Lecture 14: Addiction: Animal Models and the Development of Treatments

How is Addiction Measured in Humans and Animals ?

• In humans, measurements are symptom-oriented with subjective reports of pleasure and withdrawal (insight into addictive potential)

  • Assessments into the extent of dopamine system activation and the amount of DA released in the nucleus accumbens (NAc)

  • Ease of becoming hooked and degree of physical/cognitive harm

• Animals cannot report subjective feelings → rely on physiological or behavioural measures

  • Dopamine release in NAc correlates with addictive potential

  • Withdrawal symptoms indicate the addiction level

  • Physical and cognitive harm correlates with extent of addiction

  • Must translate human measures to observable animal behaviours

• Correlations exist between pleasurable effects, aversive effects, and addiction severity

Why are there multiple animal models of addiction?

• No single model captures all symptoms/behaviors at once.

• Most CNS models focus on specific symptoms or parts of the addiction cycle.

• Animal studies increase understanding of neurobiology and psychiatric connections, but have limited assistance in the development of new treatments

  • Provide insight into what areas are involved by looking at the connections that form and the adaptations that occur

• New and clinically useful treatments are limited

How effective are animal models in developing addiction treatments?

• Limited development of new treatments; most current treatments mimic the drug of abuse in a controlled manner to reduce withdrawal/addiction → does not interfere with the whole addiction cycle

• Psychedelics (e.g., ketamine) show potential to interfere with the whole addiction process, rather than mimic a particular substance.

• Predictive power for new drugs is limited due to unknown effects and difficulty interpreting and measure behaviour.

What are the limitations of animal models in addiction research?

• Predictive power is limited for new drugs (e.g., ketamine, psychedelics) because their unique effects make it difficult to determine if animal models can predict human responses.

  • Known drugs are easier to study; effects can be inhibited using drugs targeting the same systems.

• Behavioural assessments can be confounded: some drugs interfere with motivation or cause sedation, making results hard to interpret.

  • Issue with the rigour of studies and data collection

• Some construct validit since neurobiology in animals is similar to humans, so some findings translate.

• Some face validity: individual behaviours and specific parts of the addiction cycle can be modelled

• Clinical focus for psychedelics is often studied more in humans than animals due to translational challenges.

Which animal models have good face validity for the addiction cycle?

• Drug self-administration → models binge/intoxication; translatable to humans

• Deprivation-effect model → assesses withdrawal/relapse

• Reinstatement of drug-seeking → models craving

What is the self-administration model of binge/intoxication in animal models of addiction?

• A translatable model whereby humans and animals (rodents and non-primates) voluntarily self-administer drugs

• Routes of administration:

  • IV (drug reservoir; intermittent or continuous administration)

  • Oral (drug in water/food; form of administration is less predictable due to other factors driving the behaviour)

• Measures of drug-taking behaviour via

  • Lever pressing

  • Nose pokes (disrupt electrical current)

• Assessments into the different schedules within self-administration → provide information about how hard an animal will work to administer

  • Fixed-ratio (responses per reward fixed)

  • Progressive-ratio (responses increase → how hard the animal will work)

• A relatively easy and less invasive behavioural measure

How does the lowering of the reward threshold measure binge/intoxication states in addiction?

• Measured using intracranial self-stimulation (ICSS)

• An electrode stimulates the medial forebrain bundle (White matter tract)

  • Connects VTA → NAc (dopamine pathway)

• Drugs of abuse lower ICSS reward thresholds

  • Less electrical stimulation is needed to perceive reward

• Electrical stimulation mimics dopamine release, producing drug-like effects

• Adding/removing a drug shows changes in reward threshold and sensitivity

• Indicates increased reward value and reinforcing effects of drugs

What is conditioned place preference (CPP), and what does it measure in addiction models?

• Based on classical (Pavlovian) conditioning

• Pair distinct environments with drug vs non-drug states

• An animal develops preference for an environment paired with a drug

• Measures associative learning and drug reward

• Can assess both:

  • Pleasurable (rewarding) effects

  • Aversive effects of drugs

• Reflects context-drug associations, not active drug seeking

What factors must be considered when designing and conducting animal studies in addiction research?

• Cost and practicality of equipment: Expensive or complex setups reduce accessibility and use for labs

• Length and difficulty of animal training

  • Tasks like lever pressing or nose pokes require training

  • Can affect chronic studies as animals can lose motivation

  • Acute studies are often easier, depending on the model

• Ease of quantification: Behaviours must be measurable and interpretable

• Sensitivity to dose: Varies between genetic models and animal strains

• Drug effects on behaviour

  • Sedating drugs (e.g. opioids) reduce motor activity

  • Can confound measures like lever pressing or nose pokes

• Drug administration regimen

  • Humans use oral, IV, and nasal routes

  • IV is easiest to model in animals, but not always translational

How is the cocaine self-administration model conducted in animal studies?

• Used with cocaine and other addictive drugs; highly reproducible for self-administration in rodents

• Lever-press apparatus:

  • Active lever → delivers a cocaine dose

  • Inactive lever → no drug (control)

• Reinforcement schedules:

  • Continuous reinforcement: every level press delivers the drug (learn to receive more drug) → press the lever more

  • Intermittent reinforcement: drug delivered at programmed intervals (learn to re-establish and get the reward back over an acute period) → drug not given consistently

  • Progressive-ratio schedule: increasing number of presses required per dose (learn to work harder to receive the drug)

• Progressive-ratio assesses motivation and effort to obtain the drug → mirrors human behaviour to obtain the drug (not always readily available)

• IV infusion via a drug reservoir fitted to the rodent’s back, back-mounted pump

What do cocaine self-administration studies show about addiction-related behaviour?

• Animals stop pressing the lever when cocaine is replaced with saline

• Cocaine produces a progressive increase in active lever pressing over time → association with reward

  • Similar self-administration patterns are seen with heroin (opiates) and cannabis

• Different drugs show different rates and steepness of response

• Clear separation between saline and drug curves

• Indicates the reinforcing properties of cocaine and addiction-like behaviour

How is intracranial self-stimulation (ICSS) conducted in animal models of cocaine addiction?

• Animals press a lever to receive electrical stimulation

• A microelectrode was implanted into the medial forebrain bundle (MFB)

• Electrical stimulation activates dopaminergic fibres (VTA → NAc)

  • Stimulation mimics and replaces drug-induced dopamine release

• An electrode can be implanted in different regions of the MFB

• Electrode placement is critical for comparing results across studies

What do ICSS studies show about cocaine’s effects on reward and addiction?

• Cocaine lowers ICSS reward thresholds

  • Less stimulation is needed for reward perception

• Indicates enhanced reward sensitivity and reinforcing effects

• An electrode can substitute for and add to the drug effects

• Used as a measure of drug reward and the degree of addiction to the drug

• Ventromedial hypothalamus region of the MFB shows the most consistent effects when the electrode is placed

How is the conditioned place preference (CPP) model conducted in cocaine studies?

• Three-chamber apparatus: front, middle (connective), and end chambers

• Chambers differ in colour, texture, and/or patterns

• Animals are first habituated to the apparatus

• This is followed by a conditioning phase:

  • One chamber paired with cocaine → association with pleasureable feeling

  • The other chamber paired with saline → no response

• Pairing creates an association between drug effects and the environment

What does CPP testing show about cocaine reward and addiction?

• After conditioning, animals are then injected and placed in the middle chamber

• Animals freely explore and settle in the drug-paired compartment (associated with the pleasureable feeling)

• Cocaine-treated animals:

  • Spend significantly more time in the drug-paired chamber

• Saline-treated animals:

  • No strong preference for chamber; less exploration

• Pre-test: no preference for a compartment

• Test: clear preference for drug-associated side

• Time spent in the compartment reflects drug reward and addiction potential

Why is CPP a useful animal model for studying addiction?

• Simple and cost-effective technique

• Equipment can be easily built and validated in the lab

• Preference can be confirmed to be drug-related, not environmental

• Requires less expertise than self-administration or ICSS

• No surgery, electrodes, or complex equipment needed

How Is Withdrawl Assessed in Animal Models of Addiction

• Can assess withdrawal following binge/intoxication stages

• Focus on physical withdrawal signs→ visually monitored and scored against a scale, e.g.

  • Piloerection

  • Paw tremors

  • Seizures (severe cases)

• Easy to perform, but labour-intensive and observer variability is present (different raters may score differently)

• Standard rating scales available and can reduce variability, but it do not eliminate

• Telemetry devices (implanted under the skin) wirelessly monitor blood pressure, heart rate, and temperature

  • It is continuous, remote, and a quantitatively reliable measure

What behavioural and reward-related changes are seen during withdrawal in animal models?

• Effects are often opposite to those seen for acute drug effects

• Disruption of operant responding → reduced lever pressing / nose poking during self-administration as they feel unwell

• Conditioned place aversion → Animals avoid drug-paired compartment during withdrawal; more time spent in other chamber

• ICSS → Increased reward thresholds

  • Thresholds higher than both drug-present and drug-absent states

• Reflects a negative affective state during withdrawal

What patterns are seen in ICSS reward thresholds during withdrawal from different drugs of abuse?

• Withdrawal causes an increase in the baseline ICSS reward threshold

• Indicates reduced reward sensitivity (anhedonia)

• Post-withdrawal, thresholds gradually return toward normal baseline levels

  • Initial withdrawal effects are strongest and decrease over time

• Pattern is consistent across drugs, but magnitude and duration vary

  • Larger increases with amphetamine, cocaine, and opiates

  • Smaller or slower changes with ethanol and nicotine

• Threshold increases during withdrawal are greater than saline controls

Why is craving a critical and difficult stage of addiction to overcome?

• Craving is driven by associations between the drug, pleasure, and (social) cues

  • The biggest driver of continued substance use

• Initially motivates use to avoid withdrawal, but can persist long after

• Environmental and social cues strongly promote continued use

• A major contributor to relapse in individuals

• Particularly strong for drugs that are readily availabile and daily cue exposure (e.g. smoking) → common within the enviroment and becomes assoaicted with daily activated

How is drug-induced reinstatement used to model craving in animals?

• The drug is self-administered, then removed after a period

• The extinction phase occurs → drug-seeking behaviour declines → return to control levels

• The drug is reintroduced, and assessments of the reinstatement of drug-seeking behaviour are made

• Increase in drug-seeking behaviour indicates craving and relapse-like behaviour f

• Craving in models is driven by direct drug exposure

How does cue-induced reinstatement model craving?

• Drug-seeking behaviour is paired with a specific cue

  • Cue associated with a particular drug and behaviour

• Cue used to train self-administration is stopped → lever pressing no longer delivers drug → extinction occurs

• Cue is re-presented and drug-seeking behaviour resumes (lever pressing/nose pokes)

• Models craving triggered by environmental or contextual cues

• CIR and DIR while simillar, look at different neurocircuits

How is stress-induced reinstatement used to study craving and relapse?

• Drug-seeking behaviour is first extinguished

• A stressor is introduced (e.g. foot shock)

  • In humans, stress refers to life events that make an individual want to take the drug again (stressors not identical)

  • Translation differs, as exact nature of the stressors differs, but the underlying neurobiology is similar

• Drug-seeking behavior restarts

How Else Can Animal Models Be Used to Research Addiction Beyond Single Stage Studies?

• Traditional models can examine specific stages of the addiction cycle

• Models can be adapted and combined to study the entire addiction cycle

• This allows for the integration of data across binge/intoxication, withdrawal and craving

• Findings can be supplemented with additional data

  • From the same animals (longitudinal studies)

  • Or from different animal cohorts

• Helps build a more complete and coherent understanding of addiction

How can behavioural and neurochemical techniques be combined in animal models of addiction?

• Behavioural models (e.g. self-administration) are combined with neurochemical measurements (NT levels)

• Behaviour is often measured first, followed by a neurochemical assessment via micro dialysis → avoids implanting microdialysis probes during behaviour

• Microdialysis measures neurotransmitter levels (e.g. dopamine) in NAc

  • Probe with a semipermeable membrane allows DA diffusion

  • Fluid collected and analysed for DA concentration

• Measuring the degree of DA release in the NAc alongside drug administration to indicate the addictive potential of a drug

• Can target different brain regions: NAc (ventral striatum); Dorsal striatum; Prefrontal cortex (PFC); Other neurotransmitters (e.g. glutamate)

• Helps identify neural circuits involved in addiction

How are combined techniques used to study relapse and craving in animal models?

• Extinction of drug-seeking behaviour is first established

• A stressor is introduced (e.g. foot shock)

• Drug-seeking behaviour reinstates

• Neurochemical measures (e.g. DA release) can be taken alongside behaviour

• Allows linking stress-induced relapse to changes in brain chemistry

How are ex vivo techniques used to complement behavioural and microdialysis studies in addiction research?

• Performed after behaviour or microdialysis, once animals are culled

• The brain is dissected and analysed using immunohistochemistry, mRNA analysis and Tissue homogenates

• Assess brain morphology (Cell shrinkage and Dendritic branching)

• Measure gene expression and signalling pathways, e.g. cAMP, ΔFosB (delta phospho-B):

  • Early gene indicating synaptic activity

  • Increased expression of drugs of abuse

  • Used as a biomarker of addiction-related neural activation

  • Not fully selective for addiction

What is ΔFosB (delta phospho-B)?

• An early gene indicating synaptic activity

• Increased expression of drugs of abuse

• Used as a biomarker of addiction-related neural activation and synaptic activity

• Not a fully selective marker for addiction

What in vivo techniques are used alongside behavioural models to study addiction?

• Used to provide information on the neurobiological mechanisms in living animals

• Techniques include:

  • Lesioning

  • CRISPR/Cas9 gene editing

  • Environmental manipulations

  • Imaging

• Can be conducted alongside or in addition to behavioural tests

• Help identify causal relationships between brain changes and addictive behaviour

Why is Alcohol Use Disorder Difficult to Models in Animals

• It is a heterogeneous disorder with complex aetiology and mechanisms

• No one person has the same type → high interindividual variability → difficult to model and low face validity

• Animals differ in sensitivity and response to alcohol

  • Genetic variation across strains of inbred, outbred, and alcohol-preferring rodents

  • May not fully mirror human alcohol use patterns

• Palatability issues → Low alcohol doses are pleasureable; higher concentrations are aversive due to bitter taste

• Palatability is a larger issue in rodents than in humans as humans mask taste with mixers

  • Adding mixers in animal models confounds interpretation

• Overall, these factors limit the translational relevance of AUD animal models

What factors are involved in bridging the gap between preclinical animal studies and human addiction research?

• Translatability is critical when developing new treatments

• Preclinical (animal) studies allow for investigations of multiple different aspects of addiction and the combination of many different experimental manipulations

• Human studies are governed by the pharmacokinetics and pharmacodynamics of alcohol/drugs, which guide safe and appropriate dosing within a particular test

  • Behavioural assessments used are often similar to those used in animal tasks

• Careful design of clinical trials to improve predictability and clinical relevance of treatments

Why Are Clinical Trials for Addiction Difficult To Design

• High dropout rate

• Inconsistent and unreliable adherence to clinical trial → low

• Ethical issues e.g. inducing withdrawl → seizure risk

What is the Aim of Treatments for Alcohol Use Disroder

• Treatments interfere with the pleasureable effects of alcohol

• Examples include

  • Acamprosate

  • Naltrexone

  • Disulfaram

What is Acamprosate?

• The first drug specifically designed for AUD treatment → maintains abstinence after ceasing alcohol consumption

• Mechanism of action

  • NMDA receptor antagonist

  • Acts on calcium channels

  • Acts via GABA A and B receptors

    • Restores and rebalances the abnormal glutamatergic and GABAergic activity

What is Naltrexone?

• MU-opioid antagonist

• Blocks the pleasureable effects of ethanol induced B-endorphin release

What is Disulfiram?

• An irreversible inhibitor of ALDH enzyme (responsible for alcohol breakdown)

• Prevents the conversion of acetaldehyde to acetate

  • Acetaldehyde accumulates and is aversive and toxic in humans

• Discourages alcohol intake through excessive sweating, increased HR, BP, nausea (and vomiting)

• Effective in preventing alcohol consumption; but is limited by an individuals willingness to s to take the drug → if someone wants to take alochol they can stop taking the drug

Why do CCP addiction models show good translatability between animals and humans?

• Conditioned Place Preference (CPP): Similar setup and scenario in animals and humans with similar results seen, suggest good translatability

• Preclinical species used → Mice, rats, macaques, zebrafish

• Human studies are difficult to conduct t due to time, space, and setup requirements

  • Virtual reality (VR) has been developed to simulate alcohol/cue pairings

How does acamprosate affect ethanol-conditioned place preference (CPP) in mice, and what does this indicate?

• Acamprosate reduces ethanol-associated CPP

  • Mice show less preference for the ethanol-paired chamber

• The effect is due to acamprosate blocking ethanol’s rewarding effects, not producing its own reward or aversion

• Acamprosate alone:

  • No effect on CPP or aversion

  • Confirms that results reflect the drug’s ability to prevent ethanol’s effect, not rather than by acamprosate’s direct effects

What is the Alcohol Deprivation Effect (ADE) Model and How Can It be Used in AUD Treatment Testing?

• Rodents conditioned with ethanol vapours in a chamber

• Two-bottle choice given intermittently (24h access, 3 days/week)

• Mimics relapse after abstinence

• Treatment testing:

  • Acamprosate blocks reinstatement

  • Demonstrates effectiveness in maintenance therapy and reducing ethanol reward → blocks effects

What is the Drug Prived Reinstatement Model?

• A model where drug-seeking behaviour resumes after extinction when the drug itself is reintroduced

• Measures craving and loss of control over substance consumption (e.g., ethanol)

• It shows similar results in humans and animals → translational model

  • Effects are only observed in the presence of the drug stimulus

• Useful for testing treatments that block relapse

What is Methadone?

• A commonly used treatment for opioid addiction → dose can be controlled and gradually decreased, to reduce addiction

• It is an opioid full agonist

• Slow onset of action

• Long half-life (longer than heroin) → blocks the effect of opioids

• Not a perfect treatment

  • Addictive and abuse potenital

  • Side effects

• Effective if managed, prescribed and tapered down properly

What is Buprenorphine?

• Partial opioid agonist used in the treatment of opioid addiction→ reduces withdrawal symptoms and cravings

• Long half-life → maintains stable opioid effect, helping interfere with the addiction cycle

• Reinforcing effect → keeps patients engaged in treatment

• Produces less neuroadaptation → lower risk of long-term brain changes

• Safer at higher doses → reduced risk of overdose compared to full agonists

What is Varenicline?

• An a4b2 partial nicotinic agonist, commonly used in the treatment of nicotine addiction

  • It prevents withdrawal by gradually reducing the dose and stopping the addiction process from occurring

    • Mimics the actions of nicotine in a controlled way

• High density of a4b2 in the VTA are pushed into a desensitised state

  • Reduces DA in the NAc; reduces cravings and withdrawal

What is Memantine:

• An experimental drug not approved for human treatment of addiction

• It is a low-affinity, uncompetitive NMDA receptor antagonist which transiently blocks NMDA channels without disrupting normal synaptic transmission

• May help when the addiction cycle results in hyperglutamatergic state → suggesting its use for severa ldiffernt drugs

What Are Limitations of Memantines Use for Addiction Treatment?

• Ineffective in hypoglutamatergic states → not useful for treating all aspects of the addiction stages/cycle

• Varying results across drugs and between animal and human studies

• Pharmacokinetics in people with SUD not been well studied