John D. Salamone - US grants

The proposed study will investigate how dopamine, an important neurotransmitter in the brain, is involved in the regulation of motivated behavior. The proposed studies will investigate which brain areas are involved in this behavior, and will study the effects of different drugs in this paradigm. These results will increase our knowledge of how dopamine is involved in motor processes that are related to motivation.

There is considerable evidence to suggest that brain dopamine systems are involved in appetitively motivated behavior. DA antagonist drugs impair the performance of behavior maintained by a variety of reinforcers, and reinforcing stimuli have been shown to increase the release of DA in some brain structures. These data often are interpreted as supporting the hypothesis that DA, especially in nucleus accumbens, mediates the rewarding properties of stimuli such as food, water and drugs of abuse. However, evidence also indicates that DA is involved in aversive motivation. Administration of DA antagonists or depletion of brain DA can disrupt performance on avoidance tasks, and stressful stimuli also can increase accumbens DA release. Taking all these data into account, it is not clear that DA in any brain region has some selective role in mediating the effects of rewarding stimuli. For this reason, it is important to provide a detailed characterization of the role of accumbens, striatal and amygdala DA in appetitive and aversive instrumental behavior. The first proposed experiment is designed to study the effects of administration of the DA antagonist haloperidol on lever pressing for food and lever pressing to avoid shook. For this experiment, haloperidol will be administered directly into the nucleus accumbens, striatum and amygdala via chronic indwelling cannulae. In the second experiment, dialysis perfusion methods will be used to study the changes in DA release in nucleus accumbens, striatum and amygdala that are produced by lever pressing for food and lever pressing to avoid shock. The results of these experiments will indicate if DA in these brain regions has a selective involvement in appetitive motivation, or if DA is involved in behavioral processes that are related to stressful as well as rewarding conditions.

Salamone 9511247 While changes in neurochemicals typically have effects on behavior, they may have a very large effect or a minor effect depending upon behavioral context. Dopamine is one such neurochemical. It is secreted in the forebrain and is related to an animal's tendency to produce a response in order to receive a reward. In this project, Dr. Salamone will examine the significance of brain dopamine to behavioral performance in situations where the context of performance (costs versus benefits) is manipulated. These studies will provide new information to distinguish dopamine effects on neural systems controlling motor performance as distinct from those that influence motivation. ***

Considerable evidence indicates that substantia nigra pars reticulata (SNr) acts as an output area for direct and indirect connections originating from several striatal regions. SNr contains a very high concentration of dopamine (DA) D1 receptors, and detailed anatomical studies of the cellular localization of DA receptors have demonstrated that D1 receptors are present on GABA-containing striatonigral terminals in SNr. The proposed research is designed to determine if SNr is an important anatomical site of action for some of the behavioral effects of D1 antagonists. In addition, the proposed research will investigate the interaction between DA and GABA in SNr. It is hypothesized that local injections of the D1 antagonist SCH 23390 into SNr will be highly potent at producing behavioral effects relative to injections in other brain sites. Also, it is hypothesized that D1 antagonism will decrease SNr GABA release as measured by microdialysis, and that the effects of intra-nigral infusions of SCH 23390 will be reversed by co- administration of the GABA-A agonist muscimol. The results of these experiments could help to modify current views on the anatomical and neurochemical basis of the behavioral effects of D1 antagonism, and could expand our knowledge of the behavioral significance of dendritically released DA in SNr . These concepts have important implications for our understanding of the pathophysiological significance of dopamine depletions in Parkinson's disease, and may help to identify novel therapeutic strategies for the treatment of parkinsonian symptoms.

Previous studies involving response choice tasks or various schedules of reinforcement have demonstrated that the effects of nucleus accumbens dopamine (DA) depletion interact strongly with the work requirements of the task. Rats with accumbens DA depletion are impaired on behavioral tasks characterized by high levels of work output, and on choice procedures. Accumbens DA depletion causes animals to shift their relative response allocation in the direction of the task with lower work requirements. On lever pressing tasks, rats with accumbens DA depletion appear to have deficits in maintaining the high work output necessary for emitting large numbers of responses. Although it is difficult to identify precisely the behavioral functions that are affected by accumbens DA depletion, the current working hypothesis for this research is that accumbens DA depletion reduces the propensity for expending energy or effort. A series of experiments is proposed to determine if rats with accumbens DA depletion have problems responding to delayed or intermittent reinforcement, or if their central problem is responding to large work requirements, including force requirements. It is hypothesized that rats with accumbens DA depletion do not have "anhedonia" (i.e., a lack of emotional response to food); indeed, rats with accumbens DA depletion remain directed towards the acquisition and consumption of food provided that the work requirement is relatively low. Instead, it is thought that rats with accumbens DA depletion have "anergia" (i.e., less likely to work for food). This function can be conceived of as a higher-order sensorimotor process, which is very similar to the concept of "activational" aspects of motivation. This research could identify mechanisms in the brain that regulate energy expenditure, and modulate how effort is allocated into various response alternatives.

DESCRIPTION: (Verbatim from the Applicant's Abstract) Clozapine is an antipsychotic drug with a unique clinical, biochemical and behavioral profile. However, clozapine also produces agranulocytosis, which has led to a vigorous search for other "atypical" antipsychotics with properties that resemble clozapine. Previous research has indicated that research on tremulous jaw movements in rats may be useful for identifying drugs with motor properties similar to clozapine. Unlike typical antipsychotics, clozapine generally fails to induce tremulous jaw movements in repeated administration procedures. In addition, clozapine acts to suppress the jaw movements induce by the anticholinesterase tacrine. The suppression of tacrine-induced jaw movements by clozapine occurs at a relatively low dose; clozapine suppressed tacrine-induced jaw movements with an ED50 lower than that observed for suppression of lever pressing. Using acute injection procedures, olanzapine and risperidone also were relatively potent at suppressing jaw movements, while haloperidol was ineffective. In studies involving repeated 14 day injection procedures, the rank order of the ratios of the ED50s for suppression of tacrine-induced jaw movements and lever injection procedures, the rank order of the ratios of the ED50s for suppression of tacrine-induced jaw movements and lever pressing was (from lowest to highest) as follows: clozapine < olanzapine < thioridazine < haloperidol. This pattern is consistent with human clinical data on the production of extrapyramidal side effects. Thus, it is reasonable to suggest that behavioral studies involving tremulous jaw movements could be useful for characterizing the pharmacological characteristics that underlie the suppression of jaw movement activity by atypical antipsychotics are uncertain. Although there is considerable evidence indicating that the anticholinergic properties of clozapine may contribute to the motor effects of this drug, less is known about the possible contribution of serotonergic antagonist properties. In view of the studies indicating that 5-HT systems may be involved in mediating some of the motor effects of atypical antipsychotics, the proposed studies will investigate the involvement of 5-HT mechanisms in the suppression of jaw movement activity. The first group of studies will assess the effects of additional antipsychotic drugs that have 5-HT antagonist properties. Tests will be conducted after both acute and repeated dosing procedures. The second group of proposed experiments is designed to investigate the effects of 5-HT2a/2c antagonism on tacrine- and haloperidol-induced jaw movements. The third group of proposed studies will employ intracranial drug injections to determine that brain loci at which 5-HT2a/2c antagonists and atypical antipsychotics suppress jaw movements activity. Taken together, the results of these experiments will help to identify the pharmacological characteristics and brain circuits that underlie the motor effects of clozapine.

DESCRIPTION (provided by applicant): Symptoms of parkinsonism, such as akinesia, bradykinesia, and tremor, can be caused by degeneration of dopamine (DA) neurons, or by administration of DA antagonist drugs. Parkinsonism is characterized by a cascade of neurochemical events that reflect interactions between several neurotransmitters in the circuitry of the basal ganglia, including DA, acetylcholine, serotonin, GABA and adenosine. Within the last few years, increasing evidence has accumulated indicating that central adenosine neurons play an important role in modulating the functional circuitry of the basal ganglia. Several subtypes of adenosine receptors are involved in motor function, and anatomical studies have demonstrated that the adensonine A2A receptor subtype has a relatively high degree of expression within the striatum. Although several types of striatal cells contain some adensonine A2A receptors, these receptors are present in very high densities on striatopallidal neurons, which also tend to co-express DA D2 receptors and enkephalin. It has been suggested that antagonists of adenosine A2A receptors could have some potential utility as antiparkinsonian drugs. In a recent study from our laboratory, it was demonstrated that IP injections of the adenosine A2A antagonist, KF17837, also suppressed haloperidol-induced tremulous jaw movements, and reversed the locomotor suppression induced by this D2 antagonist. This profile of activity is consistent with the hypothesis that antagonism of adenosine A2A receptors can result in antiparkinsonian effects in animal models. The proposed experiments are designed to investigate the role of the striatopallidal GABAergic pathway as a possible mediator of the putative antiparkinsonian effects of adenosine A2A antagonists. These proposed studies will focus on the tremulous jaw movement model, which is related to parkinsonian tremor. It is hypothesized that adenosine A2A antagonists are acting on striatopallidal GABAergic neurons that also express DA D2 receptors. In view of research showing that haloperidol increases extracellular GABA in globus pallidus, and that haloperidol-induced tremulous jaw movements are reduced by pallidal injections of bicuculline, it is hypothesized that doses of adenosine A2A antagonists that reduce jaw movement activity also will reduce haloperidol-induced increases in GABA release in globus pallidus. In addition, it is hypothesized that adenosine agonists and antagonists will interact to regulate the behavioral and neurochemical effects of haloperidol. These hypotheses will be investigated using studies that involve both systemic and intrastriatal injections of drugs that act upon A2A receptors, and the proposed work will involve a combination of behavioral pharmacology and microdialysis methods. This research is designed to enhance our understanding of the neurotransmitter interactions that are involved in the generation of tremulous movements, and to foster the development of new drugs for the treatment of parkinsonism.

[unreadable] DESCRIPTION (provided by applicant): The notion that motivated behaviors have an energetic or activational component is a recurring theme in the literatures of psychology and psychiatry. Many researchers have emphasized that the vigor or persistence of work output in stimulus-seeking behavior is a fundamental aspect of motivation. Organisms continually make effort-related decisions based upon cost/benefit analyses, allocating resources into goal- directed behaviors based upon assessments of motivational value and response costs. In the clinical literature, psychiatrists and psychologists have come to emphasize the importance of energy-related dysfunctions, such as psychomotor slowing and apathy, in various clinical syndromes. Because of the scientific importance and clinical relevance of behavioral activation processes, it is critical to investigate the brain mechanisms involved. Considerable evidence indicates that dopamine (DA) in nucleus accumbens regulates behavioral activation processes. The effects of accumbens DA depletions on food-seeking behavior depend greatly upon the work requirements of the task, and interference with accumbens DA transmission exerts a powerful influence over effort-related decision making. Nevertheless, it should be recognized that accumbens DA only functions as one component of the brain circuitry regulating activational aspects of motivation. Mesolimbic DA is embedded into a larger network of neural processes involving other brain areas and transmitters. Evidence indicates that there is a functional interaction between DA and adenosine A2A receptors in striatal areas, including the nucleus accumbens. This interaction typically is studied in the context of animal models related to parkinsonism, but less is known about the effort-related functions of adenosine A2A receptors in nucleus accumbens. Therefore, the proposed experiments will focus upon the effort-related functions of adenosine A2A receptors in the nucleus accumbens. It is hypothesized that blockade of adenosine A2A receptors in nucleus accumbens will reverse the behavioral effects of DA antagonism and accumbens DA depletion, and that local stimulation of adenosine A2A receptors will alter the exertion of effort and effort-related decision making in a manner that is similar to the effects of interference with DA transmission. From a mental health perspective, it is vital to study the role of adenosine A2A receptors in effort-related processes because such studies could provide information about a fundamental aspect of motivation, and also because they could lead to the development of novel treatments for energy-related disorders such as psychomotor slowing in depression, anergia or apathy. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Clinical psychologists and psychiatrists have come to emphasize the importance of effort-related symptoms, such as anergia, apathy, and fatigue, in major depression. Nevertheless, despite the large number of tests that are currently available for the assessment of antidepressant drugs in animal models, none of the widely used tasks is specifically focused upon effort-related functions in animals. Over the last few years, our laboratory has developed behavioral models in animals that are useful for assessing the role of various neurotransmitters, such as dopamine (DA) and adenosine, in the exertion of effort and effort-related choice behavior. Considerable evidence indicates that these basic studies in animals are potentially relevant for understanding effort-related dysfunction in humans. However, it is critical to determine if these procedures can lead to the development of novel models of the effort- related symptoms of depression. For these reasons, the goal of the proposed investigation is to study the ability of several conditions that are related to depression to induce effort-related dysfunctions in rats. One group of studies will assess the effects of low doses of the monoamine-depleting agent reserpine on tasks involving exertion of effort and effort-related choice behavior. Additional experiments will study the behavioral characteristics of two genetic models of depression (Congenitally Helpless Rats; Flinders Sensitive line rats). The final group of studies will assess the effects of a novel treatment strategy for depression (adenosine A2A antagonists) and two well known drugs (bupropion and fluoxetine), using behavioral models of effort-related function. The proposed experiments are designed to test the feasibility of using these procedures as potential models of the effort-related symptoms of depression and other disorders.

The initiation and maintenance of motivated behaviors often are characterized by a high degree of vigor,speed, persistence and work output, and organisms frequently make effort-related decisions based upon cost/benefit analyses. In addition, clinicians emphasize the importance of effort-related symptoms, such as anergia, psychomotor retardation, apathy and fatigue, in major depression and other disorders. These motivational symptoms are among the most common symptoms of depression; they can strongly interfere with activities of daily living, and can be highly resistant to treatment. While catecholamine uptake inhibitors appear to be somewhat effective drugs for treating the psychomotor/motivational symptoms of depression, clinical studies indicate that serotonin transport (SERT) inhibitors such as fluoxetine are relatively poor at treating anergia and fatigue, and in fact, can induce or exacerbate these symptoms. Our laboratory has developed behavioral tasks in rodents that assess the brain mechanisms regulating the exertion of effort and effort-related choice behavior. These tests of effort-related choice behavior allow animals to make choices between high-effort alternatives that lead to more highly valued rewards vs. low-effort alternatives that lead to less valued reward (i.e., less preferred or lower in magnitude). Recently, our laboratory has demonstrated that tests of effort-based choice behavior can be used as animal models of the effort-related motivational symptoms of depression. A number of conditions associated with depression in humans can alter effort-related choice in rats, and bias animals towards low-effort options. For example, rats treated with vesicular monoamine transport inhibitor tetrabenazine, which induces or exacerbates symptoms of depression in humans, can alter effort-related choice, reducing selection of the high effort alternative. These effects can be reversed by co-administration of putative antidepressants such as the adenosine A2A antagonist MSX-3, the established antidepressant bupropion (Wellbutrin), which inhibits catecholamine uptake, and the selective dopamine (DA) uptake blockers GBR12909 and PRX-14040. Importantly, research from our laboratory shows that SERT inhibitors, rather than reversing the effort-related effects of tetrabenazine, actually tend to exacerbate them. Furthermore, our recent studies show that the SERT inhibitor fluoxetine reduces effort expenditure and lowers extracellular DA in nucleus accumbens, which is consistent with the relatively poor effects of these drugs on motivational symptoms in depressed people. The goal of the proposed investigation is to determine if the 5-HT2C receptor mediates these effects by studying the ability of 5-HT2A and 5-HT2C antagonists, injected systemically or intracranially, to reverse the effects of fluoxetine on effort-related decision making and extracellular DA in nucleus accumbens. This research could lead to a greater understanding of the neurochemical mechanisms that underlie the regulation of effort-related aspects of motivation, and foster the development of treatments that augment the therapeutic efficacy of 5-HT uptake inhibitors.

PROJECT SUMMARY: The initiation and maintenance of motivated behaviors often are characterized by a high degree of vigor, speed, persistence and work output, and organisms frequently make effort-related decisions based upon cost/benefit analyses. Clinical neuroscience research has characterized effort-related symptoms (anergia, psychomotor slowing, fatigue, lassitude) in psychopathology. These motivational symptoms interfere with activities of daily living, and can be highly resistant to treatment. Tests of effort-related choice allow animals to choose between high-effort alternatives that lead to more highly valued rewards vs. low-effort alternatives that lead to less valued reward (i.e., less preferred or lower in magnitude). The ability to exert effort and select high-effort options is dependent upon neural circuits that involve mesolimbic dopamine (DA), ventral striatum, ventral pallidum, amygdala, and prefrontal cortex. Our laboratory has developed formal animal models that involve assessment of brain mechanisms regulating effort-related choice behavior. For example, rats treated with vesicular monoamine transport inhibitor tetrabenazine (TBZ), which induces or exacerbates symptoms such as fatigue in humans, can alter effort-related choice, reducing selection of the high effort alternative. These effects can be reversed by co- administration of bupropion (Wellbutrin), which inhibits catecholamine uptake, and several dopamine transport (DAT) inhibitors. This pattern of effects is consistent with data from human clinical neuroscience on effort-related motivational dysfunctions in psychopathology, and the effects of drugs that act on dopamine (DA). However, what is missing from this picture is the development of a physiological marker in the animal studies that can be easily translatable to human clinical research. Recent studies have demonstrated that there are electroencephalographic (EEG) markers of frontal cortex activity that are characteristic of engagement in motivated behavior and anticipation of reward, and that these markers are reduced in depressed people. Moreover, these effects are significantly correlated with dysphoria and lassitude in depressed individuals. Therefore, the proposed studies will focus on the development of frontal cortex EEG and local field potential (LFP) activity in behaving rats that are performing effort-based decision-making tasks. These rats will be tested under baseline conditions, and also after pharmacological challenges that produce effort-related motivational effects, including challenges to relevant forebrain circuits using chemogenetic methods. This research could lead to a greater understanding of the circuit mechanisms that underlie the regulation of effort-based aspects of motivation, and may provide valuable preclinical animal data that would contribute to the development of biomarkers for the therapeutic effects of agents that act upon motivational symptoms in psychopathology. This combination of physiological, behavioral, pharmacological and chemogenetic methods meets the ultimate goal of this RFA, in the sense that it may provide preclinical procedures and circuit level findings that ultimately can be translatable to methods that are applicable to human clinical neuroscience.