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Intracellular Chloride Activity and the Effect of 5‐Hydroxytryptamine on the Chloride Conductance of Leech Retzius Neurons

Intracellular Cl‐ activity (aCli) and 5‐hydroxytryptamine (5‐HT)‐induced membrane currents of Retzius neurons in the central nervous system of the medicinal leech were measured using Cl‐ sensitive microelectrodes and a two‐microelectrode voltage‐clamp technique. At the membrane of Retzius neurons Cl‐ ions were not passively distributed. Under different conditions the chloride equilibrium potential (ECl, ‐60.1 mV for isotonic saline and ‐57.8 mV for a hypertonic saline) was negative with respect to the membrane potential (Em‐55 ± 3.8 and ‐47 ± 3.4 mV respectively). The endogenous neurohormone 5‐HT always polarized the membrane of Retzius neurons in the direction of ECl. When voltage‐clamping the membrane of Retzius neurons near the resting potential both in situ and in primary culture, application of 5‐HT produced an outward current (l5‐HT) and increase in membrane conductance. Current ‐ voltage relationships for l5‐HTshowed a slight outward rectification and reversal potentials of ‐61.6± 3.1 mV in situ and ‐66± 3.1 mV in primary culture, both values being comparable to the Ecl of Retzius neurons as measured in situ. The results indicate that 5‐HT increases the Cl‐ conductance of Retzius neurons, thereby hyperpolarizing the cell membrane and affecting both the excitability of the neuron and 5‐HT release from it. This could affect the feeding and swimming behaviour of the leech.