Neurotransmitters
Two general types: Small, nitrogen-containing moleculesNeuroactive peptides
Synthesis of small molecule NTs
Molecules and enzymes necessary for synthesizing small molecule NTs are contained in the presynaptic terminalThis ensures that the supply of NT can keep up with electrical activityNT synthesis is regulated by neuronal activity levels: synthesis occurs in the cytosol, but then NTs are packaged in vesicles to protect them from degradation and to prepare them for release
Cofactors in NT synthesis
Folic acidSAM(S-adenosylmethionine)O2Cu2Vitamins: C, B6 and B12
Storage in vesicles
NTs are concentrated into presynaptic vesicles, which are assembled in the terminal through a process of endocytosis that provides a mechanism for recycling materialNTs enter vesicles using transporter proteins in the vesicular membraneTransport depends on a vesicular ATPase that pumps protons into the lumenThe transporter exchanges H+ in the lument with NT in the cytoplasm This efficient mechanism allows vesicle sot concentrate NTs to 50-100mM levels
Reserpine � action
Blocks the vesicular transporterPrevents refilling of vesicles and inhibits synaptic transmission
Neurotransmitter Release
Depends on Ca influxFate of NT once released: 1. binding to presynaptic receptors2. binding to postsynaptic receptors3. diffusion out of synaptic cleft4. enzymatic degradation5. reuptake across the plasma membrane
Inotropic receptors
Composed of 4-5 subunits that form a pore in the membrane for passage of ionsDiversity comes from the variety of forms each subunit may haveSo different receptor sybtypes can exist in different locations with different physiological/pharmacological properties and functional roleSubunit composition can change developmentally
Metabotropic receptors
Transmitter binding is coupled to G protein activation and second messenter pathwaysDiversity comes form the different typs of G proteins that are coupled to receptors and the specific subunits associated with them
Reuptake by Plasma Membrane (pm) Transporters
Plasma membrane transporters efficiently allow nts and other molecules to cross the cell membranePm transporters depend oncotransport of Na+ and other ions to move transmitters into the terminal against their concentration gradientsThey can produce a 10,000 x increase in presynaptic nT concentration compared to extracellular spaceThis is a diverse group f molecules that is expressed within and outside the nervous system. Some transmitters have several transporter subtypes, which vary in location, specificity, and pharmacologyImplicationL the CNS changes molecular structure to match specific needs at specific locations � most drugs canno take advantage of these differences and consequently affect the entire class of molecules
Function of NT reuptake
1. terminate action of NT at receptor2. prevent NT diffusion to other synapses3. recycle supply of NT in presynaptic terminal
pm transporters can run in reverse
when NT levels are high intracellularly!
Some molecules: �false NTs�
Tyramine, guanethidine, ephedrine, amphetamineMimic NTs and bind to pm transporter to enter teminals and then bind to the vesicular transporter to enter vesiclesThey displace the real NTs, which accumulate in the terminal cytosolThis can result in a large, nonvesicular leak of real NT out of the terminal and massive stimulation of receptors. In addition, the false NTs may be released from synaptic vesicles to have reduced effects on post synaptic receptors (inhibitor or partial agonist)
Criteria for NTs
Synthesized in neuronStored in nerve terminalReleased in quantities sufficient to affect postsynaptic cellExogenous application mimics actionMechanism for removal
Amino Acid Neurotransmitters
Glutamate, aspartate, GABA, glycineThese AAs are common to all cells/neuronsTo be a transmitter, must be taken up into synaptic vesiclesEssential AAs cross the blood brain barrier (BBB) via transporters to enter brainHowever, AA neurotransmitters DO NOT CROSS BBB!: ! They are restricted from entryNTs must be synthesized by neurons and glia from TCA intermediates and other AAs
Glutamate Synthesis
70% synthesized from glutamine by glutaminase: .Glucose converted to alpha-ketoglutarate to glutamateThe major excitatory NT in the CNS
Glutamate receptors
Ionotropic � 14 possible subunits arranged in groups of 4 to 3 types: AMPA, Kainate, NMDAMetabotropic � 8 types: mGluR1-R8SIGNIFICANCE: potentially, many types of tetra/penta-meric ionotropic receptors can be made from different combinations of subunits, all responding to the same neurotransmitter
Reuptake by plasma membrane Glu transporter
The primary mechanism for inactivation of Glu in the synapsePmGlu transporter is found primarily on astrocytes (few on neurons)Glia: big role in Glu inactivation and recyclingAstrocytes take up glutamate, convert it to glutamine via glutamine synthetase and transport it out to extracellular environmentNeurons take up glutamine via a glutamine transporter and convert it to glutamate5 subtypes differing in affinity, specificity, locationHighly effective at lowering extracellular Glu concentrationElevated Glu levels are neurotoxic: !!! Glu transporter is important in buffering Glu especially if released in excessive amounts by neurons in pathological conditions
GABA Synthesis
Gamma-amino butyric acidGlutamate is converted to GABA by glutamic acid decarboxylase: (and from glutamine)The major inhibitory neurotransmitter in CNS; major importance in controlling potential for seizures, anxiety, sedation. Drugs facilitate receptor function
GABA vesicular transporter
Concentrates GABA in vesicles
GABA receptors
Ionotropic � GABA-A, GABA-C; metabotropic � GABA-B
GABA inactivation
Reuptake by neurons and glia4 different pmGABA transporters identified that differ in structure, type of cell found (neuron/glia/other), pharmacology
Glycine synthesis
Synthesized from glucose via glycolytic intermediates2- or 3-phosphoglycerate converted to Serine and Serine converted to glycine by addition of FH4Inhibitory transmitter in the brainstem and spinal cord
Glycine transported by
GABA receptors also transport glycine
Glycine receptor
One type ionotropic
Glycine Inactivation
Via pmGlycine transporter (several types) on neurons and glia
Monoamine Neurotransmitters
Dopamine, norepinephrine, epinephrine, serotonin
Catecholamines � 3 NTs and general characteristics
Dopamine, Norepinephrine, EpinephrineSynthesized by a small percentage of neurons but terminals have wide distribution to large areas of brainAct as excitatory and inhibitory neurotransmitters, but they also have powerful, modulatory effect (ingluence release of other transmitters) that influence motor activity, emotion, mood, attention, and arousalAll based on structure of catechol
Synthesis of catecholamines
All synthesized from tyrosine: or indirectly from phenylalanineRemember: phenylalanine (essential AA) converted to Tyrosine (nonessential AA) via PAHThe disorder PKU (PAH defect): results in low catecholamine levels
Transporter for catecholamines
One type of vesicular transporter in brain for ALL monoamines: butSecond type of vesicular monoamine transporter in adrenal medulla
Reserpine action
Inhibits vesicular transporter
Catecholamine receptors
All receptors are metabotropic (G-protein coupled)Affect ion channels directly or indirectly via second messenger pathwaysReceptor activation is complex, can cause excitation in some neurons, inhibition in others
Major mechanism for stopping synaptic action of monoamines
Reuptake into cellTwo types of pmCatecholamine transporters: dopamine and NE/E transporterImportance: 1. Terminates synaptic action 2. Limits diffusion to other synapses 3. Recycles unmetabolized transmitter for packaging in vesicles and its reuse
Catecholamine degradation
2 enzymes: MAO and COMT found intra and extracellularly in neurons and other cellsMAO: monoamine oxidaseCOMT: catechol-O-methyltransferase
Dopamine synthesis
Synthesized from tyrosine Tyrosine to Dopa: by tyrosine hydroxylase (usually saturated, which is why you give Dopa for Rx, not more tyrosine..)Then Dopa to Dopamine: by L-aromatic amino acid decarboxylase
Tyrosine hydroxilase
Rate-limiting enzyme in dopamine synthesisIt�s activity is saturated at normal levels of tyrosine in neuronTyrosine and pnehylalanine: cross the BBB via a single transporter � this transporter is also saturated at normal blood AA levelsTherefore, catecholamine synthesis cannot be increased by raising tyrosine levels!: !
L-aromatic amino acid decarboxylase (AADC)
has broad specificity for amino acid substratesalso present in many cell types outside of the nervous system
Carbidopa
doesn�t cross BBBinhibits peripheral AADC to prevent conversion of Dopa to dopamine peripherallyperipheral dopamine affects gut and causes nausea/vomiting
Dopamine source
Midbrain is the major source of dopaminergic neurons Also some in hypothalamus
Dopamine receptors
Metabotropic D1-5 Excitatory or inhibitory, depending on receptor type
Dopamine inactivation
By reuptake Via pmDopamine Transporter (DAT)Pm Transporter specific for dopamine: inhibited by cocaineAmphetamines interact with dopamine and NE transportersNeurotoxin MPP is a substrate for MPDopamine transporter: selectively kills dopaminergic neurons when internalized
Norepinephrine synthesis
Dopamine-Beta-HydroxylaseIs unique: bound to inner surface of synaptic vesicleNE is synthesized inside vesicle from dopamine: by Dopamine-beta-hydroxylaseThus, uses vesicular monoamine transporter
Norepinephrine source
Pons (locus ceruleus): is the major source of NE cell bodies for CNS. The locus ceruleus influences arousal. Also in postganglionic sympathetic neurons
Norepinephrine receptors
Multiple types of alpha/beta adrenergic receptors
Inactivation of Norepinephrine
Inactivation by reuptake via pmNE transporter (NET): Inhibited by several classes of antidepressants: tricyclics � imipramine, amytriptyline and Selective NE reuptake Inhibitors (SNRIs) � venflaxine, reboxetine; and cocaineThese drugs inhibit NE, DA and SERT transporters to varying degrees
Epinephrine synthesis
Pnehylalanine-N-methyltransferase converts NE to ERequires NE to exit vesicleUndergo conversion, and then transported back into the vesicle
Epinephrine source
Few E-neurons in the CNS, Epinephrine is primarily synthesized in adrenal medulla
Epinephrine receptors
Alpha/beta adrenergic receptors
Degradation of monoamines
by MAO and COMT
MAO
Present in neurons and most mammalian cellsIntracellular and extracellular locationIntracellularly localized to outer mitochondrial membrane: degrades monoamines not protected inside vesicles by deamination to aldehyde
Functions of MAO:
degrades monoamines in neurons/regulates general neurotransmitter levelDietary monoamines act as �false neurotransmitters�MAO also:1. decreases availability of dieatary monoamines in peripheral tissues (gut)2. prevents their entry across BBBMAOa and MAOb forms: differ in CNS location, substrate specificity, pharmacology
MAOa
Distribution:CNS and gutSubstrate specificity: all monoamines BUT preference serotonin > NE > DopaminePresent in gut and liver to breakdown dietary monoaminese.g. tyramine in cheese and transporter and concentrates in vesicles via the vesicular monoamine transporter where it displaces NEIrreversibly inhibited by clorgyline: !
MAOb
Distribution: CNS (astrocytes, serotonergic neurons, histaminergic neurons)Specificity: all monoamines but preference for beta-phenylethylamineIrreversibly inhibited by selegiline: !
MAO inhibitors (MAOIs)
Nonspecific irreversible inhivitors: tranylcypromine, phenelzine, isocarboxazidNewer MAOIs are more selective and reversibleIncrease presynaptic concentration of neurotransmitters and prolong availability of released neurotransmitter
Caution on dangerous interactions
When combined with foods containing tyramine (beer, red wine, cheese, salami, soy sauce, fava beans, liver), may result in release of large amounts of NE, inducing hypertensive crisis.Why: MAO normally metabolizes tyramine in gut. Excess tyramine displaces NE in sympathetic vesicles and NE is released at synapses by reversal of the pmNE transporter
COMT
Present in nervous system and peripheral tissues; present extracellularly in synaptic cleft and degrades neurotransmitter after releaseBroad catechol substrate specificityMethylates (SAM cofactor) one of the catechol hydroxyl groupsInhibitors include: entacapone, tolcapone
Indolamines: Serotonin synthesis
5-hydroxytryptamine/5-HTfrom tryptophan, an essential AA transported across BBBsynthesis similar to dopamine: tryptophan hydroxylase is rate-limiting enzyme
Serotonin concentrated by
Vesicular monoamine transporter
Serotonin Source:
serotonergic cell bodies: are located in midline (raphe) nuclei of the pons and medullaaxons distribute widely to the cortex and spinal cord
Serotonin Receptors
14 different receptoras identified so far (5-HT1, etc): all metabotropic, except 5-HT3(inotropic) � different roles in brain functionAll hallucinogenic drugs are 5HT2Apartial agonistsMany antipsychotics are 5-HT2A and D2 dopamine receptor antagonists
Inactivation of serotonin
Synaptic action stopped primarily by reuptake: via specific pmSerotonin transporter (SERT)Inhibitors: many antidepressants (SSRIs � selective serotonin reuptake inhibitors, tricyclics) bind with high affinity; also cocaineThese drugs inhibit NE, DA, and SERT transporters to varying degreesSerotonin also metabolized: by MAO
Histamine Synthesis
From histidine (essential AA) via AADC
Histamine Source:
Most histaminergic cell bodies are located in the Hypothalamus: !
Histamine vesicular and pm reuptake transporters
Are presumed but have not been identified
Histamine receptors
2 sybtypes of metabotropic receptors
Histamine metabolized by
MAO and histamine methyltransferase
Histamine Functions:
Involved in circuits in hypothalamus that control/maintain arousalCommon antihistamines cross BBB and are neuronal H1 receptor antagonistsThey cause sedation. Newer, 2nd generation antihistamines (loratidine) don�t cross BBB and don�t sedate
Acetylcholine (ACh)
The first identified neurotransmitter by Otto Loewi working on vagus nerve
ACh synthesis
Acetyl-CoA + Choline -> CoA + acetylcholine -> choline + acetateOne enzymatic step involving: choline acetyltransferase
Choline:
derived primarily from the diet and is transported across the BBBAlternatively, choline can also be synthesized from the membrane lipid phosphatidylcholine via phosphatidylethanolamine (requires folate and vitamin B12)Synthesis of acetylcholine is limited by availability of cholineCholine enters neuron via pmCholine transporter
Source of ACh
In CNS: cholinergic cell bodies are located primarily in nuclei in the pons and lower frontal lobePeripherally: all cranial nerve and spinal motoneurons, preganglionic sympathetic and parasympathetic neurons, and postganglionic parasympathetic neurons
Vesicular ACh transporter
Concentrates ACh into vesiclesIs inhibited by vesamicol: causes depletion of ACh from vesicles
ACh receptors
Ionotropic: nicotinic � composed of combinations of subunits, which could result in many potential types of inotropic ACh receptorsMebatropic: muscarinic: 5 subtypes (G-protein coupled) identified M1-M5
ACh Inactivation
Ionotropic ACh receptors desensitize rapidlySo ACh MUST be removed quickly fro synapsesEnzymatic degradation: major sroute for inactivation of ACh by acetylcholine esterase, which is located in the synaptic cleft (concentrated in postsynaptic membrane) and also intracellularlyThe resulting choline (generated by degradation) undergoes reuptake via a pmCholine transporterReuptake is the major regulator for ACh synthesisNO REUPTAKE TRANSPORTER FOR ACh, but have pmCholine transporter
Anticholinesterases
Block enzymatic activity of acetylcholine esteraseCause accumulation of ACh at synapses, aCh receptor desensitization and inactive receptors
Reversible inhibitors of anticholinesterases:
Block activity for several hours or lessphysostigmine and tacrine: Cross BBB!Neostigmine and edrophonium: don not cross BBB
Irreversible inhibitors of anticholinesterases
Completely inhibit ACh breakdown and require new synthesis of acetylcholine esterase to replenish normal enzymatic activityThese include: insecticies and nerve gases such as sarin, which can result in death within 5 mins due to respiratory failureAntidote involves treatment with nicotinic and muscarinic antagonists (atropine)
Other types of small molecule neurotransmitters
Purines: ATP, adenosineImportant neurotransmitter in pain system. Peripheral pain fibers have purinergic receptors damaged tissues release ATP, causing excitation!Adenosine receptors: are metabotropic and caffeine is an antagonistMembrane-soluble molecules: NO and arachidonic acid
Neuroactive Peptides
Small polypeptides of 5-41 AAs act as NTs to adjacent neurons, they can enter the circulation to act as hormones on distant target organs in the body, and they act as neuromodulators of activity nad behavior by influencing release of other transmitters over long periods of timeTheir synthesis, packaging into vesicles, processing in presynaptic terminals, and function are DIFFERENT from that of the small molecule NTs
Neuropeptide synthesis
Requires DNA transctiprion and mRNA translationTo produce a proteinGenerally neuropeptides are synthesized as large precursor polypeptides (prepropeptides): that are subsequently cleaved into smaller molecules in a multistep process. Thus, each precursor may give rise to many different smaller peptides that each have bioactivityLocation of synthesis: in cell body on ribosomes, they subsequently are processed through the endoplasmic reticulum, and then are transferred to the Golgi Apparatus where they are packaged into vesicles.Vesicles containing neuropeptides travel by axoplasmic transport down the axon to presynaptic terminals
Presynaptic vesicles of neuropeptides
Undergo caocium-dependent release
Inactivation of neuropeptides
Is slow and depends on extracellular proteases, which results in long-lasting effectsThere are NO reuptake transporters for neuropeptides: so they cannot re-enter the presynaptic terminal Synaptic transmission with neuropeptides: therefore depends on a continuous supply from the cell body
Neuropeptides additional info
Can be co-released with small molecule neurotransmitters from the same terminalThey utilize a large variety of receptors, which are metabotropic, G-protein coupled
Mechanisms of action of some drugs at the synapse
1.NT synthesis: levodopa is converted to dopamine and therefore increases the levels of dopamine; also MAO-I � selegiline2.Storage in Vesicles: reserpine � blocks the concentration of NT in vesicles � blocks vesiculat transport of NTs3.Ca entry: none really4.Neurotransmitter release: amphetamine, amantadine5.Binding to receptors: agonists � benzodiazepens, baclofen, opiods; antagonists � antipsychotics, pyridostigmine, edrophonium, naloxone6.Degradation in cleft, metabolism, or diffusion: MAO-I � selegiline; COMT-I � entacapone; AChE-I � neuromusc/CNS7.Reuptake transporter: cocaine, antidepressants � decrease the function of the reuptake transporter, therefore increasing the concentreation of NT in the synaptic cleft8.Recycling vesicles: no drugs really9.Modulation by presynaptic receptors10.Alpha-2 agonist/antagonist