Osmotic stress and cardiovascular-related peptides

The neuronal activity of magnocellular neurosecretory cells in the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus is known to be modulated directly and/or indirectly by osmotic challenge. The influence of osmotic stimulation on neural inputs to magnocellular neurosecretory cells in the SON and on the expression of cardiovascular-related peptides was examined by electrophysiological and histochemical studies. Electrophysiological recording showed that hypertonic stimulation directly influences excitatory inputs to the magnocellular neurosecretory cells by an action on presynaptic terminals and enhances excitatory synaptic events. Magnocellular neurosecretory cells receive synaptic inputs from other brain sites, including the circumventricular organs which are very sensitive to osmotic challenge. Our study focused on the role of the subfornical organ (SFO), one of the circumventricular organs, on the regulation of body fluid balance. Immunohistochemistry and in situ hybridization study showed that pituitary adenylate cyclase-activating polypeptide (PACAP) levels, and PACAP gene expression, were significantly increased in the SFO after water deprivation for 24 or 48 h. Neuropeptides such as PACAP in the central nervous system may thus play an important role in the humoral and neural changes associated with responses to osmotic stress.