Differential startle magnitude in mice selected for high and low swim analgesia is not related to difference in nociception

The acoustic startle response (ASR) elicited by 110 dB 10-ms pulses was studied in relation to pain sensitivity in mouse lines selectively bred for high (HA) and for low (LA) swim analgesia. The magnitudes of ASR, similarly as hot-plate latencies, differed between the lines in the rank order HA > unselected controls (C) > LA. The animals’ nociception did not change after the ASR session consisting of a sequence of 20 acoustic stimuli. Morphine hydrochloride (5 and 10 mg/kg IP) increased hot-plate latencies in the order of HA > C > LA, and was not effective on ASR magnitude in HA as well as in C mice. In the LA line, 10 mg/kg of morphine slightly attenuated ASR, but caused only a little analgesia. We conclude that (1) the difference in ASR between the selected lines is inversely correlated with the difference in pain sensitivity; (2) the magnitude of ASR is not altered by morphine analgesia; (3) the procedure of ASR using brief acoustic pulses is not stressful enough to elicit a form of stress analgesia. The lack of a direct relationship between the readiness to startle and pain sensation may be beneficial for an animal’s survival in dangerous situations. It is beneficial when the startle to a warning signal precedes defensive behaviours and it often must be effectuated in a state of decreased nociception.