Give an account of how the monoamines Essay Example for Free

Give an account of how the monoamines Essay Before any detailed explanations of how monoamines act as transmitters and their prime functions in the brain, I feel it is appropriate to describe what transmitters are and outline their functions in the nervous system as well as the criteria needed for a transmitter. The definition of a neurotransmitter is chemicals that are used to relay, amplify and modulate electrical signals between neurons. (wikipedia, 2006) Moreover, for these substances to be classified as a neurotransmitter, certain criteria must be met as follows. Firstly, it is synthesized when required endogenously at the pre-synaptic side of the cell and stored in vesicles. Secondly, an adequate amount can be released to cause a response on the post-synaptic neuron. Finally, an inactivation mechanism is required to terminate its effect upon cell. Neurotransmitters are agonists and may produce fast or slow sequence of events which could be either excitatory or inhibitory. The effect is largely determined by the receptors that the neurotransmitters bind onto. Neurotransmitters are usually small molecules and are packaged into small vesicles. When a neuron receives an action potential that travels along the axon reaching the synapse, the pre-synaptic membrane of the neuron becomes depolarized. This leads to the opening of calcium channels and an influx of calcium ions, which causes the vesicles to be released by exocytosis to the synaptic cleft and the neurotransmitter molecules are then free to bind to receptors that sits on the post-synaptic membrane of the neuron. Neurotransmitter receptors are commonly divided into two types: the ionotropic receptors and the metabotropic receptors. The first group of receptors is ligand-gated ion channels and its opening or closure of channel depends on the binding of neurotransmitters. The rate of transmission for ionotropic receptors is fast. The latter group passes on signals through second messengers such as G-proteins and they have slower neuromodulation. There are three major categories of neurotransmitters: the amino acids, peptides, monoamines and the last group we will be concentrated on. Monoamines include chemicals such as dopamine (DA), norepinephrine (NA) and serotonin (5-HT). Dopamine is responsible for emotional arousal; NA causes wakefulness and arousal whereas 5-HT has a role in memory, patterns in sleeping and emotions. Each of the neurotransmitters will now be introduced in more details. Dopamine is naturally synthesized inside the body by rewarding experiences; it is a neurotransmitter that acts on the brain as well as a neurohormone that is released by the hypothalamus. Generally, functions of DA in the brain include movement, cognition, prolactin secretion regulation plus motivation and pleasure. DA have other functions in the brain, however, it centrally plays a role in the pleasure reward system. The presence of an unanticipated reward induces activations to DA neurons and we tend to repeat certain behaviors to gain rewards again due to the feelings of reinforcement and motivations. Thus DA is believed to set up new motor sequences for repeated behaviors and reward system. Often this system has an association with the use of drugs such as cocaine and amphetamines which can increase the concentration of DA in the brain leading to addiction of drugs. On the other hand DA is also released with the occurrence of unpleasant experiences and we learn to keep away or remove the unlikable stimuli. The most common and recognizable symptom of Parkinsons disease is the shaky hands, where patients are less able to control their motor movement. The cause of the disease is principally understood as shortage of DA and death of DA neurons in nigrostriatal pathway. Evidentially DA has an affect on the basal ganglia motor loop which impinges on the brain to control our motor movements. The frontal lobes of the brain are the region responsible for most of the neurocognitive functions, where DA controls the information flow from other sections of the brain. Any disorder in this area can bring about reduced functions in memory and attention. As mentioned before, DA plays a role in regulation of prolactin secretion in which DA act as a neuroendocrine regulator to inhibit prolactin release from anterior lobe of pituitary gland. Receptors for DA include D1, D2, D3, D4 and D5 and D2 receptors are associated with sociability. Anxiety patients are found to have low D2 receptor binding. Other behavior disorders such as depression, schizophrenia and psychosis are related to DA system disruption. Treatment for these disorders often uses the blockade mechanism of the D2 receptors or blocking the reuptake of DA transmitters. Norepinephrine (NA) belongs to the catecholamine family, which are chemicals that are made from the amino acids tyrosine. In the nervous system, NA acts as neurotransmitters and the noradrenergic neurons are responsible for the release of NA. The function of NA in the brain is as a stress hormone, where one is subjected to stressful environment, NA is released. It acts on the adrenoreceptors which are divided into the alpha and beta type and each of these have their own subdivisions. Norepinephrine also implies a role in depression and the chemical serotonin-norepinephrine reuptake inhibitors (SNRIs) are used to treat depression. SNRIs are antidepressant and they function by increasing the concentration of NA at the postsynaptic cells as well as the amount of dopamine due to the availability of NA transporters to some dopamine. SNRIs has an inhibitory effect on the NA transporter reuptake mechanism by preventing transporters to recycle NA transmitters, which stop the NA transporter to recycle dopamine as well and prolongs the effect of dopamine transmission. The correlation between the two transmitters is effective on the antidepressant aspect. Serotonin (5-HT) is derived from the essential amino acid tryptophan and it is a neurotransmitter in the central nervous system. 5-HT is made in serotonergic neurons of the Raphe nuclei in the brain. The neurotransmission for 5-HT is relatively different compared to DA and NA. Instead of releasing the neurotransmitter from a synaptic bulb, 5-HT is released from serotonergic swellings along the axon. Classically transmitters after the release stage will usually bind to receptors that sit on the post-synaptic membrane. However, in the case of 5-HT, the receptors specifically for it not only present post-synaptically but also at areas such as cell bodies and dendrites. The termination pathway of 5-HT is via the actions of the 5-HT reuptake transporter present at pre-synaptic membrane of the neuron. There is a broad classification of 5-HT receptors, from 5-HT1 to 5-HT7, and there are subdivision of the 5-HT1, 5-HT2 and 5-HT5receptors. Most of the 5-HT receptors uses the G-protein coupled mechanism traduce signals, except the 5-HT3 receptor which is a ligand-gated ion channel. For 5-HT1 receptors, the subdivision includes 5-HT1A, 5-HT1B and 5-HT1D. Generally with selected agonists binding to the receptors their functions in the central nervous system are inhibition of neuronal effects and behavioral effects for example, sleeping, and anxiety. But for 5-HT1D it has a role in cerebral vasoconstriction. At the 5-HT2A receptors it involves neuronal excitation which is the opposite of the 1A, B subtypes as well as learning. 5-HT2C receptors are responsible for secreting cerebral spinal fluid to the spinal cord for lubrication and other functions. The 5-HT3 receptors with a selection of specific drugs binding, it brings about anxiety plus neuronal excitation. For some of the other receptors of 5-HT, their roles in the nervous system are unclear and some are responsible for the gastrointestinal tract contraction and motility; vascular constriction or dilation. Clinically, 5-HT is used in the treatment of epilepsy and depression. Although histamine is the chemical that has many involvements physiologically in the human body, it also acts as a neurotransmitter. Four histamine receptors have been discovered ranging from H1-H4. However, only the H3 receptors are mainly engaged with the central nervous system. The cell bodies in the posterior hypothalamus release histamine and it plays a part in sleeping pattern modulation. Histamine releasing disorders are associated with inability of attention maintenance. Schizophrenia also has an association with low concentration of histamine in blood level. References: Book resources:Essential psychopharmacology 2nd edition, Stephen M. Stahl Online resources: http://en. wikipedia. org/wiki/Monoamine_neurotransmitter http://en. wikipedia. org/wiki/Dopamine http://en. wikipedia. org/wiki/Norepinephrine http://en. wikipedia. org/wiki/Serotonin http://en. wikipedia. org/wiki/Histamine.