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 markedly in the selected phenotypes. These phenotypes have many associated traits, five of which are studied here: emotionality (open-field defecation), Pavlovian fear conditioning (CER suppression), passive avoidance training (punishment), 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 avoidance learning, and have larger adrenal glands in which adrenal corticosterone levels are lower than those of the high-avoidance strain. A reciprocal dihybrid cross of the two strains produced F1 hybrids, which were used to produce the segregating second filial and high and low backcross generations from which animals displaying the extreme high- and low-avoidance phenotypes were selected for study of the associated traits. Measurement of the five traits in these high and low phenotypic animals indicated that all five remain 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 learning must be rejected and that the few (2-3) genetic units thought to be involved may be closely linked to those that mediate these five associated characters, or express all five pleiotropically.