Regeneration of excitatory, inhibitory and biphasic synaptic connections made by a snail giant interneuron

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Abstract
The ability of the giant Lymnaea interneuron, R.Pe.D.1, to regenerate central synaptic connections after axonal section was examined in an in vivo preparation. This cell normally makes monosynaptic excitatory, inhibitory or biphasic chemical synaptic connections with about 40 identifiable cells in the snail central nervous system. Regenerated synaptic responses occurred first after three days in cells most proximal to the site of axonal section and in more distal locations after four days. After six days cells in both locations were reliably reconnected. The responses only occurred on cells normally postsynaptic to R.Pe.D.1 and were of the right type in particular cells. At first all three types of synaptic responses were weak but progressive strengthening of the response occurred in all cell types so by day 33 all cell types were showing unitary responses with similar waveforms, latencies and summation properties to the undamaged cell. The earliest unitary response (eight days in the Agp b.p.s.p. cells) occurred in the smallest cells. Two other types of regenerated synaptic connections also occurred in the same preparations. Transitional changes in the type of spike activity shown by R.Pe.D.1 occurred so instead of single spiking, bursting occurred consistently in cells from snails 3-12 days postoperative.