The effects of delta-receptor antagonists on cocaine- and methamphetamine-induced place preferences were examined in rats. Cocaine- and methamphetamine-induced place preferences were significantly attenuated by naltrindole (NTI: a non-selective delta-opioid receptor antagonist). Furthermore, naltriben (NTB: a selective delta 2-opioid receptor antagonist), but not 7-benzylidenenaltrexone (BNTX: a selective delta 1-opioid receptor antagonist), attenuated the cocaine- and methamphetamine-induced place preferences. These results suggest that delta-opioid receptors, particularly delta 2-opioid receptors, may be involved in the reinforcing effects of cocaine and methamphetamine.

Learning to act in a multiagent environment is a difficult problem since the normal definition of an optimal policy no longer applies. The optimal policy at any moment depends on the policies of the other agents. This creates a situation of learning a moving target. Previous learning algorithms have one of two shortcomings depending on their approach. They either converge to a policy that may not be optimal against the specific opponents' policies, or they may not converge at all. In this article we examine this learning problem in the framework of stochastic games. We look at a number of previous learning algorithms showing how they fail at one of the above criteria. We then contribute a new reinforcement learning technique using a variable learning rate to overcome these shortcomings. Specifically, we introduce the WoLF principle, “Win or Learn Fast”, for varying the learning rate. We examine this technique theoretically, proving convergence in self-play on a restricted class of iterated matrix games. We also present empirical results on a variety of more general stochastic games, in situations of self-play and otherwise, demonstrating the wide applicability of this method. Author Keywords: Multiagent learning; Reinforcement learning; Game theory