Gastrointestinal and Brain Peptides

The neuropeptide CCK is an octapeptide found regionally in the gastrointestinal tract and brain (brain-gut peptide), where it acts as a neurotransmitter and neuromodulator. Its most abundant form in the brain is the C-terminal sul-fated octapeptide fragment CCK-8, which interacts with the same affinity with both CCK receptor subtypes CCK-A and CCK-B. Extensive pharmacological studies have been carried out during the past few years suggesting that CCK may participate in the neuroendocrine responses to stress (e.g., Harro et al. 1993 Dauge and Lena 1998). 

Bombesin is a tetradecapeptide isolated from the skin of the amphibian frog Bombina bombina (Anastasi et al, 1971). It has been proposed that bombesin-related peptides may be released from the gastrointestinal tract in response to ingested food, and that they bridge the gut and brain via neurocrine means to inhibit further food intake (Merah et al, 1999). 

Amine precursor uptake decarboxylase system, APUD system the nervous system and part of the gastrointestinal tract derived from the neuroectodermal cells of the primitive neural crest. Gut and brain produce several identical peptides, e. g. bombesin, cholecystokinin, neurotensin, substance P, enkephalin, gastrin, vasoactive intestinal peptide, somatostatin, etc., reflecting the common embryological origin of the two tissues. These peptides function specifically in both gut and nervous system, but their functions are generally distinct, because they are largely excluded by the blood-brain barrier. 

The principal hormones of the human posterior pituitary include the two nonapeptides, oxytocin [50-56-6] and arginine vasopressin [11000-17-2] (antidiuretic hormone, ADH). Many other hormones, including opioid peptides (see Opioids, endogenous), cholecystokinin [9011-97-6] (CCK) (see Hormones, BRAIN oligopeptides), and gastrointestinal peptides, also have been located in mammalian neurohypophysis (6), but are usually found in much lower concentrations (7). Studies have demonstrated that oxytocin and vasopressin are synthesized in other human organs, both centrally and peripherally, and there is considerable evidence for their role as neurotransmitters (see Neuroregulators) (8). 

Incretin Hormones. Figure 3 Processing of the proglucagon. The proglucagon peptide is synthesized in pancreatic cells and cells from the gastrointestinal (Gl) tract and the brain. Different proconvertases process the peptide so that in the pancreas the glucagon is produced whereas in the Gl tract and the brain, the GLP-1 and GLP-2 peptides are mainly released. 

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