A selective genetic analysis of the Syracuse high- and low-avoidance (SHA/Bru and SLA/Bru) strains of rats (Rattus norvegicus)

Selective breeding of Long-Evans rats for good and poor avoidance learning in a two-way shuttle box resulted in the Syracuse strains that differ marke dly in the selected phenotypes. These phenotypes have many associated trait s, five of which are studied here: emotionality (open-field defecation), Pa vlovian fear conditioning (CER suppression), passive avoidance training (pu nishment), size (weight) of the adrenal glands and adrenal concentration of corticosterone. Specifically, animals of the low-avoidance strain are more emotional, show greater fear conditioning, exhibit faster passive avoidanc e learning, and have larger adrenal glands in which adrenal corticosterone levels are lower than those of the high-avoidance strain. A reciprocal dihy brid cross of the two strains produced F-1 hybrids, which were used to prod uce the segregating second filial and high and low backcross generations fr om which animals displaying the extreme high- and low-avoidance phenotypes were selected for study of the associated traits. Measurement of the five t raits in these high and low phenotypic animals indicated that all five rema in significantly associated with the avoidance phenotypes, in the expected direction, and comparably in all three segregating generations. The results indicate that the hypothesis of a major gene controlling avoidance learnin g must be rejected and that the few (2-3) genetic units thought to be invol ved may be closely linked to those that mediate these five associated chara cters, or express all five pleiotropically. (C) 1999 Elsevier Science B.V. All rights reserved.