The Brattleboro rat carries as a recessive trait the inability to synthesize hypothalamic vasopressin and its related neurophysin but is able to synthesize oxytocin and its neurophysin. Brattleboro rats homozygous for this trait have no immunologically detectable circulating vasopressin and manifest a complete syndrome of diabetes insipidus, which is corrected with vasopressin replacement therapy. Such a defect could be due to absence of the gene encoding vasopressin, the presence of an abnormal gene, or a variety of transcriptional or posttranscriptional abnormalities. We report here that the hypothalamus of the Brattleboro rat contains detectable, although markedly reduced, levels of an mRNA indistinguishable in size with and similar in sequence to authentic vasopressin mRNA. Corresponding levels of oxytocin mRNA were the same in Brattleboro and normal rat hypothalami. These findings indicate that the Brattleboro rat expresses a vasopressin gene, but at a reduced level.Vasopressin (VP) and oxytocin (OT), nonapeptides synthesized in the magnocellular neurons of the hypothalamus, are secreted via axons in the neurohypophysis and regulate water conservation, cardiovascular functions, and smooth muscle contractions (1). Both peptides are synthesized as precursors in covalent linkages with neurophysin peptides, termed preproVP-neurophysin and preproOT-neurophysin (2-4). The Brattleboro rat carries as a semirecessive trait the inability to synthesize VP or its related neurophysin but is able to synthesize OT and its neurophysin (5). Brattleboro rats homozygous for this trait have severe diabetes insipidus, which is completely corrected by VP replacement therapy. Although the Brattleboro rat has been used extensively in the study of VP deficiency, nothing is known regarding the nature of its genetic defect. Therefore, we examined this issue using molecular biological techniques. Such studies might shed some light on the nature of the analogous genetic defect in hereditary human VP deficiency. Our findings indicate that the Brattleboro rat does possess the gene for VP and expresses this gene, although at a markedly reduced level. MATERIALS AND METHODSPreparation and 5'-End Labeling of Synthetic Oligonucleotides. Oligonucleotides were synthesized by the triester method and were purified by reversed-phase high-pressure liquid chromatography on C18 resin (Waters Associates) (6).For use in subsequent studies, the oligonucleotides were 5'-end-labeled using [y32P]ATP (Amersham) and T4 polynucleotide kinase (Bethesda Research Laboratories) to a final specific activity of 4-6 x 106 cpm/pmol (7). Nucleotide sequences of the oligonucleotides were confirmed using the chemical degradation method (8).Preparation of Hypothalamic mRNA. Adult female (180-200 gm) homozygous Brattleboro rats were deprived of water for 24 hr prior to decapitation. They were confirmed to be homozygous for the Brattleboro trait by two criteria: (i) they all lost >20% of their body weight after 24 hr of water deprivation, compared with <5% loss in ...