Divalent cations and the action potential of leech Retzius cells

Abstract

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.

Segregation of leech neurones by the effect of sparteine on action potential duration.

“…Taken together Figs. 3-6 support the hypothesis that when SPT blocked repolarization, Ca and Sr were capable of carrying current through the manganese sensitive divalent cation channel previously characterized in the R and N cells (Kleinhaus & Prichard, 1975;1977b;Kleinhaus, 1976). Mn, however, could'not carry current as well through it ( Fig.…”

“…A large portion of the prolongation of the action potential in R and N cells was dependent on Ca or Sr and could be blocked by Mn 3 mm. It was probably due to inward current flowing through the divalent conductance channels previously described in these neurones (Kleinhaus & Prichard, 1975, 1977bKleinhaus, 1976). The hypothesis formulated to explain the differential sensitivities of this group of neurones to TEA rested upon presumed variations among them in the density of divalent cation channels in the order R > N > P > T (Kleinhaus & Prichard, 1977b).…”

“…Fast green (2%) was added to the solution for confirmation of successful ejection (Bowling, Nicholls & Parnas, 1978). During pressure ejection from the micro-electrode the electrical activity of the cell was monitored continuously either through the ejecting barrel or (Kleinhaus, 1976) were within 10% of those obtained by the digital method used in the present experiments. Statistical comparisons among groups of cells were by two-tailed t-test.…”

“…In this preparation about 70 of 350-400 neurones have been characterized as to function and electrophysiological properties (Nicholls & Baylor, 1968;Baylor & Nicholls, 1969a, b, c;Nicholls & Purves, 1970;Jansen & Nicholls, 1973;Van Essen 1973;Thompson & Stent, 1976a, b, c). Among these, the Retzius cell (R cell), which probably controls mucus secretion from the skin (Lent, 1973), and the mechanosensitive neurones responding to touch (T cells), pressure (P cells) and noxious (N cells) stimuli were previously used in a series of studies on the action of tetraethylammonium chloride (TEA) (Kleinhaus & Prichard, 1975, 1977bKleinhaus, 1976). These studies showed that TEA delayed repolarization in all four neurones as it does in a wide variety of excitable tissues, presumably by interfering with a K conductance normally involved in this process.…”

Inhibition of Ca2+ conductance in identified leech neurons by benzodiazepines.

“…Long-lasting regenerative Ca2+ potentials, revealed by blocking outward K+ currents, have previously been characterized for leech neurons (5)(6)(7) and for neurons in several other invertebrate and vertebrate preparations (8). Nanomolar-affinity BZ receptors do not exist in the leech (9) or other closely related annelids (10).…”