Development of learning and memory in Aplysia. III. Central neuronal correlates

The defensive withdrawal reflex of the mantle organs of Aplysia californica has 2 major components, siphon withdrawal and gill withdrawal. In the previous paper of this series (Rankin and Carew, 1987), the development of 2 forms of nonassociative learning, habituation and dishabituation, was examined in the siphon withdrawal component of the reflex. In the present study we examined these same forms of learning in the gill withdrawal component of the reflex. The purpose of these experiments was 2-fold: to examine the development of learning in the other major component of the reflex; and to establish preparations in which it is possible to carry out a cellular analysis of the development of learning in the CNS. We first established that the gill withdrawal reflex in intact animals exhibited significant habituation in response to repeated tactile stimulation of the siphon and significant dishabituation in response to tail shock. We next determined the contribution of the CNS to the gill withdrawal reflex by surgically removing the abdominal ganglion from intact animals. Using the same stimulus intensity (4 mg) that produced habituation in the previous experiments, we found that the CNS accounted for approximately 95% of the reflex. Finally, we developed 2 preparations that allowed us to relate behavioral observations of learning directly to neural plasticity exhibited in the CNS. In a semi-intact preparation gill withdrawal was behaviorally measured as in the intact animal, but tactile stimulation of the siphon (to produce habituation) and shock to the tail (to produce dishabituation) were replaced by electrical stimulation of the siphon nerve and left connective, respectively. Stimulation parameters were matched to produce behavioral responses comparable with those in the intact animal. In an isolated CNS preparation the same nerve stimuli were used as in the semi-intact preparation, but the response measure used was the evoked neural discharge recorded in an efferent nerve innervating the gill. Both preparations exhibited response decrement and facilitation that was quantitatively as well as qualitatively similar to that observed in intact animals, indicating that 2 simple forms of learning exhibited by the gill withdrawal reflex in juvenile Aplysia can be localized to neural circuits within the abdominal ganglion.