Effects of pH, changes in potassium concentration and metabolic inhibitors on the after-potentials of mammalian non-medullated nerve fibres

Studies on the tachyphylaxis to nicotinic stimulant drugs in the perfused hypogastric ganglion of the guinea-pig

1 The guinea-pig hypogastric ganglion, perfused through its vasculature with physiological saline solutions, will respond repeatedly to nicotinic stimulant drugs injected at 4 min intervals into the perfusion stream. Stimulation is indicated by contraction of the vas deferens. After a varying number of doses of nicotinic stimulants, complete tachyphylaxis usually develops, and normally this persists for at least 1 hour. During this time, however, electrical stimulation of the hypogastric nerve is still fully effective.2 Tachyphylaxis may be reversed for varying periods of time by all of the following procedures: (a) tetanic stimulation of the hypogastric nerve; (b) injection of large doses of histamine, methacholine, or potassium chloride; (c) perfusion with saline containing ouabain 0.1 mug/ml or 80-100 mM lithium chloride, or, to a lesser extent, with potassium-free saline.3 Tachyphylaxis is always re-established with further doses of nicotinic stimulants, except during perfusion with lithium solutions.4 Atropine hastens the onset of tachyphylaxis, but does not prevent its reversal by hypogastric nerve stimulation or by histamine or potassium. However, it does prevent the reversal by methacholine and by lithium.5 Tachyphylaxis occurs in decentralized ganglia, and can be reversed by histamine, methacholine, and by tetanic stimulation of the hypogastric ganglion. Hence tachyphylaxis and its reversal cannot be due to an effect on presynaptic fibres.6 Potassium chloride stimulates the ganglion both before and after tachyphylaxis has developed to nicotinic drugs, and hence this is unlikely to be due to a depolarization block.7 It is concluded that tachyphylaxis could be due to a prolonged ganglionic hyperpolarization.

Cyclic variation of potassium conductance in a burst-generating neurone in Aplysia

1. The hyperpolarization between bursts in the R 15 cell of Aplysia is accompanied by an increase in membrane slope conductance.2. The post-burst hyperpolarization can be observed with ouabain, lithium, or potassium-free solution if artificial inward current is applied. The hyperpolarization can be observed with dinitrophenol or cooling to 10 degrees C, with no injected current. Thus, the hyperpolarization apparently is not due to the cyclic activity of an electrogenic pump.3. A reversal potential for the post-burst hyperpolarization can be demonstrated by passage of inward current during the inter-burst period. The reversal of direction of the potential depends on recent occurrence of a burst.4. The reversal potential varies with external potassium concentration, but not with chloride or sodium.5. The post-burst hyperpolarization is not blocked by external tetraethylammonium at a concentration which greatly prolongs the action potentials.6. During the onset of spike blockage by, and recovery from, calcium-free+tetrodotoxin saline, the bursts of action potentials appear to be driven by endogenous waves of membrane potential.7. The hyperpolarizing phase of the waves in calcium-free+tetrodotoxin medium is accompanied by an increased slope conductance.8. A reversal potential can be demonstrated for the hyperpolarization following a wave in calcium-free+tetrodotoxin medium by applying inward current during the interwave period.9. The waves in calcium-free+tetrodotoxin medium are blocked by ouabain but can be reinstated by artificial hyperpolarization.10. The post-burst hyperpolarization and the post-wave hyperpolarization appear to result from a periodic increase in membrane conductance, primarily to potassium ions.

Some further observations on the electrogenic sodium pump in non-myelinated nerve fibres

1. A study has been made of the hyperpolarization that follows a period of electrical activity (post-tetanic hyperpolarization) and of the hyperpolarization which develops when potassium is readmitted after bathing the desheathed vagus nerve of the rabbit in potassium-free Locke solution during 15 min (potassium-activated response).2. Reduction of the external chloride concentration increases the membrane resistance and the potassium-activated response without changing the time constant of the response. A linear relation between the amplitude of the potassium-activated response and the membrane resistance was found. When chloride was replaced completely by isethionate (sodium salt) and by sulphate (other salts) the potassium-activated response increased by a factor of 5.3. The membrane resistance is decreased during the post-tetanic hyper-polarization elicited in isethionate Locke solution: the decrease is more pronounced after a longer period of electrical stimulation of the nerve.4. A small increase of the membrane resistance was found during the potassium-activated response. The changed membrane potential during the response can account for the alteration of the membrane resistance observed.5. The amplitude of the potassium-activated response is increased during hyperpolarization and reduced during external depolarization of the nerve, whereas the time constant is not affected. The potassium-activated response appears to be independent of polarization of the membrane after correction for the changed membrane resistance.6. The maximum amplitude of the activated response and the external potassium concentration are related following Michaelis-Menten kinetics; the time constant of the response is inversely related to the external potassium concentration.7. The area of the electrogenic response activated by high potassium concentrations (5.6-20 mM) is almost constant, but is reduced at lower potassium concentrations. The amplitude and area of the thallium-activated response are increased (about 1.5 times) compared with the potassium-activated response.8. It was concluded that the electrogenic response, reflected by post-tetanic hyperpolarization, is not directly related to activity of the electrogenic pump, which is probably due to accumulation of potassium in the periaxonal space; that the potassium activated response is produced entirely by activity of the electrogenic sodium pump; and that the current produced by activity of the electrogenic sodium pump is independent of the electrochemical gradient and membrane resistance.

Post-stimulus hyperpolarization and slow potassium conductance increase in Aplysia giant neurone

1. Intracellular records from Aplysia giant (R2) cell somata showed long lasting 4-10 mV hyperpolarizations after passage of outward current through a second intracellular electrode.2. An increase in membrane slope conductance occurred simultaneously with the post-stimulus hyperpolarization (PSH).3. Both the PSH and conductance-increase varied strongly with stimulus amplitude and duration.4. Both the PSH and the conductance increase occurred in Ca-free medium containing tetrodotoxin, when action-potential production was completely blocked.5. The PSH persisted in the presence of ouabain or DNP, with cooling, with removal of external K(+), and in media where all the Na(+) was replaced with Li(+), suggesting that it was not due to the activity of an electrogenic pump.6. A reversal potential for the PSH was demonstrated by application of maintained inward current following the end of an outward-directed stimulus.7. The PSH reversal potential varied with [K](o), but not with [Cl](o) or [Na](o), suggesting that the PSH was mainly due to an increase in K conductance.8. The PSH and the conductance increase were reduced strongly when all the Na(+) was replaced with Tris, and only slightly when Na(+) was replaced with sucrose.

Heterosynaptic facilitation and post-tetanic potentiation in Aplysia nervous system

1. Heterosynaptic facilitation was defined as an increase of amplitude of a test excitatory post-synaptic potential (EPSP) after the activation of a pathway (heterosynaptic pathway) different from that which produced the test EPSP. This phenomenon has been studied in Aplysia central nervous system under conditions which excluded the participation of post-tetanic potentiation.2. A unitary test was produced in the left and right giant cells, by indirect stimulation of an interneurone located in the peri-oesophageal ring.3. During heterosynaptic stimulation, orthodromic and antidromic activation of the test interneurone was prevented by (1) isolating the synaptic afferent region of the test interneurone from the tested synapse on the right giant cell by a sucrose block applied on the left pleurovisceral connective, and (2) using physiological stimulation of a piece of skin as a heterosynaptic stimulus. Under these conditions which prevented any firing in the test interneurone, heterosynaptic facilitation is observed as a 200% increase of amplitude of the test EPSP in the right cell which lasted more than 15 min. When instead of the physiological stimulus a supramaximal electrical stimulation of the nerves afferent to the abdominal ganglion was used, the increase of amplitude of the test EPSP could reach as much as 500% of its original amplitude. The effectiveness of such heterosynaptic stimulus was smaller when it was applied in the absence of a block of the left pleuro-visceral connective.4. It was possible to produce heterosynaptic facilitation when the preparation was cooled to 7-9 degrees C or if Li(+) replaced Na(+) in the medium. Both of these changes suppressed post-tetanic potentiation.5. It was concluded that heterosynaptic facilitation is a phenomenon different from post-tetanic potentiation. Heterosynaptic facilitation is similar to heterosynaptic inhibition seen in other cells in the same preparation, except for the polarity of action. Both phenomena seem to result from comparable mechanisms, probably acting on the quantity of transmitter released.

On the electrogenic sodium pump in mammalian non-myelinated nerve fibres and its activation by various external cations

1. A study has been made of the hyperpolarization that follows a period of electrical activity (the post-tetanic hyperpolarization) in mammalian non-myelinated nerve fibres.2. Evidence is presented that under certain circumstances this postetanic hyperpolarization is a result of activity of an electrogenic sodium pump that normally is absolutely dependent on the external presence of potassium.3. When the external chloride is replaced by sulphate or by isethionate the post-tetanic hyperpolarization, which in normal Locke solution is only a few millivolts in amplitude, is increased usually to about 20 mV, and on occasion to 35 mV.4. This effect of removing the chloride takes several minutes to develop and is consistent with the idea that the increase in the post-tetanic response is the result of removing the short-circuiting effect of internal chloride ions (by their being washed out into the chloride-free bathing medium).5. Small anions, such as chloride, nitrate, iodide, bromide, and thiocyanate can short-circuit the electrogenic pump, whereas larger anions such as sulphate and isethionate cannot. The bicarbonate ion, which is larger than chloride, short-circuits the pump but less effectively.6. In Locke solution containing 5 mM potassium the post-tetanic hyperpolarization declines exponentially, with a time constant of about 1-3 min. The time constant is inversely related to the external potassium concentration.7. However, when the external potassium concentration is zero the hyperpolarization declines rapidly to a very small value. Subsequent addition of potassium to the bathing medium causes a marked redevelopment of the hyperpolarization.8. This potassium-activated response declines exponentially with a time constant that is inversely related to the potassium concentration. When the added potassium concentration is 5 mM, the time constant is 1.9 min.9. The amplitude of the potassium-activated response increases with increasing concentrations of potassium.10. Other cations can produce this activated response. Thus, thallium is more effective than, rubidium as effective as, caesium and ammonium about 1/10 as effective as, and lithium ions about 1/30 as effective as potassium in producing the activated response. Choline is quite ineffective.11. The size of the post-tetanic response is little affected by changes in the duration of the period of stimulation. However, increasing the duration definitely increases the time constant of recovery.12. Reducing the external sodium concentration increases the size of the post-tetanic hyperpolarization (by about 25%), but the effect is complex and requires further study.13. Reducing the calcium of the Locke-solution from 2.2 to 0.2 mM has no appreciable effect on the post-tetanic response, nor has increasing the pH of the Locke from 7.2 to 9.2.14. When the membrane potential is increased or decreased, by externally applied currents, there is relatively little change in the post-tetanic response.15. A mathematical model of the electrogenic pump, devised to mimic the experimental results, was analysed with an analogue computer. A satisfactory agreement between model and experiment was achieved by a model in which: (1) the rate of extrusion of sodium ions depends on the degree to which a pool of carrier molecules on the inside surface of the membrane is combined with sodium; (2) each carrier molecule transfers three sodium ions at a time; (3) the rate constant for extrusion of sodium ions also depends on the presence externally of potassium ions, which combine with some sites on the external surface of the membrane that are half-saturated when the external concentration of potassium is 2.8 mM.

The oxygen consumption of mammalian non-myelinated nerve fibres at rest and during activity

1. A study has been made of the oxygen consumption of non-myelinated nerve fibres of rabbit desheathed cervical vagus nerves at rest and during activity.2. The average resting oxygen consumption (Q(r)) was 0.0924 mumole/g. min at 21 degrees C. Stimulation for 1-3 min at 3/sec caused an extra oxygen consumption (Q(s)) of 816 p-mole/g.shock.3. When the frequency of stimulation was increased, to 10/sec and 30/sec, Q(s) fell. When the frequency was decreased, to 1/sec and 0.3/sec, Q(s) increased slightly.4. When the temperature was decreased, Q(r) fell; when the temperature was increased, Q(s) also increased. Temperature similarly affected Q(s) with high frequencies of stimulation, but had relatively little effect on Q(s) at low frequencies of stimulation.5. An isolated single shock seemed to produce an increase in oxygen consumption of about 1200 p-mole/g, and this value was largely independent of temperature.6. When part of the sodium in the Locke solution was replaced by barium, Q(r) decreased (by 12%) whereas Q(s) increased (by 87%).7. Veratrine (1 mug/ml.) increased both Q(r) (by 142%) and Q(s) (by 361%).8. Acetylcholine (1.7 mM) increased Q(r) (by 32%).9. When nerves were transferred to potassium-free solutions there was little change in Q(r), and Q(s) fell slightly (by 8%).10. When the potassium concentration in the Locke solution was increased 4-fold, Q(r) increased (by 27%).11. Salicylate (1-10 mM) increased Q(r) (by 24%) and abolished Q(s).12. When the sodium of Locke solution was replaced by lithium, Q(r) decreased (by 19%) and Q(s) was abolished.13. In sodium-Locke solution ouabain (100 muM) decreased Q(r) (by 26%) and abolished Q(s). In lithium-Locke solution ouabain also decreased Q(r) (by 28%).14. All or nearly all of the oxygen consumed at rest or during activity seemed to be used to pump potassium ions into, and sodium ions out of, the axoplasm.15. The K/O(2) ratio during pumping was about 5.0.

The origin of the post-tetanic hyperpolarization of mammalian motor nerve terminals

1. Motor nerve terminals in magnesium-poisoned rat hemidiaphragm-phrenic nerve preparations in vitro were stimulated with short depolarizing pulses of approximately threshold strength and the evoked antidromic responses recorded from the phrenic nerve. The percentage of these 1/sec or 0.5/sec stimuli to which there was no antidromic response was used as a quantitative measure of the terminal excitability. After standard tetanic stimulation (1000 impulses at 100/sec) the excitability of the terminals was depressed for an average duration of 60-70 sec, during most of which time no antidromic responses to stimuli of pretetanic intensity were recorded. There was no significant interaction between stimuli to the terminals at rates of 1 or 0.5/sec.2. Potassium-free solutions at first increased, then decreased, the post-tetanic depression of excitability. Raising [K](o) threefold (15 mM) abolished the post-tetanic depression and often converted it to an exaltation of excitability.3. Polarizing currents were applied to the terminals with a second electrode. Depolarizing currents increased, while hyperpolarizing currents decreased, the post-tetanic depression of excitability.4. In solutions with 70% of the normal NaCl content replaced by sucrose, the post-tetanic depression of excitability was reversibly prolonged.5. In the presence of 7.7 x 10(-6)M digoxin or 0.42 mM ouabain there was a small reversible reduction of post-tetanic excitability.6. After exposure to solutions containing no glucose or to solutions containing 3-5 mM sodium azide the excitability of the terminals was not altered by the tetanus. After washing with the control solution, post-tetanic depression of excitability returned. Antimycin-A (1.8 x 10(-6)M) had little or no effect upon post-tetanic excitability.7. It was concluded that the post-tetanic depression of excitability reflected hyperpolarization of the terminals and that this hyperpolarization was caused by a shift of the membrane potential towards the potassium equilibrium potential because of an increase in potassium permeability.

An investigation of the post-tetanic potentiation of end-plate potentials at a mammalian neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded intracellularly from neuromuscular junctions in curarized or Mg-paralysed rat diaphragm-phrenic nerve preparations in vitro. In Mg-paralysed preparations after 1000 impulses at 100/sec the amplitude of e.p.p.s elicited at 1/sec before and after the tetanus was on average greater than the control amplitude for 120 +/- 30 sec.2. The post-tetanic potentiation (P.T.P.) of e.p.p. amplitudes was not thought to be dependent upon post-tetanic hyperpolarization (P.T.H.) of nerve terminals as it lasted longer than the hyperpolarization generated by an identical tetanus; was unaffected by hyperpolarizing currents which reduced P.T.H. or depolarizing currents which prolonged P.T.H.; and was diminished in solutions containing 30% of the normal NaCl concentration or 1% ethyl alcohol, both of which procedures prolong P.T.H. The magnitude and duration of P.T.P. were influenced by the pH of the bathing solution in the range 7-7.5 although there was no change in P.T.H. under these conditions. The inability of polarizing currents to influence P.T.P. was also thought inconsistent with the hypothesis that P.T.P. is due to an increase in available transmitter.3. P.T.P. was not thought to be due to sodium accumulation in nerve terminals, for P.T.P. was reduced or abolished by procedures which would be expected to increase the intraterminal sodium ion concentration. These procedures were: exhibition of metabolic inhibitors (1.8 x 10(-6)M antimycin A, 3-5 mM sodium azide or 1 mM sodium iodoacetate), exhibition of cardiac glycosides (7.7 x 10(-6)M digoxin or 0.42 mM ouabain), and omission of glucose or potassium ions from the bathing solution. Abolition of P.T.P. by potassium-free solutions was also thought to be inconsistent with the hypothesis that P.T.P. is due to a reduction in the potassium concentration in nerve terminals.4. P.T.P. was not thought to be due to terminal volume changes, for no consistent effect upon the quantal content of e.p.p.s could be detected in hypo- or hyperosmotic solutions.5. It was concluded that the only hypothesis for P.T.P. not excluded by our experiments was that P.T.P. is due to some change in ionized calcium at a membrane site important in transmitter release.