1. Retzius cells of leech segmental ganglia were exposed to tetraethylammonium chloride (TEA) presented both extracellularly, dissolved in the perfusing fluid, and intracellulary, by iontophoresis from a microelectrode. 2. Extracellular TEA, 10 and 25 mM, greatly prolonged the cells' action potentials, and the higher concentration increased their amplitude as well. At 10 mM the characteristic changes developed gradually over a period of about half an hour, while at 25 mM they appeared much more rapidly. However, at both concentrations the changes were reversible within minutes, even after long soaks in drug-containing solution. It is therefore probable that the drug acted at the outer surface of the membrane. 3. Intracellular TEA also prolonged the action potentials but there were several differences from the response produced by extracellular application. The changes developed gradually, and for a time, each firing of the cell was a complex event consisting of several early, brief depolarizations followed by a single much larger and more prolonged one. The large, late depolarization eventually obliterated the early ones; its gradual development suggested that it was produced only after TEA diffused to some extrasomatic portion of the cell. Intracellular TEA always caused progressive depolarization; this and the changes in the action potential were both irreversible, suggesting that the site of action was on the inner surface of the membrane. 4. Manipulations of external Na and Ca provided evidence that (a) in the absence of TEA, Retzius cell action potentials were exclusively Na-dependent, (b) that the early depolarizations in the complex action potentials produced by intracellular TEA were Na-dependent, while the later, large depolarization was Ca-dependent and (c) that the prolonged action potentials produced by extracellular TEA contained a large Ca-dependent component. 5. We conclude that TEA, acting from either side of the membrane, caused a voltage-sensitive, slowly activated Ca current to become a major contributor to the inward current of the action potential, probably by blocking the outward K current which ordinarily counteracts it. However, we cannot rule out the possibility that TEA enabled a Ca current by some means independent of its presumed action on K conductance. 6. Data resembling ours in some respects have been obtained from studies of the action of TEA on frog dorsal root ganglion cells, frog neuromuscular junction, and squid stellate ganglion. No clear counterpart of our findings has been reported form experiments on squid and amphibian axons, molluscan neurones, or frog skeletal muscle fibres.

A complete understanding of animal behavior at the cellular level requires detailed information on the intrinsic biophysical properties of neurons, muscles, and the synaptic connections they make. In the past 10 to 15 years, electrophysiological studies of leech neurons have revealed a diverse array of voltage-gated ionic conductances distinguished by their pharmacological sensitivity to classic ion channel blockers. Voltage-clamp studies have provided new information about the kinetics and voltage-dependence of Na+ conductances, several K+ currents, including IA, IK and IK(Ca.), and high- and low-voltage-gated Ca2+ conductances. These studies showed that the action potentials of most leech neurons result from the usual sequence of permeability changes to Na+, K+, and Ca2+ ions. They also added insight as to the role played by particular combinations of conductances in providing individual neurons with electrical properties appropriate for the particular information they encode. Evidence is accumulating on the modulatory actions fo endogenous neurotransmitters such as FMRFamide, serotonin, and octopamine on motor behaviors in the animal. Parallel studies suggest that changes in behavior can be explained, at least in part, by the alteration of firing patterns of selected neurons and muscles resulting from modulation of multiple ion conductances. This makes the leech exceptionally attractive for neuroethological studies because it is one of the simplest organisms in which the methods of psychology and neurobiology can be combined. Information gathered from this animal will therefore increase our understanding regarding general principles underlying the cellular basis of behavior.

The electrical activities of myometrial cells of the pregnant rabbit uterus have been studied by means of sucrose-gap and intracellular microelectrode recording techniques. The resting potential of the myometrial cell was about --50 my, and it is unaffected by the duration of pregnancy or placental attachment. Action potentials of the myometrium, although dependent on external Na +, were not always of the regenerative type; preparations from nonparturient uteri often produce mainly small spikes. The mean spike amplitude was 35 my, rising at a mean maximum rate of 3 v/see. Oxytocin, in concentrations less than 500 /~U/ml, increased the mean spike amplitude to 48 mv and the mean maximum rate of rise to 7 v/see, without affecting the resting potential. The relation between membrane potential and dV/dt of the spike was steepened by oxytoein, suggesting that oxytocin increased the number of normally sparse sodium gates in the myometrial membrane. By this action, oxytocin is believed to increase the probability of successful regenerative spikes and thereby initiate electrical activity in quiescent preparations, increase the frequency of burst discharges, the number of spikes in each burst, and the amplitude of spikes in individual cells.

The effects of Sr, Ba, Mn, La and Co on the action potential of the leech Retzius cell were examined using intracellular recording techniques. A previous paper showed that these cells could fire Ca-dependent action potentials in Na-free solution provided TEA was present (Kleinhaus and Prichard, 1975). Under the same conditions Sr 1.5--20 mM was capable of substituting as a current carrier. Ba 2--25mM added to normal Ringer prolonged the duration and increased the amplitude of the action potential of the Retzius cell, and supported action potentials without requiring TEA in Na-free solutions. The overshoots of the Sr- and Ba-dependent action potentials varied with a slope of 40 mM and 75 mV, respectively per 10-fold change in divalent cation concentration. Mn and La selectively blocked that portion of the action potential resulting from an inward movement of Ca, Sr or Ba without affecting the Na-dependent depolarization. The actions of Ca 1 mM on Sr-dependent action potentials were compatible with reversible competitive antagonism. In conclusion the findings: 1. support the proposition that outward K current must be blocked in order for divalent cations to dominate the Retzius cell's behavior during excitation. 2. characterize the divalent cation conductance channel as pharmacologically distinct from the Na conductance channel in the Retzius cell and similar to those described in several other excitable membranes. 3. suggest that the current carrying divalent cations probably flow through the same channel.