Gamma radiation-induced disruption in schedule-controlled performance in rats

Adult male rats responded under a multiple fixed-interval 2-min, fixed-ratio 50 (multiple FI FR) schedule of milk delivery. Four groups of rats were given acute whole-body doses of 2.25, 4.5, 6.75, or 9.0 gray (Gy) of 60Co gamma-photon radiation; a fifth group of rats received sham irradiation. During the session that began 10 min after exposure (day 1), multiple FI FR performance was not significantly affected in any treatment group. Neither the sham nor the 2.25-Gy irradiation produced significant alterations in performance over 6 weeks postexposure. Over days 2-4 postexposure, the 4.5-Gy and 6.75-Gy doses reduced response rates approximately 50% and increased postreinforcement pause durations under both the FI and FR schedules. The 9.0-Gy dose produced a progressive decline in both FI and FR responding over the first week postexposure, with response rates decreasing to approximately 10% of pre-irradiation control levels on day 5. Frequently, FI rates decreased more than FR rates after exposure to 4.5-9.0 Gy. Substantial recovery of pre-irradiation response rates was evident in all treatment groups over weeks 2-4 postexposure; behavioral recovery was essentially complete during postexposure weeks 5 and 6. Eight weeks after irradiation, two groups of rats were irradiated a second time. In the group given two 6.75-Gy exposures, performance decrements were similar after each exposure. In the group given two 9.0-Gy exposures, performance declined more rapidly and showed less recovery after the second exposure than after the first. Re-irradiation produced a dose-dependent increase in the incidence of lethality. Overall, gamma radiation disrupted schedule-controlled responding in a dose-related manner; both the magnitude and time course of this disruption varied as a function of dose. Exposure to higher doses of gamma radiation resulted in residual damage that was expressed following re-irradiation challenge.