REGULATION OF PITUITARY VASOPRESSIN V1B RECEPTOR MESSENGER-RNA DURINGSTRESS IN THE RAT

Previous studies have shown a parallel relationship between pituitary vasopressin (VP) receptor content and responsiveness of the corticotro ph during chronic stress. The regulation of pituitary VP receptors was further studied by analysis of V1b VP receptor mRNA levels in pituita ries of rats subjected to chronic immobilization, i.p. hypertonic sali ne injection (physical stress paradigms associated with increased pitu itary responsiveness), and water deprivation, or to 2% saline in the d rinking water (osmotic stress paradigms associated with decreased pitu itary responsiveness). Northern blot hybridization with a 363 bp P-32- labelled fragment of the rV1b receptor cDNA coding sequence revealed t wo bands of about 3.7 and 3.2 Kb, whereas a probe directed to the 5' u ntranslated region recognized only the 3.7 Kb band, Repeated i.p. hype rtonic saline injection: 3 times in 24 h at 8 h intervals, or daily fo r 8 days, increased the intensity of the 3.7 Kb band by 155+/-17.5% (P <0.01) and 118+/-14.6% (P<0.01), respectively, while the 3.2 Kb band i ncreased by 122+/-39.3% (P<0.01) only after 3 times injection. Smaller increases of 39+/-11 and 33+/-9% (P<0.05) in the 3.7 Kb band were fou nd after repeated immobilization 3 times in 24 h and 2 h for for 8 day s respectively, In situ hybridization studies confirmed significant in creases (P<0.05) in V1b receptor mRNA levels after 8 and 14 days repea ted immobilization (63+/-19% and 83+/-10%) or i.p. hypertonic saline i njection (110+/-13% and 73+/-20%). In response to acute stress, V1b re ceptor mRNA increased by 77+/-5% (3.7 Kb band) after 4 h immobilizatio n for 1 h, whereas both bands were reduced by 49+/-5% and 45+/-5%, 4 h after a single i.p. hypertonic saline injection, The decrease in V1b receptor mRNA following a single i.p. hypertonic saline injection was prevented by pretreatment with a V1 receptor antagonist, suggesting th at increased VP secretion may account for this effect. In spite of the decrease in V1b receptor mRNA following i.p. hypertonic saline inject ion, VP binding in pituitary membrane rich fractions, and VP-stimulate d inositol phosphate formation in quartered hemipituitaries were incre ased by 24 and 39%, respectively, V1b receptor mRNA levels were unchan ged or decreased following prolonged osmotic stimulation. These studie s suggest that increased V1b receptor mRNA levels contribute to the VP receptor upregulation observed during repeated immobilization and i.p . hypertonic saline injection, whereas the lack of parallelism between V1b receptor mRNA and VP binding indicates that regulation of steady- state levels of V1b receptor mRNA is not a primary determinant in the control of pituitary VP receptor concentration during stress.