Effect of 6-hydroxydopamine lesions of the amygdala on intravenous cocaine self-administration under a progressive ratio schedule of reinforcement

Dopamine and Addiction

Addiction is commonly identified with habitual nonmedical self-administration of drugs. It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Such addictions can also be defined in terms of the brain mechanisms they activate; most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig; they have only unconditioned consummatory reflexes. Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.

Projection-Target-Defined Effects of Orexin and Dynorphin on VTA Dopamine Neurons

Circuit-specific signaling of ventral tegmental area (VTA) dopamine neurons drives different aspects of motivated behavior, but the neuromodulatory control of these circuits is unclear. We tested the actions of co-expressed lateral hypothalamic peptides, orexin A (oxA) and dynorphin (dyn), on projection-target-defined dopamine neurons in mice. We determined that VTA dopamine neurons that project to the nucleus accumbens lateral shell (lAcbSh), medial shell (mAcbSh), and basolateral amygdala (BLA) are largely non-overlapping cell populations with different electrophysiological properties. Moreover, the neuromodulatory effects of oxA and dyn on these three projections differed. OxA selectively increased firing in lAcbSh- and mAcbSh-projecting dopamine neurons. Dyn decreased firing in the majority of mAcbSh- and BLA-projecting dopamine neurons but reduced firing only in a small fraction of those that project to the lAcbSh. In conclusion, the oxA-dyn input to the VTA may drive reward-seeking behavior by tuning dopaminergic output in a projection-target-dependent manner.

The basolateral amygdala in reward learning and addiction

Sophisticated behavioral paradigms partnered with the emergence of increasingly selective techniques to target the basolateral amygdala (BLA) have resulted in an enhanced understanding of the role of this nucleus in learning and using reward information. Due to the wide variety of behavioral approaches many questions remain on the circumscribed role of BLA in appetitive behavior. In this review, we integrate conclusions of BLA function in reward-related behavior using traditional interference techniques (lesion, pharmacological inactivation) with those using newer methodological approaches in experimental animals that allow in vivo manipulation of cell type-specific populations and neural recordings. Secondly, from a review of appetitive behavioral tasks in rodents and monkeys and recent computational models of reward procurement, we derive evidence for BLA as a neural integrator of reward value, history, and cost parameters. Taken together, BLA codes specific and temporally dynamic outcome representations in a distributed network to orchestrate adaptive responses. We provide evidence that experiences with opiates and psychostimulants alter these outcome representations in BLA, resulting in long-term modified action.

Inactivation of the central nucleus of the amygdala reduces the effect of punishment on cocaine self-administration in rats

Continued cocaine use despite the negative consequences is a hallmark of cocaine addiction. One such consequence is punishment, which is often used by society to curb cocaine use. Unfortunately, we know little about the mechanism involved in regulation by punishment of cocaine use. The fact that cocaine addicts continue to use cocaine despite potentially severe punishment suggests that the mechanism may be impaired. Such impairment is expected to critically contribute to compulsive cocaine use. This study was aimed at testing the hypothesis that the central nucleus of the amygdala (CeN) plays a critical role in such regulation. To this end, rats were trained to press a lever to self-administer cocaine under a chained schedule: a response on one lever (cocaine-seeking lever) led to access to the other lever (cocaine-taking lever), on which a response was reinforced by cocaine and cues. Thereafter, responses on the seeking lever were punished by footshock with a probability of 0.5. Cocaine self-administration (SA) was significantly suppressed by punishment in an intensity-dependent manner. Interestingly, rats trained with daily 6-h (extended access) but not 2-h (limited access) sessions showed resistance to the lower intensity of punishment. Inactivation of the CeN induced a robust anti-punishment effect in both groups. These data provided evidence that the CeN is a critical neural substrate involved in regulation by punishment of cocaine SA. Rats with a history of extended cocaine SA appeared to be less sensitive to punishment. The decreased sensitivity could result from the neuroplastic changes induced by extended cocaine SA in the CeN.

Microinjection of the delta-opioid receptor selective antagonist naltrindole 5'-isothiocyanate site specifically affects cocaine self-administration in rats responding under a progressive ratio schedule of reinforcement

Whether the delta-opioid receptor (DOR) system can modulate behavioral effects of cocaine remains equivocal. We examined whether site- and subtype-selective blockade of DORs within the rat mesocorticolimbic system affects cocaine self-administration. The DOR antagonist naltrindole 5'-isothiocyanate (5'-NTII; 5nmol) was microinjected into the nucleus accumbens (NAcc), ventral tegmental area (VTA), or amygdala (AMYG) in rats self-administering 1.5mg/kg cocaine under a progressive ratio (PR) schedule. Intra-NAcc 5'-NTII significantly decreased cocaine self-administration, while 5'-NTII administration into the VTA significantly increased cocaine-maintained responding. 5'-NTII administration into the AMYG produced no effect. These data support a site-specific role of DORs in cocaine's behavioral effects.

The mesolimbic dopamine system: the final common pathway for the reinforcing effect of drugs of abuse?

In this review we will critically assess the hypothesis that the reinforcing effect of virtually all drugs of abuse is primarily dependent on activation of the mesolimbic dopamine system. The focus is on five classes of abused drugs: psychostimulants, opiates, ethanol, cannabinoids and nicotine. For each of these drug classes, the pharmacological and physiological mechanisms underlying the direct or indirect influence on mesolimbic dopamine transmission will be reviewed. Next, we evaluate behavioral pharmacological experiments that specifically assess the influence of activation of the mesolimbic dopamine system on drug reinforcement, with particular emphasis on animal experiments using drug self-administration paradigms. There is overwhelming evidence that all five classes of abused drugs increase dopamine transmission in limbic regions of the brain through interactions with a variety of transporters, ionotropic receptors and metabotropic receptors. Behavioral pharmacological experiments indicate that increased dopamine transmission is clearly both necessary and sufficient to promote psychostimulant reinforcement. For the other four classes of abused substances, self-administration experiments suggest that although increasing mesolimbic dopamine transmission plays an important role in the reinforcing effects of opiates, ethanol, cannabinoids and nicotine, there are also dopamine-independent processes that contribute significantly to the reinforcing effects of these compounds.

Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.

Effect of HD-23, a potent long acting cocaine-analog, on cocaine self-administration in rats

RATIONALE

"Agonist" therapy for drug addiction proposes that a long acting analog, with similar properties to the abused substance might serve as a useful therapeutic agent. HD-23 is a very long acting tropane analog that displays a neurochemical profile similar to cocaine.

OBJECTIVE

To determine, using self-administration procedures and three different schedules of reinforcement, the effect of HD-23 on rate of cocaine intake (fixed ratio), the effect of HD-23 on the motivation to respond (progressive ratio) and the time course of HD-23 pretreatment (discrete trials).

METHODS

Male Sprague-Dawley rats were implanted with chronically indwelling intravenous cannulae and trained to self-administer cocaine (1.5 mg/kg per infusion) on a fixed ratio schedule. After a stable baseline was established, separate groups of rats ( n=6-8) were given access to various doses of cocaine (0.37, 0.75, 1.5 or 3.0 mg/kg per injection) on a fixed ratio schedule during daily 3-h sessions, or to various doses of cocaine (0.18, 0.37, 0.75, 1.5 mg/kg per injection) on a progressive ratio schedule during daily 5-h sessions. A separate group of rats ( n=10) was tested using a discrete trials procedure; animals were given the opportunity to self-administer cocaine (1.5 mg/kg per injection) during 10-min trials which were initiated every 20 min throughout the day/night cycle.

RESULTS

On the FR schedule, pretreatment with HD-23 (1.0 mg/kg) decreased the rate of cocaine intake. HD-23 shifted the dose-response curve on the PR schedule to the left. On the discrete trials schedule, animals displayed a circadian pattern of drug intake; pretreatment with HD-23 significantly increased cocaine intake for about 8 h during the light phase when the probability of responding would otherwise have been very low. Animals pretreated with HD-23 displayed a high probability of cocaine self-administration for about 14 h.

CONCLUSIONS

The results are consistent with the idea that an acute pretreatment with the long-acting agonist, HD-23, augmented rather than diminished the motivation to self-administer cocaine.

Enhanced motivation to self-administer cocaine is predicted by self-grooming behaviour and relates to dopamine release in the rat medial prefrontal cortex and amygdala

Rats, like humans, show strong individual differences in their response to anxiogenic and stressful stimuli. In the present study we evaluated whether differences in stress-induced self-grooming behaviour may predict an individual's vulnerability to engage in drug self-administration behaviour. From a population of Wistar rats, the lower and upper quartile with respect to time spent self-grooming on an elevated plus maze (EPM) were selected and trained to intravenously self-administer cocaine under fixed and progressive ratio schedules of reinforcement. High grooming (HG) rats reached considerably higher breakpoints than low grooming (LG) rats but showed no differences in acquisition rate and dose-response relationships. Further, EPM exposure elicited higher anxiety levels and enhanced plasma corticosterone secretion in HG rats. In addition, HG rats did not display enhanced novelty-seeking and still spent more time self-grooming during an EPM re-test following the cocaine self-administration procedure, indicating that stress-induced self-grooming is a stable behavioural trait marker. Neurochemically, electrically evoked [(3)H]dopamine release in vitro was profoundly lower in brain slices from the substantia nigra, medial prefrontal cortex and amygdala of naive HG rats as compared to LG rats, whereas no differences were found in the nucleus accumbens shell and core, the ventral tegmental area and caudate putamen. In conclusion, stress-induced self-grooming specifically predicts enhanced motivation to self-administer cocaine rather than sensitivity to its reinforcing effects. Responsiveness of dopaminergic nerve terminals in the medial prefrontal cortex and amygdala may represent pre-existing underlying factors.

The dorsomedial shell of the nucleus accumbens facilitates cocaine-induced locomotor activity during the induction of behavioral sensitization

The mesolimbic dopamine system has been intensely studied as the neural circuit mediating the locomotor response to psychostimulants and behavioral sensitization. In particular, the dopaminergic innervation of the nucleus accumbens has been implicated as a site responsible for the manifestations of behavioral sensitization. Previous studies have demonstrated an augmented release of dopamine in the nucleus accumbens upon a systemic injection of a psychostimulant. In addition, alterations in the dopaminergic innervation patterns in this brain region have been demonstrated in animals that received repeated injections of cocaine. Furthermore, lesions of projection sites that have terminations in the nucleus accumbens have demonstrated alterations in psychostimulant induced locomotion, both acutely, as well as in sensitization paradigms. Since dopamine in the nucleus accumbens is believed to regulate several excitatory amino acid inputs, the present study examined the effects of a localized electrolytic lesion in the dorsomedial shell of the nucleus accumbens in order to better understand the functional role this brain region has in behavioral sensitization. All animals received bi-daily injections of 15 mg/kg i.p. cocaine. Only those demonstrating behavioral sensitization after a subsequent challenge dose were included in the analysis. Following acute exposure to cocaine, lesioned animals did not show any difference in their locomotor response when compared with sham controls. However, after repeated exposure to cocaine, sensitized animals demonstrated a significant attenuation in locomotor behavior when compared with sensitized sham controls. This decrease in horizontal locomotion persisted 2 days into withdrawal, yet dissipated in the sensitized animals that were challenged 2 weeks following their last injection. The data presented here demonstrate that the dorsomedial shell of the nucleus accumbens plays an important role in the initial stages of behavioral sensitization to cocaine.

Assessment of tyrosine hydroxylase immunoreactive innervation in five subregions of the nucleus accumbens shell in rats treated with repeated cocaine

To explore the effects of behavioral sensitization on the anatomy of the nucleus accumbens shell, we employed a typical cocaine dosing paradigm and assessed tyrosine hydroxylase immunoreactive varicosities in five different areas of the shell, as well as the core of the nucleus accumbens. Rats were given bidaily injections of either saline (1 ml/kg i.p.) or cocaine (15 mg/kg i.p.) for 5 consecutive days, and sacrificed either 2 or 14 days from the last injection. Sections of the nucleus accumbens were processed for tyrosine hydroxylase immunoreactivity and the number of immunoreactive varicosities in contact with neuronal cell bodies was quantified in each of the subregions of the shell, as well as the core of the nucleus accumbens. Compared to saline controls, the cocaine-treated animals showed a significant augmentation in tyrosine hydroxylase immunoreactivity in two of the five subregions after 2 days of withdrawal in the shell, but not in the core. No differences were found in any region tested after 14 days of withdrawal. These data are the first to suggest that increases in nucleus accumbens presynaptic tyrosine hydroxylase may play a role in the development of behavioral sensitization, but not in the long-term expression of this phenomenon.

GABA modulation of cocaine self-administration

Recent experiments suggest that GABA compounds produce a clinically relevant modulation of cocaine reinforcement. This review summarizes the results of a number of studies that examined the effect of the GABAB agonist baclofen on cocaine self-administration using a variety of schedules of reinforcement. The results demonstrate that baclofen causes a dramatic reduction in cocaine self-administration which does not appear to be accounted for by a general disruption of behavior. However, the effect is dependent on the unit injection dose of cocaine and on the response requirements of the schedule. These results predict that in a clinical setting any potential therapeutic effect of baclofen will interact with the cost and availability of cocaine.

Naltrexone affects cocaine self-administration in naïve rats through the ventral tegmental area rather than dopaminergic target regions

Behavioural studies have shown an involvement of central endogenous opioid systems in experimental cocaine addiction. Seeking to further localize the attenuating effect of opioid blockade on the reinforcing effects of cocaine, naltrexone was administered locally to different regions of the mesocorticolimbic system, which are thought to be critically involved in cocaine self-administration behaviour. Both cell body and nerve terminal regions of this system were targeted. Using a model for the initiation of cocaine self-administration behaviour, no effect of naltrexone was found in caudate, amygdaloid or accumbens nuclei, nor in the medial prefrontal cortex. However, blockade of endogenous opioid receptors in the ventral tegmental area region attenuated cocaine self-administration. With the initiation model, this finding reflects an attenuating effect on the reinforcing effects of cocaine. The attenuation of self-administration was dependent on the naltrexone dose. The present findings suggest that endogenous opioid systems in the ventral tegmental area modulate the reinforcing efficacy of cocaine.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Effects of the self-administration of ethanol and ethanol/sucrose on rates of local cerebral glucose utilization in rats

In a previous study, the voluntary ingestion of ethanol by rats was found to be associated with a discrete pattern of changes in functional activity that included the nucleus accumbens, medial prefrontal cortex, basolateral and central nuclei of the amygdala, as well as the ventral midbrain. Rats in this study, however, consumed a combination of ethanol in a sucrose vehicle. The purpose of the present experiment was to characterize the role of sucrose in determining the effects of orally self-administered ethanol using the quantitative autoradiographic 2-[14C]deoxyglucose (2DG) method for measurement of rates of local cerebral glucose utilization. A modified sucrose-substitution procedure was employed to train three groups of Wistar rats to self-administer either water, 10% ethanol (10E), or a 10% ethanol/2% sucrose solution (10E/2S) in daily sessions. An additional group of rats was trained using a modified acclimation procedure (home cage) in order to determine if any exposure to sucrose would alter rates of glucose utilization. Once stable rates of consumption were established, the 2DG method was applied immediately following completion of the final test session. Rats received a dose of ethanol equivalent to 0.5 g kg-1 on the day of the procedure or a comparable volume of water. Rates of energy metabolism were significantly increased in all three groups of rats that consumed ethanol (10E/2S, 10E, and home cage), as compared to rates in rats that consumed water. The areas of significant change included the rostral pole and posterior shell of the nucleus accumbens, medial prefrontal cortex, the basolateral and central nuclei of the amygdala, the ventral tegmental area, and the substantia nigra pars compacta. Thus, the pattern of changes in functional brain activity that accompanies voluntary ingestion of ethanol is independent of the vehicle in which the ethanol is presented or the procedures used to initiate consumption. Furthermore, these data demonstrate that it is the simultaneous activation of an interrelated network of limbic brain regions that serves as the substrate of the effects of ethanol self-administration.

A critique of fixed and progressive ratio schedules used to examine the neural substrates of drug reinforcement

This paper is a critique of fixed and progressive ratio schedules used to examine the neural substrates of cocaine reinforcement. The discussion focuses on problems encountered while examining the effects of neurotoxic lesions and pharmacological pretreatments on cocaine reinforcement. We review the theoretical and interpretational problems associated with the use of the fixed ratio (FR) schedules that have been used in the majority of studies, and we conclude that rate of drug intake cannot directly address the issue of increased or decreased reinforcer efficacy. The progressive ratio (PR) schedule offers some advantages over FR schedules, although it is now clear that the same implementation cannot be applied across all drug classes. It is likely that the motivation to self-administer psychostimulant vs. opiate drugs is qualitatively different. We conclude that there is no single schedule that can quantify all aspects of drug reinforcement and that behavioral paradigms will need to be adapted according to the particular question under study.

Progressive ratio schedules in drug self-administration studies in rats: a method to evaluate reinforcing efficacy

Drug self-administration studies have recently employed progressive ratio (PR) schedules to examine psychostimulant and opiate reinforcement. This review addresses the technical, statistical, and theoretical issues related to the use of the PR schedule in self-administration studies in rats. Session parameters adopted for use in our laboratory and the considerations relevant to them are described. The strengths and weaknesses of the PR schedule are also discussed.

Effect of nucleus accumbens dopamine depletion on motivational aspects involved in initiation of cocaine and heroin self-administration in rats

The involvement of mesolimbic dopamine (DA) systems in motivational aspects of drug-taking behavior during initiation of drug self-administration was investigated using a recently developed behavioral paradigm. In separate experiments animals were allowed to self-administer cocaine or heroin (0.16 and 0.32 mg . kg-1 per inf) during 5 consecutive daily 3-h sessions. During a 15-min period preceding the last four self-administration sessions lever-press behavior was measured in absence of the drug as an index of the motivational aspects involved in drug-taking behavior. The effect of 6-hydroxydopamine (6-OHDA) lesion of the nucleus accumbens (NAC) on lever-press behavior before and during self-administration was measured. Destruction of DA terminals in the NAC did not affect initiation of heroin self-administration nor the lever-press behavior during the period preceding the self-administration sessions. In cocaine animals 6-OHDA lesion of the NAC decreased the total intake of cocaine during the self-administration sessions and impaired discriminative lever-responding for the drug, both during cocaine self-administration, and during preceding periods when no cocaine was available. It is concluded that DAergic systems in the NAC might be involved in the reinforcement and/or motivational processes underlying cocaine self-administration. The present findings, however, do not support the notion of a critical role of NAC DA in the motivational aspects of drug-taking behavior in general.

Memory for the changing cost of a reward is mediated by the sublenticular extended amygdala

The aim of this study was to examine the role of the sublenticular extended amygdala (SEA) in processes of reward and reinforcement. Previous studies have examined the effects of ibotenate lesions in this area on motivation for cocaine reward. In this study, animals were trained to work for sucrose pellets, rather than a drug, on a progressive-ratio schedule of reinforcement. Bilateral intracerebral infusions of ibotenic acid (lesion group) or vehicle (control group) were made into the SEA, following the same procedures as used in previous studies. After recovery from surgery, animals were tested for six sessions on the progressive ratio schedule. The lesion did not result in motivational impairments of the kind that have previously been reported: rather than decreases in breaking point (a measure of motivational strength), the lesion resulted in greater variability of breaking points, with a tendency for lesioned animals to work harder for reward than controls. The SEA-lesioned rats did not show the increase in postreinforcement pause that usually accompanies the increase in perceived work as the number of bar presses for a reward increases. Histological analyses showed that the ibotenate lesions had successfully destroyed the SEA and that damage was also present in adjacent structures. The results are interpreted in terms of a mnemonic, rather than a motivational, deficit.

Effects of the dopamine D-1 antagonist SCH 23390 microinjected into the accumbens, amygdala or striatum on cocaine self-administration in the rat

This study tested the hypothesis that blockade of D-1 dopamine receptors in the nucleus accumbens shell, central nucleus of the amygdala or dorsal striatum by intracerebral microinjection of the dopamine antagonist SCH 23390 produces an attenuation of the effects of self-administered cocaine. Microinjection of SCH 23390 (0-4.0 micrograms total dose) into any of the three brain regions dose-dependently increased the rate of cocaine self-administration, consistent with a partial attenuation of the effects of cocaine under these conditions (0.25 mg cocaine i.v.; fixed-ratio 5 timeout 20 s). The regional rank order potency of SCH 23390 was accumbens > amygdala > striatum, striatal injections being equipotent with subcutaneous administration. Moreover, SCH 23390 produced rapid effects on cocaine self-administration only when injected into the accumbens or amygdala. The time course of this regional selectivity was consistent with the rate of diffusion of SCH 23390 from the site of injection as measured by quantitative autoradiography, demonstrating that the regional selectivity of intracerebral injections of SCH 23390 is time-dependent. These results support a role for D-1 dopamine receptors in the nucleus accumbens and amygdala in the effects of self-administered cocaine, and suggest that D-1 receptors in certain portions of the 'extended amygdala' may be an important substrate for the reinforcing actions of cocaine.

GABAA receptor antagonism in the extended amygdala decreases ethanol self-administration in rats

The present experiments examined the role of the extended amygdala GABAA receptors in the regulation of ethanol consumption in rats. The areas of the extended amygdala studied included the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and the shell of the nucleus accumbens. The effects of bilateral microinjections of a competitive GABAA receptor antagonist, 2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide (SR 95531), on ethanol consumption were assessed in Wistar rats that were trained to respond for oral ethanol (10% w/v) in a two-lever, free-choice operant task during 30-min sessions using a saccharin fading procedure. Injections of SR 95531 into the central amygdaloid nucleus decreased ethanol responding significantly at doses of 2 and 4 ng without affecting water responding. SR 95531 injections into the bed nucleus of the stria terminalis reduced ethanol responding significantly at the 8 ng and 16 ng dose, while only the 16 ng dose produced a significant effect in the shell of the nucleus accumbens. Cumulative response patterns showed that intra-amygdaloid injections did not disrupt the initiation of responding. Injections into the bed nucleus of the stria terminalis and the nucleus accumbens, however, suppressed both ethanol and water responding at the highest SR 95531 doses during the first minutes. These findings suggest that GABAA receptors in the extended amygdala may be involved in the mediation of some aspects of ethanol reward.

Effect of medial prefrontal cortex injections of SCH 23390 on intravenous cocaine self-administration under both a fixed and progressive ratio schedule of reinforcement

Rats were trained to self-administer cocaine (1.5 mg/kg i.v.) under either a fixed ratio (FR) or progressive ratio (PR) schedule of reinforcement. Following acquisition, bilateral intracerebral cannulae were directed at the medial prefrontal cortex (mPFC) or striatum (STRM). Intracranial injections of the D1 receptor antagonist, SCH 23390 (0-2.0 micrograms/0.5 microliter/side), were made immediately prior to a self-administration session. Under the FR 1 schedule, SCH 23390 caused a dose-dependent increase in the rate of cocaine intake following injection into either the mPFC or the STRM. In contrast, under the PR schedule similar injections into the STRM had no effect on the breaking point, but large decreases were produced following injection into the mPFC. These results indicate that dopaminergic systems within the mPFC have a significant role to play in the reinforcing properties of cocaine. Moreover, the STRM does not appear to have a role in those reinforcing properties of cocaine measured under the PR schedule. The significant rate-increasing effects obtained from the STRM under the FR schedule, indicate that the two schedules must measure different aspects of cocaine's CNS action.