Neuropeptide synthesis

Dyn is not yet known, it is likely that such changes reflect variations in the activity of the associated pathways. One possible explanation is that increases in neuropeptide tissue levels are due to decreased release of the transmitter, which dunmishes the extracellular peptide metabolism and results in accumulation of these peptide substances. Another possible contributing factor is a drug-related alteration in neuropeptide synthesis. For example, Bannon et al. (1987) reported that METH administration increased the quantity of striatal messenger RNA for the SP precursor preprotachykinin. Thus, increases in peptide synthesis might contribute to increases in peptide content caused by treatment with METH or the other amphetamine analogs. 

In the case of neuropeptides, presynaptic neurotransmission synthesis occurs only in the cell body because the complex machinery for neuropeptide synthesis is not transported into the axon terminal. Synthesis of a specific neuropeptide begins with the pre-propeptide gene in the cell nucleus (Fig. 1 —9). This gene is transcribed into primary ribonucleic acid (RNA), which can be rearranged, or edited, to create different versions of RNA, known as alternative splice variants, such as prepropeptide RNA. 

FIGURE 1—9- Neurotransmitter synthesis in a neuropeptidergic neuron. Neurotransmitter synthesis occurs only in the cell body because the complex machinery for neuropeptide synthesis is not transported into the axon terminal. Synthesis of a specific neuropeptide begins with the transcription of the pre-propeptide gene in the cell nucleus into primary RNA, which can be rearranged or edited to create different versions of RNA, known as alternative splice variants or pre-propeptide RNA. Next, RNA is translated into a pre-propeptide, which enters the endoplasmic reticulum, where its peptide tail is clipped off by an enzyme called a signal peptidase to form the propeptide, the direct precursor of the neuropeptide neurotransmitter. Finally, the propeptide enters synaptic vesicles, where it is converted into the neuropeptide itself. Synaptic vesicles loaded with neuropeptide neurotransmitters are transported down to the axon terminals, where there is no reuptake pump for neuropeptides. The action of peptides is terminated by catabolic peptidases, which cut the peptide neurotransmitter into inactive metabolites. 

A striking feature of neurotransmitter ch istry is the precise regulation of synthesis, that, for example, results in a few neurons synthesizing a particular neuropeptide while all the rest do not. It appears from the vertebrate literature that mature neurons also precisely regulate the rate of neuropeptide synthesis... 

Neuropeptides constitute the largest and most diverse class of signaling substances known in metazoans. Over the last 20 yr it has become apparent that neuropeptides have important roles as neurohormones, neuromodulators, cytokines, morphogenetic factors, and possibly in some cases, as true neurotransmitters. Each neuropeptide may even be multifimctional and exist in several isoforms in a given animal species. In the search for functions of neuropeptides, it has been critical to be able to localize sites of synthesis and release. Immunocytochemistry (ICC) has been instrumental in the accurate mapping of the cellular and subcellular distribution of neuropeptides in tissue. Other immunological assays, such as radioimmunoassay (RIA) and immxmo-enzymatic assay (ELISA) provide powerful complements for quantification of neuropeptides. Several important discoveries related to neuropeptides have relied on ICC, for example Different neuropeptides have very specific distributions in small populations of neurons (1—3), neuropeptides are commonly colocalized with low-mol-wt neurotransmitters or other neuropeptides (4), the chemical diversity of neurons is far greater than previously suspected (2,3), and neuropeptide synthesis and release can be episodic (5). 

The synthesis of peptide antagonists has greatiy aided in the development of knowledge of other neuropeptide systems as well. 

CCK is found in the digestive tract and the central and peripheral nervous systems. In the brain, CCK coexists with DA. In the peripheral nervous system, the two principal physiological actions of CCK are stimulation of gaU. bladder contraction and pancreatic enzyme secretion. CCK also stimulates glucose and amino acid transport, protein and DNA synthesis, and pancreatic hormone secretion. In the CNS, CCK induces hypothermia, analgesia, hyperglycemia, stimulation of pituitary hormone release, and a decrease in exploratory behavior. The CCK family of neuropeptides has been impHcated in anxiety and panic disorders, psychoses, satiety, and gastric acid and pancreatic enzyme secretions. 

Neuropeptide Y. Neuropeptide Y [82785 5-3] (NPY) (255) is a 36-amiao acid peptide that is a member of a peptide family including peptide YY (PYY) [81858-94-8, 106338-42-5] (256) and pancreatic polypeptide (PPY) [59763-91-6] (257). In the periphery, NPY is present in most sympathetic nerve fibers, particulady around blood vessels and also in noradrenergic perivascular and selected parasympathetic nerves (66). Neurons containing NPY-like immunoreactivity ate abundant in the central nervous system, particulady in limbic stmctures. Coexistence with somatostatin and NADPH-diaphorase, an enzyme associated with NO synthesis, is common in the cortex and striatum. 

Several independent laboratories have now demonstrated that both lithium and valproate (VPA) exert complex, isozyme-specific effects on the PKC (protein kinase C) signaling cascade (reviewed in [3, 5, 11-13]). Not surprisingly, considerable research has recently attempted to identify changes in the activity of transcription factors known to be regulated (at least in part) by the PKC signaling pathway - in particular the activator protein 1 (AP-1) family of transcription factors. In the CNS, the genes that are regulated by AP-1 include those for various neuropeptides, neurotrophins, receptors, transcription factors, enzymes involved in neurotransmitter synthesis, and proteins that bind to cytoskeletal elements [14]. 

The supply of conventional neurotransmitters in small synaptic vesicles is replenished in nerve terminals by local synthesis, and many conventional neurotransmitters are recaptured after secretion. In striking contrast, neuropeptides are initially synthesized in the cell soma, sequestered within the lumen of the secretory pathway and transported down the axon while undergoing cleavages... 

Neuropeptides play key roles in appetite regulation and obesity. Many genes for neuropeptides and neuropeptide receptors have been implicated in obesity and cachexia, anorexia and bulimia [34]. For example,NPY administration into the CNS causes overeating and obesity. A second peptide involved in obesity is leptin, a product of adipocytes and the stomach. The leptin gene is defective in the ob/ob mouse but in normal mice leptin binds to its receptor in the hypothalamus, causing a decrease in the synthesis and release of hypothalamic NPY. 

From these studies, some patterns of effect potentially mediated by the endocrine system have been observed. These include the accumulation of vitellogenin in ticks [50-52] and accelerated ovarian development in beetles and ticks [51, 53]. Different researchers have attributed these effects to a range of different mechanisms, including induced excitation of neurosecretory cells releasing juvenile hormone [50], and ecdysteroid disruption either by blocking the neuropeptide itself or at the epidermal site of synthesis [51]. Other researchers have not corroborated these results and have even found contradictory effects, e.g., suppressed ovarian development [52], Therefore at present for SPs, there is no clear evidence for adverse population-relevant effects with an underlying endocrine mode of action. 

Glycine plays an important inhibitory role in the lower brain stem and spinal cord. Little is known about the synthesis of glycine. It activates a Cl- channel, which is antagonized by strychnine. Other endogenous amino acids may activate the glycine channel, such as taurine and j8-alanine. Neuropeptides... 

One of the best-characterized effectors and second messenger systems is the cAMP cascade that can be either activated or inhibited by neurotransmit-ter/neuropeptide receptors, including those implicated in anxiety/stress such as CRE Receptors that activate cAMP synthesis couple with the stimulatory G protein, Gsa, and those that inhibit this second messenger couple with the inhibitory G protein, Gia, and these either stimulate or inhibit adenylyl cyclase, the effector enzyme responsible for synthesis of cAMP (Duman and Nestler 1999). There are at least nine different forms of adenylyl cyclase that have been identified by molecular cloning, each with a unique distribution in the brain. The different types of adenylyl cyclase are activated by Gsa as well as the diterpene forskolin, but are differentially regulated by Gia, the Py subunits, Ca, and by phosphorylation. This provides for fine control of adenylyl cyclase enzyme activity and regulation by other effector pathways. 

Huag T, Storm JF (2000) Protein kinase A mediates the modulation of the slow Ca2+-dependent K+ current, lsAHP,by the neuropeptides CRF, VIP, and CGRP in hippocampal pyramidal neurons. J Neurophysiol 83 2071-2079 Huang Y, Li X-C, Kandel ER (1994) cAMP contributes to mossy fiber LTP by initiating both a covalently mediated early phase and macromolecular synthesis-dependent late phase. Cell 79 69-79... 

Fig. 2 Increased synthesis, content and release of vasopressin (AVP) in the PVN of high-anxiety (HAB) vs low-anxiety (LAB) rats under basal circumstances. Above left in situ hybridization. Above right immunocytochemistry (courtesy of Dr. N. Singewald, University of Innsbruck). Middle and below intra-PVN release of AVP and oxytocin (OXT) measured by in vivo microdialysis under basal conditions and in response to hypertonic stimulation to reveal the releasable neuropeptide pool. p<0.05, p<0.01 vs LAB. (Adapted from Wigger et al. 2004)...

The methods of peptidic synthesis, in the liquid phase as well as in the solid phase, have led to the preparation of numerous polypeptides that contain a fluorinated amino acid. Most of the examples concern peptidic hormones and neuropeptides, with replacement of either (1) a phenylalanine (or of a tyrosine) by an analogue containing a... 

In my admittedly biased view, the most coherent approach is that of a profoundly disturbed stress system that under specific conditions paves the way to development of mood disorders. These stress-system alterations can be genetic or acquired through trauma in early life or even in utero. Consistent with this neuroendocrine hypothesis are findings that centrally released neuropeptides that drive the HPA system also have behavioral effects that are similar to affective symptoms. This view is further supported by the documented ability of various antidepressants to enhance corticosteroid receptor synthesis and efficacy. Moreover, the stress system, particularly the corticosteroids and their receptors, interferes with all of the neurotransmitter receptor systems, including intracellular signaling, that have been considered in the context of mood disorders. New drugs targeted directly to various elements of the stress system will constitute a major step forward. 

Enzymes responsible for the synthesis and degradation of an expanding list that includes neurotransmitters, neuropeptides, and neurohormones... 

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