Michela Marinelli - US grants

[unreadable] DESCRIPTION (provided by applicant): There is growing concern that adolescence is associated with greater sensitivity to psychostimulant drugs and increased risk of addiction. Such increased susceptibility to drug self-administration could be due to differences in the way the adolescent brain reacts or adapts to repeated drug exposure compared with the adult brain. The overall goal of this proposal is to understand the neurophysiological and behavioral determinants that contribute to adolescent susceptibility to cocaine self-administration, using rodent models. The dopamine system will be the focus of these studies, as it plays a central role in mediating the rewarding effects of addictive drugs. The specific aims are: to determine whether adolescence is characterized by heightened excitability of dopamine cells, and whether this is mediated by decreased intrinsic inhibitory properties of these neurons (Aim 1) and/or increased excitatory synaptic input to these cells (Aim 2). Aim 3 will establish whether changes in dopamine neuron function translate into differences in cocaine self-administration behavior. Throughout these aims, adolescent and adult rats will be compared both before and after repeated cocaine exposure to appraise whether individuals exhibit inherent differences, and/or show differential cocaine- induced neuroadaptations that could facilitate self-administration behavior. Neuronal activity will be assessed using in vivo and in vitro electrophysiology techniques; behavioral tests will consist of a variety of self-administration procedures that allow estimating the rewarding effects of cocaine and motivation to self-administer the drug. Overall, using a multidisciplinary approach this proposal will examine, from the cellular to the behavioral level, potential mechanisms underlying adolescent/adult differences in cocaine self-administration. These studies will use behavioral models that are relevant to the human condition to provide insight into both the causes and effects of adolescent drug use. In the long run, these findings will facilitate the development of new targeted therapeutic strategies for the treatment or prevention of cocaine addiction. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): The activity of dopamine cells of the ventral tegmental area (VTA) plays a critical role in reward and motivation. In vivo, these cells fire irregularly, with interspersed "burst" events;this pattern of activity is the consequence of excitatory and inhibitory inputs arising locally and from distal structures. In addition, plasticity of these inputs occurs after exposure and withdrawal from addictive drugs. However, understanding the functional role of specific afferents to the VTA, both in baseline conditions and after exposure to drugs, has been limited by current technology. These studies will establish how the activity of the VTA is modulated by specific afferents, before or after cocaine self-administration in rats. To dissect the role of specific afferents, we will use the technique of optogenetics whereby the activity of selective pathways can be increased with pulses of light applied to the terminals that express the algae protein channelrhodopsin 2 (ChR2). Specific brain regions of adult rats will be infected with an adeno-associated virus for ChR2 expression in neurons. We will then determine the functional role of specific pathways by applying light pulses in the VTA, to stimulate the afferents from each of these regions. We will examine inputs to the VTA from (i) the pedunculopontine tegmental nucleus (PPTg), a mixed glutamatergic/cholinergic population that responds to salient stimuli;(ii) local VTA glutamate cells, which have recently been described but whose functional role is unclear;and (iii) the rostromedial tegmental nucleus (RMTg), a hindbrain structure recently identified that sends important GABAergic projections to the VTA but whose functional role is unknown. We will evaluate the consequence of pathway-specific stimulation on cell activity (firing rates and patterns) measured in vivo in anesthetized rats. Then, we will determine the nature of these inputs in vitro, by measuring synaptic potentials generated upon stimulation of these pathways. We will also determine the manner in which such synaptic input is integrated. Finally, we will further test the nature of the pathways by performing immunohistochemistry studies. Aim 1 will examine the role of these pathways in drug-naove rats. Aim 2 will examine it in rats that have self-administered cocaine. These studies will be the first to determine the functional role of unexplored and novel afferents to the VTA. They will also determine how cocaine self-administration modifies the way dopamine cells are excited by specific afferents. This will provide important information on the manner in which brain reward pathways function in baseline conditions, and their plasticity after self-administration. These studies will provide novel insights on neural circuits that control addictive behavior. PUBLIC HEALTH RELEVANCE: These studies will establish how the activity of a brain reward area known as the ventral tegmental area is modulated by specific inputs before or after cocaine self-administration in rats. To dissect the role of specific inputs, we will use a novel technique called optogenetics whereby the activity of selective brain cells can be increased with pulses of light applied in proximity of the brain cells. These studies will provide novel insights on neural circuits that control addictive behavior.

DESCRIPTION (provided by applicant): Exposure to methylphenidate (a dopamine reuptake inhibitor) mimics several behavioral and molecular effects of cocaine. However, other cocaine-like effects are minimal, possibly because serotonin is not elevated by methylphenidate. This notion is supported by recent findings produced in the applicants' laboratories, showing that elevating serotonin with selective serotonin reuptake inhibitors (SSRIs) potentiates methylphenidate-induced psychomotor activity and neuronal adaptations in brain regions implicated in psychostimulant addiction. It is currently unknown if increasing the serotonin transmission with SSRIs in conjunction with methylphenidate could also promote addiction. This is of concern because many individuals are being exposed to a drug combination of methylphenidate (Ritalin) plus an SSRI. The objective of this application is to determine how methylphenidate+SSRI exposure modifies indicators of cocaine addiction liability. The hypothesis is that SSRIs potentiate the effects of methylphenidate, making it more cocaine-like. Repeated exposure to methylphenidate+SSRI will thus facilitate the subsequent development of cocaine self-administration behavior, which is an indicator of cocaine addiction liability. This will be tested in rats, using paradigms that allow inferring sensitivity and motivation to self-administer cocaine (Aim 1a) and compulsive drug taking (Aim 1b). Exposure to methylphenidate+SSRI could also trigger relapse in ex-cocaine users. This will be tested in rats, using paradigms that measure reinstatement of drug seeking behavior (Aim 2). Successful completion of these aims is significant, as it will inform a decision of whether or not methylphenidate+SSRI drug combinations should be avoided in the future because of their potential to facilitate behavioral changes related to psychostimulant addiction.

Project Summary/Abstract Adolescence is a period of heightened susceptibility to cocaine addiction. Certain aspects of addiction are also more severe in females than in males. Several factors have been proposed to mediate such heightened susceptibility in adolescents and females. What is unknown is if adolescents and females are less sensitive to conditions of external adversity associated with the process of obtaining cocaine. Perseveration of drug seeking and taking despite adversity and hazardous conditions is a hallmark of addiction, and has been hypothesized to be a key factor underlying susceptibility to develop addiction. The hypothesis of this proposal is that adolescents and females are less sensitive to adverse events required to obtain cocaine. Surprisingly, this hypothesis has not been tested extensively. Testing this hypothesis is important because it would underline that adolescents and females are a vulnerable population and provide an important reason as to why that is. This hypothesis will be tested by modifying an existing test, whereby rats need to surmount an ?aversive obstacle? in order to obtain cocaine. The aversive obstacle is in the form of an electric barrier (a portion of electrified floor) that rats need to traverse to obtain the drug. The intensity of the electricity is increased incrementally, until the rats reach abstinence. Ages (adolescents and adults) and sexes (males and females) will be compared for their ability to suppress cocaine intake in conditions of increased adversity to obtain the drug. The project is innovative in what it studies (age- and sex-differences in taking cocaine in conditions of adversity to obtain the drug). It is also innovative in the manner in which it implements and modifies an existing behavioral model to test the pursuit of drug taking despite adversity associated with the process of obtaining the drug. The project is significant, as it will provide important insight into understanding the nature of adolescent and female susceptibility to addiction.

Project Summary/Abstract The lateral preoptic area (LPO) is a hypothalamic structure whose function is largely unknown. There is evidence that stimulating the LPO elicits locomotion, and that this structure supports intracranial self- stimulation, suggesting that the LPO could be part of the ?brain reward system?. The LPO also sends strong projections to the ventral tegmental area (VTA), a key member of the brain reward circuit. This further positions the LPO as a potential member of the brain reward system and, in particular, as a modulator of VTA activity. This is significant because structures that modulate VTA activity have been shown to play a critical role in cocaine seeking; increases in VTA dopamine neuron activity trigger relapse (i.e. increase drug seeking), whereas decreases reduce seeking. It is unknown if the LPO functionally regulates the activity of VTA neurons and cocaine seeking behavior. The objective of this proposal is to understand if and how the LPO regulates the activity of VTA neurons (Aim 1) and cocaine seeking behavior (Aim 2). In addition, the proposal will examine the contribution of the LPO- VTA pathway in any observed effect, and the neurotransmitters (GABA or glutamate) involved. This will be achieved by manipulating (increasing or decreasing) the activity of the LPO and LPO-VTA pathway; this will determine if this structure and pathway are sufficient and/or necessary for regulating the activity of VTA neurons and cocaine seeking behavior. Activity will be manipulated using three approaches: pharmacology, chemogenetics, and optogenetics. The activity of VTA neurons (GABA and dopamine) will be evaluated with in vivo extracellular recordings in anesthetized rats. Cocaine seeking will be evaluated after cocaine self-administration, by examining responding in the absence of cocaine. Sucrose seeking will also be tested, to examine if the role of this structure and pathway extends to seeking for natural rewards. The neurochemical nature of the LPO-VTA pathway will be probed (i) using pharmacology, (ii) by measuring synaptic currents, and (iii) by performing immunohistochemistry. We predict that stimulating the LPO increases the activity of dopamine neurons in the VTA and seeking behavior. This occurs via a GABAergic LPO-VTA pathway. Specifically, the LPO sends inhibitory GABAergic projections to the VTA. These synapse onto GABA neurons of the VTA resulting in dis-inhibition (i.e. excitation) of dopamine neurons. We expect these studies to establish a novel role for the LPO as a key modulator of VTA neuron activity and cocaine seeking behavior. This provides a significant advancement in understanding circuits and mechanisms of addiction. In addition, these studies will advance our understanding of the brain, by discovering a role of a structure whose function is largely unknown.