Regulation of hypothalamic magnocellular neuropeptides and their mRNAs in the Brattleboro rat: coordinate responses to further osmotic challenge

A paradigm was developed for the chronic osmotic stimulation of homozygous diabetes insipidus rats of the Brattleboro strain, a strain that fails to synthesize vasopressin. This study examines the adaptation of 2 sets of coexisting peptide hormone magnocellular neurons in the hypothalamoneurohypophyseal system (HNS) of Long Evans (LE), Brattleboro heterozygote (HZ), and Brattleboro homozygote (DI) rats: (1) the arginine8-vasopressin (AVP)/dynorphin (DYN) neurons, and (2) the oxytocin (OT)/cholecystokinin (CCK8) neurons of the paraventricular and supraoptic nuclei, which project to the posterior pituitary. The regimen of chronic intermittent salt-loading (CISL) involved the replacement of 2% saline for normal drinking water for 18 hr/d. This protocol effectively increased plasma levels of AVP and OT in LE and HZ rats, oxytocin in DI rats, and maintained the posterior pituitary in a state depleted of AVP, OT, CCK, and peptides derived from pro-dynorphin: DYN A 1-17, DYN A 1-8, and DYN B 1-13. The ratio of pituitary DYN A 1-17 to DYN A 1-8 content in DI rats or in LE, HZ, and DI rats following 6 d of CISL suggests a preferential release of DYN A 1-17 during periods of chronic secretory activity. In response to chronic secretory activity, mRNAs for AVP, OT, DYN, and CCK increased 1.5-2-fold in all 3 AVP rat strains, with mRNAs for coexisting peptide hormones displaying parallel increases. Mutant AVP mRNA in the DI rat was expressed at very low levels and DYN mRNA in very high levels, with each of these mRNAs continuing to be regulated by CISL in a normal manner. These results suggest a regulatory relationship between AVP and OT neurons, in which vasopressin neurons are feedback-regulated by AVP, most likely via plasma osmolarity, and that oxytocin neurons are modulated by peptides derived from pro-dynorphin.