Serum Prolactin Levels after Administration of the Alimentary Opioid Peptide Gluten Exorphin B4 in Male Rats

Abstract

"Gluten Exorphins are opioid peptides identified in enzymatic digests of gluten. The effects of Gluten Exorphins are still largely unknown. It has been shown that Gluten Exorphin B5 (Tyr-Gly-Gly-Trp-Leu) stimulates Prolactin secretion in male rats. In this study, we have evaluated the Prolactin response to Gluten Exorphin B4, another exorphin whose structure (Tyr-Gly-Gly-Trp) is identical to that of the NH(2)-terminal sequence of GlutenExorphin B5. To this aim, five groups of male rats were given the following intravenous treatments: vehicle, Gluten Exorphin B5 3 mg kg-1 body weight, Gluten Exorphin B4 at the doses of 3, 6 and 9 mg kg-1 body weight. At the dose of 3 mg kg-1 body weight, Gluten Exorphin B5 induced a significant increase in Prolactin levels. Gluten Exorphin B4 could not modify Prolactin secretion, even when administered at doses three times higher than those effective for Gluten Exorphin B5. The present study: (1) indicates that Gluten Exorphin B4 does not modify Prolactin secretion in male rats; (2) confirms the ability of Gluten Exorphin B5 to exert a stimulatory action on Prolactin release; (3) suggests that the presence of the carboxy-terminal leucine in Gluten Exorphin B5 is essential for its action on Prolactin secretion."

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Opioid Peptides Derived from Food Proteins: The Exorphins

Abstract

"Peptides with opioid activity are found in pepsin hydrolysates of wheat gluten and alpha-casein. The opioid activity of these peptides was demonstrated by use of the following bioassays: 1) naloxone-reversible inhibition of adenylate cyclase in homogenates of neuroblastoma X-glioma hybrid cells; 2) naloxone-reversible inhibition of electrically stimulated contractions of the mouse vas deferens; 3) displacement of [3H]dihydromorphine and [3H-Tyr, dAla2]met-enkephalin amide from rat brain membranes. Substances which stimulate adenylate cyclase and increase the contractions of the mouse vas deferens but do not bind to opiate receptors are also isolated from gluten hydrolysates. It is suggested that peptides derived from some food proteins may be of physiological importance."

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Gluten exorphin B5 stimulates prolactin secretion through opioid receptors located outside the blood-brain barrier.

Abstract

"Gluten exorphin B5 (GE-B5) is a food-derived opioid peptide identified in digests of wheat gluten. We have recently shown that GE-B5 stimulates prolactin (PRL) secretion in rats; this effect is abolished by preadministration of the opioid receptor antagonist naloxone. However, since the structure of naloxone allows it to cross the blood-brain barrier (BBB) and antagonize opioid effects centrally as well as peripherally, it could not established, on the basis of those data, if GE-B5-induced PRL release is exerted through sites located inside or outside the BBB. In this study, we sought to determine the site of action of GE-B5 on PRL secretion, by pretreating male rats with naloxone methobromide (NMB), an opioid antagonist that does not cross the BBB. Four groups of rats were given the following treatments: 1) intravenous vehicle; 2) intravenous GE-B5 (3 mg kg(-1) body weight); 3) intraperitoneal NMB (5 mg kg(-1) body weight), followed by vehicle; 4) NMB, followed by GE-B5. Blood samples for PRL were taken at intervals for 40 minutes after vehicle or GE-B5 administration. GE-B5 stimulated PRL secretion; the effect was statistically significant at time 20. NMB preadministration completely abolished PRL response. Our experiment indicates that GE-B5 stimulates PRL secretion through opioid receptors located outside the BBB. Since opioid peptides do not exert their effect on PRL secretion directly, but via a reduced dopaminergic tone, our data suggest that GE-B5 can modify brain neurotransmitter release without crossing the BBB."

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