• serotonin;
  • membrane potential;
  • voltage-clamp;
  • G-protein-coupled receptor;
  • Hirudo medicinalis


5-Hydroxytryptamine (5-HT), a neurotransmitter and neuromodulator in the central nervous system of the leech Hirudo medicinalis hyperpolarizes the giant glial cell in the neuropil of segmental ganglia at micromolar concentrations. The 5-HT-evoked glial response (EC50 ≈2.5 μM) is mediated by a non–desensitizing, G-protein-coupled receptor and due to activation of a Ca2+-independent K+ conductance. The adenylyl cyclase inhibitor SQ22,536 blocks the response to 5-HT; in the presence of 1 mM db-cAMP, but not of 1 mM db-cGMP, the glial response is suppressed. The 5-HT-evoked response is reduced by Ba2+ with half-maximal inhibition at 50 μM Ba2+. The results suggest that release of 5-HT from serotonergic neurons, or the maintenance of micromolar levels of extracellular 5-HT in the ganglion, may help to set the glial membrane potential close to the K+ equilibrium potential. © 2004 Wiley-Liss, Inc.