Nuero (3)- Chemical Communication

Chemical Communication

Overview

  • Focus area: Neurotransmitters and Psychopharmacology.

  • Topic: Types of Neurotransmitters and the Drugs that Modulate their Actions.

Definitions

Neurotransmitters

  • Definition: Neurotransmitters (NTs) are chemical messengers that transmit signals across synapses from one neuron to another.

  • Synthesized in axon terminals and stored in vesicles until released by action potentials.

Receptor Types in Activation of Neurotransmitters

  • Ionotropic Receptors: These are receptor channels that open directly upon binding with neurotransmitters, leading to the generation of inhibitory postsynaptic potentials (IPSPs) or excitatory postsynaptic potentials (EPSPs).

  • Metabotropic Receptors: These receptors activate G-proteins upon neurotransmitter binding, leading to second messenger cascades that can open ion channels or influence cellular functions such as protein synthesis.

Historical Context

Early Evidence for Neurotransmitters

  • By 1906, Ramon y Cajal established that the nervous system comprises individual neuronal units.

  • Debate over the nature of synaptic communication lasted until the 1950s between the concepts of 'sparks' (electrical communication) versus 'soups' (chemical communication).

  • Otto Loewi conducted experiments providing evidence for the chemical nature of neurotransmission.

Defining Neurotransmitters

  • NTs are synthesized and stored in presynaptic neurons.

  • Released upon arrival of action potentials.

  • Cause changes in postsynaptic neurons crucial for communication; blocking these changes inhibits neuronal communication.

Application and Effects of Neurotransmitters

Botulinum Toxin

  • Definition: A potent neurotoxin from the bacteria Clostridium botulinum, responsible for botulism.

  • Mechanism: Prevents synaptic vesicles from fusing with the presynaptic membrane, inhibiting neurotransmitter release.

  • Botox®: A dilute form of the toxin used medically to prevent muscle communication.

Receptor Functions

Mechanisms of NT Action

Receptors

  • Function: Receive chemical signals from presynaptic neurons.

  • Types:

    • Ionotropic (Ligand-gated ion channels)

    • Metabotropic (G-proteins and second messengers)

Inactivation of Neurotransmitters

  • After NT release, inactivation occurs via:

    • Enzymatic Degradation: Breakdown by enzymes in the synaptic cleft.

    • Reuptake: Transporter molecules on presynaptic neurons take NTs back into the cell.

  • Critical for ensuring temporal precision in neuronal communication.

Psychopharmacology

Overview

  • Definition: The study of drugs that impact thought and behavior, primarily through neurotransmitter modulation.

Subfields

  • Pharmacokinetics: Concerns the absorption, distribution, metabolism, and excretion of drugs (motion).

  • Pharmacodynamics: Focuses on how drugs exert effects on their sites of action (change within a system).

Receptor Binding

Ligands

  • Definition: Molecules that bind to receptor sites.

Types of Ligands

  • Endogenous Ligands: Naturally produced molecules (e.g., neurotransmitters).

  • Exogenous Ligands: External substances (e.g., drugs).

  • Types:

    • Agonists: Mimic the endogenous ligand.

    • Antagonists: Block the action of endogenous or agonist ligands.

Dose-Response Relationships

Dose-Response Curve

  • Definition: A graph indicating how the amount of a drug administered correlates with its physiological effects.

  • Influences affected by pharmacodynamics (interaction with receptors) and pharmacokinetics (speed to site of action).

Therapeutic Index

Overview

  • Definition: The margin of safety for a drug, characterized by the range between the effective dose for 50% of subjects (ED50) and the lethal dose for 50% of subjects (LD50).

  • Example: Benzodiazepines have a broader therapeutic index than barbiturates, making them safer options for anxiety relief.

Effects of Repeated Drug Exposure

Key Concepts

  • Sensitization: Increased effects with repeated exposure; shifts the dose-response curve to the left.

  • Tolerance: Decreased effects with repeated exposure; shifts the dose-response curve to the right.

  • Dependence: Body adapts, exhibiting withdrawal symptoms upon cessation, usually opposite the drug's effects.

Summary of Neurotransmitter Actions

  • NTs act via ionotropic and metabotropic receptors.

  • Inactivated by enzymatic degradation and reuptake.

  • Psychopharmacology studies how drugs influence NT systems.

  • Pharmacokinetics focuses on drug movement, while pharmacodynamics examines their effect mechanisms.

Types of Neurotransmitters

Overview

Classification

  • Amino Acids: (e.g., Glutamate, GABA)

  • Amines: (e.g., Dopamine, Serotonin)

  • Neuropeptides: (e.g., Endorphins)

Classical Neurotransmitters

Synthesis

  • Each neuron typically produces one type of NT, synthesized in axon terminals before vesicular packaging.

Example: Monoamines

  • Synthesized from the precursor tyrosine from food sources, leading to dopamine, norepinephrine, and epinephrine.

Non-Classical Neurotransmitters

Overview

  • Neuropeptides: Large molecules created in the cell body; synthesized from precursor proteins, followed by transport to axon terminals.

  • Significance: Involved in pain modulation and reward systems (e.g., Endorphins).

Glutamate

Characteristics

  • Classical NT and primary excitatory neurotransmitter in the brain.

Receptor Types

  • Ionotropic: AMPA (sodium channel), NMDA (calcium channel).

    • AMPA mediates EPSPs; NMDA is crucial for synaptic plasticity associated with learning.

  • Metabotropic Receptors: Influence excitatory or inhibitory effects based on G-protein activation.

Ketamine

Overview

  • An NMDA receptor antagonist, classified as a dissociative anesthetic.

Effects

  • Senses of detachment, hallucinations, cognitive disorientation; low doses may have antidepressant effects (as seen in Spravato inhaler).

GABA

Characteristics

  • Primary inhibitory neurotransmitter in the brain, synthesized and acting via GABA receptors.

Receptors

  • Ionotropic GABAA: Chloride channel, essential for IPSPs.

  • Metabotropic GABAB: Inhibit neural activity through G-proteins.

  • Agonist/Antagonist effects influence seizure activity and anxiety levels (e.g., picrotoxin).

Benzodiazepines

Overview

  • Allosteric agonists for GABA, binding different from GABA to enhance its receptor activity.

Clinical Use

  • Diazepam (Valium), lorazepam (Ativan) for anxiety management; risk of habituation with prolonged use.

Acetylcholine

Characteristics

  • Quaternary amine operating in both central and peripheral nervous systems.

Roles

  • At neuromuscular junctions affects motor neuron communication; arousal and memory in CNS.

  • Engagement through ionotropic nicotinic and metabotropic receptors.

Alzheimer's Treatments & Nerve Gas

Mechanisms

  • ACh is fully degraded by enzymes in the synapse, playing a role in dementia treatment by inhibiting its breakdown to increase synaptic availability.

Nerve Gas Effects

  • Sarin as a potent inhibitor, highly toxic, resulting in paralysis and suffocation.

Monoamine Neurotransmitters

Types

  • Dopamine, norepinephrine, and serotonin are synthesized in specific clusters in the mid and hindbrain.

  • Signing of Influence: Monoamines influence extensive brain regions despite being numerically sparse.

Dopamine

Functions

  • Essential for motor control, motivation, and behavioral reinforcement.

Pathways

  • Nigrostriatal Pathway: From substantia nigra to striatum, important for movement control.

  • Mesolimbic Pathway: From the ventral tegmental area to the nucleus accumbens and amygdala for reward processing.

Parkinson's Disease

Description

  • Associated with degeneration in the nigrostriatal pathway, resulting in motor control issues such as akinesia and bradykinesia.

Reinforcing Brain Stimulation

Findings

  • Rats pressed levers for electrical stimulation in the mesolimbic pathway even above basic needs, blocked by dopamine antagonists.

Dopamine Receptors

Classification

  • All are metabotropic; categorized into D1-like (excitatory) and D2-like (inhibitory) subgroups.

Drugs and DAT

Mechanism

  • Dopamine is removed by a transporter (DAT); cocaine acts as an antagonist inhibiting DAT, increasing synaptic dopamine levels, leading to reinforcement.

Norepinephrine

Functions

  • Role in arousal, wakefulness, and alertness, primarily dispatched from locus coeruleus to cortex and limbic regions.

Cognitive Behavioral Influence

  • Represented by the Yerkes-Dodson law, indicating optimal arousal levels for complex task performance.

Norepinephrine Receptors & Drugs

Types

  • Metabotropic, categorized into alpha and beta receptors; sympathetic communication increases physiological arousal through norepinephrine.

  • Beta-blockers: Antagonists that mitigate sympathetic response, lowering blood pressure.

Serotonin

Functions

  • Distributed by axons from raphe nuclei to many CNS areas; involved in mood regulation and satiation.

Receptor Diversity

  • Predominantly metabotropic receptor types; SSRIs used in antidepressant therapies hinder serotonin reuptake.

Neuropeptides

Overview

  • Synthesized from precursor proteins and trafficked in vesicles; play roles in reward and pain management.

  • Endogenous Opioids: (e.g., enkephalins) mediate analgesia via metabotropic receptors in specific brain areas.

Exogenous Opioids

Examples

  • Opioid drugs like heroin and morphine act as agonists on opioid receptors, facilitating pain relief and euphoria while presenting addiction risks.

Naloxone

Mechanism and Use

  • An opioid antagonist effective against overdose; it outcompetes opioids for receptor binding, reversing their effects.

Conclusion

Summary

  • Classical neurotransmitters are synthesized in neuron terminals with inactivation mechanisms; glutamate serves primary excitatory function while GABA serves inhibitory roles.

  • Monoamines (dopamine, norepinephrine, serotonin) show significant brain influence, while neuropeptides contribute to the modulation of pain and reward.

Additional Insights

Neural Communication Statistics

  • Average neuron receives input from around 1000 other neurons; total neurons in the human nervous system exceed 100 billion, generating vast numbers of action potentials.

Neuromyths

Clarification on Brain Usage

  • Misconception that only 10% of the brain is utilized; functional imaging shows that all parts are active, needed for various processes including maintaining ion gradients.

  • Discussion on misconceptions tracing to various quotes and media figures, affirming that wasteful energy usage wouldn't survive natural selection.