The effects of nucleus accumbens μ-opioid and adenosine 2A receptor stimulation and blockade on instrumental learning

Abstract

Prior research has shown that glutamate and dopamine receptors in the nucleus accumbens (NAcc) core are critical for the learning of an instrumental response for food reinforcement. It has also been demonstrated that μ-opioid and adenosine A2A receptors within the NAcc impact feeding and motivational processes. In these experiments, we examined the potential roles of NAcc μ-opioid and A2A receptors on instrumental learning and performance. Sprague-Dawley rats were food restricted and trained to lever press following daily intra-accumbens injections of the A2A receptor agonist CGS 21680 (at 0.0, 6.0, or 24.0ng/side), the A2A antagonist pro-drug MSX-3 (at 0.0, 1.0, or 3.0μg/side), the μ-opioid agonist DAMGO (at 0.0, 0.025, or 0.025μg/side), or the opioid receptor antagonist naltrexone (at 0.0, 2.0 or 20.0μg/side). After five days, rats continued training without drug injections until lever pressing rates stabilized, and were then tested with a final drug test to assess potential performance effects. Stimulation, but not inhibition, of NAcc adenosine A2A receptors depressed lever pressing during learning and performance tests, but did not impact lever pressing on non-drug days. Both μ-opioid receptor stimulation and blockade inhibited learning of the lever-press response, though only naltrexone treatment caused impairments in lever-pressing after the task had been learned. The effect of A2A receptor stimulation on learning and performance were consistent with known effects of adenosine on effort-related processes, whereas the pattern of lever presses, magazine approaches, and pellet consumption following opioid receptor manipulations suggested that their effects may have been driven by drug-induced shifts in the incentive value of the sugar reinforcer.

Keywords: Adenosine; Learning; Motivation; Nucleus accumbens; mu-Opioid receptor.

Figure 1

Representative photomicrographs and histological placements of microinjector tips within the NAcc for Experiments 1 (left panel) and Experiment 2 (right panel). Numbers at the top right of each section represent millimeters anterior to bregma. Open circles represent vehicle dose injection sites, gray circles represent injection sites for rats who received 6.0 ng/0.5μL/side of CGS 21680 (Experiment 1, left) or 1.0 μg/0.5μL/side of MSX-3 (Experiment 2; right), and black circles represent injection sites for rats who received 24.0 ng/0.5μL/side of CGS 21680 (Experiment 1, left) or 3.0 μg/0.5μL/side of MSX-3 (Experiment 2; right). This figure represents placements/doses for the drug training phase; the same rats were re-randomized into drug groups during performance testing.

Figure 2

Representative photomicrographs and histological placements of microinjector tips within the NAcc for Experiment 3 (left panel) and Experiment 4 (right panel). Numbers at the top right of each section represent millimeters anterior to bregma. Open circles represent vehicle dose injection sites, gray circles represent injection sites for rats who received 0.025 μg/0.5μL side of DAMGO (Experiment 3, left) or 2.0 μg/0.5μL/side of naltrexone (Experiment 4; right), and black circles represent injection sites for rats who received 0.25 μg/0.5μL of DAMGO (Experiment 3, left) or 20.0 μg/0.5μL/side of naltrexone (Experiment 4; right).

Figure 3

Effects of adenosine receptor stimulation or blockade on the learning and performance of an instrumental learning task. Stimulation of NAcc A2A receptors with 24.0 ng/0.5μL/side of CGS 21680 inhibited the acquisition of lever press and nosepoking behavior across the first five days of instrumental learning. Furthermore, drug treatment tended to reduce both lever pressing (A, left panel) and nose pokes (A, right panel) during the performance test (PT), which was performed after the rats had learned the task. In a separate group of animals, injections of the A2A antagonist prodrug MSX-significantly increased lever pressing on the first acquisition day (compared to control); thereafter no further effect of MSX-3 injections were seen for either lever pressing or nosepoke behavior (B). FTD = Final training day; *p < .05, **p < .01 for main drug effects; single and double crosses demark p < .05 and p < .01 for drug x time interaction effect, respectively. Stars above or below the data for a given day on the line graph denote significant drug group differences from the vehicle control group on that day, as assessed by Tukey's HSD (p < .05).

Figure 4

Effects of μ-opioid receptor stimulation or blockade on the learning and performance of an instrumental learning task. Activation of NAcc μ-opioid receptors with DAMGO decreased lever pressing (Figure 4A) during learning. Rats who had received 0.025 μg/0.5μL also lever pressed significantly fewer times than controls on Day 13, even after eight days of drug-free training. Mu-opioid receptor stimulation caused a significant drug X day interaction effect on nosepoking (see text). Following learning of the response, there was no effect of DAMGO on either lever pressing (A) or nosepoking (B) during the final performance assessment. Futhermore, blockade of NAcc opioid receptors with naltrexone decreased both lever pressing (A) and nosepoking (B) during instrumental learning and during task performance. Statistical symbols as for Figure 3.