Separation of two voltage‐sensitive potassium currents, and demonstration of a tetrodotoxin‐resistant calcium current in frog motoneurones.

Abstract

SUMMARY1. Depolarization-induced voltage and conductance changes were studied in frog motoneurones in isolated, perfused spinal cord slices. Two types of afterhyperpolarization are observed following action potentials in normal Ringer, a fast afterhyperpolarization lasting 5-10 msec and a slow afterhyperpolarization lasting 60-200 msec. Both afterhyperpolarizations are mediated by an increased K+ conductance.2. The slow afterhyperpolarization and the conductance increase underlying it are selectively and reversibly inhibited by perfusion with solutions containing low [Ca2+] (< 0-2 mM) or the Ca2+ antagonists Mn2+ (1 mM) or Co2+ (5 mM), and are enhanced by perfusion with high [Ca2+].3. Addition of 2-5 mm tetraethylammonium ion (TEA+) to the perfusing solution prolongs the falling phase of the action potential and abolishes the fast afterhyperpolarization, but does not inhibit the slow afterhyperpolarization.4. When the voltage-dependent Na+ current is blocked by perfusion with TTX (10-M), intracellularly applied depolarizing current steps evoke fast and slow hyperpolarizations with kinetics and pharmacological sensitivities similar to those of the fast and slow afterhyperpolarizations, respectively. The fast hyperpolarization is maximally activated by brief, intense depolarizations, the slow hyperpolarization by prolonged, less intense depolarizations.5. These pharmacological and kinetic data demonstrate that in frog motoneurones the repolarization-fast afterhyperpolarization sequence and the slow afterhyperpolarization are produced by different K+ conductance * Present address. ELLEN F. BARRETT AND J. N. BARRETT systems. The fast K+ conductance activates rapidly on depolarization, decays rapidly on repolarization, and is TEA+ sensitive, while the slow K+ conductance activates and decays more slowly and is Ca2+-dependent.6. Motoneurones perfused with TEA+ and TEA often show a slow, regenerative depolarizing response to applied depolarizing currents. These regenerative depolarizations are probably produced by an influx of Ca2+, because they persist in isotonic CaCl2 and are blocked by Mn2+ or low [Ca2+]. The Ca2+-dependence of the slow afterhyperpolarization and the increase in slow afterhyperpolarization magnitude observed following the slow Ca2+ potentials suggest that a depolarization-evoked Ca2+ influx activates the K+ conductance underlying the slow afterhyperpolarization.7. Motoneurones in which the slow Ca2+ and K+ conductance systems have been enhanced by high [Ca2+] or blocked by Mn2+ show altered discharge patterns in response to intracellularly applied depolarizing current steps. Perfusion with twice normal [Ca2+] (4 mM) causes motoneurones to discharge more slowly at all current intensities, and reduces the slope of the 'steady-state' frequency-current relationship. Mn2+-perfused motoneurones exhibit fairly normal high-frequency discharge at the onset of the current step, but unlike normal motoneurones, do not discharge at frequencies below 60/sec. These data indicate that the slow Ca2+ and K+ conduct...

Mechanisms of post‐synaptic excitation in amphibian motoneurones.

“…As at other peripheral and central synaptic junctions with a chemical mode of transmission the effect of Ca2+ on evoked transmitter release occurs also at synapses of frog motoneurones (Katz & Miledi, 1963;Grinnell, 1966;Dambach & Erulkar, 1973;Barrett & Barrett, 1976 The activity of presynaptic fibres from the dorsal root was practically unaffected by reduction in bath Ca2+ or by Mg2+ (5-10 mM) and Mn2+ (2 mM) as evidenced by the amplitude and duration of the presynaptic spike. However, Co2+ (5 mM) or high Mg2+ (20 mM) induced a decrease and prolongation of the presynaptic spike.…”

Effect of repetitive activation on the afterhyperpolarization in dorsal spinocerebellar tract neurones.

“…is activated by a single spike (Kernell & Sj6holm, 1972;Baldissera, Gustafsson & Parmiggiani, 1976). This latter mechanism has now been demonstrated in frog spinal motoneurones (Barret & Barret, 1976 (Gustafsson, B., unpubl.). A decrease in the peak amplitude of the second a.h.p.…”

“…In motoneurones the a.h.p. is mediated by an influx of Ca2+ ions (Krnjevic, Puil & Werman, 1975;Barret & Barret, 1976). If this is also the case for DSCT neurones then the depression may be due to some factor affecting the kinetics of the calcium conductance, for example a longlasting Ca2+ conductance inactivation.…”

“…Calvin & Sypert, 1976, Fig. 5 (Barret & Barret, 1976), a separate conductance from the potassium conductance giving rise to the falling phase of the spike. The present study gives no clue regarding the possible mechanism underlying the depression.…”

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

“…Kolnodin & Skoglund (1958) also differentiated between tr,¡o phases of spike repoJ.arization: an initialrapid repolarization which remained very constant regardless of the way in which the action potential was ini--6-tiatedt and a second, later and longer Lasting phase which !¡as nore variable. Barrett & Barrett (1976) later demonstrated in the frog that the post-spike hyperpolari zation can indeed be separated into two conponents, the early repoJ-arization conponent followed-by the later a.h.p. conponent.…”