Ming-Hu Han, Ph.D - US grants

DESCRIPTION (provided by applicant): There is an urgent need for more effective medications for major depressive disorder (MDD) treatment, as less than 50% of depressed patients achieve full remission and many are not responsive, with currently available antidepressants. It is known that prolonged stressful events are an important cause of MDD. However, there is an intriguing difference in individual responses to stress: most people experiencing stressful events maintain normal psychological functioning (resilience to stress), whereas others develop depression (susceptibility to stress). Many psychosocial skills have been successfully used in our daily life to promote stress resiliency. Recent studies have begun to reveal the neurobiological basis for these psychosocial resilient factors, and show that positive emotions and mutual cooperation are linked to the function of the mesolimbic reward neural circuit. Consistent with this idea, we previously found that the activity of ventral tegmental area (VTA) dopamine (DA) neurons in the same reward circuit is a key determinant of susceptibility vs. resilience to social defeat stress. The firing rate of these neurons was significantly increased by chronic defeat in susceptible but not resilient mice. Furthermore, experimentally induced decreased firing promoted resilience, while increased firing promoted susceptibility. Surprisingly, at the molecular level, chronic defeat regulated more genes in resilient mice than in the susceptible subgroup, and induced dramatic upregulation of several K+ channels only in resilient mice, which may drive the higher firing back to normal levels. These findings strongly support the notion that a resilience phenotype is not simply a passive absence of stress-induced pathophysiology, but a promotable and active brain function by which animals successfully cope with stressful conditions via activation of more genes. In the current project, we ask: (1) whether the physiologically important firing patterns of VTA DA neurons encode the signal of stress vulnerability and play a role in active coping or deleterious behaviors; (2) whether we can find potential drug targets by understanding the molecular (ion channel and receptor) mechanisms of susceptibility and active resilience. Accordingly, we propose to use advanced optogenetic techniques to directly link specific firing patterns to stress susceptibility and resilience in freely-moving animals. We will also intensively explore the channel and receptor basis of defeat-induced changes in the firing properties of VTA DA neurons and particularly investigate the ionic mechanisms of active resiliency. Moreover, the roles of these new ionic and receptor mechanisms in mediating standard antidepressant action will be systematically investigated. These proposed molecular and cellular studies will provide very useful and highly novel information, both for improving our knowledge of depression and for identifying new drug targets to develop more effective treatments for depression. Such treatments would be based on imitating active coping mechanisms of naturally occurring resilience and therefore might be likely to be more effective and less prone to side effects.

DESCRIPTION (provided by applicant): Alcohol-use disorders create a huge global health care burden, which ranks number two in the mental, neurological and substance-use disorders, indicating an urgent need for more effective treatments. It is well known that alcohol-use is very different from other drugs of abuse such as cocaine in multiple aspects: whereas some individuals drink alcohol for decades in a controlled manner and without developing dependence, others have an uncontrollable desire to drink and develop severe alcohol addiction. To understand the neurophysiological mechanisms that underlie the evidently different drinking behaviors, we hypothesize that there is alcohol drinking variability even in genetically identical inbred mice. This hypothesis is important because previous efforts to understand the drinking variations in animal models have encountered huge challenges, which is in part induced by the variable gene backgrounds and the unknown complex interactions between genes and the environment. In our preliminary studies toward testing this hypothesis, we observed that in C57BL/6J mice, an inbred strain typically used in alcohol research because of its high ad libitum consumption of alcohol, roughly 10% had lower alcohol drinking behaviors (preference and consumption). This provides us with an exceptional experimental model to explore the individual variations in alcohol drinking behaviors. To investigate the neurophysiological basis underlying variable alcohol drinking behaviors, we focus on the brain's reward circuit, a well-known neural system that is critically involved in mediating natural reward and drug reinforcement. We propose to: (1) characterize the differences in the firing activity of ventral tegmental area (VTA) dopamine neurons in low and high alcohol drinking mice, and, more specifically, of the subpopulations of VTA neurons projecting to the nucleus accumbens (NAc), medial prefrontal cortex (mPFC) and amygdala (BLA); (2) optogenetically dissect the functional roles of VTA dopamine neurons and their specific projections to NAc, mPFC and BLA in mediating the variable alcohol drinking behaviors; (3) intensively explore the ion channel and receptor mechanisms underlying the differences in the firing properties of VTA dopamine neurons and projection-specific neurons in the VTA to identify possible novel drug targets. In this project, we will provide the direct causal links between alcohol drinking behaviors and the specific roles of firing patterns, cell types and neural circuits via the combined use of state-of-the-art electrophysiological and sophisticated optogenetic approaches in freely behaving mice. These proposed molecular, cellular and neural circuit studies will provide very useful and highly novel information, both for improving our knowledge of variable alcohol drinking behaviors and for identifying new drug targets to develop more effective, individualized treatments for alcohol-use disorders.

PROJECT SUMMARY Anxiety disorders are the most common psychiatric illness afflicting 273 million people worldwide. The symptoms of anxiety disorders are highly complex and the causes are poorly understood. Additionally, a substantial number of patients suffering from anxiety disorders also present with depressive-like symptoms, which make the diagnosis and treatment even more complicated. In this project, we propose to utilize the repeated social defeat stress (RSDS) model that induces anxiety or mixed anxiety/depression phenotypes in separate subgroups of mice to investigate neural mechanisms regulating these behaviors. Following RSDS, mice are separated into two subgroups based on their social interaction test behavior: profound avoidance-displaying mice (susceptible) and non-avoidance-displaying mice (resilient). While susceptible mice display several depression-related behaviors, including social avoidance, anhedonia and despair, these depressive abnormalities are absent in resilient mice. However, RSDS induces severe anxiety behaviors in both susceptible and resilient mice. Thus, in this project, we label them as anxiety/depression (A/D) and anxiety (A) subgroups. An increasing number of studies have implicated the role of mesocorticolimbic ventral tegmental area (VTA) dopamine (DA) reward circuitry in anxiety and depression. Utilizing neural circuit-probing techniques, we previously observed that maladaptive firing activity occurred in the VTA DA neurons projecting to the medial prefront cortex (VTA-mPFC) and VTA DA neurons projecting to the nucleus accumbens (VTA-NAc) selectively in A/D mice (depression-susceptible mice), but not in the A-mice (the depression-resilient group). Our optogenetic studies further demonstrated the causal link between the firing maladaptations in these circuits and depression-related behaviors. However, we strikingly found that the firing activity of VTA neurons projecting to the amygdala (VTA-Amg) was dramatically decreased in both A/D- and A-mice. Based on these unexpected preliminary findings, our central hypothesis is that the VTA-Amg circuit may play a crucial role in mediating the anxiety-like behaviors observed in both A/D- and A-mice following RSDS. To test this, we propose two Specific Aims: (a) to investigate the pathological alterations of VTA-Amg DA circuit neurons in A/D- and A-male and female mice by use of ex vivo brain slice preparation and in vivo optrode recordings from intact animals; and (b) to determine the functional role of VTA-Amg DA neurons in mediating RSDS-induced anxiety behaviors by optogenetically manipulating these circuits in male and female mice. By utilizing these cell type- and circuit-specific electrophysiological and optogenetic techniques, we will determine if a causal relationship exists between the neuronal activity of VTA-Amg DA circuit and anxiety-related behaviors.

PROJECT SUMMARY Current antidepressant medications for major depressive disorder (MDD) take several weeks to months to achieve therapeutic effect. This inevitable delay for drug efficacy not only prolongs distress and impairment for depressed patients, but is also life threatening for MDD patients with suicidal ideation. This delay has led to the widely accepted theory that the therapeutic efficacy of antidepressants can only be achieved by chronic treatments. However, an increasing body of clinical studies, including deep brain stimulation, ketamine and scopolamine therapies, has demonstrated the ability to regulate mood states within minutes to hours. These groundbreaking findings provide new hope in minimizing MDD disease burden. The main purpose of this application is to explore a novel drug target, which offers the potential for rapid antidepressant efficacy. There are early lines of evidence linking the midbrain dopamine system mechanistically in rapid depression treatment. Consistent with this, various studies showed that optogenetically activating or inhibiting dopamine neurons in the ventral tegmental area (VTA) circuits, a brain?s reward system, rapidly and bi-directionally regulated depression-related behaviors in rodent models of depression. In a repeated social defeat stress (RSDS) model of depression, we previously demonstrated that pharmacological inhibition of hyperpolarization- activated cyclic nucleotide-gated (HCN) channels in the VTA reversed the pathophysiological hyperactivity of VTA dopamine neurons and achieved antidepressant-like effects within one hour. In our initial follow-up studies, we find that one single intra-VTA infusion of a HCN blocker induces rapid and long-lasting antidepressant-like effects. The single infusion-induced antidepressant efficacy lasts ~2 weeks. Similarly, one single systemic administration (intraperitoneal injection) of this HCN blocker also induces rapid and ~2 weeks long antidepressant-like effects, which is evidently different from typical antidepressants such as SSRIs that took two weeks to gain antidepressant-like efficacy in the same model. Focusing on the rapid and long-lasting treatment effects, the overall objectives of this application are: (1) Drug Effect: to systematically define dose- dependent effects of three selected HCN blockers on the VTA dopamine neuron activity and depression- related behaviors; and (2) Drug Mechanism: to determine the cellular and circuit mechanisms that underlie the long-lasting antidepressant-like efficacy induced by a single exposure to HCN blockers. Upon the completion of this project, the proposed studies will provide highly novel HCN channel mechanisms for rapid and long-lasting treatment effects. Additionally, we also expect novel information to improve our knowledge of dopamine circuit mechanisms of depression.