Neurotransmitters and Synaptic Activity

• A neurotransmitter must be present in the presynaptic neuron, released during synaptic activity, and bind to receptors on the postsynaptic neuron
• Neurons can release more than one type of neurotransmitter, known as co-transmitters
• The criteria ensures that the substance functions as a chemical messenger in neural communication

Types of Neurotransmitters

• Small molecule neurotransmitters: low molecular weight, synthesized in the axon terminal, stored in small, clear vesicles, and released in response to low-frequency stimulation, including biogenic amines, amino acids, purines, and acetylcholine
• Neuropeptides: larger molecules composed of 3-36 amino acids, synthesized in the cell body, stored in large, dense vesicles, and require high-frequency stimulation for release, including enkephalins and gastrin

Acetylcholine (ACh) – Cholinergic Pathway

• Synthesis: Choline + Acetyl-CoA → ACh
• Enzyme: Choline acetyltransferase (ChAT)
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular acetylcholine transporter (VAChT) takes the ACh up into storage vesicles
• Inactivation is done by Acetylcholinesterase (AChE) via hydrolysis of ACh to choline and acetate
• Recycling: Choline transporter (CHT) recaptures choline for reuse

Glutamate (Glu) – Glutamatergic Pathway

• Synthesis: Glutamine → Glutamate
• Enzyme: Glutaminase
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular glutamate transporters (VGLUTs) move glutamate into synaptic vesicles
• Inactivation: Glutamate transporters (EAATs) remove glutamate from the synaptic cleft
• Recycling: Glutamine Synthetase (GS) converts glutamate back to glutamine, which is then transported back to neurons

GABA – GABAergic Pathway

• Synthesis: Glutamate → GABA
• Enzyme: Glutamic acid decarboxylase (GAD)
• Cofactor: Pyridoxal phosphate (Vitamin B6)
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular GABA transporter (VGAT) moves GABA into synaptic vesicles
• Inactivation: GABA transporters (GATs) remove GABA
• Recycling: GABA transaminase converts GABA to glutamate, then to glutamine for reuse within the body

Glycine – Glycinergic Pathway

• Synthesis: Serine → Glycine
• Enzyme: Serine Hydroxymethyltransferase (SHMT)
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular inhibitory amino acid transporter (VIAAT) loads glycine into synaptic vesicles
• Inactivation and recycling: Glycine transporters (GlyTs) remove glycine and return it to neurons or glial cells

Dopamine (DA) – Dopaminergic Pathway

• Synthesis: Tyrosine → L-DOPA → Dopamine (DA)
• Enzyme: Tyrosine hydroxylase (TH; rate-limiting step) and DOPA decarboxylase (AADC)
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular monoamine transporter (VMAT2) loads dopamine into vesicles
• Reuptake: Dopamine transporter (DAT) removes dopamine
• Degradation: Monoamine oxidase (MAO) → DOPAC and Catechol-o-methyltransferase (COMT) → 3-Methoxytyramine (3-MT)

Norepinephrine (NE) – Noradrenergic Pathway

• Synthesis: Tyrosine → L-DOPA → Dopamine (DA) → Norepinephrine (NE)
• Enzyme: Dopamine ẞ-Hydroxylase (DBH) inside the vesicles
• Location: Inside synaptic vesicles
• Vesicle packaging: Vesicular monoamine transporter (VMAT2) loads norepinephrine into vesicles
• Reuptake: Norepinephrine transporter (NET) removes norepinephrine
• Degradation: MAO and COMT → Metanephrine, Vanillylmandelic acid (VMA)

Serotonin (5-HT) – Serotonergic Pathway

• Synthesis: Tryptophan → 5-Hydroxytryptophan (5-HTP) → Serotonin (5-HT)
• Enzyme: Tryptophan Hydroxylase (TPH) and Aromatic L-amino acid decarboxylase (AADC)
• Location: Cytoplasm of the presynaptic neuron
• Vesicle packaging: Vesicular monoamine transporter (VMAT2) loads serotonin into vesicles
• Reuptake: Serotonin transporter (SERT) removes 5-HT
• Degradation: MAO → 5-Hydroxyindoleacetic acid (5-HIAA)

Nicotinic Acetylcholine Receptors (nAChRs) – Ionotropic

• Type: Ligand-gated ion channels
• Location: Found at neuromuscular junctions, autonomic ganglia, and CNS
• Mechanism: Binding of ACh allows for Na+ and Ca2+ influx, causing rapid depolarization and leading to an excitatory response
• Function: Muscle contraction at neuromuscular junctions, autonomic signaling, and cognitive function in the brain

Muscarinic Acetylcholine Receptors (mAChRs) – Metabotropic

• Type: G-protein-coupled receptors (GPCRs)
• Location: Found in the CNS, heart, smooth muscle, and glands
• Mechanism: ACh binding activates second messenger systems that lead to slower and longer-lasting effects
• Function: Modulates heart rate, gland secretion, and smooth muscle contraction
• Myasthenia Gravis (MG) affects nicotinic ACh receptors at the neuromuscular junction
• Cause: Autoimmune antibodies block or degrade nAChRs, preventing ACh from binding and leading to weak muscle contractions

Symptoms and Treatment of Myasthenia Gravis (MG)

• Symptoms: muscle weakness, fatigue, difficulty speaking and swallowing
• Treatment: Acetylcholinesterase inhibitors increase ACh availability at neuromuscular junction

Glutamate (Excitatory) Receptors

• AMPA: asymmetric tetramers that form an ion channel, generating relatively fast excitatory postsynaptic currents
• NMDA: generates relatively slow excitatory postsynaptic currents, channel pore admits Ca2+, Na+, and K+
• During hyperpolarization, MG2+ blocks the channel; depolarization pushes the MG2+ out
• Glutamate + depolarization indicates some kind of information storage mechanism is happening; requires a coagonist, glycine
• Kainate: generate excitatory postsynaptic currents that rise quickly but decay slowly

GABA Receptors

• GABA^ Receptor – Ionotropic (Ligand-gated channels)
• Ion movement: Cl- influx
• Effect: Hyperpolarization (IPSP) makes the neuron less likely to fire
• Fast inhibition (milliseconds)
• Target of benzodiazepines and alcohol
• GABAB Receptor – Metabotropic (G-protein coupled channels) Ion movement: K+ efflux
• Effect: Hyperpolarization; inhibits Ca2+ influx, leading to reduced neurotransmitter release
• Slower, prolonged inhibition
• Target of muscle relaxant drugs, such as baclofen

Biosynthetic Pathway for Catecholamines (DA, NE, Epi)

• Step 1: Tyrosine → L-DOPA
  • Enzyme used: Tyrosine hydroxylase (TH); rate-limiting step
  • Cofactors: O2, Fe2+, tetrahydrobiopterin (BH4)
  • Location: Cytoplasm
• Step 2: L-DOPA → Dopamine (DA)
• Enzyme used: DOPA decarboxylase (AADC)
• Cofactors: Pyridoxal phosphate (Vitamin B6)
• Location: Cytoplasm
• Step 3: Dopamine (DA) → Norepinephrine (NE)
• Enzyme used: Dopamine 𝛽-hydroxylase (DBH)
• Cofactors: O2, Cu2+, Ascorbic acid (Vitamin C)
• Location: Inside vesicles
• Step 4: Norepinephrine (NE) → Epinephrine (Epi)
• Enzyme used: Phenylethanolamine N-methyltransferase (PNMT)
• Cofactor: S-adenosylmethionine (SAM)
• Location: Cytoplasm
• Vesicular storage: Catecholamines are packaged in vesicles via vesicular monoamine transporters (VMAT)
• Catecholamines released by exocytosis upon stimulation

Functions of DA, NE, and Epi and impacting Molecules

-Dopamine (DA) -Main functions: motor control, reward/motivation, cognition, attention, and hormonal regulation

• Increase DA activity: Psychostimulants (cocaine, amphetamines), L-DOPA, nicotine, and opioids (morphine, heroin) -Decrease DA activity: Antipsychotics and reserpine
• Norepinephrine (NE) -Main functions: arousal, alertness, fight-or-flight, mood regulation, memory, and attention -Increase NE activity: Psychostimulants (cocaine, amphetamines), SNRI antidepressants, MAO inhibitors (MAOIs), and beta-agonists -Decrease NE activity: Beta-blockers and alpha-2 agonists -Epinephrine (Epi) -Main functions: Fight-or-flight response (increases heart rate and blood pressure), metabolism, vasoconstriction, vasodilation, and anti-inflammatory effects -Increase Epi activity: Epinephrine injection, beta-agonists, and MAO inhibitors (MAOIs) -Decrease Epi activity: Beta-blockers and alpha-blockers
• The reuptake of Dopamine is done by Dopamine transporter (DAT) and the reuptake of Norepinephrine is done by Norepinephrine transporter (NET)
• Enzymatic breakdown: Monoamine oxidase (MAO) and Catechol-o-methyltransferase (COMT)

Biosynthetic Pathway for Serotonin

• Serotonin (5-hydroxytryptamine, 5-HT) is synthesized from the amino acid tryptophan through a series of enzymatic steps
• Step 1: Tryptophan → 5-Hydroxytryptophan (5-HTP)
  • Enzyme: Tryptophan hydroxylase (TPH)
  • Cofactors: O2, tetrahydrobiopterin (BH4)
  • Location: Cytoplasm
  • Hydroxylation of tryptophan at the 5-position
  • Rate-limiting step!
• Step 2: 5-Hydroxytryptophan (5-HTP) → Serotonin (5-HT)
  • Enzyme: Aromatic L-amino acid decarboxylase (AADC)
  • Cofactor: Pyridoxal phosphate (Vitamin B6)
  • Location: Cytoplasm
  • Decarboxylation of 5-HTP

Processing Pathway of Neuropeptides

• Neuropeptides are large, protein-like neurotransmitters synthesized from precursor proteins.
• Their synthesis, processing, packaging, and release follow a distinct pathway compared to small molecule neurotransmitters
• Step 1: Synthesis
  • Cell body (rough ER)
  • Translation of mRNA into prepropeptide (precursor protein)
• Step 2: Processing
  • Golgi apparatus
  • Cleavage of signal peptide, further processing into propeptide and active neuropeptides
• Step 3: Packaging
  • Golgi apparatus → vesicles
  • Neuropeptides are packaged into dense core vesicles
• Step 4: Transport
  • Axon -Vesicles are transported to axon terminals via microtubules
• Step 5: Release
  • Presynaptic terminal → synaptic cleft
  • Neuropeptides released via exocytosis (Ca2+-mediated)
• Step 6: Receptor binding
  • Postsynaptic neuron
  • Binding to GPCRs, leading to slow modulatory effects
• Step 7: Inactivation
  • Synaptic cleft, presynaptic terminal
  • Degradation by peptidases and recycling

Modulation of GABAergic Nerve Endings by Endocannabinoids

• Endocannabinoids are lipid-based neurotransmitters that act as retrograde signals
• They are released from the postsynaptic neuron and travel backwards to presynaptic terminals
• They modulate neurotransmitter release, interact with cannabinoid receptors to influence a variety of neural processes, including GABAergic transmission -Synthesis: Endocannabinoids are synthesized on-demand in postsynaptic neurons -Release: Endocannabinoids are released retrogradely from the postsynaptic neuron -CB1 receptor activation: Endocannabinoids bind to CB1 receptors on GABAergic presynaptic terminals -Intracellular signaling: CB1 activation inhibits adenylyl cyclase, activates MAPK, affects ion channels -Inhibition of GABA release: Reduced calcium influx inhibits GABA vesicle release, leading to decreased GABAergic signaling -Effect on synaptic transmission: The reduction of GABA release results in disinhibition and potential enhancement of excitatory activity -The endocannabinoid system can influence anxiety and stress responses as well seizure activity

Use of Nitric Oxide (NO) as a Neurotransmitter

One advantage of using nitric oxide (NO) as a neurotransmitter is its ability to diffuse long distances directly across cell membranes without the need for receptors on the postsynaptic membrane

• Advantages include rapid and localized signaling, regulation of blood flow, plasticity and learning, and the coordination of complex processes

Forms of Chemical Communication

• Endocrine signaling: hormones are released by endocrine glands into the bloodstream, travel through the circulatory system to target cells or organs, trigger specific hormonal responses; long-distance signaling, slower response with longer duration, and affects cells with specific receptors for the hormone
• Paracrine signaling: signaling molecules are secreted by a cell and act on nearby target cells; travel short distances through the extracellular fluid; neurotransmitters in synaptic signaling; short-range signaling, affects cells in close range, and often involved in developmental processes and immune responses within the body
• Synaptic signaling: specific type of paracrine signaling that occurs in the nervous system; neurotransmitters are released from presynaptic neurons and cross the synaptic cleft to bind to receptors on postsynaptic neurons or other target cells; fast signaling across a synapse, affects target cells with high precision
• Intracellular signal transduction: signaling processes that occur inside the cell to regulate its activities, coordinate responses, and maintain homeostasis; involves the transmission of signals within the cell, typically triggered by external signals, essential for cell functions such as growth, metabolism, gene expression, and apoptosis

Advantages of Chemical Communication

• Coordination of complex processes: coordinate complex processes across different systems in the body, enabling seamless function and adaptation to environmental or internal changes
• Long-distance communication: chemical signals, particularly hormones, allow long-distance communication within the body; important for maintaining systemic balance across different organs and tissues
• Precision and specificity: chemical communication can be highly specific; only cells with the appropriate receptors respond to certain signals, ensuring the right response is triggered in the right place at the right time
• Flexibility and regulation: chemical signaling pathways can be modulated, allowing for a range of responses depending on the strength, duration, and timing of the signals; this gives the body flexibility in adapting to various conditions and stimuli
• Integration and homeostasis: chemical communication plays a central role in maintaining homeostasis, integrating signals from various organs and tissues to ensure the body remains in a stable, balanced state

Classes of Signaling Molecules

• Cell impermeant: Chemical diffuses across space to bind to receptors on target cells; large, polar, charged molecules that cannot easily cross the cell membrane so these molecules require membrane-bound receptors
• Cell-permeant: Chemical diffuses out of one cell to another by diffusion through both cell membranes; small, nonpolar, lipophilic molecules that can easily diffuse through the lipid bilayer
• Cell-associated: Signaling molecules are anchored to the cell membrane and are not released into the extracellular space; instead, they are typically involved in cell-to-cell communication through direct contact with cell surface receptors on adjacent cells

G-Proteins

• Heterotrimeric G-proteins: consist of three subunits: alpha, beta, and gamma
• Alpha binds/hydrolyzes GTP, which activates/deactivates signaling pathways
• Beta and gamma subunits help anchor the protein to the membrane and assist in regulating the alpha subunit
• Alpha subunit activates an effector protein that produces a 2nd messenger
• Monomeric G-proteins: smaller and consist of a single subunit that binds GTP
• Involved in intracellular signaling and play roles in cell growth, differentiation, and cytoskeletal regulation

Neuronal second Messengers

• Neuronal second messengers play a critical role in intracellular signaling and help transmit signals from cell surface receptors to intracellular pathways, leading to cellular responses
• Ca2+
• Sources: Plasma membrane and endoplasmic reticulum
• Targets: Calmodulin, protein phosphatases, ion channels, synaptotagmins
• Removal: Plasma membrane, endoplasmic reticulum, and mitochondria
• Cyclic AMP
• Sources: Adenylyl cyclase acts on ATP
• Targets: Protein kinase A and cyclic nucleotide-gated channels
• Removal: CAMP phosphodiesterase
• Cyclic GMP
• Sources: Guanylyl cyclase acts on GTP
• Targets: Protein kinase G and cyclic nucleotide-gated channels
• Removal: cGMP phosphodiesterase IP3
• Sources: Phospholipase C acts on PIP2
• Targets: IP3 receptors on endoplasmic reticulum Removal: Phosphatases
• Diacylglycerol
• Sources: Phospholipase C acts on PIP2
• Targets: Protein kinase C

Signal Transduction Pathways

• Kinases and phosphatases are essential regulators of signal transduction pathways because they control the phosphorylation state of proteins, which regulates protein activity
• Kinases add a phosphate group to a target protein, a process known as phosphorylation; this occurs on the hydroxyl group of amino acids
• Phosphatases remove phosphate groups from proteins, a process known as dephosphorylation
• Both kinases and phosphatases are crucial for controlling responses to signals
• Protein Kinase A is activated through the cAMP (cyclic AMP) signaling pathway
• This involves the activation of GPCRs, activation of adenylyl cyclase, increase in CAMP levels, and PKA activation
• PKA is activated, phosphorylates CREB which then binds to a specific DNA sequence known as the cAMP Response Element (CRE), located in the promoter region of genes
• CREB (cAMP Response Element-Binding Protein): transcription factor activated by phosphorylation
• CRE (cAMP Response Element): A DNA sequence found in the promoter region of target genes
• Cognitive kinases, like PKA, CaMKII, and MAPK are essential for synaptic plasticity, which is the ability of synapses to change in response to activity

Regulation of Gene Expression

• CREB is a transcription factor that regulates gene expression in response to signal transduction pathways
• Activated by phosphorylation, binds to specific DNA sequences called CRE, found in the promoter regions of target genes
• Nuclear receptors are ligand-activated transcription factors that regulate gene expression by binding to specific hormones or other small molecules
• When a ligand enters a cell, it binds to the nuclear receptor and induces a conformational change in the receptor, allowing it to dimerize, translocate to the nucleus, and bind to specific DNA sequences called HREs

Neuronal Function and Structure

• Nerve Growth Factor (NGF): crucial for the survival and maintenance of sensory neurons and sympathetic neurons
• Long-Term Depression (LTD): reduces strength synaptic connections, important for learning and memory
• Phosphorylation of Tyrosine Hydroxylase: Increased dopamine or norepinephrine synthesis due to TH phosphorylation enhances neurotransmitter availability at synapses

Short Term Plasticity

• Synaptic facilitation: a rapid increase in synaptic strength due to residual calcium buildup in the presynaptic terminal from closely spaced action potentials
• Synaptic depression: a transient decrease in synaptic strength during repetitive stimulation due to depletion of readily releasable vesicles
• Post-Tetanic Potentiation (PTP): a form of short-term enhancement that lasts tens of seconds to minutes following a high-frequency train of action potentials
• Synaptic Augmentation: a slower and longer-lasting increase in synaptic strength compared to facilitation, lasting a few seconds

Habituation and Sensitization

• Habituation is a decrease in response to a repeated, benign stimulus over time
• Sensitization is an increase in response to a stimulus due to prior exposure to a strong or noxious stimulus
• Associative learning in Aplysia occurs when an initially neutral stimulus becomes associated with a strong, meaningful stimulus
• Short-term sensitization is mediated by serotonin via activation of cAMP and PKA, leading to increased NT release
• Long-term sensitization involves CREB activation, leading to protein synthesis and structural changes in synapses

Short Term Sensitization (STS) and Long Term Sensitization (LTS)

• Both STS and LTS are triggered by noxious stimuli, involve presynaptic facilitation of sensory neuron-motor neuron synapse, rely on serotonin release from modulatory interneurons, activate cAMP-PKA signaling cascade, cause an enhanced withdrawal reflex by increasing synaptic strength
• STS lasts minutes to hours, while LTS lasts days to weeks
• STS requires a single tail shock, while LTS requires repeated tail shocks
• STS involves modifications of existing proteins, while LTS requires new protein synthesis
• STS involves no new synapses, while LTS involves new synaptic growth

Long-Term Potentiation (LTP)

• Long-term potentiation is a long-lasting increase in synaptic strength following high-frequency stimulation
• LTP is a model for understanding learning and memory, involves persistent changes in synaptic efficacy; highlights the connection between learning and memory

Anatomy of Hippocampal Brain Slice

• Dentate Gyrus: receives input from the entorhinal cortex
• CA3: contains pyramidal neurons; involved in pattern completion and memory retrieval
• CA1: receives input from CA3; involved in memory consolidation and spatial navigation
• Subiculum: the main output region; sends information to the entorhinal cortex and other brain regions
• The trisynaptic circuit describes the flow of information through the hippocampus

The Relevance of the CA3-CA1 Synapse

• The CA3-CA1 synapse is one of the most well-studied circuits for understanding long-term potentiation, which is critical for learning and memory

Long-Term Potentiation (LTP) and Coincidence Detection

• A persistent increase in synaptic strength following high-frequency stimulation, underlying learning and memory
• Coincidence detection refers to the process by which a synapse strengthens only when presynaptic activity postsynaptic depolarization happen simultaneously

Temporal Domains of Long-Term Potentiation (LTP)

• Early-LTP / Short-term phase
• Lasts minutes to an hour
• Requires NMDA receptor activation and Ca2+ influx
• Late-LTP / Long-term phase
• Lasts hours to days or longer
• Requires new gene transcription and protein synthesis
• Very-Late LTP / Systems-level phase
• Days to weeks
• Involves synaptic tagging and capture, ensures that only relevant synapses receive long-term modifications Associativity means that a weakly stimulated synapse can undergo LTP if it is activated simultaneously with a strongly stimulated synapse
• Input selectivity ensures that only synapses that receive stimulation are strengthened, preventing random or unnecessary synaptic changes
• The Hebbian rule states that neurons that fire together, wire together

Calcium Ions and LTP

• LTP is induced because of accumulation of postsynaptic Ca2+ from its influx through NMDA receptors
• NMDAR: upon activation, these receptors allow the influx of calcium ions and sodium ions into the postsynaptic cell, and the efflux of potassium ions
• AMPAR: activated by the neurotransmitter glutamate and allow the influx of sodium ions into the postsynaptic cell, leading to depolarization of the postsynaptic membrane

Long-Term Depression (LTD)

• Long-term depression is a form of synaptic plasticity characterized by a long-lasting decrease in the strength of synaptic transmission
• LTD in the hippocampus is induced by low-frequency