Inhibition of vasopressin secretion when dehydrated rats drink water

The present study determined whether vasopressin (VP) secretion is inhibited by an oropharyngeal signal associated with swallowing fluids when dehydrated rats drink water, as it is when dehydrated dogs are used as experimental subjects (Thrasher, TN, Keil LC, and Ramsay DJ. Am J Physiol Regul Integr Comp Physiol 253: R509–R515, 1987). VP levels in systemic plasma (pVP) fell rapidly when rats drank water after overnight water deprivation. Systemic plasma Na+ concentration (pNa) also fell, but that change likely contributed little to the early inhibition of VP secretion. In contrast, consumption of water by dehydrated rats with an open gastric fistula had no effect on pVP, nor did consumption of isotonic saline by dehydrated rats; in neither case was pNa affected by fluid consumption. These findings provide no evidence that the act of drinking inhibits VP secretion in dehydrated rats. Thus some postgastric effect of the ingested water seems to be responsible for the inhibitory signal. These results are consistent with previous suggestions that an early inhibitory stimulus for VP secretion in rats is provided by postgastric visceral osmo- or Na+ receptors that sense the composition of the ingested fluid.

Observations on the Effect of an Excitatory-Inhibitory Stimulus on the Landing Reaction of the Fly, Calliphoridae

This research investigates at a behavioral level the sensitivity to movement of the visual system of Calliphora, inferred from the motor response of the landing reaction, to observe the variations of the threshold of velocity of the landing reaction during simultaneous introduction of both excitatory (landing) and inhibitory (antilanding) stimuli. Analysis supports the hypothesis that the interaction of opposing stimuli decreases the sensitivity to movement. Moreover, no effect was observed when the number of stimuli per unit time increased.