons regulates food intake and locomotor sensitization to cocaine

While it is known that mice lacking melanocortin 4 receptor (MC4R) expression develop hyperphagia resulting in early-onset obesity, the specific neural circuits that mediate this process remain unclear. Here, we report that selective restoration of MC4R expression within dopamine-1 receptor-expressing neurons [MC4R/dopamine 1 receptor (D1R) mice] partially blunts the severe obesity seen in MC4R-null mice by decreasing meal size, but not meal frequency, in the dark cycle. We also report that both acute cocaine-induced anorexia and the development of locomotor sensitization to repeated administration of cocaine are blunted in MC4R-null mice and normalized in MC4R/D1R mice. Neuronal retrograde tracing identifies the lateral hypothalamic area as the primary target of MC4R-expressing neurons in the nucleus accumbens. Biochemical studies in the ventral striatum show that phosphorylation of DARPP-32(Thr) (-34) and GluR1(Ser) (-845) is diminished in MC4R-null mice after chronic cocaine administration but rescued in MC4R/D1R mice. These findings highlight a physiological role of MC4R-mediated signaling within D1R neurons in the long-term regulation of energy balance and behavioral responses to cocaine.

The effects of locus coeruleus and norepinephrine in methamphetamine toxicity

The activity of locus coeruleus (LC) neurons has been extensively investigated in a variety of behavioural states. In fact this norepinephrine (NE)-containing nucleus modulates many physiological and pathological conditions including the sleep-waking cycle, movement disorders, mood alterations, convulsive seizures, and the effects of drugs such as psychostimulants and opioids. This review focuses on the modulation exerted by central NE pathways on the behavioural and neurotoxic effects produced by the psychostimulant methamphetamine, essentially the modulation of the activity of mesencephalic dopamine (DA) neurons. In fact, although NE in itself mediates some behavioural effects induced by methamphetamine, NE modulation of DA release is pivotal for methamphetamine-induced behavioural states and neurotoxicity. These interactions are discussed on the basis of the state of the art of the functional neuroanatomy of central NE- and DA systems. Emphasis is given to those brain sites possessing a remarkable overlapping of both neurotransmitters.

The alpha-1 adrenergic antagonist doxazosin for treatment of cocaine dependence: A pilot study

BACKGROUND

Medications decreasing central noradrenergic activity have been associated with attenuation of cocaine effects.

AIMS

This pilot study examined the efficacy of doxazosin versus placebo for reducing cocaine use in treatment-seeking cocaine dependent persons.

METHODS

We screened 108 cocaine dependent subjects and assigned 35 participants to receive either doxazosin (8mg/day) or placebo for 13 weeks. Participants were titrated on the study medication according to two different schedules. During the initial phase of the study, patients were titrated onto the study medication over an 8-week period (DOX-slow). After reviewing data from our human laboratory study, a second phase was initiated, wherein titration was accelerated to a 4-week period (DOX-fast). All participants received weekly cognitive behavioral therapy. Urine toxicology was performed thrice weekly.

RESULTS

Baseline subject characteristics were comparable. Thirty subjects entered the study: 8 subjects in DOX-slow, 9 subjects in DOX-fast, and 13 subjects in placebo. Total number of cocaine-negative urines was significantly increased in the DOX-fast group; and percentage of total cocaine-negative urines by group were 10% for DOX-slow group, 35% for DOX-fast group, and 14% for placebo (χ(2)=36.3, df=2, p<0.0001). The percentage of participants achieving two or more consecutive weeks of abstinence by group was 0% for DOX-slow group, 44% for DOX-fast group, and 7% for placebo (χ(2)=7.35, df=2, p<0.023).

CONCLUSIONS

This pilot study suggests the potential efficacy of doxazosin when rapidly titrated in reducing cocaine use.

Carvedilol inhibits the cardiostimulant and thermogenic effects of MDMA in humans

BACKGROUND AND PURPOSE

The use of ± 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') is associated with cardiovascular complications and hyperthermia.

EXPERIMENTAL APPROACH

We assessed the effects of the α(1) - and β-adrenoceptor antagonist carvedilol on the cardiostimulant, thermogenic and subjective responses to MDMA in 16 healthy subjects. Carvedilol (50 mg) or placebo was administered 1 h before MDMA (125 mg) or placebo using a randomized, double-blind, placebo-controlled, four-period crossover design. KEY RESULTS Carvedilol reduced MDMA-induced elevations in blood pressure, heart rate and body temperature. Carvedilol did not affect the subjective effects of MDMA including MDMA-induced good drug effects, drug high, drug liking, stimulation or adverse effects. Carvedilol did not alter the plasma exposure to MDMA.

CONCLUSIONS AND IMPLICATIONS

α(1) - and β-Adrenoceptors contribute to the cardiostimulant and thermogenic effects of MDMA in humans but not to its psychotropic effects. Carvedilol could be useful in the treatment of cardiovascular and hyperthermic complications associated with ecstasy use.

The α1 Antagonist Doxazosin Alters the Behavioral Effects of Cocaine in Rats

Medications that target norepinephrine (NE) neurotransmission alter the behavioral effects of cocaine and may be beneficial for stimulant-use disorders. We showed previously that the short-acting, α1-adrenergic antagonist, prazosin, blocked drug-induced reinstatement of cocaine-seeking in rats and doxazosin (DOX), a longer-acting α1 antagonist blocked cocaine’s subjective effects in cocaine-dependent volunteers. To further characterize DOX as a possible pharmacotherapy for cocaine dependence, we assessed its impact on the development and expression of cocaine-induced locomotor sensitization in rats. Rats (n = 6–8) were administered saline, cocaine (COC, 10 mg/kg) or DOX (0.3 or 1.0 mg/kg) alone or in combination for 5 consecutive days (development). Following 10-days of drug withdrawal, all rats were administered COC and locomotor activity was again assessed (expression). COC increased locomotor activity across days indicative of sensitization. The high dose (1.0 mg/kg), but not the low dose (0.3 mg/kg) of DOX significantly decreased the development and expression of COC sensitization. DOX alone did not differ from saline. These results are consistent with studies showing that α1 receptors are essential for the development and expression of cocaine’s behavioral effects. Results also suggest that blockade of both the development and expression of locomotor sensitization may be important characteristics of possible pharmacotherapies for cocaine dependence in humans.

Cocaine self-administration is not dependent upon mesocortical α1 noradrenergic signaling

The rewarding properties of psychomotor stimulants are traditionally thought to be independent of norepinephrine. Recent findings, however, suggest that local noradrenergic signaling through α1 receptors in the medial prefrontal cortex and the ventral tegmental area - brain regions critically important in natural and drug rewards - is in a position to influence stimulant reward. Despite this controversy, the contribution of this targeted signaling to stimulant self-administration has not been directly assessed. We have thus examined whether pharmacological blockade of α1 receptors in the medial prefrontal cortex and ventral tegmental area alters cocaine self-administration. Rats were trained to lever-press for cocaine (1.0 mg/kg/infusion) under a fixed ratio 1 schedule of reinforcement for 10 days. After training, the rats received a bilateral microinjection of an α1 noradrenergic antagonist (terazosin: 1.0, 5.0, or 10 mM/side), a D1 dopaminergic antagonist (SCH23390: 12.3 mM/side), or saline into either the medial prefrontal cortex or ventral tegmental area immediately before a cocaine self-administration session. Although SCH23390 significantly increased cocaine self-administration when injected into either brain region, terazosin, at all doses and sites tested, failed to alter this behavior. Thus, the maintenance of cocaine self-administration appears to be under the influence of D1 dopaminergic, rather than α1 noradrenergic, signaling at these mesocortical sites.

Role for monoaminergic systems in the antidepressant-like effect of ethanol extracts from Hemerocallis citrina

ETHNOPHARMACOLOGICAL RELEVANCE

Hemerocallis citrina, a traditional herbal medicine, has been used for the improvement of emotions in Eastern-Asia countries.

AIM OF THE STUDY

Herein, we explored the antidepressant-like effect and its monoaminergic mechanism of the ethanol extracts from Hemerocallis citrina (HCE).

MATERIALS AND METHODS

Effect of HCE (90, 180 and 360 mg/kg, p.o.) on the immobility time was assessed in the mouse forced swim test (FST) and tail suspension test (TST), and locomotor activity was evaluated in the open-field test (OFT). Additionally, the monoamine neurotransmitters serotonin (5-HT), noradrenaline (NA) and dopamine (DA) levels involved in the antidepressant-like effect of HCE were also measured in the mice brain regions of frontal cortex and hippocampus.

RESULTS

HCE (90, 180 and 360 mg/kg, p.o.) administration significantly reduced the immobility time in both the FST and TST without accompanying changes in locomotor activity in the OFT. The pretreatment of mice with WAY 100635 (0.1 mg/kg, s.c., a 5-HT(1A) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) receptor antagonist), prazosin (62.5 μg/kg, i.p., an α(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an α(2)-adrenoceptor antagonist), propranolol (5 mg/kg, i.p., a β-adrenoceptor antagonist) or sulpiride (50 mg/kg, i.p., a dopamine D(2) receptor antagonist), but not SCH23390 (0.05 mg/kg, s.c., a dopamine D(1) receptor antagonist) prevented the antidepressant-like effect of HCE (360 mg/kg, p.o.) in the TST. In addition, HCE enhanced 5-HT and NA levels in the frontal cortex and hippocampus as well as elevated DA levels in the frontal cortex.

CONCLUSION

The results indicate that the antidepressant-like effect of HCE is dependent on the serotonergic (5-HT(1A) and 5-HT(2) receptors), noradrenergic (α(1)-, α(2)- and β-adrenoceptors) and dopaminergic (D(2) receptor) systems as well as the elevation of 5-HT, NA and DA levels in the mouse brain.

Subchronic steroid administration induces long lasting changes in neurochemical and behavioral response to cocaine in rats

The abuse of anabolic androgenic steroids (AASs), such as nandrolone, is not only a problem in the world of sports but is associated with the polydrug use of non-athletes. Among other adverse effects, AAS abuse has been associated with long term or even persistent psychiatric problems. We have previously found that nandrolone decanoate treatment could produce prolonged changes in rats' brain reward circuits associated to drug dependence. The aim in this study was to evaluate whether AAS-induced neurochemical and behavioral changes are reversible. The increases in extracellular dopamine (DA) and serotonin (5-HT) concentration, as well as stereotyped behavior and locomotor activity (LMA) evoked by cocaine were attenuated by pretreatment with nandrolone. The recovery period, which was needed for the DA system to return back to the basic level, was fairly long compared to the dosing period of the steroid. In the 5-HT system, the time that system needed to return back to the basal level, was even longer than in the DA system. The attenuation was still seen though there were no detectable traces of nandrolone in the blood samples. Given that accumbal outflow of DA and 5-HT, as well as LMA and stereotyped behavior are all related to reward of stimulant drugs, this study suggests that nandrolone decanoate has significant, long-lasting but reversible effects on the rewarding properties of cocaine.

Cocaine withdrawal impairs metabotropic glutamate receptor-dependent long-term depression in the nucleus accumbens

Neuroadaptation in the nucleus accumbens (NAc), a central component of the mesolimbic dopamine (DA) system, has been implicated in the development of cocaine-induced psychomotor sensitization and relapse to cocaine seeking. However, little is known about the cellular and synaptic mechanisms underlying such adaptation. Using a mouse model of behavioral sensitization, we show that animals withdrawn from repeated cocaine exposure have a selective deficit in the ability to elicit metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) in the shell of the NAc in response to bath application of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG). Experiments conducted in the presence of the selective mGluR1 antagonists 7-(hydroxyimino)cyclopropachromen-carboxylate ethyl ester and (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid, or the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine, demonstrated that the impaired DHPG-LTD is likely attributable to a loss of mGluR5 function. Quantitative real-time reverse transcriptase-PCR and Western blot analysis revealed significant downregulation of mGluR5, but not mGluR1, mRNA and protein levels in the NAc shell. The inhibitory effect of repeated cocaine exposure on DHPG-LTD was selectively prevented when cocaine was coadministered with the selective D(1)-like DA receptor antagonist (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine. Furthermore, the levels of brain-derived neurotrophic factor (BDNF) protein in the NAc shell increased progressively after cocaine withdrawal, and the impairment of DHPG-LTD in the NAc shell was not found in slices from BDNF-knock-out mice after cocaine withdrawal. These results suggest that withdrawal from repeated cocaine exposure may result in increased BDNF levels in the NAc shell, which leads to a selective downregulation of mGluR5 and thereby impairs the induction of mGluR-dependent LTD.

Noradrenergic alpha1 receptors as a novel target for the treatment of nicotine addiction

Nicotine is the main psychoactive ingredient in tobacco and its rewarding effects are considered primarily responsible for persistent tobacco smoking and relapse. Although dopamine has been extensively implicated in the rewarding effects of nicotine, noradrenergic systems may have a larger role than previously suspected. This study evaluated the role of noradrenergic alpha(1) receptors in nicotine and food self-administration and relapse, nicotine discrimination, and nicotine-induced dopamine release in the nucleus accumbens in rats. We found that the noradrenergic alpha(1) receptor antagonist prazosin (0.25-1 mg/kg) dose dependently reduced the self-administration of nicotine (0.03 mg/kg), an effect that was maintained over consecutive daily sessions; but did not reduce food self-administration. Prazosin also decreased reinstatement of extinguished nicotine seeking induced by either a nicotine prime (0.15 mg/kg) or nicotine-associated cues, but not food-induced reinstatement of food-seeking, and decreased nicotine-induced (0.15 mg/kg) dopamine release in the nucleus accumbens shell. However, prazosin did not have nicotine-like discriminative effects and did not alter the dose-response curve for nicotine discrimination. These findings suggest that stimulation of noradrenergic alpha(1) receptors is involved in nicotine self-administration and relapse, possibly via facilitation of nicotine-induced activation of the mesolimbic dopaminergic system. The findings point to alpha(1) adrenoceptor blockade as a potential new approach to the treatment of tobacco dependence in humans.

Subchronic nandrolone administration reduces cocaine-induced dopamine and 5-hydroxytryptamine outflow in the rat nucleus accumbens

RATIONALE

The abuse of anabolic androgenic steroids (AASs) is not only a problem in the world of sports but is associated with the polydrug use of nonathletes. Investigations of the neurochemical effects of AAS have focused in part on the monoaminergic systems, involving, among other things, the development of dependence. We have previously shown that pretreatment with nandrolone decanoate attenuates dose-dependently the increase in extracellular dopamine (DA) concentration evoked by amphetamine and 3,4-methylenedioyxymethamphetamine in the nucleus accumbens (NAc).

OBJECTIVES

The aim of this study was to investigate whether the nandrolone pre-exposure modulates the acute neurochemical and behavioral effects of cocaine in rats and whether the effects are long lasting.

METHODS

DA, 5-hydroxytryptamine (5-HT), and their metabolites were measured from samples collected from the NAc by microdialysis. The behavior of the animals was recorded.

RESULTS

The present study demonstrates that five injections of nandrolone (5 and 20 mg/kg) inhibited cocaine-evoked DA and 5-HT outflow in the NAc, locomotor activity (LMA), and stereotyped behavior in experimental animals, and that these effects are seen even after elimination of nandrolone from bloodstream.

CONCLUSIONS

Given that accumbal outflow of DA and 5-HT, as well as LMA and stereotyped behavior, is related to gratification of stimulant drugs, this study suggests that nandrolone, at the doses tested, has a significant effect on the pleasurable properties of cocaine. Furthermore, because neurochemical and behavioral responses were still attenuated after a fairly long recovery period, it seems that nandrolone may induce long-lasting changes in the brains of rat.

Balancing risk and reward: a rat model of risky decision making

We developed a behavioral task in rats to assess the influence of risk of punishment on decision making. Male Long-Evans rats were given choices between pressing a lever to obtain a small, 'safe' food reward and a large food reward associated with risk of punishment (footshock). Each test session consisted of 5 blocks of 10 choice trials, with punishment risk increasing with each consecutive block (0, 25, 50, 75, 100%). Preference for the large, 'risky' reward declined with both increased probability and increased magnitude of punishment, and reward choice was not affected by the level of satiation or the order of risk presentation. Performance in this risky decision-making task was correlated with the degree to which the rats discounted the value of probabilistic rewards, but not delayed rewards. Finally, the acute effects of different doses of amphetamine and cocaine on risky decision making were assessed. Systemic amphetamine administration caused a dose-dependent decrease in choice of the large risky reward (ie, it made rats more risk averse). Cocaine did not cause a shift in reward choice, but instead impaired the rats' sensitivity to changes in punishment risk. These results should prove useful for investigating neuropsychiatric disorders in which risk taking is a prominent feature, such as attention deficit/hyperactivity disorder and addiction.

Repeated cocaine administration decreases 5-HT(2A) receptor-mediated serotonergic enhancement of synaptic activity in rat medial prefrontal cortex

Neural adaptations in the medial prefrontal cortex (mPFC) are thought to be crucial in the development and maintenance of addictive behaviors. The mPFC receives a dense serotonergic (5-hydroxytryptamine, 5-HT) innervation from raphe nuclei and 5-HT exerts complex actions on mPFC pyramidal neurons. The present study, using a rat model of behavioral sensitization to cocaine, was designed to determine whether repeated cocaine exposure in vivo is capable of altering 5-HT-induced regulation of glutamatergic transmission in the mPFC. In layer V pyramidal neurons of the mPFC, application of 5-HT, through activation of 5-HT(2A) receptors, induced a massive enhancement of spontaneous excitatory postsynaptic currents (sEPSCs). Repeated cocaine administration for 5 days resulted in an attenuation in the ability of 5-HT to enhance sEPSCs. This effect was prevented when cocaine was co-administered with the selective 5-HT(2A) receptor antagonist ketanserin and was mimicked by repeated 5-HT(2A) receptor agonist (-)4-iodo-2,5-dimethoxyphenylisopropylamine administration. Repeated cocaine administration is not associated with any changes in the levels of 5-HT(2A) receptors or regulator of GTP-binding protein signaling 4. These results suggest that cocaine-induced inhibition of 5-HT(2A) receptor-mediated enhancement of glutamatergic transmission in the mPFC may be caused, at least in part, by the impairment of coupling of 5-HT(2A) receptors with GTP-binding proteins during cocaine withdrawal. These alterations in 5-HT(2A) receptor responsiveness in the mPFC may be relevant to the development of behavioral sensitization and withdrawal effects following repeated cocaine administration.

Changes in feeding and locomotion induced by amphetamine analogs in rats

Studies of the biobehavioral actions of psychostimulants commonly focus on locomotion and less commonly on feeding, and only rarely are these measures considered in conjunction within the same animal. The present study compared the impact of (+)-amphetamine and three amphetamine analogs, PAL-287, PAL-313, and PAL-353, on eating and locomotion assessed concurrently using an automated activity/feeding chamber during a daily 45 min session. Each analog is a potent releaser of norepinephrine and of dopamine, but exerts differential serotonin-releasing activity (PAL-287>PAL-313>amphetamine>PAL-353). Rats were tested with each of five doses of drug (0, 2, 4, 8, or 16 micromol/kg, i.p.), given in equimolar concentrations and in random dose order. PAL-353, an analog with minimal serotonin-releasing capacity, markedly stimulated forward locomotion at 2, 4, 8 and 16 micromol/kg, as did amphetamine, whereas PAL-287 and PAL-313 did not. In contrast to the locomotor findings, all four amphetamine-like drugs exerted similar effects on the suppression of food intake. These results suggest that the capacity of an amphetamine analog (i.e. amphetamine and PAL-353) to stimulate serotonin release can diminish its psychostimulant action on locomotion, but does not reliably augment drug-induced hypophagia.

The alpha1-adrenergic receptor antagonist, prazosin, reduces alcohol drinking in alcohol-preferring (P) rats

BACKGROUND

Preliminary evidence suggest that noradrenergic signaling may play a role in mediating alcohol drinking behavior in both humans and rats. Accordingly, we tested the hypothesis that blockade of alpha(1)-adrenergic receptors will suppress alcohol drinking in rats selectively bred for alcohol preference (P line).

METHODS

Adult male P rats were given 24-hour access to food and water and scheduled access to a 15% (v/v) alcohol solution for 2 hours daily. Rats were injected IP with the alpha(1)-adrenergic receptor antagonist, prazosin (0, 0.5, 1.0, 1.5, or 2.0 mg/kg body weight), once a day at 15 minutes prior to onset of the daily 2-hour 2-bottle choice, alcohol versus water, access period for 2 consecutive days and then 3 weeks later for 5 consecutive days.

RESULTS

Prazosin significantly reduced (p < 0.01) alcohol intake during the initial 2 daily administrations, and this reduction of alcohol intake was maintained for 5 consecutive days by daily prazosin treatment in the subsequent more prolonged trial (p < 0.05). The prazosin-induced reduction of alcohol intake was not dependent upon drug-induced motor impairment since increases in water drinking (p < 0.05) were exhibited during the 2-hour access periods during both 2- and 5-day prazosin treatment.

CONCLUSIONS

The results indicate that the noradrenergic system plays a role in mediating alcohol drinking in rats of the P line and suggest that prazosin--a safe, well-characterized, and well-tolerated drug--may be an effective pharmacotherapeutic agent for the treatment of alcohol use disorders.

The alpha1 adrenergic receptor antagonist prazosin reduces heroin self-administration in rats with extended access to heroin administration

Previous studies have reported that noradrenergic antagonists alleviate some of the symptoms of opiate withdrawal and dependence. Clinical studies also have shown that modification of the noradrenergic system may help protect patients from relapse. The present study tested the hypothesis that a dysregulated noradrenergic system has motivational significance in heroin self-administration of dependent rats. Prazosin, an alpha1-adrenergic antagonist (0.5, 1.0, 1.5 and 2.0 mg/kg, i.p.), was administered to adult male Wistar rats with a history of limited (1 h/day; short access) or extended (12 h/day; long access) access to intravenous heroin self-administration. Prazosin dose-dependently reduced heroin self-administration in long-access rats but not short-access rats, with 2 mg/kg of systemic prazosin significantly decreasing 1 h and 2 h heroin intake. Prazosin also reversed some changes in meal pattern associated with extended heroin access, including the taking of smaller and briefer meals (at 3 h), while also increasing total food intake and slowing the eating rate within meals (both 3 h and 12 h). Thus, prazosin appears to stimulate food intake in extended access rats by restoring meals to the normal size and duration. The data suggest that the alpha1 adrenergic system may contribute to mechanisms that promote dependence in rats with extended access.

Alpha 1-noradrenergic system role in increased motivation for cocaine intake in rats with prolonged access

In rodents, extended access to cocaine produces an escalation in cocaine self-administration that has face and construct validity for human compulsive drug intake. Here we report that rats with six-hour access (long access, LgA) to cocaine self-administration produced a higher breakpoint for cocaine using a progressive-ratio schedule than rats with one-hour access (short access, ShA), and prazosin (alpha 1 receptor antagonist) reduced the higher breakpoint for cocaine in LgA rats. Additionally, the number of neurons with alpha 1-adrenergic receptor-like immunoreactivity in the bed nucleus of stria terminalis (BNST) was found to be much lower in LgA rats than in ShA and drug-naive rats. In contrast, UK14304 (alpha 2 receptor agonist) and betaxolol (beta 1 receptor antagonist) had no effect on cocaine self-administration in either group. The data suggest that activation of the alpha 1-noradrenergic system, perhaps in the BNST, is associated with increased motivation for cocaine in rats with extended access.

Lobeline attenuates progressive ratio breakpoint scores for intracranial self-stimulation in rats

The alkaloid lobeline inhibits the function of vesicular monoamine and dopamine transporters and diminishes the behavioral and neurochemical effects of nicotine and amphetamines. In the present study, we examined the interaction of systemic administration of lobeline on breakpoint scores on a progressive ratio (PR) schedule of intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB). Rats were run in two 30 min sessions, separated by a 10 min timeout period. At the end of the first session, each rat was injected with either 0, 0.5, 1.0 or 2.0 mg/kg (i.p.) lobeline. Positive controls known to suppress and to augment ICSS responding included the adrenergic antagonist prazosin (0, 0.5 and 2.0 mg/kg, i.p.) and the psychostimulant cocaine (0, 1.25, and 5.0 mg/kg, i.p.). Analyses of changes in average PR breakpoint scores between the 2 sessions revealed that lobeline significantly suppressed PR scores at doses of 0.5, 1.0 and 2.0 mg/kg, as did 0.5 mg/kg and 2.0 mg/kg prazosin. These changes are unlikely to reflect motoric effects of these drugs inasmuch as neither lobeline nor prazosin alter locomotion at these doses. In contrast, PR breakpoint scores were significantly increased at 5.0 mg/kg cocaine, a dose that is sufficient to elevate locomotion in the rat. These results are consistent with the view that lobeline modulates brain reinforcement processes.

Effects of ICV administration of the alpha1A-adrenoceptor antagonist 5-methylurapidil on concurrent measures of eating and locomotion after cocaine in the rat

Psychostimulants including amphetamine and cocaine induce locomotion and stereotypy and suppress eating. Although the capacity of cocaine to alter locomotion is usually viewed as related to dopamine neurotransmission, recent studies suggest that norepinephrine, acting through alpha1-adrenergic receptors (alpha1-ARs) can facilitate cocaine-stimulated locomotion. Of the three alpha1-AR subtypes (alpha(1A), alpha(1B), and alpha(1D)) identified to date, inactivation of the alpha(1B)-AR subtype diminishes cocaine-stimulated locomotion, whereas the impact of inactivation of the alpha(1A)-AR subtype on either eating or locomotion is unknown. In the present study, we assessed the relative impact of ICV administration of the alpha(1B)-AR antagonist 5-methylurapidil (5-MU) on cocaine-stimulated hyperlocomotion and hypophagia, using a concurrent method [Wellman, P.J., Ho, D.H., Davis, K.W., 2005. Concurrent measures of feeding and locomotion in rats. Physiology of Behavior 84 (5), 769-774.]. Rats were infused ICV with one of 3 doses of 5-MU (0, 3, or 30 nmol) and then injected (i.p.) with 0, 2.5, 5.0, 10.0, or 20.0 mg/kg cocaine HCl on each of five tests. Rats always received the same 5-MU dose, but a different cocaine dose on each trial. Feeding and locomotion were assessed concurrently during a 45-min postinjection period. Significant suppression of eating was noted at 2.5 mg/kg cocaine, a dose that does not alter forward locomotion in the rat. Administration of 5-MU did not alter locomotion in rats treated with saline, but did significantly increase baseline food intake. Neither cocaine-induced hypophagia nor hyperlocomotion was altered by ICV administration of 5-MU. These results suggest that the capacity of alpha1-AR agonists (e.g. phenylpropanolamine) to suppress eating may be related to activation of the alpha(1A)-AR subtype, whereas cocaine does not act through the alpha(1A)-AR subtype to suppress eating nor does this subtype modulate cocaine-induced hyperlocomotion.

Systemic ghrelin sensitizes cocaine-induced hyperlocomotion in rats

The feeding-relevant pathway by which food restriction (FR) augments cocaine action is unknown. Systemic administration of the 28-amino acid acylated peptide ghrelin (1-10 nmol) increases food intake in rats and circulating levels of rat ghrelin are up-regulated by FR. The present experiment examined the impact of repeated administration of ghrelin or vehicle on the subsequent capacity of cocaine to enhance locomotion in rats. Male Sprague-Dawley rats were pretreated daily for seven days with 0, 5 or 10 nmol rat ghrelin (i.p.) in the home cage. On the 8th day, rats were transported to a testing room, placed in a locomotion chamber for 15 min, and then injected (i.p.) with 0, 7.5, or 15 mg/kg cocaine hydrochloride. Locomotor activity was monitored over a 45 min post-cocaine period. Pretreatment with 5 or 10 nmol ghrelin alone did not significantly increase basal locomotion relative to that of the 0 nmol ghrelin group. Rats pretreated with 5 nmol or 10 nmol ghrelin showed an enhanced locomotor response after treatment with 15 mg/kg cocaine relative to rats treated with 0 nmol ghrelin. These results indicate that acute injection of ghrelin, at a feeding-relevant dose, can augment the acute effects of cocaine on locomotion in rats.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

There and back again: a tale of norepinephrine and drug addiction

Fueled by anatomical, electrophysiological, and pharmacological analyses of endogenous brain reward systems, norepinephrine (NE) was identified as a key mediator of both natural and drug-induced reward in the late 1960s and early 1970s. However, reward experiments from the mid-1970s that could distinguish between the noradrenergic and dopaminergic systems resulted in the prevailing view that dopamine (DA) was the primary 'reward transmitter' (a belief holding some sway still today), thereby pushing NE into the background. Most damaging to the NE hypothesis of reward were studies demonstrating that NE receptor antagonists and NE reuptake inhibitors failed to impact drug self-administration. In recent years new tools, such as genetically engineered mice, and new experimental paradigms, such as reinstatement of drug seeking following withdrawal, have propelled NE back into the awareness of addiction researchers. Of particular interest is disulfiram, an inhibitor of the NE biosynthetic enzyme dopamine beta-hydroxylase, which has demonstrated promising efficacy in the treatment of cocaine dependence in preliminary clinical trials. The purpose of this review is to synthesize the new data linking NE to critical aspects of DA signaling and drug addiction, with a focus on psychostimulants (eg, cocaine), opiates (eg, morphine), and alcohol.

Previous exposure to cocaine enhances cocaine self-administration in an alpha 1-adrenergic receptor dependent manner

Noradrenergic transmission is implicated in the biochemical and behavioral effects of cocaine. Recently, we demonstrated that the alpha 1-adrenergic receptor antagonist prazosin attenuates cocaine-induced reinstatement of drug-seeking behavior. We now assessed whether prazosin could counter the effect of previous exposure to cocaine to enhance subsequent self-administration behavior. Rats were pre-exposed to systemic injections of either saline, prazosin (0.3 mg/kg), saline+cocaine (10 mg/kg), or prazosin+cocaine for 5 days. Starting 15-18 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.5 mg/kg/infusion) under a fixed ratio 3 (FR3) schedule of reinforcement. Several tests were conducted. First, responding for cocaine under an FR3 schedule was assessed across several doses (0.125-1.0 mg/kg/infusion). Second, responding for cocaine (0.5 mg/kg/infusion) under a progressive-ratio (PR) schedule was examined for 6 consecutive days. Finally, responding for cocaine (0, 0.5, and 1.0 mg/kg/infusion) was determined under the PR schedule of reinforcement. Results showed that cocaine pre-exposed rats self-administer more cocaine compared to saline pre-exposed rats when tested under both the FR and PR schedules. Rats pre-exposed to cocaine plus prazosin did not show enhanced cocaine self-administration. These rats, as well those pre-exposed to prazosin alone, showed levels of cocaine self-administration similar to saline pre-exposed rats. Thus, previous exposure to cocaine enhanced cocaine self-administration, an effect that appears to involve activation of alpha 1-adrenergic receptors. These data, along with several recent studies, show further support for the contribution of noradrenergic transmission in the behavioral effects of cocaine.

Alpha1-adrenergic receptors mediate the locomotor response to systemic administration of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) in rats

The recreational drug 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) increases locomotor activity when administered to rats. Although the published pharmacology of MDMA has focused almost exclusively on the roles of serotonin and dopamine, in vitro studies indicate that MDMA induces serotonin and norepinephrine release with equal potency. The present experiments tested the hypothesis that blockade of alpha(1)-adrenoceptors with systemic or local administration of the antagonist prazosin would attenuate the locomotor response to systemic administration of (+/-)-MDMA. Pretreatment with systemic prazosin (0.5 mg/kg) or microinjections into either the prefrontal cortex or ventral tegmental area completely blocked the locomotor stimulant effects of 5 mg/kg (+/-)-MDMA, assessed using a computerized Behavioral Pattern Monitor. Prazosin was more potent in blocking the locomotor stimulant effects of (+/-)-MDMA than a 2 mg/kg dose of (+)-amphetamine that produced a similar locomotor activity increase. These results indicate that activation of alpha(1)-adrenoceptors in both the prefrontal cortex and ventral tegmental areas modulates the locomotor response to MDMA.

Alpha-noradrenergic receptors modulate the development and expression of cocaine sensitization

The increased activity and stereotyped behaviors that result from repeated administration of cocaine is called cocaine sensitization. This sensitized response has been postulated as one of the basic pathophysiological mechanisms in drug addiction. Recent evidence indicates that noradrenergic neurotransmission might be implicated in some of the behavioral effects of cocaine. The present article examined the role of alpha-adrenergic receptor agonists and antagonists in the development and expression of cocaine sensitization. Rats were injected once per day, for 7 consecutive days, with the alpha-1 receptor antagonist prazosin (0.5 mg/kg, i.p.) 15 min before cocaine administration (15 mg/kg, i.p.). After 8 days, animals received a cocaine challenge (15 mg/kg, i.p.) and were tested for locomotion. Following a 7-day withdrawal period rats received a second cocaine challenge. One day after the last challenge, rats were reinstated to the initial protocol for 1 day. In another set of experiments, rats were injected twice per day with the alpha-2 receptor antagonists yohimbine (5 mg/kg, i.p.), idazoxan (0.25 mg/kg, i.p.), or with the alpha-2 agonist clonidine (0.025 mg/kg, i.p.), followed by cocaine injections (15 mg/kg, i.p.), for 7 consecutive days. Thereafter, the protocol was similar to that following prazosin administration. The results demonstrated that the alpha-1 receptor antagonist prazosin blocked the development and expression of cocaine sensitization. On the other hand, both alpha-2 antagonists failed to inhibit the development or the expression of cocaine sensitization. Instead, they produced an increase in locomotor activity during the first day of experimentation. The alpha-2 agonist clonidine attenuated the acute response to cocaine on day 1 and retarded the increased locomotor activity on the following 2 days. There was a dramatic increase in the level of sensitization after the first cocaine challenge. However, it inhibited the expression of cocaine sensitization during the reinstatement protocol. These results suggest that alpha adrenoreceptors play an important role in modulating different stages of cocaine sensitization and probably cocaine addiction.

Differential effects of cocaine on firing rate and pattern of dopamine neurons: role of alpha1 receptors and comparison with L-dopa and apomorphine

Psychostimulants, including cocaine, have two opposing effects on dopamine (DA) neurons: a DA-mediated inhibition and a non-DA-mediated excitation. The latter, expressed as an increase in both firing rate and a slow oscillation (SO) in firing pattern, has been shown to require forebrain inputs to DA neurons and activation of adrenergic alpha(1) receptors. However, since the effect was observed when the DA-mediated inhibition was blocked by a D2 antagonist, it is uncertain whether the underlying mechanism also plays a role in cocaine's effects in normal animals where D2-like receptors are not blocked. This study showed that under such conditions, cocaine decreased firing rate and bursting without significantly inhibiting the SO in DA neurons recorded in the ventral tegmental area. Different from cocaine, l-dopa and apomorphine, two nonpsychostimulant DA agonists known to lack the alpha(1)-mediated excitatory effect, consistently inhibited all three measures of DA cell activity. Blockade of alpha(1) receptors by prazosin did not enhance cocaine's ability to inhibit firing rate and bursting, but it did enable cocaine to inhibit the SO. These results suggest that in control rats where D2-like receptors are not blocked, alpha(1) receptors play an important role in cocaine's effect on the SO but not in its effect on firing rate and bursting of DA neurons. The maintained SO after cocaine injection may reflect continued modulation of DA neurons by forebrain inputs, regulate the pattern of DA release, and provide a temporal structure for selection of synaptic inputs to DA neurons.

Dysregulation of dopamine signaling in the dorsal striatum inhibits feeding

Dopamine signaling is an important component of many goal-directed behaviors, such as feeding. Acute disruption of dopamine signaling using pharmacological agents tends to inhibit normal feeding behaviors in rodents. Likewise, genetically engineered dopamine-deficient (DD) mice are unable to initiate sufficient feeding and will starve by approximately 3 weeks of age if untreated. Adequate feeding by DD mice can be achieved by daily administration of L-3,4-dihydroxyphenylalanine (L-dopa), a precursor of dopamine, which can be taken up by dopaminergic neurons, converted to dopamine, and released in a regulated manner. In contrast, adequate feeding cannot be restored with apomorphine (APO), a mixed agonist that activates D1 and D2 receptors. Viral restoration of dopamine production in neurons that project to the dorsal striatum also restores feeding in DD mice. Administration of amphetamine (AMPH) or nomifensine (NOM), drugs which increase synaptic dopamine concentration, inhibits food intake in virally rescued DD mice (vrDD) as in control animals. These results indicate that the dysregulation of dopamine signaling in the dorsal striatum is sufficient to induce hypophagia and suggest that regulated release of dopamine in that brain region is essential for normal feeding and, probably, many other goal-directed behaviors.

Concurrent measures of feeding and locomotion in rats

Psychostimulants including amphetamine and cocaine induce locomotion and stereotypy and suppress eating. Studies of the biobehavioral actions of psychostimulants commonly focus on locomotion and less commonly on feeding, and only rarely are these measures considered in conjunction within the same animal. Inasmuch as hyperactivity induced by a psychostimulant may compete with other motor behaviors, including eating, it would be important to concurrently assess changes in eating and locomotion after psychostimulant treatment. The present paper describes a modification of an automated activity chamber in which minute-by-minute recordings of food consumption are gathered in parallel with an assessment of locomotion. The present experiment illustrates the method by characterizing the temporal changes in locomotion and eating produced by administration of hypophagic doses of nicotine tartrate (0.28 mg/kg (as the base), IP) or cocaine hydrochloride (7.5 mg/kg, IP). At these doses, nicotine suppressed eating and locomotion, whereas cocaine suppressed eating, but facilitated forward locomotion. These outcomes support the viability of this apparatus and the concurrent method for the dissociation of feeding and locomotion.

Dysregulation of striatal dopamine signaling by amphetamine inhibits feeding by hungry mice

Amphetamine (AMPH) releases monoamines, transiently stimulates locomotion, and inhibits feeding. Using a genetic approach, we show that mice lacking dopamine (DA-deficient, or DD, mice) are resistant to the hypophagic effects of a moderate dose of AMPH (2 microg/g) but manifest normal AMPH-induced hypophagia after restoration of DA signaling in the caudate putamen by viral gene therapy. By contrast, AMPH-induced hypophagia in response to the same dose of AMPH is not blunted in mice lacking the ability to make norepinephrine and epinephrine (Dbh(-/-)), dopamine D(2) receptors (D2r(-/-)), dopamine D(1) receptors (D1r(-/-)), serotonin 2C receptors (Htr2c(-/Y)), neuropeptide Y (Npy(-/-)), and in mice with compromised melanocortin signaling (A(y)). We suggest that, at this moderate dose of AMPH, dysregulation of striatal DA is the primary cause of AMPH-induced hypophagia and that regulated striatal dopaminergic signaling may be necessary for normal feeding behaviors.