Neuropeptides

I. Definition

A. Compounds of a peptide nature produced by nerve cells and acting within the nervous system mainly via direct peptidergic pathways.
B. Peptides lends themselves to analysis by immunohistochemistry and RIA
1. Positive immunoreactive response does not prove that expected compounds is present; antibodies are raised to recognize an antigenic site and not a compound.
2. Negative response does not prove that a specific compound is not there; fixation may diminish antigenicity. C. Now can measure in situ distribution of mRNA for a specific peptide.

II. Hypothalamic-pituitary function was first known role for brain peptides.

A. Hypothalamus-posterior pituitary relationship

1. Cell bodies in paraventricular and supraoptic nuclei send axonal projections directly into posterior pituitary.
2. Upon activation, neurohypophyseal cells release peptide from terminals into pituitary; vasopressin or oxytocin. Peptide is taken up by general circulation and distributed to target areas, eg., kidney, mammilary gland. B. Hypothalamus-anterior pituitary relationship
1. Cell bodies in hypothalamus send axons to median eminence; activation of cells leads to release of substance into hypothalamic-pituitary portal circulation. Peptide is carried to pituitary. Hypothalamic peptides include corticotrophin releasing factor (CRF), thyrotropin releasing hormone (TRH), Gonadotrophin or lutenizing hormone releasing hormone (LHRH), growth hormone release inhibiting factor (somatostatin)
2. Peptide acting on pituitary cells promotes or inhibits the release of trophic factors (ACTH, thyrotropin, lutenizing and follicle stimulating hormone) and other pituitary hormones, growth hormone, prolactin. C. Many hypothalamic peptides now found in extrahypothalamic locations (cell bodies identified in many regions of the brain). Hypothalamic cells send projections to areas other than pituitary.

III. Production and regulation (emphasis on opiod peptides)

A. Peptides are formed by transcription of DNA to mRNA. Thus production must take place in cell body. However, it is not transmitter peptides that are synthesized by this process, but larger propeptides.
B. Neuropeptides are cleaved from larger propeptide by specific endopeptidases and some carboxy peptidases. There are 3 opiod propeptides: Proenkephalin, Prodynorphin and Pro- Opiomelanocortin (POMC). Antibodies against each propeptide indicate separate distribution profiles.
1. POMC is localized to pituitary and to cell bodies in hypothalamus that have diffuse projections.
2. Proenkephalin is localized primarily to interneurons throughout the brain
3. Prodynorphin distribution overlaps considerably with proenkephalin cells C. post-translational cleavage of propeptide is tissue specific.
1. Propeptide for ACTH and b-endorphin is POMC
a. Anterior lobe of pituitary: POMC is cleaved to form ACTH and b-lipotropin, 30% of which is further split to release b-endorphin.
b. Intermediate lobe of pituitary: All ACTH is cleaved to produce a-MSH and CLIP, no steroid activity from this tissue. All of b-lipotropin is cleaved to produce b-endorphin. Propeptide cleaved similarly in arcuate nucleus of hypothalamus. 2. Same propeptide thus can serve as precursor to different peptides in different tissues.
3. Amino acid sequence of propeptides encodes information on where endopeptidases will act. D. Transcription is subject to regulation from circulating hormones. Circulating corticosteroids, eg., feed back onto pituitary and brain to regulate transcription of POMC.
1. Transcription regulation is tissue specific
ACTH is released in response to stressors and its release is triggered by CRF. ACTH stimulates secretion of glucocorticoids from adrenal cortex; glucocorticoids feed back and shut down ACTH production and release. They do this by inhibiting transcription of the POMC gene. Adrenalectomy leads to a dramatic increase in POMC mRNA and in POMC-derived peptides in the anterior pituitary, after lag of several hrs. Injection of glucocorticoids leads to a return of mRNA levels to normal. Can get adrenalx effect in cultured tissue without hypothalamus. Adrenalx has no effect on POMC mRNA levels in intermediate lobes. E. Post-translational modification of neuropeptides can influence biologic reactivity.
1. N-acetylation of b-endorphins makes them inactive in binding assays and in test for analgesic properties, but acetylated opioids retain immunological properties.
2. acetylation of endorphins probably takes place with acetylation of MSH. Acetylation of MSH is necessary for important biologic activity to that compound.

III. Inactivation of released peptides

A. No identified high affinity uptake sites
B. Could diffusion away from terminal be sufficient? Responses to peptides are longlasting, perhaps because of time needed for diffusion or perhaps because of slow dissociation from receptor.
C. Peptidases may be important
peptides introduced into brain seem to be rapidly hydrolyzed.

IV. Colocalization

A. Most peptides co-exist in neurons with other messengers.
B. Only a subpopulation of certain systems, however, exhibit certain combination of messengers.
1. Only certain mesolimbic DA projections co-localize CCK
2. In cortex, only about 10% of GAD-positive cells contain a peptide, but all SRIF cells in cortex also stain for GAD.
Not all GAD-ir cells contain peptides, maybe only 10% C. Functions of colocalized substances
1. Peptide may have specific function to perform
a.VIP present in parasympathetic ACh projections to salivary glands; ACh simulates secretion from glands and VIP regulates secretion-related vasodilation as well as facilitates action of ACh on secretory cells.
b. Substance P in primary afferent pain fibers may have a role in inflammation responses rather than mediate primary pain information.
c. VIP released from preganglionic sympathetic terminals may mediate trans-synaptic induction of CA synthesizing enzymes in post-synaptic cell. 2. Peptides may be released at different firing frequency from primary transmitter. Could act postsynaptically to modify response to primary messenger or could act presynaptically to modify synthesis or release of primary messenger. Once peptide is released, it is lost to the presynaptic neuron. It may be economical to the cell to release peptides only under more severe conditions. D. Peptides colocalized with primary transmitter may have no role
1. Concept of superfluous neurotransmitter
Peptides in unexpected places could represent inadvertent gene expression as part of the normal process of gene regulation.
2. Aberrant expression of a single neuropeptide gene in a neuron is expected to produce the same gross neuronal phenotype as would functional gene expression. Peptide cells do not require multiple enzymes for synthesis as do CA, do not express specific uptake mechanisms in vesicles and terminal membrane, nor do they express specific degrading enzymes.
3. Peptide can be released in a Ca++ dependent manner and still have no function.
4. Superfluous expression may be simply less costly to a cell than inhibiting expression of that gene.
5. Superfluous expression could explain so-called mismatch between peptide-containing cells and their receptors. Either the peptide or the receptor could have been superfluously expressed.

Last modified: 9/15/97