Amphetamine infused into the ventrolateral striatum produces oral stereotypies and conditioned place preference

Egr2 induction in spiny projection neurons of the ventrolateral striatum contributes to cocaine place preference in mice

Drug addiction develops due to brain-wide plasticity within neuronal ensembles, mediated by dynamic gene expression. Though the most common approach to identify such ensembles relies on immediate early gene expression, little is known of how the activity of these genes is linked to modified behavior observed following repeated drug exposure. To address this gap, we present a broad-to-specific approach, beginning with a comprehensive investigation of brain-wide cocaine-driven gene expression, through the description of dynamic spatial patterns of gene induction in subregions of the striatum, and finally address functionality of region-specific gene induction in the development of cocaine preference. Our findings reveal differential cell-type specific dynamic transcriptional recruitment patterns within two subdomains of the dorsal striatum following repeated cocaine exposure. Furthermore, we demonstrate that induction of the IEG Egr2 in the ventrolateral striatum, as well as the cells within which it is expressed, are required for the development of cocaine seeking.

The human brain is ever changing, constantly rewiring itself in response to new experiences, knowledge or information from the environment. Addictive drugs such as cocaine can hijack the genetic mechanisms responsible for this plasticity, creating dangerous, obsessive drug-seeking and consuming behaviors.

Cocaine-induced plasticity is difficult to apprehend, however, as brain regions or even cell populations can react differently to the compound. For instance, sub-regions in the striatum – the brain area that responds to rewards and helps to plan movement – show distinct responses during progressive exposure to cocaine. And while researchers know that the drug immediately changes how neurons switch certain genes on and off, it is still unclear how these genetic modifications later affect behavior.

Mukherjee, Gonzales et al. explored these questions at different scales, first focusing on how progressive cocaine exposure changed the way various gene programs were activated across the entire brain. This revealed that programs in the striatum were the most affected by the drug.

Examining this region more closely showed that cocaine switches on genes in specific ‘spiny projection’ neuron populations, depending on where these cells are located and the drug history of the mouse. Finally, Mukherjee, Gonzales et al. used genetically modified mice to piece together cocaine exposure, genetic changes and modifications in behavior. These experiments revealed that the drive to seek cocaine depended on activation of the Egr2 gene in populations of spiny projection neurons in a specific sub-region of the striatum. The gene, which codes for a protein that regulates how genes are switched on and off, was itself strongly activated by cocaine intake.

Cocaine addiction can have devastating consequences for individuals. Grasping how this drug alters the brain could pave the way for new treatments, while also providing information on the basic mechanisms underlying brain plasticity.

Lesions of ventrolateral striatum eliminate lose-shift but not win-stay behaviour in rats

Animals tend to repeat actions that are associated with reward delivery, whereas they tend to shift responses to alternate choices following reward omission. These so-called win-stay and lose-shift responses are employed by a wide range of animals in a variety of decision-making scenarios, and depend on dissociated regions of the striatum. Specifically, lose-shift responding is impaired by extensive excitotoxic lesions of the lateral striatum. Here we used focal lesions to assess whether dorsal and ventral regions of the lateral striatum contribute differently to this effect. We found that damage to ventrolateral striatum reduced lose-shift responding without impairing win-stay, motoric, or motivational aspects of behaviour in the task, whereas lesions confined to the dorsolateral striatum significantly impaired the ability of rats to complete trials of the task. Moreover, lesions to the dorsomedial striatum had no effect on either lose-shift or win-stay responding. Together, these data suggest a novel role of the ventral portion of the lateral striatum in driving lose-shift decisions.

Targeted ablation of cholinergic interneurons in the dorsolateral striatum produces behavioral manifestations of Tourette syndrome

Gilles de la Tourette syndrome (TS) is characterized by tics, which are transiently worsened by stress, acute administration of dopaminergic drugs, and by subtle deficits in motor coordination and sensorimotor gating. It represents the most severe end of a spectrum of tic disorders that, in aggregate, affect ∼ 5% of the population. Available treatments are frequently inadequate, and the pathophysiology is poorly understood. Postmortem studies have revealed a reduction in specific striatal interneurons, including the large cholinergic interneurons, in severe disease. We tested the hypothesis that this deficit is sufficient to produce aspects of the phenomenology of TS, using a strategy for targeted, specific cell ablation in mice. We achieved ∼ 50% ablation of the cholinergic interneurons of the striatum, recapitulating the deficit observed in patients postmortem, without any effect on GABAergic markers or on parvalbumin-expressing fast-spiking interneurons. Interneuron ablation in the dorsolateral striatum (DLS), corresponding roughly to the human putamen, led to tic-like stereotypies after either acute stress or d-amphetamine challenge; ablation in the dorsomedial striatum, in contrast, did not. DLS interneuron ablation also led to a deficit in coordination on the rotorod, but not to any abnormalities in prepulse inhibition, a measure of sensorimotor gating. These results support the causal sufficiency of cholinergic interneuron deficits in the DLS to produce some, but not all, of the characteristic symptoms of TS.

A spiking network model of decision making employing rewarded STDP

Reward-modulated spike timing dependent plasticity (STDP) combines unsupervised STDP with a reinforcement signal that modulates synaptic changes. It was proposed as a learning rule capable of solving the distal reward problem in reinforcement learning. Nonetheless, performance and limitations of this learning mechanism have yet to be tested for its ability to solve biological problems. In our work, rewarded STDP was implemented to model foraging behavior in a simulated environment. Over the course of training the network of spiking neurons developed the capability of producing highly successful decision-making. The network performance remained stable even after significant perturbations of synaptic structure. Rewarded STDP alone was insufficient to learn effective decision making due to the difficulty maintaining homeostatic equilibrium of synaptic weights and the development of local performance maxima. Our study predicts that successful learning requires stabilizing mechanisms that allow neurons to balance their input and output synapses as well as synaptic noise.

Gamma-hydroxybutyrate does not maintain self-administration but induces conditioned place preference when injected in the ventral tegmental area

Gamma-hydroxybutyric acid (GHB) is an endogenous brain substance that has diverse neuropharmacological actions, including rewarding properties in different animal species and in humans. As other drugs of abuse, GHB affects the firing of ventral tegmental neurons (VTA) in anaesthetized animals and hyperpolarizes dopaminergic neurons in VTA slices. However, no direct behavioural data on the effects of GHB applied in the VTA or in the target regions of its dopaminergic neurons, e.g. the nucleus accumbens (NAc), are available. Here, we investigated the effects of various doses of intravenous GHB in maintaining self-administration (from 0.001 to 10 mg/kg per infusion), and its ability to induce conditioned place preference (CPP) in rats when given orally (175-350 mg/kg) or injected directly either in the VTA or NAc (from 10 to 300 microg/0.5 microl per side). Our results indicate that while only 0.01 mg/kg per infusion GHB maintained self-administration, although not on every test day, 350 mg/kg GHB given orally induced CPP. CPP was also observed when GHB was injected in the VTA (30-100 microg/0.5 microl per side) but not in the NAc. Together with recent in-vitro findings, these results suggest that the rewarding properties of GHB mainly occur via disinhibition of VTA dopaminergic neurons.

Expression of c-fos mRNA in the basal ganglia associated with contingent tolerance to amphetamine-induced hypophagia

Tolerance to the hypophagic effect of psychostimulants is contingent on having access to food while intoxicated. Rats given chronic injections of such drugs with access to food learn to suppress stereotyped movements, which interfere with feeding. In contrast, controls given the drug after food access do not learn to suppress stereotypy and, therefore, do not become tolerant. To determine the role of the basal ganglia in this phenomenon, we used in situ hybridization to measure the expression of c-fos mRNA, a marker for neural activation, in the brains of tolerant and nontolerant rats. Rats given chronic amphetamine injections prior to food access learned to suppress stereotyped movements, whereas yoked controls given the drug after feeding did not. Following an acute injection of amphetamine, both of these groups had higher levels of c-fos mRNA than saline-treated controls throughout the striatum, in the nucleus accumbens core, the ventral pallidum and layers V-VI of the motor cortex. In contrast, tolerant rats, which had learned to suppress stereotypy, had higher levels of c-fos mRNA than both amphetamine- and saline-treated controls in the entopeduncular nucleus, globus pallidus, subthalamic nucleus, pedunculopontine nucleus, nucleus accumbens shell, olfactory tubercle, somatosensory cortex, and layers II-IV of motor cortex. These data suggest that the learned suppression of amphetamine-induced stereotypy involves the activation of dorsal striatal pathways previously implicated in response selection as well as the ventral striatum, long implicated in appetitive motivation and reinforcement.

Adolescent maturation of cocaine-sensitive neural mechanisms

Both clinical and animal studies have shown that adolescents undergo a late maturation of the central nervous system, which may underlie adolescent typical behaviors. In particular, decreased behavioral response to cocaine has been found in adolescents as compared to adults. In the present study, cocaine was used as a tool to explore adolescent brain maturation. Juvenile (postnatal day (P) 27), adolescent (P37), and adult (P90) male Sprague-Dawley rats were treated acutely with cocaine (750 microg/kg/injection x 2, i.v.), and c-fos mRNA expression, a marker of neuronal activation, was evaluated by in situ hybridization. Cocaine-induced c-fos mRNA was similar across ages in the dorsal caudate putamen (CPu), nucleus accumbens, and lateral bed nucleus of the stria terminalis. In contrast, there was a diminished response in juvenile/adolescent ventral CPu and in juvenile central nucleus of the amygdala, and an increased response in juvenile/adolescent cortex. Further studies evaluated the mechanism of the late maturation of cocaine response in ventral CPu. No significant age differences were observed in regional dopamine (DA) transporter binding. Although striatal DA content was significantly reduced at P27 as compared to adult, there was no difference between dorsal and ventral subregions. In contrast, basal- and cocaine-induced extracellular DA overflow, as measured by in vivo microdialysis, was lower in juvenile ventral CPu than in the adults. This age difference was not observed in dorsal CPu. These findings suggest that impulse activity in DA afferents to ventral CPu is immature in adolescents. In conclusion, the present study showed that cocaine-sensitive neuronal circuits continue to mature during adolescence.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Effects of ventrolateral striatal inactivation on predatory hunting

Previous studies from our laboratory have shown that insect hunting is associated with a distinct Fos up-regulation in the ventrolateral caudoputamen at intermediate rostro-caudal levels. It is largely known that ventrolateral striatum participates in the control of orofacial movements and forepaw usage accompanying feeding behavior, but there has been no study investigating its possible roles during predatory hunting. We have presently examined the role of the ventrolateral striatum during roach hunting by using the reversible blockade with lidocaine. Accordingly, non-treated and saline-treated animals performed the insect hunting quite well, displaying a rather stereotyped form of motor actions for chasing, capturing and killing the prey. During the bilateral blockade of the ventrolateral striatum, the animals showed a significantly longer latency to start hunting and to capture the first prey. The lidocaine-treated animals captured the prey by using mostly the mouth, with little forepaw assistance, and were less effective in capturing the roaches. Moreover, while handling the prey, animals with ventrolateral striatal inactivation kept biting several parts of the prey, but failed to deliver the killing bite to the head, leaving them alive and moving, more likely to escape. Overall, the present findings suggest that the ventrolateral striatum implements the stereotyped actions seen during prey capture and handling, and may influence the motivational drive to start attacking the roaches, as well.

Relation between rotation in the 6-OHDA lesioned rat and dopamine loss in striatal and substantia nigra subregions

The relation between the rotation response to drug-induced activation of the dopamine (DA) receptor in the rat unilaterally lesioned with 6-hydroxydopamine (6-OHDA) in the substantia nigra (SN) and the loss of DA in subregions of the SN and caudate-putamen (C/PUT) is not clear. Here this relation was examined in 23 rats classified as rotators to amphetamine (5 mg/kg). After their response was characterized in terms of ipsilateral rotation, contralateral rotation, and oral stereotypy in one place, they were divided into high, medium, low, and very low rotators. The loss of DA in each group was visualized on brain sections immunoreacted to tyrosine hydroxylase (TH). The density of the TH label on the side of the lesion was compared to that on the intact side. In the ventral midbrain, the density was determined in the SN subdivided into far lateral, lateral, central, and medial subregions and also in the ventral tegmental area (VTA). In the forebrain, it was determined in the C/PUT subdivided into lateral, central, and medial subregions and also in the nucleus accumbens (ACC). These measurements led to three principal findings. The first was a positive overall correlation between rotation and loss of TH label. The second was a correlation between rotation and penetration of the loss from the lateral subregions into more medial areas. The third was a larger loss in SN and VTA (midbrain) than in C/PUT and ACC (forebrain). These findings show that rotation depended not only on the overall loss of DA but also on its distribution across subregions. The loss in the lateral subregion, always the largest regardless of the rate of rotation, may have been the first step in inducing the motor abnormality, and the loss in the central and medial subregions may have served to enhance the abnormality due to the loss in the lateral subregion.

Functional mapping of the prosencephalic systems involved in organizing predatory behavior in rats

The study of the neural basis of predatory behavior has been largely neglected over the recent years. Using an ethologically based approach, we presently delineate the prosencephalic systems mobilized during predation by examining Fos immunoreactivity in rats performing insect hunting. These results were further compared with those obtained from animals killed after the early nocturnal surge of food ingestion. First, predatory behavior was associated with a distinct Fos up-regulation in the ventrolateral caudoputamen at intermediate rostro-caudal levels, suggesting a possible candidate to organize the stereotyped sequence of actions seen during insect hunting. Insect predation also presented conspicuous mobilization of a neural network formed by a distinct amygdalar circuit (i.e. the postpiriform-transition area, the anterior part of cortical nucleus, anterior part of basomedial nucleus, posterior part of basolateral nucleus, and medial part of central nucleus) and affiliated sites in the bed nuclei of the stria terminalis (i.e. the rhomboid nucleus) and in the hypothalamus (i.e. the parasubthalamic nucleus). Accordingly, this network is likely to encode prey-related motivational values, such as prey's odor and taste, and to influence autonomic and motor control accompanying predatory eating. Notably, regular food intake was also associated with a relatively weak Fos up-regulation in this network. However, during regular surge of food intake, we observed a much larger mobilization in hypothalamic sites related to the homeostatic control of eating, namely, the arcuate nucleus and autonomic parts of the paraventricular nucleus. Overall, the present findings suggest potential neural systems involved in integrating prey-related motivational values and in organizing the stereotyped sequences of action seen during predation. Moreover, the comparison with regular food intake contrasts putative neural mechanisms controlling predatory related eating vs. regular food intake.

Putting a spin on the dorsal-ventral divide of the striatum

Since its conception three decades ago, the idea that the striatum consists of a dorsal sensorimotor part and a ventral portion processing limbic information has sparked a quest for functional correlates and anatomical characteristics of the striatal divisions. But this classic dorsal-ventral distinction might not offer the best view of striatal function. Anatomy and neurophysiology show that the two striatal areas have the same basic structure and that sharp boundaries are absent. Behaviorally, a distinction between dorsolateral and ventromedial seems most valid, in accordance with a mediolateral functional zonation imposed on the striatum by its excitatory cortical, thalamic and amygdaloid inputs. Therefore, this review presents a synthesis between the dorsal-ventral distinction and the more mediolateral-oriented functional striatal gradient.

Differential effects of intraspecific interactions on the striatal dopamine system in social and non-social voles

We used in vivo microdialysis to examine the responses to intraspecific social interactions in the striatal dopamine systems of females of two vole species displaying vastly different social structures. Both highly social prairie voles and asocial meadow voles had similar increases in extracellular dopamine associated with mating. There was a species-specific effect of social condition on extracellular dihydroxyphenylacetic acid (DOPAC). Exposure to a conspecific male significantly decreased extracellular DOPAC in female prairie voles isolated for approximately 18 h during surgical recovery. Such decrease in DOPAC was not seen if females experienced continued isolation or if they were housed with a sibling during surgical recovery. No changes in extracellular DOPAC were seen in meadow voles after manipulations of social environment. Together, our data indicate that mating-associated dopamine release is independent from mating systems. However, species-specific patterns of extracellular DOPAC suggest that social isolation may be a more stressful stimulus for the social prairie vole than for the asocial meadow vole.

Chronic access to a sucrose solution enhances the development of conditioned place preferences for fentanyl and amphetamine in male Long-Evans rats

Consumption of palatable food and fluids alters the behavioral consequences of psychoactive drugs. To further investigate the effects of intake of palatable nutrients on the rewarding properties of these drugs, the effects of chronic intake of a sweet sucrose solution on the development of conditioned place preferences (CPP) to a mu-opioid agonist, fentanyl, and to a stimulant drug, amphetamine, were examined. Male Long-Evans rats consumed laboratory chow and water or chow, water, and a 32% sucrose solution. CPP testing was conducted in a three-chamber apparatus. In Experiment 1 (over four conditioning days), rats received saline, 0.004, or 0.016 mg/kg sc fentanyl citrate before being placed on the nonpreferred side of the apparatus and saline (subcutaneously) before being placed on the preferred side during a separate session on the same day. When given access to all three chambers, rats injected with 0.016 mg/kg fentanyl spent significantly more time on the drug-paired side than rats injected with saline. Furthermore, sucrose-fed rats displayed a significantly greater CPP than chow-fed rats. After conditioning, rats were tested for fentanyl-induced antinociception using the tail-flick test. Using a cumulative dose procedure, fentanyl (0.003, 0.010, 0.030, and 0.100 mg/kg sc) led to dose-dependent increases in tail-flick latencies. Rats fed with sucrose displayed significantly greater responses to fentanyl than those in the chow group. In Experiment 2, rats spent significantly more time on the drug-paired side of the CPP apparatus following injections of 0.33 or 1.0 mg/kg amphetamine than after saline injections. Additionally, following injection of 0.33 mg/kg amphetamine, sucrose-fed rats spent significantly more time on the drug-paired side of the chamber than chow-fed rats.

Localization of brain reinforcement mechanisms: intracranial self-administration and intracranial place-conditioning studies

Intracranial self-administration (ICSA) and intracranial place conditioning (ICPC) methodologies have been mainly used to study drug reward mechanisms, but they have also been applied toward examining brain reward mechanisms. ICSA studies in rodents have established that the ventral tegmental area (VTA) is a site supporting morphine and ethanol reinforcement. ICPC studies confirmed that injection of morphine into the VTA produces conditioned place preference (CPP). Further confirmation that activation of opioid receptors within the VTA is reinforcing comes from the findings that the endogenous opioid peptide met-enkephalin injected into the VTA produces CPP, and that the mu- and delta-opioid agonists, DAMGO and DPDPE, are self-infused into the VTA. Activation of the VTA dopamine (DA) system may produce reinforcing effects in general because (a) neurotensin is self-administered into the VTA, and injection of neurotensin into the VTA produces CPP and enhances DA release in the nucleus accumbens (NAC), and (b) GABA(A) antagonists are self-administered into the anterior VTA and injections of GABA(A) antagonists into the anterior VTA enhance DA release in the NAC. The NAC also appears to have a major role in brain reward mechanisms, whereas most data from ICSA and ICPC studies do not support an involvement of the caudate-putamen in reinforcement processes. Rodents will self-infuse a variety of drugs of abuse (e.g. amphetamine, morphine, phencyclidine and cocaine) into the NAC, and this occurs primarily in the shell region. ICPC studies also indicate that injection of amphetamine into the shell portion of the NAC produces CPP. Activation of the DA system within the shell subregion of the NAC appears to play a key role in brain reward mechanisms. Rats will ICSA the DA uptake blocker, nomifensine, into the NAC shell; co-infusion with a D2 antagonist can block this behavior. In addition, rats will self-administer a mixture of a D1 plus a D2 agonist into the shell, but not the core, region of the NAC. The ICSA of this mixture can be blocked with the co-infusion of either a D1 or a D2 antagonist. However, the interactions of other transmitter systems within the NAC may also play key roles because NMDA antagonists and the muscarinic agonist carbachol are self-infused into the NAC. The medial prefrontal (MPF) cortex supports the ICSA of cocaine and phencyclidine. The DA system also seems to play a role in this behavior since cocaine self-infusion into the MPF cortex can be blocked by co-infusing a D2 antagonist, or with 6-OHDA lesions of the MPF cortex. Limited studies have been conducted on other CNS regions to elucidate their role in brain and drug reward mechanisms using ICSA or ICPC procedures. Among these regions, ICPC findings suggest that cocaine and amphetamine are rewarding in the rostral ventral pallidum (VP); ICSA and ICPC studies indicate that morphine is rewarding in the dorsal hippocampus, central gray and lateral hypothalamus. Finally, substance P mediated systems within the caudal VP (nucleus basalis magnocellularis) and serotonin systems of the dorsal and median raphe nuclei may also be important anatomical components involved in brain reward mechanisms. Overall, the ICSA and ICPC studies indicate that there are a number of receptors, neuronal pathways, and discrete CNS sites involved in brain reward mechanisms.