Progressive recovery of learning during regeneration of a single synapse in the medicinal leech.
The leech escape reflex-shortening of the body-can change with nonassociative conditioning, including sensitization, habituation, and dishabituation. Capacity for sensitization, which is an enhancement of the reflex, is lost when a single S-interneuron is ablated, but the reflex response itself remains. In the present experiments, the S-interneuron's axon in the living leech was filled with 6-carboxyfluorescein (6-CF) dye and cut with an argon laser microbeam (lambda = 488 nm). In contrast to sham-operated animals, axotomized preparations did not sensitize, reflecting the key role of the S-cell. By 2 weeks or more, S-cell axons had regenerated and reestablished synapses at their usual locations with neighboring S-cells. By 4 weeks, this restored the ability to sensitize to a level indistinguishable from that of controls, but an intermediate state of recovery was seen from 2-3 weeks after injury-a period not previously examined. The small capacity for sensitization among newly regenerated preparations was significantly lower than in sham controls but appeared higher than in animals whose cut S-cell axon had not regenerated its synapse. The results confirm the crucial role of the S-cell in sensitization. Moreover, full sensitization does not occur immediately upon synapse regeneration.
Burrell
Brian D
BD
Department of Physiology & Biophysics, University of Miami School of Medicine, Miami, Florida 33136, USA. burrell@purdue.edu
Sahley
Christie L
CL
Muller
Kenneth J
KJ
D000818:Animals; D019771:Axotomy; D004594:Electrophysiology; D007395:Interneurons / Q000502:physiology*; D007858:Learning / Q000502:physiology*; D007865:Leeches / Q000502:physiology*; D009416:Nerve Regeneration / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D013569:Synapses / Q000502:physiology*