INTRACELLULAR CHLORIDE ACTIVITY AND THE EFFECT OF 5-HYDROXYTRYPTAMINEON THE CHLORIDE CONDUCTANCE OF LEECH RETZIUS NEURONS

Intracellular Cl- activity (aCl(i)) and 5-hydroxytryptamine (5-HT)-ind uced membrane currents Of Retzius neurons in the central nervous syste m of the medicinal leech were measured using Cl--sensitive microelectr odes and a two-microelectrode voltage-clamp technique. At the membrane of Retzius neurons Cl- ions were not passively distributed. Under dif ferent conditions the chloride equilibrium potential (E(Cl), -60.1 mV for isotonic saline and - 57.8 mV for a hypertonic saline) was negativ e with respect to the membrane potential (E(m), -55 +/- 3.8 and -47 +/ - 3.4 mV respectively). The endogenous neurohormone 5-HT always polari zed the membrane of Retzius neurons in the direction of E(Cl). When vo ltage-clamping the membrane of Retzius neurons near the resting potent ial both in situ and in primary culture, application of 5-HT produced an outward current (I5-HT) and an increase in membrane conductance. Cu rrent-voltage relationships for I5-HT showed a slight outward rectific ation 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 E(Cl) of Retzius neurons as measured in situ. The results indicate that 5-H T increases the Cl- conductance of Retzius neurons, thereby hyperpolar izing the cell membrane and affecting both the excitability of the neu ron and 5-HT release from it. This could affect the feeding and swimmi ng behaviour of the leech.