The use of x-rays as the unconditioned stimulus in five-hundred-day-old rats.

Attempts were made to resolve conflicting reports on the threshold dose for the unconditioned stimulus (US) of ionizing radiation as an aversion to saccharin. Earlier studies employed both the simultaneous presentation of conditioned stimulus (CS) and US and the trace conditioning technique, and although some investigators have been unable to demonstrate the conditioned aversion with doses lower than 200 r, others were able to show it with total doses as low as 10 r of gamma radiation over a wide variety of dose rates. These investigations, however, were limited to animals ranging in age from 60 to 120 days. Since effects of x irradiation on rats have been shown to be a function of age, the present study was undertaken in an attempt to condition an aversion to saccharin fluid with x rays as the US in rats older than those used in previous studies. In addition, the strength of the saccharin aversion was studied as a function of both dose and amount of saccharin fluid (CS) ingested prior to irradiation. Male Wistar rats, 500 days old, were whole-body irradiated with 50- to 350-r doses of 250-kv x rays at 30 r/min, and control rats were sham- irradiated. On themore » day of irradiation, 0.1% saccharin fluid was offered the animals. Twenty-four hr after irradiation, two bottles were attached to each cage, one containing tap water and the other containing saccharin fluid. At the end of a 48-hr preference test the bottles were removed and the total fluid consumption and the S score were determined. The S score is equal to the proportion of the total fluid consumption that was saccharin. There was no significant difference between the S scores or total fluid intake for the 6 irradiated groups conditions gave rise to significantiy different consumption of saccharin fluid presented as a CS, the deprivation condition has no apparent effect on the amount of fluid intake or the S score for a postirradiation test. When the groups are reconstituted on the basis of the amount of saccharin fluid consumed in the CS, again no effect is noted on the two postirradiation measures. The threshold for the conditioned aversion was between 0 and 50 r and an increase in the roentgen dose up to 350 r had no effect. Conditioned aversion can be obtained as readily with mature animals as with young animals. The threshold for this effect is close to that found earlier in 60- to 120-day-old rats ( approximates 10 r). (BBB)« less