7. Ion Channels and Synapses: Pharmacology

Introduction and Review Opportunities

• This lecture concludes the sequence on ion channels, synapses, and neurons. Specifically, it brings together the knowledge on electrical signaling in neurons, synaptic transmission, and the key role of ion channels in these fundamental processes.

• The material covered has been quite complex, involving intricate molecular mechanisms and physiological concepts; therefore, misunderstanding is common and active engagement is encouraged.

• Feedback is crucial: Students are strongly encouraged to actively ask questions during sessions and utilize available resources to ensure a solid understanding of the material. Clear communication of difficulties is essential for effective learning.

• Today's session is presented as a valuable opportunity for questions, as it is expected to finish early, allowing ample time for discussion.

  • A dedicated review session is scheduled for Friday, offering further opportunities for clarification and deeper understanding before assessments.

  • Emailing questions is also highly encouraged to address specific concerns or concepts that require individual attention.

• Two-way communication, both in-person and via email, is essential for building student confidence and ensuring a comprehensive understanding of the complex material.

General Pharmacology of Ion Channels

• Ion channels are absolutely central to the physiology of the entire nervous system, encompassing individual neurons, glial cells, and the brain as a whole. Their omnipresence underlies almost every aspect of neuronal function.

• They meticulously provide neurons with their unique electrical properties, defining their excitability and responsiveness to stimuli.

  • At synapses, ion channels are the primary molecular machinery that translates chemical signals (neurotransmitters) into precise electrical signals (postsynaptic potentials), bridging the gap between neurons.

  • Voltage-gated ion channels, in particular, endow neurons with intrinsic electrophysiological properties, meaning neurons can exhibit complex electrical behaviors (like rhythmicity or pacemaking) even in the absence of external synaptic input.

• Due to their profound importance in biological systems, nature has independently evolved an astonishing array of numerous toxins specifically targeting ion channels, often with extreme potency and selectivity.

• Beyond natural toxins, ion channels are also crucial and highly validated targets for numerous therapeutically used compounds across a wide range of medical conditions.

Natural Toxins Acting on Ion Channels

• Tetrodotoxin (TTX) (from pufferfish, newts, and other animals):

  • An extremely potent and highly selective inhibitor of voltage-gated sodium channels, particularly those found in nerve and muscle cells.

  • It functions by physically blocking nerve impulses, binding to and plugging up the outer opening of the sodium channel pore, thus preventing sodium ion influx.

  • Possesses very high affinity for its target: It effectively blocks brain sodium channels at concentrations as low as $10^{-9}$ to $10^{-8}$ molar ($1$ to $10$ nanomolar), making it incredibly dangerous.

• Batrachotoxin (from South American tree frogs, Phyllobates genus):

  • Considered one of the most extremely deadly neurotoxins known to science.

  • Its mechanism of action involves irreversibly activating voltage-gated sodium channels and simultaneously preventing their normal inactivation mechanism.

  • This leads to channels opening spontaneously at more negative, resting membrane potentials (e.g., $-50$ mV instead of a typical threshold of $-20$ mV) and remaining open indefinitely. The consequence is a constant, unregulated sodium influx, causing persistent neuronal depolarization, paralysis, and cardiac arrest.

  • Often cited as the world's deadliest non-protein toxin.

• Scorpion toxins: A diverse family of neurotoxins, many types of which act on various ion channels, including voltage-gated sodium, potassium, and calcium channels. They can alter channel gating, leading to hyperexcitability or paralysis.

• Cone snail toxins (Conotoxins): A vast library of highly specific peptides that target a wide range of ion channels, including sodium, calcium, and potassium channels, as well as acetylcholine receptors. Their precision makes them valuable tools in neuroscience research.

• Bandit Krait snake toxin (α-Bungarotoxin): A potent neurotoxin that acts specifically on nicotinic acetylcholine receptors at the neuromuscular junction, leading to competitive antagonism. This binding blocks neurotransmission, causing severe paralysis of skeletal muscles, including those essential for respiration, and ultimately death.

• Pyrethroids (synthetic derivatives from natural pyrethrins found in chrysanthemums):

  • Widely used as insecticides due to their effectiveness and relatively low toxicity to mammals compared to insects.

  • They primarily activate voltage-gated sodium channels in insects (and to a lesser extent, in people), causing repetitive firing of neurons and ultimately paralysis and death in insect pests.

Therapeutic Compounds Targeting Ion Channels

• GABA receptors (GABA-A type):

  • These ligand-gated chloride channels are major inhibitory receptors in the CNS.

  • They are primary targets for anxiolytics and sedatives such as benzodiazepines (e.g., Valium, Xanax, clonazepam) and barbiturates (e.g., phenobarbital, pentobarbital).

  • These compounds act as positive allosteric modulators, potentiating the GABA receptor's response by increasing the frequency (benzodiazepines) or duration (barbiturates) of chloride channel opening. This enhancement of inhibitory signaling leads to increased neuronal inhibition, promoting feelings of calmness, sedation, and sleepiness (a similar general effect is seen with ethanol consumption).

• NMDA receptors (N-methyl-D-aspartate receptors):

  • These are ionotropic glutamate receptors crucial for synaptic plasticity and learning.

  • They are targets for dissociative anesthetics and hallucinogens like ketamine and phencyclidine (PCP).

  • These compounds act as non-competitive antagonists, blocking the NMDA receptor pore from within, thereby preventing calcium and sodium ion influx. This blockade can lead to profound psychotic hallucinations, altered perceptions of reality, and out-of-body experiences, useful for anesthesia but problematic for recreational abuse.

• Voltage-gated calcium channels:

  • Various subtypes of these channels regulate calcium entry into cells, crucial for neurotransmitter release, muscle contraction, and cardiac function.

  • They are targets for dihydropyridines (e.g., nifedipine, amlodipine), which are widely used for the treatment of cardiac conditions like hypertension and angina, by reducing calcium influx into vascular smooth muscle and cardiac cells, leading to vasodilation and decreased cardiac workload.

• Voltage-gated sodium channels:

  • These channels are fundamental for the initiation and propagation of action potentials.

  • They are primary targets for local anesthetics (e.g., lidocaine) and a number of crucial anti-epileptic/anti-seizure medications (e.g., phenytoin, carbamazepine).

Voltage-Gated Sodium Channel Modulators: Local Anesthetics and Anti-Epileptics

Local Anesthetics

• Examples: Lidocaine, Benzocaine, Tetracaine, Cocaine. These drugs are weak bases, often used in their protonated form.

• They are commonly administered by dentists (e.g., Novocain, which is procaine) to numb sensory nerves, for instance, those innervating teeth prior to procedures.

• Mechanism: Local anesthetics, such as lidocaine, work by reversibly inhibiting or blocking voltage-gated sodium channels. This blockade prevents the rapid influx of sodium ions required for action potential generation and propagation along nerve axons, effectively halting signal transmission beyond the injection site.

  • For example, when injected at a dental site, sensory neurons responsible for transmitting pain signals from the teeth and gums are blocked, and nearby motor neurons responsible for jaw movement might also be temporarily affected, leading to localized numbness and paresis.

• Availability: Benzocaine is readily available over-the-counter in various topical preparations for temporary relief from toothaches, sore throats, or skin irritation due to its localized anesthetic effect.

• Cocaine: While infamous for its recreational use as a stimulant due to its effects on the brain's dopamine system (blocking dopamine reuptake), cocaine was historically one of the first effective local anesthetics and still functions as one (e.g., causes profound numbness if snorted due to direct blockade of sodium channels in nasal mucosa).

Anti-Epileptic Drugs

• Examples: Phenytoin (Dilantin), Carbamazepine (Tegretol), Lamotrigine. These are cornerstone medications in the management of epilepsy.

• Functionally, these anti-epileptic drugs operate with a similar fundamental mechanism to local anesthetics in how they target and block sodium channels, albeit with different selectivity and kinetics that make them suitable for chronic neurological conditions.

• Phenytoin: A crucial therapeutic compound, widely prescribed and globally utilized for the prophylactic prevention of epileptic seizures.

  • It is typically taken prophylactically every day by individuals diagnosed with epilepsy to maintain a steady therapeutic concentration and stabilize neuronal excitability, thereby preventing seizure onset.

  • Key Advantage: A significant breakthrough in its development was its ability to effectively block seizures without causing debilitating general sedation, which was a common side effect of earlier anti-seizure medications (like barbiturates). While benzodiazepines can suppress acute seizures, they typically induce significant sleepiness or drowsiness, making them less suitable for long-term daily prophylactic use.

  • Developed in the 1940s, its precise molecular mechanism of action as a state-dependent sodium channel blocker was only fully elucidated decades later, illustrating a common pattern in pharmacology where clinical efficacy is established through empirical observation long before the intricate molecular underpinnings are completely understood.

Mechanism of Sodium Channel Block by Local Anesthetics and Phenytoin

Basic Blockage

• Similar to the deadly TTX, these therapeutic drugs also function to block sodium channels, but they are significantly less potent in doing so. This difference in potency is critical for their safety profile.

• For instance, Lidocaine blocks sodium currents at concentrations around $10^{-4}$ molar, which is approximately four orders of magnitude less potent than TTX, which blocks at $10^{-8}$ molar. This lower potency is exactly what makes them therapeutically useful and manageably safe rather than posing an immediate lethal toxicity risk in clinical settings.

• Location of binding: Local anesthetics and phenytoin uniquely bind within the cytoplasmic (intracellular) side of the channel pore, specifically within the inner vestibule. Access to this binding site is typically granted when the channel opens, allowing the drug molecule to enter and then bind more tightly when the channel transitions to the inactivated state.

Voltage and Frequency-Dependent Block

Local anesthetics and phenytoin exhibit a characteristic use- or frequency-dependent block, meaning their inhibitory effect is more pronounced when neurons are firing frequently or when the membrane is depolarized.

Experimental Observations

1. Voltage Dependence (Holding Potential):

   • Experiment 1 (Holding at $-100$ mV):

     • When a cell (e.g., a neuron or a cell line expressing voltage-gated sodium channels) is electrically clamped and held at a hyperpolarized membrane potential of $-100$ mV, all sodium channels are in their closed, resting state and readily available to open.

     • Upon a sudden, brief depolarization to $0$ mV, a large, rapid, and transient inward sodium current is observed as nearly all channels open simultaneously and then quickly inactivate.

     • When lidocaine at a concentration of $10^{-4}$ molar is applied under these conditions, it results in only a slight reduction (typically a few percent) in the peak current amplitude, indicating minimal block.

   • Experiment 2 (Holding at $-50$ mV):

     • When the same cell is held at a more depolarized resting potential of $-50$ mV, prior to the depolarizing pulse to $0$ mV, the control current (without the drug) is already significantly smaller than what was seen in Experiment 1.

     • This reduction in control current indicates that at $-50$ mV, a substantial fraction of sodium channels have already shifted from the closed state to the inactivated state, even before the stimulating pulse. Consequently, fewer channels are available to open.

     • Crucially, in the presence of lidocaine, the block observed is significantly more potent and pronounced compared to Experiment 1, with the current being much smaller. This shows that the more depolarized holding potential enhances the drug's effect.

   • Conclusion: The block by local anesthetics is distinctly voltage-dependent; it is considerably more effective at depolarized holding potentials where sodium channels spend a greater proportion of time in the inactivated state.

2. Frequency/Activity Dependence (Repetitive Stimulation):

   • Experiment: The cell is held at a hyperpolarized potential (e.g., $-90$ mV) where all sodium channels are initially in the closed state. A series of repetitive depolarizing pulses to $0$ mV are then applied at a high frequency (e.g., $10$ Hz, meaning $10$ pulses per second).

   • Observation: With each successive depolarizing pulse in the train, the degree of block progressively increases. This is manifested as a gradual and cumulative decrease in the peak sodium current amplitude across the series of pulses.

   • Conclusion: This phenomenon clearly demonstrates that the block by these drugs accumulates with repeated channel activation cycles (opening and inactivation), making it intensely frequency or activity-dependent. The drug's effect strengthens with increased neuronal firing.

Sodium Channel States and Drug Binding Affinity

• Voltage-gated sodium channels dynamically transition between three fundamental conformational states:

  1. Closed (or Resting): At hyperpolarizing (negative) membrane potentials (e.g., $-90$ mV), channels are closed but fully primed and available to open upon depolarization.

  2. Open: Channels rapidly transition to this state upon membrane depolarization (e.g., from threshold to peak action potential voltage). This state is very short-lived, transiently lasting less than $1$ millisecond (<1 ms), allowing a rapid influx of sodium ions.

  3. Inactivated: Immediately following the open state, channels rapidly inactivate. In this state, the channel pore is blocked internally, and it cannot open again, even if the membrane remains depolarized, until it returns to the closed state by repolarization.

• Equilibrium between Closed and Inactivated States: The open state is incredibly transitory. Consequently, channels spend the vast majority of their existence either in the closed state or the inactivated state. They dynamically cycle between these two primary states, with their distribution depending critically on the prevailing membrane potential.

  • At $-100$ mV: Nearly $100\%$ of voltage-gated sodium channels are in the closed (resting) state, fully available for subsequent activation.

  • At $0$ mV: At steady-state depolarization, almost $100\%$ of channels are in the inactivated state, exhibiting complete channel inactivation.

  • At intermediate membrane potentials (e.g., $-50$ mV): Channels exist in a dynamic equilibrium, with a mix of closed and inactivated states (e.g., approximately $50\%$ closed, $50\%$ inactivated, depending on the specific channel type and temperature). Only channels in the closed state can open; thus, at $-50$ mV, the maximum achievable sodium current is inherently smaller because fewer channels are available to transition to the open state.

• Inactivation Curve (or Steady-State Inactivation Curve): This essential graph plots the fraction of sodium channels that are available to open (i.e., in the closed state, or alternatively, the fraction that are inactivated) as a function of the membrane holding potential (or conditioning potential).

  • At $-100$ mV: $100\%$ of channels may be available (closed state).

  • At $-20$ mV: $0\%$ of channels may be available, meaning virtually all are in the inactivated state at steady state.

  • At approximately $-60$ mV: About $50\%$ of channels are in the closed state, and $50\%$ are in the inactivated state.

• Drug Binding Preference: The defining characteristic of local anesthetics and phenytoin is their profoundly higher affinity for the inactivated state of the sodium channel, in stark contrast to their relatively low affinity for the resting (closed) state.

  • When sodium channels transition into the inactivated state, the internal pore configuration and amino acid residues involved in drug binding change, creating a transient but high-affinity binding site for these drug molecules in the inner vestibule.

  • During repetitive neuronal stimulation (as occurs in high-frequency firing), each action potential causes channels to open very briefly and then rapidly inactivate. This repeated entry into the inactivated, high-affinity state allows drug binding to accumulate with each cycle, leading to the observed frequency-dependent block.

Kinetic Explanation: Shifting the Equilibrium

• To understand the voltage and frequency dependence, consider the channel as existing in various states, including drug-bound states:

  1. Closed (C): Channels are available but not conducting.

  2. Open (O): Channels are conducting. Drug binds to open state, but with lower affinity.

  3. Inactivated (I): Channels are non-conducting after opening. Drug binds to inactivated state with very high affinity.

  4. Inactivated-Drug Bound (I-D): The channel is inactivated and has a drug molecule bound within its pore.

• Dynamic Equilibrium (without drug): The channel constantly equilibrates between the closed and inactivated states (e.g., $C ightleftharpoons I$) based on membrane potential. The open state (O) is a transient transition between C and I.

  • At $-50$ mV, in a healthy neuron, there might be approximately $50\%$ C channels and $50\%$ I channels, dynamically interconverting.

• Effect of Drug: When a local anesthetic (LA) or phenytoin is added, it binds tightly to the 'I' state, forming the highly stable 'I-D' state. The rate of drug binding to 'I' is significantly higher than its binding to 'C' or 'O' (though some binding to 'O' does occur and contributes to the block).

  • This tight binding to 'I' continually removes 'I' channels from the dynamic $C ightleftharpoons I$ equilibrium. According to Le Chatelier's principle, this effectively pulls the entire equilibrium towards 'I' (and subsequently towards 'I-D'), acting as a 'trap' for channels in the inactivated state.

  • The overall kinetic pathway can be simplified as $C ightarrow O ightarrow I ightarrow I-D$. Once a channel is in the I-D state, it is effectively blocked and unavailable to open again.

• Shift in Inactivation Curve: This preferential binding to the inactivated state leads to a characteristic leftward shift (towards more negative potentials) in the inactivation curve. In the presence of the drug, at any given membrane potential, a greater fraction of channels will be forced into (or shifted towards) the inactivated-drug bound state, becoming unavailable to open. This results in a reduction in the total number of functional, available closed channels, leading to a macroscopic current reduction.

• Summary: The clinically significant block observed with local anesthetics and phenytoin is voltage and frequency-dependent. This is not due to an intrinsic voltage sensor on the drug molecule itself, but rather because the drug's effect is fundamentally state-dependent, exhibiting dramatically high-affinity binding to the inactivated channel state and comparatively low-affinity binding to the resting (closed) state. This state preference ensures that the drug primarily affects neurons that are depolarized or firing frequently.

Therapeutic Significance of Phenytoin's Mechanism

• How Phenytoin Treats Epilepsy Without Significant Sedation (Unlike General CNS Depressants):

  • Normal Brain Activity: In an individual without epilepsy, or during periods between seizures in an epileptic patient, neurons generally maintain relatively negative resting membrane potentials (e.g., $-70$ mV to $-90$ mV) and fire action potentials at reasonable, physiological frequencies (e.g., $1$-$50$ Hz).

    • Under these normal conditions, phenytoin has minimal effect. The majority of voltage-gated sodium channels are in the closed, resting state, for which phenytoin has a very low binding affinity. Therefore, normal neuronal function, thought processes, and alertness are largely preserved.

  • Seizure Activity: A seizure is fundamentally characterized by an abnormal, excessive, and synchronized high-frequency discharge of a population of neurons in the brain, often involving rapid, repetitive action potential firing at very high frequencies (e.g., $100$-$200$ Hz) and sustained membrane depolarization. These pathological conditions include:

    • Depolarization: During a seizure, neurons become persistently depolarized, pushing a larger fraction of sodium channels toward the inactivated state.

    • High-Frequency Firing: The rapid, repetitive firing means that sodium channels are constantly cycling through their open and inactivated states at an accelerated rate.

  • Phenytoin's Targeted Action in Seizures: It is precisely under these pathophysiological conditions (depolarization and high-frequency firing) that phenytoin's state-dependent mechanism becomes highly effective.

    • The sustained depolarization during a seizure increases the fraction of channels in the inactivated state, providing more high-affinity binding sites for phenytoin.

    • The rapid, repetitive firing forces channels to quickly transition through the open and into the inactivated state repeatedly, allowing more frequent access for phenytoin to its high-affinity binding site. With each cycle, more drug molecules bind, leading to a cumulative, frequency-dependent block.

    • By preferentially blocking sodium channels under these hyperexcitable conditions, phenytoin effectively reduces the ability of neurons to sustain high-frequency firing, aborting or preventing the synchronized bursts that characterize seizures.

  • Therapeutic Ratio: This selective action on hyperexcitable neurons provides a favorable therapeutic index for phenytoin: it effectively suppresses pathological seizure activity with minimal disruption to normal brain function, thus avoiding the profound sedation and cognitive impairment associated with less selective CNS depressants.