Figure 1.

Cranial irradiation decreases adult neurogenesis. A, IRR-CSA (n = 21) and Sham-CSA (n = 18) rats were habituated to the animal facility for 1 week, anesthetized (50 mg/kg sodium pentobarbital, i.p.), irradiated, and left in the home cage to recover for 4 weeks (with handling every 3 d). Rats completed food training, were implanted with an intravenous catheter, and allowed to recover for several days. Three weeks of CSA followed. Rats were then split into groups for progressive-ratio testing (PR) or dose–response and reinstatement. Progressive-ratio rats had 6 d of testing and then were killed the next day (IRR, n = 7; Sham, n = 5). The remaining rats were given a dose–response test on the 16th day of CSA (IRR, n = 14; Sham, n = 13) and then were placed in the home cage for 4 weeks of WD. Daily reinstatement testing followed, which consisted of an extinction session for at least 1 h, followed by reinstatement testing to several variables as described in Materials and Methods: the context itself, the cue light, saline, intraperitoneal cocaine (5 or 15 mg/kg), and finally footshock stress. A locomotor test (basal and after a 15 mg/kg cocaine injection) was done on the last day. Rats were killed 24 h later. B, The experimental design for IRR-SSA (n = 21) and Sham-SSA (n = 18) rats was similar to A, except that rats were not implanted with an intravenous catheter after food training, and locomotor testing occurred after the last day of food training. Rats completed 3 weeks of SSA, followed by progressive-ratio for sucrose pellets while food restricted for 3 d and while fed ad libitum for 3 d. Rats were then food restricted and restabilized on FR5 timeout 15 s sucrose self-administration for 3 d. Reinstatement testing consisted of extinction of sucrose-seeking, cue-induced, and sucrose-induced reinstatement testing. C, CSA-WD/IRR (n = 15) and CSA-WD/Sham (n = 9) rats were irradiated after 3 weeks of CSA and, after 4 weeks of WD coincident with the recovery period, were then tested for reinstatement. All rats were age matched to the beginning of CSA or SSA. Scale bar, 50 μm. Lightning bolt, 2 d of 10 Gy cranial irradiation, in a 1-cm-diameter circle on the dorsal surface of the head to target the hippocampus as described previously (Snyder et al., 2005). H, Habituation; FT/S, food training and intravenous catheter surgery; r, recovery; sac, sacrifice by intracardial perfusion; Reinstate, reinstatement testing.