Neurochemistry I

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Three Characteristics of Neurotransmitters
-Stored in Presynaptic Vesicles
-Released in Ca²⁺-dependent manner
-Capable of interacting with a membrane-bound receptor to produce an effect

Additionally, a substance that meets these criteria and also interacts with the receptors to produce the same physiological effect as activations of presynaptic axons is considered to be the transmitter at that synapse.

Small Molecule Neurotransmitters
Acetylcholine, Amino Acids and Amino Acid Derivatives
Norepinephrine
Two Types of NT’s and EM Appearance
SMALL MOLECULE – Appear as clear vesicles
PEPTIDE – Appear as Large, dense vesicles
Biosynthesis of Small Molecule NT’s
Enzymes for converting the AA’s are made in the nucleus and modified in the cell body prior to being taken to axon terminals.

Uptake molecules in the Axon terminal membrane move AA precursors or actual NT’s into vesicles or other organelles where reactions can take place.

Biosynthesis of Peptide NT’s
Formed from cleavage of larger peptides in the rough ER and Golgi and are then packaged into vesicles and transported to the axon terminals
Relationship Between Quanta Release and Vesicle Fusion
Linear because each Quanta comes from one vesicle fusion
Steps in Presynaptic Neurotransmitter release
1. AP reach Axon terminal
2. Voltage-Gated Ca²⁺ Channels Open leading to Influx
3. Synaptic Vesicles fuse with Presynaptic Membrane
4. NT Released
Relationship Between Presynaptic Voltage and Postsynaptic EPSP Amplitude (Na⁺ and K⁺ Channels Blocked)
The greater the Presynaptic depolarization, the larger the EPSP amplitude. The curve levels off at a certain positive value, as it is approach the Equilibrium Potential of Ca²⁺
Otto Loewi Experiment
Proved that Neurons were separate cells whose firing was regulated by Neurotransmitters, rather than solely by Gap Junctions, as the former was thought to be too slow. (cajal)

Involved two frog hearts beating within separate tissue cultures baths.

First Frog Heart: Stimulate Parasympathetic input so that contractions slow.

-He then removed some solution from the first bath and placed it in the second-

Second Frog Heart: The contractions also slowed

Basically, some of the ACh being released by the vagus nerve in the first bath didn’t synapse on the heart and was put into solution. This was then moved to the second bath, where it did synapse, slowing the heart rate.

Postsynaptic potentials and Amplitude
PSP’s may be excitatory or inhibitory, depending on which ions permeate the membrane when the transmitter binds to a receptor which regulates an ion channel.

EPSP – Depolarizing; increases likelihood of AP; typically involves elevated Na+ (excitatory)

IPSP – Hyperpolarizing; decreases likelihood of AP; typically involves elevated K⁺ or Cl⁻ (inhibitory)

AMPLITUDE – Unlike AP’s, PSP’s are GRADED, in that stronger inputs will produce stronger outputs. Individual EPSP’s are generally Subthreshold and need to Summate with others to produce an AP;

Events of Synaptic Delay
Ca²⁺ entry into the axon, vesicle release, NT accumulation and diffusion across the synaptic cleft, binding to and and activation of post-synaptic receptors
Characteristics of Fast Transmitters
Short time to Peak Effect
Quickly inactivated
Typically associated with Ionotropic Receptors
Ex fEPSP: nAchR
Ex fIPSP: GABA A,
Characteristics of Slow Transmitters
Longer time to Peak Effect and long time to inactivation
Typically a characteristics of the receptor rather than the NT
Muscarinic/Metabotropic Receptors
Peptide Transmitters are typically slow type
Often act following Fast Transmitters
Ex sEPSP: G protein coupling, mAchR
Ex sIPSP: GABA B
Postsynaptic Receptor Locations and Activation
Postsynaptic Receptors can be located on Somas or Dendrites (including Dendritic Spines)

Can be activated by either an NT different from the one synthesized by the neuron or by the same NT synthesized by the Neuron.

Axo-Axonic Synapse
Synapses where the Pre- and Post-Synaptic Structures are both axons.

Autoreceptors and Heteroreceptors

Presynaptic Autoreceptors
When the Axon terminal has receptors for the NT it releases on the same terminal.

Generally, this serves a negative-feedback control

Presynpatic Heteroreceptors (Presynaptic Inhibition)
When the Axon terminal has receptors for an inhibitor NT released from a separate Axon terminal at the same synapse.
Ionotropic Nuerotransmission
Ligand-gated Ion channels
neurotransmitter “ligand” functions to open ion channel, and are either fast EPSP’s or fast IPSP’s following NT binding.
-multiple homologous subunits
-1 or 2 binding pockets on receptors for ligand.
(name ligand binding is confusing because metabatrophic channels use ligands like Ach)
Structure of G Protein Receptors/Metabotrophic NT’s
Single Subunit
Seven Transmembrane Regions
Coupled to Heterotrimeric G Proteins

*Do not have a channel pore associated*

G-Protein Cycling
BASAL STATE: α subunit bound to GDP molecule

ACTIVATED STATE: NT binds, causing α subunit to lose GDP and bind GTP. The subunit also dissociates, and carries out some enzymatic process to activate a second messenger

RECYCLE: α subunit re-binds, GTP is converted to GDP, NT dissociates

G Protein Subtypes in the Brain
Gs—- increases adenylyl cyclase
Gi— decreases adenylyl cyclase
Gq/p– increases phospholipase C
Go— increases K and Ca channels
Gs
:Cyclic nucleotide gated channels (in retina photoreceptors and olfactory neurons)
↑ Adenylyl Cyclase -> cAMP -> open channels*
* Cyclic nucleotide gated channels conduct Na, K, Ca fairly non selectively when bound by cGMP and cAMP
d1 dopamine Receptor
Gi
↓ Adenylyl Cyclase
mAChR, d2 dopamine R
Gq/p
↑ Phospholipase C -> DAG + IP³

-DAG then increases PKA, which binds to cAMP to Phosphrylate other proteins
-IP₃ binds to a receptor on the ER to mobilize Ca2+ from storage organelles
—Ca targets: PKC, Calmodulin, CaMK
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Go
↑ K⁺, directly opens Ca²⁺ Channels
Two Modes of G-Protein Operation
Activation of Ion Channels (Go)
Activation of Second Messangers, typically kinases (Gs, Gi, Gq)
Second Messangers of Neurons
cAMP, DAG, IP₃, Ca²⁺
Cyclic AMP (cAMP)
PRECURSOR: ATP
ENZYME OF SYNTHESIS: Adenyl Cyclase *
G PROTEIN: Gs or Gi
TARGET: Cyclic Nucleotide-Gated Ion Channels or cAMP-dependent Protein Kinase (Protein Kinase A)
Diacylglycerol (DAG)
PRECURSOR: PIP₂
ENZYME OF SYNTHESIS: Phospholipase C
G PROTEIN: Gp
TARGET: Protein Kinase C (PKC)
Inosito Triphosphate (IP₃)
PRECURSOR: PIP₂
ENZYME OF SYNTHESIS: Phospholipase C
G PROTEIN: Gp
TARGET: Receptor of Endoplasmic Reticulum (causes release of Ca²⁺)
Ca²⁺
SOURCES: Voltage-Gated Channels, NMDA Receptors and ER
TARGETS: PKC, Calmodulin, Ca²⁺/Calmodulin Protein Kinase (CaMK)
Adenylate Cyclase Activity
STIMULATORY: Gs
INHIBITORY: Gi

Adenylate Cyclase produce cAMP from ATP

Cyclic Nucleotide Gated Channels
Bind cAMP to open channels that can conduct Na⁺, K⁺ and Ca²⁺

Belong to the voltage gated ion channel family even through their activity isn’t dependent on voltage.

Found in Retinal Photoreceptors and Olfactory Sensory neurons. Mutations in these give rise to blindness.

Protein Kinase A (PKA)
Tetramer that binds for molecules of cAMP. When the two regulatory molecules do this, they dissociate, exposing the two catalytic subunits which can phosphorylate a variety of proteins.
PIP₂
Hydrolyzed to produce DAG and IP₃
Phospholipase C
In response to Gp, it stimulates the reaction:

PIP₂ → DAG + IP₃

Calmodulin (CaM)
Has four Ca²⁺ binding sites that when bound regulates several things:

Calcium/Calmodulin-Dependent Protein Kinase (CaMK)
ATPase Pump
Adenylyl Cyclase
Phosphodiesterase

Tyrosine Hydroxylase
Phosphorylated by Kinase-Type second messengers, activating it to synthesize some NT’s
Microtubule Associated Proteins (MAPs, Tau)
Phosphorylated by Kinase-Type second messengers, changing/maintaining the neuronal structure
cAMP Response Element Binding Protein (CREB)
Phosphorylated by Kinase-Type second messengers, altering gene expression
Dopamine – Receptor Types and Effect
D1 & D5: Increase Adenylyl Cyclase
D2, D3, D4: Decrease Adenylyl Cyclase
Norepinephrine – Receptor Types and Effect
α1: ↑ PLC
α2: ↓ Adeynyl Cyclase
β: ↑Adenylyl Cyclase
Acetylcholine – Receptor Types and Effect
Nicotinic: Na⁺/K⁺ Conductance
Muscarainic: 5 Types

m1, m3, m5: ↑ PLC
m2, m4: ↓ Adenylyl Cyclase and involved with G Protein coupling to ion channels

γ-Aminobutyric Acid (GABA)
GABA(a) – Cl⁻ Conductance (FAST)
GABA(b) – K⁺/Ca²⁺ Conductance (G Protein-Coupled)
GABA(c) – Cl⁻ Conductance (SLOW)
Glutamate
AMPA/Kainic Acid – Na⁺/K⁺ Conductance
NMDA – Na⁺, K⁺, Ca²⁺ Conducatnace
Metabotropic – ↑ Phospholipase C
Categories: Neurochemistry