Beyond memory consolidation? Varied effects of post-training heroin on drug place conditioning and escape learning

To test the hypothesis that opiates reinforce self-administration through modulation of memory consolidation, three experiments in male Sprague-Dawley rats explored whether post-training heroin administration could facilitate drug place conditioning and escape learning in the Barnes maze. Immediately following heroin- (1 mg/kg; Experiment 1) or cocaine-compartment (20 mg/kg; Experiment 2) conditioning sessions, rats received vehicle, 1 mg/kg heroin, or 1 mg/kg heroin in combination with 3 mg/kg naloxone. All groups were tested after one pairing (Test 1) and again after three additional pairings (Test 2). Experiment 3 assessed the effects of vehicle, 1 and 2 mg/kg heroin administered immediately following each of 10 escape training sessions. All groups were tested drug-free on a final Probe test when the escape location was moved to the opposite quadrant of the maze. Contrary to predictions, in both place conditioning experiments there was no significant drug-compartment preference in rats that received post-training 1 mg/kg heroin, and this effect was reversed by naloxone. In the Barnes maze, post-training 1 mg/kg heroin did enhance investigation of the escape location during training. However, during the Probe test, 1 mg/kg increased while 2 mg/kg decreased investigation of the original escape location, and only 1 mg/kg enhanced locomotion within the original training quadrant. Taken together, this complex pattern of results cannot be interpreted simply from a memory consolidation perspective and suggests that, even when administered post-training, heroin can influence subsequent task performance through incidental learning.