ALTERATIONS IN BEHAVIORAL-RESPONSES TO STRESSORS FOLLOWING EXCITOTOXIN LESIONS OF DORSOMEDIAL HYPOTHALAMIC REGIONS

The dorsomedial hypothalamus is important for regulation of cardiovasc ular responses associated with emotional arousal. This region has also been identified as a component of neural circuitry involved in fear/a nxiety, yet clear evidence as to the effects of lesioning on stress-re lated behaviors is missing. In this study, we lesioned the dorsomedial hypothalamic region with the neurotoxin, ibotenic acid (IBO; 2.0 mu g in 0.2 mu l), and studied the impact on spontaneous and unlearned beh avioral responses to stressors. In the open field test, we observed no n-generalized increases in motility parameters in the IBO rats with th e differences occurring in the latter two-thirds of the test. In the e levated plus-maze, the IBO rats displayed a classic anxiolytic respons e with a greater proportion of entries into (and greater time spent in ) the open arms of the maze. In the environment-specific social intera ction (SI) test, the IBO rats showed a normal familiar/unfamiliar envi ronment discrimination with respect to Total SI; however, the composit ion of the behaviors ('curiosity' vs. physical contact) by the IBO rat s was markedly altered, with there being a 2-fold increase in non-viol ent physical interactions. Additionally, the differences in these trad itional indices of anxiety were associated with lesioned animals exhib iting greater acoustic startle responsiveness than controls as a funct ion of prepulse intensity. Overall, the results following IBO lesions indicate an altered responsiveness to sudden stressors, particularly a s relates to novelty or exploration-oriented behaviors. The hypothalam ic lesion may, therefore, have resulted in a disinhibition of normally suppressed responding to innate fear or challenging stimuli. This stu dy contributes to those that have begun to define neural interactions that are essential for integrated stress responses.