The influence of salt loading on vasopressin gene expression in magno- and parvocellular hypothalamic neurons: an immunocytochemical and in situ hybridization analysis

Arginine vasopressin peptide and messenger RNA expression were examined at the cellular level in the magnocellular and parvocellular neurons in the rat paraventricular nucleus after dehydration and rehydration, employing immunocytochemistry and in situ hybridization histochemistry on the same tissue sections. Most magnocellular vasopressinergic neurons of control animals expressed both vasopressin-like immunoreactivity and messenger RNA. However, neurons negative for vasopressin-like immunoreactivity but expressing messenger RNA were also detected, and their number increased during dehydration. In contrast, almost all of the parvocellular vasopressinergic neurons of dehydrated animals expressed vasopressin messenger RNA alone, with continued increase in their number after rehydration, despite return of the number of magnocellular vasopressinergic neurons to the control level. Vasopressin messenger RNA and corticotropin releasing factor-like immunoreactivity were co-localized in the same parvocellular neurons, and vasopressin-immunoreactive nerve terminals were detected in the external zone of the median eminence. These findings suggest that magno- and parvocellular vasopressinergic neurons are differentially activated during dehydration/rehydration. Osmotic stimuli activate all magnocellular vasopressinergic neurons, but the effect is not simultaneous in all of these neurons. Parvocellular vasopressinergic neurons are also activated by the stress of dehydration which effect appears to last longer than in the magnocellular system.