Some previous findings indicate that
Some previous findings indicate that GALP has an influence on AVP and OT secretion from the posterior pituitary gland. Cunningham et al. (2004) noted that the expression of GALP mRNA was increased in the neurohypophysis of lactating rats compared to non-lactating rats, whereas GALP mRNA expression in the ARC was unaffected by lactation. Onaka et al. (2005) demonstrated that icv administration of GALP caused significant increases of plasma concentrations of neurohypophyseal hormones in male rats. Shen et al. (2001) showed that the expression of galanin-like peptide mRNA in the rat posterior pituitary gland was markedly upregulated by chronic osmotic stimuli such as dehydration, salt-loading and intraperitoneal administration of lipopolysaccharide: challenges that stimulate the secretion of vasopressin and oxytocin. Kawasaki et al. (2007) examined whether the expression of the GALP gene in the NH and ARC would be induced after intraperitoneal (ip) administration of hypertonic saline, that is, acute osmotic stimulus, in rats.
Previous findings indicate the ability of Gal and GALP to alter AVP and OT secretion from the posterior pituitary gland, but several questions remain unclear. An important question is why Gal and GALP sometimes share such similar features as stimulation of gonadotropin secretion and orexigenic effects in rats but differ in their effects, such as regulation of neurohypophysial hormones. As it might be the case that these effects are based on interactions with different receptors, the aim of the present in vitro study was to investigate the effect of various concentrations of Gal and GALP on basal and K+-stimulated AVP and OT secretion from isolated rat neurohypophysis and hypothalamo-neurohypophysial explants. To investigate whether the presence of galanin receptors is required for Gal and GALP to exert an influence on AVP/OT secretion, the role of the non-selective galanin receptor antagonist, galantide (M15; galanin(1–12)-Pro-substanceP(5–11)-amide) was also examined.
Materials and methods
Discussion The results of the present in vitro study clearly demonstrate the roles played by both Gal and GALP in the processes of AVP and OT release from the rat neurohypophysis or hypothalamo-neurohypophysial explants incubated in normal or modified Krebs–Ringer fluid. The use of galantide, an antagonist of galanin receptors, indicates that Gal modifies the release of both neurohormones via its binding sites but GALP action is exerted by another unidentified specific receptor. The neurohypophysis incubated in vitro provides an excellent model for studying the regulation of secretion at the terminal level, free from the complex synaptic effects present throughout the rest of the CNS (Ciosek, 2007, Juszczak, 2002). The NPS-2143 sale of these neurons in the posterior lobe of the pituitary maintain the ability to secrete AVP and OT in response to appropriate stimuli for at least a few hours (Ciosek, Drobnik, 2013, Ciosek, Guzek, 1992). In this incubation model, the differences in AVP and OT secretion are due to the direct influence of various biologically active compounds, Gal or GALP in the present study, on AVP-ergic and OT-ergic neuron endings creating the neurohypophysis. The whole isolated hypothalamo-neurohypophysial system may be also used for studying the in vitro release of AVP and OT (Ciosek, Gałecka, 2011, Izdebska, Ciosek, 2010). In such a prepared specimen, the permanence of neural hypothalamo-neurohypophysial tracts is maintained, as are most processes related to the biosynthesis of these neurohormones, their transport in the infundibular stalk and the secretion into the medium. It is also assumed that most neuron fibers secreting different neuromodulators/neuromediators and impinging on the AVP-ergic and OT-ergic cell bodies within the paraventricular and supraoptic nuclei of the hypothalamus are intact (Gregg and Sladek, 1984). Some data from earlier in vivo and in vitro experiments confirm that Gal plays a modulatory role in the release of AVP and OT from the neurohypophysis. The results of the in vivo studies are ambiguous. Kondo et al. (1991) showed Gal to have an inhibitory effect on AVP release in hypertonic saline-treated rats. Balment and al Barazanji (1992) observed a transitory diuresis after central Gal infusion, which may be the result of impaired AVP secretion. Similarly, a significant reduction of OT plasma level has been observed in rats after icv injection of Gal (Björkstrand et al., 1993). However, Gayman and Falke (1990) reported Gal to have no influence on OT release from posterior lobe of the pituitary in rats. Landry et al. (1995) demonstrated that the expression of AVP mRNA (but not OT mRNA) in PVN and SON of the rat hypothalamus decreased following an icv injection of Gal into dehydrated rats. Moreover, the increase of AVP mRNA level was observed after administration of the galanin receptor antagonist M15 (Landry et al, 1995, Landry et al, 2000). However, Molnár et al. (2005) did not report any significant influence of Gal on AVP release, i.e. an intravenous (iv) injection of 1.0 nmol of Gal did not modify the plasma basal AVP level and AVP release evoked by osmotic factor. An icv injection of 100 pmol of Gal did not influence the basal AVP plasma level. Ciosek and Cisowska (2003) reported that Gal injected icv did not affect OT content in the Hth and NH but diminished the hypothalamic AVP content without any change of neurohypophysial storage in rats which were not dehydrated. However, the same treatment distinctly inhibited AVP and OT release from the Hth–NH system in dehydrated rats (Ciosek and Cisowska, 2003). Furthermore, icv injection of Gal to salt-loaded rats caused a marked increase of both AVP and OT in the Hth and NH and a decrease in blood plasma (Cisowska-Maciejewska and Ciosek, 2005). Gal was found to have a similar impact on AVP and OT release from the Hth–NH system into the circulation in hemorrhaged rats (Ciosek et al., 2003).