Lecture 24 Neuropeptides and gaseous neurotransmitters

Type 1 and 2 neurotransmitters vs. neuropeptides

Type 1 and 2: classical neurotransmitters- monoamines, amino acids. etc

• precursor molecules before neurotransmitter (memorize those)

• synthesis happening in the axon terminal of the neuron- Where NT is close to the small clear vesicles they are transported in

• vesicles: small, clear

Neuropeptides

• precursor molecules are synthesized at the cell body

• transported down axon to axon terminal

• processing of the protein into neuropeptide takes place in vesicle as its being transported down the axon

• large, dense core vesicles

Compare and contrast Type 1 and 2 vesicles vs neuropeptides

small clear:

• released from active zone

• primed at active zone

large, dense core

• bigger

• contain neuropeptides

• released extrasynaptically

Neuropeptide synthesis

1. protein that is precursor- synthesized in cell body

   • nucleus, ER, golgi (protein) -

   • protein in vesicle is called: prepropeptide

   • pre- signal sequence (not a part of final product)

   • pro: peptide is NOT complete yet

   • peptide: final product

2. vesicle buds off golgi and becomes transport vesicle to the axon terminal where transport vesicle is now dense core vesicle

Post-translational processing of neuropeptide

*happening inside vesicle as it is transported down the axon

• inside transport vesicle— there must be synthetic enzymes to process propeptide to final

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Diversity in neuropeptides created here:

EXMAPLE:

1. precursor pepride: POMC

   • depending on enzymes that are put in vesicle with the POMC, the POMC can be converted to a variety of products (ACTH or b-LPH)

2. Those peptides can be further processed by additional enzymes based on end product- (ex. in the intermediate lobe of a gland)

   • further diversity is possible, converted into different compounds

3. One precursor with a lot of final signalling neuropeptides

Alternative splicing of neuropeptides

Happening at mRNA level- pre-translation

• different coding regions in the same gene can code for different neuropeptides

• they can be spliced to be separated

example:

CALC 1 MRNA is spliced to have calcitonin coding region and cGRP mRNA region

means a smaller portion of genes can give rise to more neuropeptides

Neuropeptide release and regulation of synthesis (ca2+ relationship and production of neuropeptide)

Dense core vesicles are not docked at active zone but instead released extrasynaptically

• require higher concentrations of ca2+ in the terminal to trigger NP’s to be released

  • when there is low frequency activity- the ca2+ released in the presynaptic cell is quickly taken up by buffers and etc

  • it is not able to travel through the axon terminal

  • need high enough concentration that it can easily saturate buffers and diffuse to extra active zone locations of large dense core vesicles and trigger them to be released in areas outside of the active zone

    • this comes from high firing rate

ALSO:

• increased ca2+ levels can also increase neuropeptide PRODUCTION

  • high frequency activity is when dense core vesicles will be released, increase synthesis of peptides

    • to produce more neuropeptide, signal has to get back to SOMA

Concentrations and binding affinities for neuropeptides vs Type 1 and 2 neurotransmitters

Type 1 and 2:

• high concentration of vesicle released

• acting right at the synapse, across from active zone

Neuropeptides:

• always neuromodulators

• always act on metabotropic receptors

• metabotropic receptors have high binding affinity, therefore, the receptor will be activated with lower concentrations of it (low kd)

• even though they are larger vesicles, less vesicles are released, therefore NP is in lower concentration because receptors have higher binding affinity

Neuropeptide termination/degradation

No, there is no reuptake/recycling of neuropeptides

enzymatic degradation happens through peptadases

Neuropeptides are critical for function

NPY receptor activated, targets GIRK channel:

Feeding behavior

homeostasis

circadian rhythm

stress and anxiety- increased levels of NPY affect anxiety and stress

Gaseous neurotransmitters

Nitric oxide- vasodilation

Carbon monoxide

hydrogen sulfide

• they are free radical compounds thought to contribute to neurodegeneration

Nitric oxide synthesis

• Is produced in the process of arginine turning into citruline

• enzyme: nitric oxide synthetase

• Neuronal NOS: found in neurons

• endothelial NOS- blood vessel endothelium

• Inducible NOS: found after inflammation

Regulation of Nitric oxide synthesis and termination of action

NO synthesis must be tightly controlled because:

• NO cannot be stored in the body

• It is not inactivated by normal mechanisms such as reuptake

• NO can damage cells at high concentrations

• May contribute to neuronal damage after other injuries

Regulated by:

• Intracellular ca2+ (ca2+ activates the channel)

• gene expression

Termination of action:

• reaction with amino acids

• activity of phosphodiesterases

Arguments for and against Nitric oxide as a neurotransmitter

Against:

• not released from vesicles

• no transmembrane receptors

For:

can affect neuronal signaling by:

• activation of guanylyl cyclase (Fe2+ binds to NO, then gmp turns into cGMP which is second messenger and causes signalling cascade)

• s-nitrosylation of proteins (causes conformational change that alters function)

Nitric oxide as a neurotransmitter in the PNS

1. presynaptic cell becomes depolarized

2. Ca2+ enters the cell

3. nNOS activated by Ca2+ and calmodulin

   • NO is lipid soluble and easily passed through membranes

4. goes to smooth muscle, turns GTP into cGMP and causes vasorelaxation

NO as a neurotransmitter in CNS

1-2% of CNS neurons have NOS

1. nNOS activated in the post-synaptic terminal by ca2+ that NMDA passes through

2. can travel to adjacent neurons and presynaptic neurons

effects of NOS:

• neurotransmitter release

• synaptogenesis

• apoptosis

• synaptic plasticity

Carbon Monoxide SYNTHESIS

• Iron and CO are the end products of heme catabolism

• Heme goes through heme oxygenase to make Co and Fe

CO2 as neurotransmitter in PNS

1. ca2+ comes to presynapse

2. activates PKC

3. PKC activates HO2

4. creates CO

5. CO goes to smooth muscle in intestine and causes vasorelaxation

   • GTP to cGMP activates K+ channels (hyperpolarization)

Hydrogen sulfide

• enhances NMDA receptors

• increases LTP induction

  • dont know if it is neuroprotective or degenerative yet

Scenarios where multiple transmitters are released from a single neuron

1. More than one NT at each synapse in different vesicles

2. More than one transmitter at each synapse in the same vesicles

3. A different transmitter at each synapse

4. a mix of NT at each synapse

Purine transmitters

ATP is broken down into adenosine by ectoATPase and ectonucleotidease

adenosine receptors on membrane

Evidence that GABA and Glycine are released from the same vesicle

when you apply GABA receptor antagonists (biccuculine), you just get glycine fast current

When you apply glycine receptor antagonists (strychnine), you get slow release GABA

Nerve terminals in the spinal cord can release GABA and Glycine by expressing the required pre and postsynaptic proteins for both

refer to pic

• transporters for glycine and the precursor for GABA are both present in mixed synapses that has both of them in vesicles present

Can the golgi preferentially sort vesicles

yes

• golgi can send different transmitter containing vesicles to different axon collaterals at different synapses