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An acetyltransferase is another name for a transacetylase, an enzyme which catalyzes the transfer of an acetyl group from one molecule to another.
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What does sat1 mean?
"Sat1" is the name of a television broadcaster in Germany.or"Spermidine/spermine N1-acetyltransferase 1", a human gene.
What has the author Paul H Volkman written?
Paul H. Volkman has written: 'Neurinal regulation of pineal N-acetyltransferase activity'
Melatonin comes from where?
Melatonin is made from serotonin (5-hydroxytryptamine) and N-acetylserotonin in the pineal gland by arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase.
What agent blocks enzymes resulting in a cholinergic crisis?
There are many. The enzymes that can be affected are choline acetyltransferase (for making acetylcholine), and acetylcholinesterase (for breaking down acetylcholine). The most commonly used enzyme inhibitors affecting the cholinergic system are the acetylcholinesterase inhibitors, such as physostigmine, or neostigmine, etc.
Where is melatonin made?
Melatonin is made from the amino acid tryptophan. Tryptophan is an essential amino acid, that is, the body cannot make it; we need to get it through the foods we eat. Tryptophan is found in a wide variety of foods. As we consume tryptophan during the day, the body converts it into serotonin, an important brain chemical involved with mood. Serotonin, in turn, is converted into melatonin. This conversion occurs most efficiently at night.
What is oligonucleotides?
Transcription Factor oligodeoxynucleotide decoys (ODN decoys). Use our GeneDetect® transcription factor decoys to inhibit specific transcription factors in cell culture. Complete list of Transcription Factor decoy Products. IntroductionTranscription factor ODN decoy approachAdvantages and disadvantages of the ODN decoy approach for studying cellular gene expressionODN Decoys available from GeneDetect.comHow are these decoys used?ControlsReferencesIntroductionCells can respond to stimuli (normal or pathological) by changing the levels of expression of specific genes. The cellular proteins that regulate changes in gene expression are called transcription factors. Transcription factors are generally nuclear and can either be constitutively expressed within the cell (present under basal conditions, for example CREB) or themselves inducible (for example AP-1). These transcription factor proteins bind specific sequences found in the promoter regions of genes (target genes) whose expression they then regulate (switch on or off). These binding sequences are generally 6-10 base pairs in length and are occasionally found in multiple copies within the promoter regions of target genes. Although the transcription factor protein-DNA interaction is sequence-specific, the binding site for one given transcription factor may vary by several base pairs within different target genes. Therefore when we describe the specific DNA binding sequence for a transcription factor we refer to the non-variable part of the binding sequence, that is, the transcription factor consensus sequence. For example, the AP-1 transcription factor made up of Fos and Jun proteins binds to the TGACTCA consensus sequence. In comparison the consensus sequence for the Smad transcription factor family which mediate TGF-b, activin and BMP induced changes in gene expression is CAGACA. Fig. 1. Transcription factor ODN decoy approach. The basic theory behind the transcription factor ODN decoy approach involves flooding the cell with competing synthetic, transcription factor-specific consensus sequences. These synthetic decoys "compete" for binding of the transcription factor with consensus sequences in target genes. If delivered into the cell in sufficient concentrations these "decoys" thus have the potential to attenuate the binding of the transcription factor to promoter regions of target genes and thus attenuate the function of the transcription factor to regulate the expression of its target gene(s). Transfected at high concentrations these decoys have been reported in the literature to completely block transcription factor function. Clearly they represent powerful research tools for studying gene regulation both in vitro and also more recently in vivo (for Reviews see Moshita et al., 1998, Mann and Dzau, 2000). Fig. 2. Advantages and disadvantages of the ODN decoy approach for studying cellular gene expression. Advantages. # ODN decoys offer a means of specifically inhibiting transcription factor function in living cells. # Inexpensive compared to other more classical methods of investigating gene expression such as chloramphenicol acetyltransferase and luciferase constructs in promoter-reporter gene transfection experiments. # Allows for investigation of both endogenous and pathological gene regulation # Proven to be highly effective and selective within in vitro experiments. # Easy to use. Disadvantages. # An emerging technology that has not yet been fully characterized # Issues of decoy synthesis. High levels of purity and stability required. # Transfection issues. Which method is best. How to optimize transfection. # Issue of controls. What controls are needed. ODN Decoys available from GeneDetect.com Click here for a full listing. We have designed ODN decoys to over 45 common transcription factors. Our decoys are double-stranded synthetic phosphorothioate deoyxynucleotides which range in length from 20-28 base pairs. The transcription factor consensus sequence occurs within the middle of the decoy sequence and is flanked by carefully selected base-pairs that allow for "optimized" transcription factor binding. These ODN decoys are also available labeled so that you are able to optimize your specific transfection technique by imaging the passage of the decoy into the cell (for example by fluorescence microscopy). Our ODN decoys are purified by HPLC and assessed by gel electrophoresis to ensure that >99% of decoy supplied represents full length, double stranded, functional decoy. As a control, matching mutant decoys are available for each transcription factor. Mutant decoys have the same flanking sequences but contain a disrupted consensus sequence in comparison with the (wild type) ODN decoy. How are these decoys used?The majority of experiments to date have used transcription factor ODN decoys to examine gene regulation in cultured primary cells and cell lines. The most important variables involved in determining whether or not your ODN decoy performs its required function include (a) the combination of cell type/cell density and transfection reagent used (b) the time cells are incubated with ODN decoy and (c) the concentration of ODN decoy used. A. The combination of cell type/cell density and transfection reagent used. While some investigators have achieved success by simply adding naked ODN decoys directly into the cell culture media, the most common method of in vitro ODN decoy transfection is to mix the ODN decoy with a cationic lipid to form a liposome complex before adding the ODN decoy/liposome mixture directly to the media. To aid transfection therefore we recommend mixing of your ODN decoy with an effective liposome-based carrier substance. One transfection reagent we have had good success with in our laboratories is the OligofectAMINE reagent. This is a proprietary formulation available from Invitrogen that is designed to optimize transfection of phosphorothioate ODNs into eukaryotic cells. Stable complexes are formed between the lipid and the ODN permitting efficient delivery of the ODN into mammalian cells. This product represents an improvement over the Lipofectin reagent in respect to transfection of ODNs. Please follow the manufacturers guidelines for use of this product. Product sheets are available via their website. Other transfection reagents we have had previous success with include FuGene 6 from Roche Diagnostics and Superfect Transfection Reagent from Qiagen. Obviously certain cell types are more susceptible to transfection than others and certain liposome "carriers" perform better with certain cell types. Therefore an amount of trial and error may be required to optimize transfection under your specific conditions. It is therefore helpful to have a way of measuring the kinetics and efficiency of transfection of your ODN decoy. One way of doing this is to use either biotin or fluorescently labeled ODN decoys. After incubation of ODN decoys with your cells you can assess transfection efficiency by fluorescent microscopy or biotin detection. With successful transfection you should expect to see a strong nuclear signal with weaker but noticeable signal in the cytoplasm in 60-90% of your cells. We have noticed that the transfection efficiency of ODN decoys and indeed ODNs in general (for example antisense ODNs) is much more sensitive to cell density than that of plasmid DNA. Therefore we recommend that a standard seeding protocol be maintained from experiment to experiment and that cell density be varied, if required, to optimize transfection efficiency. B. The time cells are incubated with ODN decoy. The time of incubation of cells with ODN decoys is critical. While there is no standard time of incubation due to the many other variables that can affect the incubation time required (including but not limited to ODN decoy concentration, cell type and transfection reagent used) an incubation time of 8hrs (minimum) to 24-28 hrs (maximum without re-addition of ODN decoy) is suggested. Significant ODN decoy degradation has been reported to occur after incubation periods of longer than 24 hrs. Obviously frequent re-addition of ODN decoy could be used to provide continuous blockade of transcription factor functionality beyond 24 hrs if required. C. The concentration of ODN decoy used. Within the recent literature ODN decoy concentrations of up to 5mM appear to be well tolerated and highly effective in most cell types with little or no observable effect on cell viability. With the newer transfection reagents (such as OligofectAMINE) a final ODN decoy concentration within the range of 0.1-2µM will be sufficient to block transcription factor activity without inducing non-specific cellular toxicity. Controls. To confirm that the effects of the ODN decoy are due to a consensus sequence-specific inhibition of transcription factor functionality rather than a non-specific effect of the ODN decoy on cell viability or functioning we recommend using our matching mutant ODN decoys as controls in each experiment. Mutant decoys have the same flanking sequences but contain a disrupted consensus sequence that does not bind transcription factor. References Morishita, R., Higaki, J., Tomita N. and Ogihara T. (1998) Application of transcription factor "decoy" strategy as means of gene therapy and study of gene expression in cardiovascular disease. Circ Res 82, 1023-1028. Mann, M.J. and Dzau, V.J. (2000) Therapeutic applications of transcription factor decoy oligonucleotides. J. Clin. Invest. 106, 1071-1075. Products | Accounts | FAQ | Contact | Search | Home Terms of Use | Privacy Policy | Shopping Basket | Quotes | CheckoutCopyright © 2000-2007 GeneDetect.com Limited
What is decoy oligonucleotides?
Transcription Factor oligodeoxynucleotide decoys (ODN decoys). Use our GeneDetect® transcription factor decoys to inhibit specific transcription factors in cell culture. Complete list of Transcription Factor decoy Products. IntroductionTranscription factor ODN decoy approachAdvantages and disadvantages of the ODN decoy approach for studying cellular gene expressionODN Decoys available from GeneDetect.comHow are these decoys used?ControlsReferencesIntroductionCells can respond to stimuli (normal or pathological) by changing the levels of expression of specific genes. The cellular proteins that regulate changes in gene expression are called transcription factors. Transcription factors are generally nuclear and can either be constitutively expressed within the cell (present under basal conditions, for example CREB) or themselves inducible (for example AP-1). These transcription factor proteins bind specific sequences found in the promoter regions of genes (target genes) whose expression they then regulate (switch on or off). These binding sequences are generally 6-10 base pairs in length and are occasionally found in multiple copies within the promoter regions of target genes. Although the transcription factor protein-DNA interaction is sequence-specific, the binding site for one given transcription factor may vary by several base pairs within different target genes. Therefore when we describe the specific DNA binding sequence for a transcription factor we refer to the non-variable part of the binding sequence, that is, the transcription factor consensus sequence. For example, the AP-1 transcription factor made up of Fos and Jun proteins binds to the TGACTCA consensus sequence. In comparison the consensus sequence for the Smad transcription factor family which mediate TGF-b, activin and BMP induced changes in gene expression is CAGACA. Fig. 1. Transcription factor ODN decoy approach. The basic theory behind the transcription factor ODN decoy approach involves flooding the cell with competing synthetic, transcription factor-specific consensus sequences. These synthetic decoys "compete" for binding of the transcription factor with consensus sequences in target genes. If delivered into the cell in sufficient concentrations these "decoys" thus have the potential to attenuate the binding of the transcription factor to promoter regions of target genes and thus attenuate the function of the transcription factor to regulate the expression of its target gene(s). Transfected at high concentrations these decoys have been reported in the literature to completely block transcription factor function. Clearly they represent powerful research tools for studying gene regulation both in vitro and also more recently in vivo (for Reviews see Moshita et al., 1998, Mann and Dzau, 2000). Fig. 2. Advantages and disadvantages of the ODN decoy approach for studying cellular gene expression. Advantages. # ODN decoys offer a means of specifically inhibiting transcription factor function in living cells. # Inexpensive compared to other more classical methods of investigating gene expression such as chloramphenicol acetyltransferase and luciferase constructs in promoter-reporter gene transfection experiments. # Allows for investigation of both endogenous and pathological gene regulation # Proven to be highly effective and selective within in vitro experiments. # Easy to use. Disadvantages. # An emerging technology that has not yet been fully characterized # Issues of decoy synthesis. High levels of purity and stability required. # Transfection issues. Which method is best. How to optimize transfection. # Issue of controls. What controls are needed. ODN Decoys available from GeneDetect.com Click here for a full listing. We have designed ODN decoys to over 45 common transcription factors. Our decoys are double-stranded synthetic phosphorothioate deoyxynucleotides which range in length from 20-28 base pairs. The transcription factor consensus sequence occurs within the middle of the decoy sequence and is flanked by carefully selected base-pairs that allow for "optimized" transcription factor binding. These ODN decoys are also available labeled so that you are able to optimize your specific transfection technique by imaging the passage of the decoy into the cell (for example by fluorescence microscopy). Our ODN decoys are purified by HPLC and assessed by gel electrophoresis to ensure that >99% of decoy supplied represents full length, double stranded, functional decoy. As a control, matching mutant decoys are available for each transcription factor. Mutant decoys have the same flanking sequences but contain a disrupted consensus sequence in comparison with the (wild type) ODN decoy. How are these decoys used?The majority of experiments to date have used transcription factor ODN decoys to examine gene regulation in cultured primary cells and cell lines. The most important variables involved in determining whether or not your ODN decoy performs its required function include (a) the combination of cell type/cell density and transfection reagent used (b) the time cells are incubated with ODN decoy and (c) the concentration of ODN decoy used. A. The combination of cell type/cell density and transfection reagent used. While some investigators have achieved success by simply adding naked ODN decoys directly into the cell culture media, the most common method of in vitro ODN decoy transfection is to mix the ODN decoy with a cationic lipid to form a liposome complex before adding the ODN decoy/liposome mixture directly to the media. To aid transfection therefore we recommend mixing of your ODN decoy with an effective liposome-based carrier substance. One transfection reagent we have had good success with in our laboratories is the OligofectAMINE reagent. This is a proprietary formulation available from Invitrogen that is designed to optimize transfection of phosphorothioate ODNs into eukaryotic cells. Stable complexes are formed between the lipid and the ODN permitting efficient delivery of the ODN into mammalian cells. This product represents an improvement over the Lipofectin reagent in respect to transfection of ODNs. Please follow the manufacturers guidelines for use of this product. Product sheets are available via their website. Other transfection reagents we have had previous success with include FuGene 6 from Roche Diagnostics and Superfect Transfection Reagent from Qiagen. Obviously certain cell types are more susceptible to transfection than others and certain liposome "carriers" perform better with certain cell types. Therefore an amount of trial and error may be required to optimize transfection under your specific conditions. It is therefore helpful to have a way of measuring the kinetics and efficiency of transfection of your ODN decoy. One way of doing this is to use either biotin or fluorescently labeled ODN decoys. After incubation of ODN decoys with your cells you can assess transfection efficiency by fluorescent microscopy or biotin detection. With successful transfection you should expect to see a strong nuclear signal with weaker but noticeable signal in the cytoplasm in 60-90% of your cells. We have noticed that the transfection efficiency of ODN decoys and indeed ODNs in general (for example antisense ODNs) is much more sensitive to cell density than that of plasmid DNA. Therefore we recommend that a standard seeding protocol be maintained from experiment to experiment and that cell density be varied, if required, to optimize transfection efficiency. B. The time cells are incubated with ODN decoy. The time of incubation of cells with ODN decoys is critical. While there is no standard time of incubation due to the many other variables that can affect the incubation time required (including but not limited to ODN decoy concentration, cell type and transfection reagent used) an incubation time of 8hrs (minimum) to 24-28 hrs (maximum without re-addition of ODN decoy) is suggested. Significant ODN decoy degradation has been reported to occur after incubation periods of longer than 24 hrs. Obviously frequent re-addition of ODN decoy could be used to provide continuous blockade of transcription factor functionality beyond 24 hrs if required. C. The concentration of ODN decoy used. Within the recent literature ODN decoy concentrations of up to 5mM appear to be well tolerated and highly effective in most cell types with little or no observable effect on cell viability. With the newer transfection reagents (such as OligofectAMINE) a final ODN decoy concentration within the range of 0.1-2µM will be sufficient to block transcription factor activity without inducing non-specific cellular toxicity. Controls. To confirm that the effects of the ODN decoy are due to a consensus sequence-specific inhibition of transcription factor functionality rather than a non-specific effect of the ODN decoy on cell viability or functioning we recommend using our matching mutant ODN decoys as controls in each experiment. Mutant decoys have the same flanking sequences but contain a disrupted consensus sequence that does not bind transcription factor. References Morishita, R., Higaki, J., Tomita N. and Ogihara T. (1998) Application of transcription factor "decoy" strategy as means of gene therapy and study of gene expression in cardiovascular disease. Circ Res 82, 1023-1028. Mann, M.J. and Dzau, V.J. (2000) Therapeutic applications of transcription factor decoy oligonucleotides. J. Clin. Invest. 106, 1071-1075. Products | Accounts | FAQ | Contact | Search | Home Terms of Use | Privacy Policy | Shopping Basket | Quotes | CheckoutCopyright © 2000-2007 GeneDetect.com Limited
What breaks down neurotransmitters?
After Galvani had shown, in 1742, that electrical stimulation of the nerve to the muscle of a frog's leg caused the muscle to twitch, the idea gained ground that transmission from nerves to the 'end organ' was an electrical process. Today we know that only in very rare instances is transmission across a http://www.answers.com/topic/synapse - that is, between the end of a nerve and whatever it innervates - an electrical event. Virtually all neurotransmission is chemical. Nerves release one or more neurotransmitters, which act chemically on receptors in the membrane of the cells across the synaptic cleft. To detect neurotransmitters is a difficult task as the amounts released are minute and mechanisms exist that quickly remove the transmitter, leaving the system in a state of readiness for the arrival of the next nerve impulse. The first steps came from finding chemicals which could mimic the action of neurotransmitters. Muscarinewas able to mimic the effects of stimulation of the http://www.answers.com/topic/heart by the vagus nerves, and its actions were blocked by atropine. Similarly, nicotinemimicked the effect of stimulating motor nerves to http://www.answers.com/topic/skeletal-muscle, and this was blocked by curare.Du Bois Reymond, in 1877, declared that chemical or electrical transmission were the only two real alternatives, although there was no data to decide unequivocally between the two. In 1904 Elliot suggested that adrenaline might be the transmitter in the sympathetic nervous system, and in the next year a nicotine-like substance was postulated by Langley as the transmitter in motor nerves to muscles. Dixon, in 1907, tried to extract from a heart the neurotransmitter released by the vagus, measuring the effect of the extract on another heart, but reached no conclusions. It was 1921 before Loewi performed the celebrated experiment in which two beating frog hearts were superfused in series, the perfusate of the first flowing over the second. Stimulation of the vagus nerve to the first heart caused its beat to slow and, after a brief interval, the second heart slowed too. While this experiment did not prove that the substance flowing over from the first heart was released from the vagus nerve, clearly something chemical rather than electrical slowed the second heart.In the 1930s Dale and his school in England showed that http://www.answers.com/topic/acetylcholine was the transmitter at motor nerve endings in skeletal muscle (http://www.answers.com/topic/neuromuscular-junction-1) and Cannon and his colleagues in the US demonstrated that an adrenaline-like substance was the transmitter in the sympathetic nervous system. These discoveries depended on the development of sensitive bioassays for transmitter substances. Dale was not convinced that the sympathetic neurotransmitter was http://www.answers.com/topic/adrenaline itself, because he demonstrated subtle differences between the responses to the transmitter and to adrenaline. Von Euler, in 1946, finally showed that the sympathetic neurotransmitter was http://www.answers.com/topic/noradrenaline-2 (non-methylated adrenaline).After this period there was a lull in which many thought that acetylcholine and noradrenaline were the only two neurotransmitters. After all, the transmitter at the neuromuscular junction in skeletal muscle, in the synapses in ganglia of the autonomic nervous system, and at the ends of parasympathetic nerve fibres was shown to be acetylcholine, and noradrenaline was the transmitter at sympathetic nerve endings. So all types of synapse appeared to be accounted for. There was also evidence that acetylcholine was a transmitter in the brain. Furthermore, by then, criteria for showing that an agent was a neurotransmitter had been laid down and these were quite difficult to fulfill, especially in complex situations like the neural pathways in the brain. These five criteria are demonstrations that show:(i) the putative transmitter is released when the nerve trunk is stimulated;(ii) application of the transmitter to the post-synaptic structure causes the same effect as nerve stimulation;(iii) the nerve fibres have a mechanism for making, storing, and releasing the transmitter;(iv) a mechanism is present for rapidly terminating the actions of the neurotransmitter; and(v) an appropriate antagonist is equally effective at blocking both neurotransmission and exogenous application of the neurotransmitter.Consider how these criteria are met for acetylcholine and for noradrenaline. We have already seen that these two substances were detected by sensitive bioassays in perfusates from neuronally-stimulated systems. To meet the second criterion it is necessary to show identity of action of the neurotransmitter and exogenously-applied transmitter substance. While this was straightforward with acetylcholine, Dale's insistence that adrenaline was not the neurotransmitter was based on differences between the responses to adrenaline and to nerve stimulation. This was an essential step leading to the discovery of noradrenaline.The third criterion is satisfied because nerves that release acetylcholine (cholinergic nerves) have an enzyme (choline acetyltransferase) in the nerve terminals that synthesizes acetylcholine from choline, while noradrenergic nerves have a series of enzymes that synthesize noradrenaline from the amino acid tyrosine. The enzymes are produced in the nerve cell bodies and pass down the axon to the nerve terminals in the so-called axoplasmic flow (they can travel a considerable distance - for example, from the spinal cord to the foot muscles).With respect to the fourth criterion: after release of acetylcholine from the nerve terminals it is attacked by acetylcholine esterase, which breaks down the neurotransmitter to choline and an acetate group, thus quickly terminating its action at the muscle membrane receptors. The choline is taken up into the terminal and recycled.The mechanism for terminating transmitter action is different in the noradrenergic system. Seventy per cent of the released noradrenaline is taken back up into the nerve terminals and stored for later reuse. The rest is metabolized either extracellularly or after uptake into the end organ.We have seen earlier that there are specific antagonists for acetylcholine, which also antagonize the effects of cholinergic stimulation. Similarly there are antagonists, such as the b-blockers, which block both noradrenaline and noradrenergic stimulation, thus meeting the fifth criterion.In 1934 Dale enunciated a principle which stated that a nerve liberates the same neurotransmitter at all its terminations. Thus if a nerve branches, each branch will release the same neurotransmitter. This principle has remained a truism and it would be difficult to imagine a nerve cell that could exclusively send a different set of synthetic enzymes, in the axoplasmic flow, down different branches. However, until relatively recently it was assumed that any one nerve produced only one type of neurotransmitter. As usual in science, advances arose when observations were made which failed to fit the established dogma. It was recognized that in some systems antagonists that were able to block exogenously applied transmitter were not able to block nerve stimulation completely. There was a residual activity with nerve stimulation that was resistant to the block. The term NANC transmission was coined, standing for 'nonadrenergic non-cholinergic transmission'. As the responses to nerve stimulation were partially blocked by antagonists with known specificities, the corollary was that the nerve must liberate more than one transmitter, but would do so from all its branches. Thus the concept of co-transmission was born, in which nerve stimulation could, in some instances, co-release more than one transmitter. In the peripheral autonomic nervous system - at the site where the nerves reach the tissue that they act upon - a great number of NANC transmitters have been claimed including, among others, ATP, VIP (vasoactive intestinal peptide), 5-HT, GABA, and http://www.answers.com/topic/dopamine-2. Undoubtedly some of these substances are transmitters, but few have yet met all the five criteria required to confirm their bona fides.What advantages might accrue from co-transmission? First, the small molecular weight transmitters (amine transmitters) and one of the peptide transmitters are likely to have very different kinetics (fast and slow effects). Secondly, the receptor targets for the two transmitters may have different locations, for example one on the end organ and the other on the nerve terminal itself, allowing feedback control and finally 'traffic neuromodulation'. This last results from the different ways in which the peptide transmitters and amine transmitters are handled. The 'machinery' needed to synthesize peptides like VIP is considerable. Consequently, these transmitters are synthesized in the nerve cell body and pass to the terminal in the axoplasmic flow, where they are stored ready for release. If there is heavy traffic in the nerve then the supply of the peptide neurotransmitter will soon be depleted. It is more difficult to deplete the supply of amine transmitters, which are made in the nerve terminal itself, so the ratio of the two transmitters released by nerve impulses will change.While the criteria for proving that a chemical agent acts as a neurotransmitter in the periphery are not easy to achieve, the technical difficulties in the brain and spinal cord are much greater. Here there is a mass of neural tissue with intricate interconnections and ramifications within the brain, as well as the connections made with incoming and outgoing neural pathways. However, there is overwhelming evidence for many neurotransmitters in the central nervous system, even though not all the five criteria above have been satisfied. Histochemical methods have been used to demonstrate the localization of neurotransmitters in particular types of nerve cells, coupled with electrophysiological methods in which the physiology of a single identified cell is studied with microelectrodes. Finally, it is now possible to suck a few nanolitres of intracellular substance (cytosol) from a single, identified neuron in a brain slice and to determine which genes are activated, including those coding for proteins associated with neurotransmission (receptors, enzymes, etc.). Useful information can also be obtained by a study of disease states. For example, there can be no doubt that a lack of dopamine transmission gives rise to Parkinson's disease. Evidence from post-mortembrains and comparison of the dopamine concentrations in normal and diseased brains locates the dopaminergic pathways involved in the disease.A potential forty neurotransmitters have been postulated to exist in the brain, of which ten are of the amine type with a small molecular weight. The amine types include acetylcholine, noradrenaline, dopamine, 5-HT, and histamine, and there are also excitatory and inhibitory amino acids. Glutamate and aspartate are the principal fast-acting excitatory transmitters in the brain, while GABA and glycine are the main inhibitory transmitters. Initially there was great reluctance to accept that these simple amino acids could act as neurotransmitters. Identified neurons were excited or inhibited when these amino acids were squirted onto them from very fine micropipettes. However, the presence of a pharmacological response does not prove physiological relevance. The development of selective agonists and antagonists has, subsequently, established that the four amino acids are true neurotransmitters. The remaining thirty-odd neurotransmitters in the central nervous system are mainly peptides, but much more evidence is needed before their true roles are unravelled.
What destroys neurotransmitters?
After Galvani had shown, in 1742, that electrical stimulation of the nerve to the muscle of a frog's leg caused the muscle to twitch, the idea gained ground that transmission from nerves to the 'end organ' was an electrical process. Today we know that only in very rare instances is transmission across a http://www.answers.com/topic/synapse - that is, between the end of a nerve and whatever it innervates - an electrical event. Virtually all neurotransmission is chemical. Nerves release one or more neurotransmitters, which act chemically on receptors in the membrane of the cells across the synaptic cleft. To detect neurotransmitters is a difficult task as the amounts released are minute and mechanisms exist that quickly remove the transmitter, leaving the system in a state of readiness for the arrival of the next nerve impulse. The first steps came from finding chemicals which could mimic the action of neurotransmitters. Muscarinewas able to mimic the effects of stimulation of the http://www.answers.com/topic/heart by the vagus nerves, and its actions were blocked by atropine. Similarly, nicotinemimicked the effect of stimulating motor nerves to http://www.answers.com/topic/skeletal-muscle, and this was blocked by curare.Du Bois Reymond, in 1877, declared that chemical or electrical transmission were the only two real alternatives, although there was no data to decide unequivocally between the two. In 1904 Elliot suggested that adrenaline might be the transmitter in the sympathetic nervous system, and in the next year a nicotine-like substance was postulated by Langley as the transmitter in motor nerves to muscles. Dixon, in 1907, tried to extract from a heart the neurotransmitter released by the vagus, measuring the effect of the extract on another heart, but reached no conclusions. It was 1921 before Loewi performed the celebrated experiment in which two beating frog hearts were superfused in series, the perfusate of the first flowing over the second. Stimulation of the vagus nerve to the first heart caused its beat to slow and, after a brief interval, the second heart slowed too. While this experiment did not prove that the substance flowing over from the first heart was released from the vagus nerve, clearly something chemical rather than electrical slowed the second heart.In the 1930s Dale and his school in England showed that http://www.answers.com/topic/acetylcholine was the transmitter at motor nerve endings in skeletal muscle (http://www.answers.com/topic/neuromuscular-junction-1) and Cannon and his colleagues in the US demonstrated that an adrenaline-like substance was the transmitter in the sympathetic nervous system. These discoveries depended on the development of sensitive bioassays for transmitter substances. Dale was not convinced that the sympathetic neurotransmitter was http://www.answers.com/topic/adrenaline itself, because he demonstrated subtle differences between the responses to the transmitter and to adrenaline. Von Euler, in 1946, finally showed that the sympathetic neurotransmitter was http://www.answers.com/topic/noradrenaline-2 (non-methylated adrenaline).After this period there was a lull in which many thought that acetylcholine and noradrenaline were the only two neurotransmitters. After all, the transmitter at the neuromuscular junction in skeletal muscle, in the synapses in ganglia of the autonomic nervous system, and at the ends of parasympathetic nerve fibres was shown to be acetylcholine, and noradrenaline was the transmitter at sympathetic nerve endings. So all types of synapse appeared to be accounted for. There was also evidence that acetylcholine was a transmitter in the brain. Furthermore, by then, criteria for showing that an agent was a neurotransmitter had been laid down and these were quite difficult to fulfill, especially in complex situations like the neural pathways in the brain. These five criteria are demonstrations that show:(i) the putative transmitter is released when the nerve trunk is stimulated;(ii) application of the transmitter to the post-synaptic structure causes the same effect as nerve stimulation;(iii) the nerve fibres have a mechanism for making, storing, and releasing the transmitter;(iv) a mechanism is present for rapidly terminating the actions of the neurotransmitter; and(v) an appropriate antagonist is equally effective at blocking both neurotransmission and exogenous application of the neurotransmitter.Consider how these criteria are met for acetylcholine and for noradrenaline. We have already seen that these two substances were detected by sensitive bioassays in perfusates from neuronally-stimulated systems. To meet the second criterion it is necessary to show identity of action of the neurotransmitter and exogenously-applied transmitter substance. While this was straightforward with acetylcholine, Dale's insistence that adrenaline was not the neurotransmitter was based on differences between the responses to adrenaline and to nerve stimulation. This was an essential step leading to the discovery of noradrenaline.The third criterion is satisfied because nerves that release acetylcholine (cholinergic nerves) have an enzyme (choline acetyltransferase) in the nerve terminals that synthesizes acetylcholine from choline, while noradrenergic nerves have a series of enzymes that synthesize noradrenaline from the amino acid tyrosine. The enzymes are produced in the nerve cell bodies and pass down the axon to the nerve terminals in the so-called axoplasmic flow (they can travel a considerable distance - for example, from the spinal cord to the foot muscles).With respect to the fourth criterion: after release of acetylcholine from the nerve terminals it is attacked by acetylcholine esterase, which breaks down the neurotransmitter to choline and an acetate group, thus quickly terminating its action at the muscle membrane receptors. The choline is taken up into the terminal and recycled.The mechanism for terminating transmitter action is different in the noradrenergic system. Seventy per cent of the released noradrenaline is taken back up into the nerve terminals and stored for later reuse. The rest is metabolized either extracellularly or after uptake into the end organ.We have seen earlier that there are specific antagonists for acetylcholine, which also antagonize the effects of cholinergic stimulation. Similarly there are antagonists, such as the b-blockers, which block both noradrenaline and noradrenergic stimulation, thus meeting the fifth criterion.In 1934 Dale enunciated a principle which stated that a nerve liberates the same neurotransmitter at all its terminations. Thus if a nerve branches, each branch will release the same neurotransmitter. This principle has remained a truism and it would be difficult to imagine a nerve cell that could exclusively send a different set of synthetic enzymes, in the axoplasmic flow, down different branches. However, until relatively recently it was assumed that any one nerve produced only one type of neurotransmitter. As usual in science, advances arose when observations were made which failed to fit the established dogma. It was recognized that in some systems antagonists that were able to block exogenously applied transmitter were not able to block nerve stimulation completely. There was a residual activity with nerve stimulation that was resistant to the block. The term NANC transmission was coined, standing for 'nonadrenergic non-cholinergic transmission'. As the responses to nerve stimulation were partially blocked by antagonists with known specificities, the corollary was that the nerve must liberate more than one transmitter, but would do so from all its branches. Thus the concept of co-transmission was born, in which nerve stimulation could, in some instances, co-release more than one transmitter. In the peripheral autonomic nervous system - at the site where the nerves reach the tissue that they act upon - a great number of NANC transmitters have been claimed including, among others, ATP, VIP (vasoactive intestinal peptide), 5-HT, GABA, and http://www.answers.com/topic/dopamine-2. Undoubtedly some of these substances are transmitters, but few have yet met all the five criteria required to confirm their bona fides.What advantages might accrue from co-transmission? First, the small molecular weight transmitters (amine transmitters) and one of the peptide transmitters are likely to have very different kinetics (fast and slow effects). Secondly, the receptor targets for the two transmitters may have different locations, for example one on the end organ and the other on the nerve terminal itself, allowing feedback control and finally 'traffic neuromodulation'. This last results from the different ways in which the peptide transmitters and amine transmitters are handled. The 'machinery' needed to synthesize peptides like VIP is considerable. Consequently, these transmitters are synthesized in the nerve cell body and pass to the terminal in the axoplasmic flow, where they are stored ready for release. If there is heavy traffic in the nerve then the supply of the peptide neurotransmitter will soon be depleted. It is more difficult to deplete the supply of amine transmitters, which are made in the nerve terminal itself, so the ratio of the two transmitters released by nerve impulses will change.While the criteria for proving that a chemical agent acts as a neurotransmitter in the periphery are not easy to achieve, the technical difficulties in the brain and spinal cord are much greater. Here there is a mass of neural tissue with intricate interconnections and ramifications within the brain, as well as the connections made with incoming and outgoing neural pathways. However, there is overwhelming evidence for many neurotransmitters in the central nervous system, even though not all the five criteria above have been satisfied. Histochemical methods have been used to demonstrate the localization of neurotransmitters in particular types of nerve cells, coupled with electrophysiological methods in which the physiology of a single identified cell is studied with microelectrodes. Finally, it is now possible to suck a few nanolitres of intracellular substance (cytosol) from a single, identified neuron in a brain slice and to determine which genes are activated, including those coding for proteins associated with neurotransmission (receptors, enzymes, etc.). Useful information can also be obtained by a study of disease states. For example, there can be no doubt that a lack of dopamine transmission gives rise to Parkinson's disease. Evidence from post-mortembrains and comparison of the dopamine concentrations in normal and diseased brains locates the dopaminergic pathways involved in the disease.A potential forty neurotransmitters have been postulated to exist in the brain, of which ten are of the amine type with a small molecular weight. The amine types include acetylcholine, noradrenaline, dopamine, 5-HT, and histamine, and there are also excitatory and inhibitory amino acids. Glutamate and aspartate are the principal fast-acting excitatory transmitters in the brain, while GABA and glycine are the main inhibitory transmitters. Initially there was great reluctance to accept that these simple amino acids could act as neurotransmitters. Identified neurons were excited or inhibited when these amino acids were squirted onto them from very fine micropipettes. However, the presence of a pharmacological response does not prove physiological relevance. The development of selective agonists and antagonists has, subsequently, established that the four amino acids are true neurotransmitters. The remaining thirty-odd neurotransmitters in the central nervous system are mainly peptides, but much more evidence is needed before their true roles are unravelled.
