Acoustic startle and open-field behavior in mice bred for magnitude of swim analgesia

Acoustic startle response (ASR) and open-field activity was examined in the 46th generation of mice that have been selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 degreesC w ater. These lines were earlier found to differ in brain opioid receptor den sity and in the expression of opioid-mediated phenomena, as analgesic sensi tivity to opiates and reversibility of swim stress-induced analgesia (SSIA) by naloxone. For comparison, a randomly bred control (C) line was used. To measure the amplitude of ASR, the mice were exposed to IIO-dB acoustic sti muli in a Coulboum apparatus. In saline-injected mice, the ASR force was fo und significantly lower in the LA than in the HA, as well in the C line, bu t did not differ between the two last lines. Naltrexone hydrochloride (10 m g/kg IP 30 min before ASR testing) augmented the startle in the opioid rece ptor-dense HA line, but had no effect in the opioid receptor-deficient LA l ine, as well in the C line; therefore, the ASR magnitude in naltrexone-inje cted HA mice was significantly higher compared to the C line. HA mice displ ayed less activity in an open-field test; that is, they remained immobile l onger in the center of the field, and thereafter performed less ambulation and less rearing against the wall compared to the LA line. Naltrexone faile d to modify the open-field activity in any line. The results confirm that t he pattern of ASR depends on the genetic makeup of the animals. The higher amplitude of ASR, taken together with the lower open-field activity of HA m ice, can be interpreted in terms of higher anxiety level, compared to the L A line. It is suggested that the higher ASR in HA mice relies on a nonopioi d mechanism, which is tonically inhibited by the opioid system. (C) 2000 El sevier Science Inc. All rights reserved.