M.Prasad NaiduMSc Medical Biochemistry,

Ph.D.Research Scholar

Definition : - The chemical substance helpful for signal transmission in central nervous system &peripheral nervous system (via) the chemical synapses is neurotransmitters.

Synaptic transmission is the predominant means by which neurons communicate with each other.

The criteria for Chemical neurotransmitter1) found in presynaptic axon terminal.2) enzymes necessary for synthesis are

present in presynaptic neuron .3) stimulation under physiological conditions

results in release.4) mechanism exist for rapid termination of

action.5) direct application to postsynaptic terminal

mimics the activation of nerve stimulus.

6) drugs that modify metabolism of the neurotransmitter should have predictable physiological effects invivo assuming that the drug is transported to the site where neurotransmitter acts.

Not all neuron to neuron transmission is by neurotransmitters , gap junctions provides direct neuron to neuron electrical conduction.

Neurotransmitter is stored in synaptic vesicle released in response to nerve impulse & controled by calcium influx.

Release of neurotransmitter is quantal event , that is a nerve impulse reaching presynaptic terminal result in release of transmitter from a fixed number of synaptic vesicle.

Neurotransmitter action is terminated by metabolic degradation , reuptake , or diffusion into other cell types.

Class :- 1 acetylcholineClass : -2 The biogenic amines norepinephrine , epinephrine dopamine , serotonin .Class : - 3 amino acids gamma amino butyric acid (GABA) , glycine , glutamate , aspartate.Class : - 4 nitric acid (NO) carbonmonoxide ( co )

In addition to classical neurotransmitters many neuropetides are identified as definite or probable neurotransmitters,

eg : - substance p , neurotensin , enkephalin , β – endorphin , histamine,

vasoactive intestinal polypeptide, cholecystokinin , neuropeptide Y & somatostatin.

Neurotransmitters modulate the function of post synaptic cells by binding to specific receptors of 2 types

1) ionotropic receptors ( direct ion channels that open after binding of neurotransmitters. )

2) metabotropic receptors ( interact with G proteins stimulating production of second messengers & activating protein kinases , which modulate the cellular events. )

G proteins couple several receptors to intra cellular signaling system , linking neuronal excitability to energy metabolism & second messenger systems.

G protein binding receptors include adenosine , Ach ( muscarnic ), norepinephrine , dopamine , serotonin

Kinetics of ionotropic receptors are fast , (< 1 ms ) , because neurotransmitters directly alter the electrical property of the postsynaptic cell.

Kinetics of metabotropic receptors functions over longer time periods.

This contributes to the potential for selective & finely modulated signaling by neurotransmitters

The membrane of neuronal cell maintains an asymmetry of inside outside voltage , & is electrically excitable.

Neuronal membranes are polarized to a potential of -90 mV by the activity of Na+_k+ ATPase transport system.

Factors that control the neuroexcitability 1)voltage gated ion channels 2) neurotransmitter activated ion

channels. 3)neuromodulators 4)second messenger system.

The control of neuronal activity within normal limits is by the modulation of excitatory & inhibotory events simultaneously.

Ligand gated channels are responsible for communication between cells.

Voltage gated sodium channels are involved in propagation of action potential , rapid activation is at -60mV due to opening of fast transient channels.

Voltage gated potassium channels contribute to repolarization ,this regulate repeated firing of action potential by prolonging after spike repolarization.

Voltage dependent calcium channels trigger neurotransmitter release , at rapid activation is around -70mV.

Autoantibodies to ca++ channels in motor nerve terminal leads to decreased release of Ach from nerve terminal , this is seen in eaton lambert myasthenic syndrome.

Voltage gated channels determine how inhibitory & excitatory influences are integrated .

Acetyl cholineAcetyl choline is the neurotransmitter used

by all motor axons that arise from spinal cord, that is at neuromuscular junction.

Junction consist of a single nerve terminal separated from post synaptic region by synaptic cleft.

Motor end plate is the specialized portion of the muscle membrane involved in the junction.

Junctional folds are prominent they contain high density of Ach receptors.

Synthesis of Ach takes place in cytosol of nerve terminal .

choline acetyl transferase acetyl coA+ choline Ach + coAAch is incorporated into membrane bound

particle called synaptic vesicles.Assembly of synaptic vesicle with cell

membrane resembles assembly of transport vesicle involving SNAREs.

Release of Ach into synaptic cleft occurs by exocytosis , which involves fusion of vesicle with presynaptic membrane.

Nerve ending is depolarized by transmission of nerve impulse this opens the voltage gated Ca++ channels , permitting influx of Ca++ from synaptic cleft to nerve terminal , this Ca++ plays a role in exocytosis of Ach vesicle.

Approximately 200 vesicles are released into synaptic space.

Each vesicle contains 10000 molecules of Ach.

Ach binds Ach receptor , receptor undergoes conformational change opening the channel in the receptor that allows entry of Na+, k+ resulting in depolarization of muscle membrane.

Properties of Ach receptor of NMJ : nicotinic receptor (nicotine is an agonist

for the receptor) a membrane glycoprotein containing 5

subunits. ( 2αβγδ subunits). only α subunit binds Ach with high affinity. 2 molecules of Ach binds receptor to open

the ion channel which permits Na+ , K+ the receptor is thus transmitter gated ion channel.

autoantibodies to receptors are implicated in causation of myasthenia gravis

Snake venom α bungarotoxin binds tightly to the α subunit & can used to label the receptor .

Formation of autoantibodies to Ach receptors in NMJ

damage to receptors by autoantibodies

reduction in number of receptors

Episodic weekness of muscles supplied by cranial nerves

When the channel closes Ach dissociates & it is hydrolyzed by acetyl choline esterase.

acetyl choline esterase Ach + H2O Acetate +choline

Choline is recycled into nerve terminal by active transport , it can be used for synthesis of Ach.

The classical neurotransmitter of autonomic ganglia whether sympathetic or parasympathic is acetyl choline.

2 classes of receptors are present in autonomic nervous system.

1) nicotinic eceptors ,2) muscarnic recptors.

Nicotinic receptors in autonomic ganglia are different from those on skeletal muscle.

Nicotinc & muscarnic receptors mediate excitatory postsynaptic potentials (EPSP) , but these potential have different time course.

Stimulation of presynaptic neuron elicits a fast EPSP followed by a slow EPSP.

Fast EPSP results from activation of nicotinic receptors which cause of ion channels to open.

Slow EPSP is mediated by activation of muscarnic receptors that inhibit the M current , a current that is produced by K+ conductance.

Besides acetyl choline sympathetic preganglion neurons may release enkephalin , substance p , LHRH , neurotensin or somatostatin.

Neurotransmitter in parasympathetic postganglionic neurons is acetyl choline.

Actions are mediated by 3 types of muscarnic receptors.

1) M1 receptor (neural ) produces slow excitation of ganglia.

2) M2 receptor (cardiac) activation slows the heart.

3) M3 receptor (glandular) , causing secretion, contraction of visceral smooth muscle , vascular relaxation.

Muscarnic Ach receptors act by way of inosine triphosphate system & they may also inhibit adenyl cyclase & thus decreasing cAMP synthesis.

Muscarnic recptors also open or close ion channels particularly K+ or Ca++ this action occurs through G proteins.

Muscarnic receptors relax smooth muscle by an effect on endothelial cells which produces nitric oxide (NO) .

Nitric oxide ( NO ) relaxes smooth muscles by stimulating guanylate cyclase & there by increasing levels of cGMP which in turn activates cGMP dependent protein kinases.

The number of muscarnic receptors are regulated & exposure to muscarnic agonist decreases the number of receptors by internalization of rceptor.

The betz cells of motor cortex uses acetyl choline as their neurotransmitter.

Acetyl choline probably acts as an imporatant neurotransmitter in basal ganglia which is involved in control of movements.

Deficits in cholinergic path way in the brain implicated in some form of Alzheimer's disease.

GABA major fast inhibitory neurotransmitter in the fore brain. 30% synapses of C.N.S contain GABA.

Glutamic acid dehydrogenase synthesizes GABA from glutamate in nerve terminal .

3 types of receptors GABA a GABA b GABA c

GABA a & GABA c are ionotropic receptors & are post synaptic linked to chloride channel.

GABA b receptors are metabotropic may be pre or post synaptic & are coupled to ca+ or k+ ion channels via GTP proteins.

Presynaptic GABA b receptors serve autoreceptors to inhibit release from nerve terminal.

Binding of GABA leads to an opening of chloride channels & resultant hyperpolarization.

Glycine is inhibitory neurotransmitter in brain stem & spinal cord.

Post synaptic receptor for glycine is ligand gated chloride channel that allows influx of Cl- to hyperpolarize the postsynaptic neuron

Glutamate & aspartate are excitatory neurotransmitters.

Glutamate is responsible for 75% of excitatory neurotransmission in brain.

Synthesis of glutamate & aspartate within central neuron & glial cells is from carbohydrates involved in TCA cycle.

Mitochondrial enzyme aspartate transaminase interconverts glutamate & aspartate.

Glia contains glutamine synthase which converts glutamate to glutamine.

Glutamine is subsequently transferred to neuron where it is deaminated to glutamate by glutaminase.

Glial inactivation & specific uptake systems for glutamate reduces interstitial glutamate levels to terminate neurotransmitter action & prevent excitotoxic damage.

Monosodium glutamate produces migrainous head ache.

Excessive glutamate can result in neurotoxicity , celldeath & neurodegeration seen alzheimer’s disease.

The receptors are subdivided into 5 classes.1 )NMDA (N – methyl –D –aspartate )2 )AMPA (α amino 3 hydroxy 5 methyl 4

isoxazole propionic acid )3 )The kainate recptor ( isolated from sea

weed)4 )L –AP 4 ( synthetic agonist )5 )Metabotropic receptors.First four receptors are cation channels .

Metebotropic receptors are linked to intracellular production of diacylglycerol,& inositol triphosphate by phosphoinositide path way.

NMDA is receptor is complex contains 5 distinct sites for binding

1 ) site for transmitter binding glutamate 2 ) a regulatory site that binds glycine. 3 ) a voltage dependent Mg++ binding site 4 ) a site that binds phencyclidine 5 ) a site that binds Zn++.

NMDA receptor opens when glutamate binds & allows influx of Ca++ & Na++ into the cell.

Mg++ , zn++ , poly amines , & steroids can also modulate NMDA.

one of the most important controls on the ionic conductance through the NMDA receptor is voltage sensitive blocking by Mg++

Activation of AMPA receptor channels may depolarize the neuron sufficiently to remove the voltage dependent Mg++ block & activate NMDA channels.

AMPA & NMDA are co activated & are present on the same part of the neuron.

A separate site that modulates the gating of NMDA channel binds polyamines such as spermine & spermidine which are synthesized by neurons.different concentration dependent effects have observed.

Endogenous Zn reduces NMDA activated current.

Zinc is present in high concentrations in the hippocampus & released with some neurotransmitter in nervous system.

Hydrogen ions also modulate the ion conductance which is maximal at slightly alkaline pH , & reduces with increasing acidity.

During hypoxic ischemic injury , progressive acidification resulting from glycolytic metabolism , may turn off the NMDA receptor channel.

AMPA receptor is coupled to both Na++ , & K ++ channels. it’s activation opens the above channels, depolarizes the neuronal cell rapidly, it is responsible for the majority of rapid excitatory neurotransmission.

Kainate receptor is also coupled to Na++ , k++ channels.

Kainate receptor has slower rate of depolarizing capacity than AMPA receptor.

Excitatory aminoacids are also able to interact with metabotropic recptors that activate the second messenger system, these receptors are found both pre & post synaptically.

Activation result in presynaptic inhibition & post synaptic excitation.

The spectrum of neurological disorders mediated by excitotoxicity include epilepsy, stroke , neurodegerative disorders ( parkinson’s disease , amyotropic lateral sclerosis , AIDS dementia )

Most strokes are caused by thromboembolic events causing diminished perfusion resulting in reduction of supply of oxygen & glucose.

3 subsequent stages are there in the development of brain damage caused by ischemia.

1)induction ,2)amplification ,3)expression.

Induction : ischemia causes depolarization of the neuronal membrane leading to release of glutamate.

Glutamate overexcites the NMDA receptors in adjacent neuron , leading to abnormally large influxes of Ca++ & Na+ and resultant cell injury or death .

In addition glutamate stimulate AMPA – kainate receptor ( leading to additional influx of Na+ ) & also metabotropic receptors , causing the release of ITP & diacylgycerol.

Amplification : further build up of intra cellular calcium occurs by following mechanism ,

1) increased intracellular Na+ activates Na+ - Ca++ transporters.]

2)voltage gated Ca++ channels are activated by depolarozation.

3) ITP release Ca++ into cytosol from within endoplasmic reticulum.

Expression : high levels of intra cellular Ca++ activates Ca++ dependent nucleases , proteases , & phospholipases.

Degradation of phospholipids

formation of platelet activating factor (PAF) & release of arachidonic acid

eicosanoids (vasoconstriction)

damage by oxygen free radicals This is called glutamate cascade.

Huntington disease characterized by selective neuronal death in corpus striatum & glial proliferation .

Apoptosis , protein aggregation , & excitotoxins may all contribute cell death in huntington disease.

Excitotoxicity is by glutamate cascade.

The dopaminergic neurons are found in nigrostrital , mesolimbic , mesocortical tuberohypophysial systems.

Dopamine synthesis occurs from tyrosine , tyrosine hydroxylase is rate limiting enzyme in formation.

Entry of dopamine into synaptic vesicle is is driven by pH gradient established by a protein in vesicular membrane that pumps protons into vesicle at the expense of ATP

Release of dopamine involves exocytosis.

Dopamine has 5 post synaptic recptors D 1 receptor family(D1 & D5) D 2 receptor family(D2, D3, D4)D4 receptor exhibits 5 polymorphic variants.

The effect of dopamine is to increase direct path way by D 1 recptor, & supress indirect path way by D 2 receptor.

D 1 receptor activation augments adenylate cyclase ( linked to stimulatory G protein).

D 2 receptor activation decreases the activity of adenylate cyclase ( linked to inhibitory G protein ).

ATP dependent reuptake of dopamine achieved by a high affinity transporter in presynatic membrane , this is incorporated into vesicles & reused again.

Degradation of dopamine occurs within synaptic cleft or following reuptake ,within presynatic terminal.

Mono amino oxidase B present in the outer membrane of mitochondria & also in synaptic cleft.

MAO –B & MAO – A are distinguished from each other by preference for different substrates & by their different susceptibility to various inhibitors.

Both the above enzymes acts on dopamine to produce 3 – hydroxyphenyl acetaldehyde (DOPAC).

DOPAC converted to homovanillic acid by the action of catechol o methyl transferase.

parkinson disease is due to loss of dopaminergic activity & excessive cholinergic activity in basal ganglia.

Signs of parkinson disease reflects a deficiency of dopamine in the substantia nigra , corpus striatum ( caudate nucleus & putamen )

Basal ganglia are important for motor control they include putamen

caudate nucleus globus pallidum substantia nigra subthalamic nucleus .All circuits in basal ganglia are inhibitory

utilizing GABA except glutamatergic subthalamic input to globus pallidum internum(GPi) which is excitatory.

Cell damage in parkinson disease reflect a process of ageing , 13 % of cells of substantia nigra are lost per decade from 25 age onwards . ( parkinson disease rarely occurs befor 40 years )

Mutattions in gene encoding α synuclein , a presynaptic protein involved in neuronal plasticity is associated with parkinson disease.

Lewy bodies are found strongly stained with antibodies of α synuclein .

Signs of parkinson disease appear when the level of dopamine is droped in nigrosriatal system by 80%.

Exposure to high levels of Manganese ( miners) leads to parkinson disease.Reserpine inhibit dopamine storage & many

neuroleptics block dopamine receptors.

Schizophrenia is a manifestation of hyperdopaminergia .

Measurement of dopamine metabolite homovanillic acid in CSF is high in schizophrenics.

Level of D2 receptors appears to be increased in the brains of schizophrenics.

Dopamine mimetic drugs ( L dopa) induces schizophrenia

Low dopamine activity in prefrontal cortex of the brain of schizophrenics correlate well with the negative symptoms .

Low dopamine activity in prefrontal cortex releases the inhibitory action on subcortical dopamine neurons resulting in elevated dopaminergic activity.

Adrenergic neurotransmission is by norepinephrine & epinephrine.

The adrenergic neurons of locus ceruleus , pons , & medulla project to every area of brain & spinal cord.

Sympathetic postganglionic neurons typically release norepinephrine.

NE & E serve important role in the regulation of blood volume & blood pressure.

Norepinephrine is synthesized from tyrosine . dopamine β hydroxylase dopamine norepinephrine cu dopamine β hydroxylase is bound to inner

membrane of synaptic vesicle & release norepinephrine in a tetrameric glycoprotein form.

The overall system of epinephrine synthesis , storage & secretion from adrenal medulla are regulated by neuronal controls & also by glucocorticoid hormones synthesized in & secreted from adrenal cortex in response stress.

Secretion of epinephrine is signaled by neural response to stress , which is transmitted to adrenal medulla by way of a preganglionic acetyl cholinergic neuron.

A small number of neurons in the medulla contain phenyl ethenolamine –N- methyl tranferase enzyme that converts norepinephrine to epinephrine with SAM as methyl donor.

These neurons project to the thalamus, brainstem , spinal cord.

Concentration of epinephrine secreting terminals in the paraventricular nucleus suggests a role in secreton of oxytocin & vasopressin.

Dense innervation of dorsal motor nucleus of vagus , & nucleus solitarius suggets role in regulating cardiovascular & respiratory reflexes.

Receptors on target cells may be either α or β adrenergic receptors.

Receptors are further sub divided into α1, α2, β1 , β2 , β3.

α1 receptor are located postsynaptically but α2 receptors may be either pre or postsynaptic .

Receptors located presynaptically are autoreceptors inhibit release of neurotransmitter.

The effects of α1 receptors are mediated by activation of ITP/ diacyl glycerol second messenger system.

β receptors can be antagonized by action of α1 receptor.

α2 receptors decease the rate of synthesis of cAMP through an action on inhibitory G protein.

β1&β2 receptors activates stimulatory G protein to increase cellular cAMP levels.

Activation of β receptor result in coactivation of β adrenergic receptor kinase (BARK), this phosphorylates the receptor.

Phophorylation is prominent mechanism of receptor desensitization.

Number of β receptors is regulated .

β receptor is phosphorylated & desensitized their number also decreased if they become internalized.

β receptors can also be increased by denervation.

The number of α receptor is also regulated.

β1 adrenergic receptors principally found in heart & cerebral cortex.

β2 receptors principally found in lung & cerebellum.

β1 receptors equally prefer NE & E as agonist.

β2 receptors prefer epinephrine to norepinephrine.

In synaptic neurons norepinephrine decreases the amplitude of calcium spikes.

Excitatory effects of norepinephrine in various parts of CNS & sympathetic ganglion neuron results from α1 receptor activation. This activity primarily depends on blockade of a resting K+ conductance as a result neuron depolarizes & firing rate increases.

Inhibitory effects of norepinephrine results from α2 receptor activation , which results in increase of K+ conductance this hyperpolarizes the neuron & decreases it’s firing rates.

NE acting at α2 receptor also block Ca++ current.

Both the above inhibitory mechanisms account for the autoreceptor function of α2 receptors which decreases neurotransmitter release.

α1 adreno receptor activation results in , 1)vasoconstriction ,

2) relaxation of gastrointestinal smooth muscle,

3) salivary secretion , 4)hepatic glcogenolysis.

α2 adreno receptor activation results in 1) inhibition of transmitter release,

(including NE & Ach from autonomic nerves) 2) platelet aggregation 3) contraction of vascular smooth muscle. 4) inhibition of insulin release.

β1 adreno receptors activation results in 1) increased cardiac rate & force

β2 adreno receptors activation results in 1) bronchodilatation ,

2) vasodilation, 3) relaxation of visceral smooth muscle, 4) hepatic glycogenolysis, 5) muscle tremor. β3 adreno receptor activation results in

lipolysis.

The action of catecholamine neurotransmitters is terminated by reuptake into presynaptic neuron by specific transporters.

Enzymes involved in metabolism are catechol 0 methyl transferase & monoamino oxidase .

End product of norepinephrine & epinephrine metabolism is 3 methoxy 4 hydroxy mandelic acid.

serotoninMore than 95% of body’s serotonin is stored

in platelets & GI tract , only 5% is seen in brain.

Serotonin is distributed in brain regions that affect behaviour especially the hypothalamus & limbic system.

Availability of tryptophan is the main factor regulating synthesis of tryptophan.

Process of synthesis , storage , release , reuptake & degradation are similar to catecholmines.

Urinary 5 HIAA provides a measure of 5 –HT turn over.

Functions associated with 5-HT path ways 1)hallucination & behavioural changes, 2)sleep, wakefulness & mood , 3) feeding behaviour, 4)control of sensory pathway including

nociception, 5) vomiting.Serotonin receptors are metabotropic.Melatonin a derived product of 5-HT has

role in establishing circadian rhythm.

Histamine has neurotransmitter role in brain. Acts on metabotropic receptors.H1 receptors are excitatory & H2 , H3

receptors are inhibitory.H1 receptors in cortex & RAS contributes to

arousal & wakefulness.Has role in food & water intake,

thermoregulation

Nitric oxide synthase is present in many CNS neurons.

NO production is increased by mechanisms that raise intracellular Ca++ concentration( eg: transmitter action).

NO affects neuronal functions by increasing cGMP formation ,producing both inhibitory & excitatory effects on neurons.

ATP functions as neurotransmitter , it acts via ionotropic receptors as fast excitatory transmitter , via metabotropic receptors acts as neuro modulator.

Adenosine exerts inhibitory effects through metabotropic receptors.

Neurons contain CO generating enzyme, heme oxygenase , have role in cerebellum & olfactory neurons which have cGMP sensitive ion channels.