Following pre-exposure to objects that underwent CS-UCS conditioning, B6 mice expressed freezing responses to the CS-only (test trials 1-6), whereas B6 mice that were pre-exposed to non-distressed objects did not respond appreciably to presentation of the CS.
Difficulties of gene expression profiling by microarray on FFPET include the inability to obtain adequate RNA for gene microarray profiling from most archival samples [25] and a lack of sensitivity for identifying genes known to be expressed from frozen tissue in matched FFPET samples [26].
Thus, while B6 subjects expressed enhanced freezing responses following pre-exposure to object distress, we cannot rule out the possibility that BALB mice may also express empathy.
Upon repeated administration of the paired CS-UCS, a fear response is acquired, which is expressed as freezing in response to presentation of the CS-only.
B6 subjects expressed longer freezing responses than BALB subjects after exposure to objects receiving the CS-UCS contingency (Fig. 3a; effect of subject genotype, F[1,124] = 53.8, P<0.0001).
B6 subjects again expressed longer freezing relative to BALB subjects (Fig. 3c) in response to presentation of the CS-only (orthogonal contrast, P = 0.003) and the paired CS-UCS (orthogonal contrast, P<0.001).
When test mice were assessed during the first 28 s of a CS that was forward paired with a UCS (the UCS was a 2-s shock) for 10 consecutive trials, test mice from both strains expressed longer freezing responses across successive conditioning trials (Fig. 2b).
Even after pre-exposure to objects experiencing this unpaired CS-UCS, B6 subjects expressed longer freezing responses than BALB subjects (Fig. 3b) during several of the CS-only (orthogonal contrast, P = 0.03) and paired CS-UCS (orthogonal contrast, P<0.001) test trials.
Conditioning was assessed by the analysis of the fear response expressed as freezing behavior with the aid of the FreezeFrame program (v. 3.06, Actimetrics).
Furthermore, after pre-exposure to object mice receiving the CS-only (Fig. 1e), subjects from both strains did not express a freezing response to the CS (orthogonal contrast, P = 0.43) and their responses to the paired CS-UCS were also similar (Fig. 3d; orthogonal contrast, P = 0.48).
