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Signal Transduction in Neuropharmacology

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According to Rosenzweig-Lipson, signal transduction is the process by which a cell can convert an input into an appropriate output, such as, the conversion of muscarinic stimulation of glandular tissue into a secretory response (Rosenzweig-Lipson, S. et al 239). In this study we examine the conformational change occurring in the receptor protein which is a consequence of agonistic binding that enables the receptor to interact with a second element in the system. It has been observed that G- protein transduces a signal to a third component “an amplifying enzyme” that elaborates an additional messenger which is the final component of a system, and activates a cascade of enzymes that leads to an increase in protein phosphorilation and an output or response (Sitaramayya and Ari 8).  “The membrane of a normal cell acts as a pathway for signals to pass through the intercellular molecules and it is that which activates transcription” (Gilman 615–649). These signals follow a certain rhythm and this series is referred to as signal transduction. Hence in this study, proteins are key components since they play very important roles in various inter-neuronal communications

Membrane receptors, Second messengers, and Signal Transduction pathways

When the neurotransmitter binds on to the receptor, ion channels permit the flow of ions. The receptors act on the protein channels by using a more indirect second messenger system. This is the G proteins inclusion in the transduction of from the membrane receptors to intercellular effectors. The binding of a neurotransmitter to a receptor activates the G protein this in turn causes the protein channels that gate the ion flow to open and cause migration of either ion.

A new component of GPCR signal transduction

Proteins are involved in several vital roles in the inter-neuronal communication. The ion channels found in the cell membranes are proteins. G proteins are heterotrimers that consist of three subunits, which include alpha, beta, and gamma (%uF061- (45 to 47 kD), %uF062- (35 kD), and %uF067- (7 to 9 kD). The alpha unit is an active subunit, which binds the guanine diphosphate in its stationary state. On the other hand, it exchanges the guanine diphosphate for the guanine triphosphate when active. During this active mode, it acts as the courier between the receptor sites and the effectors. The other sub units, which include beta and gamma, help the alpha subunit to attach to membranes. Therefore, beta and gamma subunits are deemed as passive units in the G proteins.

The alpha unit activates the effectors and gets back to its resting state. The alpha unit returns to its resting state through cleaving the guanine triphosphate, attaching again to the guanine diphosphate, and joining up with the betagamma dimer (Malbon & Morris 5). The %uF061-subunit binds GDP or GTP and has an intrinsic, slow GTPase activity. The G%uF061%uF062%uF067 complex in the inactivated state comprises of GDP at the nucleotide site. Binding of hormone to receptor stimulates a rapid exchange of GTP for GDP on G%uF061. When GTP binds, it causes G%uF061 to dissociate from G%uF062%uF067 and to associate with an effector protein like the adenylyl cyclase. Binding of G%uF061 (GTP) activates adenylyl cyclase. The adenylyl cyclase actively synthesizes cAMP as long as G%uF061(GTP) remains bound to it. However, the intrinsic GTPase activity of G%uF061 eventually hydrolyzes GTP to GDP, leading to dissociation of G%uF061(GDP) from adenylyl cyclase and re-association with the G%uF062%uF067 dimer, regenerating the inactive heterotrimeric G%uF061%uF062%uF067 complex.

 In order to facilitate neuron communication, the post-synaptic neuron should posses the receptor sites for the neurotransmitters discharged from the pre-synaptic neuron. Through binding on to the receptors, the neurotransmitters are expected to facilitate change in the post-synaptic neuron, which may cause a potential action in the post-synaptic neuron. G protein receptors are engaged in numerous diseases. An approximate of 30% of the medicinal drugs target the G protein coupled drugs. “The G protein-coupled receptors engage two principal transduction pathways, which include the camp signal pathway and phosphatidylinositol” (Malbon & Morris 8). 

A simple process takes place for neuron communication to take place. The postsynaptic neuron must have receptor sites for neurotransmitters that were released by the pre-synaptic neurons. The neurotransmitters must bind on to the receptors hereby bringing about a change in the, post synaptic neuron and this is called an excitatory post synaptic potential or (EPSP). At the same time, it could be an inhibitory post synaptic potential or (IPSP). These two are produced depending on the concentrations of either sodium or chlorine ions existing in the neuron. According to Jeong, the change in concentration happens in the channeling of either sodium or chlorine ions that migration across the cell membrane (Jeong & Ikeda pp. 335–347).

            The neurotransmitter-receptor binding facilitates the opening of the protein channels. The receptor is sited on the top of the protein channel to simplify the opening of the protein channels. The neurotransmitter-receptor binding enables the protein channel to allow the ion flow. The protein channels can also be acted upon by the receptors in an indirect fashion through a second messenger system. The second messenger system is exemplified by a G protein’s addition in the transduction of “signals from the transmembrane receptors to intracellular effectors.” Clearly, the neurotransmitter-receptor binding stimulates the G protein, which facilitates the opening of the protein channels for ion flow (Sitaramayya and Ari, 23).

                        In a second messenger system, a ligand binds to a receptor while the G protein attaches to the receptor. The guanine diphosphate that is bound to the alpha unit is replaced by the guanine triphosphate thus the unit detaches from the rest of the G protein. In this process, the next session differs from the simple messenger system. The alpha unit fails to attach to a protein channel and instead binds to another membrane protein known as adenylyl cyclase. The Adenylyl cyclase converts adenosine triphosphate to cyclic monophosphate. The process is initiated after the alpha unit latches on to adenylyl cyclase. Furthermore, the cyclic adenosine monophosphate initiates another protein in the neuron known as protein kinease. Protein kinease is a component that is composed of two elements, which include regulatory unit and catalytic unit. Commonly, the regulatory unit scrutinizes the catalytic unit. The cyclic adenosine monophosphate, however, causes the two units to detach. The catalytic unit, which is the active component moves to the protein channels in the membrane of the ion and triggers them to open. The ion flow finally occurs and the excitatory post-synaptic potential or inhibitory post-synaptic potential are stimulated. Several second messenger systems similarly work effectively.

Proteins: transducers of receptor-generated signals


            The G protein initiates a series of changes in the neuron and the ion channels open up. A receptor encounters a G protein in its active state. It is clear that the receptors and the G proteins are pre-coupled. The signal transduction relies on the form of the G protein. Adenylate cyclase enzyme is a type of a cellular protein that is regulated by the G protein. The activation of the Adenylate cyclase enzyme terminates when the G protein gets to the guanine diphosphate bound state. The Adenylate cyclases can also be activated in other methods, which regulate their activity. The heterotrimeric G proteins can engage in functional roles independent of the G proteins-coupled receptors. Apart from the heterotrimeric G proteins, other types of G proteins also play vital roles in the cell function. These proteins belong to a certain group known as “small G proteins.” They also perform their activities like the heterotrimeric G proteins.

A G protein coupled receptor is a receptor that mediates the opening of the ion channels through a G protein. This component is involved in a system that renovates the external signals into an intracellular second messenger system. In this second messenger system, the neurotransmitter binds on to a receptor while the G protein attaches on to the receptor and becomes triggered. The alpha subunit, then binds on to the protein ion channel and cause it to open thus permitting ion flow. Therefore, signal transfer in this case is the vital role for the G protein. The function of G protein in the other version of the second messenger system is less undeviating. In this case, it does not activate the protein channels in the membrane. It activates a series of events that cause the opening of protein channels.

This constitutes about seven membrane domains interconnected by three intercellular domains and three extracellular loops, an extra cellular N-terminal domain and an intracellular C- terminal domain. Each of the seven interconnected domains is composed of about 20- 27 amino acids. The GPRS span three environments and these are extracellular, intra-membrane as well as the intra-cellular. The conformal change of the central core will be responsible for the conformation of the intracellular loops and this will activate the binding and processes of the G protein. The activation of these proteins is then responsible for the activation of various intracellular signaling pathways.  G protein-coupled receptors (GPCRs) have been identified to be the target for more than half of the drugs currently on the market; this includes approximately 25% of the 100 top-selling drugs. Hence, they are considered the most important molecules in drug discovery, due to their role as receptors in many processes in the body and their presence in all the body tissues.

G-protein specificity

 “The binding of the GPCR to some of the different proteins will determine the various inter-membrane and intracellular members resulting in the localizing of the receptors and the G protein independent signaling” (Hildebrandt & Iyengar, 4). These proteins include the tyrosine protein kinesis and scaffolding proteins. GPCR’s can be classified according to five groups. These five families are classified according to their phylo-genetic origins. They are glutamate, rhodopsin, frizzled, adhesive, and secretin and these form the GRAFS classification system.

The rhodopsin family makes up the largest portion and was formerly named as family one or family A. It has the largest number of receptors in the group and 700 members. The group has some similarities of phylogenetic characteristics, but also has some intrinsic characteristics. There are about four more classifications from the rhodopsin group. There are three classes of adrenergic receptors of the super family of G- protein coupled receptors and they mediate a large variety of peripheral as well as the central responses to the endogenous catecholamines. All of the adreno-receptor subfamilies are comprised of three receptor subtypes that that are categorized by a distinct heterotrimeric G protein coupling.

One concept that shares characteristics with inverse agonism is that of protean agonism. In this way, some ligands display both characteristics of both agonism and inverse agonism in a single GPCR. This event is quite rare and has only been observed at a handful of GPCR’s and is not well understood at the present, however, it provides some notion into how the ligands modulate the GPCR behavior. In normal theorem, any preparation that is used in the measurement of GPCR stimulation by agonists can also may be used to study the inhibitory effects that come about from inverse agonism.

This is because both groups of agonists and inverse agonists produce their effects from balancing or modulating the active as well as the inactive receptors. However, there should be an amount of spontaneous receptor activity needed to measure individual effects of either class of ligand. Similarly, the detection of inverse agonism requires that spontaneous receptor activity be quite distinguishable from that of background noise. This according to experience will not be the actual case. The measuring of inverse agonism and its effects intrinsically is quite difficult. The conditions for doing the same therefore will be the same as by measuring the receptor activation from the agonists.

When a ligand is found to have a negative effect on an agonist, independent GPCR then the one carrying out the experiment must find out if it is genuine. The presence of endogenous activating ligands also needs to be ruled out of the equation. If that is not possible then one need to demonstrate the effects of a strong inverse those of the neutral antagonists or weak inverse agonists overshadow agonists. At the same time, the confusing effects of the related receptor subtypes if any are present at the time should be considered.

There is a differential set for G protein partners, which allow it to initiate different signaling pathways in order to trigger diverse and even opposed functional outcomes in response to the same stimuli. “The distribution of tissue according to the different adrenal receptor subtypes varies quite differently while conferring to the different catelochamines” (Ciruela 31). G protein coupled receptors can activate G proteins and initiate the signaling in the presence of the agonist. The receptor activity can be quite spontaneous but can be regulated by certain antagonists. A process that many refer to as inverse agonism causes this and the inverse agonist’s job is to produce biochemical effects that are opposite to the agonists. This will dispute with the theorem that many support as to state the antagonists lack intrinsic activity. This would suggest their work is to preclude the binding of agonists to the receptors solely. The work of inverse agonism has been displayed in a wide variety of systems. These include endogenously and heterogeneously expressed GPCR’s. On the other hand, the contribution of inverse agonism to the overall therapeutic effects of antagonism is hard to determine considering the continual presence of the endogenous agonists in the normal physiological conditions that are set in the state. However, the proof to such evidence of awaits the confirmation and evidence that spontaneous receptor signaling is significant or it changes in terms of receptor density or distribution. These have to be attributable to the effects of inverse agonism at the cellular and the sub cellular level. These also occur in intact organisms  

Thus, inverse agonism can be assessed at the level of the receptor, the G protein, and the effector as well as the effects that follow downstream. When considering the technique that works best in this issue it might depend on the system that is currently under investigation. Mall of the ends in this case, may not necessarily yield the equivalent results that many expect thus, it would be worthwhile trying to assign the ligand activity to multiple levels. The receptor, however, facilitates binding of the inverse agonists which increases by the guanine nucleotides whereas the agonists have an opposite reaction of decreased binding.

Receptor-dependent specificity

The past decade has seen many events whereby cell surface carbohydrates play a role in signal transduction events. This was hypothesized some time ago that the complex structure of cell surface carbohydrates was there to play an informational role (Haltiwanger 593). The numerous developmentally relevant pathways are affected by proteoglycans as well as the recent importance demonstrations of fucose modifications in notch and nodal signaling. This suggests that the time for uncovering the involvement of the glycans in signaling is just beginning.

Many other proteins are involved in developmental or signaling events that are predicted to go through modification with O-fucose. At the same time, the EGF sites are also modified at consensus sites between the first as well as the second conserved cysteines. Many of the predicted O-glucose sites on the notch receptor have conservation because of evolution. This proposes that the modifications will play an important role in notch signaling. The other important form of O-fucose is mapped on to the consensus site on the thrombospondin type 1 repeats. However, this type repeats on a heparan-sulfate-binding domain and that suggests that the alteration may activate reactions at the site.

These results and progress suggests that the surface has hardly been scratched the surface in terms of the role of glycosolation in the regulation of signal transduction. The objective is to find a link between these factors and the brain in neuropharmacology. Thus, we must become fully aware of what it means. Neuropharmacology is study of drugs that interact with neurons in the brain and this affects the mood, sensation, thinking and overall behavior. There are some of these alterations in behavior that result from drug intake but not all.

RGS proteins as drug target

“GPCRs are excellent targets for pharmaceutical treatments since they comprise of the most widely screened classes of signal transduction targets” (Auld, et al., 2002). Major diseases involve the malfunction of the receptors; hence they become the most important drug target for pharmacological intervention. Thus, changes result in the actions of the brains chemical transmitters, which is under the field of brain neuro-chemistry. A lot of interest is coming to the fore over molecular neuroscience in both clinical as well as basic research. This is because of new findings on topics such as neuro-peptides, neuro-hormones that have been disseminated into a number of topics that keeps on increasing with the time. The topic focuses on new drugs that will be used in neurology which some consider as the final frontier of the medical world. The aim is to also modify the processes of the brain on the premise of improving on some characteristics.  These include memory; mood as well as attention deficits that people may have and give enhancements in these sectors. However, they would not be impaired by disease or illness beforehand thus; it would cause much debate on the ethical field. There is also a field dedicated to this known as neuro-ethics. This field is defined as the study of legal and social questions that arise when scientific study and discoveries of the brain’s processes come into the medical field in practice. The problem in the real essence lies with the organ.

It is the brain, which we are dealing with and it is responsible for making all of us unique in our own individual way. This gives us our personality’s emotions, memories emotions, dreams, and creative abilities, (Neuro-ethics 1). Therefore, it is the driving force behind the motives of our individual actions. Many out there would support the medical development of the sector of neuroscience in order to help deal with mental illness as well as basic research to further our understanding. However, they would rather the journey end there, as they are not too comfortable with getting to know too much about the human brain.

The human brain is a special piece of art and holds a special place because it gives everyone his or her own autonomy. Manipulating this organ is not the same as operating and altering another part of the body like the lungs, kidney, or the heart. It is because of its sensitive nature that not too many people would be thrilled about giving stimulating drugs to alter intelligence of mood set up. This would be manipulating a person’s character and would cause many uncomfortable feelings. Enhancement at this stage is not confined to neuroscience but there is a sense that something is quite distinct about enhancing things to do with cognitive functions as well as behavior.

RGS inhibitors as clinical therapeutics

Neural disorders for one have their origin in form of shifts and delicate balances in the neuro-chemicals that could originate from cell damage of degeneration. However, the brain is quite flexible and capable of restoration of certain imbalances. This can be done by increasing the sensitivity of the brain to a certain neuro-transmitter. However, this is not to say that it cannot also fail. When a substance such as dopamine is depleted, especially when talking about Parkinson’s disease, then a disease or severe neural disorder may come, as a result.

The fundamental research in neuropharmacology investigates the processes going on the brain and how drugs introduced to that system interact with those processes. One of the research tools that many like to employ is the building models of the neuro-chemical as well as the neuro-physiological processes. These aim to fit the data on the laboratory studies on the animal models. In a certain pharmacy department in a university, this happened by electrophysiological and microdyalisis methods in order to track the nerve signals. Nerves propagate their electric signals by conducting an electric pulse and this is known as an action potential.

Endogenous RGS-protein function

The signal in turn activates release of some transmitter chemicals at the terminals of the cell that affect the receptors of the next nerve cell in the relay that in turn continues the cycle. On the other hand, placing an electrode inside the brain will allow the individual to monitor further electrical activity. The release of the transmitters will be monitored while using the micro dialysis probe. This probe would also be used to release these chemicals locally and measure the full effects. At the pharmacy department at this university, neuro-physiological pathways are studied using two techniques.

The specific studies that occur when relating several variables together that gives an understanding of the function of the brain area. To give a basic visual description of the neural circuits, bow and arrow models can be drawn that represent negative and positive influence relations. These models will be further tested for their correct nature and in this way, used to explain the functioning of this system. In this way, newly developed drug compounds will give a foothold in this sector because they will play the role of revising and refining the model as well as the experiments that are conducted.

The model in turn will play the role of getting to know the effects. This would be in such a way , that if a drug used were to selectively favor one particular type of pathway then, it could still be used to further explore the function of that pathway. The data acquired from this may serve to refine the model and in this way, the effects may be explained as well as predicted. At this university, there is a group of nuclei called the base ganglia. These nuclei play an all-important role in the control of voluntary behavior.

Scope of review in Parkinson’s disease

In Parkinson’s disease a portion of them that do not survive decay due to a cause that yet remains unknown. However, this portion is a supplier of an important neuro-transmitter called dopamine. This transmitter is postulated to perform a modulating function. It is also thought to create a delicate balance in the signals that are bound for the cortex. In order for this to work, however, a schematic model should be used to represent neural activity in the brain ganglia especially according to Parkinson’s disease. If this were visually illustrated on a paper, an arrow would serve as a neural pathway.

This would consist of a bundle of individual nerve cells. The box in this case, would represent the nucleus or a clustering of nerve cells. The model gives a dual function to dopamine. It shows a direct path from the striatum to the SNR while inhibiting the indirect path via the STN. The balance creates an inhibition in two fronts, which are the thalamus and the brainstem. However, when it becomes nearly depleted then the balance becomes disrupted once again and this results in the increase of activation in an area known as the SNR.

This state of hyper-activation in this region causes inhibition of brainstem neurons, which go hand in hand with some symptoms of Parkinson’s disease. Most of the traditional research that goes to Parkinson’s disease focuses on restoring levels of dopamine in the brain. However, the compound cannot be administered as an oral substitute because it cannot go through the blood brain barrier. On the other hand, it was found that L-dopa could pass through the barrier and it metabolizes while in the brain to form Dopamine. Thus, at the present it is the most successful method of dealing with Parkinson’s symptoms.

Possible clinical uses of RGS inhibitors

Similarly administering L-dopa to the brain raises dopamine levels in other parts of the body. This high concentration of dopamine in the body on the other hand, causes feelings of nausea as a side effect. This is due to stimulation of dopamine receptors in other parts of the body. The therapeutic effect however, declines over a period like three to five years. Further research into the issue is finding out the effects of highly selective dopamine receptors that interact with certain dopamine receptors. The functions of the brain are quite dynamic. In neuro-biology, it is described as complex dynamic system.

In physics the tools that people use for modeling dynamic systems is by employing differential equations. The variables show the properties of the system. This variables change over the course of time. When the relationship between those variables is ascertained then one can get the value of these values and thus, one can see the original state of the system. Therefore, empirical study of both the brain and the behavior of Parkinson’s research will result in correlating variables of the activation frequency of the nuclei as well as the neural pathways and local concentrations of different types on neurotransmitters.


RGS–Gα interaction site the A-site (mark & Herlitze (Eur. J. Biochem)

However, these relations are not enough to dub the equation as quantitative thus; the relation is only seen as qualitative. There are many results on study on the brain whereby the change of one variable in one direction would cause another variable to be perceived as moving in another direction, as a result. Here the theory would explain why in Parkinson’s disease, the activation of the thalamus decreases with the decrease of concentration of DA in the striatum (Peijnenburg 351). In the event that these events are insufficient to provide a model with the help of an ordinary differential equation, they can still be represented by a more abstract qualitative equation.

G protein signaling in the C

Until the recent past, it was thought that synaptic transmission was conceptualized some processes where the neurotransmitters would act through their receptors and cause changes in conductances of certain ion channels to cause excitatory or inhibitions on the postsynaptic potential. If taken in this view the human brain would seem to be a very complex diagram whereby the complexity of the visual representation would be in the complex wiring. It has become quite evident over the past twenty years that neurotransmitters elicit complicated effects on the neurons that serve as their targets.

This led to a better and complete understanding of synaptic transmission. In addition to the fast elicitation of the post synaptic potentials and the neuro-transmitter receptor interactions influence most of the activities of the target neuron through a complicated network of intracellular messenger systems. The activation of most catecholamine receptors and most of the other types of receptors is transmitted to the intracellular sites via the G proteins. These in turn couple the receptors to other effector proteins. These would include the numerous intracellular second messenger pathways as well as the ion channels. The generation of these second messengers leads to various physiological effects courtesy to their arrival.

            In most of the cases, the intracellular cascades would involve the changes that occur in protein phosphporylation. This would be the removal of protein phosphatases from the target phosphoproteins or the addition happening through protein kinases. The altered phosphorylation of the phosphoproteins could also be considered as a third messenger. This also alters the physiological nature and activity. This happens with all of the neurotransmitters. Catelochamine regulation of the second messenger and the protein pathways influence a big part of the neuron function through the phosphorylation of different types of the neural proteins.

“These types of intracellular processes produce some sort of quick response to the neurotransmitter and this can be regulation of the ion channel or neuronal firing rate” (Ross & Wilkie, 795–827). At the same time, these processes may produce a short-term modulatory effect on the neuronal function such as the regulation of the response of the neuron relating to similar or different neurotransmitters. These changes can also cause long-term modulation effects on the functioning of the neuron and this can be expressed through gene expression regulation. These changes would require altered synthesis of receptors as well as ion channels, cellular proteins and other forms of learning.

There may be an exception of synaptic transmission that is mediated through receptors containing intrinsic enzyme activity or ion channels. The family of the trans-membrane could be involved in signaling in the nervous system. The G proteins get their name from their ability to bind with   guanine nucleotides, which are guanine diphosphate and triphosphate as termed earlier. Since more and more is becoming known in the field, additional information is being revealed on certain materials otherwise thought to be dormant. Zinc for one, has long been thought to be a mere static component of the protein family.

It was thought to only provide catalytic or structural functions to the group. There is recent research that indicates there is a further use for the metal, especially a particular type known as an intracellular pool of labile zinc, in signal transduction. The homeostasis process of zinc is under tight control, whereby, a specialized system of transport proteins regulates the entry and the export through the plasma membrane and through intracellular distribution. While in the cell, zinc can be stored in the vesicles. These are called zincosomes and the regulation on the labile zinc in the plasma is observed (Wu et al 1q31.).

There are several reports that describe the fluctuations of labile zinc happening after the stimulation of cells on several established pathways. These include mitogen activated protein kinases and calcium that has been showing up. The important point is that is has been discovered that labile zinc is essential for well-known and established physiological signals. The investigations of the molecular mechanism of how zinc exerts the effects have come to identify that it directly reacts with the several components of signaling the pathways, which include the interleukin receptor associated kinase 1. These observations conclude that zinc is a second bivalent metal ion.

Actions of the ANS on the cardiovascular system

It also has the function as the second messenger in the cardiovascular system. In any case, it would seem that signal transduction is the key. The term first made the mark in biological literature in the seventies. It appeared as a title word as early as 1979. Most [physical scientists and electronic engineers had earlier used the term to conversion of energy or information from one form to another. An example being that a microphone transduces sound waves to electric signals. However, attention went to the biological application and the GTP and GTP binding proteins in the metabolic regulation. The term was borrowed from other fields to describe its role. The use of the term grew in momentum and by the year, 2000 about 12 percent of all of the papers that used the word cell had mentions of the word transduction. In the main issue when considering signal transduction, there is concern about the external influences and this could mean the presence of specific hormones and how they could affect what happens inside the target cells. There is some difficulty however, since the hormones being hydrophilic meaning the substances are unable to pass through.

Thus, their influence is exerted from the outside and the membranes of the cells are very thin, and effectively impermeable to ions as well as polar molecules. Even foe small molecules such as urea, the permeability is still about ten thousand times lower than that of water. Thus for a molecule like that of adrenaline the permeation is still quite too low to measure. Thus, the evolution of these receptors has accompanied the development of the mechanisms and it is these, which permit the external chemical signaling molecules. These are the first messengers to direct the activities within the cells in many ways with high specificity. Signal transduction has delved into the world of the intracellular activities as those within the cell. There should also be a consideration of the adherence of the cell surface to other cells and enquires how this works in terms of asking how it affects the responses to the soluble agonists such as growth factors as well as the how the soluble agonists affect the cellular adherence. The event itself is an important in the maintenance of the stem cell compartments as well as the epithelial mesenchymal transition. These molecules affect the adhesion and therefore serve as the targets for the signals that are generated within the cell (Zhong & Neubig, pp. 837–845).

These are two exemplified aspects in regulation of survival, regulation as well as leukocyte trafficking. Adhesion molecules are very important especially when it comes to the nerve cell differentiation, functioning of the synapse as well as gene expression in the epithelial cells. At first, these adhesive molecules were thought to be a form of glue but this turned out to be an untruth. At the present, they are recognized as receptors and the act as signaling molecules. However, the ligands that are involved with these adhesion molecules are insoluble. They are presented themselves on the adjacent cells by an extracellular matrix that happens to be on the surface of epithelia, or by a mass of connective tissue. These adhesive molecules development became known in the 1970s as a result of an investigation interestingly about the brain. It was realized that the organizational system of the cells that were in the central nervous system needed a systematic and dynamic process of cell guidance as well as cell adhesion. This would go on to drive the direction seeking processes of neutrite growth and the formation of synapses.

On this account, there were two main ideas that were considered. One was suggested during the development in order to establish precise cell-to-cell contacts. The interacting cells would each present the adequate unique adhesion molecules that would fit into each other like a lock and key simulation. The second idea would be that the set of adhesion molecules would be limited although their building capacity is limited over time. For instance, one could say that offering the same molecule during the process of outgrowth. This would be interpreted as a low affinity state.

The cell would then turn to convert the adhesion molecule into a high affinity state. This might help in binding to a counter part in a nearby cell. It appears that there is truth to both of the propositions. The number of the adhesion molecules is limited and their capacity to interact with the counter receptors is regulated at the levels of expression and by their state of binding. In the part of immunology, the set of adhesion molecules that is set on the cell surface as well as the state of activation is dubbed as the area code (De Vries et al, pp 235-271).

Some chemicals in the brain assist with the functions of releasing the synapses with the neurotransmitters. Neuro-peptides are released at the synapse region along with the neurotransmitters. They differ from the neurotransmitters in the fact that they are found at lower concentrations and are derived from larger inactive molecules that would be precursors, synthesized in the soma. These neuro-peptide precursors are transported down the axon while they are in the process of getting processed and cleaved. They are then degraded right after the synaptic release. The precursor peptides can then be processed differently according to the different neurons.  They can act as multiple sites whereby they include the pre synaptic and post synaptic sites this could then include cells that are far away from their site of release. Peptide release however, requires calcium. The quantity of the release in this case is proportional to the neuronal firing. Most of the peptide binding sites are G- protein coupled receptors that activate enzyme effectors or ion channels. Many psychoactive drugs alter the neuronal signaling by acting as endogenous transmitters to activate neurotransmitter receptors. The other drugs act by altering synthesizing the release, uptake, or degradation of a neurotransmitter.

The chronic use of psychotropic drugs exerts the common effects through the regulation of neuronal signaling elements that would occur at a variety of levels. These include the neurotransmitters, G proteins, second messengers, as well as the protein kinases. Chronic drug agonist treatment introduces the down regulation of the receptors by reducing the receptor synthesis that takes place (Hammer and Kaplan, 59). Other drugs such as anti psychotics may produce effects at a variety of levels, which include genomic, neuro-chemical, as well as neuro-physiological. At this, level the acute treatment antipsychotic treatment of the dopaminmergic neurons. On the other hand, chronic antipsychotic drug treatment produces the delayed inactivation of dopamine neuron firing.

 The condition of the brain and the reactions to different drugs affect whether one would be an addict or not. For one, neurosomatic patients do not fit the criteria for addiction prone people. This is in the same way, that chronic pain patients are not prone to addictions. Drug abuse in terms of medicine would be termed as craving higher amounts and doses of drugs that would cause acute damage to the patient upon intake (Goldstein, 331). Some chronic pain patients would argue their dosage requires escalation on a regular basis; however, they still hold to the line. These individuals are the ones at risk and have the most cases, of dropping from counseling. The sensitization of behavior encompasses the ability of stimulated cells NAc cells to secrete peptides as well as related substances. Signal transducing pathways may also include the regulation of alternative splicing. In this way, the use of alternative exons keeps changing during the development or in response to other stimuli. Neurons can change their molecular structure when they become activated, (Chen & Lambert, 2000). This can be done by communication with other neurons or by receiving information from the outside.

In the basic definition of the sense, a drug is a substance that brings about a biological change within the functions of a cell. The agonist, in this case, binds to and activates the receptor. The agonists come in various forms whereby a full agonist binds and activates the receptor with full efficiency. A good example in this case, of a full agonist is isoproterenol and its mimics the ligand epinephrine. Other drugs act as pharmaceutical antagonists. They might bind the receptors but do not lead to any signaling by the cell. On the contrary, they interfere with the ability of an agonist to activate the receptor.  In this case, the effect of a pure or neutral antagonist depends entirely on it preventing the binding of the agonist molecules and blocking their biological functions. An endogenous ligand like serotonin may bind and stimulate the receptors that couple to different subsets of the G proteins. It is becoming apparent that the relative efficacy of compounds may actually depend on the conditions used in the experiment. Inverse agonists on the other hand, are easier to identify with functional screens versus classical binding essays. When a receptor lacks the constitutive activity, a neutral agonist in turn acts as the neutral competitive agonist (Chan & Otte, pp. 10-20). This was emphasized by a recent report where it was thought to be the neutral antagonists of the 5HT, which were shown to be the inverse agonists.

Future prospects

The regulations of signal transduction within critical regions of the brain have effects on the intracellular signal generated by multiple neurotransmitter systems. The effects represent putative mediators of the therapeutic actions of the available antidepressants and mood stabilizers, mediated by their effects on a network of interconnected neurotransmitter pathways. For many refractory patients with brain disorders, new drugs that resemble the  "traditional" drugs that alter neurotransmitter levels either directly or indirectly and those that bind to cell surface receptors which are of little medical importance (Arshavsky & Pugh, pp. 11-14). This happens so because of the assumption that the target receptors are functionally intact, furthermore the altered synaptic activity will be transduced to modify the postsynaptic "throughput" of the system. However, abnormalities existing in signal transduction pathways suggest that improved refractory to conventional medications is obtained by direct targeting of postreceptor sites.

Discoveries on a variety of mechanisms involved in the formation and inactivation of second messengers allows for development of pharmacological agents designed to "site-specifically" target signal transduction pathways. However, this is more complex than the development of receptor-specific drugs. This has made it possible to design novel agents that may have implications on the second messenger systems due to their heterogeneous nature at the molecular and cellular level linked to receptors in various ways and expressed in different cell types in different stoichiometries. Furthermore, since signal transduction pathways show unique characteristics depending on the rate of guanine nucleotide exchange, G protein conformational states, GTP hydrolysis, interaction with different RGS proteins, and cytosol-to-membrane translocation of PKC isozymes and receptor kinases, and many other ways, they provide in built-in targets for relative specificity of action, that relies on the "set point" of the substrate. In this study, we can summarize that due to technological advances in both biochemistry and molecular biology, understanding the complexities of the regulation of neuronal function is greatly enhanced in our abilities. 

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