Striatal serotonin depletion facilitates rat egocentric learning via dopamine modulation

Egocentric spatial learning has been defined as the ability to navigate in an environment using only proprioceptive information, thereby performing a motor response based on one's own movement. This form of learning has been associated with the neural memory system, including the striatum body. Cerebral serotonin depletion induces better performance, both in tasks with strong egocentric components and in egocentric navigation in the Morris' maze. Based on this, we propose that the striatal serotonergic depletion must facilitate egocentric learning. Fifteen female Sprague Dawley rats weighing 250–350 g and maintained under standard conditions were chronically implanted with infusion cannulas for bilateral application of drugs into the striatum. The animals were evaluated for egocentric navigation using the Morris' maze, under different conditions: saline solution infusion, serotonin depletion by infusion of 5,7-Dihydroxytryptamine (25 μg of free base solved in 2.5 μl of ascorbic acid 1% in saline solution), infusion of mixed dopamine D1 and D2 receptor antagonists (0.5 μl/min during 5 min of mixed spiperone 20 μM and SCH23390 10 μM), or serotonin depletion and dopamine blockade simultaneously. Striatal serotonin depletion facilitated egocentric learning, which was demonstrated as shorter escape latencies and the display of a defined sequence of movements for reaching the platform. The facilitation was not observed under condition of simultaneous dopamine blockade. Striatal serotonin depletion produced a dopamine-dependent facilitation of egocentric learning. A role for serotonin in the inhibition of striatal-mediated learning strategies is proposed.