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-α 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 neurohypophyses 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 ([Ca²⁺]_i) increase induced by salusin-β on freshly-isolated single nerve terminals from the neurohypophysis devoid of pars intermedia. Furthermore, this salusin-β-induced [Ca²⁺]_i increase was blocked in the presence of high voltage activated Ca²⁺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. [ABSTRACT FROM AUTHOR]