Self-administration of small amounts of morphine through glass micropipettes into the ventral tegmental area of the rat.

A micropipette-system was used to investigate intracranial self-injection of morphine in the rat. The system consisted of a glass micropipette (tip dia 5 microns) connected by flexible tubing and a swivel to a pressure source. Using nose-poking through a hole in one wall of the cage as the operant, rats self-injected quantities as small as 50 ng/5 nl of morphine into the ventral tegmental area. In contrast, rats in the saline-treated group and yoked-control group did not increase their rates of nose-poking behaviour above baseline levels. Intermittent reinforcement in the morphine-treated group resulted in a further increase in the rate of nose-poking. Histological inspection of the site of injection revealed minimal damage to brain tissue and likely confinement of the injected solution to the area around the tip of the pipette.