Reciprocal changes in dopamine responsiveness in the nucleus accumbens shell and core and in the dorsal caudate-putamen in rats sensitized to morphine

Selegiline potentiates cocaine-induced increases in rodent nucleus accumbens dopamine

Selegiline has been proposed as a treatment for cocaine addiction and studies in humans suggest that it attenuates cocaine's reinforcing effects. Here we assessed the effects of selegiline treatment on cocaine-induced increases in nucleus accumbens (NAc) dopamine (DA) in freely moving rodents. Chronic treatment with selegiline (L-deprenyl, 0.25/mg/kg, 24 days) potentiated cocaine-induced increases in NAc DA from 350-600%. However, this enhanced response was abolished when animals were treated chronically with both cocaine and selegiline. Inasmuch as increases in NAc DA are associated with the reinforcing effects of cocaine, these results obtained in rodents suggest that MAO-A and -B inhibition may not be a suitable strategy to antagonize cocaine's reinforcing effects during cocaine detoxification. On the other hand, chronic selegiline treatment may improve DA deficits, which are thought to contribute to relapse through a decreased response to natural rewards.

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.

Amphetamine-sensitized rats show sugar-induced hyperactivity (cross-sensitization) and sugar hyperphagia

The goal was to determine the locomotor and consummatory effects of sugar in amphetamine-sensitized rats. Following a 30-min locomotor activity baseline using a photocell cage, male rats were administered either 3.0 mg/kg amphetamine or saline i.p. daily for 6 days. On the final day of injections, locomotor activity was measured again to affirm amphetamine sensitization. Experiment 1: Seven days later, half of each group was offered 10% sucrose or water for 1 min in the home cages, followed by a 30-min locomotor activity test to determine whether or not the animals had become hyperactive in response to sugar. Results showed that amphetamine-sensitized animals were hyperactive following a taste of sugar, but not water. Experiment 2: All subjects were then given access to 10% sucrose for 1 h daily for five consecutive days. Results showed that the amphetamine-sensitized group consumed more sucrose across the 5-day measurement period. These results suggest that sugar may be acting on the same system as amphetamine to trigger hyperactivity, and that alterations in this system caused by repeated doses of amphetamine can instigate an appetite for sugar that persists for at least a week.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Influence of acute or repeated restraint stress on morphine-induced locomotion: involvement of dopamine, opioid and glutamate receptors

The development of restraint stress-induced sensitization to the locomotor stimulating effect of morphine (2 mg/kg i.p.) was investigated. In experiment 1, both a single restraint session (2 h) and a repeated restraint stress (2 h per day for 7 days), similarly enhanced the effects of morphine on motor activity. In experiment 2, we observed that this sensitization was prevented by administration of both D(1) and D(2) dopaminergic antagonist [SCH-23390 (0.5 mg/kg i.p.) and (+/-)-sulpiride (60 mg/kg i.p.)] 10 min prior to the stress session. In experiment 3, we showed that an opioid antagonist pretreatment [naltrexone (1 mg/kg i.p.) 10 min prior to stress session, suppressed the stress-induced sensitization after morphine administration. In experiment 4, pretreatment with a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptors [(+)-MK-801 (0.1 mg/kg i.p.)], 30 min prior to the acute restraint session, prevented the development of sensitization to morphine. All these results suggest that: (1) sensitization to morphine on stimulating locomotor effect does not depend on the length of exposure to stress (acute vs. repeated); (2) stimulation of both D(1) and D(2) dopaminergic receptors is necessary for the development of restraint stress-induced sensitization to morphine; (3) an opioid system is also involved in this sensitization process; and (4) the stimulation of glutamatergic NMDA receptors is involved in this acute restraint-induced effect.

Dopamine release in the dorsal striatum during cocaine-seeking behavior under the control of a drug-associated cue

Compulsive drug use is characterized by a pattern of drug seeking and consumption that becomes progressively habitual and less and less modifiable by external and internal factors. Although traditional views would posit that nigrostriatal dopamine (DA) neurons originating in the substantia nigra and innervating the dorsal striatum are primarily concerned with motor functions, recent studies have implicated the dorsal striatum in mediating stimulus-response (habit) learning. In this study, in vivo microdialysis in combination with a second-order schedule of cocaine reinforcement was used to investigate the role of the dorsal striatal dopamine innervation in well established drug-seeking behavior under the control of a drug-associated cue [light conditioned stimulus (CS+)]. Rats were initially trained to self-administer cocaine under a continuous reinforcement schedule where a response on one of two identical levers led to a 20 sec presentation of a light CS+ and an intravenous cocaine infusion (0.75 mg/kg). The response requirement for the CS+ and cocaine was then progressively increased until stable responding was established under a second-order schedule of reinforcement. During microdialysis, rats were presented with the cocaine-associated CS+ either noncontingently or contingent on responding during a session of cocaine-seeking behavior. The results showed a marked increase in DA release in the dorsal striatum during drug-seeking, when cocaine cues were presented contingently, but not when the same cue was presented noncontingently. These data indicate a possible involvement of the dopaminergic innervation of the dorsal striatum in well established, or habitual, cocaine-seeking behavior.

A single exposure to restraint stress induces behavioral and neurochemical sensitization to stimulating effects of amphetamine: involvement of NMDA receptors

Evidence indicates that repeated exposure to stressful events sensitizes the motor and addictive effects of drugs of abuse in rats. Regarding a single exposure to one restraint stress, previous findings have shown that it is sufficient to induce behavioral sensitization to stimulating and reinforcing properties of abuse drugs (e.g., amphetamine and morphine), as measured by locomotor activity and conditioned place preference, respectively. It is well known that enhanced dopaminergic neurotransmission in the nucleus accumbens and striatum plays a critical role in the development and/or expression of repeated stress-induced or drug-induced sensitization. In addition, involvement of NMDA receptors has been implicated in its development. However, whether sensitization induced by a single restraint stress exposure represents the same neurobiologic phenomenon is unknown. We studied the following issues: (a) influence of a single restraint exposure on the stimulating effects of amphetamine on dopamine release by microdialysis from striatum and (b) involvement of glutamatergic pathways, specifically those innervating striatum, on stress-induced sensitization to amphetamine, by administering MK-801 ip (0.1 mg/kg) or intrastriatally (1 microg/0.5 microL) previous to an acute restraint stress. For microdialysis studies (a) or intrastriatal administration of MK-801 (b), Wistar rats (250-330 g) were implanted stereotactically under anesthesia with a guide cannula in the striatum. After 2 days, animals were immobilized for 2 hours in a Plexiglas device. Control animals remained in their home cages. The following day we evaluated the stimulating effect of amphetamine on (a) dopamine release from striatum or (b) locomotor activity. In studies (a), dialysis probes were inserted into the guide cannula, and baseline dopamine levels were collected for 2 hours before a challenge of amphetamine (1.5 mg/kg i.p.). Dialysates were then collected by 3 hours. Amphetamine challenge induced a significantly higher increase in dopamine release and locomotor activity in animals previously subjected to one restraint stress exposure, relative to that observed in the no-restraint stress group. MK-801 administered i.p. or intrastriatally blocked the restraint stress-induced sensitization to amphetamine. First, our results point out that a single restraint stress exposure is a pertinent stimulus to induce sensitization of amphetamine's stimulating effects on dopaminergic neurotransmission in the striatum. Secondly, NMDA-glutamatergic receptors, specifically those placed in the striatum, are implicated in the development of stress restraint-induced sensitization.

Motivational effects mu- and kappa-opioid agonists following acute and chronic restraint stress: involvement of dopamine D(1) and D(2) receptors

The influence of both acute and chronic restraint stress on the rewarding properties of morphine (1, 2 or 3 mg/kg i.p.) and the aversive effects of naloxone (0.5 mg/kg i.p. x3 or 1.0 mg/kg i.p.) or bremazocine (0.4 mg/kg i.p.) was investigated. An acute (2 h) but not chronic restraint (2 h daily for 7 days) enhanced the morphine place preference, and elicited a place aversion with a subthreshold dose of bremazocine. This enhancing effect on the reinforcing properties induced by the drugs was prevented by either R(+)-SCH-23390 hydrochloride (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H3-benzazepine, 30 microg/kg i.p.) or (+/-)-sulpiride (60 mg/kg i.p.), 10-20 min prior to the stress session. Naltrexone pretreatment (1 mg/kg i.p.) abolished the stress effect on morphine place preference but not that on bremazocine aversion. Instead, nor-BNI (30 microg/3 microl i.c.v.) abolished the stress's effects on bremazocine aversion, but did not modify those on morphine preference. These results show that: (1) acute stress enhanced the morphine and bremazocine conditioned reinforcing effects meanwhile chronic stress did not modify them; (2) the stimulation of D(1) and D(2) dopamine receptors is necessary for the development of restraint stress-induced sensitization to the conditioned reinforcing effects of drugs; and (3) the stimulation mu/delta- and kappa-opioid receptors seems to be differentially involved.

Heroin sensitization as mapped by c-Fos immunoreactivity in the rat striatum

Immunohistochemistry was used to map the induction of c-Fos protein in the forebrain of rats treated with heroin. Acute injection of heroin to drug-naive rats caused significant induction of c-Fos protein in the nucleus accumbens shell, whereas the same dose of heroin given to drug-sensitized rats significantly increased c-Fos immunoreactivity in the dorsomedial caudate-putamen. These results show that the heroin-induced pattern of c-Fos protein in the rat striatum differs according to the rat's drug history. These findings may represent a neural correlate of the motor components of heroin sensitization.

Increased conditioned fear response and altered balance of dopamine in the shell and core of the nucleus accumbens during amphetamine withdrawal

It has been suggested that neuroadaptations within the nucleus accumbens (NAC) dopaminergic (DA) projection contribute to the negative affect associated with psychostimulant withdrawal. The present study assessed the effects of amphetamine (AMPH) withdrawal on behavioral and NAC DA responses to conditioned fear stress. Animals injected with escalating-dose AMPH (1-5mg/kg, three injections/day, 6 days) or saline (SAL) acquired a tone-shock association on withdrawal day 3 and were tested for extinction of conditioned freezing to the tone on withdrawal day 4. Extracellular levels of NAC shell and core DA were monitored using in vivo microdialysis on both days. AMPH-withdrawn animals exhibited more conditioned freezing than SAL animals during both acquisition and extinction. During acquisition, DA increased more in the shell than the core of the NAC in both AMPH and SAL groups. During extinction to the tone, shell DA increased in SAL- but not AMPH-treated animals, whereas core DA activity was greater in AMPH than SAL animals. These data demonstrate that AMPH withdrawal alters the balance between shell and core DA transmission while increasing the behavioral expression of conditioned fear. Such drug-induced neuroadaptations in the NAC stress response may be involved in the exacerbation of negative emotions associated with drug withdrawal and stimulant-induced psychosis.

Repeated cocaine administration attenuates group I metabotropic glutamate receptor-mediated glutamate release and behavioral activation: a potential role for Homer

The present study aimed to characterize a functional role for group I metabotropic glutamate receptors (mGluRs) in the nucleus accumbens and the capacity of repeated cocaine to elicit long-term changes in group I mGluR function. Reverse dialysis of the group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) into the nucleus accumbens resulted in an increase in extracellular glutamate levels that was mediated by the mGluR1 subtype and depended on voltage-dependent Na(+) and Ca(2+) conductance. At 3 weeks after discontinuing 1 week of daily cocaine injections, the capacity of DHPG to induce glutamate release was markedly reduced. Similarly, DHPG induced an mGluR1-dependent increase in locomotor activity after microinjection into the nucleus accumbens that was significantly blunted 3 weeks after repeated cocaine administration. Signaling through group I mGluRs is regulated, in part, by Homer proteins, and it was found that the blunting of group I mGluR-induced glutamate release and motor activity after repeated cocaine was associated with a reduction in Homer1b/c protein that was selective for the medial nucleus accumbens. These data show that repeated cocaine produces an enduring inhibition of the neurochemical and behavioral consequences of stimulating mGluR1 that is accompanied by changes in the mGluR scaffolding apparatus.

Biphasic dose-related effects of morphine on dopamine release

The effects of morphine on extracellular dopamine levels in brain have never been studied over a wide range of doses within a single study. This has made it difficult to make definitive interpretations of drug interactions with morphine. An inhibition of morphine-induced increases in dopamine could be interpreted as either antagonism or potentiation depending the shape of the morphine dose-response curve. Accordingly, the aim of the present study was to determine the effects of a wide range of morphine doses (0, 5, 10, 20 and 30 mg/kg, i.p.) on extracellular dopamine, DOPAC and HVA levels in the nucleus accumbens and striatum of awake and freely moving female Sprague-Dawley rats. The results show that, in both brain regions, the dose-response curve for morphine-induced increases in dopamine is non-monotonic while the dose-response curve for morphine-induced increases in DOPAC and HVA is monotonic in the nucleus accumbens. The results of this study are discussed in terms of their implications for interpreting drug interactions with morphine and with relationship to morphine's mode of action at mu and kappa opioid receptors.

Dopamine characteristics in rat genotypes with distinct susceptibility to epileptic activity: apomorphine-induced stereotyped gnawing and novelty/amphetamine-induced locomotor stimulation

Rat genotypes differ in their susceptibility to spontaneously occurring spike-wave discharges and in their dopaminergic properties. In a previous study, it was found that spike-wave discharge incidence decreased in the following order in four rat genotypes during baseline and following injection with the dopamine antagonist haloperidol: apomorphine-susceptible (APO-SUS) > WAG/Rij > apomorphine-unsusceptible (APO-UNSUS) and ACI rats. The question in the present study was to what extent certain dopaminergic properties are pathognomonic for epileptic rats. Therefore, behavioral responses were assessed in order to investigate the dopaminergic properties in the four rat genotypes. Apomorphine-induced gnawing data imply that the dopamine activity of the nigrostriatal system in the WAG/Rij rats is higher than in APO-SUS but lower than in the ACI and APO-UNSUS rats. Furthermore, in previous studies APO-SUS have been shown to have a higher novelty/amphetamine-induced locomotion, indicative of a higher dopamine reactivity of the mesolimbic system as compared to APO-UNSUS rats. Results from the present study showed that WAG/Rij rats have a higher locomotor responsiveness to novelty/amphetamine, indicating a higher dopamine reactivity of the mesolimbic system in comparison to the ACI rats. It is suggested that the functional dopaminergic mesolimbic dominance is an important factor in the susceptibility to show spontaneously occurring spike-wave discharges.

The psychoactive ingredient of marijuana induces behavioural sensitization

Here we describe, for the first time, the occurrence of behavioural sensitization after chronic exposure to Delta9-tetrahydrocannabinol. Rats were treated twice a day, for five days, with increasing doses (5, 10, 20, 40, 40 mg/kg i.p.) of Delta9-tetrahydrocannabinol or its vehicle and after 20 days of withdrawal, animals were challenged with 5 mg/kg (i.p.) of the drug and their behaviour was assessed. Contrary to the motor inhibition induced in control rats, challenge with Delta9-tetrahydrocannabinol in pre-exposed animals elicited a complex behavioural syndrome mainly characterized by oral stereotyped items. Due to the relevance of behavioural sensitization in drug-seeking behaviour that persists long after discontinuation of drug use, our findings suggest that cannabinoids could trigger neurobiological alteration not dissimilar from those observed with more harmful abused drugs.

Behavioral effects of psychomotor stimulants in rats with dorsal or ventral subiculum lesions: locomotion, cocaine self-administration, and prepulse inhibition of startle

Compelling evidence suggests a primary role for the mesoaccumbens dopaminergic pathway in the behavioral effects of amphetamine and cocaine, but the roles of other projections to the accumbens, including those arising in the hippocampal formation, are less clear. The authors evaluated the effects of discrete excitotoxic lesions of either the dorsal or ventral subiculum on the locomotor activating, reinforcing, and sensorimotor gating-disruptive effects of psychomotor stimulant drugs. Whereas dorsal subiculum-lesioned rats were hyperactive in tests of exploratory locomotion and startle reactivity, ventral subiculum-lesioned rats exhibited an attenuated locomotor response to amphetamine, moderately impaired acquisition of cocaine self-administration, and reduced levels of prepulse inhibition of startle. These 2 behavioral profiles overlap considerably with those previously observed in rats with lesions of the rostrodorsal and caudomedial accumbens, respectively, and suggest that projections from dorsal subiculum to accumbens core and ventral subiculum to accumbens shell exert distinct influences on behavioral responses that are amplified by psychomotor stimulant drugs.

Behavioral sensitization to heroin by cannabinoid pretreatment in the rat

The behavioral consequences of acute heroin challenge (0.5 mg/kg, s.c.) were measured in rats previously submitted to repeated administration of increasing doses of the synthetic cannabinoid receptor agonist, R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN55212.2) (first day 2 mg/kg, second day 4 mg/kg, third day 8 mg/kg) or vehicle. Heroin administration to rats pretreated with vehicle produced catalepsy. The same dose of heroin in WIN55212.2-pretreated rats was followed by a marked increase of locomotor activity with stereotyped and non-stereotyped behaviors. These effects were blocked by the opioid receptor antagonist, naloxone. These findings indicate that pretreatment with WIN55212.2 produces cross-sensitization to heroin in the rat. These changes might reflect long-lasting changes of receptor population or transcriptional mechanisms in the mesolimbic system.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

Differential changes in synaptic terminal protein expression between nucleus accumbens core and shell in the amphetamine-sensitized rat

Repeated, intermittent administration of psychostimulant drugs such as D-amphetamine (AMPH) produces a state of behavioral sensitization to the drug that can last up to weeks to months. The molecular basis of this enhanced sensitivity to AMPH is poorly understood; however, adaptive changes in the mesocorticolimbic dopamine system has been postulated to be of primary importance. In the present investigation we used Western blotting to examine the expression of candidate presynaptic proteins involved in regulating neurotransmitter release and synaptic plasticity. Specifically, syntaxin 1, synaptophysin and synapsin I protein levels were examined in the nucleus accumbens (Nacc) and ventral tegmental area (VTA) of Sprague-Dawley rats following AMPH-sensitization. Animals received five repeated administrations of AMPH (1.5 mg/kg, i.p. on alternate days) followed by 14 days of withdrawal. Levels of syntaxin 1 and synaptophysin were found to be significantly reduced in the Nacc core of sensitized animals compared to saline-treated and untreated controls. However, syntaxin 1 expression was significantly increased in the Nacc shell subregion of sensitized animals. No significant difference in the level of synapsin I was noted in any of the brain regions. Further, expression of none of the synaptic proteins was significantly altered in the VTA of sensitized animals. Given the importance of syntaxin and synaptophysin in learning and memory processes and in the regulation of neurotransmitter release, changes in these proteins suggest their involvement in the associative learning aspects of sensitization and differential neurotransmitter release in the Nacc subregions.

Ultrastructural features of the nitric oxide synthase-containing interneurons in the nucleus accumbens and their relationship with tyrosine hydroxylase-containing terminals

The ultrastructural features of neuronal nitric oxide synthase (NOS) -immunoreactive interneurons of rat nucleus accumbens shell and core were studied and compared. The NOS-containing subpopulation displayed characteristics similar to those previously described for nicotinamide adenine dinucleotide phosphate diaphorase-, neuropeptide Y, or somatostatin-containing striatal neurons, but also showed properties not previously associated with them, particularly the formation of both asymmetric and symmetric synaptic junctions. Inputs derived mainly from unlabeled terminals, but some contacts were made by NOS-immunolabeled terminals, by means of asymmetric synapses. Immunopositive endings that formed symmetric synapses were mainly onto dendritic shafts, whereas those that formed asymmetric synapses targeted spine heads. Morphometric analysis revealed that the core and shell NOS-stained neurons had subtly different innervation patterns and that immunostained terminals were significantly larger in the shell. A parallel investigation explored synaptic associations with dopaminergic innervation identified by labeling with an antibody against tyrosine hydroxylase (TH). In both shell and core, TH-positive boutons formed symmetric synapses onto NOS-containing dendrites, and in the core, TH- and NOS-immunolabeled terminals converged on both a single spiny dendrite and a spine. These results suggest that, in the rat nucleus accumbens, NOS-containing neurons may be further partitioned into subtypes, with differing connectivities in shell and core regions. These NOS-containing neurons may be influenced by a dopaminergic input. Recent studies suggest that nitric oxide potentiates dopamine release and the current study identifies the medium-sized, densely spiny neurons as a possible site of such an interaction.

Dissociation in conditioned dopamine release in the nucleus accumbens core and shell in response to cocaine cues and during cocaine-seeking behavior in rats

The dopaminergic innervation of the nucleus accumbens is generally agreed to mediate the primary reinforcing and locomotor effects of psychostimulants, but there is less consensus on conditioned dopamine (DA) release during drug-seeking behavior. We investigated the neurochemical correlates of drug-seeking behavior under the control of a drug-associated cue [a light conditioned stimulus (CS+)] and to noncontingent presentations of the CS+ in the core and shell subregions of the nucleus accumbens. Rats self-administered cocaine under a continuous reinforcement schedule in which a response on one of two identical levers led to an intravenous cocaine infusion (0.25 mg/infusion) and a 20 sec light CS+. Response requirements for cocaine and the CS+ were then progressively increased until stable responding was established under a second-order schedule of reinforcement. During microdialysis, rats were presented noncontingently with a set of 10 sec CS+ and neutral tone stimuli (CS-) before and after a 90 min period during which they responded for cocaine under a second-order schedule. Results showed the following: (1) nucleus accumbens DA increased in both the core and shell during intravenous cocaine self-administration; (2) noncontingent presentations of a cocaine-associated CS+ led to increased DA release selectively in the nucleus accumbens core; and (3) extracellular DA levels were unaltered in both core and shell during a protracted period of drug-seeking behavior under the control of the same cocaine-associated cue. These results indicate that the mesolimbic dopamine system is activated after exposure to drug-associated stimuli under specific conditions.

Neurotoxic regimen of methamphetamine produces evidence of behavioral sensitization in the rat

A neurotoxic regimen of methamphetamine (MA) produces long-term depletions in neostriatal dopamine and serotonin concentrations. In addition to evidence of dopaminergic and serotonergic neurotoxicity, there is evidence of MA-induced behavioral changes. In this regard, stereotypic behavior elicited by MA is greater in rats treated previously with a neurotoxic regimen of MA than in control animals. The present study was designed to determine whether the enhanced stereotypy observed in MA-treated rats is due to the MA-induced loss of dopamine (neurotoxicity) or to the repeated exposure to MA (sensitization). Rats were treated with MA (10 mg/kg every 2 h for four injections) or vehicle at either a normal (24 degrees C) room temperature or a cold (4 degrees C) room temperature, which has been shown to attenuate the MA-induced loss of dopamine. Stereotypy was assessed 7 days after treatment. Rats that had received a neurotoxic regimen of MA at 24 degrees C exhibited 49% and 45% reductions in neostriatal dopamine and serotonin concentrations, respectively, whereas rats treated with MA at 4 degrees C had no significant neurochemical depletions. Stereotypy elicited by MA (5.0 mg/kg) was significantly greater in rats treated with a neurotoxic regimen of MA regardless of the initial treatment temperature. In addition, an injection of apomorphine (0.5 mg/kg) elicited an enhanced stereotypic response in MA-treated rats. These data suggest that the augmented stereotypic behavior observed in rats treated with a neurotoxic regimen of MA is not due to the loss of dopamine, but rather the manifestation of behavioral sensitization, possibly due to an increase in dopamine receptor sensitivity.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Effects of repeated morphine on cerebral dopamine release and metabolism in AA and ANA rats

Cerebral dopaminergic mechanisms were studied in the nucleus accumbens and caudate-putamen of alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats after 4-day repeated morphine treatment. This treatment has been shown to enhance the locomotor activity stimulating effect of morphine in the AA but not in the ANA rats. Morphine (1 or 3 mg/kg) or saline was administered subcutaneously once daily and the extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured, in freely moving rats by in vivo microdialysis on days 1 and 4. Morphine increased accumbal DA, DOPAC and HVA similarly in rats of both lines, and no sensitization of DA release or metabolism was seen in rats of either line given morphine repeatedly. In the caudate-putamen, morphine increased DA, DOPAC and HVA significantly only in the AA rats. During repeated treatment, the morphine-induced elevation of DA metabolites, but not that of DA, was enhanced similarly in rats of both lines. These results suggest that the effects of acute morphine administration on nigrostriatal dopaminergic mechanisms are stronger in the AA than in the ANA rats, whereas the effects of morphine on mesolimbic dopaminergic systems do not differ. Furthermore, in rats of both lines, repeated morphine treatment enhanced the responses of the nigrostriatal dopaminergic systems similarly, but no enhancement occurred in the mesolimbic systems of rats of either line. These findings do not support the critical role of accumbal dopaminergic systems in morphine-induced behavioural sensitization.

The effects of nicotine on locomotor activity and dopamine overflow in the alcohol-preferring AA and alcohol-avoiding ANA rats

The aim of the study was to investigate the importance of the interaction between central dopaminergic and cholinergic mechanisms for ethanol reinforcement. This was done by comparing the effects of nicotine on locomotor activity and release of dopamine in the nucleus accumbens of the alcohol-preferring Alko Alcohol (AA) and alcohol-avoiding alko non-alcohol (ANA) rats. Nicotine was administered acutely (0.25, 0.50 or 0.75 mg/kg, s.c.) or repeatedly once daily (0.5 mg/kg, s.c.) for 8 days. An acute dose of nicotine increased locomotor activity and the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) measured with in vivo microdialysis suggesting stimulation of dopamine release by nicotine. No difference in the stimulation of locomotor activity or in the increase in the extracellular concentrations of dopamine or its metabolites by nicotine was found between the rat lines. The concentrations of nicotine in the plasma were also identical. The rats treated repeatedly with nicotine showed a progressive increase in locomotion. On the challenge day, 1 week after termination of nicotine or saline injections, rats previously treated with nicotine were activated more by nicotine than saline-treated rats. This behavioral sensitization was not accompanied by an increase in the amplitude of the neurochemical response to nicotine, but the duration of the increase in the levels of DOPAC was longer in the nicotine than saline-treated animals. The increases in locomotor activity and metabolite levels were, however, similar in both rat lines. These data suggest that differences in the interaction of central dopaminergic and cholinergic mechanisms probably do not contribute to the difference in ethanol self-administration between the AA and ANA rat lines.

Long-term lithium administration abolishes the resistance to stress in rats sensitized to morphine

Morphine sensitized rats appear protected from the sequelae of an unavoidable stress: when exposed to stress (after a 7-day morphine wash-out) and then tested for escape, they perform like naive animals. This protection appears similar to that induced by chronic imipramine treatment, as it is antagonized by the inhibition of D(1)-dopamine receptors before exposure to unavoidable stress. Repeated unavoidable stress induces in rats a condition characterized by hyporeactivity to noxious stimuli and reverted by long-term antidepressant treatments, and this state is regarded as an experimental model of depression. The resistance to stress in morphine sensitized rats could be considered as the behavioral counterpart of the sensitivity to stress in control rats, i.e. as a model of mania. The aim of the present study was to validate such a putative model by studying whether the resistance to stress induced by morphine sensitization would respond to a long-term administration of lithium, the reference antimanic drug. Long-term lithium treatment induces in rats a condition of hyporeactivity to noxious stimuli, accompanied by decreased levels of dopamine in the nucleus accumbens shell. In morphine sensitized rats chronic lithium abolished the resistance to stress, but it did not modify the D(1)-dopamine receptor mediated response to morphine, nor did it modify the levels of extraneuronal dopamine in the nucleus accumbens shell. Thus, lithium treatment abolished the resistance to stress in morphine sensitized rats, conferring predictive validity to the paradigm. Moreover, it did so through a mechanism which appeared to be independent of D(1)-dopamine receptor activity.

Interactions between 18-methoxycoronaridine (18-MC) and cocaine: dissociation of behavioural and neurochemical sensitization

The phenomenon of sensitization has been implicated in various aspects of drug addiction. As such, the present study determined the effects of a potential anti-addictive agent, 18-methoxycoronaridine (18-MC; 40 mg/kg, IP, 19 h earlier), on the expression of sensitization following the repeated administration of cocaine (COC; five once daily injections of 15 mg/kg, IP) or saline. The effects of 18-MC on COC metabolism were also assessed. Compared to vehicle controls, 18-MC significantly enhanced the expression of COC-induced locomotion (0, 10, 20 and 40 mg/kg, IP) in chronic COC treated rats only. In both acute and chronic COC rats, 18-MC potentiated the stereotypy induced by higher COC doses (20 and 40 mg/kg, IP). In contrast, 18-MC abolished the sensitized dopamine (DA) response in the nucleus accumbens (NAC) to COC (20 mg/kg), without altering the DA response of acute COC rats. None of the interactions between 18-MC and COC appear to be related to alterations in COC metabolism as no effect of 18-MC pretreatment was observed on extracellular levels of COC or two of its metabolites, benzoylecogonine and norcocaine. From the present findings, it is concluded that the enhancement of COC-induced behaviour produced by 18-MC pretreatment is independent of effects on either COC pharmacokinetics or COC-induced alterations in DA transmission. However, given that 18-MC decreases the self-administration of COC in laboratory animals, it is proposed that the anti-addictive efficacy of 18-MC might be related to an ability to selectively block the expression of sensitized extracellular levels of DA in the NAC in rats with previous COC experience.

Differential effects of ibogaine on behavioural and dopamine sensitization to cocaine

To investigate a possible basis for the proposed anti-addictive property of ibogaine, the effects of ibogaine (40 mg/kg, i.p., 19 h earlier) on the expression of sensitization induced by cocaine were investigated. Ibogaine pretreatment potentiated the increase in the stereotypic effects of a cocaine challenge (20 mg/kg) in both sensitized (5 x 15 mg/kg, i.p.) and acutely treated rats. However, while ibogaine pretreatment did not significantly alter the dopamine response in the nucleus accumbens to acute cocaine, it abolished the expression of cocaine-induced dopamine sensitization. This result demonstrates that ibogaine pretreatment can reverse one of the neuroadaptations produced by chronic cocaine administration, an effect that may contribute to its putative anti-addictive property.

State-dependent behavioural sensitization: evidence from a chlordiazepoxide state

The present study investigated the hypothesis that behavioural sensitization to psychomotor stimulants is expressed only if the internal state of the animal is the same as it was at the time of sensitization development. This state-dependency hypothesis has previously been put forward to explain the apparent blockade of sensitization by N-methyl-D-aspartate (NMDA) receptor antagonists. The present study used amphetamine (0.375 mg/kg) as the stimulant and chlordiazepoxide (CDP) as a secondary stimulus. Mice were given double injections of either amphetamine + CDP, amphetamine + saline, CDP + saline, or saline + saline daily over 8 days. On the ninth and tenth days, all mice were challenged with amphetamine + saline and with CDP + saline, in a counterbalanced design. CDP does not show evidence of locomotor sensitization and does not act at NMDA receptors, but in other studies it has been shown to give rise to state-dependent-like effects. Thus any progressive augmentation of the response to repeated amphetamine + CDP treatment would be attributable to amphetamine sensitization, and any blockade of sensitization would not be due to NMDA receptor antagonism. Both groups receiving amphetamine became sensitized over the first 8 days (shown by progressive increases in locomotion). When challenged with amphetamine alone, the amphetamine + CDP group failed to show a sensitized response. This observation supports the state-dependency hypothesis and emphasizes the importance of considering the context provided by drug-induced internal states in studies of sensitization.

Role of dopamine in the behavioural actions of nicotine related to addiction

Experimental impairment of dopamine function by 6-hydroxydopamine lesions or by dopamine receptor antagonists shows that dopamine is involved in nicotine's discriminative stimulus properties, nicotine-induced facilitation of intracranial self-stimulation, intravenous nicotine self-administration, nicotine conditioned place-preference and nicotine-induced disruption of latent inhibition. Therefore, nicotine depends on dopamine for those behavioural effects that are most relevant for its reinforcing properties and are likely to be the basis of the abuse liability of tobacco smoke. On the other hand, in vivo monitoring studies show that nicotine stimulates dopamine transmission in specific brain areas and in particular, in the shell of the nucleus accumbens and in areas of the extended amygdala. These effects of nicotine resemble those of a reward like food except that nicotine-induced release of dopamine does not undergo single-trial, long-lasting habituation. It is speculated that repeated non-habituating stimulation of dopamine release by nicotine in the nucleus accumbens shell abnormally facilitates associative stimulus-reward learning. Acute effects of nicotine on dopamine transmission undergo acute and chronic tolerance; with repeated, discontinuous exposure, sensitization of nicotine-induced stimulation of dopamine release in the nucleus accumbens core takes place while the response in the shell is reduced. It is speculated that these adaptive changes are the substrate of a switch from abnormal incentive responding controlled by consequences (action-outcome responding) into abnormal habit responding, triggered by conditional stimuli and automatically driven by action schemata relatively independent from nicotine reward. These two modalities might coexist, being utilized alternatively in relation to the availability of tobacco. Unavailability of tobacco disrupts the automatic, implicit modality of abnormal habit responding switching responding into the explicit, conscious modality of incentive drug-seeking and craving.

Psychostimulant sensitization: differential changes in accumbal shell and core dopamine

The nucleus accumbens has been subdivided into a shell and a core compartment on the basis of histochemical and connectional differences. Recently, we reported that behavioral sensitization to morphine is associated with an increased dopamine transmission in the caudate-putamen and in the nucleus accumbens core as well as a decreased response in the nucleus accumbens shell following acute morphine challenge. We have now performed a similar study in rats sensitized to amphetamine and to cocaine. Behavioral sensitization was induced by daily administration of a single dose of 1 mg/kg s.c. of amphetamine for 10 days or of 10 mg/kg i.p. of cocaine twice a day for 14 days. Microdialysis was performed 10-14 days after the last injection of amphetamine and 7-10 days after the last injection of cocaine. Both schedules resulted in robust behavioral sensitization in response to challenge with 0.25 and 0.5 mg/kg of amphetamine and to 5 and 10 mg/kg of cocaine, respectively. Subjects pre-exposed to amphetamine showed a sensitization of dopamine transmission in the nucleus accumbens core but not in the nucleus accumbens shell. Subjects pre-exposed to cocaine showed sensitization of dopamine transmission in the core only to the lower dose of cocaine. In the shell no change was observed after the lower dose of cocaine while a significant reduction of the dopamine response was observed after the higher dose. These results suggest that behavioral sensitization might result from reciprocal changes in the response of nucleus accumbens dopamine in the shell and in the core to drug challenge.

Differential changes in accumbens shell and core dopamine in behavioral sensitization to nicotine

Repeated treatment with nicotine has been shown to sensitize rats to its locomotor stimulant effects and to its properties to stimulate mesolimbic dopamine transmission. We investigated the relationship between sensitization of nicotine induced locomotor stimulation and activation of dopamine transmission in the nucleus accumbens shell and core. Rats were administered daily for 5 days with 0.4 mg/kg s.c. of nicotine or with saline and 24 h later, dopamine was monitored by microdialysis in the shell and in the core of nucleus accumbens and behavioral activity was scored after challenge with nicotine (0. 4 mg/kg s.c.). Behavioral sensitization to nicotine was associated with a reduced response of dopamine transmission in the shell and with an increased one in the core of nucleus accumbens.

Drug addiction as dopamine-dependent associative learning disorder

Natural rewards preferentially stimulate dopamine transmission in the nucleus accumbens shell. This effect undergoes adaptive changes (one-trial habituation, inhibition by appetitive stimuli) that are consistent with a role of nucleus accumbens shell dopamine in associative reward-related learning. Experimental studies with a variety of paradigms confirm this role. A role in associative stimulus-reward learning can provide an explanation for the extinction-like impairment of primary reinforcement that led Wise to propose the 'anhedonia hypothesis'. Addictive drugs share with natural rewards the property of stimulating dopamine transmission preferentially in the nucleus accumbens shell. This response, however, in contrast to that to natural rewards, is not subjected to one-trial habituation. Resistance to habituation allows drugs to activate dopamine transmission in the shell non-decrementally upon repeated self-administration. It is hypothesized that this process abnormally strengthens stimulus-drug associations thus resulting in the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behaviour acquired by drug-related stimuli as a result of abnormal associative learning following repeated stimulation of dopamine transmission in the nucleus accumbens shell.

Drug addiction as a disorder of associative learning. Role of nucleus accumbens shell/extended amygdala dopamine

Conventional reinforcers phasically stimulate dopamine transmission in the nucleus accumbens shell. This property undergoes one-trial habituation consistent with a role of nucleus accumbens shell dopamine in associative learning. Experimental studies with place- and taste-conditioning paradigms confirm this role. Addictive drugs share with conventional reinforcers the property of stimulating dopamine transmission in the nucleus accumbens shell. This response, however, undergoes one-trial habituation in the case of conventional reinforcers but not of drugs. Resistance to habituation allows drugs to repetitively activate dopamine transmission in the shell upon repeated self-administration. This process abnormally facilitates associative learning, leading to the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behavior acquired by drug-related stimuli as a result of abnormal strenghtening of stimulus-drug contingencies by nondecremental drug-induced stimulation of dopamine transmission in the nucleus accumbens shell.