PREFERENCE FOR SELF-ADMINISTRATION OF A LOW-DOSE OF MORPHINE INTO THEVENTRAL TEGMENTAL AREA RATHER THAN INTO THE AMYGDALA IN MICE

BALB/c mice were bilaterally implanted with two guide cannulae, the ti ps of which were positioned either 1.5 mm above the amygdala (AMY) and the ventral tegmental area (VTA)(AMY-VTA subjects) or 1.5 mm above th e AMY and 2.3 mm above the VTA (D.vta)(AMY-D.vta subjects). On each ex perimental day, a stainless steel injection cannula was inserted into each brain structure. The experiment was carried out in a Y-maze. Duri ng a preliminary phase, which lasted 4 days, animals were allowed to s elf-inject morphine successively into the AMY and into the VTA, or int o the AMY and into the D.vta. From the 5th day, animals of each group were given the possibility of choosing between the two sites. Four sub groups were constituted depending on the dose of morphine used (5 and 50 ng: AMY5ng-VTA5ng, AMY50ng-VTA50ng, AMY5ng-D.vta5ng, and AMY5ng-D.v ta50ng). The AMY5ng-VTA5ng group rapidly differentiated between the tw o injection sites and showed a marked preference for self-injection in to the VTA. In the AMY50ng-VTA50ng group, no significant preference wa s observed, with the animals tending to alternate self-injection into the AMY and VTA. The AMY5ng-D.vta5ng group discriminated between the t wo sites and self-injected morphine preferentially into the AMY. The d iscrimination performance of the AMY5ng-D.vta50ng group was not statis tically different from that at chance level. These results demonstrate that mice are capable of discriminating, at the intracerebral level, the motivational or rewarding components of morphine when the dose ava ilable is low (5 ng). The preference manifested is highly influenced b y the location of injection cannulae. The positive effect of a low dos e of morphine appeared stronger in the VTA than in the AMY. However, t he location of injection cannulae 0.8 mm above the VTA induced a marke d preference for self-injection into the AMY. Consequently, the reward ing effects of morphine into the VTA probably results from a local act ion of the drug and not from a dorsal diffusion.