Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system

Abstract

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.

G protein activation by uncaging of GTP-γ-S in the leech giant glial cell

“…We have previously shown in the leech giant glial cell in situ that stimulation of the Leydig neuron elicits a membrane hyperpolarization due to an increased K + conductance in the giant glial cell (Britz et al, 2002). We were able to show that Leydig neurons release the peptide myomodulin, which activates metabotropic membrane receptors in the glial cell membrane and results in the activation of G protein-coupled adenylyl cyclase, leading to an increase in K + conductance and a membrane hyperpolarization (Britz et al, 2004).…”

“…The steps of dissection and the preparation have been described before (Britz et al, 2002). Isolated ganglia were stored in leech saline at 4°C, before being transferred into the experimental chamber.…”

“…A Leydig neuron was stimulated with single square current pulses (2-4·nA, 10-40·ms) via an intracellular sharp microelectrode, controlled by digitimers (D4030 and DS2; Digitimer Limited, Welwyn Garden City, UK), to induce firing frequencies of 10·Hz (for details, see Britz et al, 2002;Britz et al, 2004). KCl.…”

“…The DMSO concentration never exceeded 0.2% in the experimental saline. The leech myomodulin was synthesized according to Wang et al (Wang et al, 1998) as described before (Britz et al, 2002). All experiments were done at room temperature (22-24°C).…”

“…), N indicates the number of experiments. The glial responses were measured either in the anterior or in the posterior giant glial cell, and the results were pooled, since no differences in the responses of the two glial cells can be observed (Schmidt and Deitmer, 1999;Britz et al, 2002). The statistical significance of the differences between mean values was tested using a standard, or a paired, t-test, if applicable.…”