MORPHOLOGICAL-CHANGES AND FUNCTIONAL RECOVERY FOLLOWING AXOTOMY OF A SEROTONERGIC CEREBROBUCCAL NEURON IN THE LAND SNAIL ACHATINA-FULICA

We have examined an identified serotonergic neurone in Achatina fulica and described the normal morphological and physiological characterist ics of this cell. Injury-induced changes in this neurone following in vivo recovery are described and compared with in vitro gastropod model s of regeneration. Nickel-lysine and biocytin dye-fills of the metacer ebral giant (MCG) neurone, together with serotonin-like immunoreactivi ty, revealed an extensive innervation of the ipsilateral buccal gangli on, much greater than that previously reported. Labelled MCG fibres we re seen to ramify throughout the ganglion, providing extensive neuropi lar innervation. Serotonin-immunoreactive fibres were seen not only wi thin the neuropile but also within the cell body layer of the buccal g anglia, surrounding many of the cell bodies with varicose fibres. Dye- fills also revealed a minor contralateral buccal innervation not previ ously described. This view of a predominantly ipsilateral innervation of the buccal ganglia by the MCG was supported by electrophysiological measurements. The ipsilateral buccal follower cell B1 displayed an in crease in depolarization in response to repeated trains of action pote ntials to the MCG, whereas the contralateral B1 showed only a weak dep olarization in response to the identical stimuli. Following a crush to the cerebral-buccal connective (CBC), the MCG rapidly regenerated its injured projections, displaying both morphological and physiological recovery within 5-10 days. The original, severed fibres of the MCG wer e, however, replaced by a multitude of smaller neurites, which persist ed for up to 3 months (the longest recovery period examined). Despite this morphological difference between normal and regenerated fibres, t he MCG re-established functionally equivalent connections upon B1. In contrast with previous in vitro studies using gastropods, serotonin-li ke immunoreactivity revealed that severed distal fibres from the MCG r apidly degenerated (2-6 days), resulting in a transient unilateral dep letion of serotonin in the buccal ganglia. We suggest that this loss o f serotonin in the lesioned ganglion may play a functional role in reg eneration, as has been suggested in vitro.