Mary C. Olmstead

Associative Processes in Addiction and Reward The Role of Amygdala‐Ventral Striatal Subsystems

ABSTRACT: Only recently have the functional implications of the organization of the ventral striatum, amygdala, and related limbic‐cortical structures, and their neuroanatomical interactions begun to be clarified. Processes of activation and reward have long been associated with the NAcc and its dopamine innervation, but the precise relationships between these constructs have remained elusive. We have sought to enrich our understanding of the special role of the ventral striatum in coordinating the contribution of different functional subsystems to confer flexibility, as well as coherence and vigor, to goal‐directed behavior, through different forms of associative learning. Such appetitive behavior comprises many subcomponents, some of which we have isolated in these experiments to reveal that, not surprisingly, the mechanisms by which an animal sequences responding to reach a goal are complex. The data reveal how the different components, pavlovian approach (or sign‐tracking), conditioned reinforcement (whereby pavlovian stimuli control goal‐directed action), and also more general response‐invigorating processes (often called “activation,”“stress,” or “drive”) may be integrated within the ventral striatum through convergent interactions of the amygdala, other limbic cortical structures, and the mesolimbic dopamine system to produce coherent behavior. The position is probably not far different when considering aversively motivated behavior. Although it may be necessary to employ simplified, even abstract, paradigms for isolating these mechanisms, their concerted action can readily be appreciated in an adaptive, functional setting, such as the responding by rats for intravenous cocaine under a second‐order schedule of reinforcement. Here, the interactions of primary reinforcement, psychomotor activation, pavlovian conditioning, and the control that drug cues exert over the integrated drug‐seeking response can be seen to operate both serially and concurrently. The power of our analytic techniques for understanding complex motivated behavior has been evident for some time. However, the crucial point is that we are now able to map these components with increasing certainty onto discrete amygdaloid, and other limbic cortical‐ventral striatal subsystems. The neural dissection of these mechanisms also serves an important theoretical purpose in helping to validate the various hypothetical constructs and further developing theory. Major challenges remain, not the least of which is an understanding of the operation of the ventral striatum together with its dopaminergic innervation and its interactions with the basolateral amygdala, hippocampal formation, and prefrontal cortex at a more mechanistic, neuronal level.

The development of a conditioned place preference to morphine : Effects of microinjections into various CNS sites

Experiment 1 examined whether microinjections of morphine (1 microg in 0.5 microl over 1 min x 2 pairings) into 13 different CNS sites produced a conditioned place preference (CPP). Injections into the lateral ventricles (LV), ventral tegmental area (VTA), or periaqueductal gray (PAG) produced a CPP; injections 1 mm dorsal to the PAG or VTA, or into the caudate putamen, medial frontal cortex, hippocampus, lateral nucleus of the amygdala, lateral hypothalamus, pedunculopontine tegmental nucleus, posterior hypothalamus, ventral palladium, or nucleus accumbens septi (core or shell) did not. In Experiment 2, morphine 0.2 microg produced a CPP when injected into the VTA but not in the PAG, while 5.0 microg was effective in both sites. The CPP induced by systemic morphine (4 mg/kg x 1 pairing) was blocked by naloxone methiodide (NM) injected (2 nmol in 0.5 microl) into the VTA. PAG injections of 2 nmol reduced, and 5 nmol NM eliminated, the CPP. The results confirm that morphine injections into the VTA or the PAG are rewarding, that blockade of opioid receptors in either site disrupts a morphine-induced CPP, and that the VTA is more sensitive to both effects.

Ultra-low-dose naloxone suppresses opioid tolerance, dependence and associated changes in mu opioid receptor–G protein coupling and Gβγ signaling

Opiates produce analgesia by activating μ opioid receptor-linked inhibitory G protein signaling cascades and related ion channel interactions that suppress cellular activities by hyperpolarization. After chronic opiate exposure, an excitatory effect emerges contributing to analgesic tolerance and opioid-induced hyperalgesia. Ultra-low-dose opioid antagonist co-treatment blocks the excitatory effects of opiates in vitro, as well as opioid analgesic tolerance and dependence, as was demonstrated here with ultra-low-dose naloxone combined with morphine. While the molecular mechanism for the excitatory effects of opiates is unclear, a switch in the G protein coupling profile of the μ opioid receptor and adenylyl cyclase activation by Gβγ have both been suggested. Using CNS regions from rats chronically treated with vehicle, morphine, morphine+ultra-low-dose naloxone or ultra-low-dose naloxone alone, we examined whether altered μ opioid receptor coupling to G proteins or adenylyl cyclase activation by Gβγ occurs after chronic opioid treatment. In morphine-naive rats, μ opioid receptors coupled to Go in striatum and to both Gi and Go in periaqueductal gray and spinal cord. Although chronic morphine decreased Gi/o coupling by μ opioid receptors, a pronounced coupling to Gs emerged coincident with a Gβγ interaction with adenylyl cyclase types II and IV. Co-treatment with ultra-low-dose naloxone attenuated both the chronic morphine-induced Gs coupling and the Gβγ signaling to adenylyl cyclase, while increasing Gi/o coupling toward or beyond vehicle control levels. These findings provide a molecular mechanism underpinning opioid tolerance and dependence and their attenuation by ultra-low-dose opioid antagonists.

Effects of Pedunculopontine Tegmental Nucleus Lesions on Responding for Intravenous Heroin under Different Schedules of Reinforcement

The pedunculopontine tegmental nucleus (PPTg) is believed to play important roles in reward and learning. We examined the effect of PPTg lesions (0.5 μl of 0.1 m NMDA injected bilaterally over 10 min) on the learning of an operant response for opiate reward. In 14 adult male Long–Evans rats, bilateral lesions of the PPTg disrupted the acquisition of responding for intravenous heroin (0.1 mg/kg infused at a rate of 0.25 ml/28 sec) on a fixed ratio-1 (FR-1) schedule of reinforcement. The 12 remaining lesioned animals increased their heroin intake over the acquisition sessions but did not reach the response levels of sham-lesioned animals on the 15th and final session. The sham- and PPTg-lesioned animals that learned the FR-1 task exhibited similar patterns of responding during extinction and reacquisition sessions. When tested on a progressive ratio (PR) schedule of reinforcement, however, PPTg-lesioned animals had lower break points than sham-lesioned animals. Asymmetric lesions, which destroyed the majority of the nucleus in one hemisphere only, did not produce any behavioral deficits. Rats that were lesioned after training also did not show deficits in responding under either FR or PR schedules. These findings suggest that PPTg lesions reduce the rewarding effect of opiates but do not disrupt the ability either to learn an operant response or the response requirements of a PR schedule.

The development of a conditioned place preference to morphine: effects of lesions of various CNS sites.

This study examined the neural substrates underlying the development of a conditioned place preference (CPP) to morphine (2 mg/kg x 3 pairings) by testing whether lesions of 7 different neural sites block a morphine-induced CPP. Lesions of the pedunculopontine tegmental nucleus (PPTg), the periaqueductal gray (PAG), or the fornix reduced the preference for a morphine-paired compartment. When they were retested following morphine administration, fornix- or PAG-lesioned animals exhibited a CPP indicating that lesions did not block morphine-induced reward or the ability to associate this effect with salient environmental cues. PPTg-lesioned animals did not express a CPP during state-dependent testing, suggesting that the lesions may attenuate the rewarding effect of the drug. Lesions of the mesolimbic dopamine system, the ventral pallidum, the lateral nucleus of the amygdala, or the caudate putamen had no effect on a morphine-induced CPP.

Selective deficits in attentional performance on the 5-choice serial reaction time task following pedunculopontine tegmental nucleus lesions.

Sustained attention requires the integrity of basal forebrain cholinergic systems. The pedunculopontine tegmental nucleus (PPTg) has direct and indirect connections (via the thalamus) with the basal forebrain, suggesting that the PPTg may also play an important role in attentional processes. We examined this hypothesis by testing the effects of PPTg lesions in rats on performance in the 5-choice serial reaction time test. Bilateral lesions reduced accuracy, increased errors of omission, and increased the latency to correct responses. The deficits were more severe when neuronal damage was bilateral and concentrated in the posterior PPTg. Attentional demands of the task were increased by decreasing the stimulus duration, the stimulus brightness, or the inter-trial interval, and by introducing random bursts of white noise. These challenges impaired performance of all animals, but the magnitude of deficit was increased in the lesioned group. Conversely, lesion-induced deficits were partially alleviated when the attentional demands of the task were reduced. This pattern of results suggests that PPTg lesions produce a global deficit in attention, rather than a specific impairment in one process. The PPTg may control attentional processes through its direct projections to the forebrain cholinergic system or, indirectly, through activation of thalamocortical projections.

Lesions of the pedunculopontine tegmental nucleus block drug-induced reinforcement but not amphetamine-induced locomotion

It has been proposed that the positive reinforcing and motor stimulating effects of drugs involve the activation of a common neural substrate. Reinforcing effects of food, drugs and brain stimulation are blocked by lesions of the pedunculopontine tegmental nucleus (PPTg), which is a component of the mesencephalic locomotor region. This has suggested that the PPTg may be involved in both positive reinforcement and forward locomotion. In four separate experiments, rats were prepared with NMDA (0.5 microliters of 0.1 M solution) or sham lesions of the PPTg. Animals in the first two experiments were tested for the development of a conditioned place preference (CPP) to morphine (2 mg/kg x 3 pairings) or amphetamine (1.5 mg/kg x 3 pairings). Ten days later, spontaneous motor activity (SMA) was assessed in these animals following a subcutaneous injection of saline or amphetamine (1.5 mg/kg). In two further experiments, drug-naive lesioned and control animals were tested for SMA only (saline or 1.5 mg/kg amphetamine in Experiment 3, and saline, 0.5 mg/kg, or 3 mg/kg amphetamine in Experiment 4). Lesions of the PPTg blocked the development of a CPP to both morphine and amphetamine. In contrast, lesions had no effect on saline or amphetamine-stimulated SMA. The PPTg, therefore, appears to be involved in the reinforcing effects of amphetamine and morphine, but is not necessary for the expression of amphetamine-induced activity.

Effects of chronic cocaine on impulsivity: relation to cortical serotonin mechanisms.

Drug addiction can be considered an impulse control disorder in that addicts exhibit increased impulsivity on both behavioural and self-report measures. We investigated whether chronic cocaine affects delay of gratification and/or behavioural disinhibition in rats using the delayed reinforcement and Go/No-go paradigms. Animals were treated with saline or cocaine (15 mg/kg) three times per day for 14 days; all behavioural tests occurred prior to daily injections. To assess the effectiveness of the cocaine treatment, sucrose intake, behavioural sensitization and serotonin (5-HT)-dependent (dorsal raphe-stimulated) cortical activation were also measured. Chronic cocaine caused a transient (days 7-8) increase in impulsivity in the delayed reinforcement paradigm, but did not influence behaviour in the Go/No-go paradigm. As expected, chronic cocaine increased behavioural sensitization scores, although it did not affect sucrose consumption. Although, cocaine treatment did not affect dorsal raphe-stimulated electrocorticographic activation, the serotonergic receptor antagonist methiothepin (0.1 mg/kg) was more effective in blocking cortical activation in cocaine- than in saline-treated animals. The electrocorticographic changes may be the result of a pre-synaptic 5-HT deficit and the compensatory supersensitivity of post-synaptic 5-HT receptors. Given the differential time courses of the behavioural and electrocorticographic data, however, this change probably does not mediate the effects of chronic cocaine in the delayed reinforcement paradigm.

Cocaine seeking by rats is a goal-directed action.

In two experiments rats were trained to self-administer intravenous cocaine on chained schedules using different responses in the initial (drug-seeking) and terminal (drug-taking) links. In both between- (Experiment 1) and within-subject designs (Experiment 2), the drug-taking response was then either extinguished or reinforced in the absence of the opportunity to perform the seeking response. In a subsequent extinction test with the seeking manipulanda alone, the rate of drug seeking was reduced after the prior extinction of the associated taking response. An additional group trained with a sucrose reinforcer showed a comparable devaluation effect. These findings demonstrate that seeking responses for cocaine and food rewards are mediated by a representation of the contingency between seeking responses and the opportunity to take the reward.

Repeated exposure to stress across the childhood-adolescent period alters rats' anxiety- and depression-like behaviors in adulthood: The importance of stressor type and gender.

This research tests the hypothesis that specific forms of adversity in early life map onto behavioral signs analogous to depression versus anxiety in later life. Male and female rats were exposed to either severe sporadic stress or chronic mild stress during the childhood-adolescent period, and their behavior was tested in adulthood. Males in the severe sporadic stress group showed exaggerated anxiety-related behaviors, as indicated by increases in shock-probe burying and escape-like responses (jumps) from the open arms of the elevated plus-maze. Females exposed to severe sporadic stress displayed no change in burying behavior but did display increases in escape behavior. These same females also exhibited behaviors analogous to depression that manifested as decreased sucrose consumption. The chronic mild stress regime produced effects only in females, including reduced burying, decreased sucrose consumption, and an exaggerated corticosterone response to cold-water immersion stress. Findings reiterate the importance of early life experience to the development of adult psychopathologies and emphasize the need to consider both the type of early experience and gender differences in these analyses.