Serotonin depletion impairs but does not eliminate classical conditioning in the leech Hirudo medicinalis

The goal of these experiments was to test the role of serotonin (5HT) in classical conditioning of the touch-elicited shortening reflex in the leech (Hirudo medicinalis). The toxin 5,7-dihydroxytryptamine (5,7-DHT) was used to deplete serotonin. The results indicated that 5HT depletion significantly impairs the expression of conditioned responding; however, depleted leeches experiencing conditioned stimulus-unconditioned stimulus (CS-US) pairings still performed significantly better than depleted leeches experiencing unpaired CS-US presentations, suggesting that a 5HT-dependent mechanism does not account fully for learning in this preparation. Moreover, the residual pairing dependent effect is observed, although the depletions eliminate sensitization, suggesting that the amplification of sensitization may not be sufficient to account for classical conditioning of this reflex. Histological analyses of the ganglia revealed an absence of staining in 100% of the Retzius cells in the toxin group.