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Mission

We use genetically engineered mice with altered noradrenergic systems to study how norepinephrine influences various aspects of physiology, behavior, and neurochemistry.  Our main areas of interest are drug addiction, epilepsy, depression, and neurodcgenerative disease.

Background

General

My approach is to use model systems to better understand genes involved in human disease. In particular, I believe that genetic models combined with pharmacological tools represents a powerful way to answer biological questions. I have focused this approach on various aspects of neurobiology and used it to study antidepressant drugs, epilepsy, and drug addiction.

Introduction

Norepinephrine (NE) is one of the most abundant neurotransmitters in the central and peripheral nervous systems, and has been implicated in many aspects of physiology and behavior. I have taken a genetic approach to studying NE by using dopamine beta-hydroxylase knockout (Dbh -/-) mice. Dbh functions in the NE biosynthetic pathway and is required to converts dopamine to NE; thus, Dbh -/- mice completely lack NE and provide a good model to study NE function. I have been primarily using the Dbh -/- mice in behavioral, pharmacological, and molecular paradigms to study central NE function.

Why study NE?

NE was the originally characterized in the peripheral nervous system and was one of the first neurotransmitter discovered. It has profound effects on almost all aspects of the sympathetic nervous system, including regulation of cardiovascular function and energy metabolism. NE is also abundant in the central nervous system. Most noradrenergic neurons originate in the brainstem in a region called the locus coeruleus (LC). These neurons project to almost every region of the brain; in fact, it is nearly impossible to find a brain structure that completely lacks noradrenergic input. Because it was the first neurotransmitter discovered and is so widespread, NE has been extensively studied for over 30 years by various means. There are three major types of NE receptor, all of which are seven-pass transmembrane, G protein-coupled receptors. Stimulation of alpha-1 and beta-noradrenergic receptors increase intracellular Ca++ and cAMP respectively, and are localized on target cells of noradrenergic neurons. Activation of alpha-2 receptors decreases cAMP. These receptors function as inhibitory autoreceptors and are also found on target neurons. There are multiple subtypes within each of these receptor classes, and the total number of identified NE receptors now stands at 9. Fairly specific agonists and antagonists exist for most of these receptors and have been extensively characterized. There are also neurotoxins that are specific for noradrenergic neurons that have been used for many years to study the consequences of destroying NE-containing neurons. Knockout technology has resulted in the generation of mice lacking two different NE biosynthetic enzymes (Dbh and tyrosine hydroxylase) as well as almost every receptor subtype. What makes this system so enticing is that because of the rich history of NE experimentation, there is an abundance of pharmacological tools available for its study.

Why use genetics?

Neurotransmitter function has classically been studied in one of three ways: neuronal lesioning, which eliminate the cells and their transmitters, neuronal stimulation, which activates neurons and causes neurotransmitter release, and pharmacological intervention with agonists and antagonists that activate or block neurotransmitter receptors. Using these techniques, a lot has been learned about the role of NE in many processes. However, there are caveats associated with these techniques that make the interpretation of some experiments difficult. Cell lesions deplete not only the neurotransmitter of interest, but also any other transmitters co-expressed in the neurons, and similarly neuronal stimulation causes release of all transmitters. It is nearly impossible to eliminate all of the neurons of a specific transmitter type by lesioning, and the ones that remain can activate mechanisms to compensate for cell loss. Pharmacological agents can suffer from lack of both receptor and regional specificity.

Over the last 15 years, transgenic and knockout technology has established a viable alternative for studying neurotransmitter function. Genes coding for biosynthetic enzymes required for neurotransmitter synthesis, neuropeptides, and neurotransmitter receptors have been successfully knocked out in mice. Dbh -/- mice have been useful in determining many critical functions of NE in vivo. These include roles predicted by lesioning and pharmacology such as cardiovascular function, smooth muscle contraction, and brown fat thermogenesis, and novel roles such as embryonic development, maternal behavior, and immune function.

The major concern with using neurotransmitter knockouts is that because the animals develop in the absence of the transmitter, compensation by other transmitters may mask some phenotypes and create others de novo. In an effort to circumvent these problems, conditional knockouts have been developed that can restrict the gene deletion to particular times during the life cycle and regions of the brain. The Dbh -/- mouse is, in essence, a reversible knockout. A compound called L-threo-3,4-dihydroxyphenylserine (DOPS) can be converted to NE by the enzyme L-aromatic acid decarboxylase (AADC), thus bypassing the requirement for DBH. Acute administration of DOPS to adult Dbh -/- mice that have lacked NE since birth is able to reverse nearly all of the phenotypes associated with the mutation. This demonstrates that the defects caused by knocking out the Dbh gene are solely due to a specific lack of NE and are not caused by developmental abnormalities or compensatory changes in other neurotransmitters. Therefore, the principal caveat to using genetics to study neurotransmitter function is addressed in Dbh -/- mice. Combining the pharmacological tools made available by years of NE study with newly developed molecular genetic tools such as Dbh -/- mice has created a powerful system with which to investigate NE function.