Síntesis, metabolismo y acción de los neurotransmisores.Parte II:Glutamato, GABA, Glicina y Péptidos

Neurotransmitter Synthesis, Metabolism, and Action

Introduction to Neurotransmitters

• Professor Pedro Penzini introduces the topic of neurotransmitter synthesis, metabolism, and action, focusing on metabolic pathways.

Types of Neurotransmitters

• Discussion on key neurotransmitters: glutamate, GABA (gamma-aminobutyric acid), glycine, and neuropeptides. Glutamate and GABA are highlighted as the most prevalent in synapses within the nervous system.

Synaptic Structure

• Importance of understanding synapse structure is emphasized; classifications based on electron microscopy are introduced.

• Gray Type 1 synapses are described as asymmetric with distinct pre-synaptic and post-synaptic densities featuring multiple active zones.

• Gray Type 2 synapses show symmetry in density between pre-synaptic and post-synaptic areas with fewer vesicles compared to Type 1.

Neurotransmitter Classification Criteria

• Focus shifts to how substances are synthesized into vesicles at axon terminals and their subsequent metabolism after release into the synaptic cleft.

• Key criteria for classifying a molecule as a neurotransmitter include its presence in synaptic vesicles, ability to mimic nerve stimulation effects, release upon stimulation, synthesis location within neurons (especially for neuropeptides).

Classical vs. Non-Classical Neurotransmitters

• Classical neurotransmitters (Type 1), such as acetylcholine and monoamines, have small molecular weights and are produced solely at the synapse.

• Non-classical neurotransmitters (Type 2), including glutamate and glycine, differ by being produced outside the synapse.

Receptor Families

• Discussion on receptor families that interact with neurotransmitters; nicotinic acetylcholine receptors characterized by five subunits each containing four transmembrane segments.

• Similarities noted between various receptors like GABA receptors which also belong to the nicotinic family due to structural similarities.

Specific Receptors for Glutamate

• Overview of ionotropic glutamate receptors categorized into three types: AMPA, Kainate, and NMDA. Each type has specific blocking compounds associated with them.

Summary of Amino Acid Neurotransmitters

Glutamate Metabolism and Neurotransmission

Overview of Glutamate's Role in Cells

• Glutamate is classified as a type 2 neurotransmitter because it is widely present in cells and forms part of many proteins, not just synapses.

• Pyruvate, a product of glucose metabolism, can be converted into acetyl-CoA through the pyruvate dehydrogenase complex, which plays a crucial role in energy production.

The Krebs Cycle Initiation

• Acetyl-CoA combines with oxaloacetate to form citrate (a six-carbon compound), initiating the Krebs cycle (also known as the tricarboxylic acid cycle).

• Citrate undergoes decarboxylation to form alpha-ketoglutarate (five carbons), which can then be converted into glutamate.

Enzymatic Conversion Processes

• The enzyme glutamate decarboxylase is found only at GABAergic synaptic terminals, indicating that GABA is classified as a class 1 neurotransmitter.

• Glutamate synthesis occurs via three pathways:

• From alpha-ketoglutarate via glutamate dehydrogenase.

• By combining alpha-ketoglutarate with aspartate through aspartate aminotransferase.

• From glutamine via glutaminase.

Packaging and Release Mechanisms

• After synthesis, glutamate must be packaged into vesicles using mechanisms similar to other neurotransmitters.

• Vesicles contain a hydrogen ion pump that uses ATP for active transport to concentrate hydrogen ions inside the vesicle.

Transporter Functionality

• The vesicular transporter for glutamate operates by exchanging hydrogen ions for glutamate against concentration gradients, ensuring efficient storage within vesicles.

• Upon neuronal signaling, these processes facilitate the release of glutamate into the synaptic cleft where it binds to specific receptors on postsynaptic membranes.

Reuptake and Recycling of Glutamate

• Following its action on receptors, glutamate is recaptured either directly by neurons or by surrounding glial cells (astrocytes).

• Astrocytes utilize excitatory amino acid transporters (EAAT), specifically EAAT1 and EAAT2, for reuptake from the synapse back into their cells.

Synthesis of Glutamine from Glutamate

• Within astrocytes, an enzyme called glutamine synthetase converts excess glutamate back into glutamine by adding an amino group.

• This newly synthesized glutamine is secreted back into the extracellular space where it can be taken up again by neurons through specific transporters like SAT2.

Regulation of Glutamatergic Activity

Glutamatergic Synapse and Its Mechanisms

Energy Source for Neuronal Activity

• The primary energy source for neuronal activity is mainly derived from glycolysis.

Role of Glycolysis in Glutamate Production

• An increase in glycolysis leads to a rise in substrates necessary for glutamate production.

Glutamate Release and Receptor Interaction

• In the glutamatergic synapse, glutamate release has a direct relationship with receptor types, acting through ionotropic or metabotropic receptors.

• Released glutamate can be converted into glutamine by the presynaptic terminal, which is then secreted and reabsorbed to regenerate glutamate.

Types of Glutamate Receptors

Ionotropic Receptors

• There are three types of ionotropic receptors: NMDA, AMPA, and Kainate. Each type has variations in subunit composition that affect their excitability when activated by glutamate.

Metabotropic Receptors

• Metabotropic receptors are classified into three classes:

• Class 1 (e.g., mGluR1 and mGluR5): Associated with IP3 signaling pathways.

• Class 2: Linked to decreased cyclic AMP levels via G-proteins.

• Class 3: Related to increased cyclic AMP levels.

Functional Characteristics of Receptors

• The specific functions of these proteins can be explored further in cellular signaling classes.

Classification of Glutamate Channels

• Glutamate channels can be categorized based on substances that block them (e.g., AMPA, NMDA, Kainate).

Differences Between NMDA and Non-NMDA Channels

• NMDA channels require glycine as a co-activator alongside glutamate. They also have unique structural features allowing magnesium ion attraction.

Voltage Dependence of NMDA Channels

• NMDA channels exhibit both ligand-dependent activation (requiring glutamate and glycine) and voltage dependence due to membrane polarization changes expelling magnesium ions.

Permeability Characteristics of Channels

• AMPA and Kainate channels primarily allow sodium (Na+) and potassium (K+) passage; some AMPA channels may also permit calcium (Ca2+).

Activation Mechanism of AMPA Channels

• For NMDA channel activation, they must associate with AMPA-type receptors that bind glutamate, leading to conformational changes that open the channel.

Ionic Current Dynamics

• When the membrane potential is far from equilibrium potentials for Na+ and K+, more Na+ will flow through than K+. As depolarization occurs, currents will balance until reaching reversal potential near zero millivolts.

Importance of Reversal Potential

Mechanisms of Calcium Entry in Neuronal Channels

Role of Calcium in Cellular Signaling

• The electrostatic repulsion generated by the NMDA channel allows for the removal of magnesium when glutamate and glycine bind, facilitating ion passage according to their equilibrium potential.

• Calcium acts as a second messenger that activates various proteins within the cell, influencing phosphorylation processes and altering cellular behavior.

Structure of Glutamate Receptors

• Glutamate receptors consist of four distinct transmembrane subdomains, differing from acetylcholine and glycine receptors.

• Each subunit has four transmembrane passes; the M2 subdomain is crucial for channel formation while M1 features a long N-terminal domain contributing to receptor structure.

Activation Mechanism of Glutamate Receptors

• The extracellular connection between M3 and M4 forms a binding site for glutamate, which upon binding alters receptor configuration.

• This conformational change leads to movement between domains (M1 and M2), resulting in channel opening that permits ion flow.

Kinetics of Channel Activation

• Different channels exhibit varying activation kinetics; some activate quickly with high current levels while others are slower with lower currents.

• AMPA channels activate and deactivate rapidly compared to NMDA channels, which have slower activation and deactivation rates.

Conductance Properties Under Different Conditions

• In magnesium-free solutions, channels behave like ohmic conductors; however, with magnesium present, they act as rectifiers conducting electricity more easily at certain potentials.

• At membrane potentials below -50 mV, channels remain inactive due to electrostatic attraction between negative charges on the membrane and magnesium ions.

Summary on Glutamate Functionality

Understanding GABA: Synthesis, Function, and Pharmacology

GABA Synthesis and Mechanism

• The enzyme glutamate decarboxylase converts glutamate into gamma-aminobutyric acid (GABA), which is the primary inhibitory neurotransmitter in synapses. It acts on ionotropic channels for negative ions and metabotropic channels that conduct potassium ions.

• Pyridoxine serves as a cofactor for the decarboxylation reaction to produce GABA from glutamate.

Storage and Release of GABA

• After synthesis, GABA is concentrated within vesicles via an antiporter protein that exchanges hydrogen ions for GABA.

• This transporter, known as VGAT (Vesicular GABA Transporter), actively imports GABA into vesicles while exporting protons.

Action at Synaptic Sites

• Upon receiving a signal, vesicles fuse with the presynaptic membrane to release GABA into the synaptic cleft where it interacts with postsynaptic receptors.

• Two types of transporters remove excess GABA from the synaptic cleft: one located at the terminal and another in glial cells.

Enzymatic Breakdown of GABA

• The enzyme GABA transaminase breaks down excess GABA in the synapse, contributing to its regulation.

Structural Characteristics of GABA

• The nomenclature "gamma-aminobutyric acid" reflects its structure: it has four carbons (1-alpha, 2-beta, 3-gamma), with carboxylic and amino groups defining its classification.

Receptor Interaction and Variability

• When released, GABA binds primarily between alpha and beta subunits of its receptors. Variability among receptor subunits influences pharmacological effects.

Pharmacological Implications

• Different drugs interact variably with these receptors; benzodiazepines bind between specific subunit combinations affecting their therapeutic outcomes such as sedation or anxiety reduction.

Effects of Various Substances on GABAA Channels

• Medications targeting different sites on the GABAA channel can lead to diverse effects like sleep induction or anxiolytic properties due to their interaction with various receptor configurations.

Integration of Inhibitory Signals

Neuronal Activity and Neurotransmitter Dynamics

Understanding Chloride Ion Equilibrium Potential

• The equilibrium potential for chloride ions is approximately -70 mV, which is crucial for understanding changes in voltage within motor neurons.

• When the membrane potential remains at -70 mV due to inhibitory neurotransmitter release, there is no net current through the channel, maintaining resting potential.

Effects of Membrane Voltage Changes

• If the membrane voltage rises to -65 mV or even -40 mV, it can lead to negative voltage changes, indicating a shift from equilibrium.

• Hyperpolarization can occur if chloride channels open when the neuron is already hyperpolarized, leading to depolarizing currents instead of hyperpolarizing ones.

Role of GABA in Neuronal Development

• In immature neurons, GABA acts as an excitatory neurotransmitter due to specific sodium-potassium transporters that alter ion concentrations across membranes.

• These transporters influence synaptic behavior; when synapses switch from inhibitory to excitatory during development, it can lead to severe epileptic crises in neonates.

Mechanisms of Inhibitory Neurotransmitters

• The effectiveness of inhibitory neurotransmitters like glycine depends on both resting membrane potential and ion concentration gradients rather than just their action on channels.

• Glycine is synthesized from serine via the enzyme serine hydroxymethyltransferase and plays a significant role in spinal cord and peripheral nervous system signaling.

Glycine Transport Mechanisms

• Glycine interacts with receptors primarily in the spinal cord and utilizes vesicular amino acid transporters for packaging and release.

• Two types of glycine transporters exist: GlyT1 (found in glial cells) and GlyT2 (located at presynaptic terminals), differing mainly by their coupling with sodium ions.

Importance of Interneurons in Spinal Circuits

• Interneurons known as Renshaw cells are critical circuits associated with alpha motor neurons; they provide feedback inhibition essential for muscle control.

Tetanus Toxin and Neuropetides

Mechanism of Tetanus Toxin

• The tetanus toxin causes the Renzo cell to stop releasing vesicles filled with glycine, affecting alpha motor neurons that control body balance.

• This leads to hyperactivity in these neurons, resulting in a characteristic posture known as "sardonic laughter" due to hyper-contraction.

Differences Between Neurotransmitters and Neuropeptides

• Neuropeptides differ from conventional neurotransmitters; they are released not at active zones but at various other sites.

• Neuropeptides are synthesized from protein precursors by ribosomes, packaged in vesicles within the endoplasmic reticulum before being transported through the Golgi apparatus.

Synthesis and Release of Neuropeptides

• Pre-pro-peptides are formed initially, which undergo modifications via enzymes to produce the final neuropeptide that binds to receptors.

• These peptides have lower concentrations but higher affinities compared to traditional neurotransmitters, influencing their action sites.

Action Mechanisms of Neuropeptides

• Neuropeptide release is triggered by significant cellular stimulation and elevated calcium levels, differing from rapid-release neurotransmitters.

• The expression of genes for neuropeptides involves mRNA synthesis, which then guides ribosomal assembly for protein production.

Functional Implications of Neuropeptide Activity

• After synthesis, neuropeptides are transported along microtubules and released away from active zones during high-frequency neuronal firing.

• A single precursor can yield multiple neuropeptides (e.g., pro-opiomelanocortin can produce several hormones), each acting on specific receptors.

Summary of Key Neurotransmitter Characteristics

• Conventional neurotransmitter release occurs primarily at active zones with rapid response times; however, neuropeptide actions result in slower but more enduring changes in target cells.

Neurotransmitters and Their Functions

Key Neurotransmitters in the Nervous System

• Glutamate is identified as the primary excitatory neurotransmitter in the nervous system, possessing both ionotropic and metabotropic receptors.

• GABA (gamma-aminobutyric acid) serves as the main inhibitory neurotransmitter in the central nervous system, featuring various types of ionotropic and metabotropic receptors that enhance potassium conductance.

• Glycine is noted as the principal inhibitory neurotransmitter within the spinal cord, playing a crucial role in motor control and reflexes.

Neuropéptides and Their Role

• Neuropeptides are co-released with classical neurotransmitters from clear vesicles; they differ by being produced in the cell body rather than at synaptic terminals.

• The release sites for neuropeptides are distinct from active zones, indicating a complex mechanism of action within neural communication.

Conclusion