Stressors produce a concurrent decrease and increase in reflex amplitude in rats treated with naloxone.
The effect of inescapable foot-shock on the tail-flick response and on the startle response to brief shocks and brief tones was studied in rats. In the first experiment, 25 minutes of inescapable foot shock (stressor) produced a significant increase in tail-flick latency which was antagonized by the opioid antagonist naloxone (2.0 mg/kg). In the second experiment, the startle response to an electric shock to the tail was significantly diminished by the stressor, and this effect was not significantly reduced by naloxone. However, the size of the startle response to a brief tone was significantly increased in rats treated with naloxone. Thus, rats injected with naloxone had a decreased startle to shock but an increased startle to tone following inescapable foot shock. Finally, tones which preceded shocks by one second produced a facilitation of the startle response to the shocks in tests that followed exposure to the stressor. This facilitation was not affected significantly by naloxone. These results indicate that the changes in the startle response following the stressor were not mediated exclusively by endogenous opioids.
D000161:Acoustic Stimulation; D000818:Animals; D004597:Electroshock; D008297:Male; D009270:Naloxone / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013216:Reflex, Startle / Q000187:drug effects / Q000502:physiology*; D013312:Stress, Physiological / Q000503:physiopathology*
D009270:Naloxone