Glial hyperpolarization upon nerve root stimulation in the leech Hirudo medicinalis

Hyperpolarizing responses in neuropil glial cells evoked by nerve root stim ulation were studied in the central nervous system of the leech Hirudo medi cinalis using intracellular recording and extracellular stimulation techniq ues. From a mean resting potential of -60.5 +/- 1.0, the glial membrane was hyperpolarized by -8.6 +/- 0.8 mV, via stimulation of the dorsal posterior nerve root in an isolated ganglion. Nerve root stimulation evoked biphasic or depolarizing responses in glial cells with resting potentials around -7 0 mV (Rose CR, Deitmer JW. J. Neurophysiol. 73:125-131, 1995). The hyperpol arizing response was reduced by the ionotropic glutamate receptor antagonis t CNQX (50 mu M) to 58% of its initial amplitude. In 15 mM Ca2+/15 mM Mg2+- saline the hyperpolarization was reduced by 44%. The hyperpolarization that persisted in high-divalent cation saline was not affected by CNQX. Bath-ap plied glutamate (500 mu M) and kainate (2 mu M) elicited glial hyperpolariz ations that were sensitive to CNQX and 10 mM Mg2+/1 mM Ca2+-saline. The Ei- HT-antagonist methysergide did not affect the hyperpolarizations evoked by nerve root stimulation. The results show that in the leech glial membrane r esponses to neuronal activity include not only depolarizations, as shown pr eviously, but also hyperpolarizations, which are mediated by direct and ind irect neuron-glial communication pathways. In the indirect pathway, glutama te is a transmitter between neurons. (C) 1999 Wiley-Liss, Inc.