Ionizing Radiation Impairs the Formation of Trace Fear Memories and Reduces Hippocampal Neurogenesis Pragathi Achanta University of Texas at San Antonio and Johns Hopkins University Martin Fuss Oregon Health & Science University Joe L. Martinez, Jr. University of Texas at San Antonio Long-term cognitive impairments are a feared consequence of therapeutic cranial irradiation in children as well as adults. Studies in animal models suggest that these deficits may be associated with a decrease in hippocampal granule cell proliferation and survival. In the present study the authors examined whether whole brain irradiation would affect trace fear conditioning, a hippocampal-dependent task. Pread- olescent (postnatal Day 21, PD 21), adolescent (PD 50), and postadolescent (PD 70) rats received single doses of 0 Gray (Gy), 0.3 Gy, 3 Gy, or 10 Gy whole brain irradiation. Three months after radiation treatment, a significant dose-dependent decrease in bromo-deoxyuridine positive cells was observed. Irradiation produced a dose-dependent decrease in freezing in response to the conditioned stimulus in all age groups. Interestingly, the authors found no differences in context freezing between irradiated and control groups. Further, there were no differences in delay fear memories, which are independent of hippocampus function. Our results strongly indicate that irradiation impairs associative memories dependent on hippocampus and this deficit is accompanied by a decrease in granule cell neurogenesis indicating that these cells may be involved in normal hippocampal memory function. Keywords: ionizing radiation, hippocampus, fear conditioning, neurogenesis, learning and memory Following cranial radiation impairment of cognitive develop- ment is commonly observed in children and has also been de- scribed in adults. Currently, no “safe” whole brain radiation dose, yielding a zero probability of cognitive deficits, has been identi- fied. Even at reduced total whole brain irradiation (WBI) doses, a negative effect on young children can be measured by neurocog- nitive testing procedures (Fuss, Poljanc, & Hug, 2000; Roman & Sperduto, 1995). Cognitive decline is mainly attributed to a dimin- ished capability to learn and memorize new tasks and information, as well as to reductions in full-scale IQ (Macedoni-Luksic, Jereb, & Todorovski, 2003; Schatz, Kramer, Ablin, & Matthay, 2000). Radiation-induced cognitive changes are often manifested as deficits in hippocampally dependent functions of learning and memory (Abayomi, 1996; Crossen, Garwood, Glatstein, & Neu- welt, 1994; Lee, Hung, Woo, Tai, & Choi, 1989; Roman & Sperduto, 1995; Surma-aho et al., 2001). Doses of cranial radiation that are subthreshold for demyelinative pathology do cause delayed deficits in performance in hippocampal- dependent behavioral tasks (Hodges et al., 1998). These deficits are especially relevant regarding potential long-term conse- quences of prophylactic and lower dose of WBI prescribed to lower the incidence of brain involvement of hematological malignancies and the delayed occurrence of brain metastases in selective solid tumors. Both neurogenesis and performance in behavioral tasks that test hippocampal function decrease in a radiation dose-dependent manner (Sienkiewicz, Haylock, & Saunders, 1994). The underlying mechanisms for these effects have remained unknown, although it was suggested that changes in neuronal precursor cells in the dentate subgranular zone of the hippocampus might be involved. Evidence in animal models supports the importance of hippocampal neurogenesis to normal cognitive functioning (Madsen, Kristjansen, Bolwig, & Wortwein, 2003; Mizumatsu et al., 2003). Understanding how irradiation affects normal hippocampus functioning is of utmost importance in developing potential strategies to reduce cognitive impairments in humans. In this translational animal study we assessed the age dependent/dose dependent impact of ionizing radiation on hippocampus-dependent associative learn- ing and memory and granule cell neurogenesis. Pragathi Achanta, Department of Biology, the University of Texas at San Antonio, and Department of Neurosurgery, Johns Hopkins University; Martin Fuss, Department of Radiation Medicine, Oregon Health & Science University; and Joe L. Martinez, Jr., Department of Biology, the University of Texas at San Antonio. This research was supported by the RO1 DA04195 (JLM), the Ewing Halsell and Kleberg Foundations (JLM), the San Antonio Life Sciences Institute (SALSI)–Research Enhancement Fund and San Antonio Neuro- science Alliance (SANA). We thank Dr. Steve Maren from the University of Michigan for the Excel file used to calculate freezing percentages in our behavioral analyses. We thank Dr. Heather Cameron (NIMH) for assistance with BrdU immu- nohistochemistry. We thank Dr. Angela Sikorski (UTSA) for assistance with statistical analysis, and Myra Guzman for her technical assistance. Correspondence concerning this article should be addressed to Joe L. Martinez, Jr., Department of Biology, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249. E-mail: jmartinez@utsa.edu Behavioral Neuroscience © 2009 American Psychological Association 2009, Vol. 123, No. 5, 1036 –1045 0735-7044/09/$12.00 DOI: 10.1037/a0016870 1036