PHYLOGENETIC ANALYSIS OF LEARNING-RELATED NEUROMODULATION IN MOLLUSCAN MECHANOSENSORY NEURONS

In spite of significant advances in our understanding of mechanisms of learning and memory in a variety of organisms, little is known about how such mechanisms evolve. Even mechanisms of simple forms of learnin g, such as habituation and sensitization, have not been studied phylog enetically. Here we begin an evolutionary analysis of learning-related neuromodulation in species related to the well-studied opisthobranch gastropod, Aplysia californica. In Aplysia, increased spike duration a nd excitability in mechanosensory neurons contribute to several forms of learning-related changes to defensive withdrawal reflexes. The modu latory transmitter serotonin (5-hydroxytryptamine, or 5-HT), is though t to play a critical role in producing these firing property changes. In the present study, we tested mechanosensory homologs of the tail-wi thdrawal reflex in species related to Aplysia for 5-HT-mediated increa ses in spike duration and excitability. Criteria used to identify homo logous tail-sensory neurons included position, relative size, resting electrical properties, expression of a sensory neuron specific protein , neuroanatomy, and receptive field. The four ingroup species studied (Aplysia californica, Dolabella auricularia, Bursatella leachii, and D olabrifera dolabrifera) belong to two clades (two species each) within the family Aplysiidae. In the first clade (Aplysia/Dolabella), we fou nd that the tail-sensory neurons of A. californica and tail-sensory ho mologs of a closely related species, D. auricularia, responded to bath -applied serotonin in essentially similar fashion: significant increas es in spike duration as well as excitability. In the other clade (Dola brifera/Bursatella), more distantly related to Aplysia, one species (B . leachii) showed spike broadening and increased excitability. However , the other species (D. dolabrifera) showed neither spike broadening n or increased excitability. The firing properties of tail-sensory homol ogs of D. dolabrifera were insensitive to 5-HT over a wide range of co ncentrations. We also performed experiments on two outgroup species (A kera bullata and Bulla gouldiana) and found that spike duration was un affected by 5-HT, whereas excitability was increased. This study sugge sts that 5-HT-induced spike broadening arose more recently in opisthob ranch evolution, whereas 5-HT-induced excitability increase is a more ancestral trait that may have been expressed in the earliest opisthobr anchs. Both traits are absent in the aplysiid species D. dolabrifera, demonstrating that a lineage can lose learning-related mechanisms. The phylogenetic variation observed in the present study presents the opp ortunity to test general models about learning mechanisms and their ev olution in unique ways.