Threshold For Action Potential Generation Research Articles

Antidromic indentification of hypothalamic neurosecretory cells in rats

Characteristics of antidromic action potentials of neurons of the paraventricular and supraoptic nuclei of the rat hypothalamus were studied during stimulation of the hypothalamo-hypophyseal tract by stimuli of varied amplitude and frequency. Step-like changes were found in spike latency in response to an increase in strength (up to 1.5–2.5 thresholds) or frequency (over 100 Hz) of stimulation, as well as cases with variation of the degree of division of the peak into A and B components. Injection of leu-enkephalin analog into the third ventricle or intravenous injection of NaCl solution (1 M) caused reversible changes in the level of excitability of antidromically activated neurons: leu-enkephalin mainly increased the latent period and threshold of action potential generation and reduced the reproducible frequency of stimulation to 10 Hz, whereas NaCl had the opposite effect. The results indicate that when the adopted criteria of antidromic identification of neurosecretory cells are used the level of their excitability must be taken into account.

Enkephalin opens potassium channels on mammalian central neurones.

Enkephalin and opiate analgesic drugs exert their effects on the brain by interacting with receptors located on neuronal membranes. An almost immediate consequence of this interaction is an inhibition of action potential discharge of individual nerve cells1. This could result from a direct hyperpolarization of the neurone which bears the opiate receptors, thereby shifting its membrane away from the threshold for action potential generation. On the other hand, it is well known that opiates and enkephalin depress the release of neurotransmitters2,3, and this could indirectly result in an inhibition of cell firing. Here we describe experiments which indicate that Met5-enkephalin and a stable analogue, D-Ala2-D-Leu5-enkephalin (DADLE), as well as narcotic analgesics, increase the potassium conductance of neurones in the rat locus coeruleus and thereby inhibit then-spontaneous firing. This effect of the opioids results from an interaction with a receptor having a high affinity for naloxone.

Effects of lanthanum on the electrical and mechanical activities of frog ventricular muscle.

The effects of lanthanum on the resting membrane potential, action potential, membrane resistance, twitch tension, and potassium contracture were investigated and the localization of the drug was studied electron microscopically in isolated frog ventricular muscle. Lanthanum in concentration of 0.2 to 5 mM decreased the resting potential by about 5-8 mV, which was accompanied by an increase in the membrane resistance of about 43% for the depolarizing and 40% for the hyperpolarizing direction. Lanthanum caused a decrease in height and a prominent shortening of the action potential, and also, a depression of the plateau level. In addition, it increased the threshold for action potential generation depending on its concentration. The slow response action potential was inhibited by lanthanum in parallel with twitch inhibition. This finding suggests that the twitch inhibition resulted from the suppression of the slow inward calcium current. In contrast, potassium contracture was not inhibited by lanthanum. When the muscle preparation was treated with neuraminidase, the twitch inhibition caused by lanthanum was strongly depressed. Electron microscopic observation revealed that the precipitates of lanthanum were localized on the external lamina of myocytes as well as in the extracellular spaces but could never be found within the cytoplasm. No such precipitates could be detected in the neuraminidase-treated muscle. From these results it is suggested that lanthanum takes the place of calcium at the membrane surface: it modifies permeabilities to sodium, potassium and calcium ions and the excitation-contraction coupling of the ventricular muscle by replacing calcium bound to the membrane-surface.

Localization of sodium channels in cultured neural cells

Sodium channels in cultured neural cells were localized by light microscopic autoradiography of specifically bound 125I-scorpion toxin. Ninety percent of the cell-bound 125I-scorpion toxin was associated specifically with sodium channels as assessed by the blocking of autoradiographic labeling by unlabeled scorpion toxin and by depolarization. Sodium channels were distributed uniformly in the surface membrane of neurites and cell bodies of both morphologically differentiated and undifferentiated cells of clone N18 of mouse neuroblastoma C1300. Sodium channels were distributed nonuniformly in many cultured spinal cord neurons. Visual observation indicated that 37 +/- 5% of cultured spinal cord neurons had a higher sodium channel density on the initial segment of one or more neurites than on the cell body. For these neurons, the density on one neurite initial segment averaged 7.4 +/- 1.9-fold greater than on the adjacent cell body. This increased sodium channel density may be the basis of the lower threshold for action potential generation at the axon initial segment of motor neurons and some spinal interneurons in vivo.

Convulsant and possible anticholinergic actions of dendrotoxin in the amphibian spinal cord.

1 Dendrotoxin (DTOX)6, 6a and 5,6-1, fractions of the venom isolated from the green mamba (Dendroaspis viridis) promoted both spontaneous and stimulus-coupled rhythmic activity and antagonized the cholinergically mediated ventral root-dorsal root potential (VR-DRP) of frog spinal cord. The different time course and reversibility of these two effects indicates that the toxin has two entirely separate sites of action on the frog spinal cord. 2 Since DTOX 6 neither blocked nor enhanced responses of ventral and dorsal roots to glutamate, gamma-aminobutyric acid (GABA), beta-alanine, glycine or aspartate, it is unlikely that its convulsant action resulted from an alteration of the postsynaptic actions of inhibitory or excitatory amino acids. 3 An alteration in the threshold for action potential generation could perhaps contribute to the convulsant action of DTOX 6, although other mechanisms such as blockade of the release of unspecified inhibitory substances cannot be excluded. 4 In addition to the lack of effect on amino acid responses, DTOX failed to block the polysynaptic DR-VRP or DR-DRP pathways, which are mediated at least in part by amino acid neurotransmitters. Although this would be consistent with a specific action of DTOX at the cholinergic synapse of the VR-DRP pathway, this site of action has not yet been demonstrated unequivocally. Other possible mechanisms whereby DTOX could block VR-DRP are discussed.

Comparison of the action of La3+ and Ca2+ on contraction threshold and other membrane parameters of frog skeletal muscle.

The influence of La3+ on contraction threshold, on membrane input resistance, and on action potential parameters was investigated in fibers of the sartorius muscle of the frog, and it was compared to that of Ca2+. The dependence of the contraction threshold on [La3+]0 in the presence of 0.5mM Ca3+ gave a sigmoid relationship between 0.1 and 5 mM La3+ with a shift of 23 to 34 mV to less negative potentials following a 10-fold increase of [La3+]0. The membrane input resistance was increased to various degrees in La-containing solutions, the increase being irreversible. The threshold of action potential generation was shifted to less negative potentials by 28 mV, and the duration at half-maximal amplitude was tripled by 0.5 mM La3+. In comparison a 10-fold increase of [Ca2+]0 in the range of 0.5 to 50 mM shifted the contraction threshold by 15 mV to less negative potentials. 17 mM Ca3+, a concentration having the same effect on contraction threshold as 0.5 mM La3+, increased membrane input resistance reversibly, shifted the action potential threshold by 16 mV to less negative potentials, and had only minor effects on action potential duration. Conduction was never blocked by Ca3+ as it was with 1 mM La3+. In the theoretical treatment, it is shown that the influence of Ca3+ on contraction threshold, but not that of La3+, may be accounted for by its screening and binding to negative surface charges according to the Gouy-Chapman theory of the diffuse double layer. To describe the action of La3+ on the contraction threshold an additional interaction of La3+ with neutral but amphoteric sites was considered.

Effects of adenosine and adenine nucleotides on synaptic transmission in the cerebral cortex.

Adenosine and the adenine nucleotides have a potent depressant action on cerebral cortical neurons, including identified corticospinal cells. Other purine and pyrimidine nucleotides were either weakly depressant (inosine and guanosine derivatives) or largely inactive (xanthine, cytidine, thymidine, uridine derivatives). The 5'-triphosphates and to a lesser extent the 5'-diphosphates of all the purine and pyrimidines tested had excitant actions on cortical neurons. Adenosine transport blockers and deaminase inhibitors depressed the firing of cortical neurons and potentiated the depressant actions of adenosine and the adenine nucleotides. Methylxanthines (theophylline, caffeine, and isobutylmethylxanthine) antagonized the depressant effects of adenosine and the adenine nucleotides and enhanced the spontaneous firing rate of cerebral cortical neurons. Intracellular recordings showed that adenosine 5'-monophosphate hyperpolarizes cerebral cortical neurons and suppresses spontaneous and evoked excitatory postsynaptic potentials in the absence of any pronounced alterations in membrane resistance or of the threshold for action potential generation. It is suggested that adenosine depresses spontaneous and evoked activity by inhibiting the release of transmitter from presynaptic nerve terminals. Furthermore, the depressant effects of potentiators and excitant effects of antagonists of adenosine on neuronal firing are consistent with the hypothesis that cortical neurons are subject to control by endogenously released purines.

Effects of droperidol on neuromuscular transmission and muscle membrane

The effect of droperidol on miniature endplate potential (mepp) frequency and amplitude, acetylcholine sensitivity, action potential generation, twitch tension, input resistance and alteration of acetylcholine release were studied in frog muscle. Interaction with d-tubocurarine was studied in the cat sciatic gastrocnemius preparation. Between 10 −11 and 10 −7 g/l droperidol did not change mepp frequency or amplitude. Between 7.5 × 10 −6 and 2.5 × 10 −5 g/l mepp amplitude, endplate sensitivity to acetylcholine and twitch tension were depressed. At 5 × 10 −5 g/l the threshold for action potential generation increased while amplitude and rate of rise decreased. Input resistance was unaltered. With indirect interrupted tetanic stimulation of the muscle for 30 min droperidol 10 −6 to 10 −4 g/l reduced the contraction height while acetylcholine release was concomitantly depressed. Droperidol 10 mg/kg in cats did not block muscle contraction, but combination with d-tubocurarine prolonged recovery time.

Reticulospinal and propriospinal monosynaptic influences on motoneurons in frogs

Monosynaptic effects evoked by electrical stimulation of suprasegmental structures and the ventral and lateral columns were recorded intracellularly from motoneurons of the lumbar and cervical enlargements after isolation of the spinal cord and medulla in frogs. Reticulospinal fibers arising from cells of the medial reticular formation of the medulla and running in the ventro-lateral columns evoke monosynaptic excitation of cervical and lumbar motoneurons. The reticulo-motoneuronal E PSPs do not exceed 2–3 mV in amplitude and do not reach the threshold for action potential generation. Division of the spinal cord and interaction between all synaptic inputs tested in chronic experiments showed that monosynaptic E PSPs evoked by direct stimulation of the ventral and lateral columns are due to activation of the descending system of propriospinal fibers. By transmembrane polarization experiments the equilibrium potentials of the reticulo-motoneuronal and propriospinal monosynaptic E PSPs could be determined.

Calcium, magnesium and the electrical activity of guinea-pig olfactory coex in vitro.

1. Evoked electrical activity was observed in slices from guinea-pig olfactory cortex maintained in vitro. This evoked activity was studied in saline solutions containing various concentrations of Ca(2+) and Mg(2+).2. The evoked potentials recorded from the surface of the prepiriform cortex comprised a negative wave (N-wave) of about 10-15 msec duration upon which a variable number of short duration positive notches were superimposed. The N-wave was identified as a population excitatory post-synaptic potential (EPSP) of the olfactory cortex neurones and the positive notches were identified with synchronous discharge of cortical neurones.3. The relation between the EPSP amplitude and saline Ca(2+) concentration was sigmoid. The N-wave amplitude (EPSP) was proportional to [Ca(2+)](o) (n). When saline Ca(2+) was between 0.4 and 1 mMn averaged 2.5 (range 1.5-4.3).4. The EPSP amplitude was reduced by increasing saline [Mg(2+)]. High Mg(2+) salines (about 10 mM) abolished the EPSP.5. Both Ca(2+) and Mg(2+) in high concentrations depressed the positive notches.6. Salines containing little or no Ca(2+) and Mg(2+) caused the preparation to show signs of hyperexcitability. Under these conditions the evoked potentials were very variable and unstable. ;Ca(2+)-free' salines caused a reversible loss of excitability.7. The results indicated that Ca(2+) and Mg(2+) had antagonistic effects on the release of the transmitter substance(s). Increased Ca(2+) concentrations increased the output of transmitter, increased Mg(2+) concentrations reduced the output of transmitter. Both Ca(2+) and Mg(2+) in increasing concentration increased the threshold for action potential generation.

Effects of pH and uranyl ions on action potential generation and acetylcholine sensitivity of skeletal muscle

Over the range pH 9.0–5.2 only a pH below 6 affected action potential generation. The effects were principally an elevation of the depolarization level at which an action potential was triggered and a reduction in the rate of rise of the spike. The acetylcholine sensitivity of the muscle membrane was progressively reduced below pH 6.5 and at pH 5.2 it was only 10% of its value at pH 7. The effects of pH alteration were maximal within 60 sec and were completely reversible. Uranyl ions (0.2–0.4 mM) increased the threshold for action potential generation and depressed the rate of rise of the spike. At a concentration of 0.2 mM the sensitivity of the membrane to acetylcholine was reduced to about 15% of the control level. The similar effect of pH alteration on action potential generation and acetylcholine sensitivity suggests that the membrane structures responsible for these two processes have a similar pK value. The finding that uranyl ions depressed both mechanisms indicates that these membrane structures are phosphate-containing macromolecules.