EVOLUTION OF NONASSOCIATIVE LEARNING - BEHAVIORAL-ANALYSIS OF A PHYLOGENETIC LESION

A recent phylogenetic analysis of two learning-related neuromodulatory traits in mechanosensory neurons of species related to the marine mol lusk Aplysia californica identified one species, Dolabrifera dolabrife ra, which lacked both neuromodulatory traits. Since these traits are t hought to contribute importantly to certain forms of learning and memo ry in the defensive withdrawal reflexes of Aplysia, in the present stu dy, I tested the prediction that facilitatory nonassociative learning would be reduced or absent in Dolabrifera. I tested the tail-mantle wi thdrawal reflex in Dolabrifera and size-matched Aplysia for three form s of nonassociative learning and memory: dishabituation and short- and long-term sensitization. I found that the same protocols that produce d significant dishabituation, short-term sensitization, and long-term sensitization in Aplysia failed in all three cases to produce signific ant learning in Dolabrifera. Thus, the prediction from the prior mecha nistic analysis is confirmed: Dishabituation and short- and long-term sensitization are significantly reduced and perhaps abolished in Dolab rifera. Although not conclusive, this phylogenetic correlation between the absence of behavioral changes and the absence of neural mechanism s thought to underlie the behavioral changes gives support to the cont emporary neuromodulatory model of dishabituation and sensitization in Aplysia. Furthermore, these results raise the possibility that evoluti onary alteration of two specific neuromodulatory mechanisms may have d irectly contributed to evolutionary change in behavioral plasticity. ( C) 1998 Academic Press.