Vasopressin and oxytocin neurones of hypothalamic supraoptic and paraventricular nuclei co-express mRNA for type-1 andType-2 corticotropin-releasing hormone receptors

The presence of corticotropin-releasing hormone (CRH) receptors type-1 (CRH R-1) and type-2 (CRHR-2 alpha) in the hypothalamic supraoptic (SON) and par aventricular (PVN) nuclei, and the effects of i.c.v. injection of CRH and u rocortin on arginine vasopressin (AVP) and oxytocin release, have suggested that CRH ligands have a role in osmoregulation. In this study, double labe lling in situ hybridization using S-35-labelled CRHR-1 or CRHR-2 alpha and digoxigenin-labelled AVP, oxytocin or CRH riboprobes was employed to examin e the localization of CRHR-1 or CRHR-2 alpha mRNA in the SON and PVN of con trol and osmotically stimulated rats. Rats received an i.p. hypertonic sali ne (1.5 M) injection or isotonic saline injection (controls), or 2% NaCl in take (salt loading) or tap water (controls) for 12 days. While CRHR-1 mRNA was undetectable in the SON and PVN in control rats, its expression was inc reased markedly at 4 h after i.p. hypertonic saline injection or after 12 d ays salt loading. Of the cells labelled with digoxigenin-AVP, 53% in the SO N and 90% in the PVN coexpressed CRHR-1 mRNA after i.p. hypertonic saline i njection. In oxytocinergic neurones, 73% in the SON and 91% in the PVN show ed CRHR-1 autoradiographic grains higher than background levels after i.p. hypertonic saline injection. In addition, i.p. hypertonic saline induced CR HR-1 mRNA expression in digoxigenin-CRH stained cells in the parvocellular PVN. CRHR-2 alpha transcripts were present in both the SON and PVN under ba sal conditions, and salt loading, but not acute i.p. hypertonic saline inje ction, further stimulated this expression. Double labelling in situ hybridi zation showed colocalization of CRHR-2 alpha mRNA with AVP and oxytocin mRN A in the SON. These studies support a role for CRH and urocortin regulating the hypothalamo-neurohypophyseal system, and suggest a direct action of th e peptides in the magnocellular neurones.