Ion Channel Kinetics and Manipulation of Neuronal Activity

The lecture discusses manipulating neuronal activity via receptors, specifically chemogenetic and optogenetic approaches.

Designer Receptors Exclusively Activated by Designer Drugs (DREADs):
- These receptors are activated by a compound called CNO (Clozapine N-Oxide).
- DREADs are coupled with either stimulatory G proteins or inhibitory G proteins.
- Useful for controlling entire neuronal circuits, such as in the hippocampus or amygdala.
Administration:
- CNO can be delivered orally or injected, ensuring wide distribution to receptors.

Involves ion channels that respond to specific wavelengths of light.
Key Channels:
- Channelrhodopsin: Responds to blue light, opens a sodium channel.
- Halorhodopsin: Responds to yellow light, opens a chloride channel.
Optogenetics allows real-time neural activation and inhibition, focusing on specific brain regions.

Chemogenetics provide longer-lasting effects due to the sustained presence of CNO in circulation, which has a half-life of around 24 hours (dependent on method of administration).
Optogenetics provides precise control of neuronal activity at specific times.

SNARE Proteins: Exist on presynaptic membranes and synaptic vesicles.
- v-SNAREs: Synaptobrevin and Synaptotagmin (calcium sensor on synaptic vesicles).
- t-SNAREs: Located on the presynaptic membrane for docking.
Synaptotagmin is activated when calcium enters the presynaptic terminal, catalyzing vesicle fusion with the presynaptic membrane.

Resting Membrane Potential: Generally around -70 mV.
- Determined by ion permeability: the cell’s potential aligns with the ion equilibrium that it is most permeable to.
Nernst Equation: Used to calculate equilibrium potentials: $E{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]{outside}}{[ion]_{inside}} \right)$
- Where:
- R: Gas constant
- T: Temperature in Kelvin
- z: Ion charge
- F: Faraday's constant
Calculated for 310.15 Kelvin (37°C) or 298 Kelvin (22°C).

Driving Force: Indicates the strength of ion movement across the membrane.
- Calculated as:
  $\text{Driving Force} = V{membrane} - E{ion}$
- A positive driving force indicates cations exit and anions enter; a negative driving force indicates cations enter and anions exit.
- Example: Sodium has a driving force of -140.84 mV (indicative of entry); Calcium is -201 mV; Potassium is +18 mV (indicative of exit).

Horizontal Plane: Front of the brain is rostral/anterior; back is caudal.
Sagittal Section: Cut down the midline of the brain.
Coronal Sections: Cut creating sections similar to wearing a crown.
Dorsal and Ventral: Dorsal indicates the top of the brain; ventral indicates the bottom.

Rare conditions associated with frequent seizures (epileptiform discharges), intellectual disabilities, and developmental delays.
Patients may plateau in development and show aggressive behavior.

T1-Weighted Images: Dark cerebral spinal fluid (CSF), light white matter, gray cortex.
T2-Weighted Images: Bright CSF, dark gray white matter, brighter than white matter inflammation.
Assessment: Observe the presence of CSF and inflammation; indicates demyelination or atrophy.

Ion channels influence depolarization/hyperpolarization and timing of action potentials—critical for neural network function.
Disruption of action potential timing can lead to seizures, particularly through uncoordinated firing.

KV2.1 is a key delayed rectifier channel responsible for repolarization and resting memory potential control.
A mutation in the KCNB1 gene can disrupt function, leading to seizures and impaired synaptic plasticity.

Method for studying ion channel behaviors by injecting current and recording responses.
Patch Clamp Techniques:
- Cell-Attached: Records neuronal firing without breaking the membrane.
- Whole Cell: Breaks the membrane to measure internal current.
- Inside-Out/Outside-Out: Investigates specific channel activity by manipulating membrane patches.

Activation: Opening of an ion channel in response to depolarization.
Deactivation: Closing of an ion channel when repolarization occurs.
Inactivation: Channel becomes deactivated even in the presence of its ligand; can happen in both open and closed states.
- Channels may exhibit Open State Inactivation or Closed State Inactivation.

Increased temperature accelerates ion channel kinetics, affecting activation, closing, deactivation, and inactivation rates.
Fever can complicate this relationship, potentially increasing seizure risk despite making activation harder due to altered equilibrium for ions like potassium.

Current-voltage relationship graphs illustrate differences between normal and mutated channels in activation and deactivation kinetics.
Slowed activation and deactivation may lead to longer repolarizations, potentially affecting neuronal firing coordination across neural networks.

Slow activation/deactivation/inactivation can lead to longer action potential times, disrupting synchrony within networks, which can result in seizures and impaired cognitive function.