Predicting population responses to environmental change from individual-level mechanisms: towards a standardized mechanistic approach.

Animal populations will mediate the response of global biodiversity to environmental changes. Population models are thus important tools for both understanding and predicting animal responses to uncertain future conditions. Most approaches, however, are correlative and ignore the individual-level mechanisms that give rise to population dynamics. Here, we assess several existing population modelling approaches and find limitations to both 'correlative' and 'mechanistic' models. We advocate the need for a standardized mechanistic approach for linking individual mechanisms (physiology, behaviour, and evolution) to population dynamics in spatially explicit landscapes. Such an approach is potentially more flexible and informative than current population models. Key to realizing this goal, however, is overcoming current data limitations, the development and testing of eco-evolutionary theory to represent interactions between individual mechanisms, and standardized multi-dimensional environmental change scenarios which incorporate multiple stressors. Such progress is essential in supporting environmental decisions in uncertain future conditions.

Development and initial characterization of a novel ghrelin receptor CRISPR/Cas9 knockout wistar rat model.

BACKGROUND/OBJECTIVES: Ghrelin, a stomach-derived hormone implicated in numerous behaviors including feeding, reward, stress, and addictive behaviors, acts by binding to the growth hormone secretagogue receptor (GHSR). Here, we present the development, verification, and initial characterization of a novel GHSR knockout (KO) Wistar rat model created with CRISPR genome editing. METHODS: Using CRISPR/Cas9, we developed a GHSR KO in a Wistar background. Loss of GHSR mRNA expression was histologically verified using RNAscope in wild-type (WT; n = 2) and KO (n = 2) rats. We tested the effects of intraperitoneal acyl-ghrelin administration on food consumption and plasma growth hormone (GH) concentrations in WT (n = 8) and KO (n = 8) rats. We also analyzed locomotion, food consumption, and body fat composition in these animals. Body weight was monitored from early development to adulthood. RESULTS: The RNAscope analysis revealed an abundance of GHSR mRNA expression in the hypothalamus, midbrain, and hippocampus in WTs, and no observed probe binding in KOs. Ghrelin administration increased plasma GH levels (p = 0.0067) and food consumption (p = 0.0448) in WT rats but not KOs. KO rats consumed less food overall at basal conditions and weighed significantly less compared with WTs throughout development (p = 0.0001). Compared with WTs, KOs presented higher concentrations of brown adipose tissue (BAT; p = 0.0322). CONCLUSIONS: We have verified GHSR deletion in our KO model using histological, physiological, neuroendocrinological, and behavioral measures. Our findings indicate that GHSR deletion in rats is not only associated with a lack of response to ghrelin, but also associated with decreases in daily food consumption and body growth, and increases in BAT. This GHSR KO Wistar rat model provides a novel tool for studying the role of the ghrelin system in obesity and in a wide range of medical and neuropsychiatric disorders.

A single high dose of methamphetamine increases cocaine self-administration by depletion of striatal dopamine in rats.

Psychostimulant addicts often take high doses of drugs, and high doses of psychostimulants such as methamphetamine (METH) are neurotoxic to striatal dopamine (DA) terminals. Yet, the effects of high doses of METH on drug-seeking and drug-taking behavior have not been examined. In the present study, we found that single high doses of METH in rats (10-20 mg/kg) dose-dependently increased cocaine self-administration under fixed-ratio 2 (FR2) reinforcement conditions, while higher doses (40 mg/kgx1 or 10 mg/kg/2 hx4) caused high mortality among rats maintained on daily cocaine self-administration. The increased cocaine self-administration appeared to be a compensatory response to reduced cocaine reward after METH, because the same doses of METH caused a dose-dependent reduction both in "break-point" levels for cocaine self-administration under progressive-ratio reinforcement and in nucleus accumbens DA response to acute cocaine. Further, METH (10-20 mg/kg) produced large DA release (4000%-6000% over baseline), followed by a significant reduction in striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) contents, but without significant changes in striatal DA transporter levels. These findings suggest that the present high doses of METH caused striatal DA depletion or hypofunction without severe damage in DA terminals, which may contribute to the increased cocaine-taking behavior observed in the present study. Provided that the present doses of METH may mimic METH overdose incidents in humans, the present findings suggest that METH-induced DA depletion or neurotoxicity may lead to an increase in subsequent drug-taking and drug-seeking behavior.

The selective sigma(1) receptor agonist, 1-(3,4-dimethoxyphenethyl)-4-(phenylpropyl)piperazine (SA4503), blocks the acquisition of the conditioned place preference response to (-)-nicotine in rats.

We examined the effect of the sigma(1) receptor agonist, 1-(3,4-dimethoxyphenethyl)-4-(phenylpropyl)piperazine (SA4503), on the acquisition of the conditioned place preference response to subcutaneously administered (-)-nicotine in rats. (-)-Nicotine, but not SA4503 or vehicle, produced a significant conditioned place preference response. Pretreatment of animals with either 1 or 3 mg/kg of SA4503 significantly attenuated the conditioned place preference response to (-)-nicotine.

Gamma-vinyl GABA (GVG) blocks expression of the conditioned place preference response to heroin in rats.

We examined the effect of gamma-vinyl GABA (GVG) on the expression of the conditioned place preference response to intraperitoneally (i.p.) administered heroin in rats. Heroin, but not vehicle, produced a significant conditioned place preference response. Pretreatment of animals with 300 mg/kg of GVG significantly attenuated the expression of the heroin-induced conditioned place preference response. These results are the first to suggest that systemic GVG may provide an effective alternative to methadone maintenance in the treatment of heroin addiction, since it is without abuse potential and can be used for treatment outside an institutional setting.

Relapse to cocaine-seeking after hippocampal theta burst stimulation.

Treatment efforts for cocaine addiction are hampered by high relapse rates. To map brain areas underlying relapse, we used electrical brain stimulation and intracranial injection of pharmacological compounds after extinction of cocaine self-administration behavior in rats. Electrical stimulation of the hippocampus containing glutamatergic fibers, but not the medial forebrain bundle containing dopaminergic fibers, elicited cocaine-seeking behavior dependent on glutamate in the ventral tegmental area. This suggests a role for glutamatergic neurotransmission in relapse to cocaine abuse. The medial forebrain bundle electrodes supported intense electrical self-stimulation. These findings suggest a dissociation of neural systems subserving positive reinforcement (self-stimulation) and incentive motivation (relapse).

GABAergic blockade of cocaine-associated cue-induced increases in nucleus accumbens dopamine.

Environments previously associated with drug use can become one of the most common factors triggering relapse to drug-seeking behavior. To better understand the neurochemical mechanisms potentially mediating these cues, we measured nucleus accumbens dopamine levels in animals exposed to environmental cues previously paired with cocaine administration. In animals exposed to a cocaine-paired environment nucleus accumbens dopamine increased by 25%. When administered 2.5 h prior to presentation of the environmental trigger, racemic vigabatrin (an irreversible inhibitor of gamma-aminobutyric acid (GABA)-transaminase) abolished this cue-induced increase. Conversely, R-(-)-vigabatrin, the inactive enantiomer, had no effect. Combined with our earlier findings, these studies support the potential therapeutic benefit of this enzyme-based GABAergic strategy to modulate brain dopamine and the subsequent treatment of drug addiction.

Heterogeneity of the mesotelencephalic dopamine fibers: physiology and pharmacology.

The mesotelencephalic dopamine (DA) system is heterogeneous with respect to nuclei, terminal loci, DA receptor subtypes, electrophysiological characteristics and response patterns, and neuropharmacological response to a range of agents. The majority of mesocortical and mesolimbic DA neurons originate in the ventral tegmental area. Mesostriatal DA neurons originate in substantia nigra pars compacta. DA neurons originating from the retrorubal field primarily innervate subcortical limbic and neostriatal loci. Mesostriatal terminal loci have relatively low densities of D3 and D4 receptors, compared to mesolimbic and mesocortical loci. The D1 and D2 receptors appear more homogeneously distributed. Electrophysiologically, mesostriatal DA neurons show more regularity in firing pattern (fewer bursting events), and a lower basal firing rate than mesolimbic or mesocortical neurons. Neuropharmacologically, mesocortical DA neurons are less responsive to intravenous d-amphetamine, (+)apomorphine, and chronic antipsychotic drug treatment. Mesocortical DA neurons are also relatively insensitive to iontophoretically applied DA, a finding congruent with their reported relative lack of somatodendritic autoreceptors. Neurochemically, mesoaccumbens DA neurons are more sensitive to systemic administration of drugs with addictive liability.

Slow-onset, long-duration 3-(3',4'-dichlorophenyl)-1-indanamine monoamine reuptake blockers as potential medications to treat cocaine abuse.

A series of 3-(3',4'-dichlorophenyl)-1-indanamine monoamine reuptake blockers have been synthesized in an effort to develop a compound that could be used as a maintenance therapy to treat cocaine abuse. Since the effects of cocaine on dopamine (DA) and serotonin (5HT) transporters are important components of its pharmacological activity, the focus was on nonselective inhibitors of monoamine transport. To reduce or eliminate the abuse potential of a DA reuptake blocker, the compounds were designed to be slow-onset, long-duration prodrugs whose N-demethylated metabolites would have increased activity over the parent compound with the ideal being a parent compound that has little or no activity. To achieve this, pairs of compounds with different groups on the amine nitrogen and with and without an additional N-methyl group were synthesized. All of the synthesized compounds were screened for binding and reuptake at the cloned human DA, 5HT, and norepinephrine (NE) transporters. As previously found, trans isomers are nonselective blockers of DA, 5HT, and NE reuptake, cis isomers with small N-alkyl groups are selective blockers of 5HT reuptake, and tertiary amines of the trans compounds are less potent than the corresponding N-demethylated secondary amines as blockers of DA reuptake. Larger N-alkyl groups in both the trans and cis series were found to reduce activity for the 5HT and NE transporters with less effect at DA transporters. Selected trans compounds were also screened for locomotor activity in mice and generalization to a cocaine-like profile in rats. With intraperitoneal administration, all of the trans isomers showed a slow onset of at least 20 min and an extremely long duration of action in the locomotor assays. Several of the trans compounds also fully generalized to a cocaine-like pharmacological profile. An initial lead compound, the N,N-dimethyl analogue trans-1b, was resolved into chirally pure enantiomers. Surprisingly, both enantiomers were found to have significant affinity for the DA transporter and to cause locomotor activation. This is in contrast to the N-methyl compound in which only the (+)-enantiomer had significant activity. The absolute configuration of the more active enantiomer was determined by X-ray crystallography to be 3R,1S.

What we have learned about addiction from animal models of drug self-administration.

Self-administration of addictive drugs in laboratory animals has been widely used for decades as a tool for studying behavioral, neurobiological, and genetic factors in addiction. From such studies has come an enormous amount of information on brain mechanisms involved in addiction, on vulnerability factors in the addictive process, and on possible pharmacotherapeutic treatments for addiction. Modifications of the laboratory animal self-administration paradigm--including progressive ratio break-point models and the "reinstatement" model of relapse to drug-seeking behavior--are currently increasing our knowledge of incentive motivational factors in addiction and of the mechanisms underlying relapse to drug self-administration behavior.

Enhancement of conditioned place preference response to cocaine in rats following subchronic administration of 3, 4-methylenedioxymethamphetamine (MDMA).

In this study, we measured conditioned place preference (CPP) responses to cocaine following subchronic administration of the recreationally abused drug (+/-)-3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") in male Sprague-Dawley rats. Animals were given either vehicle (1 ml/kg of distilled water, s.c.) or MDMA (20 mg/kg, s.c.) twice a day for 4 consecutive days. Two weeks later, CPP responses to cocaine (5, 10, or 20 mg/kg, i.p.) were measured. The MDMA-treated animals showed a significantly greater CPP response to cocaine than the vehicle-treated animals. Since conditioned place preference is believed to be a measure of appetitive behavior, these results suggest that MDMA abuse could lead to an increased vulnerability to the rewarding actions of cocaine and, hence, to increased vulnerability to cocaine addiction and dependence.

Gamma-vinyl GABA inhibits methamphetamine, heroin, or ethanol-induced increases in nucleus accumbens dopamine.

We examined the acute effect of the irreversible GABA-transaminase inhibitor, gamma-vinyl GABA (GVG, Sabril((R)), Vigabatrin((R))) on increases in nucleus accumbens (NAc) dopamine (DA) following acute administration of methamphetamine, heroin, or ethanol. Methamphetamine (2.5 mg/kg) produced a dose-dependent increase (2, 700%) in NAc DA. GVG preadministration (300 or 600 mg/kg), however, inhibited this response by approximately 39 and 61%, respectively. The lower dose of methamphetamine (1.25 mg/kg), increased DA by 1, 700%. This response was inhibited to a similar extent (44%) regardless of the GVG dose preadministered (300 or 600 mg/kg). In addition, heroin-induced increases in NAc DA (0.5 mg/kg, 170%) were inhibited or completely abolished by GVG (150 or 300 mg/kg, 65 and 100%, respectively). Finally, at half the dose necessary for heroin, GVG (150 mg/kg) also completely abolished ethanol-induced increases in NAc DA following a 0.25 g/kg challenge dose (140%). Taken with our previous findings using nicotine or cocaine as the challenge drug, these results indicate that GVG attenuates increases in NAc DA by a mechanism common to many drugs of abuse. However, it appears unlikely that an acute dose of GVG can completely inhibit increases in NAc DA following challenges with a drug whose mechanism of action is mediated primarily through the DA reuptake site.

A pharmacologic strategy for the treatment of nicotine addiction.

Like many psychostimulant drugs, nicotine elevates extracellular and synaptic dopamine (DA) concentrations in the nucleus accumbens (NAc). This elevation has been linked to its reinforcing properties. Dopaminergic transmission within the NAc is modulated by gamma-aminobutyric acid (GABA). Therefore, we examined the utility of gamma vinyl-GABA (GVG, Vigabatrin) for inhibiting nicotine's biochemical effects on NAc DA as well as its effects on behaviors associated with these biochemical changes. Given 2.5 hours prior to nicotine, GVG (75 mg/kg) had no effect on nicotine-induced increases in extracellular NAc DA. However, at 90 mg/kg, GVG significantly inhibited nicotine-induced increases by approximately 50% while at 100 or 150 mg/kg, GVG completely abolished nicotine-induced increases in both naive and chronically nicotine-treated animals. When given 12 or 24 hours prior to nicotine administration at a dose of 100 mg/kg, GVG-induced inhibition was diminished or abolished, respectively. In addition, at a dose of 18.75 mg/kg GVG abolished the expression of nicotine-induced conditioned place preference (CPP) while a dose of 75 mg/kg abolished the acquisition phase of CPP. Finally, using positron emission tomography (PET) and 11C-raclopride in primates, GVG (100 mg/kg) abolished nicotine-induced increases in synaptic DA while having no effect on the rate of metabolism of the radiotracer or its regional distribution. Together, these data suggest that GVG may be useful for the treatment of nicotine addiction and further support the strategy of targeting the GABAergic system with a suicide inhibitor of GABA-transaminase for the treatment of drug addiction.

A novel strategy for the treatment of cocaine addiction.

Cocaine's addictive liability has been linked to its pharmacologic actions on mesotelencephalic dopamine (DA) reinforcement/reward pathways in the central nervous system (CNS). Dopaminergic transmission within these pathways is modulated by gamma-aminobutyric acid (GABA). With this knowledge, we examined the utility of gamma vinylGABA (GVG), a selective and irreversible inhibitor of GABA-transaminase (GABA-T) known to potentiate GABAergic inhibition, to alter cocaine's biochemical effects as well as its effects on behaviors associated with these biochemical changes. GVG significantly attenuated cocaine-induced increases in neostriatal synaptic DA in the non-human primate (baboon) brain as assessed by positron emission tomography (PET) and abolished both the expression and acquisition of cocaine-induced conditioned place preference (CPP). It had no effect on CPP for a food reward, the delivery of cocaine to the brain or locomotor activity. These findings suggest the possible therapeutic utility in cocaine addiction of a pharmacologic strategy targeted at the GABAergic neurotransmitter system, a system distinct from but functionally linked to the DA mesotelencephalic reward/reinforcement system. However, rather than targeting the GABA receptor complex with a direct GABA agonist, this novel approach with GVG takes advantage of the prolonged effects of an irreversible enzyme inhibitor that raises endogenous GABA levels without the addictive liability associated with GABA agonists acting directly at the receptor itself. Human trials with GVG are currently being developed to directly examine the utility of this novel strategy for the treatment of cocaine addiction.

Cannabinoid transmission and reward-related events.

The reward/reinforcement circuitry of the mammalian brain consists of synaptically interconnected neurons associated with the medial forebrain bundle, linking the ventral tegmental area, nucleus accumbens, and ventral pallidum. Electrical stimulation of this circuit supports intense self-stimulation in animals and, in humans, produces intense pleasure or euphoria. This circuit is strongly implicated in the neural substrates of drug addiction and in such addiction-related phenomena as withdrawal dysphoria and craving. This circuit is also implicated in the pleasures produced by natural rewards (e.g., food, sex). Cannabinoids are euphorigenic in humans and have addictive liability in vulnerable persons, but were long considered "anomalous" drugs of abuse, lacking pharmacological interaction with these brain reward substrates. It is now clear, however, that cannabinoids activate these brain substrates and influence reward-related behaviors. From these actions, presumably, derive both the abuse potential of cannabinoids and the possible clinical efficacy in dysphoric states.

(-)-Nicotine produces conditioned place preference in Lewis, but not Fischer 344 rats.

In this study, we sought to determine if Fischer 344 (F344) and Lewis rats showed different conditioned place preference (CPP) responses to subcutaneously administered (-)-nicotine. Lewis rats displayed a CPP response to (-)-nicotine after five pairings, whereas F344 rats showed no preference for nicotine compared to vehicle. After 10 pairings, the F344 rats showed a conditioned place aversion to (-)-nicotine, whereas the Lewis rats still displayed a significant CPP response. These results suggest that the differences in appetitiveness shown between Lewis and F344 rats to other drugs of abuse extend also to (-)-nicotine.

The effect of intravenous administration of delta-9-tetrahydrocannabinol on the activity of A10 dopamine neurons recorded in vivo in anesthetized rats.

In this pilot study, we examined the effect of the intravenous administration of delta-9-tetrahydrocannabinol (THC) on the activity of spontaneously active A10 dopamine (DA) neurons of anesthetized, male albino rats. This was accomplished using the technique of in vivo extracellular single-unit recording. The administration of THC (0.05-1.6 mg/kg i.v.) did not significantly alter either the basal firing rate or the firing pattern of spontaneously active A10 DA neurons compared to vehicle controls. Our findings suggest that, unlike a number of drugs of abuse, THC does not alter the activity of A10 DA neurons and that the previously reported THC-induced increase in brain DA levels is not due to its action on firing rate or pattern in A10 DA neurons.

Genetic differences in delta 9-tetrahydrocannabinol-induced facilitation of brain stimulation reward as measured by a rate-frequency curve-shift electrical brain stimulation paradigm in three different rat strains.

Lewis, Fischer 344, and Sprague-Dawley rats were implanted with electrodes in the medial forebrain bundle and trained to lever press for brain stimulation reward using a rate-frequency curve-shift electrical brain stimulation paradigm based on a series of 16 pulse frequencies ranging from 25 to 141 Hz in descending order. Once reward thresholds were stable, rats were given 1.0 mg/kg delta 9-tetrahydrocannabinol (delta 9-THC), the psychoactive constituent in marijuana and hashish, or vehicle, by intraperitoneal injection. Lewis rats showed the most pronounced delta 9-THC-induced enhancement of brain reward functions. Sprague-Dawley rats showed an enhancement of brain reward functions that was approximately half that seen in Lewis rats. Brain reward functions in Fischer 344 rats were unaffected by delta 9-THC at the dose tested. These results are consistent with previous work showing Lewis rats to be highly sensitive to the rewarding properties of a variety of drugs of abuse, including opiates, cocaine, and alcohol, while Fischer 344 rats are relatively less sensitive. They extend such previous findings to cannabinoids, and further suggest that genetic variations to other cannabinoid effects may also exist.

Systemic cocaine challenge after chronic cocaine treatment reveals sensitization of extracellular dopamine levels in nucleus accumbens but direct cocaine perfusion into nucleus accumbens does not: implications for the neural locus of cocaine sensitization.

Rats were treated chronically with 20 mg/kg/day cocaine (by intraperitoneal injection) for 16 days, followed by 7 days of cocaine wash-out. On the next day, rats were challenged with an acute dose of cocaine administered by one of two routes (systemic or intracranial), and extra-cellular dopamine (DA) in the nucleus accumbens (Acb) was measured by in vivo microdialysis. Rats acutely challenged systemically with 20 mg/kg cocaine showed enhanced Acb extracellular DA levels (compared to control rats that had not previously received chronic cocaine). However, rats acutely challenged with intracranial cocaine by perfusion of 10(-5) M cocaine directly into the Acb did not. It is suggested that both the development and triggering of cocaine sensitization, as manifested by enhanced Acb DA content to subsequent acute cocaine challenge, may involve more than just neural mechanisms occurring locally within the Acb.