Aggression and violence pose a significant public health concern to society. Aggression is a highly conserved behavior that shares common biological correlates across species. While aggression developed as an evolutionary adaptation to competition, its untimely and uncontrolled expression is maladaptive and presents itself in a number of neuropsychiatric disorders. A mechanistic hypothesis for pathological aggression links aberrant behavior with heightened dopamine function. However, while dopamine hyper-activity is a neural correlate of aggression, the developmental aspects and circuit level contributions of dopaminergic signaling have not been elucidated. In this dissertation, I aim to address these questions regarding the specifics of dopamine function in a murine model of aggressive behavior. In chapter I, I provide a review of the literature that describes the current state of research on aggression. I describe the background elements that lay the foundation for experimental questions and original data presented in later chapters. I introduce, in detail, published studies that describe the clinical manifestation and epidemiological spread, the dominant categories, the anatomy and physiology, and the pharmacology of aggression, with a particular emphasis on the dopaminergic system. Finally, I describe instances of genetic and environmental risk factors impacting aggression, concluding with studies revealing an important role for interactions among genetics, environmental factors, and age in the development of aggression. In chapter II, I investigate the developmental origins of aggression by examining sensitive periods during which perturbations to the dopaminergic system impact adult aggressive behavior. Previous work in our laboratory has concluded that periadolescent (postnatal days 22-41) elevation in dopamine, via transient dopamine transporter blockade, leads to increased adult aggression and heightened response to amphetamine. I expanded on these findings by temporally refining the opening and closing of this window of sensitivity, specifically to postnatal days 32 to 41, during which increases in dopaminergic tone increase adult aggression and behavioral sensitivity to psychostimulants. The potentiated response to amphetamine indicated to us a state of altered dopaminergic physiology. We next validated this hypothesis and found increased firing rate (in vitro), and increased bursting and population activity (in vivo) at baseline. These data indicate that elevated periadolescent dopamine impacts maturation of the dopamine system, leading to a hyper-active dopaminergic and aggressive predisposition. In conclusion, this chapter introduces a developmental component to the hyper-dopaminergic model of aggression. In chapter III, I report a series of experiments exploring the direct and causal involvement of dopamine in driving aggression. While dopamine hyper-activity is a neural correlate of aggression, the precise brain circuits involved have not been elucidated. Using optogenetics, I established a causal role for the ventral tegmental area (a key source of dopamine) in aggression modulation. I further advanced this finding by demonstrating that the modulatory role of dopamine, is population- and projection-specific. I found that activity of ventral tegmental area, but not substantia nigra, dopamine neurons promotes aggression. Furthermore, controlled stimulation of ventral tegmental area dopaminergic terminals in the lateral septum, but not the nucleus accumbens, mediates increased aggression. I selectively traced connectivity between the lateral septum and the ventral tegmental area using a Cre-driven, population-specific viral vector. I used this virus to show that anatomically distinct clusters of ventral tegmental area dopamine cells send projections to the lateral septum and the nucleus accumbens, thereby dissociating the two target sites both behaviorally and anatomically. Furthermore, I found that while local dopamine release in the lateral septum increases aggression, it has no bearing on reward behaviors thus indicating a stronger association with impulsive, and not motivated, aggression. In conclusion, this chapter offers causal evidence for dopamine’s role in modulating impulsive aggression by identifying a distinct pathway from the ventral tegmental area to the lateral septum that controls aggression. In the work described in chapter IV, my aim was to determine the mechanism underlying ventral tegmental area to lateral septum dopamine-mediated aggression. I first characterized the expression of dopamine receptors in the lateral septum and found that D2 receptors heavily colocalize with the dominant population of neurons in the lateral septum, i.e. GABAergic cells. Moreover, the D2 receptors are perfectly aligned with incoming dopamine afferents. Next we investigated, in acute brain slices, how D2 signaling affects lateral septum function. We revealed that activating D2 receptors hyperpolarizes D2-expressing lateral septum neurons. This effect was abolished with bath application of the D2 receptor antagonist, sulpiride. We validated the functional involvement of post-synaptic D2 signaling in a behavioral test, and found that the aggression induced by direct terminal release of dopamine at the lateral septum is reversed by acutely blocking local D2 receptor signaling. In conclusion, this chapter demonstrates that the ventral tegmental area to lateral septum dopamine pathway, via D2-mediated inhibition of GABAergic lateral septum neurons, is necessary to drive ventral tegmental area-triggered aggression. In chapter V, I engage in a general discussion addressing how the findings from each chapter can be linked to provide a more comprehensive outlook on environmental and genetic risk factors that can modulate ventral tegmental area-triggered aggression. I discuss possible pre- and post-synaptic mechanisms that could impact the functionality of the identified dopaminergic ventral tegmental area to lateral septum pathway. Moreover, in distinguishing this specific dopamine circuit and lateral septum D2 signaling as an underlying correlate of violent pathology, this dissertation aims to evoke deeper understanding of the mechanism of current antipsychotics used to manage aggression. I end this dissertation by proposing new empirical questions, techniques and lines of research that could further develop my findings as well strengthen the links between dominant models of aggression that exist in the field today.

Motivation has been defined as a set of processes which enables organisms to overcome obstacles by energizing behavior in the pursuit of a goal. There are several important observations about motivated behavior which provide insight into the neural mechanisms underlying goal-directed motivation. First, motivation serves two important functions, as it both energizes behavior and also directs it toward or away from specific stimuli. Many of the behavioral tasks used to assay motivation in laboratory rodents do not specifically aim to measure these two distinct aspects of motivation. A second feature of goal-directed motivation is that it is sensitive to both costs and benefits of a given situation, enabling animals to make cost-benefit decisions. Again, many of the behavioral tasks which study cost-benefit decision making do not specifically aim to independently measure the impact of cost manipulations and benefit manipulations in an isolated manner. Here, I first develop behavioral measures which aim to specifically dissociate activational and directional effects of motivation. By characterizing a novel behavioral measure known as a Progressive Hold Down (Ph.D.) task, and using this task in parallel with a more traditionally used Progressive Ratio (PR) task, I show that methamphetamine robustly enhances activational effects of motivation, leading to increased response rates in both the Ph.D. and PR task, but mice are not more goal-directed in the Ph.D. task. I next develop and characterize two novel behavioral assays which are specifically used to examine effort and value contributions to cost-benefit decision making. The Concurrent Effort Choice (CEC) task measures how changes in effort levels impact decision making whereas the Concurrent Value Choice (CVC) task measure how changes in reward value impact decision making. Using these novel assays to examine specific processes important for goal-directed motivation, I carefully examine the role of manipulation of the Dopamine D2 receptor (D2R) in a mouse model which over-expresses the D2R within the striatum (D2R-OE), and the role of pharmacological manipulation of the Serotonin 2C receptor (5-HT2CR) with the functionally selective ligand SB242084. Whereas D2R-OE specifically impacts sensitivity to changes in effort levels which decrease overall levels of goal-directed motivation, selective modulation of the 5-HT2CR via treatment with SB242084 increases response vigor through enhanced dopamine release in the dorsomedial striatum, but this increase in response vigor does not alter sensitivity to effort or value changes when working for rewards. Together, these studies demonstrate the benefits of developing a more nuanced understanding of how specific manipulations impact motivated behavior by examining the specific underlying processes being altered.

Psychotic symptoms are prevalent in a wide variety of psychiatric and neurological disorders. Yet, despite decades of research, the neurobiological mechanisms via which these symptoms come to manifest themselves remain to be elucidated. I argue in this thesis that using a mechanistic approach towards understanding psychosis that borrows heavily from the predictive coding framework, can help us understand the relationship between neurobiology and symptomology. In the first results chapter I present new data on a biomarker that has often been cited in relation to psychotic disorders, which is glutamate levels in the anterior cingulate cortex (ACC), as measured with magnetic resonance spectroscopy. In this chapter I aimed to replicate previous results that show differences in glutamate levels in psychosis and health. However, no statistically significant group differences and correlations with symptomology were found. In order to elucidate the potential mechanism underlying glutamate changes in the anterior cingulate cortex in psychosis, I tested whether a pharmacological challenge of Bromocriptine or Sulpiride altered glutamate levels in the anterior cingulate cortex. However, no significant group differences were found, between medication groups. In the second results chapter I aimed to address a long-standing question in the field of computational psychiatry, which is whether prior expectations have a stronger or weaker influence on inference in psychosis. I go on to show that this depends on the origin of the prior expectation and disease stage. That is, cognitive priors are stronger in first episode psychosis but not in people at risk for psychosis, whereas perceptual priors seem to be weakened in individuals at risk for psychosis compared to healthy individuals and individuals with first episode psychosis. Furthermore, there is some evidence that these alterations are correlated with glutamate levels. In the third results chapter I aimed to elucidate the nature of reward prediction error aberrancies in chronic schizophrenia. There has been some evidence suggesting that schizophrenia is associated with aberrant coding of reward prediction errors during reinforcement learning. However it is unclear whether these aberrancies are related to disease years and medication use. Here I provide evidence for a small but significant alteration in the coding of reward prediction errors that is correlated with medication use. In the fourth results chapter I aimed to study the influence of uncertainty on the coding of unsigned prediction errors during learning. It has been hypothesized by predictive coding theorists that dopamine plays a role in the precision-weighting of unsigned prediction error. This theory is of particular relevance to psychosis research, as this might provide a mechanism via which dopamine aberrancies, might lead to psychotic symptoms. I found that blocking dopamine using Sulpiride abolishes precision-weighting of unsigned prediction error, providing evidence for a dopamine mediated precision-weighting mechanism. In the fifth results chapter I aimed to extend this research into early psychosis, to elucidate whether psychosis is indeed associated with a failure to precision-weight prediction error. I found that first episode psychosis is indeed associated with a failure to precision-weight prediction errors, an effect that is explained by the experience of positive symptoms. In the sixth results chapter I explore whether the degree of precision-weighting of unsigned prediction errors is correlated with glutamate levels in the anterior cingulate cortex. Such a correlation might be plausible given that psychosis has been associated with both. However, I did not find such a relationship, even in a sample of 137 individuals. Thus I concluded that anterior cingulate glutamate levels might be more related to non-positive symptoms associated with psychotic disorders. In summary, a mechanistic approach towards understanding psychosis can give us valuable insights into the disease mechanisms at play. I have shown here that the influence of expectations on perception is different across disease stage in psychosis. Furthermore, aberrancies in prediction error mechanisms might explain positive symptoms in psychosis, a process likely mediated by dopaminergic mechanisms, whereas evidence for glutamatergic mediation remains absent.

Repeated daily treatments of perinatal rats with the dopamine D2-receptor (D2-R) agonist quinpirole for a week or more produces the phenomenon of ‘priming’—gradual but long-term sensitization of D2-R. In fact a daily dose of quinpirole as low as 50 µg/kg/day is adequate for sensitizing D2-R. Primed rats as neonates and in adolescence, when acutely treated with quinpirole display enhanced eating/gnawing/nursing on dams, also horizontal locomotor activity. Between 3 and 5 weeks of age, acute quinpirole treatment of primed rats produces profound vertical jumping with paw treading—a behavior that is not observed in control rats. At later ages acute quinpirole treatment is associated with enhanced yawning, a D2-R-associated behavior. This long-term D2-R supersensitivity is believed to be life-long, despite the relatively brief period of D2-R priming near the time of birth. D2-R supersensitivity is not associated with an increase in the number or affinity of D2-R, as assessed in the striatum of rats; nor is it induced with the D3-R agonist 7-OH-DPAT. However, quinpirole-induced D2-R supersensitivity is associated with cognitive deficits, also a deficit in pre-pulse inhibition and in neurotrophic factors, and low levels of the transcript regulator of G-protein signaling (RGS) RGS9 in brain; and acute reversal of these alterations by the antipsychotic agent olanzapine. In sum, rats ontogenetically D2-R supersensitized have face validity, construct validity and predictive ability for schizophrenia.

The prevailing view is that enhancement of dopamine (DA) transmission in the mesolimbic system leads to the rewarding properties of alcohol. The mesolimbic DA system, which plays an important role in regulating reward and addiction, consists of DA neurons in the midbrain ventral tegmental area (VTA) that innervate the nucleus accumbens (NAc). It is believed that VTA DA neurons are inhibited by local gamma-aminobutyric acid (GABA) interneurons that express connexin-36 (Cx36) gap junctions (GJs). We have previously demonstrated that blocking Cx36 GJs suppresses electrical coupling between VTA GABA neurons and reduces ethanol intoxication and consumption suggesting that electrical coupling between mature VTA GABA neurons underlies the rewarding properties of ethanol. The aim of this study was to further investigate the role of VTA GABA neurons expressing Cx36 GJs in regulating DA neuron activity and release and mediating ethanol effects on VTA GABA neurons. To this end, we customized a Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) viral vector to target VTA GABA neurons expressing Cx36 GJs in order to chemogenetically modulate their activity. In order to more conclusively demonstrate the role of this sub population of VTA GABA neurons in regulating DA neural activity and release we used electrophysiology to characterize the electrical changes that occur in VTA DA and GABA neurons when Cx36-expressing VTA GABA cells were selectively activated. In addition, we evaluated the effects of activation of VTA GABA neurons on brain stimulation reward and alcohol consumption in ethanol naive and dependent mice. Results indicate that there are two populations of GABA neurons in the VTA, one that is GAD65+/Cx36+ and one that is GAD67+/Cx36-. Activation of Cx36+ VTA GABA neurons by clozapine-n-oxide (CNO) in mice injected with Gq DREADD activated VTA DA neurons and subsequent DA release in the NAc, suggesting that Cx36-containing GABA neurons are inhibiting non-Cx36 GABA neurons to disinhibit DA neurons. In hM3Dq animals, CNO administration provided a rewarding stimulus in the conditioned pace preference paradigm, and reduced consumption in the drink-in-the-dark ethanol consumption paradigm in dependent and naïve mice. A better understanding of the circuitry of the mesolimbic DA system is key to understanding the mechanisms that lead to addiction and may ultimately lead to improved therapies for substance abuse.

The prevailing view is that enhancement of dopamine (DA) transmission in the mesolimbic system leads to the rewarding properties of alcohol. The mesolimbic DA system, which plays an important role in regulating reward and addiction, consists of DA neurons in the midbrain ventral tegmental area (VTA) that innervate the nucleus accumbens (NAc). It is believed that VTA DA neurons are inhibited by local gamma-aminobutyric acid (GABA) interneurons that express connexin-36 (Cx36) gap junctions (GJs). We have previously demonstrated that blocking Cx36 GJs suppresses electrical coupling between VTA GABA neurons and reduces ethanol intoxication and consumption suggesting that electrical coupling between mature VTA GABA neurons underlies the rewarding properties of ethanol. The aim of this study was to further investigate the role of VTA GABA neurons expressing Cx36 GJs in regulating DA neuron activity and release and mediating ethanol effects on VTA GABA neurons. To this end, we customized a Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) viral vector to target VTA GABA neurons expressing Cx36 GJs in order to chemogenetically modulate their activity. In order to more conclusively demonstrate the role of this sub population of VTA GABA neurons in regulating DA neural activity and release we used electrophysiology to characterize the electrical changes that occur in VTA DA and GABA neurons when Cx36-expressing VTA GABA cells were selectively activated. In addition, we evaluated the effects of activation of VTA GABA neurons on brain stimulation reward and alcohol consumption in ethanol naive and dependent mice. Results indicate that there are two populations of GABA neurons in the VTA, one that is GAD65+/Cx36+ and one that is GAD67+/Cx36-. Activation of Cx36+ VTA GABA neurons by clozapine-n-oxide (CNO) in mice injected with Gq DREADD activated VTA DA neurons and subsequent DA release in the NAc, suggesting that Cx36-containing GABA neurons are inhibiting non-Cx36 GABA neurons to disinhibit DA neurons. In hM3Dq animals, CNO administration provided a rewarding stimulus in the conditioned pace preference paradigm, and reduced consumption in the drink-in-the-dark ethanol consumption paradigm in dependent and naïve mice. A better understanding of the circuitry of the mesolimbic DA system is key to understanding the mechanisms that lead to addiction and may ultimately lead to improved therapies for substance abuse.

Methylphenidate (MPH) is the most commonly-prescribed medication for treating ADHD. Despite high prescription rates among kindergarten-aged children, MPH was not approved for use in children younger than nine, and research into its long-term consequences is lacking. Here, we examined the effects of early-life MPH exposure on anxiety-like behaviors in adulthood in normal rats and rats with dysfunctional central dopamine. On postnatal day (PD) 3, male and female rat pups were injected intracisternally with 6-OHDA or vehicle to generate normal and dopamine-deficient groups. In an initial pair of experiments, 6-OHDA (50, 100 and 150 µg/10µL infusion) was assessed for its ability to induce an ADHD-like phenotype. Subsequently, rats were lesioned with 6-OHDA (100 µg/infusion) or vehicle on PD3 and given MPH (0, 0.5, 2 or 5 mg/kg, i.p.) once daily for 10 days, starting on PD11. On PD60, anxiety-like behavior was assessed with light/dark box or social interaction tests. On PD65, all rats were tested on the elevated plus maze (EPM). Rats with neonatal 6-OHDA lesions exhibited anxiety-like behavior in the light/dark box test and on the EPM. However, there was a complex interaction between sex, lesion, and drug dose in the social interaction test. Pretreatment with 2 mg/kg MPH increased investigatory behaviors in non-lesioned females and decreased investigatory behaviors in lesioned females, suggesting that the long-term effects of early-life MPH in females depend on normal dopamine levels. Together, these experiments support the efficacy of preclinical ADHD models and diverse measures of anxiety-like behaviors when studying the effects of early-life MPH exposure.

No description available.

tDCS brain stimulation does not always work in the intended direction. It has been found to sometimes worsen behavior rather than improve it. A preliminary study shows that people high on sensation-seeking and lack of premeditation were prone to reverse effects of tDCS on performance on a Stop Signal Task. Both of these constructs are related to dopamine levels. Study 2 seeks to intentionally cause a reverse effect of tDCS by increasing participants’ dopamine levels via caffeine. There was not a significant interaction between tDCS and caffeine on errors on the Stop Signal Task in this study. However, other factors interacted with tDCS and caffeine including lack of premeditation. This two study package suggests the effects of tDCS are variable across individuals, with personality and neurochemistry both affecting behavioral outcomes of tDCS.

The neural mechanisms mediating one-trial and multi-trial behavioral sensitization during early ontogeny are poorly understood. The purpose of this thesis was to assess the importance of D2-like receptors for the induction of cocaine- and methamphetamine-induced one-trial and multi-trial behavioral sensitization during the middle and late preweanling period. In a series of four experiments, rats were injected with saline or the selective dopamine D2-like receptor antagonist raclopride 15 min prior to treatment with the indirect dopamine agonists cocaine or methamphetamine. Acute control groups received two injections of saline. The pretreatment regimens occurred on either PND 16 or PND 20 (one-trial behavioral sensitization) or PND 13-16 or PND 17-20 (multi-trial behavioral sensitization). On PND 17 or PND 21, rats were challenged with either cocaine or methamphetamine and sensitized responding was assessed. With only a single exception, both one -trial and multi-trial cocaine- and methamphetamine-induced sensitization was evident on PND 17 and PND 21. Importantly, the D2-like receptor antagonist raclopride did not prevent the induction of cocaine- or methamphetamine-induced one-trial behavioral sensitization. In regards to multi-trial behavioral sensitization, raclopride failed to inhibit cocaine -induced sensitized responding on PND 17 and PND 21. Interestingly, higher doses of raclopride (0.5 and 1 mg/kg) were able to prevent the induction of multi-trial methamphetamine-induced sensitization on PND 17. Therefore, D2-like receptor antagonism differentially affected methamphetamine -induced behavioral sensitization depending on whether a one-trial or multi-trial paradigm was employed. When considered together, these results suggest that the neural mechanisms underlying the methamphetamine -induced behavioral sensitization of preweanling rats differs depending on the type of experimental paradigm (one- vs multi-trial) being used. Other potential explanations (i.e., nonspecific antagonist effects, impact of contextual conditioning, etc.) for this interesting effect are presented in the Discussion.