Neurotransmission and Ion Transport Mechanisms in the Nervous System

Posted on May 20, 2024 in Biology

NeuroTrans

Transporters

• Uniporter: Same ions, different directions
• Antiporter: Different ions, different directions
• Symporter: Different ions, same direction

IonTrans

• Passive
• 1st Active (ATP)
• 2nd Active (Indirect)

1st Active Transport

2 P-types (phosphorylate conserved aspartate) =

1. Na-K ATPase: 3 Na⁺ out, 2 K⁺ in, electrogenic (inside electronegative, 1 ion charge movement), ↑ infinity Na⁺, rest -60 to -90 mV, K⁺ leak channels @ rest
2. Ca²⁺ ATPase: Ca²⁺ out, need low intracellular Ca²⁺, surface of ER, muscle cells.

Domains:

• T: Core, binding site
• S: Support for T
• P: Key, aspartic acid residue, allows conformational changes (E1/E2)
• N: ATP binding pocket, kinase to phosphorylate P
• A: Phosphatase to dephosphorylate P, TGE loop

Conformational States:

• E1: Ion X⁺ binds to M domain, Mg²⁺ATP from N to P domain.
• E1-P: Aspartate phosphorylated
• E1P to E2P: P reorients from E1-E2, A rotates bringing TGE loop to protect from hydrolysis & shut ion channel, ADP dissociates, X⁺ released extracellularly, 2Y⁺ binds from outside.
• E2: Hydrolysis of phosphorylated Asp, Mg²⁺ dissociates, reverts to E1 state, Y⁺ released into cell.

Na⁺ Injection: Increased intracellular Na⁺, hyperpolarization because the pump is activated to push out Na⁺. Na⁺ is critical for the pump, K⁺ is not.

Configuration of Na-K Pump:

• α subunit: Enzymatic activity, binding sites, 10 transmembrane segments, large M4-M5 loop (phosphorylation)
• β subunit: Glycosylation, pump function, 1 transmembrane domain

Ion Translocation by Na-K ATPase

1. A) Inward-facing binding sites ↑ Na⁺ affinity
2. B) 3 Na⁺ binding induces phosphorylation of the enzyme
3. C) Conformational change, binding site opens outward
4. D) Outward-facing binding sites ↓ Na⁺ affinity
5. E) Dephosphorylation from K⁺ binding
6. F) Repeat

Intracellular Ca²⁺

Functions:

• Neurotransmitter release
• Activate ion channels
• 2nd messenger regulating cytoplasmic enzymes
• Muscle contraction

Sources:

• Entry through the plasma membrane
• Release from intracellular stores

Transport Mechanism: 1st active, Ca²⁺-ATPase, ↑ intracellular compartments in ER/SAR/mitochondria

Channel Types:

• Voltage-gated
• Ligand-gated
• Exchangers
• Ca²⁺ pump

SERCA Pump

Mechanism: ER Ca²⁺-ATPase, E1 ↑ affinity for Ca²⁺ at the intracellular binding site, E2 low Ca²⁺ affinity, 1 Ca²⁺ transported per cycle.

States:

1. E1 to E1-2Ca²⁺ATP: Cytoplasmic Ca²⁺ binds, activating the pump & ATP, leading to occlusion of the pump and ATP hydrolysis
2. E1P-Ca²⁺ADP to E2P: Spontaneous phosphoryl transfer, conversion of the phosphorylated pump to E2P, opening of the luminal channel & release of Ca²⁺
3. E2 to E2Pi: Closing of the luminal gate & dephosphorylation, release of Ca²⁺

2nd Active Transport

Uses indirect Na⁺ gradient (Na⁺-K⁺ ATPase).

• Cotransport: Same direction
• Ion exchange: Opposite directions

Na⁺-Ca²⁺ Exchange

1. NCX: 3 Na⁺ in, 1 Ca²⁺ out, direction depends on concentration & membrane potential (V_m), electrogenic, ↓ affinity for Ca²⁺ to function longer, empty exchanger binds either 1 Ca²⁺ or 3 Na⁺, mutually exclusive, conformational change to reorient binding site, properties = extracellular N-terminus, intracellular C-terminus, Ca²⁺ regulation, splice site
2. NCKX: 4 Na⁺ in, 1 Ca²⁺ & 1 K⁺ out, retinal cells, V_m -40 mV, empty transporter binds either 1 Ca²⁺/1 K⁺ or 4 Na⁺, reorients binding site.

Others: Na⁺-Cl^–, Na⁺-HCO₃^–, Na⁺-H⁺ (intracellular pH), extracellular N/C-terminus.

Extracellular Concentration Effects:

• Reduce extracellular Ca²⁺ = ↓ intracellular Ca²⁺
• ↓ extracellular Na⁺ = ↑ intracellular Ca²⁺ by ↓ outward Ca²⁺ transport

Reversal of Na⁺-Ca²⁺ Exchange

Na⁺ = Ca²⁺ energy, no exchange, transport direction depends on 3 Na⁺ entry.

Neurons: V_r -74 mV (rest), E_Na⁺ +58 mV, E_Ca²⁺ +124 mV.

NCX Reversal

• Forward transport: Hyperpolarization, Na⁺ in, Ca²⁺ out
• Reverse transport: Depolarization, Na⁺ out

Retinal: If reverse direction, Ca²⁺ accumulates & continuous depolarization (NCKX used to avoid), V_m = -40 mV, V_r = 74 mV, in rods, K⁺ extruded down gradient.

Both: Homologous α1/α2 regions, critical residues for ligand binding & transport.

Cl^– Transport Mechanisms (not electrogenic)

1. Inward Na⁺-Cl^– with Na⁺-K⁺-Cl^– transporter: Uses Na⁺ gradient, 1:1 Na⁺-Cl^– (NCC) cotransport, 1:1:2 Na⁺-K⁺-Cl^– (NKCC) cotransport.
2. Outward K⁺-Cl^– cotransporter (KCC): Uses outward K⁺ gradient, cell volume (intracellular Cl^– regulation)
3. Na⁺-dependent Cl^–-HCO₃^– exchanger (NDCBE): Regulates intracellular pH, electrically neutral.

Human Isoforms

Critical for development, function of proteins

• KCC: Transport activated by dephosphorylation, inhibited by phosphorylation
• NKCC: Activated by phosphorylation, inhibited by dephosphorylation
• CCC: Cl^– cotransporter, transcripts in neocortex from early fetal to late adult.

Neuron Development

Fueled by Na⁺-K⁺ ATPase, KCC2 expression neuron-specific in CNS, similar in hippocampus/amygdala/cerebral cortex.

NKCC1 & KCC2: Control intracellular Cl^– concentration in CNS, electroneutral

Immature Neurons: ↓ active transport, Na⁺ gradient (Na⁺-K⁺ ATPase) uptake of Cl^– & generates depolarizing Cl^– current across GABA_A receptors

Mature Neurons: K⁺ gradient (Na⁺-K⁺ ATPase) & KCC2 attains ↑ expression & hyperpolarizing Cl^– current.

Neurotransmitter Transport

1. Into presynaptic vesicles
2. Transmitter reuptake transporters

Transmitter Recovery (after release)

From synaptic cleft by transporters in the plasma membrane of terminals/adjacent glial cells to:

1. Terminate synaptic action
2. Prevent diffusion
3. Repackage for re-release

Transporters:

• PNT: Plasma membrane
• VNT: Vesicular
• RNT: Receptor

Into Presynaptic Vesicles

Transmitter synthesized in the cytosol of nerve terminals & concentrated into vesicles via secondary transport mechanisms coupled to proton efflux.

Proton-coupled transporter: Energy from proton gradient from H⁺-ATPase (pumps H⁺ into vesicles from cytosol) in vesicle membranes.

Stoichiometry:

1. A) Monoamines/Acetylcholine: 2:1
2. B) GABA/Glycine: 1:1
3. C) Glutamate: 1:1 (Transmitter & Cl^–)

VMAT2 (Monoamine)

2 H⁺ out, 1 transmitter in, transports dopamine into vesicles from cytosol with H⁺-ATPase, 12 transmembrane segments, C/N-terminus inside, large extracellular loops for post-translational modifications by proteins, important for anxiety/depression/attention.

Glutamate Transport

1. Vesicles packaged
2. Glutamate release from presynaptic terminal
3. Acts on postsynaptic glutamate receptors
4. EAAT: Recycled into presynaptic terminal OR excess transported into glial astrocytes (glutamine synthase → glutamine)
5. Astrocyte: Glutamine converted to glutamate by glutaminase & packed into vesicles by VGlut transporters.

Glutamate Transporters

3 subunits, 8 transmembrane segments

• EAAT1: Glia/cerebellum, retina/cochlea, humans ↑ affinity glutamate transporters (GLAST) & trimerization domain to form
• EAAT2: Astrocytes in cortex/hippocampus
• EAAT3: Neurons in cortex/hippocampus/thalamus/superior colliculus
• EAAT4: Purkinje cells

Reuptake Transporters

Use Na⁺ electrochemical gradient

• SLC1: Inward transport of 1 glutamate coupled with 2 Na⁺ in, 1 K⁺ out & H⁺ out/in, excitatory amino acid transporter (EEAT found at active sites)
• SLC6: Uptake of transmitter with 2 Na⁺/1 Cl^– in, GABA, norepinephrine, dopamine, serotonin, glycine.

DAT Protein

12 transmembrane segments, large extracellular loop/glycosylation site, modulates trafficking/stability.

Function: Transition outward → inward facing, regulates mood/attention/reward

Amphetamine/Methamphetamine: (DAT target for medications) modulate dopamine levels, interfere with DAT so dopamine remains in the synapse, ↑ dopamine levels in DAT regions.

Dopamine Postsynaptic Receptors

= D1 & D2

Drugs on Dopamine Terminals

1. Stimulants: Affect transmission, elevated synaptic dopamine, increased postsynaptic responses
2. Cocaine: Blocks DAT, competitive reuptake inhibition to increase synaptic dopamine, does not enter presynaptic neuron
3. Amphetamine/Methamphetamine: Uptake into presynaptic terminal via DAT, competitive reuptake inhibition, lipophilic via diffusion through the membrane, enter secretory vesicles to cause dopamine efflux, reverse DAT function to increase dopamine.

Results: Enhanced signaling at D1/D2 receptors, activation/inhibition of adenylate cyclase (AC), downstream cellular responses for enhanced attention.

Antidepressants

1st Generation:

1. MAOI: Monoamine oxidase inhibitors, block in presynaptic terminal
2. TCA: Tricyclic, block reuptake

2nd Generation:

1. SSRI: Selective serotonin reuptake inhibitors
2. SNRI: Serotonin-norepinephrine reuptake inhibitors

Synaptic Transmission

• Direct: Transmitter binds to postsynaptic ionotropic receptor, receptor conformational change, channel opens, ion flow, V_m change, fast (milliseconds)
• Indirect: Transmitter binds to postsynaptic G-protein coupled receptor (GPCR), GPCR & 2nd messenger system activated, opens ion channel, V_m change, slow.

Electrical Transmission

Muscle contraction/nerve conduction, direct from presynaptic to postsynaptic, studies = frog end-plate potential (EPP) at skeletal neuromuscular junction & acetylcholine as neurotransmitter

Chemical Transmission

Transmitter release from nerve terminal by incoming action potential.

Potentials

: EPSP = if Vr more + than AP, excites postsyn, cationic channels (depol Vm), Glu in CNS activate excita iono receptors IPSP = inhibits, anionic (hyper Vm), GABA/glyc in CNS activate inhib iono recep. EPP = acetyl release from presyn to depol end-plate region of muscle following motor excita. NMJ Drugs: agonist = chemical binds/activate receptor antagonist = binds/deactive receptor potentiator = inhibit enzyme acetylCoA (responsible for hydrolysis/inactivate ACh). ACh distance from endplate: synap potent amplitude = smaller and longer peak. Volt Clamp, Vr below 0 = flow into muscle. ↑ 0 = out.