Nitrogen neurotransmission Flashcards
What is nitric oxide?
Nitric oxide is a gas and free radical, ●N=O
There are no storage pools of NO. It diffuses rapidly from the site of production and it is membrane permeant.
Labile, half-life of 100 ms
Can combine with superoxide or Hb for removal
NO is synthesised on demand and inactivated after use.
Suited for short range transmission.
Nitric oxide synthase
Multiple isoforms: neuronal, inducible, endothelial, mitochondrial.
Catalyses the reaction:
L-arginine + O2 → L-citrulline + NO
Reaction requires calcium and calmodulin.
The enzyme has recognition sites for electron donors like NADPH and FAD, FMN, BH4 and Heme.
NO signalling
NO increases the activity of soluble guanylyl cyclase 400-fold.
This generates cGMP which activates PKG, acts as a cation channel activator and increases intracellular calcium.
Phosphodiesterase (PDE) breaks down cGMP, so inhibitors can increase the actions of NO.
NO binds non-heme iron enzymes
It is involved in S-nitrosylation of proteins via cysteine groups.
Actions of glutamate in some cells in the brain cause an influx of calcium, which stimulates NOS to form NO.
NO also acts in the peripheral nervous system. ACh, VIP and NO act on the muscle cell to cause relaxation.
NO is a signalling molecule in other nerves in the PNS and has been shown to be a cotransmitter with ATP in nerves in the portal vein.
NO neurotransmission
Brain activity widespread but found in high densities in the cerebellum and accessory olfactory bulb.
Linked to NMDA signalling.
Involved in the regulation of excitability, firing, long-term potentiation and depression, learning and memory, neurotransmitter release, and development.
Excess NO is neurotoxic.
Vasodilator in cerebral vessels.
Also a vasodilator in the salivary glands, along with ACh and VIP.
NO neurotransmission occurs in myenteric neurons (GIT). Knock-out mice for nNOS show dilated stomach and pyloric hypertrophy.
In the penis, nNOS is localised to neurons of corpora cavernosae and blood vessels of penis, involved in erection.
In the urinary bladder, NO regulates outflow.
In the trachea, it causes bronchodilation.
In the rabbit portal vein, it causes vasodilation.
NOS inhibitors can be used to abolish responses and PDE inhibitors and NO donors to enhance them.
Evidence for NO as a neurotransmitter
nNOS immunolocalization
Non-adrenergic non-cholinergic (NANC) neurotransmission involved in smooth muscle relaxation is blocked by NOS inhibitors. This is reversed by L-arginine and NO donors.
nNOS knock-out mice show: decreased msucle relaxation, cGMP levels and neurophysiological response, and abnormal resting potentials. This effects are reversed by NO donors.
Cotransmission
NO is cotransmitted with: VIP, ACh, choline acetyltransferase, ATP, CO, glutamate, GABA, somatostatin, NPY.
Neuromodulation
NOS inhibitors suppress peptide outflow from parasympathetic nerves in the salivary gland, pancreas and intestine → NO enhances neuropeptide release
NO donors enhance and NOS inhibition reduces basal release of ACh in the brain→ NO enhances ACh release.
NO in disease - stroke
Brain ischemia and reperfusion leads to transient stimulation of the activity of eNOS, resulting in increases in endothelial NO generation, associated with neuroprotective actions in stroke.
In parallel, ischemic energy depletion and ROS production triggers the release of glutamate, which results in neuronal calcium overload, stimulation of nNOS and prolonged NO synthesis. iNOS is also induced in inflammatory cells.
Superoxide production is enhanced. Formation of peroxynitrite occurs, damaging lipids, proteins, DNA, and triggering the activation of PARP, contributing to neurotoxicity.
NO + O2.- → ONOO-
NO in disease - Parkinson’s
In MPTP models of Parkinson’s, there are elevated levels of nitrotyrosine in the ventral midbrain and striatum.
nNOS knockout mice were resistant to MPTP-induced neurotoxicity → pathogenic role of NO in PD.
PD brain showed ROS- and peroxynitrite-mediated oxidative and/or nitrosative damage, and increased nitrotyrosine accumulation in Lewy bodies.
Carbon monoxide as a neurotransmitter
CO is a stable neutral molecule with a relatively long half-life of 36-137 minutes.
Formed by heme oxygenase (HO)
Heme → biliverdin + Fe + CO
Reaction requires the electron donor NADPH-CYP450 reductase.
Multiple isoforms of the HO enzyme:
-HO1 – heat shock protein induced by cellular stress, concentrated in peripheral tissues e.g. spleen and liver
-HO2 – constitutive , highly concentrated in the brain.
-HO3 - constitutive
CO causes a 5-fold activation of soluble guanylyl cyclase.
HO2 knock-out mice show a 50% reduction in NANC transmission and cGMP levels in the intestine.
HO is colocalised with nNOS in myenteric neurons.
It also has actions in the Vas deferens.
HO2 is found in high densities in the brain. It is thought that CO cGMP signalling plays a role in LTP and memory.
