A two-step mechanism by which corticotropin-releasing hormone releases hypothalamic beta-endorphin: the role of vasopressin and G-proteins.
It is well established that in the pituitary gland corticotropin-releasing hormone (CRH) stimulates the release of beta-endorphin (beta-E) via a cAMP-linked mechanism. Studies of the mechanisms underlying the CRH stimulation of beta-E release from rat hypothalamic slices perifused in vitro are reported in this paper. The data indicate that both a cAMP-dependent and non-cAMP-dependent mechanism mediate the action of CRH in the hypothalamus. The presence of a cAMP-linked mechanism was suggested by the finding that cholera toxin (0.1-10 nM) and forskolin (2.5 x 10(-6) M), both of which act to raise intracellular cAMP levels, stimulated the release of beta-E. In both cases, no further stimulation was seen upon addition of CRH (10(-8)M). However, it was also found that preincubation of the tissue with pertussis toxin (PTX; 100 ng/ml) prevented both the CRH- and forskolin-stimulated release of beta-E. This indicated that, in addition to the cAMP-linked mechanism, a further messenger system which is connected to a PTX-sensitive G-protein may also play a role. The latter observation also implied that a further substance, which utilizes a separate second messenger system, might be involved in the CRH stimulation of beta-E release. In this regard the role of arginine vasopressin (AVP) was investigated due to the known interaction between CRH and AVP in the pituitary gland. AVP (10(-12) to 10(-6)M) itself potently and dose-dependently stimulated beta-E release, producing a maximal increase of 220% above basal levels. The AVP-induced release of beta-E was abolished in PTX-pretreated hypothalami. The apparently obligatory requirement of AVP for the CRH-stimulation of beta-E release was illustrated by the finding that blockade of AVP receptors using the AVP antagonist d(CH2)5 [Tyr(OEt)2,Val4]-AVP almost completely attenuated the CRH-stimulated release of beta-E. Furthermore, in the presence of a high concentration of AVP (10(-6)M) no further stimulation of release was seen with CRH (10(-8)M). These data therefore strongly indicate that CRH acts via the intermediacy of AVP to release beta-E from hypothalamic slices in vitro and that two separate second messenger systems are involved: a cAMP-linked mechanism connected to a cholera toxin-sensitive G-protein (CRH) and a second system linked to a PTX-sensitive G-protein (AVP).
Burns
G
G
Department of Neuropharmacology, Max Planck Institute for Psychiatry, Planegg/Martinsried, Federal Republic of Germany.
Almeida
O F
OF
Passarelli
F
F
Herz
A
A
D000818:Animals; D001127:Arginine Vasopressin / Q000031:analogs & derivatives* / Q000494:pharmacology* / Q000502:physiology; D002772:Cholera Toxin / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000502:physiology*; D007031:Hypothalamus / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D037342:Pertussis Toxin; D051381:Rats; D011919:Rats, Inbred Strains; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D001615:beta-Endorphin / Q000378:metabolism*
D010566:Virulence Factors, Bordetella; D001127:Arginine Vasopressin; D005576:Colforsin; D001615:beta-Endorphin; C028884:1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)-2-(O-ethyl-Tyr)-4-Val-arginine vasopressin; D002772:Cholera Toxin; D003346:Corticotropin-Releasing Hormone; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins