Lecture 24: Neuropeptides and gaseous neurotransmitters

Type 1 and 2 neurotransmitters and neuropeptides

• 1 and 2 synthesis: in nerve terminal(close to clear small vesicles that they’ll be packaged in)

• Neuropeptide synthesis is in soma → transported down axon to axon terminal; processing of protein into neuropeptide occurs in vesicle while being transported

Types of synaptic vesicles

• small clear synaptic vesicles:

  • contain “classical” and amino acid neurotransmitters(Type 1 and 2)

  • released from active zone

• large dense core vesicles

  • contain neuropeptides

  • electron dense

  • released extrasynaptically

Neuropeptide synthesis

• protein that will be neuropeptide is synthesized in the soma

• prepropeptide: initial protein product

  • “pro”: not complete peptide

• post-translational processing: in vesicle as it moves down axon terminal meaning the vesicle must also contain synthetic enzymes that process it from prepropeptide to the peptide

Post-translational processing of neuropeptides

• diversity can be created

• depending on enzyme in vesicle, prepropeptide(ex. POMC) can be converted into different things(ACTH and β-LPH)

• ACTH and β-LPH can be further processed by additional enzymes depending on the eventual target

Alternative splicing of neuropeptides

• transcription → RNA splicing → translation

• don’t need many genes for many actions

  • different ways for diversity in genes

Neuropeptide release and regulation of synthesis

• large dense core vesicles: not docked at active zone so need more calcium for release so need higher frequency action potential rate

  • calcium quickly sopped up by buffers so need enough so it can diffuse to extrasynaptic locations where large dense core vesicles are located

• increased calcium levels can also increase neuropeptide production

  • some sort of retrograde messenger that goes back to soma to increase neuropeptide production

Type 1 and 2 neurotransmitter vs neuropeptides

• neuropeptides: always neuromodulators → act on metabotropic receptors which have higher binding affinity so they’re activated at lower concentrations so neuropeptide in lower concentration

• sites of release:

  • type 1+2: AZ in small clear vesicles

  • neuropeptideds: outside active zone in dense core vesicles

• sites of action:

  • type 1+2: act right at the synapse

  • neuropeptides: extrasynaptic action

• termination/recycling:

  • no reuptake of neuropeptides, all enzymatic degradation by nonspecific peptidases

Examples of neuropeptide effects: neuropeptide Y(NPY)

• NPY can have pre-and post-synaptic effects

• NPY receptors are GPCR

• NPY can impact functions like:

  • feeding behavior

  • homeostasis

  • circadian rhythym

  • stress+anxiety

• high NPY levels=less likely to prefer alcohol

NPY regions involved in fear+anxiety

• not localized in one specific region

Gaseous neurotransmitters

• nitric oxide

• carbon monoxide

• hydrogen sulfide

Nitric oxide synthesis

• precursor: arginine

• enzyme: nitrous oxide synthase(NOS)

  • enzyme activated by Calcium-calmodulin complex

• cofactors: FAD, FMN, BH4

• arginine → citrulline with NOS enzyme

• different types of NOS

  • neuronal nitric oxide synthase(nNOS): form of NOS originally found in neurons

  • endothelial NOS: found in blood vessel endothelium

  • inducible NOS: found after inflammation

  • ALL OF THESE CATALYZE NITRIC OXIDE PRODUCTION

Regulation of nitric oxide synthesis and termination of action

• NO synthesis must be controlled because

  • NO can’t be stored

  • no enzymatic degradation or reuptake of NO

  • NO can damage cells at high concentrations

  • May contribute to neuronal damage after other injury?

• NO synthesis is regulated by:

  • intracellular calcium

    • less calcium = less activation of calcium-calmodulin = less activation of nNOS = less NO produced

  • gene expression

• Termination of action:

  • reaction with amino acids because NO is a free radical

  • activity with phosphodiesterases

    • phosphodiesterases break converts cGMP back to GMP

Is nitric oxide a neurotransmitter?

• against

  • not released from vesicles

  • no transmembrane receptor

• for:

  • can affect neuronal signaling by:

    • activating guanylyl cyclase; GTP→cGMP(second messenger molecule activated by NO) **NO activates guanylyl cyclase which catalyzes conversion of GTP to cGMP**

    • s-nitrosylation of proteins

      • conformational change in protein → changed function; a common target of s-nitrosylation is ion channels

Nitric oxide as a neurotransmitter in the PNS

• NANC: non adrenergic non cholanergic neuron

• Calcium influx → activation of Ca2+-calmodulin complex → activation of nNOS → NO production → diffuses because it’s lipid soluble to adjacent smooth muscle → s-Guanylyl activated and conversion of GTP to cGMP → cGMP opens potassium channels → mV hyperpolarizes → vasorelaxation

Nitric oxide as a neurotransmitter in the CNS

• 1-2% of CNS neurons have NOS

• NOS affects:

  • NT release

  • synaptogenesis

  • apoptosis

  • synaptic plasticity

• glutamate release from pre-synaptic → onto NMDA(SOURCE OF CALCIUM IN POST-SYNAPTIC NEURON) → Ca2+ influx in post-synaptic neuron → nNOS activation → NO produced → forms cGMP AND diffuses to adjacent neurons and glia + retrograde to the pre-synaptic neuron

Carbon monoxide

• Heme→ CO (and Fe2+) using heme oxygenase(HO2)

  • O2 converted to biliverdin

  • NADPH converted to NADP+ and H2O

• Regulation of CO synthesis:

  • phosphorylation of HO2

  • End products of heme catabolism

Carbon monoxide as a neurotransmitter in the PNS

• Ca2+ influx → PKC activated → HO2 activation which converts heme to CO → CO diffuses to adjacent smooth muscle → sGuanylyl cyclase activated by CO converting GTP to cGMP→ cGMP activated K+ channels → hyperpolarizes membrane → vasorelaxation

Hydrogen sulfide

• enhances NMDA receptors

• increases LTP production

• studies show neuroprotective AND neurodegenerative

How can multiple neurotransmitters be released: 4 theoretical situations

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

   1. differential release

2. more than one NT at each synapse in the same vesicle

   1. corelease

3. a different transmitter at each synapse

   1. spatial segregation

4. a different mix of transmitter at each synapse

Purine transmitters

• when vesicle released so is ATP onto ATP receptors

• ectoATPase+ectonucleotidase: removes phosphate from ATP to form adenosine

• adenosine can also be received onto receptor

Use of ATP and adenosine as neurotransmitters

• ionotropic and metabotropic ATP receptors

  • P2X P2Y

• metabotropic adenosine receptors

  • P1

Evidence that GABA and glycine in the same synaptic vesicle

• looking at mIPSC of both doesn’t show if it’s in the same vesicle

• if you separate using antagonists:

  • strychnine is a glycine antagonist so you see GABA mIPSC

  • biccuculine is a GABA antagonist and shows glycine mIPSC

  • neither drug blocks the whole IPSC

Nerve terminals in spinal cord can release GABA+ glycine by expressing the required pre- and post-synaptic proteins for both

• glycine needs GLYT2

• GABA needs GAD

Using the sorting of vesicles in the golgi, neurons can send two different transmitter containing vesicles to different axon collaterals for use at different synapses

• preferential targeting of neurotransmitter by cellular machinery

Not all synapses are at the end of axons - some can be on dendrites

• olfactory bulb

• granule cell dendrite is releasing GABA