CONTRALATERAL SPROUTING AND COMPENSATORY INNERVATION FOLLOWING THE PERMANENT LESION OF A GANGLIONIC CONNECTIVE IN THE SNAIL

The fate of sprouted fibres was examined following longterm recovery f rom lesions to the central nervous system of the snail Achatina fulica . Axonal dye-labelling of one of the cerebrobuccal connectives (CBC), following either a cut or a crush to the opposite CBC, revealed supern umerary labelling of neuronal elements in both the cerebral and buccal ganglia in the weeks following treatment. A part of this sprouting re sponse involved the rerouting of axonal projections from injured neuro nes that contralaterally into the uninjured CBC. In intracellular dye- fills, immunocytochemistry for detection of serotonin and electrophysi ological measurements all revealed that a contralateral, uninjured neu rone, the metacerebral giant (MCG) cell, sprouted new processes to inv ade the buccal ganglion denervated by the lesion. The contralateral MC G also increased synaptic drive over a neurone in the denervated bucca l ganglion, a cell that normally receives strong input only from the l esioned ipsilateral MCG. After 5 weeks of recovery, morphological and electrophysiological measurements returned to normal levels in animals receiving a crush to the CBC, suggesting a retraction of sprouted pro jections following successful regeneration across the lesioned pathwa. In contrast, the measurements indicative of sprouted fibres continued for up to 5 months when the regenerative response was prevented by cu tting the CBC. Together, these results suggest that both the cessation of sprouting and the eventual retraction of sprouted fibres in Achati na fulica is contingent upon successful regeneration of the damaged ax onal pathway.