Increase in basal firing rate and sensitivity to angiotensin II in subfornical organ neurones of ducks adapted to salt water.

Abstract

1. The influence of salt adaptation on spontaneous firing rate of subfornical organ (SFO) neurones and on their responsiveness to angiotensin II (AngII) (10(-10)-10(-7) M) were studied in vitro on hypothalamic brain slices taken alternately from ducks kept on either fresh water (FW ducks) or saline of 500 mosmol/kg for 8 weeks (SW ducks) as their only water supply. The animals were of the same age and were housed and fed identically. In SW ducks plasma osmolality and AngII plasma concentrations were typically increased. 2. Recording SFO single-unit activity in the AngII-free control perfusion medium disclosed significantly higher spontaneous firing rates in SW than in FW ducks with an average difference of 2.5 spikes/s. 3. With approximate threshold concentrations of AngII for activation of SFO neurones of 10(-10) M in slices from SW ducks and of 10(-9) M in slices from FW ducks, the fractions of neurones excited by AngII increased dose dependently in each group and were consistently larger in the slices from SW ducks. Average maximum increases in discharge rate during AngII-induced excitation also increased dose dependently and were higher at each AngII dose in SFO neurones from SW as compared to FW ducks. 4. Mean latencies of neuronal excitation decreased with increasing AngII doses in both groups of neurones but were significantly shorter in slices from SW than FW ducks. The shorter latencies in SW ducks corresponded to their enhanced sensitivity to AngII. Mean half-times of the disappearance of AngII-induced activation were determined after stimulation with 10(-7) M-AngII and were identical in SW and FW ducks, indicating no difference in kinetics in AngII-neurone interaction. 5. AngII-responsive SFO neurones in slices from SW ducks did not respond to AngIII and activation by AngII was abolished in the presence of the specific antagonist 1Sar-8Ile-AngII. 6. In the magnocellular portion of the paraventricular nucleus, the occurrence of AngII-responsive neurones was as infrequent in SW ducks (2 out of 45) as in FW ducks (0 out of 36). 7. The results indicate that adaptation to salt water in ducks selectively enhanced basal activity and responsiveness to AngII of the SFO neurones. This is a functional correlate of the up-regulation of AngII-receptor density observed in the SFO of the same species in the course of adaptation to salt water. Both adaptive adjustments seem to provide tighter coupling between systemic and central control of salt and fluid balance in conditions of chronic salt stress.