Evolution of learning in three aplysiid species: differences in heterosynaptic plasticity contrast with conservation in serotonergic pathways.

We investigated the neurobiological basis of variation in sensitization between three aplysiid species: Aplysia californica, Phyllaplysia taylori and Dolabrifera dolabrifera. We tested two different forms of sensitization induced by a noxious tail shock: local sensitization, expressed near the site of shock, and general sensitization, tested at remote sites. Aplysia showed both local and general sensitization, whereas Phyllaplysia demonstrated only local sensitization, and Dolabrifera lacked both forms of learning. We then investigated a neurobiological correlate of sensitization, heterosynaptic modulation of sensory neuron excitability by tail-nerve stimulation. We found (1) an increase in sensory neuron (SN) excitability after both ipsilateral and contralateral nerve stimulation in Aplysia, (2) a smaller and shorter-lasting increase in Phyllaplysia, and (3) no effect in Dolabrifera. Because sensitization in Aplysia is strongly correlated with serotonergic (5-HT) neuromodulation, we hypothesized that the observed interspecific variation in sensitization and SN neuromodulation might be correlated with variation in the anatomy and/or functional response of the serotonergic system. However, using immunohistochemistry, we found that all three species showed a similar pattern of 5-HT innervation. Furthermore, they also showed comparable 5-HT release evoked by tail-nerve shock, as measured with chronoamperometry. These observations indicate that interspecific variation in learning is correlated with differences in SN heterosynaptic plasticity within a background of evolutionary conservation in the 5-HT neuromodulatory pathway. We thus hypothesize that evolutionary changes in learning phenotype do not involve modifications of the 5-HT pathway per se, but rather, changes in the response of SNs to the activation of this or other neuromodulatory pathways upon noxious stimulation.