BY: O M A R A L R A I S

Neuropeptides

Discovering New Neuropeptides

-new neuropeptides discovered by process of analyzing tissue extract for molecules possessing some particular biological function .

-processing large amounts of hypothalamic tissue , the various hypothalamic releasing peptides were discovered.

-various biological active gut peptides were extracted from large quantities of gastrointestinal tissue.

-the neuropeptides are always formed from larger precursor prohormone polypeptides.

-the cleavage enzymes involved recognize pairs of basic amino acid residues ; these cleavage sites can be identified from DNA sequences and new peptides can be predicted.

-these peptides can be synthesized and tested for biological activity ,the synthetic material can also be used to generate antibodies that can be employed for quantitative measurements(radioimmunoassay),and qualitative distribution of the peptide by immunocytochemistry.

What is different about peptides?

-neuropeptide synthesis can take place only on ribosome's in the perikaryon or dendrites of a neuron.

-the synthesized prohormone is then processed by cleavage enzymes and packaged into vesicles in the smooth endoplasmic reticulum and transported from perikaryon to the nerve terminal.

But peptide –releasing neurons, however ,share certain basic properties with all other chemically characterized neurons.

-neuropeptides are stored in vesicles in presynaptic nerve terminals, their release is Ca+2

dependent; postsynaptically, like the monoamine neurotransmitters the effect mediated by activation of G protein-coupled receptors.

-peptides identical to those extracted from nervous tissue are often made by nonneuronal secretory cells in the endocrine glands and cells linging hollow viscera (gut-brain peptides).

Coexistence of neuropeptides

and neurotransmitters -Dale’s law “neurons released only one

transmitter”-however ,neuroactive peptides started

showing up in autonomic neurons, where there was no need for additional transmitters .the presence of coexisting peptides in central as well as peripheral neurons that already occupied by an amino acid or monoamine or even another neuropeptide is now seen as the norm.

-for example in A.N.S peptide-monoamine interaction in salivary gland ,low freq. stimulation of the parasympathetic nerves releases ACH ,but VIP also released at higher frequencies.

Another example: neuropeptide Y (NPY)found in many sympathetic neurons, is also released at higher freq. of activation and sensitizes smooth muscle target cells to the accompanying adrenergic signals.

-coexistence may be to strengthen or prolong the primary transmitter’s actions, especially when nerve is called on to fire at higher than normal frequencies.

-the past decade has seen the discovery of nonpeptide drugs that act as potent and selective agonists or antagonists at neuropeptide receptors; peptides themselves do not make good starting points for drug discovery: the molecules are too large, do not penetrate the blood-brain barrier ,and are too easily metabolized .

Cholecystokinin

-gut-brain hormone.-CCK is secreted in the periphery as local

hormone in the gut in response to food intake.

-in CNS CCK coexists with dopamine and neurotensin in the substantia nigra and ventrotegmental area ;with VIP , NPY,and GABA inthalamocortical and thalamostriatal connections; and with substance p and 5-HT in medullary neurons.

-CCK neurons in spinal cord and brain stem act as an anti-Opioid system.

-two types of CCK receptors (CCK-1,CCK-2) CCK2 predominates in the CNS of most mammalian species ,although this is not true in human spinal cord and brain stem, where CCK-1 is the most common form .

-CCK-1 antagonist (devazepide) made morphine more effective in treating pain of neuropathic origin.

-intravenous administration of small doses of C-terminal fragments of CCK or the gastrin fragment pentagastrin reliably induce a psychological panic in human subjects .the panic is dose related and short –lived.adminstration of CCK2 antagonist L-365,260 completely blocked this chemically induced panic in volunteers, how ever clinical trial of this drug in patients who suffered spontaneous panic attacks were negative.

Corticotropin-releasing factor

-is hypothalamic releasing factor.-CRF------ant. Pituitary-----ACTH----adrenal gland ------

glucocorticoids.-this is called HPA axis mediates stress reaction with

glucocorticoids production as the end result.-CRF is related to other neuropeptides the urocortins .-act on two distinct receptors CRF-1 , CRF-2.CRF-2 exist in three different variants of the same gene

CRF-2α ,β,γ.-CRF not restricted to the hypothalamus,CRF containing

nerve fibers widely distributed in cerebral cortex , cerebellum , amygdale and other limbic forebrain structures and in monoamine-rich locus ceruleus and raphe nuclei in the brain stem.

-CRF and urocortins are also widely distributed in male and female reproductive organs , skin , gut , heart ,ands muscle.

-in peripheral tissue CRF-1 and CRF-2β receptors predominate.

-brain contains mainly CRF-1and CRF-2α.-apart of HPA axis the extra hypothalamic CRF System

which is not under negative feedback control by glucocorticoids seems particularly well suited to active cortical, limbic ,autonomic , and central monoamine systems thought to be important in mediating autonomic and behavioral reaction to stress.

-clinical findings also suggest a role for CRF in some forms of depression ,anxiety disorders .depressed patient were found to have elevated concentrations of CRF in cerebrospinal fluid; and this was reversed after successful antidepressant treatment.

-non peptide drugs that act as antagonist at CRF-1receptors as potential antidepressant/anxiolitic or antistress medicines.

galanin

-initially isolated from the gut and later found present in the CNS.

-human version of galanin is a 30-amino acid residue peptide lacking the C-terminal amide.

-has 3 receptors GalR1,GalR2,GalR3.-in the CNS possible function of galanin as modulator of

pain systems in the spinal cord -Galanin strong inhibitory , reducing the excitability of

target cells or inhibiting presynaptic release of neurotransmitters or other neuropeptides.

-it is normally present in small amount in many small – diameter primary sensory fibers, but the expression of the peptide is greatly increased if the nerves are damaged .

-galanin is also present in inhibitory BABAergic interneuron's in spinal cord dorsal horn.

-in models of neuropathic pain in which sensory nerve fibers are damaged and galanin is up regulated ,spinally administrated galanin alleviates the heightened pain sensitivity; these results suggest that galanin receptor agonist might provide a noval analgesic.(Galnon,Galmic)galmic compound showed some selectivity to GalR1.Galanon displays little selectivity for galanin receptors but showed strong anticonvulsant properties.

-in brain galanin coexists with serotonin in the dorsal raphe nucleus and with norepinephrine in the locus ceruleus-nuclei------ play a key role in the actions of antidepressant drugs.

-when galanin administered into the brain , showed to be potent inhibitor of serotoninergic transmission, leading to a reduction in serotonin release and a functional blockade of 5-HT1a receptor mediated responses----depressant effects---animal.

-the peptidic galanin antagonist M35 produces antidepressant effects----animal.

neurotensin

-13-amino acid peptide another member of the gut-brain family.-in the intestine neurotensin is secreted by mucosal nonneural

endocrine cells after food ingestion.-it’s name derives from its ability to lower blood pressure and

causes a visible vasodilatation in vessels in the rabbit ear.-it is present in the CNS as neuropeptide with largest amount in

the ant. And basal hypothalamus, the nucleus accumbens and septum,the midbrain dopamine neurons (along with CCK),and selected scattered neurons in the spinal cord and brain stem.

-two G PTN coupled receptors exist, NTS1,2 and third less studied receptor

NTS3 has been reported that is not in G PTN coupled family.-centrally administered neurotensin resulted in analgesia,

hypothermia , anorexia, accentuation of barbiturate and ethanol sleeping time, and increased release of growth hormone and prolactin.

-the peptide has been implicated in pain mechanism , central control of temp. ,control of feeding behavior and in the mechanism of action of antipsychotic drugs(most research attention).

-neurotensin is present in dopamine rich areas ; in rodents it coexists with dopamine in midbrain dopaminergic neurons ,but this is not the case in human brain.

-reduced levels of neurotensin have been reported in postmortem brain samples from schizophrenic patients.

-the association with dopamine and it’s behavioral properties has led to the suggestion that it may act as an endogenous neuroleptic.

-in animals intracerebral administration of neurotensin leads to behavioral effects similar to those seen after atypical neuroleptics(blocked of apomorphine induced yawning , penile erection,climbing behavior).

-administration of neuroleptic drugs lead to increased levels of neurotensin in dopamine rich brain areas with an interesting differential distribution effect of typical drugs (basal ganglia regions)versus atypical drugs(nucleus accumbens and forebrain) this may be a factor in explaining why the atypical drugs are less prone to cause extra pyramidal motor side effects.

-drugs acting as neurotensin agonists might represent a noval approach to the development of atypical neuroleptics.

Opioid peptides

-more than 20 Opioid peptides (also known as endorphins) are now known

-4 distinct Opioid receptors : µ(MOR),δ(DOR),Κ(KOR),and ORL1.

-the Opioid peptides are grouped to several families.

-this field of research has been disappointing in that it has not led to the discovery of new medicines;althought some of the Opioid peptides are very potent (β-endorphin is more than 1000 times more potent than morphine),they are unable to penetrate into the brain after systemic administration and most are rapidly degraded metabolically.

-development of enzymes that inhibits such degradation is seen by some as a possible way of finding novel analgesics.

Substance P & other Tachykinins

-SP is one of the oldest neuropeptides.-member of a family of related peptides , the tachykinins (so called because they can

produce slow contractions of smooth muscle).-the three mammalian neuropeptides are recognized by three receptors NK-1,NK-2,NK-3.-in recent years a number of potent nonpeptide receptor antagonists have became

available with selectivity for one or other of these receptors.-research focused on SP and the NK-1 receptor (although all three peptides and their

receptors are present in mammalian CNS).-SP present in small sensory nerve fibers projecting into the spinal cord where SP may play

a neurotransmitter or neuromodulator role; such C fibers play an important role in transmitting pain information into the CNS so the idea that SP might act as some form of pain transmitter had been studied .

-a series of clinical trials of NK-1 antagonists as analgesics however proved disappointing.-blockade of SP alone is apparently not sufficient to cause analgesia although SP is up

regulated after nerve damage in conditions of neuropathic pain.-sensory neurons that contain SP also invariably contain a second neuropeptide ,calcitonin

gene-related peptide (CGRP)and is too has been implicated in pain mechanism particularly in migraine, where there is evidence for an increased release of this peptide during migraine attacks, nonpeptide antagonist of CGRP have yielded positive results in the relief of acute migraine attacks.

-possible role of SP in mediating stress responses.-NK-1 antagonists block the anxiogenic effects of such stress effect produced from

microinjections of SP into the brain region (animal experiments).-NK-1 antagonist were also reported to have positive effects as antidepressant (animal

experiments).-NK-1 antagonists (aprepitant)also controlled major depression but the results failed to be

repeated on large scale clinical trials.-aprepitant is one of the first of what is hopped to be a new generation of human medicines

based on neuropeptide research. it found clinically useful role in another context as a noval antiemetic agent used in conjunction with 5-HT3 antagonists in patients receiving chemotherapy ; the rationale for this is that SP is present in a majority of sensory fibers in the vagus nerve, which carry anxious stimuli to the brain stem vomiting center.

-nonpeptide antagonists of NK-2(saredutant)act as antidepressant and anxiolitic is in

advanced clinical trials.-nonpeptide antagonists of NK-3(e.g.: talnetant,osanetant)have been proposed as potential

antipsychotic agents.-

Vasopressin & oxytocin

-both peptides are synthesized in neurons in the hypothalamus that project to the post. Pituitary from which they are released into the circulation .

-oxytocin(OT)and arginine-vasopressin (AVP)neurons innervates various regions of the brain and spinal cord.

-AVP acts on three related receptors V1a,V2,V1b(sometimes called V3),have important role in the kidney and cardiovascular system, in the brain AVP acting at V1b receptors enhances the actions of CRF both in pituitary function of releasing ACTH and in extra hypothalamic regions.non peptide antagonists with selectivity for the V1b receptor (e.g.: SSR149415)display anxiolitic and antidepressant actions.

-OT act on a single receptor; it has important role in female reproductive system in parturition and lactation.

-AVP & OT play critical role in regulation social behaviors(pair bonding, parental care ,and territorial protection),human imaging have shown that both maternal love and romantic love activate brain regions that are rich in AVP & OT.defects in such mechanisms may underlie sexual dysfunction ,autism and social phobia.

Appetite-stimulating (Orexigenic)

peptides

-neuropeptide Y a 36-amino acid residue peptide, is one of the most abundant in hypothalamus, particularly in the arcuate nucleus, which is the key feeding control center.

-NPY on of the most potent appetite stimulants in animal studies.

-six distinct NPY receptors five of them are expressed in the hypothalamus ,the Y1&Y5 subtypes appear most important in mediating the action of NPY on appetite.

-Melanin-concentrating hormone (MCH)a cyclic 19-amino acid neuropeptide is found in other feeding centers in lateral hypothalamus and zona incerta,it stimulates energy intake in rats and decreases energy expenditure.

-MCH acts on two receptors MCH-1R & MCH-2R ,non peptide antagonists of MCH-1R prevent diet induced obesity and also have anxiolitic and antidepressant effects, they may prove valuable in the treatment of obesity .

-hypocretins (also known as Orexins) are a group of closely related neuropeptides ,{Hypocretin A (33 residues),and Hypocretin B (28 residues) } ,there are two closely related receptors OX1R & OX2R both peptides and their receptors are present in the hypothalamus and both stimulate food intake, by delaying the normal satiety mechanisms. Nonpeptide antagonists with selectivity for OX1R suppress food intake and advance the onset of satiety. Also involved in control of sleep.

Appetite-Suppressing (Anorectic) Peptides

-melanocortins are bioactive peptides derived from the precursor POMC.

-this prohormone give rise to ACTH in anterior pituitary ,but in hypothalamus it can generate bioactive peptides related to α-MSH that act on the MC4R to inhibit eating and reduce body weight.

-MC4R and related MC3R are unique in that there is an endogenous 132-residue peptide, Agouti-related peptide (AGP) that acts as an antagonist at theses receptors.

-balance between melanocortins peptides and AGP may determine at any particular time where this is an orexigenic or anorectic mechanism .

-MC4R found on fat cells on the periphery where they may respond to melanocortins released from the pituitary.

-CART (cocaine-and amphetamine-regulated transcript)is an 89-amino acid peptide that was discovered on the basis of its increased expression in rats following repeated administration of cocaine or amphetamine. Intracerebral administration of CART inhibits normal and fast induced feeding in rats; it also blocks NPY-induced feeding.

-in brain stem nucleus of the solitary tract , preproglucagon gives rise to various peptides , including the glucagon –like peptides GLP-1 and GLP-2 these play complex roles both in the brain and in the periphery in the control of appetite.

-in the hypothalamus ,GLP-1 containing fibers and the GLP-1 receptor appear to play a key role in the central anorectic actions of GLP-1.