Transmitter Action, Reversal Potentials, and Post-synaptic Responses

Transmitter Action, Reversal Potentials, and Post-synaptic Responses (BI2432)

Learning Outcomes

• Activation Mechanisms: Understand how ligand binding to receptors leads to electrical responses. Examples include Nicotinic Acetylcholine ($nACh$), $AMPA$, and $NMDA$ receptors activated by glutamate.
• Excitatory Channels: Recognize that mixed non-selective cation channels are typically excitatory.
• Inhibitory Channels (Anionic): Know that $Cl^-$ channels are usually inhibitory, such as $GABA_A$ and glycine receptors. Distinguish between hyperpolarization and shunting inhibition.
• Inhibitory Channels (Cationic): Understand that $K^+$ channels are inhibitory, such as the $GABA_B$ metabotropic receptor activated via G-proteins.
• Synaptic Integration: Understand how membrane capacitance, axial charge spread, and membrane leak conductance convert synaptic currents into Post-Synaptic Potentials ($PSPs$). Capacitance integrates current, following Ohm’s Law.
• Variable Chloride Polarity: Be aware of specific conditions under which $Cl^-$ currents can become depolarizing.
• Structural Biology: Knowledge of basic receptor structures.

Classification of Neurotransmitter Receptors

There are two primary types of neurotransmitter receptors:

1. Ionotropic Receptors:     * These are directly ligand-gated ion channels.     * They facilitate faster transmission.     * Signals (ligands) bind to the receptor, causing the channel to open, allowing ions to flow across the membrane.
2. G-protein Coupled Receptors ($GPCRs$, Metabotropic):     * Involve indirect mechanisms.     * They utilize G-proteins and second messenger systems to modify ion channels or other cellular machinery.     * They facilitate slower transmission.

Neurotransmission vs. Neuromodulation

• Neurotransmission: Involves traditional synapses and extrasynaptic receptors.
• Neuromodulation: A more diffuse process altering the input-output relations of neurons, specific neuronal compartments, or synapses.

Major Ionotropic Transmitters and Receptors

Excitatory Receptors

• Glutamate:     * AMPA Receptors: Non-selective cation channels ($Na^+$, $K^+$, and sometimes $Ca^{2+}$). Mediate fast Excitatory Post-Synaptic Potentials ($EPSPs$).     * NMDA Receptors: Slower kinetics; opened by depolarization which relieves the $Mg^{2+}$ block. Function as non-selective $Na^+/K^+$ channels with approximately $10\%$ $Ca^{2+}$ permeability. They contribute to the slow component of the $EPSP$, depolarization-activated boosting, $NMDA$ spikes, and plasticity cascades.
• Acetylcholine ($ACh$):     * Nicotinic Receptor: Non-selective monovalent cation channels ($Na^+$, $K^+$). Found at the neuromuscular junction and in the brain. Mediate fast end-plate potentials.
• Reversal Potentials ($E_{rev}$): For these excitatory receptors, $E_{rev}$ is typically around $0\,mV$ because the total cation concentration is similar on either side of the membrane.

Inhibitory Receptors

• GABA ($GABA_A$ Receptor): $Cl^-$ channels mediating fast Inhibitory Post-Synaptic Potentials ($IPSPs$). Note that these are generally slower than $AMPA$ receptor currents.
• Glycine Receptor: $Cl^-$ channels located primarily in the spinal cord; mediate fast $IPSPs$.
• Reversal Potential of Chloride ($E_{Cl}$): The "battery" potential typically varies between $-75\,mV$ and $-80\,mV$. This is due to the concentration gradient, where extracellular concentration is roughly $18 imes$ intracellular concentration. The sign of the current depends on intracellular concentration; it can be depolarizing if intracellular $[Cl^-]$ is high.

Voltage Clamp and Reversal Potentials

Using a Voltage Clamp, current-voltage ($I-V$) curves can be plotted:

• Intercept: Represents the reversal potential ($E_{rev}$, where current is zero).
• Gradient: Represents the conductance ($g$), where $g = \frac{1}{V}$.
• Equation for Clamp Current:$I_{clamp} = -g(E_x - V_m) = g(V_m - E_x)$
• The Golden Rule: The membrane potential is ALWAYS driven toward the conductance's reversal (concentration battery) potential ($E_{rev}$).     * If $V_m < E_{rev}$, the injected current is depolarizing ($E_{rev} - V_m > 0$).     * If V_m > E_{rev}, the injected current is hyperpolarizing (E_{rev} - V_m < 0).

Receptor Structure and Biophysics

Nicotinic Acetylcholine Receptor ($nAChR$)

• Features a pentameric structure (five subunits).
• This pentameric architecture is shared by $GABA_A$ and glycine receptor channels.

Excitatory Cation Channels

• Non-voltage-dependent mixed cation-selective channels act as a "hole" in the membrane.
• The concentration gradient calculation uses $\log_{10}(1) = 0$, leading to a reversal potential $E_{cat} \approx 0\,mV$.
• Equation for cation current at resting potential ($V_{rest} = -70\,mV$):     $I_{cat} = g_{cat}(E_{cat} - V_m) = -g_{cat}V_m$

Single Channel Currents

• Measured via patch clamp (e.g., $2\,\mu M$ Acetylcholine applied to an outside-out membrane patch).
• Single channel currents are probabilistic; they add up to form macroscopic postsynaptic currents ($PSC$).
• End Plate Potential ($EPP$): The depolarization (voltage) has a slower time course than the End Plate Current ($EPC$) due to membrane capacitance.

Ionotropic Glutamate Receptors: AMPA vs. NMDA

AMPA Receptors

• Fast kinetics and lower glutamate affinity.
• They undergo desensitization.
• Involved in normal communication between neurons.
• Show a linear $I-V$ plot with $E_{rev} \approx 0\,mV$ (when $NMDA$ receptors are blocked by $APV$).

NMDA Receptors

• Slower kinetics and higher glutamate affinity.
• Require both glutamate binding and depolarization to open.
• Mediators of activity-dependent plasticity, such as Long-Term Potentiation ($LTP$).
• Structure: Composed of 4 subunits (e.g., $GluN1$, $GluN2A$, $GluN2B$). They are typically a dimer of dimers.
• Pharmacology:     * Agonists: Glutamate, NMDA.     * Co-agonists: Glycine, D-serine.     * Antagonists: D-AP5, 5,7-DCKA.     * Channel Blockers: $Mg^{2+}$, Memantine.     * Modulators: Polyamines, $Zn^{2+}$, Histamine, Pregnenolone.
• Magnesium Block: The channel pore is blocked by extracellular $Mg^{2+}$ ions at resting potentials.     * Blocking is very fast ($50-100\,\mu s$).     * Un-blocking occurs as the voltage becomes more positive to $V_{rest}$.     * Higher extracellular $[Mg^{2+}]$ shifts the $I-V$ curve to the right.

Channel Dynamics: Activation and Inactivation

• Activation: Channel opens with a slight delay when the specific condition occurs (e.g., depolarization or rise in $Ca^{2+}$).
• Deactivation: Channel closes if the activation condition is reversed.
• Inactivation: The channel closes spontaneously from an activated state while the activation condition is still present (e.g., sustained depolarization). This is a slower conformational change involving different gates (several types like C- and N-type).
• De-inactivation: Recovery from inactivation once the activation condition is removed. The ion channel returns to its original closed state, ready to be activated again.
• Conductance Calculation:$\text{Conductance} = \text{max conductance} \times \text{non-inactivated fraction} \times \text{activated fraction}$

Neuromodulators and the Dopamine System

Neuromodulation regulates diverse populations of neurons using chemicals that typically bind to metabotropic $GPCRs$. The resulting second messenger cascades induce broad, long-lasting signals ($100\,ms$ to several minutes).

Examples of Neuromodulators

• Dopamine, Serotonin ($5-HT$), Acetylcholine, Histamine, Norepinephrine, and Nitric Oxide.

Dopamine Dynamics

• Functions: Executive function, motor control, motivation, arousal, and reward.
• Key Source Areas: Ventral tegmental area ($VTA$) and substantia nigra ($SN$). These innervate the striatum and frontal cortex.
• Receptors: 5 types, all metabotropic. They act via adenylate cyclase.     * D1-type: Generally excitatory.     * D2-type: Generally inhibitory.
• Schizophrenia Connection: Historically linked to the "dopamine hypothesis" because antipsychotics are often $D_2$ antagonists. It is characterized by psychosis, hallucinations, delusions, and dysfunction of the prefrontal cortex ($PFC$).

Inputs to the Prefrontal Cortex ($PFC$)

• Amygdala, Thalamus, Cortical areas, Hippocampus, and the Limbic system (including $VTA$).

Inhibitory Synaptic Transmission

GABAA Receptors

• Pentameric structure similar to $nAChR$.
• Mediate chloride influx, causing hyperpolarization.
• IPSP types:     * Hyperpolarizing inhibition: Direct injection of negative charge.     * Shunting inhibition: Also known as 'silent' or 'divisive' inhibition. It increases membrane conductance (leaking positive charge out), reducing the size of $EPSPs$ and increasing decay speed without changing the membrane potential significantly. Occurs when $V_m$ is close to $E_{Cl}$.

GABAB Receptors

• Metabotropic ($GPCR$).
• G-proteins open $KIR$ channels (inwardly-rectifying $K^+$).
• Reversal potential $E_{rev} = E_K \approx -90\,mV$.

Conversion of Synaptic Current to PSP

The process follows these sequential steps:

1. Binding: Receptors bind neurotransmitter.
2. Gating: Channels open.
3. Conductance: Synaptic conductances are established.
4. Current: Synaptic current is driven by the difference between $E_{rev}$ and $V_m$.
5. Capacitance (Local): Charge accumulates on the local membrane capacitance ($Rising Phase$).
6. Axial Spread: Charge spreads axially through the neuron.
7. Capacitance (Distant): Capacitance at other locations is charged with a delay.
8. Leak: Charge leaks out through membrane conductance, causing a slow decay.

Key Equations for Integration

• Capacitance: $V = \frac{Q}{C}$
• Axial Current: $I_{ax} = g_{ax}(V_1 - V_2)$
• Membrane Integration: The lipid bilayer acts as a capacitor, integrating currents over time and space. Capacitors enable "leaky temporal integration," whereas a simple resistor allows no temporal integration.

Synaptic Plasticity

• Long-Term Potentiation ($LTP$): A long-lasting increase in $EPSP$ amplitude following intense (tetanic) activation. Occurs when presynaptic firing precedes post-synaptic firing (causal order).
• Long-Term Depression ($LTD$): A decrease in synaptic strength. Occurs if the post-synaptic Action Potential ($AP$) precedes the presynaptic $AP$ (non-causal) or if presynaptic activation occurs without post-synaptic depolarization.
• Glutamate Connection Density: Typically $1-4$ synapses per single glutamate connection in the visual cortex.