Gregory P. Mark - US grants

Cocaine is a potent psychostimulant with high abuse liability in humans. The substantial health risk posed by chronic cocaine use has prompted a serious research effort to identify the neurobiological substrates that underlie the motivational aspects of drug seeking. Research in this area has concentrated anatomically on a ventromedial nucleus of the basal ganglia called the nucleus accumbens (NAc), which is considered to be an integral component in the link between motivation and action. Recent neurochemical investigations have suggested the importance of several neurotransmitter systems (e.g. dopamine, serotonin, glutamate) localized within the NAc in modulating drug reinforcement. Despite this substantial progress, our understanding of the neurochemical mechanisms involved in the motivational/behavioral aspects of drug seeking remains incomplete. In particular, the NAc is known to contain a population of cholinergic interneurons whose activity may have important motivational consequences. Therefore, a primary goal of the proposed studies is to characterize the extent to which ACh modulates drug self-administration in rats. Sprague- Dawley rats will be implanted with intravenous catheters and trained to lever-press for infusions of cocaine on a progressive ratio schedule of reinforcement. After a steady level of baseline responding is established, micro-infusions of ACh agonists, oxotremorine and methylcarbamylcholine or cholinergic antagonists (atropine and mecamylamine) will be delivered into the lateral ventricle or bilaterally into the NAc prior to drug availability. Responding on both a drug-active and an inactive lever will be measured for five hours following each infusion. Selective increases in responding on the drug-active lever should indicate an increase in the motivation to obtain cocaine while decreases in responding would signal a suppression of cocaine's reinforcing effects. It is hoped that the results of these studies will contribute to an understanding of the neurochemical substrates which underlie drug-seeking and potentially contribute to the development of clinically efficacious treatments for drug craving and relapse.

DESCRIPTION: (Applicant's Abstract) The substantial health risk posed by chronic cocaine use has prompted a serious research effort to identify the neurobiological substrates that underlie the motivational aspects of drug seeking and drug relapse. Research in this area has concentrated on the nucleus accumbens (NAc) which is considered to be an integral component in the link between motivation and action. The NAc receives afferents from several neurotransmitter systems (e.g. dopamine, serotonin, glutamate) and each of these has been implicated in drug reinforcement. Despite this substantial progress, our understanding of the neurochemical mechanisms involved in the motivational aspects of drug seeking remains incomplete. The NAc is known to contain cholinergic interneurons whose activity may have important motivational consequences. The first specific aim of this research is to identify the impact of cocaine self-administration on acetylcholine (ACh) release within the NAc and, as a motoric control, the dorso-lateral striatum. Sprague-Dawley rats will be implanted with intravenous catheters and trained to lever-press for infusions of cocaine. Microdialysis will be used to measure ACh at four time points: 1) during initial exposure to cocaine, 2) after 2 weeks of daily self-administration, 3) after 10 days of withdrawal and finally 4) during reinstated self-administration. The second aim of these experiments is to determine the responsiveness of ACh interneurons within the NAc to GABAergic and dopaminergic drugs following 2 weeks of cocaine self-administration and after ten days of withdrawal. Microdialysis probes will be used to administer drugs into the NAc while ACh release is measured. The third aim of this research is to characterize the extent to which ACh modulates cocaine self-administration. Rats will be trained to lever-press for intravenous infusions of cocaine on a progressive ratio schedule of reinforcement. Micro-infusions of ACh agonists and antagonists will be delivered bilaterally into the NAc prior to drug availability. Responding on drug-active and inactive levers will be measured for five hours following infusions. Selective increases in responding on the drug-active lever should indicate an increase in the motivation to obtain cocaine while decreases in responding would signal a suppression. It is hoped that these results will further our understanding of the involvement of cholinergic mechanisms in drug-seeking and potentially to the development of clinically efficacious treatments for drug craving and relapse.

The substantial health risk posed by the compulsive use of cocaine has prompted a serious research effort to identify the neurobiological substrates that underlie the development of addiction to this drug. Despite substantial progress, an understanding of the neurochemical systems that mediate the motivational aspects of drug-seeking and craving remains incomplete. To expand this understanding, this proposal will examine the involvement of four discrete cholinergicsystems in the development of compulsive cocaine intake and cocaine- seeking behaviors in rats. Experiments will focus on nicotinic acetylcholine (ACh) receptor activation in the development and expression of addictive behavior. The first specific aim is to identify the impact of escalating cocaine self-administrationon the release of ACh in amygdaloid complex (AmC), ventral tegmental area (VTA), nucleus accumbens (NAc) and prefrontal cortex (PFC). Rats will be trained to press a lever to self-administer cocaine through indwelling jugular catheters. Availability of cocaine will be increased from 1 to 6 hr per day (which has been shown to escalate daily cocaine intake) and microdialysis will measure ACh release before and after escalated cocaine intake. A second experiment will examine the impact of intracerebral microinjections of nicotine or the nicotinic antagonist mecamylamine on the escalation of cocaine intake. The second aim of this proposal is to determine if nicotine injections in VTA, NAc, PFC or AmC will prime non-reinforced lever-press behavior in cocaine addicted rats. Prior to testing, rats will receive systemic injections of nicotine or saline before self-administering cocaine for 2 weeks. On test days, rats will be given intracerebral nicotine and non- reinforced responding will be measured as an index of cocaine craving. The third aim of this research is to determine the role of AmC, VTA, NAc and PFC ACh systems in the ability of drug-associated stimuli to control non-reinforced responding. ACh will be measured during the presentation of cocaine-associated cues along with responding elicited by a cocaine-conditioned stimulus after microinjection of nicotinic drugs into AmC, VTA, NAc and PFC. Injections of nicotinic antagonists in these four areas are expected to reduce lever- press behavior in response to presentation of second-order stimuli. It is hoped that the results of these studies will further our understanding of the involvement of central cholinergic mechanisms in cocaine-reinforced behavior and drug craving and the brain site-specific influence of nicotine on cocaine addiction and relapse.

model; prefrontal lobe /cortex

DESCRIPTION (provided by applicant): Methamphetamine (METH) is an addictive psychostimulant with extremely high relapse rates. Effective pharmacotherapies to treat METH addiction, and in particular to counter relapse have not yet been developed. Research in animal models implicates the corticotropin releasing factor (CRF) peptide system in the mechanisms of extinction and reinstatement of psychostimulant self- administration (SA). These studies suggest CRF receptors could be an important target for therapeutic development. The CRF peptide system is complex, consisting of two main types of receptors, CRF1 and CRF2, and four endogenous ligands, CRF, urocortin 1 (Ucn1), urocortin 2 (Ucn2) and urocortin 3 (Ucn3). Since many brain regions are innervated by several of these endogenous ligands and often contain both receptors, the contribution of individual components of the CRF system to extinction and relapse is difficult to resolve using pharmacological approaches. Recently developed knockout (KO) mice, deficient in components of the CRF system, provide a novel approach that can complement pharmacological studies and clarify the role of components of the CRF system in behaviors such as reinstatement of drug-seeking. Here we propose using CRF1KO, CRF2 KO and Ucn1 KO to study the role of components of the CRF system in extinction from, and reinstatement of SA of METH. We hypothesize that different components of the CRF system will be differentially involved in drug- versus stress-induced reinstatement of METH SA. The project will include 3 Specific Aims: (1) to compare rates of METH SA in CRF1, CRF2 and Ucn1 KO mice using operant intravenous procedures. We hypothesize that CRF1 KO will show slightly slower acquisition rates of METH SA than other genotypes, but that all three genotypes will ultimately reach a similar level of METH SA. (2) To compare rates of extinction from METH SA in CRF1, CRF2 and Ucn1 KO mice. We hypothesize that CRF2 KO mice will show faster rates of extinction from METH SA compared to other genotypes. (3) To compare reinstatement of METH SA in CRF1, CRF2 and Ucn1 KO mice following a priming dose of METH or exposure to stress (metabolic and physical). We hypothesize that drug-induced reinstatement will be attenuated in CRF1 KO, but not in CRF2 or Ucn1 KO mice, and that stress-induced reinstatement will be attenuated in CRF2 KO mice suggesting that it involves endogenous urocortins. Analysis of stress- induced reinstatement in Ucn1 KO mice will further delineate whether this behavior is mediated by Ucn1 versus Ucn2 or Ucn3 peptides. PUBLIC HEALTH RELEVANCE: Methamphetamine (METH) is an addictive psychostimulant with extremely high relapse rates. Effective pharmacotherapies to treat METH addiction, and in particular to counter relapse, have not yet been developed. This research will use an animal model of METH self-administration to examine the involvement of the corticotropin releasing factor (CRF) peptide system in the mechanisms of extinction and reinstatement of METH-seeking behavior. These studies will determine if CRF receptors could be an important target for therapeutic development.