Chronic Osmotic Stimuli Increase Salusin-β-Like Immunoreactivity in the Rat Hypothalamo-Neurohypophyseal System : Possible Involvement of Salusin-β on [Ca2+]i Increase and Neurohypophyseal Hormone Release from the Axon Terminals

Salusin-a and -β were recently discovered as bioactive endogenous peptides. In the present study, we investigated the effects of chronic osmotic stimuli on salusin-β-like immunoreactivity (LI) in the rat hypothalamo-neurohypophyseal system. We examined the effects of salusin-β on synaptic inputs to the rat magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) and neurohypophyseal hormone release from both freshly dissociated SONs and neuro-hypophyses in rats. Immunohistochemical studies revealed that salusin-β-LI neurones and fibres were markedly increased in the SON and the magnocellular division of the paraventricular nucleus after chronic osmotic stimuli resulting from salt loading for 5 days and dehydration for 3 days. Salusin-β-LI fibres and varicosities in the internal zone of the median eminence and the neurohypophysis were also increased after osmotic stimuli. Whole-cell patch-clamp recordings from rat SON slice preparations showed that salusin-β did not cause significant changes in the excitatory and inhibitory postsynaptic currents of the MNCs. In vitro hormone release studies showed that salusin-β evoked both arginine vasopressin (AVP) and oxytocin release from the neurohypophysis, but not the SON. In our hands, in the neurohypophysis, a significant release of AVP and oxytocin was observed only at concentrations from 100 nM and above of salusin-β. Low concentrations below 100 nM were ineffective both on AVP and oxytocin release. We also measured intracellular calcium ((Ca 2+ ] i ) increase induced by salusin-β on freshly-isolated single nerve terminals from the neurohypophysis devoid of pars intermedia. Furthermore, this salusin-β-induced [Ca 2+ ] i increase was blocked in the presence of high voltage activated Ca 2+ channel blockers. Our results suggest that salusin-β may be involved in the regulation of body fluid balance by stimulating neurohypophyseal hormone release from nerve endings by an autocrine/paracrine mechanism.