Dissociation between sensitization and learning-related neuromodulation inan aplysiid species

Previous phylogenetic analyses of learning and memory in an opisthobranch l ineage uncovered a correlation between two learning-related neuromodulatory traits and their associated behavioral phenotypes. In particular, serotoni n-induced increases in sensory neuron spike duration and excitability, whic h are thought to underlie several facilitatory forms of learning in Aplysia , appear to have been lost over the course of evolution in a distantly rela ted aplysiid, Dolabrifera dolabrifera. This deficit is paralleled by a beha vioral deficit: individuals of Dolabrifera do not express generalized sensi tization (reflex enhancement of an unhabituated response after a noxious st imulus is applied outside of the reflex receptive held) or dishabituation ( reflex enhancement of a habituated reflex). The goal of the present study w as to confirm and extend this correlation by testing for the neuromodulator y traits and generalized sensitization in an additional species, Phyllaplys ia taylori, which is closely related to Dolabrifera. Instead, our results i ndicated a lack of correlation between the neuromodulatory and behavioral p henotypes; In particular, sensory neuron homologues in Phyllaplysia showed the ancestral neuromodulatory phenotype typified by Aplysia. Bath-applied 1 0 mu M serotonin significantly increased homologue spike duration and excit ability. However, when trained with the identical apparatus and protocols t hat produced generalized sensitization in Aplysia, individuals of Phyllaply sia showed no evidence of sensitization. Thus, this species expresses the n euromodulatory phenotype of its ancestors while appearing to express the be havioral phenotype of its near relative. These results suggests that genera lized sensitization can be lost during the course of evolution in the absen ce of a deficit in these two neuromodulatory traits, and raises the possibi lity that the two traits may support some other form of behavioral plastici ty in Phyllaplysia. The results also raise the question of the mechanistic basis of the behavioral deficit in Phyllaplysia. (C) 1999 Wiley-Liss, Inc.