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Updated: Aug 9, 2026
Intracellular Recording, Sensory Field Mapping, and Culturing Identified Neurons in the Leech, Hirudo medicinalis
Published on: November 4, 2013
A 3-synapse positive feedback loop regulates the excitability of an interneuron critical for sensitization in the
Kevin M Crisp1, Kenneth J Muller
1Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33136, USA. crisp@stolaf.edu
Abstract:
Sensitization of reflexive shortening in the leech has been linked to serotonin (5-HT)-induced changes in the excitability of a single interneuron, the S cell. This neuron is necessary for sensitization and complete dishabituation of reflexive shortening, during which it contributes to the sensory-motor reflex. The S cell does not contain 5-HT, which is released primarily from the Retzius (R) cells, whose firing enhances S-cell excitability. Here, we show that the S cell excites the R cells, mainly via a fast disynaptic pathway in which the first synapse is the electrical junction between the S cell and the coupling interneurons, and the second synapse is a glutamatergic synapse of the coupling interneurons onto the R cells. The S cell-triggered excitatory postsynaptic potential in the R cell diminishes and nearly disappears in elevated concentrations of divalent cations because the coupling interneurons become inexcitable under these conditions. Serotonin released from the R cells feeds back on the S cell and increases its excitability by activating a 5-HT7-like receptor; 5-methoxytryptamine (5-MeOT; 10 microM) mimics the effects of 5-HT on S cell excitability, and effects of both 5-HT and 5-MeOT are blocked by pimozide (10 microM) and SB-269970 [(R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol] (5 microM). This feedback loop may be critical for the full expression of sensitization of reflexive shortening.
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