A Ca- dependent regenerative response in rodent dorsal root ganglion cells cultured in vitro

Role of calcium in regulating primary sensory neuronal excitability

The fundamental role of calcium ions (Ca(2+)) in an excitable tissue, the frog heart, was first demonstrated in a series of classical reports by Sydney Ringer in the latter part of the nineteenth century (1882a, b; 1893a, b). Even so, nearly a century elapsed before it was proven that Ca(2+) regulated the excitability of primary sensory neurons. In this chapter we review the sites and mechanisms whereby internal and external Ca(2+) can directly or indirectly alter the excitability of primary sensory neurons: excitability changes being manifested typically by variations in shape of the action potential or the pattern of its discharge.

A Quantitative Description of Low- and High-threshold Ca2+ Spikes in Rat Sensory Neurons: A Perforated-patch Study

Action potentials generated by voltage-dependent Ca2+ conductances were studied at 25 degrees C with the perforated-patch technique, in freshly dispersed adult rat sensory neurons perfused with Na-free solutions containing tetraethylammonium. Brief depolarizing currents from membrane potentials negative to - 75 mV always elicited long (> 100 ms) plateau spikes which had different thresholds in different neurons: a low threshold around - 60/- 50 mV and a high-threshold at - 30/- 20 mV. Stimulations from potentials positive to - 55 mV, on the contrary, elicited spikes originating only in the high threshold region and sensitive to 25 microM Cd2+, designated high-threshold spikes. In neurons which showed spikes with low threshold, addition of 25 microM Cd2+ disclosed a smaller and shorter regenerative response, the low-threshold spike. Moreover, the classical 'anode-break' stimulation from - 50/- 60 mV uncovered isolated low-threshold spikes, indicating a time- and voltage-dependent de-inactivating process. From the properties of the low (LVA) and high (HVA) voltage-activated Ca2+ currents, recorded under the same extracellular conditions, a Hodgkin - Huxley model was derived and used to reconstruct all the features of the recorded spikes. The model was also able to simulate experimental blocking of LVA channels by amiloride, modulation of HVA channels by baclofen and induced oscillatory firing. This agreement between the behaviour of recorded spikes and their mathematical description led us to conclude that the LVA and HVA Ca2+ currents underlie the low- and high-threshold Ca2+ spikes, respectively. Furthermore, our data suggest that complex behaviour known to be typical of central nervous system neurons is also present in sensory peripheral neurons.

Membrane properties of dissociated trigeminal mesencephalic neurons of the adult rat

Electrophysiological properties of pseudounipolar trigeminal mesencephalic (Me5) neurons, dissociated from the rat brain, were studied under current-clamp conditions using the whole-cell configuration. Almost all Me5 neurons (37/38, 97%) exhibited a rapid adaptation in response to long depolarizing current pulses. Another firing type, slowly-adapting, was observed in only 3% of neurons (1/38). Most Me5 neurons (42/43) generated an overshooting action potential without a hump on the falling phase, and the remaining neuron (1/43) showed an action potential with a small hump. The action potential of Me5 neurons was reversibly blocked by 1 microM tetrodotoxin (TTX) or by removing Na+ from the bathing medium. When the outward K+ current was suppressed, two types of Ca2+ spikes were revealed. According to characteristic thresholds and sensitivity to inorganic (Ni2+, Cd2+) and organic (nifedipine, omega-conotoxin GVIA) Ca2+ channel blockers, these Ca2+ spikes were identified as T-type LTS (low-threshold spike) and L-type HTS (high-threshold spike). Also, a time-dependent inward rectification was observed in all Me5 neurons. It is concluded that the majority of Me5 neurons are of the rapidly-adapting type and generate a TTX-sensitive Na+ spike with negligible contribution of Ca2+, showing that the electrophysiological properties of Me5 neurons are more similar to those of CNS neurons than to those of PNS ganglion cells which have similar morphological features to Me5 neurons.

Local anaesthetic effects on tetrodotoxin-resistant Na+ currents in rat dorsal root ganglion neurones

Besides the fast tetrodotoxin-sensitive Na+ current, small dorsal root ganglion neurones of rats also possess a slower tetrodotoxin-resistant Na+ current. The blocking effect of commonly used local anaesthetics upon the tetrodotoxin-resistant Na+ current was investigated in the present paper. Dorsal root ganglia were dissected from adult rats and cells were enzymatically isolated. The whole-cell patch clamp technique was then used to measure inward Na+ currents of small dorsal root ganglion neurones. Externally applied local anaesthetics reversibly blocked the tetrodotoxin-resistant Na+ current in a dose-dependent manner. Half-maximal blocking concentrations for tonic block were: lignocaine, 326 microM; prilocaine, 253 microM; mepivacaine, 166 microM; etidocaine, 196 microM bupivacaine, 57 microM procaine, 518 microM benzocaine, 489 microM; tetracaine, 21 microM; and dibucaine, 23 microM. Blocking of the current by lignocaine was independent of temperature. The quaternary lignocaine derivative OX-314 did not have any effect upon the tetrodotoxin-resistant Na+ current when applied externally. High concentrations of tetrodotoxin also blocked the tetrodotoxin-resistant Na+ current with a half-maximal blocking concentration of 115 microM. The block by high tetrodotoxin concentrations did not compete with the lignocaine block, suggesting that there were two independent blocking mechanisms for the two substances. The tetrodotoxin-resistant Na+ currents also showed a marked sensitivity to phasic (use-dependent) block by local anaesthetics.

Tetrodotoxin-resistant sodium channels

1. Tetrodotoxin (TTX) has been widely used as a chemical tool for blocking Na+ channels. However, reports are accumulating that some Na+ channels are resistant to TTX in various tissues and in different animal species. Studying the sensitivity of Na+ channels to TTX may provide us with an insight into the evolution of Na+ channels. 2. Na+ channels present in TTX-carrying animals such as pufferfish and some types of shellfish, frogs, salamanders, octopuses, etc., are resistant to TTX. 3. Denervation converts TTX-sensitive Na+ channels to TTX-resistant ones in skeletal muscle cells, i.e., reverting-back phenomenon. Also, undifferentiated skeletal muscle cells contain TTX-resistant Na+ channels. Cardiac muscle cells and some types of smooth muscle cells are considerably insensitive to TTX. 4. TTX-resistant Na+ channels have been found in cell bodies of many peripheral nervous system (PNS) neurons in both immature and mature animals. However, TTX-resistant Na+ channels have been reported in only a few types of central nervous system (CNS). Axons of PNS and CNS neurons are sensitive to TTX. However, some glial cells have TTX-resistant Na+ channels. 5. Properties of TTX-sensitive and TTX-resistant Na+ channels are different. Like Ca2+ channels, TTX-resistant Na+ channels can be blocked by inorganic (Co2+, Mn2+, Ni2+, Cd2+, Zn2+, La3+) and organic (D-600) Ca2+ channel blockers. Usually, TTX-resistant Na+ channels show smaller single-channel conductance, slower kinetics, and a more positive current-voltage relation than TTX-sensitive ones. 6. Molecular aspects of the TTX-resistant Na+ channel have been described. The structure of the channel has been revealed, and changing its amino acid(s) alters the sensitivity of the Na+ channel to TTX. 7. TTX-sensitive Na+ channels seem to be used preferentially in differentiated cells and in higher animals instead of TTX-resistant Na+ channels for rapid and effective processing of information. 8. Possible evolution courses for Na+ and Ca2+ channels are discussed with regard to ontogenesis and phylogenesis.

C fiber generates a slow Na+ spike in the frog sciatic nerve

The present study was carried out to examine the properties of A and C fibers in the bullfrog sciatic nerves by applying several agents through perfusing solutions between stimulating and recording electrodes. The compound action potentials (CAPs) of A beta and A delta fibers were tetrodotoxin (TTX)-sensitive and were abolished in Na(+)-free solution. However, C fiber CAP was TTX-insensitive although CAP disappeared in Na(+)-free solution. Moreover, C fiber CAP was not blocked by Ca2+ channel blockers and its chronaxy (2 ms) and conduction velocity (0.70 m/s) indicate that the time constant of C fiber CAP is relatively large (2.88 ms). These suggest that a slow Na+ channel, which is TTX-resistant, contributes to C fiber action potentials.

Spontaneous rhythmic activity in the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord in vitro

Intracellular recordings from the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord slice preparation revealed a population of neurons which displayed three types of spontaneous rhythmic activity: burst firing, tonic beating and membrane oscillations. Most neurons displayed more than one of these types of activity. Neurons had mean resting potentials of -59 mV and input resistances ranging from 10 to 48 m omega. Spontaneous oscillations which were observed either independently or following hyperpolarization of neurons displaying tonic beating or bursting behaviour had a mean peak amplitude and frequency of approximately 14 mV and 1 Hz respectively. Oscillations were not obviously reversible as they were still apparent at potentials as negative as -120 to -140 mV. This suggests that the oscillations had a site of generation distant to the recording electrode. Neurons displaying tonic beating activity were characterized by low frequency firing activated at the peak of the depolarizing phase of the underlying oscillation and these neurons could be induced to exhibit burst behaviour by membrane depolarization. The frequency of firing in tonic beating neurons ranged from 0.1 to 8.8 Hz. Burst firing was characterized by: bursts of 3-17 action potentials; burst cycle frequency of approximately 1 Hz; an afterdepolarization potential mainly observed at the termination of a burst. Burst firing was abolished by cobalt and membrane hyperpolarization but not by barium, low calcium or tetraethylammonium chloride. The switch from tonic beating to burst firing may, in part, involve activation of a voltage- and calcium-dependent afterdepolarization potential. We conclude that a population of neurons in the lateral horn of the spinal cord are capable of rhythmic activity with underlying spontaneous pacemaker-like oscillations.

Electrophysiological properties of subpopulations of rat dorsal root ganglion neurons in vitro

Intracellular recordings were made from rat dorsal root ganglion neurons in vitro. Action potentials with an inflection on the falling phase occurred in all cells conducting up to 5.2 m/s and in a proportion of faster conducting cells which decreased with increasing conduction velocity, until no cells conducting faster than 31 m/s had an inflection. Overall, all C-cells (less than 1.3 m/s), 61% of A delta-cells (1.3-12 m/s) and 23% of A alpha/beta-cells (greater than 12 m/s) had inflections. A-cells with inflections were found to be electrophysiologically distinct from those without as they differed in the mean and distribution of every action potential and afterhyperpolarization parameter measured. C-cells differed from all A-cell groups, but the means and distributions of most parameters were closer to those of A-cells with inflections than of A-cells without. In addition, all A- and C-cell action potentials with inflections were tetrodotoxin resistant, while all those without were sensitive. The only parameters whose means differed between A alpha/beta- and A delta-neurons were ones which correlated with conduction velocity (action potential duration, overshoot and maximum rate of rise and fall). The response pattern to prolonged current injection did not correlate with conduction velocity, but slightly more A-cells with inflections were single firing. A-cells with long afterhyperpolarizations always fired singly, while those with shorter durations fired singly or multiply. Somatic following frequency was most strongly limited by long afterhyperpolarization duration; it was also slightly lower in A delta- than in A alpha/beta-cells, and lower in A-cells with inflections than in those without. Fibre following frequencies were highest in the fastest conduction neurons.

Modulation of calcium-currents by capsaicin in a subpopulation of sensory neurones of guinea pig

The action of capsaicin (CAP) on the total Ca2+ current was examined in internally perfused voltage-clamped dorsal root ganglion (DRG) neurones of guinea pigs. CAP changed the total Ca2+ current in about 50% of the investigated DRG neurones ("CAP-sensitive" neurones) in the following way: (I) a transient increase of the current amplitude at potentials between -35 mV and about -10 mV was accompanied by a shift of the current-voltage relation towards negative potentials by 5-8 mV; (II) the current inactivation was accelerated at potentials positive to about -35 mV; and (III) the current activation of Ca2+ currents (time to peak values) was also accelerated. Separated low voltage-activated (T-type) currents at potentials negative to about -35 mV were either not affected or reduced. It remains undecided whether CAP increases T-type currents in a particular potential range or activates an N-type current. External application of 50 microM Ni2+ blocks the effect of CAP, but does not affect the acceleration of the high voltage-activated (L-type) current inactivation induced by menthol. This appears to exclude a CAP effect on L-type current inactivation. "CAP sensitive" and "CAP insensitive" neurones could be discriminated by their different Ca2+ currents: the former demonstrate both fast and slow inactivating currents while the latter have only L-type currents. The observed changes of fast-inactivating Ca2+ currents may be related to the specific action of CAP on peptidergic sensory neurones.

Comparison of transmitter release properties of embryonic sympathetic neurons growing in vivo and in vitro

The functional behavior of embryonic chick sympathetic neurons was determined by inducing release of [3H]norepinephrine by electrical stimulation of sympathetic neurons growing in the chick heart and in culture, with and without heart cells. A very close correspondence between the functional behavior of neurons developing with the heart cells, either in vivo or in vitro, was demonstrated. For example, the outflow of tritium from [3H]norepinephrine loaded sympathetic neurons of 15-day-old chick heart was about three times more at 10 Hz than at 1 Hz. In contrast, the outflow of tritium from 12-day-old [3H]norepinephrine loaded cultured sympathetic neurons was inversely related to the frequency of stimulation (outflow at 1 Hz was about three time more than at 10 Hz). When neurons were co-cultured with the heart cells, the frequency-outflow relationship reverted to that seen in the intact heart. Electrically-evoked outflow of tritium from the heart was reduced in a concentration-dependent manner by 3-30 nM tetrodotoxin, abolished in 0.25 mM Ca medium, and potentiated by 3 mM tetraethylammonium. In sharp contrast, the outflow evoked by stimulation of cultured neurons was neither blocked by 30-300 nM tetrodotoxin, low Ca, nor potentiated by tetraethylammonium. However, when neurons were co-cultured with heart cells, the evoked outflow was blocked by 30 nM tetrodotoxin and low Ca, and potentiated by tetraethylammonium. Veratrine (10 microM) had very little effect on the outflow from cultured neurons but induced a massive outflow from co-cultures as well as hearts. Neurons grown in a medium conditioned by the heart cells were not sensitive to tetrodotoxin and veratrine. It is implied that cultured sympathetic neurons are endowed mostly with Ca channels, and that the Na channels become functional only when neurons are grown with the target cells. This dramatic alteration in the functional behavior of neurons co-cultured with heart cells indicates that the effector organ has an important role in the development of ionic conductances of sympathetic neurons growing in the body and in culture.

Voltage-activated sodium conductances in cultured normal and trisomy 16 dorsal root ganglion neurons from the fetal mouse

Current and voltage clamp recordings were made with a patch-clamp technique from large, light, dorsal root ganglia (DRG) neurons in tissue culture, derived from trisomy 16 and normal fetal mice. In a Na gradient of [52 mM]o/[28 mM]i, the action potential was accelerated, depolarization and repolarization were faster and the total Na conductance was higher in trisomic neurons. A tetrodotoxin (TTX)-sensitive, fast Na current was demonstrated, about 0.9 nA in trisomic and 0.3 nA in control neurons. The calculated mean specific membrane conductances were 0.74 mS/cm2 and 0.28 mS/cm2, respectively. A TTX-insensitive, slow Na conductance, 3-4 times the fast Na conductance and sensitive to Cd, also was demonstrated, with a 2-fold greater current density and conductance in trisomic as compared with control neurons, of 2.22 +/- 0.54 mS/cm2 and 1.26 +/- 0.09 mS/cm2, respectively. The voltage-dependence and kinetics of the TTX-insensitive, slow, Na current were similar in the two neuronal groups. The results indicate that depolarization during the action potential, in fetal mouse DRG neurons in culture, is mediated by this slow TTX-insensitive Na current. Further, acceleration of depolarization in trisomy 16 neurons is caused by a 2-fold increase in the density of the slow Na current.

The sensory-efferent function of capsaicin-sensitive sensory neurons

Capsaicin-sensitive sensory neurons convey to the central nervous system signals (chemical and physical) arising from viscera and the skin which activate a variety of visceromotor and neuroendocrine reflexes integrated at various levels (intramurally in peripheral organs, at level of prevertebral ganglia, spinal and supraspinal level). Much evidence is now available that peripheral terminals of certain sensory neurons, widely distributed in skin and viscera have the ability to release, upon adequate stimulation, their transmitter content. In addition to the well-known "axon reflex" arrangement, the capsaicin-sensitive sensory neurons have the ability to release the stored transmitter also from the same terminal which is excited by the environmental stimulus. The efferent function of these sensory neurons is realized through the direct and indirect (i.e. mediated by activation of other cells) effects of released mediators. The action of released transmitters on postjunctional elements covers a wide range of effects which may have a physiological or pathological relevance. Development of drugs capable of controlling the sensory-efferent functions of the capsaicin-sensitive sensory neurons represent a new and very promising area of research for pharmacological treatment of various human diseases.

Electrophysiological properties of cultured dorsal root ganglion and spinal cord neurons of normal and trisomy 16 fetal mice

Dorsal root ganglion (DRG) and spinal cord neurons from normal and trisomy 16 fetal mice, an animal model for human trisomy 21 (Down syndrome), were maintained in primary culture and their electrical membrane properties were compared with intracellular recording techniques. After 3-4 weeks in culture, trisomic DRG neurons had a higher mean resting potential (+10%), a higher specific membrane resistance (+50%) and higher excitability (+17%), a shorter action potential (-22%), higher maximal rates of depolarization (+39%) and of two phases of repolarization (+20% and +10%) and a lower duration (-42%) of the afterhyperpolarization, than did control DRG neurons (P less than 0.05). The duration of the action potential was 2X greater than in control neurons, when external calcium was elevated from 1.2 to 10 mM. Differences in the electrical parameters like those observed in DRG neurons also were found in cultured spinal cord neurons. These results indicate that trisomy 16 in fetal mice alters passive and active electrical membrane properties in DRG and spinal cord neurons, and suggest that some differences are related to differences in calcium currents.

The effects of ethanol on the electrophysiology of calcium channels

Acute ethanol intoxication affects many systems in the body, especially the central nervous system. Because early experiments using axonal preparations required very high concentrations of ethanol to produce ionic current alterations, researchers turned their attention away from specific effects on electrogenesis and looked for effects at the synapse. The role of Ca2+ in the release of neurotransmitters was well known and was considered a possible site of action for ethanol. Indeed, several studies demonstrated that ethanol alters Ca2+ binding or transport in synaptosomes and neural tissue. The purpose of this chapter is to present electrophysiological evidence for the acute effects of ethanol on calcium channels. It is necessary first to define the relevant ethanol concentrations and to describe the characteristics of tissue preparations that may best help to determine the effects of ethanol. A discussion of these two points along with a brief synopsis of the role of Ca2+ in excitable tissues is presented. This is followed by a discussion of the effects of ethanol on Ca2+ and Ca2+-activated conductances in both nonmammalian and mammalian cells, and a model is presented in an attempt to unify the experimental evidence of the acute effects of ethanol.

Somal action potential duration differs in identified primary afferents

In the present study we examined action potentials recorded from somata in the cat L7 and S1 dorsal root ganglia. We found (i) that there is a characteristic spike duration for each receptor type (e.g. Golgi tendon organ, D-hair) and (ii) that, for a given peripheral fiber conduction velocity, somata whose axons supply low-threshold cutaneous receptors exhibit spikes with shorter durations than those innervating high-threshold cutaneous receptors. These results suggest that there may be significant differences in somal membrane composition which are specific to the physiological receptor types innervated.

Effects of capsaicin on molluscan neurons: an intracellular study

Effects of capsaicin (CAP) on membrane properties and action potentials (AP) were studied (30-300 microM, at 22 degrees C, pH 7.4) in Helix and Aplysia neurons. CAP (100-300 microM) depolarized the cell membrane and increased the slope resistance. The neuronal firing increased and/or the spike threshold decreased. CAP differentially affected the APs generated in A- and B-cells in Helix or S- and F-cells in Aplysia. Plateau-like prolongation of the APs with a concomitant increase of the hump duration was observed in A-cells, while a significant prolongation of the spike duration was at 90% repolarization time in B-cells. The electrophysiological changes proved to be similar when CAP acted in homologous Helix and Aplysia neurons, but were less pronounced in the latter animal. CAP decreased the rate of rise and the rate of fall of the APs and shortened the action potential duration (APD) in Na-free (TEA) solution. CAP-induced events were dose-dependent and reversible.

Action potentials dependent on monovalent cations in developing mouse embryos

Action potentials were examined using intracellular recording techniques to study the ionic mechanisms of excitability in oocytes and embryos of the mouse from the 1-cell through to the 16-cell stages of development. At all stages examined, action potentials dependent on monovalent cations (Na+ or Li+) were observed under Ca2+-free conditions, and the maximum rate of rise (MRR) of the Na action potential was larger than that of the Li action potential at a given concentration of monovalent cations. Both the Na and Li action potentials were insensitive to tetrodotoxin, and they were blocked by inorganic (Co2+, Cd2+, Mn2+, La3+) and organic (diltiazem) Ca antagonists. These properties were exactly the same as those of the Ca channels present in the membranes of the mouse embryos. In addition, competition was observed between permeant monovalent and divalent cations: the overshoot and MRR of the Na or Li action potentials were reduced in the presence of Ca2+. These results suggest that Na+ or Li+ go through the Ca channels when the external Ca2+ concentration was very low, and that the Ca channels are more permeable to Na+ than to Li+. Separate Na channels could not be detected or induced at any stages of development.

Electrical properties of rat dorsal root ganglion neurones with different peripheral nerve conduction velocities

The electrical characteristics of individual rat dorsal root ganglion neurones were studied and related to the peripheral axon conduction velocity and morphological cell type. Neurones were divided into four groups based on the conduction velocity of their peripheral axons (A alpha, 30-55 m/s; A beta, 14-30 m/s; A delta, 2.2-8 m/s and C less than 1.4 m/s). Electrophysiological parameters examined included membrane potential, action potential amplitude and duration, after-potential height and duration, input resistance and the occurrence of time-dependent rectification. The mean duration of the somatic action potentials was found to be characteristic for each of the conduction velocity groupings. However, there was considerable overlap between groups. The fast-conducting (A alpha) and slowly conducting (A delta) myelinated fibres had short-duration action potentials, within the ranges 0.49-1.35 and 0.5-1.7 ms at the base respectively. The A beta and C cells had somatic action potentials with durations in the ranges of 0.6-2.9 and 0.6-7.4 ms respectively. The longer action potential durations could be related to the presence of an inflexion on the repolarizing phase seen in a third of A beta neurones (called A beta I neurones) and in all C neurones. The action potential overshoot was larger in C neurones and A beta I neurones than in the other neurone groups. The mean duration of the after-hyperpolarization was several times greater in C neurones than in A neurones. A delta neurones displayed the shortest and greatest amplitude after-hyperpolarizations. Large, long-lasting after-hyperpolarizations were not limited to neurones displaying an inflexion. The electrophysiological properties of the soma membrane of A delta neurones closely resembled those of A alpha neurones, while in several respects those of C neurones resembled the A beta I neuronal properties. The input resistance was found to be much greater in C than in A cells, although there was no significant difference between specific membrane resistance values calculated for the different groups. A number of A cells exhibited time-dependent rectification.

Ionic currents in cultured dorsal root ganglion cells from adult guinea pigs

Ionic currents in cultured dorsal root ganglion (DRG) neurons from adult guinea pigs were analyzed by voltage-clamp techniques. The Na+ inward current had a reversal potential at +33 mV, and revealed activation and inactivation kinetics similar to those of squid giant axons. A typical value for the maximum Na+ conductance was 178 mS/cm2 and the peak current was 2.5 mA/cm2. The delayed K+ outward current showed a fast and a slow phase of inactivation and was sensitive to tetraethylammonium (TEA; approximately 130 mM) and 4-aminopyridine (approximately 2 mM). The maximum K+ conductance was 26 +/- 9 (mean +/- SD) mS/cm2. The slow Ca2+ inward current was identified in Na+-free, TEA-containing solution. Its peak value was increased by 1.7-fold when [Ca2+]o was increased from 5 to 10 mM. The current was blocked by Co2+ but not by tetrodotoxin. Sr2+ and Ba2+ could substitute in carrying this current. The maximum peak of the Ca2+ current was 0.22 +/- 0.14 mA/cm2. At potentials positive to 0 mV, the Ca2+ current was often followed by a slowly developing outward current, which was also sensitive to Co2+, suggesting a Ca2+-activated outward current. It is concluded that the action potential of the adult guinea pig DRG neuron is mediated by Ca2+ as well as by Na+ and K+ currents. The current densities of these ionic channels are considered to be different from embryonic neurons and from nodes of Ranvier.

The ionic basis of action potentials in petrosal ganglion cells of the cat

The ionic conductances underlying the action potential and after-hyperpolarization of the cat petrosal ganglion neurones with myelinated axons in the carotid nerve were studied in vitro. Neurones were divided into two groups based on the presence or absence of an inflexion or hump on the spike falling phase. The application of tetrodotoxin (TTX, 3 X 10(-7)-3 X 10(-6) M) revealed the presence of a TTX-resistant component in spikes with a hump, which was abolished in Na+-free solution. The action potential without a hump was blocked by TTX. The spike hump decreased or was abolished when Ca2+-channel blockers (Mn2+, 3-4 mM or Co2+, 5 mM) or low-Ca2+ solutions (0.1-0.2 mM) were applied to the preparation. In neurones with a hump on the spike, regenerative responses were obtained in Na+-free, high-Ca2+ (8.8 mM) solution; these responses were antagonized by Mn2+, and their amplitude was proportional to the external Ca2+ concentration. It is concluded that the action potential with a hump was produced by an Na+ current, a part of which was TTX-resistant, and by a Ca2+ current which is responsible for the hump. Neurones without a hump had a TTX-sensitive Na+ spike. The spike with a hump was followed by a long-lasting after-hyperpolarization which reversed polarity at about -82 mV. During the hyperpolarization an increase in membrane conductance was observed. The amplitude and duration of the long hyperpolarizing potential decreased when Ca2+-channel blockers or low-Ca2+ solutions were applied. In Na+-free solution, regenerative responses were followed by a long hyperpolarization associated with an increase in membrane conductance. It is concluded that the long after-hyperpolarization is produced by activation of the Ca2+-dependent K+ conductance.

Potassium accumulation around individual purkinje cells in cerebellar slices from the guinea-pig

K+-selective micropipettes were used to measure external K+ concentration [( K+]o) in the immediate vicinity of Purkinje cells in slices from guinea-pig cerebellum. The cells were either spontaneously active or were polarized via a separate intracellular micro-electrode. The level of [K+]o rose by 1-3 mM around the soma and dendrites of Purkinje cells during spike activity. The increases in [K+]o were usually greater during Ca2+-mediated spikes than during Na+-mediated spikes. This was even true at the soma where the Ca2+ spike only invaded electrotonically from the dendrites, in contrast to the Na+ spikes which were generated at the soma. No [K+]o changes were seen in the vicinity of Purkinje cells when the cells were hyperpolarized with current passage nor was any [K+]o change seen during subthreshold depolarizations. In glial cells, however, a hyperpolarizing current reduced [K+]o while a depolarizing current increased [K+]o in a symmetrical manner. When Ba2+ was substituted for Ca2+ in the bathing Ringer solution, prolonged plateau-potential spikes could be evoked from Purkinje cells. These spikes were accompanied by large [K+]o elevations but the plateau potentials outlasted the [K+]o elevations. These experiments suggest that large [K+]o increases can occur in the absence of Ca2+-mediated K+ conductances. Substitution of Mn2+ for Ca2+ in the Ringer solution removed some of the [K+]o increases at the Purkinje cell soma. Addition of tetrodotoxin to normal Ringer solution also reduced, but did not abolish the [K+]o increases at the soma. These experiments confirmed that both Na+ and Ca2+ spikes were involved in the [K+]o change. The diffusion characteristics of the slices were determined by an ionophoretic method using tetramethylammonium and ion-selective micropipettes. The extracellular volume fraction of the slice averaged 0.28 while the tortuosity averaged 1.84. These values were close to those found previously in the intact rat cerebellum. These data were used to make quantitative estimates of the expected [K+]o accumulation in the vicinity of a single cell (see Appendix). Such estimates showed reasonable agreement with the measured values. Our data show that quite large increases in [K+]o may occur around single Purkinje cells. Such increases have previously only been evident during the activation of cell populations in mammalian preparations. The present results are probably due to the superior recording conditions of the slice. Implications for intercellular communication are discussed.

Opioid peptides decrease calcium-dependent action potential duration of mouse dorsal root ganglion neurons in cell culture

We investigated opioid peptide actions on somatic calcium-dependent action potentials of dorsal root ganglion (DRG) neurons grown in primary dissociated cell culture. We report that leucine-enkephalin decreased the duration and amplitude of DRG somatic calcium-dependent action potentials. The opioid peptide action was dose-dependent over 20 nM to 5 microM and was antagonized by naloxone, consistent with mediation by opiate receptors. Thus, DRG neuron membranes have opiate receptors which act to decrease calcium influx. It is more likely, therefore, that opiate receptors on the somata of DRG neurons in culture are functionally similar to opiate receptors on primary afferent terminals.

Membrane depolarization and prolongation of calcium-dependent action potentials of mouse neurons in cell culture by two convulsants: bicuculline and penicillin

The convulsant compounds bicuculline (BICUC) and penicillin (PCN) are antagonists of GABA-mediated synaptic inhibition. In addition, we have shown that BICUC and PCN produced membrane depolarization of mouse spinal cord neurons in primary dissociated cell culture by blocking a potassium conductance, a non-synaptic direct effect. Both compounds also prolonged calcium-dependent action potentials of mouse dorsal root ganglion and spinal cord neurons in cell culture. Thus, BICUC and PCN had both synaptic and non-synaptic actions. The possibility that both synaptic and non-synaptic actions of BICUC and PCN are involved in their convulsant mechanism of action is discussed.

Calcium-dependent action potentials in mouse spinal cord neurons in cell culture

Following blockade of membrane potassium conductance with tetraethylammonium ions or 3-aminopyridine, long-duration action potentials were recorded from mouse spinal cord neurons in primary dissociated cell culture. The action potentials were calcium-dependent since they: (1) were not blocked by the sodium-channel blocker tetrodotoxin, (2) could be recorded in sodium-free, calcium-containing medium (3) could not be evoked in sodium-containing, calcium-free medium, (4) were blocked by calcium channel blockers manganese and cobalt and (5) had overshoot amplitudes that varied linearly with the log of the extracellular calcium concentration (slope of 27.5 mV/decade change in calcium concentration).

Spike potentials and membrane properties of dorsal root ganglion cells in pigeons

Active and passive membrane properties of dorsal root ganglion (DRG)-cells from the intact superfused ganglion of pigeons have been compared with the conduction velocity of their centrifugal axons. About two thirds of the neurones were associated with myelinated axons and classified as A-cells; the remainder were associated with unmyelinated axons and classified as C-cells. Slowly conducting group III A-cells (5--25 m . s-1) constituted half of the A-cell population. With exception of spike duration, spike parameters and membrane properties did not differ among the A-cells. Spike duration increased with decreasing conduction velocity demonstrating a small plateau ("hump") during the fall time in group III neurones. This hump was more distinct in C-cells, resulting in a 2--5 times longer duration of action potentials. Amplitude and duration of afterhyperpolarization (AHP) of C-cells was 2--3 times that of A-cells. Administration of 10 mM CoCl2 decreased the rate of rise and the overshoot but increased the rate of fall of the action potential in C-cells and group III A-cells, largely abolishing the hump. It is suggested that the hump of the spike potential is largely produced by a Ca-current and that the resultant increase of intracellular Ca might produce the larger AHP in C-cells, secondary to an increase in K-conductance.

The effect of phenytoin on the action potential of a vertebrate spinal neuron

The effect of phenytoin (PTN) 20 microgram/ml was tested on the passive membrane properties and the action potential of the dorsal cells in the spinal cord of the river lamprey. Dorsal cells are primary sensory neurons with no synaptic input, and thus allow examination of membrane properties in the absence of contaminating synaptic currents. PTN did not affect the resting membrane potential and slightly raised the input resistance. However, it greatly raised the threshold voltage and current for activation of action potentials by intracellularly injected current. It also reduced the maximum rate of rise of the action potential, the spike overshoot and the spike undershoot, while increasing the spike duration. In contrast to findings in other vertebrate sensory neurons, dorsal cell action potentials were blocked in zero sodium or tetrodotoxin, and not affected by zero calcium or 1 mM manganese. Thus they PTN on dorsal cells is that it partially blocks the activated sodium conductance increase of the action potential. Because a long delay was observed for maximal effect of PTN and for washout, it is postulated that the drug may require partitioning into the lipid membrane, or entry into the cell for its pharmacological action.

Bioelectric activity of chick stato-acoustic ganglion cells in culture

Electrical membrane properties were recorded intracellularly from dissociated embryonic chick stato-acoustic ganglion (S-AG) cells cultivated in vitro for 8 days. Most S-AG cells exhibited the complete action potentials with overshoot by passing current and a half of them fired repetitively during single prolonged stimulation. In some S-AG cells with relatively high input resistances, long-term negative after-potentials were clearly observed at the end of repetitive discharges. The action potential was composed of fast and slow components. The former component was blocked by tetrodtoxin (TTX) and the latter one was markedly suppressed by treatment with MnCl2 or verapamil. These data indicate that the action potential from the chick S-AG perikaryon is produced by both Na and Ca mechanisms together.

Tetrodotoxin sensitivity and Ca component of action potentials of mouse dorsal root ganglion cells cultured in vitro

In the mouse dorsal root ganglia cultured in vitro, neurons were classified into 3 groups according to the responses of their action potentials to tetrodotoxin (TTX) and removal of Na ions from bathing medium: (1) the neurons whose action potentials were not affected by TTX by TTX (10(-6) - 10 (-5)g/ml) and which generated Ca-dependent regenerative responses under Na-free condition, (2) the neurons whose spike potentials were resistant to TTX but failed to survive in Na-free saline and (3) the neurons whose action potentials were suppressed by TTX(10(-8)g/ml) as well as Na removal. The mean duration of spike and after-hyperpolarization was longest in the first group of the neurons and shortest in the third, probably reflecting the difference in the contribution of Ca currents to action potentials. The unresponsiveness of the neurons to TTX was shown to be due to the insensitivity of Na as well as Ca components of action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action might be related to the neuronal development.

Action potentials of embryonic dorsal root ganglion neurones in Xenopus tadpoles

1. Several classes of action potentials can be distinguished in dorsal root ganglion cells, studied by intracellular recording techniques in Xenopus laevis tadpoles 4.5--51 days old. The ionic basis of the action potential was investigated by changing the ionic environment of the cells and applying various blocking agents. 2. The Ca2+-dependent action potential is a plateau of relatively long duration (mean 8.7 msec). It is unaffected by removal of Na+ but blocked by mM quantities of Co2+. It is present only in small cells. 3. Ca2+/Na+-dependent action potentials. Type I is a spike followed by a plateau or hump of different durations (mean 8.1 msec). The spike is selectively blocked by removal of Na+, leaving the plateau which is in turn blocked by Co2+. It is present in cells of small and intermediate size. Type II is a spike of short duration (mean 2.0 msec) with only an inflection on the falling phase. The spike is blocked by removal of Na+ and no other components can be elicited. The inflection is blocked by Co2+. It is present in cells of all sizes. Type III is similar to type I but is seen only in solutions in which the outward current is blocked. It was observed only very infrequently. 4. Na+-dependent action potentials. Type I a is a short duration spike (mean 1.1 msec). It is abolished by removal of Na+ or addition of tetrodotoxin (TTX), but largely unaffected by Co2+ or La3+. It is present in cells of all sizes. When the outward current channels are blocked and cells exposed to Na+-free solutions, all cells are capable of producing an action potential in which the inward current is carried by divalent cations. Type I b is a spike with a smooth, more slowly falling phase. It has the same pharmacological properties as type I a action potential and is present in cells of small size. 5. Na+-dependent action potentials. Type II is a spike with an inflection on the falling phase (mean duration 3.4 msec). It is prolonged by Co2+ and La3+. Removal of Na+ abolishes the spike but TTX does not block it. It is present in cells of all sizes. The mean resting potential is less than that of cells with Na+-dependent type I action potentials, while the mean input resistance is greater. 6. Tetraethylammonium chloride (TEA) prolongs the different kinds of action potentials. The amount of prolongation varies among cells with a given type of action potential, so that no distinction could be made of the different actionpotential types based on the effect of TEA. 7. The percent of cells with each kind of action potential varies with the developmental age of the animal. The number of cells with Ca2+ and Ca2+/Na+ action potentials decreases with age, while the number of cells with a Na+ type I action potentials increases. The Na+ type II action potential appears only at later stages. 8...

Rat cortical neurons in cell culture: culture methods, cell morphology, electrophysiology, and synapse formation

Rat cortical neurons from 15 day embryos are grown in dissociated cell culture and maintained in vitro for 8--12 weeks. The neurons develop into forms which resemble mature cortical neurons in situ, stain with silver and exhibit passive and active electrophysiological properties similar to those of cortical neurons. Extensive chemical excitatory and inhibitory synapses develop de novo. These cultures can provide a model for future studies of mammalian CNS neuronal physiology, transmitter pharmacology, pathophysiology and mechanisms of drug action.

Ionic currents in cultured mouse neuroblastoma cells under voltage-clamp conditions

1. Ionic currents in differentiated cells of mouse neuroblastoma clone N1E-115 have been studied under voltage-clamp conditions. 2. Depolarizing voltage steps from a holding potential of -85 mV to levels more positive than -40 mV produced fast transient inward currents followed by delayed outward currents. 3. The fast inward current is carried by Na+: it is blocked by tetrodotoxin and is absent in Na+-free solutions. Its kinetic behaviour resembles that of the Na+ current in squid giant axon. A mean value of 85 mmho/cm2 was found for the maximum Na+ conductance (GNa).4. The delayed outward current is carried primarily by K+: it is blocked by externally applied tetraethylammonium (TEA, 15 mM) and has a reversal potential (mean -71 mV) close to the theoretical K+ equilibrium potential. Its instantaneous I--V curve is linear. By analogy with the formulation of Hodgkin & Huxley (1952c), the outward current can be described by IK = -GKn2(V--EK) where GK = 12 mmho/mc2. 5. During prolonged depolarizations the delayed outward current declines. This decline, which occurs in two phases, represents a partial inactivation of the K+ conductance. 6. A weak inward current with slow activation and inactivation kinetics appears in Na+-free solution containing 10 mM-Ca2+. It is activated at a membrane potential of -55 mV and reaches its maximum at -20 mV with a time to peak of about 10 msec. This current is tetrodotoxin-resistant, reversibly blocked by Co2+ (5mM) and is suggested to be carried by Ca2+. 7. An increase in the external divalent cation concentration results in a parallel shift of the steady-state I--V curve along the voltage axis in positive direction. The activation of delayed outward currents is suggested not to depend on Ca2+ influx. 8. It is concluded that separate voltage-dependent Na+, K+ and Ca2+ channels exist in the differentiated neuroblastoma membrane with kinetic and pharmacological properties similar to those observed in non-mammalian preparations.

Ionic determinants of excitability in cultured mouse dorsal root ganglion and spinal cord cells

The ionic components of the action potentials of mouse spinal cord (SC) cells and dorsal root ganglion (DRG) cells were studied in dissociated cell cultures. It was found that the action potentials of SC cells required Na+ in the medium and were blocked by tetrodotoxin (TTX) (1 micron). Action potentials of DRG cells, on the other hand, were not blocked by TTX (up to 10 micron) and were observed in Na-free media in the presence of 8 mM Ca2+. In low Na (31 mM), low Ca2+ (0.1 mM) medium, action potentials were not observed but could be obtained if the Ca2+ concentration was increased. Action potentials of DRG cells investigated in low Na concentration in the presence of 1 mM or 8 mM Ca2+ became larger in amplitude and shorter in duration when the sodium concentration was increased. Na+ has this effect even in the presence of TTX. It is concluded that the action potentials of SC cells result mainly from a TTX-sensitive Na component. The action potentials of DRG cells on the other hand have both a TTX-insensitive Na component and a Ca2+ component.