Two-way avoidance behavior of the roman rat strains following brain lesions.

Rats of the Roman strains, Roman High Avoidance (RHA), Roman Low Avoidance (RLA) and Roman Control Avoidance (RCA) were submitted to 5 conditioning sessions in a two-way shuttle box; the sessions were separated by a 24 hr-interval. Two indices were computed: a within-session progression index W and an inter-session progression index I. Index W was comparable in all three strains; index I was strongly negative in RLA and equal to zero or slightly positive in RHA rats. Lesion of the hippocampal commissure (HC) significantly facilitated CAR acquisition in RLA by making index I less negative: performances of RCA and RHA rats were unaffected. Lesions of the centro medianum parafascicularis nuclei of the thalamus (CM) significantly impaired acquisition of CAR in RCA and RHA by making index I more negative; performances of RLA rats were unaffected. Neither lesion altered index W. These results support the "freezing hypothesis" in the explanation of the Roman strain differences. They are in agreement with recent data on the emotional characteristics of the Roman strains and on the role of octopamine in avoidance behavior.