Naltrexone fails to block shock-induced deficits in an appetitive discrimination task

Abstract

Acute stress has been shown to induce analgesia-like effects in animals. However, little attention has been paid to other effects of these stressors, for example those on complex learned behaviors. Therefore, we assessed the effects of shock-induced stress on discrimination performance in rats and established whether any of the effects were opioid mediated. Rats were first trained in a two-lever, discrete-trial procedure in which brief light presentations were discriminative stimuli for food-reinforced leverpresses. After stable accuracy levels were achieved, the animals were injected (SC) with saline or the narcotic antagonist naltrexone (NALT) (5.0 or 20.0 mg/kg), exposed to footshock (2.5 mA for 30 sec), and then tested immediately afterward. Footshock significantly disrupted accuracy and increased choice and food-retrieval latencies. The accuracy deficits were found to be independent of the alterations in choice latencies. NALT failed to attenuate the accuracy deficits and accentuated the performance deficits. We proposed that some forms of acute shock exert nonopioid mediated dissociative effects that occur in conjunction with, and may be confounded with, alterations in nociceptive processes.