Self-administration studies in rats have been performed frequently when addressing mechanisms associated with human drug-seeking behavior. In particular, self-administration on a progressive ratio (PR) schedule is an effective method for measuring the strength of reinforcement of psychostimulant drugs, including cocaine (Richardson and Roberts, 1996; Roberts 1989). In the current study, rats working for food or cocaine self-administration on a PR reinforcement schedule were microinjected with an M2 autoreceptor agonist oxotremorine sesquifumarate (OxoSQ) directly in the LDTg. We hypothesized that suppression of the cholinergic pathway would reduce the reinforcing properties of both natural rewards and cocaine, and would therefore reduce the PR break point (BP) for responding.
