Genetic Tools to Study Neural Circuits 12

Neural Circuits

  • Definition: Neural circuits are collections of interconnected neurons that work together to perform specific functions.
  • Example: A simple neural circuit involves three or four neurons that control muscle contraction and inhibit opposing muscle movement to facilitate actions like leg movement.
  • Purpose of Study: Understanding neural circuits helps uncover how neurons interact and their roles in various functions such as muscle movement, behavior, and overall nervous system operations.

Genetic Tools for Studying Neural Circuits

Transgenic Animals

  • Overview: Transgenic mice are animals with modified genes, extensively used in neuroscience research to study neural circuitry.
  • Traditional Approach:
    • Genetic modification is performed on embryos.
    • Resultant animals may exhibit early developmental adaptations due to mutations.
  • Disadvantages:
    • Gene deletion can be fatal (lethal mutations).
    • Lack of control over where modifications occur (e.g., liver vs. brain).
    • Compensatory adaptations can confound results.

CRE-LOX System

  • Concept: A genetic manipulation tool derived from phages that allows targeted gene modification in mammals.
  • Components:
    • CRE protein: A DNA recombination enzyme.
    • LOX P sequence: Short DNA sequence recognized by CRE.
  • Functionality: When CRE identifies LOX P sequences, it can remove DNA between these sequences, thus regulating gene expression.
  • Applications:
    • Gene deletion can be localized to specific tissue (e.g., targeting only neurons in the brain).
    • Can also enable gene activation or inversion based on orientation of LOX P sequences.

Reporter Genes

  • Function: Used to indicate gene expression or activity within cells.
  • Example: Genetically encoded fluorescent proteins that allow visualization and tracking of specific cells in real-time during experiments.

Viral Vectors

  • Role: Serve as tools for delivering genetic material into living cells.
  • Types: Retroviruses, lentiviruses, adeno-associated viruses (AAV).
  • Advantages:
    • AAVs are favored due to low immune response and ease of use.
    • Simplifies the process by requiring only one transgenic mouse line instead of two.

Observing and Perturbing Neural Functions

Observation Techniques

  • Neural Tracing: Chemical tracers injected into specific brain areas to visualize connections across neural circuits.
  • Tracer Types:
    • Anterograde Tracers: Trace connections from neurons to their terminals.
    • Retrograde Tracers: Trace input to specific neurons.
    • Polysynaptic vs. Monosynaptic Tracers: Determine the complexity of the traced connections (whether multiple synapses are involved).

Perturbation Techniques

  • Traditional Methods: Electrical stimulation or lesioning specific brain areas to observe resultant behavioral changes.
  • Modern Methods:
    • Optogenetics (e.g., channelrhodopsin): Activates or inhibits neurons using light to study their functions.
    • Chemogenetics: Uses designer receptors activated by synthetic drugs (DREADs) to manipulate neuron activity.

In Vivo Photometry and Calcium Indicators

  • Calcium's Role: Calcium levels within cells are indicative of cell activity; increased calcium corresponds with neuronal activation.
  • Calcium Indicators:
    • Genetically encoded indicators (e.g., GCaMP) allow for real-time monitoring of neuronal activity within live animals.
    • This can be combined with fluorescence measurement to correlate neuronal activity to behaviors.

Summary of Techniques in Neuroscience

  • Neuroscience now combines traditional methods of observation and perturbation with advanced genetic tools (transgenic mice, viral vectors).
  • The overall aim is to enhance understanding of how neurons are interconnected and how they function collaboratively in various tasks or behaviors.
  • Continued advancements in tools provide new data that can lead to novel insights into the complexities of neural circuits.