Environment-dependent Tolerance to Ethanol Produced by Intracerebroventricular Injections in Mice

In comparison to other routes of drug administration, an intracerebroventricular (ICV) injection requires the use of a very small amount of ethanol in mice to produce a brief but substantial hypothermic response. By avoiding the longer duration of drug action, use of ICV injections may facilitate the demonstration of the contribution of certain aspects of learning in the expression of tolerance. Therefore, tolerance to ethanol was developed in mice in a Pavlovian conditioning paradigm with eight ICV injections of ethanol delivered at 2-h intervals so that the animals received four injections per day for 2 days. The effect of ethanol in a different environment was tested on the following day. Environment-dependent tolerance to the hypothermic effect of ethanol and a robust conditioned compensatory response were demonstrated when the mice were trained and tested with ICV injections of 2.0 mg ethanol. The environment-dependent tolerance was also evident when the mice were trained with ICV injections and tested with intraperitoneal injections of ethanol. These results demonstrate that ICV injections are a beneficial means of clarifying the role of learning phenomena in the development of tolerance.