Investigating Methamphetamine Craving Using the Extinction-Reinstatement Model in the Rat

Distinct roles for orexin-1 and orexin-2 receptors in the dentate gyrus of the hippocampus in the methamphetamine-seeking behavior in the rats

Because of the relapsing properties of psychostimulants such as methamphetamine (Meth), there is no established pharmacotherapy for Meth addiction. The orexinergic system is a promising target for treating psychostimulant use disorders and relapse. However, to the best of our knowledge, no investigation regarding the role of orexin receptors in the dentate gyrus (DG) region of the hippocampus has been conducted in the extinction and reinstatement of Meth-seeking behavior. Two stainless-steel guide cannulae were bilaterally implanted into the DG of the rats' brains. The unbiased conditioned place preference (CPP) procedure was conducted to induce Meth conditioning. Following the five days Meth injections (1 mg/kg; sc), animals received intra-DG microinjection of SB334867 or TCS OX2 29, as orexin 1 (OX1) or orexin 2 (OX2) receptor antagonists, respectively (without Meth administration) during extinction phase to elucidate the role of orexin receptors in the latency of the extinction period in the Meth-conditioned rats. To evaluate the role of orexin receptors in the DG region in the reinstatement of Meth-seeking behavior, the extinguished rats received SB334867 or TCS OX2 29 before injecting a priming dose of Meth (0.25 mg/kg; sc). The results indicated two distinct roles for the OX1 and OX2 receptors in the DG region. TCS OX2 29 attenuated the extinction latency, and SB334867 considerably reduced the reinstatement of Meth-seeking behavior in this region. Therefore, the DG region's orexinergic system might be a potential therapeutic target for psychostimulant use disorders.

Nicotine- and cocaine-triggered methamphetamine reinstatement in female and male Sprague-Dawley rats

Preclinical studies have demonstrated a return to methamphetamine (meth)-seeking behavior (reinstatement) induced by injections of meth administered by the experimenter (drug-prime). Notably, females tend to be more sensitive to drug-prime; often displaying more reinstatement behavior when compared to males. While meth-primed reinstatement of meth-seeking behavior has been established, little is known about the ability of other drugs of abuse to substitute for meth during drug-primed reinstatement; nicotine and cocaine were the focus of the present work. We also examined if self-administration and/or reinstated meth-seeking behavior was affected by repeated nicotine administration. Male and female Sprague-Dawley rats were trained to self-administer meth during daily sessions. During this self-administration phase, rats were placed into 1 of 2 groups: saline or repeated nicotine exposure. Rats in the repeated nicotine group received nicotine injections 4h after meth self-administration sessions, whereas the remaining rats received saline. Following self-administration was extinction in which meth was no longer available and nicotine was no longer administered. After extinction, rats were tested to determine if 0 (saline), 0.2, and 0.4mg/kg nicotine reinstated meth-seeking behavior. Three days of re-extinction followed nicotine testing. Finally, rats received reinstatement tests with 0 (saline), 5, and 10mg/kg cocaine. Nicotine and cocaine reinstated meth-seeking behavior in male and female rats with no difference between the sexes. Repeated nicotine administration potentiated meth reinstatement following the 0.4mg/kg nicotine-prime. While females may be more sensitive to reinstatement triggered with the original self-administration drug, this effect may not generalize to priming with other drugs of abuse.

Compulsive methamphetamine taking under punishment is associated with greater cue-induced drug seeking in rats

Methamphetamine (METH) addicts lose control over drug consumption despite suffering multiple adverse medicolegal consequences. To mimic the negative events associated with drug addiction in humans, we recently introduced a rat model of self-administration (SA) with response-contingent punishment on METH intake. These procedures allowed us to distinguish between two addiction-like phenotypes in rats, those that sustained METH taking despite negative consequences (shock-resistant, SR) and rats that significantly reduced their METH intake (shock-sensitive, SS). Here, we further developed our adverse consequence model and examined incubation of METH craving by measuring cue-induced drug seeking in SR and SS rats. Male Sprague-Dawley rats were trained to self-administer METH (0.1mg/kg/injection) or saline intravenously (i.v.) during twenty-two 9-h sessions that consisted of 3 separate 3-h sessions separated by 30min. Subsequently, rats were subjected to incremental footshocks during thirteen additional 9-h METH SA sessions performed in a fashion identical to the training phase. Cue-induced drug craving was then assessed at 2 and 21days after the footshock phase. All rats escalated their intake of METH, with both phenotypes showing similar drug taking patterns during SA training. In addition, rats that continued their METH intake despite negative consequences showed even greater cue-induced drug craving following withdrawal than the rats that reduced METH intake following negative consequences. Taken together, our adverse consequence-based model highlights the possibility of identifying rats by addiction-like phenotypes and subsequent vulnerability to relapse-like behaviors. The use of similar SA models should help in the development of better therapeutic approaches to treat different stages of METH addiction.

Regulation of the Dopamine and Vesicular Monoamine Transporters: Pharmacological Targets and Implications for Disease

Dopamine (DA) plays a well recognized role in a variety of physiologic functions such as movement, cognition, mood, and reward. Consequently, many human disorders are due, in part, to dysfunctional dopaminergic systems, including Parkinson's disease, attention deficit hyperactivity disorder, and substance abuse. Drugs that modify the DA system are clinically effective in treating symptoms of these diseases or are involved in their manifestation, implicating DA in their etiology. DA signaling and distribution are primarily modulated by the DA transporter (DAT) and by vesicular monoamine transporter (VMAT)-2, which transport DA into presynaptic terminals and synaptic vesicles, respectively. These transporters are regulated by complex processes such as phosphorylation, protein-protein interactions, and changes in intracellular localization. This review provides an overview of 1) the current understanding of DAT and VMAT2 neurobiology, including discussion of studies ranging from those conducted in vitro to those involving human subjects; 2) the role of these transporters in disease and how these transporters are affected by disease; and 3) and how selected drugs alter the function and expression of these transporters. Understanding the regulatory processes and the pathologic consequences of DAT and VMAT2 dysfunction underlies the evolution of therapeutic development for the treatment of DA-related disorders.

Methamphetamine self-administration in mice decreases GIRK channel-mediated currents in midbrain dopamine neurons

BACKGROUND

Methamphetamine is a psychomotor stimulant with abuse liability and a substrate for catecholamine uptake transporters. Acute methamphetamine elevates extracellular dopamine, which in the midbrain can activate D2 autoreceptors to increase a G-protein gated inwardly rectifying potassium (GIRK) conductance that inhibits dopamine neuron firing. These studies examined the neurophysiological consequences of methamphetamine self-administration on GIRK channel-mediated currents in dopaminergic neurons in the substantia nigra and ventral tegmental area.

METHODS

Male DBA/2J mice were trained to self-administer intravenous methamphetamine. A dose response was conducted as well as extinction and cue-induced reinstatement. In a second study, after at least 2 weeks of stable self-administration of methamphetamine, electrophysiological brain slice recordings were conducted on dopamine neurons from self-administering and control mice.

RESULTS

In the first experiment, ad libitum-fed, nonfood-trained mice exhibited a significant increase in intake and locomotion following self-administration as the concentration of methamphetamine per infusion was increased (0.0015-0.15mg/kg/infusion). Mice exhibited extinction in responding and cue-induced reinstatement. In the second experiment, dopamine cells in both the substantia nigra and ventral tegmental area from adult mice with a history of methamphetamine self-administration exhibited significantly smaller D2 and GABAB receptor-mediated currents compared with control mice, regardless of whether their daily self-administration sessions had been 1 or 4 hours. Interestingly, the effects of methamphetamine self-administration were not present when intracellular calcium was chelated by including BAPTA in the recording pipette.

CONCLUSIONS

Our results suggest that methamphetamine self-administration decreases GIRK channel-mediated currents in dopaminergic neurons and that this effect may be calcium dependent.

Neurogenetics and Nutrigenomics of Neuro-Nutrient Therapy for Reward Deficiency Syndrome (RDS): Clinical Ramifications as a Function of Molecular Neurobiological Mechanisms

In accord with the new definition of addiction published by American Society of Addiction Medicine (ASAM) it is well-known that individuals who present to a treatment center involved in chemical dependency or other documented reward dependence behaviors have impaired brain reward circuitry. They have hypodopaminergic function due to genetic and/or environmental negative pressures upon the reward neuro-circuitry. This impairment leads to aberrant craving behavior and other behaviors such as Substance Use Disorder (SUD). Neurogenetic research in both animal and humans revealed that there is a well-defined cascade in the reward site of the brain that leads to normal dopamine release. This cascade has been termed the "Brain Reward Cascade" (BRC). Any impairment due to either genetics or environmental influences on this cascade will result in a reduced amount of dopamine release in the brain reward site. Manipulation of the BRC has been successfully achieved with neuro-nutrient therapy utilizing nutrigenomic principles. After over four decades of development, neuro-nutrient therapy has provided important clinical benefits when appropriately utilized. This is a review, with some illustrative case histories from a number of addiction professionals, of certain molecular neurobiological mechanisms which if ignored may lead to clinical complications.

Positive Allosteric Modulation of mGluR5 Accelerates Extinction Learning but Not Relearning Following Methamphetamine Self-Administration

Recent studies have implicated glutamate neurotransmission as an important substrate for the extinction of conditioned behaviors, including responding for drug reinforcement. Positive allosteric modulation of the type-5 metabotropic glutamate receptor (mGluR5) in particular has emerged as a treatment strategy for the enhancement of extinction of drug-motivated behaviors. Here, we investigated the effects of the mGluR5 positive allosteric modulator CDPPB, a compound known for its cognitive enhancing effects in rodents, on extinction learning in rats with different histories of methamphetamine (METH) training. Rats were trained to self-administer METH under two conditions: 16 daily sessions of short access (90 min/day, ShA), or eight daily sessions of short access followed by eight sessions of long access (6 h/day, LgA). Control rats self-administered sucrose pellets in daily 30 min sessions. Next, rats were administered vehicle or 30 mg/kg CDPPB prior to seven consecutive daily extinction sessions, subjected to additional extinction sessions to re-establish a post-treatment baseline, and then tested for reinstatement of behavior in the presence of METH- or sucrose-paired cues. Rats were then subjected to a second series of extinction sessions, preceded by vehicle or 30 mg/kg CDPPB, and an additional test for cue-triggered reinstatement. CDPPB treatment resulted in a more rapid extinction of responding on the active lever, especially in the early sessions of the first extinction sequence. However, treatment effects were minimal during subsequent cue reinstatement tests and non-existent during the second series of extinction sessions. Rats with histories of ShA, LgA, and sucrose training expressed similar behavioral sensitivities to CDPPB, with LgA rats demonstrating a modestly higher treatment effect. Positive allosteric modulation of mGluR5 may therefore have some beneficial effects on efforts to facilitate extinction learning and reduce methamphetamine seeking.

Attenuation of methamphetamine seeking by the mGluR2/3 agonist LY379268 in rats with histories of restricted and escalated self-administration

Recent findings implicate group II metabotropic glutamate receptors (mGluR(2/3)) in the reinforcing effects of psychostimulants and have identified these receptors as potential treatment targets for drug addiction. Here, we investigated the effects of mGluR(2/3) stimulation on cue- and drug-primed reinstatement in rats with different histories of methamphetamine (METH) self-administration training, under two conditions: 16 daily sessions of short access (90 min/day, ShA), or 8 daily sessions of short access followed by 8 sessions of long access (6 h/day, LgA). Following self-administration and subsequent extinction training, rats were pretreated with the selective mGluR(2/3) agonist LY379268 (variable dose, 0-3 mg/kg), exposed to METH-paired cues or a priming injection of METH (1 mg/kg), and tested for reinstatement of METH-seeking behavior. LgA rats self-administered greater amounts of METH during the second half of training, but when pretreated with vehicle, ShA and LgA rats showed cue- and drug-primed reinstatement at equivalent response rates. However, LgA rats demonstrated greater sensitivity to mGluR(2/3) stimulation with attenuated responding during cue-induced reinstatement after 0.3 mg/kg and higher doses of LY379268, whereas ShA rats decreased cue-induced reinstatement behavior following 1.0 mg/kg and 3.0 mg/kg LY379268. Additionally, both LgA and ShA rats exhibited decreased METH-primed reinstatement behavior following 0.3 mg/kg and higher doses of LY379268. A separate group of control rats was trained to self-administer sucrose pellets, and demonstrated attenuated cue-induced sucrose-seeking behavior following 1.0 and 3.0 mg/kg LY379268. Together, the results indicate that LY379268 has differential attenuating effects on cue-induced reinstatement behavior in rats with different histories of METH intake. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.