Functional implications of neurotransmitter expression during axonal regeneration: Serotonin, but not peptides, auto-regulate axon growth of an identified central neuron

We studied the regenerative properties of one of two electrically coupled m olluscan neurons, the serotonergic cerebral giant cells (CGCs) of Lymnaea s tagnalis, after axotomy. The CGCs play a crucial role in feeding behavior, and when both cells are disconnected from their target neurons, animals no longer feed. When one CGC was permanently disconnected from its targets and the other was reversibly damaged by a nerve crush, the latter one regenera ted over a period of 2 weeks to reform functional synapses with specific ta rget neurons. At the same time, recovery of the feeding behavior was observ ed. After the crush, neuropeptide gene expression in the CGC was downregula ted to similar to 50%. Serotonin synthesis, on the other hand, remained una ffected, suggesting that serotonin might have an active role in regeneratio n. In primary neuron culture, CGCs failed to extend neurites in the presenc e of serotonin; in cells that extended neurites in the absence of serotonin , focally applied serotonin, but not neuropeptides, induced growth cone col lapse. Using serotonin-sensitive sniffer cells, we show that CGC neurites a nd growth cones release serotonin in culture. Finally, both the spontaneous and stimulation-induced release of serotonin from CGCs in culture resulted in growth cone collapse responses that could be blocked by the serotonin r eceptor antagonist methysergide. Our data suggest that auto-released seroto nin is inhibitory to CGC neurite outgrowth in vitro. During regeneration in vivo, serotonin release might fine-tune axon guidance and branching by ind ucing local collapse responses in extending neurites.