Afferent connections of physiologically identified neuronal complexes in the paraventricular nucleus of conscious Pekin ducks involved in regulation of salt- and water-balance

Summary The afferent connections of the paraventricular nucleus (PVN) of the domestic mallard (Pekin duck), Anas platyrhynchos, were demonstrated by means of microiontophoretic injection of horseradish peroxidase (HRP). To place the HRP injection exactly into the PVN, its location was identified prior to the injection by observing antidiuretic reactions to electrostimulations within the rostral hypothalamus of conscious, hydrated animals. Antidiuresis was induced only when electrostimulation was applied to a distinct hypothalamic area. Two different patterns of antidiuresis were observed: (i) an immediate reduction in rate of production of urine, and (ii) antidiuresis preceded by a period of increase in production of urine. Repeated stimulation of the same site with the same parameters resulted in decreasing antidiuretic effects. At the site where stimulation had elicited the most pronounced antidiuresis of either response type, HRP was injected microiontophoretically.Histological examination after 3–8 days of survival revealed delicate injection sites located exclusively in the periventricular portion of the PVN. Adjacent to the dorsal portion of the PVN retrogradely labeled tanycytes and intraependymal neurons were scattered in the ventricular wall. As demonstrated in neurohistological and electron-microscopic investigations, this ependymal region exhibits a particular arrangement of tanycytes and small neurons (10–15 m in diameter), some of which belong to the neurosecretory type.Additional HRP-labeled neuronal perikarya afferent to the PVN were demonstrated in the contralateral PVN, and on the ipsilateral side in the lateral septum, lateral hypothalamic area and locus coeruleus. Within the nuclei of the solitary tract, stained nerve cells were found ipsilateral as well as contralateral to the injection site.Several of the neurons demonstrated may be considered as candidates for the transmission of signals originating from various receptive structures relevant for the control of avian salt- and water-balance. The physiological results conform to the concept that neurons of the PVN influence urine formation by controlling the release of arginine-vasotocin (AVT). Evidence that suggests additional modes of control exerted by these neurons in salt- and water-balance is presented.Supported by grants from the Deutsche Forschungsgemeinschaft (Ko 758/1; Si 230/4-4)Portions of these results were presented on the occasion of the 54th Meeting of the Deutsche Physiologische Gesellschaft (Korf et al. 1981 a) and the 76th Meeting of the Anatomische Gesellschaft (Korf et al. 1981 b)