Gregg Stanwood, Ph.D. - US grants

Biogenic amines such as dopamine regulate the formation and function of neural circuits within the developing forebrain. The pharmacological and genetic alterations of these systems during pre- and postnatal development are linked to both neurological and neuropsychiatric abnormalities. Using a rabbit model of low dose fetal cocaine exposure, we previously described specific neuroanatomical, biochemical,and behavioral deficits following permanent inhibition of dopamine D1 receptor signaling. The biobehavioral changes observed in this model reproduce some of the alterations that have been observed in the children of human mothers who have abused cocaine during a sensitive period of fetal development. The current availability of sophisticated transgenic and genetic targeting technologies affords a unique opportunity to gain additional insight into the molecular mechanisms that influence the effects of cocaine on brain development and circuit function. However, these studies require the generation of a novel model of low dose intravenous prenatal cocaine exposure in the mouse strain typically used in genetic studies of brain development and function. The experiments proposed in this application are therefore designed to develop such a murine model. In Aim 1 we will establish the successful delivery of drugs to pregnant C57BL/6J mice via chronic jugular catheters and deliver low dose, intravenous cocaine to dams during the period of peak cortical development. Plasma and brain cocaine and metabolite concentrations will be monitored to determine the extent to which specific doses mimic the pharmacokinetic responses that have been documented in rat models and in humans abusing cocaine. In Aim 2, the effects of this treatment on the physiological and neurological development of offspring will be assessed by histological, neuroanatomical, and biochemical approaches. Permanent alterations in dendritic structure of neurons and impairment of D1 receptor signaling in DA-rich cortical regions are central hallmarks of the rabbit model and we will initially survey the mouse model for similar deficits. The successful establishment of the intravenous murine model will provide the requisite preliminary data to propose a more complete series of genetic, pharmacological and behavioral studies. The mouse model will thus facilitate sophisticated molecular and cellular analyses of altered neurotransmitter signaling, and may lead to novel intervention strategies to normalize the resultant developmental disabilities.

DESCRIPTION (provided by applicant): Most mental health disorders have developmental etiologies and are produced by alterations in the formation and connectivity of specific forebrain regions including the medial frontal cortex and the striatum. Dopamine and other biogenic amines serve as neurotransmitters in the mature nervous system, and are also prominent drug targets in the treatment of neurological and psychiatric disorders. The dopamine system is expressed early in brain development, prior to the formation of synapses, and pleiotropically modulates decisions related to neuronal differentiation and circuit formation. Dopamine-dependent effects on dendritic morphology are receptor subtype-specific and brain region specific. We have gathered preliminary data suggesting that additional specificity is conferred by the stimulation of different signaling pathways depending on the receptor conformation(s) stabilized by distinct ligands (functional selectivity). The goals of this proposal are thus to identify the cellular functions of dopamine receptors during development of the frontal cortex and striatum, with direct reference to cellular subpopulations and functional selectivity. We propose three specific aims to probe the mechanisms by which dopamine receptor stimulation controls dendritic morphology. In Aim 1, we will examine the effects of activating distinct dopamine receptor subpopulations on dendritic differentiation and cell signaling responses of dissociated neurons in vitro. We will test the hypothesis that D1 and D2 receptors can produce distinct effects on dendritic growth patterns depending on which G protein signaling pathway is induced by functionally distinct ligands. In Aim 2, we will use recently created BAC reporter lines of mice (D1-tdTomato and D2-eGFP) to investigate whether spontaneous rates of process outgrowth differ as a function of dopamine receptor expression (and/or co-expression). In Aim 3, we will move into in vivo systems, testing to what degree genetic loss of the D1 and D2 receptors alters dendritic morphology in D1- and D2 receptor- expressing neurons, respectively. Our research program will thus identify cell-specific differences in developmental responsiveness to a common biological ligand, dopamine. Alterations in dopaminergic activity during development, whether produced by genetic or pharmacological means, alters circuits mediating cognitive and emotional behaviors during critical epochs of development, and may lead to subsequent psychiatric disease later in life. PUBLIC HEALTH RELEVANCE: Dysfunctions in brain catecholamine systems have been linked to both the development and expression of mental illness. The studies contained in this proposal will help elucidate how ligand- specific dopamine receptor modulation contributes to the establishment of proper brain architecture.

DESCRIPTION (provided by applicant): Psychostimulant abuse and addiction is a crushing public health problem. Our laboratories have begun to decipher the functional effects of glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) stimulation on dopamine uptake, clearance, and trafficking of presynaptic dopamine transporters. GLP-1 is an incretin hormone and neuropeptide that is released in response to food intake. GLP-1 acts through both peripheral and central mechanisms to regulate energy homeostasis and the hedonic components of food intake. We and others have hypothesized that peptides that modulate feeding behavior may also regulate brain circuitry responsible for drug reward. In fact, we recently discovered that systemic administration of the GLP-1 long-lasting analogue exendin-4, which is already used clinically in the treatment of type 2 diabetes, reduces the rewarding effects of cocaine in mice. Within the brain, GLP-1Rs are expressed within the hypothalamus, ventral tegmental area, and nucleus accumbens, but are especially enriched in the lateral septum (LS). The LS is (re)emerging as a crucial brain region involved in the hedonic properties of psychostimulants. In the current application, we will first define cellular heterogeneity in GLP-1 receptor expression patterns within the LS and test the hypothesis that GLP-1Rs modulate dopamine neurotransmission and signaling within the LS (Aim 1). These studies will use modern molecular neuroanatomical, biochemical and electrochemical methods. Next, we will test the hypothesis that local GLP-1 receptor signaling within the LS mediates the therapeutic effects of systemic exendin-4 on cocaine reward (Aim 2). We will also examine cocaine- and GLP-1 receptor agonist-induced changes in cellular activation. Our studies will use both pharmacological and genetic approaches, taking advantage of a recently created GLP-1R conditional knockout mouse. These multidisciplinary studies will provide essential foundational knowledge of the role of the GLP-1 receptor in psychostimulant abuse. The commercial availability of several FDA-approved GLP-1 agonists for the treatment of diabetes offers readily translational opportunities to improve human outcomes in psychostimulant abuse.