Ionic mechanism of a hyperpolarizing 5-hydroxytryptamine effect on leech neuropile glial cells

The ionic mechanism of a membrane effect of 5-hydroxytryptamine (5-HT) on neuropile glial (NG) cells in ganglia of the medicinal leech was investigated with conventional single-barrelled microelectrodes. Control experiments were made with double-barrelled ion-selective microelectrodes. 5-Hydroxytryptamine hyperpolarized the NG-cell membrane and increased the conductance considerably. Methysergide, a potent 5-HT antagonist, blocked the 5-HT-induced hyperpolarization completely. When leech ganglia were superfused with physiological bathing media free of 5-HT, the NG-cell membrane conductance returned to the original value, but the membrane potential recovered only partially from the hyperpolarization in most experiments. In glial membranes artificially depolarized by means of constant-current injection, the amplitude of the 5-HT response increased. The amplitude decreased with membrane hyperpolarization and reversed at - 73 mV, close to the potassium equilibrium potential. The reversal potential changed by 52 mV when the extracellular potassium concentration was altered by a factor of 10. We conclude that 5-HT increases the potassium conductance of NG-cell membranes.