Strain and substrain differences in context- and tone-dependent fear conditioning of inbred mice

The performance of C57BL/6J (65), C57BL/6N (6N), DBA/2J (2J) and DBA/2N (2N ) mice in context- and tone-dependent fear conditioning was determined 24 h after fear conditioning to evaluate and compare different behavioral measu res as indices of emotional learning. Freezing, the change in activity and the size of the explored area were evaluated as behavioral parameters indic ating fear. Additionally, the heart rate (HR) increase elicited by tone pre sentation was evaluated as an autonomic indicator of fear. During the conte xt-dependent memory test, freezing was high only in 65 and 6N mice, whereas a drop of activity and a reduced exploratory area was measured in all stra ins. During the tone-dependent memory test, high freezing, low activity, re duced exploratory area and a strong HR increase were demonstrated only in 6 N and 65 mice, whereas behavioral and HR changes of 2J and 2N mice were alw ays low. In extinction tests, context- and tone-dependent freezing of 65 mi ce decayed significantly faster than the freezing of 6N mice, whereas in bo th substrains the conditioned tachycardia to tone extinguished similarly in the home cage. The data demonstrate that monitoring of additional behavior al measures besides freezing and autonomic measures is necessary to interpr et differences in associative learning performance of mouse strains that co uld be related to a differential expression of fear. (C) 1999 Elsevier Scie nce B.V. Al rights reserved.