Receptors and Second Messengers

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biology

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published 26/11/2007

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Summary

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.

The neurotensin receptor was first characterized by photoaffinity labeling and cross-linking of radioiodinated ligands, which resulted in two labeled subunits of about 49 Kd and 51 Kd from rat brain synaptosomes.
The much more powerful tools of molecular biology have been utilized more recently.
Neuropeptide receptors have been associated with just about every type of second messenger signal transduction system that has been identified.
Peptides are degraded to smaller fragments, and eventually to single amino acids, by specific enzymes termed peptidases.
The metabolism of TRH has been investigated fairly completely, principally because of the limited number of fragments that can be generated from a tripeptide.
The peptides involved in neuroendocrine regulation have cell bodies residing in the hypothalamus that receive feedback from all levels of the endocrine axes.
Regional differences in CRF receptor regulation by corticosterone have also been reported, which have been shown to partly result from differential glycosylation of the CRF receptor.
Alzheimer's Disease Dementia of the Alzheimer's Type represents up to two thirds of the demented population encountered in clinical practice, and over half of the nursing home beds in the United States are currently occupied by such patients.
The CRF-containing interneurons of the cortex are also consistently depleted in Alzheimer's disease. As with SRIF, subcortical areas containing CRF neurons may be spared, but unlike SRIF, CRF receptors are increased in number (up-regulated) with no change in affinity.
Corticotropin-Releasing Factor After a search spanning nearly three decades, CRF was isolated and characterized in 1981 as a 41-amino acid peptide.
A series of studies have demonstrated significant elevations of CRF concentrations in the CSF of drug-free patients with major depression or following suicide.
Like many other neuropeptide transmitters, central administration of SRIF produces a variety of behavioral and physiological effects.
Decreased neurotensin concentrations in CSF have been reported in several populations of patients with schizophrenia when compared to controls or patients with other psychiatric disorders.