Neurotransmitter

http://e-neurotransmitter.blogspot.com/

Identifying neurotransmitters
Some of the properties that define a chemical as a neurotransmitter are difficult to test experimentally. For example, it is easy using an electron microscope to recognize vesicles on the presynaptic side of a synapse, but it may not be easy to determine directly what chemical is packed into them. The difficulties led to many historical controversies over whether a given chemical was or was not clearly established as a transmitter. In an effort to give some structure to the arguments, neurochemists worked out a set of experimentally tractable rules. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:

* There are precursors and/or synthesis enzymes located in the presynaptic side of the synapse.
* The chemical is present in the presynaptic element.
* It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron;
* There are postsynaptic receptors and the chemical is able to bind to them.
* A biochemical mechanism for inactivation is present.

Modern advances in pharmacology, genetics, and chemical neuroanatomy have greatly reduced the importance of these rules. A series of experiments that may have taken several years in the 1960s can now be done, with much better precision, in a few months. Thus, it is unusual nowadays for the identification of a chemical as a neurotransmitter to remain controversial for very long.
Publicat de health victorro la 08:17 0 comentarii
Etichete: Identifying, neurotransmitters
Excitatory and inhibitory
Some neurotransmitters are commonly described as "excitatory" or "inhibitory". It is important to understand what these terms mean. The only thing that a neurotransmitter does directly is to activate one or more types of receptors. The effect on the postsynaptic cell depends entirely on the properties of the receptors. It so happens that for some neurotransmitters (for example, glutamate), the most important receptors all have excitatory effects: that is, they increase the probability that the target cell will fire an action potential. For other neurotransmitters (such as GABA), the most important receptors all have inhibitory effects. There are, however, other important neurotransmitters, such as acetylcholine, for which both excitatory and inhibitory receptors exist; and there are some types of receptors that activate complex metabolic pathways in the postsynaptic cell to produce effects that cannot appropriately be called either excitatory or inhibitory. Thus, it is widely understood to be an abuse of language to call a neurotransmitter excitatory or inhibitory—nevertheless it is so convenient to call glutamate excitatory and GABA inhibitory that this usage is seen very frequently.
Publicat de health victorro la 08:16 0 comentarii
Etichete: Excitatory, inhibitory
neuromodulation
Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system affects large volumes of the brain, called volume transmission. The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system and the cholinergic system.

Drugs targeting the neurotransmitter of such systems affects the whole system; this fact explains the mode of action of many drugs. Cocaine, for example, blocks the reentering of dopamine back into the presynaptic neuron, leaving these neurotransmitters in the synaptic gap longer. Since the dopamine is in the synapse longer, the neurotransmitter rapidly hit the receptors on the postsynaptic neuron cell, and therefore causing happiness. Excess intake of cocaine can lead to physical addiction. The physical addiction of cocaine is when the neurotransmitters stay in the synapse so long , the body removes some receptors from the postsynaptic neuron. After the effects of the drug wear off, the person usually feels unhappy, because now the neurotransmitters are less likely to hit the receptor since the body removed many of them during the drug intake. Prozac is a selective serotonin reuptake inhibitor (SSRI), hence potentiating the effect of naturally released serotonin. AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.

Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
Publicat de health victorro la 08:15 0 comentarii
Etichete: neuromodulation
Actions-glutam-GABA
As explained above, the only direct action of a neurotransmitter is to activate a receptor. Therefore, the effects of a neurotransmitter system depend on the connections of the neurons that use the transmitter, and the chemical properties of the receptors that the transmitter binds to.

Here are a few examples of important neurotransmitter actions:

* Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord. It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength. Modifiable synapses are thought to be the main memory-storage elements in the brain.
* GABA is used at the great majority of fast inhibitory synapses in virtually every part of the brain. Many sedative/tranquilizing drugs act by enhancing the effects of GABA. Correspondingly glycine is the inhibitory transmitter in the spinal cord.
* Acetylcholine is distinguished as the transmitter at the neuromuscular junction connecting motor nerves to muscles. The paralytic arrow-poison curare acts by blocking transmission at these synapses. Acetylcholine also operates in many regions of the brain, but using different types of receptors.
* Dopamine has a number of important functions in the brain. It plays a critical role in the reward system, but dysfunction of the dopamine system is also implicated in Parkinson's Disease and schizophrenia.
* Serotonin has a number of important functions that are difficult to describe in a unified way, including regulation of mood, sleep/wake cycles, and body temperature. It is released during sunny weather, and also when eating chocolate or taking Prozac.
* Substance P undecapeptide responsible for transmission of pain from certain sensory neurons to the central nervous system.
Publicat de health victorro la 08:14 0 comentarii
Etichete: Actions
Degradation and elimination
Neurotransmitter must be broken down once it reaches the post-synaptic cell to prevent further excitatory or inhibitory signal transduction. For example, acetylcholine, (ACH) (an excitatory neurotransmitter), is broken down by acetylcholinesterase (AchE). Choline is taken up and recycled by the pre-synaptic neuron to synthesize more ACH. Other neurotransmitters such as dopamine are able to diffuse away from their targeted synaptic junctions and are eliminated from the body via the kidneys, or destroyed in the liver. Each neurotransmitter has very specific degradation pathways at regulatory points, which may be the target of the body's own regulatory system or recreational drugs.

• Added on: Mar 13th, 2009
• Country: Romania
• Language: English

• Author: rorac2009
• Listed in: Health Medicine
• Tags: identifying neurotransmitters excitatory and inhibitory