The cross talk between dopaminergic and serotonergic systems in the brain has multiple neurophysiological and behavioral implications. Primary neuronal cultures of embryonic wild type (+/+) and serotonin transporter knockout (-/-) mice were used as a model to elucidate the possibility of plasticity at the level of serotonin uptake. Serotonergic neurons were identified in midbrain-hindbrain cultures of both wild type and knockout mice, using polyclonal anti-serotonin antibodies. Adding serotonin (10 microM) to wild type midbrain-hindbrain cultures increased the intensity of serotonin immunostaining, but did not change the number of serotonergic neurons. This increased intensity of serotonin staining was blocked by the serotonin transporter inhibitors fluoxetine and imipramine, but not with the dopamine transporter inhibitor nomifensine. In serotonin transporter knockout cultures, however, serotonin increased both the intensity of serotonin immunostaining and the number of serotonin positive neurons, and nomifensine decreased the number of serotonin-labeled neurons. Uptake of [3H]serotonin to wild type midbrain-hindbrain cultures was completely blocked by 1 microM fluoxetine, whereas nomifensine had a very small effect. In contrast, [3H]serotonin uptake to serotonin transporter knockout cultures, although very weak, was better inhibited by nomifensine than fluoxetine. The results imply that midbrain-hindbrain neuronal cultures of knockout mice, that do not express serotonin transporters, acquire the capacity to take up serotonin, apparently via dopamine transporters.