Neuromodulation is the physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons. A neuromodulator can be conceptualized as a neurotransmitter that is not reabsorbed by the pre-synaptic neuron or broken down into a metabolite. Neuromodulators typically bind to metabotropic, G-protein coupled receptors (GPCRs) to initiate a second messenger signaling cascade that induces a broad, long-lasting signal. This modulation can last for hundreds of milliseconds to several minutes. Some of the effects of neuromodulators include: alter intrinsic firing activity, increase or decrease voltage-dependent currents, alter synaptic efficacy, increase bursting activity and reconfiguration of synaptic connectivity.
Major neuromodulators in the central nervous system include: dopamine, serotonin, acetylcholine, histamine, norepinephrine, nitric oxide, and several neuropeptides. Cannabinoids can also be powerful CNS neuromodulators.
Acetylcholine
Acetylcholinesterase (EC 3.1.1.7, e.g. from Torpedo californica, TcAChE) hydrolysizes the neurotransmitter acetylcholine , producing group. ACh directly binds (via its nucleophilic Oγ atom) within the catalytic triad of (ACh/TcAChE structure 2ace). The residues are also important in the ligand recognition. After this binding acetylcholinesterase ACh.
Acetylcholine Receptors
Nicotinic acetylcholine receptors
The receptor is a transmembrane pentameric glycoprotein. It cylindrical in appearance by electron microscopy approximately 16nm in length and 8nm in diameter. The main ion channel is composed of a water pore that runs through the entire length of the protein. If viewed from the synaptic cleft, the protein will look like a pseudo-symmetrical rosette shown in the picture below composed of 10 different alpha and 4 different beta subunits.
When cobra venom is introduced into the body is moves along the bloodstream to a diaphragm muscle. It works as a postsynaptic neurotoxin binding to the receptor as an extracellular ligand by interacting with OH group leaving the acetylcholine channel open which releases ions used in creating an action potential. There must be 5 molecules of cobra toxin (red) to block the receptor (blue) as each molecule binds with an individual alpha chain on the acetylcholine receptor. The 2nd image depicts an individual toxin binding with one chain on the receptor, both in the same color. . This representation shows each molecule of the .
Muscarinic acetylcholine receptors
M1, M3, M5 receptors are coupled with Gq proteins, while M2 and M4 receptors are coupled with Gi/o proteins. They belong to GPCRs Subfamily A18.
Acetylcholinesterase
Adrenaline (Epinephrine)/Noradrenaline (Norepinephrine)
- .
- .
- Phenylethanolamine N-methyltransferase (Noradrenaline N-Methyltransferase) catalyzes the conversion of norepinephrine (noradrenaline) to epinephrine (adrenaline). This is the last step in the conversion of tyrosine to adrenaline[1].
The adrenergic receptors are metabolic G protein-coupled receptors. They are the targets of catecholamines. The binding of an agonist to them causes a sympathetic response.
- The α-2 adrenergic receptor (A2AR) inhibits insulin or glucagons release.
- The β-1 adrenergic receptor (B1AR) increases cardiac output and secretion of rennin and ghrelin.[2]
- The β-2 adrenergic receptor (B2AR) triggers many relaxation reactions.
β1 adrenergic receptor
- β1-adrenergic agonists:
- Beta blockers:
- Metoprolol
- Atenolol
- Bisoprolol
- Propranolol
- Timolol
- Nebivolol
- Vortioxetine
β2 adrenergic receptor
β2-adrenergic agonists:
-
- Beta blockers:
- Butoxamine
- Timolol
- Propranolol
- ICI-118,551
- Paroxetine
Monoamine oxidases (MAO)
Monoamine oxidases (MAO) (EC 1.4.3.4) are a family of enzymes that catalyze the oxidation of monoamines including adrenaline, noradrenaline, serotonin and dopamine.
Dopamine
Dopamine Receptors
Dopamine receptors are a class of metabotropic G protein-coupled receptors that are important in the central nervous system. Dopamine receptors are involved in many neurological processes that comprise motivation, pleasure, cognition, memory, learning, and fine motor skills. There are five subtype dopamine receptors, D1, D2, D3, D4, and D5. The D3 receptor is a part of the D2-like family.[3]
Agonists
- Amphetamine[4]
- Methamphetamine[5]
Antagonists
- Clebopride[6]
- Nafadotride[7]
- Eticlopride.
(3pbl).
.
Parkinson's disease
DOPA decarboxylase is responsible for the synthesis of dopamine and serotonin from L-DOPA and L-5-hydroxytryptophan, respectively. It is highly stereospecific, yet relatively nonspecific in terms of substrate, making it a somewhat uninteresting enzyme to study. Although it is not typically a rate-determining step of dopamine synthesis, the decarboxylation of L-DOPA to dopamine by DDC is the controlling step for individuals with Parkinson's disease[8], the second most common neurodegenerative disorder, occuring in 1% of the population over the age of 65. The loss of dopaminergic neurons is the main cause of cognitive impairment and tremors observed in patients with the disease. The hallmark of the disease is the formation of alpha-synuclein containing Lewy bodies.
Currently, treatment for the disease is aimed at DOPA decarboxylase inhibition. Since dopamine cannot cross the blood-brain barrier, it cannot be used to directly treat Parkinson's disease. Thus, exogenously administered L-DOPA is the primary treatment for patients suffering from this neurodegenerative disease. Unfortunately, DOPA decarboxylase rapidly converts L-DOPA to dopamine in the blood stream, with only a small percentage reaching the brain. By inhibiting the enzyme, greater amounts of exogenously administered L-DOPA can reach the brain, where it can then be converted to dopamine. [9]. Unfortunately, with continued L-Dopa treatment, up to 80% of patients experience 'wearing-off' symptoms, dyskinesias and other motor complications (referred to as the "on-off phenomenon". [10]. Clearly, a better understanding of the catalytic mechanism and enzymatic activity of DDC in both healthy and PD individuals is critical to drug design and treatment of the disease.
GABA
GABA receptors
GABA (i.e. gamma-aminobutyric acid) is the primary inhibitory neurotransmitter of the vertebrate central nervous system. GABA can bind one of two different receptor proteins, each using a discrete mechanism to elicit a cellular response. Upon binding with GABA, GABAB receptors (metabotropic) utilize a second messenger amplification pathway that ultimately results in an inhibitory signal for neuronal transmission. This pathway for signal transmission differs from GABAA receptors (ionotropic), which are considered ligand-gated ion channels as the binding of GABA results in the opening of ion channels leading to the inhibition of a neuronal signal.
(PDB code 4ms3).
Histamine
.
Histamine receptors
Allergy symptoms are mostly caused by the release of histamine in response to allergens. The binding of histamine to the extracellular portion of the H1 receptor triggers a structural change of the transmembrane portion, leading to a change in the C terminal area. This c terminal region interacts with G proteins, leading to the activation of the Gq signalling pathway, which triggers allergy symptoms like itchy eyes and runny noses. Many allergy drugs are anti-histamines, in that they bind to the histamine receptor but do not cause the conformational change that leads to a response. The H1 receptor is a histamine receptor belonging to the family of rhodopsin-like G-protein-coupled receptors. The H1 receptor is linked to an intracellular G-protein (Gq) that activates phospholipase C (see Unique bidirectional interactions of Phospholipase C beta 3 with G alpha Q and the inositol triphosphate (IP3) signalling pathway. When a ligand binds to a G protein-coupled receptor that is coupled to a Gq heterotrimeric G protein, the α-subunit of Gq can bind to and induce activity in the PLC isozyme PLC-β, which results in the cleavage of PIP2 into IP3 and DAG.
Serotonin
Serotonin receptors
5-hydroxytryptamine (5-HT), Serotonin receptors are found on the membrane of neurons in the central nervous system and peripheral nervous system. These receptors allow for the body to respond to serotonin and regulate many biological pathways. Serotonin, also known as 5 hydroxytryptamine, is an endogenous neurotransmitter made from tryptophan and is largely found in the gastrointestinal tract. It is known to regulate mood, appetite, digestion, circadian rhythm, learning and internal temperature regulation. It is can be an inhibitory or excitatory neurotransmitter that is released into the synaptic space and can bind to receptors on the postsynaptic neuron or be taken back up into the presynaptic neuron via Serotonin re-uptake transporters.[11] 5-HT receptors are classified into 7 different subfamilies (5-HT1, 5-HT2, 5-HT3, etc.) by signaling mechanisms and homology of structure. All 5-HT receptors are known to have G-protein linked pathways except for the 5-HT3 receptor which acts as an ion channel. [12]
Nitric Oxide
Neuronal Nitric Oxide Synthase (Nos1) is functioning in cell signaling and communication - a vital part of the nervous tissue.
