1 Drug Experience-Dependent Plasticity

Study Tips

  • Authors and Year: Organize studies by the authors' names and publication years.

  • Behavioral Procedure: Include the method used in the studies.

  • Main Finding: Highlight the significant outcomes of each study.

  • Brain Area: Identify the specific brain regions involved in the findings.

Example Studies

  • Boudreau and Wolf (2006)
      - Procedure: Repeated cocaine injections.
      - Finding: Increased AMPAR (AMPA receptor) surface expression in the nucleus accumbens (Acb).

  • Bell et al. (2009)
      - Procedure: Cocaine self-administration.
      - Finding: Increased glutamate release in the nucleus accumbens.

  • Kourrich et al. (2007)
      - Procedure: Repeated cocaine injections.
      - Finding: Increased AMPA/NMDA ratio in the nucleus accumbens.

  • Bonci et al. (2001)
      - Procedure: Single cocaine injection.
      - Finding: Increased AMPA/NMDA ratio in the ventral tegmental area (VTA).

Transition from Casual Drug Use to Addiction

  • Stages of Use:
      - Recreational Use: Occasional, e.g., weekends.
      - Long-term Changes: Evolving towards compulsive use, cravings, and relapse.

Reward System: Nucleus Accumbens and Ventral Tegmental Area

  • Role of Dopamine: Projections from the VTA to the nucleus accumbens are crucial for experiencing the rewarding effects of various stimuli like drugs, food, and sex (Wise, 1998).

  • Excitatory Input: The VTA receives glutamate input from multiple regions including the prefrontal cortex, involved in drug-seeking behavior.

Key Functions of the Nucleus Accumbens

  • Reinforcement Learning: Excitatory transmission (glutamate) is vital for reinforcement learning, cocaine sensitization, self-administration, and drug-seeking (Stuber et al., 2011; Pierce et al., 1998; Suto et al., 2009; Knackstedt and Kalivas, 2009).

  • Neuronal Composition: 90-95% of neurons in the nucleus accumbens are medium-sized spiny neurons (MSNs).

Lecture Objectives

  • Neural Plasticity in Accumbens: Understand how psychostimulants induce structural and functional changes in both morphology and glutamate synapses.

  • Influence of Environment: Investigate how the drug administration environment affects cocaine-induced behaviors and neuronal patterns.

Neural Plasticity

  • Definition: Structural and functional changes in neurons or synapses following experiences such as learning, development, injury, and drug exposure.

  • Historical Context:
      - Emerged in the 19th century (Cajal).
      - Gained importance in cognitive functions by the mid-20th century (Hebb).

  • Hebb's Theory: Activity-induced changes in communication between neurons are fundamental for information storage.

  • Cajal’s Insights: Neurons alter their processes, impacting the amount of input received at synapses.

Drug Sensitization and Self-Administration

  • Training Days: Data on drug infusions across training days are presented (e.g., lever presses, drug-seeking behavior).

  • Addiction Dynamics: Behavioral sensitization measured over time regarding drug administration contexts.

Neuronal Morphology and Drug Effects

  • Dendritic Spines:
      - Defined as the most actin-rich structures in the brain, contributing to the postsynaptic density (PSD).
      - Altered by LTP/LTD protocols and associated with learning and memory.
      - Morphological abnormalities in mental disorders correlate with spine head size and density (Kasai et al., 2010; Macaskill et al., 2014).

Assessing Neuronal Morphology Changes

  • Utilizes Golgi-Cox staining and viral-mediated gene transfer of GFP to analyze dendritic branching and spine density.
       - Identifies how neuronal signals are modified based on their morphology.

Cocaine Impact on Neuronal Morphology

  • Findings: Repeated cocaine injections increase spine density in accumbens neurons (Robinson and Kolb, 1999).
      - Experimental Design: Involves Golgi-Cox staining over a 4-week period with 24-25 drug-free days.
      - Conclusion: Cocaine induces significant synaptic connectivity changes.

Drug-Environment Interactions

  • Theoretical Framework: Drug effects are significantly influenced by “set and setting” (Badiani et al., 1995).
      - Definitions: Home context vs. novel context (enhanced arousal and learning in novel settings leading to more robust sensitization).

Changes in Accumbens Neuronal Morphology

  • Studies indicate that cocaine injections in a novel context lead to increased sensitization and spine density (Li et al., 2004).
       - Analysis Techniques: The neurolucida program for analyzing dendritic segments and spines per unit length.
       - Sample Size: Significant cell samples from varied regions, considering tissue integrity and staining quality.

Behavioral Sensitization and Neuronal Changes

  • Significant behavioral sensitization observed in novel contexts vs. home settings across trials.
      - Statistical Findings: Two-way ANOVA results confirming interactions between groups over time, particularly notable in the novel context group.

Drug-Induced Glutamate Release

  • Cocaine's impact on glutamate release is context-dependent; enhanced release observed with prior exposure in a novel context (Bell and Kalivas, 2000).
      - Micordialysis Techniques: Measures extracellular glutamate from various brain regions.

Operant Learning Concept

  • Definition: Instrumental conditioning explains how an action (e.g., drug use) leads to a consequence (e.g., euphoria).

Cocaine Self-Administration Effects

  • Studies show that cocaine self-administration significantly reconfigures synaptic patterns in accumbens neurons (Robinson et al., 2001).
      - Electrophysiological assessments confirm alterations post-drug-naïve comparison.

Conclusion: Repeated Drug Exposure Impacts Neuronal Function

  • Repeated doses of cocaine increase AMPAR surface expression, impacting neuronal functionality and behavior sensitivity.

  • Long-term vs. short-term withdrawal effects on synaptic strength highlight critical differences in circuitry disruption and recovery potential.