Study Notes on Postsynaptic Mechanisms

Overview of Synaptic Mechanisms

• Main Steps in Synaptic Transmission: 1. Depolarization of the neuron.

  2. Ca²⁺

     channels open, allowing Ca²⁺

     to flow into the cell.

  3. Vesicle fusion occurs, leading to transmitter release.

  4. Transmitter binds to receptors on the postsynaptic membrane.

  5. Channels in the postsynaptic membrane open as a result of binding.

Characteristics of Electrical and Chemical Synapses

• Electrical Synapses:

  • Uses gap junctions for direct cell-to-cell transmission.

  • Example: Connecting the cytoplasm of cells A and B via hexameric connexons (3.5 nm wide).

  • Bidirectional signaling capability.

  • Limitations: Cannot change “signs” and less modifiable.

• Chemical Synapses:

  • More dynamic and can be modulated by various mechanisms.

  • Advantages include:

  • Ability to undergo long-term potentiation (LTP).

  • Influence from neuromodulators and hormones.

  • Can be unidirectional and can either be excitatory or inhibitory.

Synaptic Structure at Neuromuscular Junction

• Components of Neuromuscular Junction:

  • Presynaptic Terminal: Contains synaptic vesicles filled with neurotransmitters.

  • Postsynaptic Membrane: Sarcolemma of muscle fiber where receptors are located.

  • Synaptic Cleft: The space between the presynaptic terminal and postsynaptic membrane (10 μm).

• Key Structures:

  • Active Zone: Site of vesicle docking and release.

  • Myofibrils: Contractile threads in muscle fibers.

Calcium’s Role in Synaptic Transmission

• Ca²⁺

  influx is crucial for neurotransmitter release.

• Influences several processes:- Vesicle fusion and exocytosis (release of neurotransmitters).

  • The relation of [Ca²⁺

  ] concentration to endplate potentials (EPP):

  • EPP increases with higher [Ca²⁺

  ] concentrations.

  • Relationship expressed as: $EPP ext{ (mV)} = [Ca^{2+}]^4$

• Looking at synaptic vesicle dynamics, including varieties of vesicle pools: - Ready-Releasable Pool (RRP):

  • Comprises 1-2% of all vesicles.

  • Rapid release (within <1 second).

  • Recycling Pool:

  • Consists of about 10-20% of vesicles.

  • Takes a few seconds for release.

  • Reserve Pool:

  • Represents 80-90% of vesicles.

  • Slow release (tens of seconds to minutes).

Synaptic Depression and Facilitation

Depression Mechanisms

• A phenomenon where vesicle depletion occurs, leading to reduced neurotransmitter release.

• Short-term depression can be induced by multiple stimuli: - Stimulation leads to diminishing postsynaptic potential (PSP).

• Example: Starting with 6 docked vesicles and a release probability of $0.5$. The first stimulus releases 3 vesicles ($0.5 \times 6 = 3$), generating an EPSP. The second stimulus, with 3 vesicles remaining, releases ~2 vesicles ($0.5 \times 3 \approx 2$), leading to a smaller EPSP. The third stimulus, with 1 vesicle remaining, releases ~0.5 vesicle, resulting in an even smaller EPSP. This sequential decrease in available vesicles causes a diminished EPSP amplitude, illustrating depression.

Facilitation Mechanisms

• Residual calcium hypothesis suggests that no rapid sequestration of calcium allows more Ca²⁺

  to be available for subsequent action potentials, increasing the probability of neurotransmitter release.

• Example of a simulation showing effects of probability on initial amplitude and degree of facilitation with varying probability of vesicle release.

• Example: Starting with 5 vesicles and an initial release probability of $0.2$. The first stimulus releases 1 vesicle ($0.2 \times 5 = 1$), resulting in a small EPSP. Residual Ca²⁺ accumulates, increasing the release probability for the next pulse (e.g., to $0.5$). The second stimulus releases 2 vesicles ($0.5 \times 4 = 2$ if 4 vesicles remain), producing a larger EPSP (facilitation). However, if depletion is fast and probability increases (e.g., to $1$), only 2 vesicles may be left to release, leading to a maximal release of 2 vesicles, which might be similar to the previous pulse, illustrating a balance between calcium accumulation (facilitation) and vesicle depletion.

Determinants of Synaptic Potential Properties and Their Modulation

To understand EPSP amplitude and the degree of synaptic depression/facilitation, both presynaptic and postsynaptic factors are critical:

Presynaptic Determinants:

• Quantal Release: The amount of neurotransmitter in each vesicle.

• Release Probability: The likelihood of a vesicle being released upon stimulation.

• Calcium Channels: Amount of Ca²⁺ influx and proximity of channels to vesicles (localized [Ca²⁺] high concentration).

• Vesicle Pools: Especially the 'ready-releasable pool' (RRP) of docked vesicles.

• Replenishment Rate: How quickly depleted vesicles are refilled.

Postsynaptic Determinants:

• Transmitter Clearance: By diffusion or re-uptake.

• Channel Kinetics: Opening and closing rates of receptors.

• Conductance (Permeability): How much current flows through open channels.

• Number of Receptors/Channels: Available on the postsynaptic membrane.

Modulation of EPSP Amplitude and Depression/Facilitation:

Postsynaptic Potentials (PSPs)

Steps from Transmitter Binding

• Transmitter binding to its receptor initiates several processes that determine the shape of the postsynaptic potentials (PSPs):

1. Ligand Binding: Increases channel permeability.

   2. Resulting ionic flow contributes to the magnitude and direction (excitatory/inhibitory) of PSPs.

Key Voltage-Operated Concepts

• Ion channel properties:

  • Synaptic channels are analogous to voltage-gated channels, modeled with a resistor (channel) and a battery (determining current direction).

  • The key difference is that synaptic channel conductance changes with transmitter (ligand) concentration, unlike voltage-gated channels where it changes with voltage.

  • The driving force determines current flow (inward/outward) at resting potential ($V - E<em>{Na}$ or $V - E</em>{excitation}$).

  • Voltage-gated channels have different conductances (g_Na

  , g_K

  etc.).

  • Excitatory Postsynaptic Current (EPSC): Measured as inward current (nA).

  • Inhibitory Postsynaptic Current (IPSC): Measured as outward current (nA).

  • Measurement Techniques:

  • Voltage clamp (to measure current at a fixed voltage) and current clamp (to evoke presynaptic action potentials) methods are used for measuring EPSP and IPSP at resting potentials.

Reversal Potentials and Conductances

Definitions

• Reversal Potential: The voltage at which there is no net current flow through the channel, depends on ion permeability.

• Experimentally, reversal potential ($E<em>{excitation}$ or $E</em>{inhibition}$) is determined by holding the postsynaptic cell at different voltages via voltage clamp, stimulating the presynaptic cell (under current clamp to evoke an action potential), and measuring the evoked synaptic current.

• The current amplitude is plotted against the command voltage. The voltage at which the current is zero is the reversal potential (where $V = E<em>{excitation}$ or $V = E</em>{inhibition}$).

• For excitatory synapses, an inward current at rest indicates excitation (e.g., like sodium channels). As voltage steps become less hyperpolarized, the current decreases, eventually reversing from inward to outward when $V > E_{reversal}$. The reversal potential for excitatory synapses is typically $0 ext{ mV}$ (e.g., in the example discussed).

• For excitatory synapses, typically involves permeable ions, calculated using the Goldman-Hodgkin-Katz equation.

• For inhibitory synapses, mainly permeable to Cl^-

  , calculated using the Nernst equation:

  • $[Cl]<em>{out} = 120 ext{ mM}, [Cl]</em>{in}=5 ext{ mM}$

  • For inhibitory synapses, the reversal potential is typically hyperpolarized (e.g., $-80$ to $-90$ mV), often involving Cl- ions.

Neurotransmitter Diversity

Types of Neurotransmitters

• Amino Acids:

  1. Glutamate

  2. GABA

  3. Aspartate

  4. Glycine

• Monoamines:

  1. Norepinephrine

  2. Dopamine

  3. Serotonin

• Peptides:

  1. Vasopressin

  2. Somatostatin

  3. Neurotensin

• Other:

  1. Acetylcholine

  2. Nitric Oxide

Receptor Types Associated

• Glutamate Receptors: Kainate, AMPA, NMDA subtypes.

• Cys-loop Receptors: ACh, Serotonin, GABA, Glycine,

Synaptic Kinetics and Dynamics

1. Time Course of PSPs

   • Various factors such as channel kinetics, receptor dynamics, transmitter diffusion, re-uptake by glial cells, and degradation by enzymes (e.g., ACh esterase).

   • The transient, exponential-like time course of conductance resembles an alpha function and is determined by transmitter binding, channel gating kinetics, and transmitter clearance.

   • Effects of varying timing and concentration of transmitters can be studied through simulations.

2. Impact of Channel Kinetics

   • Shape of macroscopic current reflects individual channel kinetics.

   • Transmitter Binding and Gating: Neurotransmitter (T) binds to the receptor (R) on the channel, forming a bound-closed state (T-R_closed). This transitions to a bound-open state (T-R_open). The transmitter can also unbind (T-R_open to T+R_open or T-R_closed to T+R_closed). Channels flicker between open and closed states while bound.

   • Kinetic Variables: Rate constants K1 (unbound to bound), K-1 (bound to unbound), K2 (bound-closed to bound-open), and K-2 (bound-open to bound-closed) govern these transitions.

   • If K1 >> K-1, the channel has high affinity for the transmitter, staying bound longer and favoring the open state.

   • If K2 >> K-2, the channel spends more time in the open state, rarely closing. Conversely, if K-2 >> K2, the channel closes very quickly, resulting in transient currents.

   • Transmitter Clearance: Dominantly by diffusion out of the synaptic cleft, driving unbinding and channel closure. Other mechanisms include re-uptake into the presynaptic terminal.

   • Desensitization of channels affects the time course of current.

   • Simulations show the effect of probabilities on initial amplitude and depression in neurotransmitter release capacity.

   • Effects of Mutations: Mutations can alter channel kinetics, e.g., changing K2 to keep channels open longer (slow decay of macroscopic current) or increasing K-2 to cause rapid opening and closing (fast, transient macroscopic current).

3. Experimental Techniques

   • Employing patch-clamp recordings to dissect out different components of synaptic currents with pharmacological blockers.

4. NMDA and AMPA Receptor Dynamics

   • Co-localization in synapses leads to distinct time course characteristics.

   • AMPA Receptors: Rapid activation and deactivation.

   • NMDA Receptors: Slower activation/deactivation and are permeable to Ca²⁺

   , playing a role in plasticity and learning.

Summary of Key Points

• Understanding the mechanics of synaptic transmission involves grasping the roles of vesicles, calcium dynamics, and neurotransmitter types.

• The interplay between electrical and chemical synapses, alongside various receptor types, defines the functional outcome of synaptic signaling.