Drugs, Brain and Behaviour (12) - Drug Experience-Dependent Plasticity: Glutamate & Environment.

AMPA receptors

Ionotropic glutamate receptors that mediate fast excitatory transmission via Na⁺ influx.

NMDA receptors

Glutamate receptors that require depolarisation to remove Mg²⁺ block and allow Ca²⁺ influx. They trigger synaptic plasticity through calcium signalling.

Cocaine effect on Glutamate

Increases AMPA receptor expression in the nucleus accumbens.

What does AMPAR/(to)NMDAR ratio indicate?

Measure of synaptic strength; increased after cocaine exposure (high AMPAR:NMDAR = stronger transmission)

GluR2-containing AMPA receptors

Allow Na⁺ but not Ca²⁺ flow → stable signalling.

GluR2-lacking AMPA receptors

Allow both Na⁺ and Ca²⁺ → produce stronger synaptic effects → Increase neuronal excitability and plasticity.

NASPM

Selective antagonist that blocks GluR2-lacking AMPA receptors.

Cocaine Withdrawal Effect on Receptors

Increases GluR2-lacking AMPA receptors in nucleus accumbens. → drives increased drug seeking (after withdrawal).

Behavioural Effect of Cocaine Withdrawal

Increased cocaine-seeking behaviour.

What did Conrad et al., 2008 find about withdrawal?

Long-term withdrawal increases:

• Drug seeking

• GluR2-lacking AMPARs

Effect of blocking GluR2-lacking AMPARs

Blocking GluR2-lacking AMPARs reduces cocaine seeking (causal)

Effect of AMPAR plasticity on Drug Behaviours

Changes in receptor composition → increased drug seeking

Set and Setting

Where drug effects depend on psychological state and environment:

• Home environment: Familiar, low arousal environment.

• Novel environment: Unfamiliar, high arousal environment enhancing learning.

Sensitisation

Increased behavioural response after repeated drug exposure (e.g. Increased locomotor activity after repeated cocaine-use)

Environmental Effect on Sensitisation

Stronger Sensitisation occurs in novel environments

Neural Plasticity and environment

Novel environments enhance drug-induced brain changes.

Dendritic Spine Density

Number of synaptic connections on neurons.

Effect of Cocaine in Novel environments

Increases dendritic spine density in nucleus accumbens. (Novel environment + cocaine → stronger behaviour and neural changes).

Structural Plasticity

Physical changes in neuron structure due to experience.

Microdialysis

Used to collect substances from brain tissue using a semi-permeable catheter. Method used to measure glutamate release.

Environmental Effect on Glutamate

Increased glutamate release in drug-paired environments.

Drug-Context Learning

Animals associate drug effects with specific environments

Associative Learning in Addiction

Where environment becomes linked to drug effects

Neuronal Ensembles

Small groups of neurons encoding specific experiences (e.g. Different neurons respond to different visual stimuli).

Brain Encoding Mechanism

Information is stored in specific neuronal populations

Context-specific Neural Activity

Different environments activate different neuronal ensembles

Hippocampus Context Coding

Different neuron groups represent different contexts.

Drug-Context Encoding

Specific ensembles encode drug + environment associations.

Context-specific Sensitisation

Behavioural sensitisation occurs only in drug-paired environment (No sensitisation in unfamiliar context).

Fos

Immediate early gene marker of neuronal activation.

What proportion of neurons are activated?

Only ~2–3% of nucleus accumbens neurons activated → Drug memories stored in small neuronal ensembles.

Daun02 inactivation

Technique that selectively silences Fos-expressing neurons (tests causation of ensembles).

c-fos-lacZ rats

Genetically modified rats used to identify activated neurons.

Effect of Ensemble Inactivation

Eliminates sensitised drug behaviour

Neuronal Ensembles and Drugs

Specific neuron groups causally control drug-related behaviour. Drug memories are stored in discrete neuronal ensembles.

Neural Plasticity Mechanisms

• Receptor changes

• Glutamate signalling

• Structural changes

Relapse Mechanism

Triggered by reactivation of drug-context neuronal ensembles.

Therapeutic Implication of Neuronal Ensembles

Targeting drug-memory circuits may reduce relapse & Weaken/Erase drug-context associations.

Core finding on Environment & Drugs

Context strongly enhances drug effects and neural plasticity.

Core finding on Ensembles

Small neuronal populations encode and control drug behaviour

Conclusions on Addiction

Addiction is driven by causal neural plasticity shaped by experience and environment.