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Receptors and Second Messengers

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  1. Introduction
    1. A powerful tools of molecular biology
    2. Receptor population for peptides
    3. Neuropeptide and second messenger signal transduction systems
  2. Peptidases
    1. Description
    2. The metabolism of TRH
  3. Neuroendocrine secretion
    1. The peptides involved in neuroendocrine regulation
    2. Regional differences in CRF receptor regulation
  4. Neuropeptides in psychiatric disorders
    1. Neuropeptide research
    2. Alzheimer's disease
    3. The CRF-containing interneurons of the cortex
    4. Mood disorders
    5. Somatostatin
    6. Thyrotropin releasing hormone
  5. Conclusion
  6. References

Neuropeptide receptors have undergone the same process of discovery and characterization that receptors for other neurotransmitters have enjoyed. The process begins with the pharmacological characterization of the receptor's physicochemical binding properties by assessing the affinity of various metabolically derived and synthetic peptide fragments, and the native molecule, for the receptor binding site found in membrane preparations. Peptide receptor locations are mapped with radioactive or fluorescent tags that are inserted into peptide molecules, which often contain substituted amino acids at the most vulnerable peptidase cleavage sites. Previously, once the peptide receptor was characterized pharmacologically, it was usually purified from some relatively enriched biological tissue source or brain region by affinity column chromatography. After it had been purified, binding parameters and activity were recharacterized for the reconstituted purified receptor protein and structural information obtained by X-ray crystallography. This process was closely followed in the purification of the neurotensin-neuromedin N receptor.

[...] Indeed, it has been reported that there is a marked (23 percent) decrease in the density of CRF receptors in the frontal cortex of suicide victims as compared to matched control samples; these findings have been confirmed in a second study. Somatostatin Like a number of other neuropeptides, somatostatin was serendipitously discovered during attempts to purify growth hormone- releasing factor (GRF). As the name implies, SRIF inhibits the release of growth hormone from the anterior pituitary. Since its structural identification 20 years ago, SRIF has been unequivocally shown to be the major inhibitory influence on growth hormone secretion. [...]

[...] PEPTIDASES Peptides are degraded to smaller fragments, and eventually to single amino acids, by specific enzymes termed peptidases. As yet, peptides or their fragments have not been shown to be actively taken up by presynaptic nerve terminals, as is the case for the monoamines. The enzymes may be found bound to post- or presynaptic neural membranes or in solution in the cytoplasm and extracellular fluid, and they are distributed widely in peripheral organs and serum as well as in the CNS. [...]

[...] Subcortical regions containing SRIF, such as the substantia innominata, hypothalamus, and bed nucleus of the stria terminalis, were spared whereas SRIF receptors in the cortex were decreased in number. In regions such as the hippocampus, findings of SRIF depletions were less consistent than in the cortex, but when depleted in the hippocampus, the SRIF neurons colocalized with neuropeptide Y were spared. Somatostatin concentration in the CSF of Alzheimer's disease patients has also been consistently found to be decreased, and this decrease has been correlated with the magnitude of cognitive impairment. [...]

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