Longer Action Potential Duration Research Articles

A comparison of extracellular and intracellular recordings from medial septum/diagonal band neurons in vitro

Firing patterns, action potential characteristics and some active membrane properties of guinea-pig medial septum/diagonal band neurons were studied in an in vitro slice preparation. A comparison was made between several types of cells classified according to either extracellularly recorded ( n = 130) or intracellularly recorded ( n = 30) electrophysiological characteristics. Using multi-barrel extracellular electrodes, three principal cell types were distinguished: slow rhythmic firing cells (29%), fast rhythmic firing cells (65%) and burst-firing cells (6%). Most slow firing cells could also be distinguished from other cell types by their relatively longer action potential duration and a characteristic cadmium-sensitive “hump” in the repolarization phase of the action potential. These characteristics of slow firing cells matched well with the characteristics of cholinergic, slow afterhyperpolarization cells previously identified with intracellular recordings. The action potential shape, firing rate and firing pattern characteristics of about 60% of extracellularly recorded fast rhythmic firing cells matched those of previously identified non-cholinergic fast afterhyperpolarization cells. The remaining extracellularly recorded, rhythmic firing cells (about 10% of slow firing and 40% of fast firing cells) had a mixture of characteristics which precluded unequivocal classification as to cholinergic or non-cholinergic cell type. Using intracellular recording, the bee venom toxin, apamin, was shown to attenuate the characteristic post spike slow afterhyperpolarization of cholinergic cells and greatly enhanced their firing rate to depolarizing pulses. Apamin often attenuated a smaller and more transient afterhyperpolarization found in identified non-cholinergic cells, but firing rate was increased only slightly. Extracellular recordings from slow and fast rhythmic firing cells in the presence of apamin showed that excitability of slow firing cells was enhanced significantly more than fast firing cells. The apamin data support the hypothesis that extracellularly recorded slow firing cells are cholinergic. We conclude that extracellularly recorded medial septum/diagonal band cells characterized by broad action potentials, slow rhythmic firing under microiontophoresed glutamate and a signature “hump” in the falling phase of the action potential are cholinergic cells. Extracellularly recorded fast rhythmic firing cells with a narrow action potential and no “hump” in the action potential are likely to be non-cholinergic cells. This extracellular electrophysiological “fingerprint” for cholinergic medial septum/diagonal band cells in vitro may now be extended to studies in vivo where controversy remains as to the neurochemical identity of basal forebrain cells involved in control of hippocampal slow rhythmic activity.

Membrane depolarization and intracellular Ca2+ increase caused by high external Ca2+ in a rat calcitonin‐secreting cell line.

1. Calcitonin secretion is regulated by the external Ca2+ concentration ([Ca2+]o) via a rise in intracellular Ca2+ concentration ([Ca2+]i). The mechanism which couples an increase in [Ca2+]o to an increase in [Ca2+]i was explored in a rat calcitonin-secreting cell line (rMTC 44-2). [Ca2+]i was monitored using Fura-2 AM, and the membrane potential or current was simultaneously measured. 2. Using the conventional whole-cell clamp, tetrodotoxin-sensitive voltage-gated Na+ channels, T- and L-type Ca2+ channels, and three types of K+ channels, the delayed K+ channel, the A-channel and the inward-rectifying channel were observed. 3. Using the nystatin-perforated whole-cell-clamp technique, the resting potential measured under current clamp in standard extracellular medium was -59.0 +/- 5.0 mV (mean +/- S.D., n = 25), and the input resistance was 3.9 +/- 1.9 G omega (n = 10). In 0.5 mM [Ca2+]o most cells (22/25) showed spontaneous action potentials. 4. An increase in [Ca2+]o depolarized the cell membrane and elevated [Ca2+]i even in the presence of 10 microM-tetrodotoxin. The rise in [Ca2+]i was greatly reduced when action potentials were inhibited by applying hyperpolarizing current. The increase in [Ca2+]i saturated with 3-4 mM [Ca2+]o. In 3 mM [Ca2+]o, [Ca2+]i was 188.9 +/- 40.5% (n = 12) of that in 0.5 mM [Ca2+]o. 5. In high [Ca2+]o the duration of action potentials was prolonged, but the action potential frequency did not always increase. In some cases it even decreased in high [Ca2+]o. 6. Two types of action potential were observed in high [Ca2+]o, one with a shorter duration and the other with a longer duration. [Ca2+]i transiently increased in association with the long-duration action potentials. These long-duration action potentials were also accompanied by a larger after-hyperpolarization. 7. Under voltage clamp, high [Ca2+]o caused a membrane conductance increase to Na+ ions. 8. Even when the membrane potential was clamped at a level below the threshold for Ca2+ channel activation, high [Ca2+]o provoked an increase of [Ca2+]i which was composed of an initial transient increase followed by a sustained increase, indicating an involvement of mechanisms other than Ca2+ influx through voltage-gated channels. The sustained increase was more frequently observed than the initial transient increase. The amplitude of the sustained phase was dependent on [Ca2+]o, and in 5 mM [Ca2+]o it was 120.9 +/- 18.9% (103-194%) (n = 58) of that in 0.5 mM [Ca2+]o.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Propranolol on Premature Action Potentials in Canine Purkinje and Ventricular Muscle

We compared the effects of a low (0.09 microgram/ml) concentration of propranolol expected to produce only beta-adrenoceptor blockade and a high concentration (0.9 microgram/ml) expected to produce additional direct local anesthetic-like electrophysiological effects on basic and premature action potentials. Both isolated dog cardiac Purkinje and ventricular muscle fibers were examined using conventional microelectrode techniques. The low concentration of propranolol produced no significant electrophysiological change in either fiber type. The high concentration of propranolol shortened the action potential duration and refractoriness while decreasing the maximal upstroke velocity (Vmax) in both Purkinje and ventricular muscle fibers at a constant basic cycle length. In Purkinje fibers, the high concentration also slowed the kinetics of restitution of the action potential duration (tau c from 124.6 +/- 6.5 to 201.4 +/- 16.0 ms, p less than 0.01, n = 7), slowed the recovery kinetics of Vmax, and shifted the early portion of the normalized restitution curve toward longer action potential duration values in both fiber types. The range of premature action potential durations, defined as the difference between action potential durations during the first 100 ms of restitution, was decreased by the high concentration of propranolol in both Purkinje and ventricular muscle fibers by 39.5% and 33.9%, respectively. These findings indicate that (a) low concentrations of propranolol produced no direct electrophysiological effect, and (b) high concentrations of propranolol produced several potential antiarrhythmic effects in addition to the previously reported effects on Vmax and the action potential duration.

Spontaneous calcium influx and its roles in differentiation of spinal neurons in culture

Stimulation of embryonic amphibian spinal neurons has been shown to produce calcium-dependent action potentials of long duration at early stages of development. These impulses become brief and sodium-dependent upon further differentiation. The neurons are now shown to exhibit spontaneous, transient elevations of intracellular calcium in culture during the early developmental period when activity produces greatest calcium influx. Removal of extracellular calcium during this period alone is sufficient to perturb differentiation, and influx through voltage-dependent calcium channels is shown to be required for standard development of neuronal phenotypes. No large changes in steady-state calcium levels occur in the cytoplasm during the maturation of cultured neurons despite a reduction of the calcium-dependent component of the impulse. Transient elevation of intracellular calcium is necessary for standard cytodifferentiation and may provide a link between electrical activity and gene expression.

Firing patterns and synaptic potentials of identified giant aspiny interneurons in the rat neostriatum

Intracellular recordings were made in vivo from 9 giant aspiny neurons in the neostriatum of urethane-anesthetized rats. The cells were identified by intracellular staining with HRP or biocytin. The neurons exhibited morphological features typical of neostriatal cholinergic interneurons. Six of the cells were obtained from intact animals, while 3 were recorded from rats with ipsilateral hemidecortications. Giant aspiny neurons were characterized by their slow irregular but tonic (3-10/sec) spontaneous activity and long-duration action potentials. Examination of the underlying membrane potential trajectories during spontaneous firing revealed that individual action potentials were triggered from spontaneous small (1-5 mV) depolarizing potentials. These spontaneous potentials exhibited the voltage sensitivity of ordinary EPSPs. They were much less frequent during the 80-200 msec pause in tonic afferent input that follows the excitation evoked by cortical or thalamic stimulation, and were decreased in frequency in decorticate animals. Their rise times and half-widths matched those expected for unitary synaptic potentials placed proximally on the surface of the neurons. Low-intensity stimulation of neostriatal afferents produced small short-latency EPSPs that appeared to be composed of responses identical to the spontaneous depolarizing potentials. The latencies of the EPSPs evoked from the cerebral cortex and thalamus were consistent with a monosynaptic input from both structures, but the maximal size of the EPSPs was much smaller than that evoked in spiny neurons, suggesting that a smaller number of afferent inputs make synapses with each of the aspiny cells. Giant aspiny neurons exhibited much larger input resistances and longer time constants than spiny neostriatal neurons. They also exhibited relatively linear steady-state current-voltage relationship compared to spiny projection cells. Input resistances ranged from 71-105 M omega, and time constants ranged from 17.8-28.5 msec. Analysis of the charging transients in response to current pulses yielded estimates of dendritic length of approximately 1 length constant. Repetitive firing of the neurons was limited by a powerful spike afterhyperpolarization and by a strong spike frequency adaptation. The sensitivity of the giant aspiny interneuron to a relatively small number of proximal afferent synaptic contacts, its tonic firing, and its widespread dendritic and axonal fields place it in an excellent position to act as a modulator of the excitability of neostriatal projection neurons in advance of the onset of movement-related neostriatal activity.

Sodium and calcium currents in action potentials of rat somatotrophs: Their possible functions in growth hormone secretion

We report that both Na + and Ca 2+ currents are involved in the action potentials and in the hormone release from rat somatotrophs in primary culture. Single somatotrophs were identified by reverse hemolytic plaque assay (RHPA) and transmembrane voltage and curents were recorded using the whole-cell mode of the patch-clamp technique. Somatotrophs displayed a mean resting potential of −80mV and an average input resistance of 5.7GΩ. Most of the cells showed spontaneous or evoked action potentials. Single action potentials or the initial spike in a burst were characterized by their high amplitude and short duration. Tetrodotoxin (TTX, 1μM) blocked single action potentials and the initial spikes in a burst, whereas action potentials of long duration and low amplitude persisted. Cobalt (2mM) plus TTX (1μM) blocked all the action potentials. Voltage-clamp experiments confirmed the presence of both a TTX-sensitive Na + current and Co 2+ -sensitive Ca 2+ currents. TTX or Na +-free medium slightly decreased the basal release of GH but did not markedly modify hGRF-stimulated GH release. However, Co 2+ (2mM), which partially decreased the basal release, totally blocked hGRF-stimulated release. We conclude that (1) Na + currents which initiate rapid action potentials may participate in spontaneous GH release; (2) Ca 2+ currents, which give rise to long duration action potentials and membrane voltage fluctuation, are probably involved in both basal and hGRF-stimulated GH releases.

Ionic dependencies of tetrodotoxin-resistant action potentials in trigeminal root ganglion neurons

Action potentials recorded in vitro from the perikarya of trigeminal root ganglion neurons (guinea-pig) were examined for their sensitivities to blockers of specific ion channels or to removal of certain ionic species in the bathing media. The majority (~65%) of the 137 neurons exhibited action potentials following application of the Na +-channel blocker, tetrodotoxin. This group of neurons was selected for further investigation under conditions of extracellular K +-channel blockade with tetraethyl-ammonium and 4-aminopyridine. Long-duration action potentials consisting of two distinct components could be evoked under such conditions. The fast component of the spike was abolished in Na +-deficient perfusion media and was sensitive to blockade by extracellular lidocaine or intracellular QX-222 applications. It is likely that the slow component was mediated mainly by Ca 2+, but in Ca 2+-deficient media, Mg 2+-influx may have contributed to the small voltage response. The amplitude and shape of the slow component was unaffected by applications of lidocaine or QX-222. Self-sustained repetitive firing was also observed in 11 neurons in the above conditions. This activity persisted even under conditions of severe deficiencies in extracellular [Ca 2+] or [Na +]. Two distinct but overlapping K +-conductances that were sensitive to blockade by internal Cs +-application and insensitive to applications of tetraethylammonium and 4-aminopyridine, appear to mediate the afterhyperpolarization of the long-duration spike. One portion of the afterhyperpolarization was 60–150 ms in duration and was unaffected by removal of Ca 2+ from the extracellular media, while the other had a time-course lasting 150–250 ms and was abolished by removal of external Ca 2+. In some neurons, these K +-conductances were blocked by high doses of doxorubicin or cisplatin. The results show that at least two ion species (Na + and Ca 2+) contribute to the formation of the tetrodotoxin-resistant, long-duration action potential in trigeminal root ganglion neurons during selective K +-conductance blockade and also provide evidence for Mg 2+ involvement in the generation of this voltage response.

Comparison of the sodium currents in normal Purkinje fibres and Purkinje fibres surviving infarction—a pharmacological study

1. Purkinje fibres surviving infarction showed a lower maximum upstroke velocity (Vmax) and a longer action potential duration when compared to normal Purkinje fibers. A reduction in the fast sodium current and an increase in the sodium 'window' current may be responsible for the observed alterations in Vmax and action potential duration respectively. 2. Since voltage clamp studies were not feasible, a pharmacological approach was used. The responses to tetrodotoxin (TTX) and lignocaine in normal Purkinje fibres and Purkinje fibres surviving infarction were used to examine the sodium currents in these fibres. 3. Vmax, an indirect measure of the fast sodium current, was more sensitive to lignocaine in Purkinje fibres surviving infarction than in normal Purkinje fibres. The reduction in Vmax by lignocaine was more prominent at the shorter stimulation cycle length. Significant reduction of Vmax was observed with the higher concentration of TTX and no differential effect on Vmax between normal Purkinje fibres and Purkinje fibres surviving infarction was detected. 4. Reduction of action potential duration in the presence of TTX or lignocaine was used as a measure of the sodium 'window' current. A greater reduction of action potential duration by TTX and lignocaine was observed in normal Purkinje fibres than in Purkinje fibres surviving infarction. 5. The results suggested that the fast sodium current in Purkinje fibres surviving infarction is more sensitive to pharmacological agents with local anaesthetic properties and the prolonged action potential duration in these Purkinje fibres cannot be due to an increase in the sodium 'window' current. The results are compatible with an enhanced effect of antiarrhythmic drugs on Vmax and conduction in the ischaemic heart.

Effects of BAY K 8644 on Ca-channel currents and electrical activity in mouse melanotrophs

The effects of BAY K 8644 on both evoked and spontaneous electrical activity were studied in mouse melanotrophs. The action potential in these cells consisted of both a Na and a Ca component. BAY K 8644 greatly increased the duration of the evoked action potential apparently by enhancing the Ca component since this prolongation remained when the Na component was blocked by tetrodotoxin but was diminished by the Ca-channel antagonists Co 2+ or nimodipine. In addition to its effects on the evoked action potential, BAY K 8644 sometimes caused action potentials of long duration to occur in otherwise quiescent melanotrophs, even in the presence of tetrodotoxin. These action potentials appeared to be triggered by small, depolarizing prepotentials. Voltage-clamp experiments suggested the existence of 3 types of Ca-channel currents in these cells: one low-threshold current and two high-threshold currents, one of which rapidly inactivated and another which did not. BAY K 8644 caused a shift in the I–V relation for Ca-channel current to more negative values and reduced or abolished the hump in the initial part of the I–V curve produced by the low-threshold current suggesting that BAY K 8644 selectively affects high-threshold Ca current. The action of BAY K 8644 to initiate trains of long-lasting Ca spikes and thereby increase Ca influx, might explain its secretagogue effect on the unstimulated melanotroph.

Subtypes of dorsal root ganglion neurons based on different inward currents as measured by whole-cell voltage clamp.

Electrophysiological and pharmacological properties distinguished subtypes of adult mammalian dorsal root ganglion neurons (DRGn) in monolayer dissociated cell culture. By analogy of action potential waveform and duration, neurons with short duration (SDn) and long duration (LDn) action potentials resembled functionally distinct subtypes of DRGn in intact ganglia. Patch clamp and conventional intracellular recording techniques were combined here to elucidate differences in the ionic basis of excitability of subtypes of DRGn in vitro. Both SDn and LDn were quiescent at the resting potential. Action potentials of SDn were brief (less than 2 msec), sensitive to tetrodotoxin (TTX, 5-10 nM), exhibited damped firing during long depolarizations, and did not respond to algesic agents applied by pressure ejection. Action potentials of LDn were 2-6 msec in duration, persisted in 30 microM TTX, and fired repetitively during depolarizing current pulses or exposure to algesic agents (e.g., capsaicin, histamine and bradykinin). Whole-cell recordings from freshly dissociated neurons revealed two inward sodium currents (INa; variable with changes in sodium but not calcium concentration in the superfusate) in various proportions: a rapidly activating and inactivating, TTX-sensitive current; and, a slower, TTX (30 microM)-resistant INa. Large neurons, presumable SDn, had predominantly TTX-sensitive current and little TTX-resistant current. The predominant inward current of small neurons, presumably LDn, was TTX-resistant with a smaller TTX-sensitive component. By analogy to findings from intact ganglia, these results suggest that fundamentally different ionic currents controlling excitability of subtypes of DRGn in vitro may contribute to functional differences between subtypes of neurons in situ.

The role of potassium and sodium-calcium exchange currents in the action potential durations of normal Purkinje fibres and Purkinje fibres surviving infarction

Ventricular arrhythmias following myocardial infarction may originate from subendocardial Purkinje fibres in the infarcted area. The role of potassium and sodium-calcium exchange currents on action potential duration was therefore investigated in Purkinje fibres surviving infarction and in normal Purkinje fibres. Barium was used to reduce potassium conductance and replacement of sodium chloride by lithium chloride was used to reduce the sodium-calcium exchange current. Barium (1 x 10(-5) to 3 x 10(-5) M) produced a concentration dependent lengthening of action potential duration in normal Purkinje fibres but these concentrations had no effect in Purkinje fibres surviving infarction. The resting membrane potential, activation voltage, amplitude and Vmax were decreased in Purkinje fibres surviving infarction and in barium treated fibres v normal Purkinje fibres. These results show that action potential characteristics of barium treated normal Purkinje fibres closely resemble those seen in Purkinje fibres surviving infarction. A similar reduction of action potential duration was seen in normal Purkinje fibres, fibres surviving infarction and barium treated fibres when the sodium "window" current was decreased by lignocaine. Without any effect on the Vmax, the replacement of sodium chloride by lithium chloride resulted in a similar effect on the action potential duration in normal Purkinje fibres and fibres surviving infarction. The results show that the longer action potential duration found in Purkinje fibres surviving infarction can be explained by a decrease of the potassium conductance and not by an increase of the sodium "window" current or the sodium-calcium exchange current.

Some membrane properties of the circular muscle of chicken rectum and its non-adrenergic non-cholinergic innervation.

1. Membrane properties and innervation of the circular muscle of chicken rectum were investigated by recording intracellularly electrotonic potentials evoked by passing current, and excitatory and inhibitory junction potentials (EJPs and IJPs) evoked by electrical stimulation of the extrinsic or intrinsic nerves. 2. The membrane potential was -55 +/- 0.6 mV (n = 95). Action potentials of long duration (1.2-4.0 s) discharged spontaneously, or were generated when the membrane depolarization due to either electrotonic potential or EJP reached the threshold. The drug D600 blocked the generation of action potentials. 3. Electrotonic potentials spread fairly well in the longitudinal direction of the muscle fibres but not in the transverse direction. The longitudinal space constant was 1.7 +/- 0.2 mm (n = 10) and the membrane time constant was 205 +/- 21 ms (n = 10). 4. Field stimulation of intramural nerves evoked an EJP followed by a long-lasting IJP (3-12 s in total duration) in most cells, and an EJP alone or an IJP alone in a small number of cells. The EJP and IJP were preserved in the simultaneous presence of atropine and guanethidine, but abolished with tetrodotoxin. 5. Stimulation of Remak's nerve trunk or its branches produced EJPs which were atropine resistant and guanethidine resistant just like the EJP elicited by intramural nerve stimulation. The extrinsic nerve stimulation was ineffective in eliciting IJPs. 6. The EJP amplitude declined in a linear manner as the distance from the stimulating site of intramural nerves was increased. The decline was much greater along the transverse axis than the longitudinal axis of circular muscle fibres. 7. The reversal potential for the EJP was estimated by extrapolation to be about - 15.3 +/- 0.3 mV (n = 7). 8. Apamin did not inhibit the IJP. During the hyperpolarization of a single IJP or summed IJPs, electrotonic potentials remained unchanged or slightly decreased in amplitude.

Transmembrane action potentials and intracellular potassium activity of baboon cardiac tissues

Preparations of baboon cardiac tissue were studied using standard microelectrode and double barrelled potassium ion selective microelectrode techniques. Transmembrane action potentials were recorded from Purkinje fibres and from atrial and ventricular muscle cells. Purkinje fibres were impaled frequently on the ventricular septum but were sparse on the endocardium of the free wall. Purkinje fibres had longer action potential durations than ventricular muscle cells, and endocardial muscle cells had longer action potential durations than epicardial muscle cells. Tissue bundles resembling the false tendons of the canine heart were found in the ventricles. Most of these false tendons consisted of ventricular muscle as indicated by transmembrane action potentials. On rare occasions, false tendon preparations were studied in which the cells had action potentials similar to those of canine Purkinje fibres. In these preparations, normal automaticity occurred from maximum diastolic potentials greater than or equal to -85 mV. Intracellular potassium activity in the baboon ventricular muscle cells was 96(3) mmol.litre-1 (mean(SEM), n = 32), which is lower than that reported for canine tissues but higher than that reported for feline or rabbit tissues. It is concluded that, although the cellular electrophysiology of primate and canine cardiac tissues is superficially similar, consistent and perhaps important differences do occur.

Mechanics of Jet Propulsion in the Hydromedusan Jellyfish, Polyorchis Penicillatus:II. Energetics of the Jet Cycle

ABSTRACT The mechanical energy generated by the contraction of the subumbrellar swimming muscles to power the jet cycle in the hydromedusan jellyfish Polyorchis penicillatus (Eschscholtz, 1829) was measured. This energy was experimentally partitioned into three components during the contraction. The sum of these components was taken to be the mechanical energy generated by the muscles during the jet cycle and was between 8·9×10−5 and 1·4×10−4J per contraction. Energy from one of these components is stored as strain energy in the mesoglea and powers the refilling phase. The mesoglea can clearly act as an effective elastic structure to antagonize the contraction of the swimming muscles completely, and it may be designed to function at some optimum. The mechanical significance of elastic energy storage systems in jet-propelled animals is discussed, and this significance is clearly displayed in Polyorchis. The unusually long-duration action potential of the swimming muscles may be an important component of the swimming mechanism, allowing the muscles to store energy in an elastic structure at the end of the contraction phase when little hydrodynamic thrust is developed. It is suggested that the action potential of vertebrate cardiac muscle may have a similar mechanical function.

Intracellular study of rat substantia nigra pars reticulata neurons in an in vitro slice preparation: electrical membrane properties and response characteristics to subthalamic stimulation

The electrical membrane properties of substantia nigra pars reticulata (SNR) neurons and their postsynaptic responses to stimulation of the subthalamic nucleus (STH) were studied in an in vitro slice preparation. SNR neurons were divided into two types based on their electrical membrane properties. Type-I neurons possessed (1) spontaneous repetitive firings, (2) short-duration action potentials, (3) less prominent spike accommodations, and (4) a strong delayed rectification during membrane depolarization. Type-II neurons had (1) no spontaneous firings, (2) long-duration action potentials, (3) a prominent spike accommodation, (4) a relatively large post-active hyperpolarization, and (5) a less prominent delayed rectification. These membrane properties were very similar to those observed in substantia nigra pars compacta (SNC) neurons in slice preparations. Features commoon to both types of neurons include that (1) the input resistance was similar, (2) they showed an anomalous rectification during strong hyperpolarizations, and (3) they were capable of generating Ca potentials. Intracellular responses of both types of SNR neurons to STH stimulation consisted of initial short-duration monosynaptic excitatory postsynaptic potentials (EPSPs) and a short-duration inhibitory postsynaptic potential (IPSP) followed by a long-duration depolarization. The IPSP was markedly suppressed by application of bicuculline methiodide and the polarity was reversed by intracellular injection of Cl −. In the preparations obtained from internal capsule-transected rats. STH-induced EPSPs had much longer durations than those observed in the normal preparations, while the amplitude of IPSPs and succeeding small-amplitude long-duration depolarizations was small. The results indicated that SNR contains two electrophysiologically different types of neurons, and that both types of neurons receive monosynaptic EPSPs from STh and IPSPs from areas rostral to STH.

Anatomy and electrophysiology of neurons terminating in the corpora allata of the cockroach Diploptera punctata.

Intracellular recording and dye injection were used to study the structure and electrophysiological properties of individual neurons that project to the corpora allata of the cockroach, Diploptera punctata. Neurons in the pars intercerebralis generate long-duration, tetrodotoxin-sensitive action potentials. Dye injection revealed two cell types. One type extends axons to the contralateral nervi corporis cardiaci I, some of which innervate the corpora allata, and another type extends a major axon down each of the circumoesophageal connectives. Neurons in the pars lateralis also generate long-duration action potentials. These neurons extend axons to the ipsilateral nervi corporis cardiaci II, which continue on to terminate in the corpora cardiaca and the corpora allata. Small groups of all the above neuronal types are dye and electrically coupled. Penetration and dye injection into nerve terminals in the corpora allata and corpora cardiaca confirmed the innervation of the corpora allata by neurons located in the pars intercerebralis and pars lateralis and revealed a third class of neurons that have terminals in the corpora allata: intrinsic neurons of the corpora cardiaca.

Recording of mouse ventral tegmental area dopamine-containing neurons

We examined the electrophysiologic and pharmacologic properties of dopamine-containing ventral tegmental area neurons in the mouse using extracellular singleunit recording techniques in both chloral hydrate-anesthetized mice and in vitro mouse midbrain slices. In vivo the ventral tegmental area neurons had long-duration action potentials (2 to 5 ms) and discharged at 1 to 9 spikes/s with either a decremental burst pattern or a regular pattern. Systemic administration of the dopamine agonist, apomorphine, decreased their firing rate, and the dopamine receptor blocker, haloperidol, reversed this effect. Similarly, systemic administration of the dopamine-releasing agent, d-amphetamine, suppressed their discharge rate, an effect blocked by pretreatment of the animals with α-methyl- p-tyrosine. When recorded in vitro from midbrain slices, ventral tegmental area neurons showed electrophysiologic properties similar to those found in vivo; however, the neurons recorded in vitro fired at a significantly faster rate and their firing pattern tended to be more pacemaker-like, especially when recordings were made in an incubation medium that blocked synaptic transmission (i.e., low calcium/high magnesium). The activity of most of these neurons was suppressed by addition of apomorphine to the incubation medium, an effect reversed by haloperidol. Pretreatment with α-methyl- p-tyrosine produced no significant change in the discharge pattern or rate for cells recorded in vitro. These data indicate that mouse ventral tegmental area dopamine neurons in vivo exhibit the same electrophysiologic and pharmacologic properties as do rat and cat dopamine-containing neurons and that in vitro they fire with pacemaker regularity in a low-calcium/high-magnesium medium. The in vitro preparation offers an approach to examining the fundamental properties of ventral tegmental area dopamine-containing neurons in the absence of afferent inputs.

Monophasic action potential mapping in human subjects with normal electrocardiograms: direct evidence for the genesis of the T wave.

T wave concordance in the normal human electrocardiogram (ECG) generally is explained by assuming opposite directions of ventricular depolarization and repolarization; however, direct experimental evidence for this hypothesis is lacking. We used a contact electrode catheter to record monophasic action potentials (MAPs) from 54 left ventricular endocardial sites during cardiac catheterization (seven patients) and a new contact electrode probe to record MAPs from 23 epicardial sites during cardiac surgery (three patients). All patients had normal left ventricular function and ECGs with concordant T waves. MAP recordings during constant sinus rhythm or right atrial pacing were analyzed for activation time (AT) = earliest QRS deflection to MAP upstroke, action potential duration (APD) = MAP upstroke to 90% repolarization, and repolarization time (RT) = AT plus APD. AT and APD varied by 32 and 64 msec, respectively, over the left ventricular endocardium and by 55 and 73 msec, respectively, over the left ventricular epicardium. On a regional basis, the diaphragmatic and apicoseptal endocardium had the shortest AT and the longest APD, and the anteroapical and posterolateral endocardium had the longest AT and the shortest APD (p less than .05 to less than .0001). RT was less heterogeneous than APD, and no significant transventricular gradients of RT were found. In percent of the simultaneously recorded QT interval, epicardial RT ranged from 70.8 to 87.4 (mean 80.7 +/- 3.9) and endocardial RT ranged from 80 to 97.8 (mean 87.1 +/- 4.4) (p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Passive and active membrane properties of mudpuppy taste receptor cells.

1. Intracellular recordings were obtained from taste receptor cells and surface epithelial cells of isolated mudpuppy lingual epithelium. 2. Surface epithelial cells had a mean resting potential of -40.2 +/- 8.9 mV, a mean input resistance of 40.3 +/- 11.3 M omega, and a linear current-voltage (I-V) relationship. Taste receptor cells had a mean resting potential of -61.7 +/- 15 mV, a mean input resistance of 380.3 +/- 177.2 M omega, and the I-V relationship showed pronounced outward rectification; the outward rectification persisted in high-K+ saline, but was abolished by tetraethylammonium bromide (TEA). 3. Surface epithelial cells responded to depolarizing current injection with only passive membrane potential changes. Taste receptor cells responded to brief pulses of depolarizing current injection with regenerative action potentials characterized by an abrupt rising phase, an inflexion on the falling phase, and a prolonged after-potential. 4. The abrupt rising phase of the action potential was blocked by tetrodotoxin (TTX), suggesting that voltage-gated Na+ currents are responsible for the rising phase. 5. Long-duration action potentials were elicited from cells treated with TEA to block outward K+ currents and with TTX to block Na+ currents, and from cells bathed in isotonic CaCl2. These results suggest that the active membrane response contains a significant Ca2+ component. 6. The after-potential was blocked or greatly reduced by the addition of Ca2+ channel blockers to the bathing medium. In contrast, addition of TEA to the bathing medium greatly enhanced the after-potential. These data suggest that a significant portion of the after-potential is Ca2+ mediated. 7. The mean reversal potential for the after-potential (-76.8 +/- 6.0 mV) was significantly different from the mean reversal potential for the undershoot of the action potential (-86 +/- 5.6 mV). Superfusion with TEA reduced the reversal potential of the after-potential to -42.3 +/- 8.2 mV and abolished the undershoot. These results suggest that the after-potential results from at least two conductances, one which is blocked by TEA and the other which is Ca2+ dependent and involves ions other than, or in addition to K+. 8. Our data suggest that taste receptor cells, unlike surface epithelial cells, possess voltage-gated Na+, Ca2+, and K+ channels, as well as Ca2+-mediated channels. The role of the Ca2+ channels may be in part to regulate release of transmitter onto nerve terminals. The role of the other conductances in taste transduction is unknown.

Electrophysiological properties of cortically projecting histamine neurons of the rat hypothalamus

The tuberomammillary (TM) nuclei of the hypothalamus appear to be the sole histaminergic cell group in the brain. The extracellular electrophysiological properties of cortically projecting TM neurons were studied in the urethane-anesthetized rat. TM neurons, antidromically activated from either ipsi- or contralateral cerebral cortex, displayed relatively slow conduction velocities, consistent with reports suggesting that TM neurons possess unmyelinated axons. Spontaneous activity was slow and regular, with action potentials of long duration. There was often a noticeable delay between initial segment and somatodendritic portions of spontaneous action potentials, and complete loss of the somatodendritic portion of the second antidromic action potential was commonly seen when double pulse paradigms were employed. These data demonstrate that in addition to anatomical and biochemical similarities, TM neurons share a constellation of physiological properties with other central aminergic neurons.