Post-tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

Sodium pump regulation of locomotor control circuits

Sodium pumps are ubiquitously expressed membrane proteins that extrude three Na⁺ ions in exchange for two K⁺ ions, using ATP as an energy source. Recent studies have illuminated additional, dynamic roles for sodium pumps in regulating the excitability of neuronal networks in an activity-dependent fashion. We review their role in a novel form of short-term memory within rhythmic locomotor networks. The data we review derives mainly from recent studies on Xenopus tadpoles and neonatal mice. The role and underlying mechanisms of pump action broadly match previously published data from an invertebrate, the Drosophila larva. We therefore propose a highly conserved mechanism by which sodium pump activity increases following a bout of locomotion. This results in an ultraslow afterhyperpolarization (usAHP) of the membrane potential that lasts around 1 min, but which only occurs in around half the network neurons. This usAHP in turn alters network excitability so that network output is reduced in a locomotor interval-dependent manner. The pumps therefore confer on spinal locomotor networks a temporary memory trace of recent network performance.

Na(+)/K(+) pump interacts with the h-current to control bursting activity in central pattern generator neurons of leeches

The dynamics of different ionic currents shape the bursting activity of neurons and networks that control motor output. Despite being ubiquitous in all animal cells, the contribution of the Na(+)/K(+) pump current to such bursting activity has not been well studied. We used monensin, a Na(+)/H(+) antiporter, to examine the role of the pump on the bursting activity of oscillator heart interneurons in leeches. When we stimulated the pump with monensin, the period of these neurons decreased significantly, an effect that was prevented or reversed when the h-current was blocked by Cs(+). The decreased period could also occur if the pump was inhibited with strophanthidin or K(+)-free saline. Our monensin results were reproduced in model, which explains the pump's contributions to bursting activity based on Na(+) dynamics. Our results indicate that a dynamically oscillating pump current that interacts with the h-current can regulate the bursting activity of neurons and networks.

Autocrine/paracrine modulation of baroreceptor activity after antidromic stimulation of aortic depressor nerve in vivo

Activation of the sensory nerve endings of non-myelinated C-fiber afferents evokes release of autocrine/paracrine factors that cause localized vasodilation, neurogenic inflammation, and modulation of sensory nerve activity. The aims of this study were to determine the effect of antidromic electrical stimulation on afferent baroreceptor activity in vivo, and investigate the role of endogenous prostanoids and hydrogen peroxide (H2O2) in mediating changes in nerve activity. Baroreceptor activity was recorded from the left aortic depressor nerve (ADN) in anesthetized rats before and after stimulating the ADN for brief (5–20 s) periods. The rostral end of the ADN was crushed or sectioned beforehand to prevent reflex changes in blood pressure. Antidromic stimulation of ADN using parameters that activate both myelinated A-fibers and non-myelinated C-fibers caused pronounced and long-lasting (> 1 min) inhibition of baroreceptor activity (n = 9, P < 0.05), with the magnitude and duration of inhibition dependent on the duration of the stimulation period (n = 5). Baroreceptor activity was only transiently inhibited after selective stimulation of A-fibers. The inhibition of activity after antidromic stimulation of A and C fibers was prolonged after administration of the cyclooxygenase inhibitor indomethacin (5 mg/kg, IV, n = 7) and abolished after administration of PEG-catalase (104 units/kg, IV, n = 7), an enzyme that catalyzes the decomposition of H2O2 to water and oxygen. The results demonstrate a long-lasting inhibition of baroreceptor activity after antidromic stimulation of ADN and suggest that endogenous prostanoids and H2O2 oppose and mediate the inhibition, respectively. These mechanisms may contribute to rapid baroreceptor resetting during acute hypertension and be engaged during chronic baroreceptor activation therapy in patients with hypertension.

Remodeling of hyperpolarization-activated current, Ih, in Ah-type visceral ganglion neurons following ovariectomy in adult rats

Hyperpolarization-activated currents (I_h) mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels modulate excitability of myelinated A− and Ah-type visceral ganglion neurons (VGN). Whether alterations in I_h underlie the previously reported reduction of excitability of myelinated Ah-type VGNs following ovariectomy (OVX) has remained unclear. Here we used the intact nodose ganglion preparation in conjunction with electrophysiological approaches to examine the role of I_h remodeling in altering Ah-type neuron excitability following ovariectomy in adult rats. Ah-type neurons were identified based on their afferent conduction velocity. Ah-type neurons in nodose ganglia from non-OVX rats exhibited a voltage ‘sag’ as well as ‘rebound’ action potentials immediately following hyperpolarizing current injections, which both were suppressed by the I_h blocker ZD7288. Repetitive spike activity induced afterhyperpolarizations lasting several hundreds of milliseconds (termed post-excitatory membrane hyperpolarizations, PEMHs), which were significantly reduced by ZD7288, suggesting that they resulted from transient deactivation of I_h during the preceding spike trains. Ovariectomy reduced whole-cell I_h density, caused a hyperpolarizing shift of the voltage-dependence of I_h activation, and slowed I_h activation. OVX-induced I_h remodeling was accompanied by a flattening of the stimulus frequency/response curve and loss of PEMHs. Also, HCN1 mRNA levels were reduced by ∼30% in nodose ganglia from OVX rats compared with their non-OVX counterparts. Acute exposure of nodose ganglia to 17beta-estradiol partly restored I_h density and accelerated I_h activation in Ah-type cells. In conclusion, I_h plays a significant role in modulating the excitability of myelinated Ah-type VGNs in adult female rats.

Muscle receptor organs in the crayfish abdomen: a student laboratory exercise in proprioception

The primary purpose of this experiment is to demonstrate primary sensory neurons conveying information of joint movements and positions as proprioceptive information for an animal. An additional objective of this experiment is to learn anatomy of the preparation by staining, dissection and viewing of neurons and sensory structures under a dissecting microscope. This is performed by using basic neurophysiological equipment to record the electrical activity from a joint receptor organ and staining techniques. The muscle receptor organ (MRO) system in the crayfish is analogous to the intrafusal muscle spindle in mammals, which aids in serving as a comparative model that is more readily accessible for electrophysiological recordings. In addition, these are identifiable sensory neurons among preparations. The preparation is viable in a minimal saline for hours which is amenable for student laboratory exercises. The MRO is also susceptible to neuromodulation which encourages intriguing questions in the sites of modulatory action and integration of dynamic signals of movements and static position along with a gain that can be changed in the system.

Functional Organization of the Cardiac Ganglion of the Lobster, Homarus americanus

External recording and stimulation, techniques were used to determine which neurons and interactions are essential for production of the periodic burst discharge in the lobster cardiac ganglion. Burst activity can be modulated by brief single shocks applied to the four small cells, but not by similar stimulation of the five large cells, suggesting that normally one or more small cells primarily determine burst rate and duration. Repetitive electrical stimulation of large cells initiates spike activity in small cells, probably via excitatory synaptic and/or electrotonic connections which may normally act to prolong bursts and decrease burst rate. Transection of the ganglion can result in burst activity in small cells in the partial or complete absence of large cell spike activity, but large cells isolated from small cell excitatory synaptic input by transection or by application of dinitrophenol do not burst. Generally, transections which decrease excitatory feedback to small cells are accompanied by an increase in burst rate, but mean spike frequency over an entire burst cycle stabilizes at the original level within 10-30 min for various groups of cells whose spike-initiating sites are still intact. These and previous results suggest that the system is two layered: one or more small cells generate the burst pattern and impose it on the large cells which are the system's motorneurons.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

The effects of serotonin and ecdysone on primary sensory neurons in crayfish

The overall behaviors and motivational states observed during social interactions and throughout the molting cycle of crayfish have been linked to the effects of humoral neuromodulators. Both serotonin (5-HT) and a molt-related hormone, 20-hydroxyecdysone (20-HE), are known to be present in the hemolymph of crustaceans. To determine if they alter the activity of a primary sensory neuron that monitors proprioceptive information, we examined their effects on the activity of the slow-adapting muscle receptor organ (MRO) of the crayfish abdomen, a model sensory system that has been extensively studied. 5-HT within the range of 100 nM to 1 microM, increases the firing frequency of the neuron during sustained stimulation. In experiments in which 20-HE was added alone, an increase in the firing frequency also occurred, although to a lesser degree than that for 5-HT at the same concentrations. When the MRO is first exposed to 20-HE, followed sequentially by 5-HT, the activity increases to about the same degree as in the reverse order of exposure. This outcome indicates that mixtures of these endogenous neuromodulators, at various levels, are more important in alternating behavior than the absolute level of any one of them introduced alone.

Effects of chlorobutanol on primary and secondary endings of isolated cat muscle spindles

The effects of the preservative chlorobutanol on primary and secondary endings of muscle spindles isolated from the tenuissimus muscle of the cat were investigated in this study. Chlorobutanol was applied to the bathing solution in final concentrations of between 10 and 100 microg/ml. It induced a reversible and dose dependent decrease in the discharge frequency of both types of ending without any visible length change in the sensory region of the receptor. The initial activity, the peak dynamic discharge, the maximum static discharge value and the final static discharge value were evaluated from an ending's discharge pattern obtained during ramp-and-hold stretches. These four basic discharge frequencies decreased in parallel with increasing concentrations of chlorobutanol. Their sensitivities to chlorobutanol were similar (mean values: -0.11 to -0.29 imp/s per microg/ml chlorobutanol) and were independent of the amplitude of stretch. The dynamic response and the static response of both primary and secondary endings remained unchanged, indicating that the sensitivity of the spindle to stretch was not influenced by chlorobutanol. Chlorobutanol also reduced the discharge activity of the muscle spindle afferents during sinusoidal stretches. The amplitude of the receptor potential (AC component) remained unchanged under chlorobutanol. With the available recording technique it was not possible to measure slow shifts of the membrane potential. However, a hyperpolarization of the ending's membrane might explain why the afferent discharge frequency is reduced by chlorobutanol. The calcium dynamics of the spindle do not appear to be altered by CB, as the effect exerted on the afferent discharge by a change in the extracellular calcium concentration and a blockage of calcium channels was different from the CB effect. As the inhibitory effect of CB was reduced by ouabain, it is possible that CB activates the electrogenic Na/K pump or affects a mechanism that is closely related to the activity of the pump. The properties of the axonal membrane appear not to be altered, as chlorobutanol did not change the shape of action potentials.

Mechanisms of afterhyperpolarization in lobster olfactory receptor neurons

In lobster olfactory receptor neurons (ORNs), depolarizing responses to odorants and current injection are accompanied by the development of an afterhyperpolarization (AHP) that likely contributes to spike-frequency adaptation and that persists for several seconds after termination of the response. A portion of the AHP can be blocked by extracellular application of 5 mM CsCl. At this concentration, CsCl specifically blocks the hyperpolarization-activated cation current (Ih) in lobster ORNs. This current is likely to be active at rest, where it provides a constant, depolarizing influence. Further depolarization deactivates Ih, thus allowing the cell to be briefly hyperpolarized when that depolarizing influence is removed, thus generating an AHP. Reactivation of Ih would terminate the AHP. The component of the AHP that could not be blocked by Cs+ (the Cs(+)-insensitive AHP) was accompanied by decreased input resistance, suggesting that this component is generated by increased conductance to an ion with an equilibrium potential more negative than the resting potential. The Cs(+)-insensitive AHP in current clamp and the underlying current in voltage clamp displayed a reversal potential of approximately -75 mV. Both EK and ECl are predicted to be in this range. Similar results were obtained with the use of a high Cl- pipette solution, although that shifted ECl from -72 mV to -13 mV. However, when EK was shifted to more positive or negative values, the reversal potential also shifted accordingly. A role for the Ca(2+)-mediated K+ current in generating the Cs(+)-independent AHP was explored by testing cells in current and voltage clamp while blocking IK(Ca) with Cs+/Co(2+)-saline. In some cells, the Cs(+)-independent AHP and its underlying current could be completely and reversibly blocked by Cs+/Co2+ saline, whereas in other cells some fraction of it remained. This indicates that the Cs(+)-independent AHP results from two K+ currents, one that requires an influx of extracellular Ca2+ and one that does not. Collectively, these findings indicate that AHPs result from three phenomena that occur when lobster ORNs are depolarized: 1) inactivation of the hyperpolarization-activated cation current, 2) activation of a Ca(2+)-mediated K+ current, and 3) activation of a K+ current that does not require influx of extracellular Ca2+. Roles of these processes in modulating the output of lobster ORNs are discussed.

Characterization of electrogenic Na/K pump in rat neostriatal neurons

The electrogenic Na/K pump current (Ip) was studied in the dissociated neostriatal neurons of the rat by using the nystatin-perforated patch recording mode. The Ip was activated by external K+ in a concentration-dependent manner with an EC50 of 0.7 mM at a holding potential (VH) of -40 mV. Other monovalent cations also caused Ip and the order of potency was Tl+>K+, Rb+>NH4+, Cs+>>>Li+. The Ip decreased with membrane hyperpolarization in an external solution containing 150 mM Na+, while the Ip did not show such voltage dependency without external Na+. Ouabain showed a steady-state inhibition of Ip in a concentration- and temperature-dependent manner at a VH of -40 mV. The IC50 values at 20 and 30 degrees C were 7.1 x 10(-6) and 1.3 x 10(-6) M, respectively. The decay of Ip after adding ouabain well fitted with a single exponential function. At a VH of -40 Mv, the association (k+1) and dissociation (k-1) rate constants estimated from the time constant of the current decay at 20 degrees C were 4.0 x10(2) s-1 M-1 and 6.3 x 10(-3) s-1, respectively. At 30 degrees C, k+1 increased to 2.8 x 10(3) s-1 M-1 while k-1 showed no such change with a value of 1.8 x 10(-3) s-1. A continuous Na+ influx was demonstrated by both the Na+-dependent leakage current and tetrodotoxin-sensitive Na+ current, which resulted in the continuous activation of the Na/K pump. It was thus concluded that the Na/K pump activity was well-maintained in the dissociated rat neostriatal neurons with distinct functional properties and that the activity of the pump was tightly connected with Na+ influxes.

Periodically modulated inhibition and its postsynaptic consequences--I. General features. Influence of modulation frequency

Our aim was to examine the relation, or "synaptic coding", between spike trains across a synapse with inhibitory postsynaptic potentials when the presynaptic rate is modulated periodically and the postsynaptic cell is a pacemaker. Experiments were on the synapse in crayfish stretch receptor organs. Spike trains were considered point processes along time; the time series of corresponding pre- and postsynaptic intervals were extracted. Analyses used displays of intervals along time and order ("basic graphs", and "rasters", respectively), displays of differences between intervals along order ("recurrence plots"), cycle histograms (as such and as Lissajous diagrams with presynaptic and postsynaptic on the abscissae and ordinate, respectively), and correlation histograms. Cycle histograms and correlation histograms demonstrated that all presynaptic modulation frequencies (1/60-10 Hz) are reflected postsynaptically; novel frequencies may arise, not always relating simply to the pre- or postsynaptic ones. The transferred frequency domain is broad and physiologically meaningful. Indeed, vitally important functions have strong periodicities in all portions of the explored domain, and so do the discharges of participating neurons. Overall, pre- and postsynaptic discharges change oppositely, one accelerating while the other slows. Locally, however, pre- and postsynaptic discharges contrast clearly in other ways. The presynaptic evolution is everywhere smooth and orderly, half-cycles usually are symmetric, and there is a single kind of discharge, as expected because the presynaptic axon follows well the controlling stimuli. The postsynaptic cycle shows marked local distortions. These involve presynaptic domains called "congruent portions" where changes are in the same sense (e.g., joint accelerations), "saturated" domains where postsynaptic discharges are arrested, and asymmetric sensitivities to presynaptic change with hysteretic loops in the Lissajous diagrams; the postsynaptic discharge is heterogeneous showing dissimilar forms in succession. Congruent portions are either "positive segments" with pre- to postsynaptic rate ratios practically 1:1, 2:1, 1:1, or parts of Lissajous loops. Different modulation frequencies have different postsynaptic consequences. Differences involve the width and number of positive segments, the proportion of the cycle with saturation, the sense, magnitude and lead-lag characteristics of the hysteretic loops, etc. Because their consequences are separable, frequencies are classified within categories labelled "low" (under 0.5 Hz), "high" (between 0.5 and 5.0 Hz) and "very high" (over 5.0 Hz). Categories arise widely but each prevails in different biological functions (e.g., low or high in, respectively, respiration or vibratory sensitivity). The refactoriness of the inhibitory fibre affects how it can be modulated: consequently, the very high category resembles pacemaker discharges and was not analysed.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural versus functional modulation of the arterial baroreflex

Structural changes in large arteries are often considered the predominant mechanism responsible for decreased baroreflex sensitivity and baroreceptor resetting in hypertension, atherosclerosis, and aging. Recent work has demonstrated that "functional" mechanisms, both at the level of the peripheral sensory endings and within the central nervous system, contribute significantly to altered baroreflex responses. We have conducted both reductive studies of mechanoelectrical transduction in cultured baroreceptor neurons and integrative studies with in vivo recordings of the activity of baroreceptor afferent fibers and efferent sympathetic nerves. Results suggest that the primary mechanism of mechanical activation of baroreceptor neurons involves opening of stretch-activated ion channels susceptible to blockade by gadolinium. Baroreceptor nerve activity is modulated by the activity of potassium channels and the sodium-potassium pump and by paracrine factors, including prostacyclin, oxygen free radicals, and factors released from aggregating platelets. Endothelial dysfunction and altered release of these paracrine factors contribute significantly to the decreased baroreceptor sensitivity in hypertension and atherosclerosis. The central mediation of the baroreflex depends on the pulse phasic pattern of afferent baroreceptor discharge. Baroreflex-mediated inhibition of sympathetic nerve activity is well maintained during pulse phasic afferent activity. Continuous, nonphasic baroreceptor discharge or a rapid (> 1.5 Hz) pulse phasic discharge results in disinhibition of sympathetic activity. This disinhibition during continuous baroreceptor input is exaggerated with aging. Thus, a defect in central mediation of the baroreflex may be a major cause of the impaired baroreflex and sympathoexcitation in the elderly. In summary, functional neural mechanisms, in addition to structural vascular changes, contribute importantly to altered baroreflex responses in normal and pathophysiological states.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature dependence of EEG frequencies during natural hypothermia

We have investigated the effects of changes in brain temperature on the electroencephalogram (EEG) during entrance into daily torpor, a natural hypothermic state, in the Djungarian hamster. A systematic shift of single EEG frequencies was found as cortical temperature decreased. The relation between EEG frequency and cortical temperature was very similar to the temperature dependence of the Na(+)-K(+)-pump, suggesting that the pump is the rate-limiting step in determining EEG frequency.

Mechanism of baroreceptor adaptation in dogs: attenuation of adaptation by the K+ channel blocker 4-aminopyridine

1. Increased arterial pressure increases baroreceptor activity but activity declines (i.e. baroreceptors adapt) as the pressure is maintained at the higher level. The purpose of this study was to investigate the role of a 4-aminopyridine (4-AP)-sensitive K+ current in causing baroreceptor adaptation. 2. Multi- and single fibre recordings of baroreceptor activity were obtained from the vascularly isolated carotid sinus in anaesthetized dogs during step increases in carotid sinus pressure sustained for periods up to 5 min. 3. Baroreceptor activity increased with the rise in pressure, declined markedly over the first minute, and continued to decline at a slower rate during the remainder of the 5 min period of elevated pressure. Exposure of the isolated carotid sinus to 4-AP (10(-5) and 10(-4) M) attenuated adaptation in a dose-dependent and reversible manner (P < 0.05). 4-AP attenuated the gradual decline in single fibre activity and also prevented derecruitment or dropout of fibres that occurred over time. 4-AP did not alter peak nerve activity measured within the first 2 s of the pressure step. 4. Ouabain (5 x 10(-7)-10(-6) M), an inhibitor of Na+,K(+)-ATPase, increased baroreceptor activity but did not attenuate baroreceptor adaptation. 5. Neither 4-AP nor ouabain altered the distensibility of the carotid sinus as measured with sonomicrometer crystals suggesting that the agents act directly on the nerve endings. 6. The results suggest that activation of a 4-AP-sensitive K+ current contributes significantly to baroreceptor adaptation with little or no contribution of Na+,K(+)-ATPase.

Lithium block of outward current channels in Helix neurons

Outward currents were studied in isolated, perfused ganglion cells from Helix aspersa. Treatment with external solutions containing 40 mM LiCl slowed and reduced the delayed outward currents after about 15 min. The leak currents were reduced by external application of Li. Electrophoretic injection of LiCl increased the leak currents and reduced the net outward currents. From fits of a model with a rapid second-order current component and a slower first-order component, the effect of Li was to reduce the asymptotic current for the first component and double the activation time constant for the second component. This was equivalent to increasing the probability of entering the blocked state in a kinetic model of the K-channels. The slowness of the block suggested that Li might act through a chemical intermediate.

A novel action of deltamethrin on membrane resistance in mammalian skeletal muscle and non-myelinated nerve fibres

The pyrethroids, deltamethrin and cismethrin, were assessed for their ability to change membrane conductance directly in skeletal muscle and indirectly in non-myelinated nerve fibre preparations from the rat. In diaphragm muscle fibres of the rat, input resistance was significantly increased (35%) by deltamethrin but not by cismethrin, compared with solvent alone. In perfused vagus nerve from the rat, the amplitude of the post-tetanic hyperpolarization was significantly increased (100%) by deltamethrin both in vitro and ex vivo but not by cismethrin or solvent. In both test systems the actions of deltamethrin were abolished by changing the perfusate to a low chloride solution. The enhancement of amplitude of post-tetanic hyperpolarization by deltamethrin was reversed by ivermectin, a compound known to increase the resting chloride flux in neuronal preparations. Depolarizing afterpotentials, indicative of a prolongation in sodium conductance, did not develop until 0.5-1.0 hr after the enhancement of the amplitude of post-tetanic hyperpolarization by deltamethrin in the vagus preparations. The amplitude of post-tetanic hyperpolarization was not enhanced by exposure of the vagus to veratrine. These observations reinforce the conclusion that the enhancement of post-tetanic hyperpolarization by deltamethrin is not the result of intracellular accumulation of sodium. In addition, the chloride-dependent nature of the effects of deltamethrin, in both muscle and non-myelinated nerve, suggests that they are in both cases due to a reduction in resting membrane chloride conductance. This novel action of deltamethrin would be expected to amplify the effect of prolonged sodium current and thus influence the actions on excitable membranes both directly and indirectly.

Peripheral and central mechanisms of baroreflex resetting

1. A change in the arterial pressure-sympathetic activity or heart rate relation (baroreflex resetting) can result from resetting of baroreceptors ('peripheral' resetting) or from an altered coupling within the central nervous system of afferent baroreceptor to efferent nerve activities ('central' resetting). 2. 'Peripheral' resetting involves a shift in the pressure-baroreceptor activity curve in the direction of the prevailing level of arterial pressure, e.g. after elevations in pressure, the baroreceptor pressure threshold (Pth) is increased and activity reduced at equivalent pressures and vascular strains. 3. 'Peripheral' resetting occurs during the diastolic phase of a cardiac cycle (instantaneous resetting), after brief exposure to elevated pressure (acute resetting), and during chronic hypertension or when chronic structural changes in the vasculature have occurred (chronic resetting). 4. Mechanisms include: (i) changes in the mechanical properties of the vessel wall that may alter the tension on the receptors; (ii) ionic mechanisms operating at the neuronal membrane such as activation of Na+,K+-ATPase; and (iii) release of endothelial factors that may modulate baroreceptor sensitivity. 5. Acute resetting of baroreceptors can be prevented or attenuated when the sustained elevations in pressure are pulsatile rather than static. Increases in flow increase carotid sinus nerve activity at constant pressure and strain and decrease the Pth of baroreceptors. 6. 'Central' resetting can involve neural-humoral interactions or an altered responsiveness of central neurons mediating the baroreflex to changes in afferent baroreceptor activity. 7. During static pressure, the continuous baroreceptor discharge causes significant 'central' resetting, i.e. sympathetic activity escapes from baroreflex inhibition. In contrast, during pulsatile pressure, the pulse phasic baroreceptor discharge minimizes 'central' resetting causing sustained sympathetic inhibition.

Hyperpolarization and short-circuiting as mechanisms of seizure prevention following febrile convulsions

Though children with febrile convulsions only have seizures in the early stage of a febrile illness and not later, these seizures have been attributed to the fever. We studied the serum electrolyte and metabolite profiles in the later stage to see if there were fuel responses resulting in electrophysiological changes which prevented further seizure activity. On admission there was intracellular glucose starvation, as evidenced by increased ketones and lactate, and the possibility of the failure of some electrolyte pumps, as suggested by hyperuricaemia (energy crisis) and decreased serum Na+, Cl- and Ca2+. However, there was adaptive hyperglycemia and decreased serum K+. It seems likely that the hyperglycemia, induced the uptake of K+ by neurones, enabling their repolarization and hyperpolarization, which prevented further seizure activity, while Cl- influx short-circuited depolarizing currents produced by Na+ influx. Studies during recovery showed a gradual return of the metabolic and electrolyte aberrations to normality, suggesting that the provision of energy through adaptation to the stress, enabled recovery of the aforementioned pumps.

Mechanisms of resetting of arterial baroreceptors: an overview

Arterial baroreceptors are reset when their afferent nerve activity is reduced at an equivalent arterial pressure and vascular strain. Resetting occurs as a result of stretch of the baroreceptors, usually during an acute or chronic rise in arterial pressure. It may be seen during the diastolic phase of a cardiac cycle (instantaneous resetting), after brief exposure to a sustained elevation of pressure (acute resetting), and after chronic elevation of pressure or in physiologic or pathologic states associated with structural changes in the vascular regions of baroreceptors (chronic resetting). The mechanisms reviewed here include mechanical, ionic and chemical factors. Viscoelastic properties of the carotid sinus and aortic arch may explain the instantaneous resetting that occurs with each cardiac cycle when activity begins in early systole and stops in early diastole. Viscoelastic properties and ionic mechanisms may play a role in acute resetting. Inhibition of Na+K+ ATPase reduces the magnitude of acute resetting. The release of chemicals from the endothelium may modulate baroreceptor activity. Exogenous prostacyclin suppresses and indomethacin augments acute resetting in the rabbit, suggesting that the release of endogenous prostacyclin during a rise in arterial pressure attenuates resetting. Changes in pulsatility and blood flow also may modulate baroreceptor activity. The addition of pulsatile pressure at an increased mean pressure attenuates resetting.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast charge translocations associated with partial reactions of the Na,K-pump: I. Current and voltage transients after photochemical release of ATP

Nonstationary electric currents are described which are generated by the Na,K-pump. Flat membrane sheets 0.2-1 micron in diameter containing a high density of oriented Na,K-ATPase molecules are bound to a planar lipid bilayer acting as a capacitive electrode. In the aqueous phase adjacent to the bound membrane sheets, ATP is released within milliseconds from an inactive, photolabile precursor ("caged" ATP) by an intense flash of light. After the ATP-concentration jump, transient current and voltage signals can be recorded in the external circuit corresponding to a translocation of positive charge across the pump protein from the cytoplasmic to the extracellular side. These electrical signals which can be suppressed by inhibitors of the Na,K-ATPase require the presence of Na+ but not of K+ in the aqueous medium. The intrinsic pump current Ip(t) can be evaluated from the recorded current signal, using estimated values of the circuit parameters of the compound membrane system. Ip(t) exhibits a biphasic behavior with a fast rising period, followed by a slower decline towards a small quasi-stationary current. The time constant of the rising phase of Ip(t) is found to depend on the rate of photochemical ATP release. Further information on the microscopic origin of the current transient can be obtained by double-flash experiments and by chymotrypsin modification of the protein. These and other experiments indicate that the observed charge-translocation is associated with early events in the normal transport cycle. After activation by ATP, the pump goes through the first steps of the cycle and then enters a long-lived state from which return to the initial state is slow.

Prevention or attenuation of baroreceptor resetting by pulsatility during elevated pressure

Acute static elevation of arterial pressure increases the pressure threshold for activation of baroreceptors (acute resetting). The purpose of this study was to test the hypothesis that pulsatility during acute elevation of pressure modifies this acute resetting. Activity was recorded in 21 single baroreceptor units from the isolated carotid sinuses of dogs anesthetized with chloralose. Single-unit pressure thresholds were determined with a slow ramp increase in pressure. After a control period of static pressure at 25 to 50 mm Hg, the pressure threshold averaged 69 +/- 4 (SE) mm Hg. Three graded levels of static pressure were held for 5 to 15 minutes. The levels averaged 76 +/- 4, 115 +/- 6, and 170 +/- 5 mm Hg. The corresponding nerve activity during these periods was 0, 44 +/- 6, and 63 +/- 6 spikes per second, and the resulting increases in pressure threshold averaged 10 +/- 1, 17 +/- 2, and 26 +/- 3 mm Hg, respectively. In contrast, during equivalent elevations of pulsatile pressure, nerve activity averaged 20 +/- 3, 37 +/- 4, and 61 +/- 5 spikes per second, and the increases in pressure threshold averaged 0 +/- 4, 14 +/- 2, and 24 +/- 2 mm Hg, respectively. In some units, the pressure threshold decreased following elevation of pulsatile pressure. The results indicate that: pulsatility during elevation in pressure prevents or attenuates the acute baroreceptor resetting except at maximal pressure; upward resetting occurs with elevation of static pressure even when there is no nerve activity during the period of elevated pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation-induced changes in the intracellular sodium activity of the crayfish stretch receptor

Changes in intracellular Na+ activity (aiNa) caused by mechanical stimulation in the slowly adapting stretch receptor of the crayfish were examined using Na+-selective microelectrodes. A series of brief stretches (each giving rise to a brief receptor potential and a single action potential) induced a reversible increase in aiNa which was proportional to the stimulation frequency in the range examined, 0-9.5 Hz. At 9.5 Hz, aiNa increased by 4-5 mM from the resting value of 7-10 mM. Tetrodotoxin (TTX) reduced, but did not abolish the stimulation-dependent increase in aiNa indicating the involvement of a Na+-influx pathway in addition to the potential-dependent, TTX-sensitive sodium channels of the neuronal plasma membrane. A likely candidate for this TTX-resistant pathway are the cationic transducer channels of the dendrites.

Voltage dependence of the rheogenic Na+/K+ ATPase in the membrane of oocytes of Xenopus laevis

Electrophysiological experiments were performed to analyze the Na+/K+-ATPase in full-grown prophase-arrested oocytes of Xenopus laevis. If the Na+/K+-ATPase is inhibited by dihydroouabain (DHO), the resting potential of the membrane of Na+-loaded oocytes may depolarize by nearly 50 mV. This hyperpolarizing contribution to the resting potential depends on the degree of activation of the Na+/K+-ATPase and varies with intracellular Na+ activity (aiNa) and extracellular K+ (K+o). It is concluded that variations of aiNa among different oocytes are primarily responsible for the variations of resting potentials measured in oocytes of X. laevis. Under voltage-clamp conditions, the DHO-sensitive current also exhibits dependence on aiNa that may be described by a Hill equation with a coefficient of 2. This current will be shown to be identical with the electrogenic current generated by the 3Na+/2K+ pump. The voltage dependence of the pump current was investigated at saturating values of aiNa (33 mmol/liter) and of K+o (3 mmol/liter) in the range from -200 to +100 mV. The current was found to exhibit a characteristic maximum at about +20 mV. This is taken as evidence that in the physiological range at least two steps within the cycle of the pump are voltage dependent and are oppositely affected by the membrane potential.

Processing vibratory stimuli in isolated frog muscle spindle

Experiments were performed on isolated frog muscle spindle receptors to study the particular transducer and encoder mechanisms involved in the signal transfer of high frequency sinusoids (vibration). In order to systematically investigate the signal transfer over the entire dynamic range of the receptor, vibration stimuli were applied to the intrafusal muscle bundle at different prestretch levels, so that the isolated receptor potential or the afferent impulse train were recorded at different operating points. The vibration-induced receptor potential displayed severe distortion, because the depolarization during stretch rose steeply, whereas the repolarization transient during release of stretch declined more slowly. The positive peak velocity values of the depolarization transient increased with increasing stimulus frequency, although the ac-component of the receptor potential decreased. The negative peak velocity values of the repolarization transient remained constant throughout the frequency range. The amplitude of the receptor potential grew larger when vibration of constant amplitude was applied at increasing levels of prestretch, revealing another non-linearity of the transducer. These two types of non-linearity were influential in determining the afferent discharge pattern. Each fast depolarization transient facilitated the generation of a single action potential, which therefore could be firmly phase-locked to a small segment of the vibratory movement. Due to its short rise-time, the depolarization transient tended to prevent multiple firing during one stimulus cycle. The prolonged depolarizing afterpotential of the evoked action potential operated in the same direction. Increasing prestretch greatly enhanced the responsiveness of the spindle to vibration. Thus, under appropriate conditions, the afferent discharge was driven in 1:1 synchrony with the vibration. An analysis is given of the after-effects of repetitive activity at the receptor site. The progressive decline of the mean membrane voltage during long lasting stimulation and the "post-tetanic" hyperpolarization ("off-effect") on termination of the vibration suggest the action of an electrogenic pumping mechanism. As a consequence, the afferent impulse train possessed a complex structure segmented into several transient and steady states, which differed in impulse rate, phase response, and in the degree of phase-locking.

Sustained sensitivity modifications induced by brief length perturbations in the crayfish slowly adapting stretch receptor

These experiments in the slowly adapting stretch receptor of crayfish test the effects of brief length perturbations (i.e., pulses) when presented in isolation at different constant elongations or superimposed on trapezoidal stretches of different amplitudes. Within "in vivo" lengths, during static responses, perturbations reduced firing rates to below control values and, in extreme cases, could silence the receptor. This effect, or "down-step," was sustained, occurred above a threshold pulse amplitude and background stretch, and increased with both stimulus characteristics, but was not present during dynamic responses. Beyond "in vivo" lengths, and in a few cases within those limits but close to the extremes, the receptor was silent but perturbations could restore activity. Lengthening pulses were more effective than shortening ones in generating after-effects. Perturbations change, during indefinitively long periods, the receptor's length or static sensitivity acting as a negative feedback which tends to maintain the discharge rate within fixed values. Perturbations disclose marked nonlinearities, which suggest that the classical view of a proportional control in the reflex loop in which the receptor participates may not operate in natural conditions.

Intracellular pH regulation in the sensory neurone of the stretch receptor of the crayfish (Astacus fluviatilis)

The ionic mechanisms of intracellular pH (pHi) regulation were studied in the slowly adapting sensory cell of the crayfish stretch receptor by using pH-, Na+- and Cl(-)-sensitive liquid ion exchanger electrodes. Under control conditions a mean pHi of 7.23 +/- 0.12 (S.D.) at a mean membrane potential of 68.3 +/- 4.1 mV S.D. was found in sixteen cells. Thus pHi is about 1 pH unit more alkaline than predicted from passive distribution, implying the presence of an acid extrusion mechanism. In order to acidify the cytoplasm, the cell was either acid-loaded by NH4Cl or exposed to CO2 and CO2/HCO3- solutions. During CO2 exposures pHi was regulated only if calculated amounts of HCO3- were added to keep external pH (pHo) constant. The pHo per se was found to be an important determinant of pHi and its regulation. Substitution of external Na+ by choline inhibited pHi recovery almost completely. As soon as Na+ was readmitted H+ extrusion occurred immediately at a rate similar to that of the control. The internal Na+ activity (aiNa) ranged between 6 and 13 mM with a mean of approximately 9.1 +/- 2.5 mM (S.D.; n = 8). The effects of various solutions on aiNa and the temporal relationship between aiNa and pHi in NH4Cl acid-loaded cells were investigated. The amount of aiNa increased during cell internal acidification and recovered in parallel with pHi recovery in NH4Cl acid-loaded cells. Experiments with 10(-4) M-ouabain and K+-free conditions suggest that neither the Na+-K+ pump nor external K+ are directly involved in pHi regulation. The internal chloride activity (aiCl), which was lower than predicted from a passive distribution, fell during exposure to HCO3-/CO2. Regulation of pHi was inhibited if the cell was completely depleted of Cl- by prolonged exposures to Cl(-)-free solution (isethionate and/or gluconate substituted). The pHi-regulating system of the sensory cell requires Na+ and Cl- which probably operate in a combined mechanism such as Na+ -H+-Cl(-)-HCO3- or an equivalent.

Calcium-dependent after-potentials in visceral afferent neurones of the rabbit

Intracellular recordings were made from neurones in nodose ganglia excised from rabbits. In C neurones, 1-60 action potentials were followed by an after-hyperpolarization with a peak amplitude of 16 mV and a time constant of decay ranging from 3 to 10 s. In A neurones, the action potentials were followed only by a brief (up to 50 ms) after-hyperpolarization. The after-hyperpolarization was associated with an increase in the membrane conductance to potassium ions; it reversed polarity at the potassium equilibrium potential. The increase in conductance following the action potentials was blocked by removal of calcium ions, or addition of cobalt to the extracellular solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) abolished the after-hyperpolarization; intracellular injection of calcium mimicked the after-hyperpolarization. It is concluded that calcium entry during the action potential leads to a long-lasting increase in potassium conductance in visceral afferent C neurones.

Post-tetanic hyperpolarization evoked by depolarizing pulses in crayfish stretch receptor neurones in tetrodotoxin

A post-tetanic hyperpolarization (p.t.h.) that is quantitatively identical to that evoked by a train of action potentials in stretch receptor neurones of crayfish Procambarus clarki and Pacifastacus leniculus is evoked when the normal Na+ influx is blocked with tetrodotoxin (TTX) and a train of depolarizing pulses is used to simulate a train of action potentials. The p.t.h. evoked by depolarizing pulses in the presence of TTX is attributable to an electrogenic Na-K pump, because it (a) is abolished by strophanthidin, (b) is abolished by removal of external K+, (c) depends in magnitude on internal Na+ concentration, (d) is not associated with a change in membrane conductance and (e) does not exhibit a reversal potential. When each action potential in the stimulus train is followed by a hyperpolarizing pulse, generation of the p.t.h. is prevented even though the action potentials are unchanged. The time constant for build-up of the p.t.h. is longer than the time constant of decay. Increasing the magnitude of depolarizing pulses increases the magnitude of the p.t.h. response in the presence of TTX and also increases the time constant for its build-up. The suppression of the p.t.h. occurring in low external Na+ appears to represent a response to a change in internal Na+ concentration, characterized by a time constant much longer than the decay of the p.t.h. The activity of the pump appears to be regulated by two mechanisms: a Na+-sensitive mechanism with a time constant of the order of a minute and an apparently voltage-sensitive mechanism with a time constant of about 5 s. The hypothesis is proposed that changes in the transmembrane electric field influence the enzymatic systems of the pump and disrupt the steady-state distribution of conformation states. The decay of the p.t.h. represents a relaxation back to the resting distribution.

Aortic wall properties and baroreceptor behaviour at normal arterial pressure and in acute hypertensive resetting in dogs

In order to throw light on the mechanism of acute hypertensive baroreceptor resetting, we examined the relationship between aortic baroreceptor firing and aortic wall properties in anaesthetized dogs as pressure was varied in a number of ways. We recorded baroreceptor impulses from the left aortic nerve, and measured aortic pressure with a catheter-tip transducer and external aortic diameter with ultrasonic transit-time transducers. Narrow anticlockwise hysteresis loops were evident in the pressure-diameter relationship of the upper thoracic aorta, both during the rapid pulsatile pressure changes of the cardiac cycle and during the slow excursions of mean pressure imposed for construction of baroreceptor pressure--response curves. In contrast to the 'phase-lag' response of diameter to pressure, the baroreceptor response was 'phaselead' in character, decreasing when stress-induced creep occurred in the aortic wall. When the mean arterial pressure set-point was increased from 100 to 125 mmHg for 20 min, the hysteresis loops relating mean diameter to mean pressure in the range 60-200 mmHg were displaced along the diameter axis in the direction of wall creep. A reduction in the baroreceptor response to pressure (i.e. resetting) always accompanied this displacement. Administration of ouabain (25-35 micrograms/kg) had no consistent effect on baroreceptor resetting. It has been suggested that acute baroreceptor resetting is akin to adaptation. To investigate the possibility that the two processes are accompanied by similar changes in aortic wall properties, we converted the aorta into a closed sac and distended it with a square wave of pressure. Like resetting, adaptation of the baroreceptor response to maintained pressure was associated with a small degree of creep of the aortic wall. Our results are compatible with the hypothesis that acute hypertensive resetting of aortic baroreceptors is similar to adaptation, both phenomena being attributable to relaxation of viscoelastic coupling elements, leading to a reduction of strain at the receptor membrane. Whether viscoelastic processes alone can account for acute resetting, or whether changes in ionic balance are involved also, baroreceptor responsiveness is a function of the stress history of the wall, the pressure-response curve moving along the pressure axis in the direction of the prevailing set-point. Hence, in early hypertension physiological resetting of baroreceptors will precede pathological resetting, and may even promote an upward movement of set-point.

White noise analysis of pace-maker-response interactions and non-linearities in slowly adapting crayfish stretch receptor

Input-output relations were investigated in the slowly adapting stretch receptor organ of crayfish using a Gaussian white noise length input with a 0.03-12.5 Hz band width and the resulting action potential output. The noise input was presented to the de-efferented receptor in situ, at three mean elongations and at four different amplitudes. The three mean elongations were set within the normal range in vivo, two at the extremes close to the minimum and maximum physiological lengths and the other in the mid-range. With white noise inputs there is a finite probability that the system will be tested in all possible conditions within the chosen band width because white noise has the advantage that it contains, with a finite probability, all possible stimulus wave forms at random. The analysis indicated similarities between the effects of the input variables, namely white noise amplitude and mean elongation. With low input variables the activity was periodic. With larger inputs, impulse rates were higher and irregular. The average length trajectories leading to a spike (i.e. the average stimulus) were either biphasic with high inputs or multiphasic and periodic with lower input variables. The frequency of periodicity increased with mean elongation. Although for a given length and noise amplitude a variety of individual length trajectories preceded spikes, the final biphasic shortening-lengthening average stimulus sequence before a spike was similar in all cases irrespective of the input variables. The number of possible trajectories decreased with increments in the input variables. The standard deviation of length values for each average stimulus was computed and displayed as a function of time relative to the spike. It was first constant, and decreased gradually to a minimum value at the spike reference. Standard deviation values were lower for higher white noise amplitudes and mean elongation. Simple, short-lasting stimulus wave forms in the white noise were isolated and the corresponding responses were computed. Responses were periodic at low input variables and aperiodic with larger inputs and also asymmetric, being larger and briefer for lengthenings than for shortenings. Asymmetries augmented with input increments. Investigation of the interactions between the pace-maker and the white noise effects showed that lengthenings were more effective when they occurred just after a spike while the effectiveness of shortenings was greater just before a spike.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of lithium on electrical activity and potassium ion distribution in the vertebrate central nervous system

Three different regions of the vertebrate central nervous system maintained in vitro (frog spinal cord, guinea pig olfactory cortex and hippocampus) have been used to investigate how Li+ influences membrane potential, membrane resistance, action potentials, synaptic potentials and the transmembrane K+-distribution of neurons and glial cells. In view of the therapeutic action of Li+ in manic-depressive disease, a special effort was made to determine the threshold concentration for the actions of Li+ on the parameters described above. It was observed that Li+ induced a membrane depolarization of both neurons and glial cells, a decrease of action potential amplitudes, a facilitation of monosynaptic excitatory postsynaptic potentials and a depression of polysynaptic reflexes. The membrane resistance of neurons was not altered. Li+ also induced an elevation of the free extracellular potassium concentration and a decrease of the free intracellular potassium concentration. Furthermore, in the presence of Li+ a slowing of the recovery of the membrane potential of neurons and glial cells, and of the extracellular potassium concentration after repetitive synaptic stimulation was observed. The threshold concentrations for the effects of Li+ were below 5 mmol/l in the frog spinal cord and below 2 mmol/l in the guinea pig olfactory cortex and hippocampus. The basic mechanism underlying the action of Li+ may be an interaction with the transport-function of the Na+/K+ pump.

Hyperpolarization following long-lasting tetanic activation of hippocampal pyramidal cells

Long-lasting spike activation of CA3 hippocampal pyramidal neurons is shown to cause the development of a large and long-lasting (greater than 50 s) membrane hyperpolarization (PTH). Under normal conditions this PTH is mainly given by a relatively potential-independent process, presumably an electrogenic sodium pump. Following reduction in pump activity (cooling, ouabain), the PTH remains but is mainly produced by a conductance process, presumably a K conductance increase resulting from a sodium-induced calcium release from intracellular stores.

Intracellular ion control in lobster stretch receptor neurone

The control of intracellular ion concentrations by means of passive and active transmembrane ion transports was investigated in the lobster stretch neurone using electrophysiological and pharmacological techniques in combination with recording with ion-sensitive microelectrodes. In resting conditions [Na+]i, [K+]i, and [Cl-]i were, in both slowly and rapidly adapting cells, found to be in the order of 20, 155, and 50 mM, respectively. In the slowly adapting cell impulse firing at stationary frequencies of 7-10 Hz caused an increase in [Na+]i and a decrease in [K+]i of 20-30 mM; [Cl-]i was only little affected, the rise in [Na+]i led to an enhanced Na-K pump activity noticeable as an increase in pump current production. In stationary conditions the quotient between pump current and Na+ influx increments was about 0.3, which is compatible with 3:2 Na-K pumping ratio in the present preparation. From measurements of the pump current activation during stationary firing at maximum tolerable frequencies an estimate was made of the cell's maximum pump current production. The measurements were used in the formulation of a mathematical model of the intracellular ion control in which expressions of active and passive transmembrane ion transports are incorporated into the continuity equation for the ion fluxes involved.

Modifications of dynamic and static behavior by small-length perturbations in crayfish stretch receptor organs

To investigate whether static and dynamic sensitivities of slowly and rapidly adapting stretch receptor organs (SAO and RAO, respectively) or crayfish are different when perturbed compared with those in conventional laboratory experiments, receptors were submitted to ramplike length changes of different velocities separated by long-duration, constant lengths of different values. They were perturbed at random by fast, small-amplitude length variations called "jitter." First-order afferent discharges were recorded extracellularly. Quantifications involved the separate estimation of static and dynamic response components. In the SAO, jitter (1) augmented the static sensitivity, (2) decreased the dynamic sensitivity, (3) simplified response profiles by decreasing nonlinearities and increasing transduction fidelity in terms of coding length. In the RAO, jitter (1) changed the behavior from phasic to tonic with length sensitivity, (2) decreased the dynamic sensitivity to values close to those of the unperturbed SAO, (3) increased transduction fidelity for stimulus length and decreased nonlinearities. Perturbation effects suggest that differences between SAO and RAO are more quantitative than qualitative. Moreover, they have general implications and are relevant to other mechanoreceptors at other levels in the CNS.

A model for the firing pattern of a paced nerve cell

When a nerve cell is paced by another cell or by external stimuli, its firing pattern is generally not regular. In a given stimulus and frequency interval the cell responds only on a fraction of the stimuli, and this fraction is not necessarily a nice simple fraction as 1/2, 1/3, etc. but may be any fraction less than one, why the firing pattern becomes accordingly irregular. The paper demonstrates the rules for these irregularities and supplies a mathematical tool to analyse the firing pattern. In the analysis only a cell with the so-called impulse dependent type of adaptation is considered, but similar analyses may be done with other cell types. The analysis is restricted to a case where a resting cell is stimulated by repetitively applied stimuli of constant strength and repetition frequency. Cases with more than one input, spontaneously firing cells, and transient phenomena are not included in the analysis.

Changes of intracellular sodium and potassium ion concentrations in frog spinal motoneurons induced by repetitive synaptic stimulation

A post-tetanic membrane hyperpolarization following repetitive neuronal activity is a commonly observed phenomenon in the isolated frog spinal cord as well as in neurons of other nervous tissues. We have now used double-barrelled Na+- and K+-ion-sensitive microelectrodes to measure the intracellular Na+- and K+-concentrations and also the extracellular K+-concentration of lumbar spinal motoneurons during and after repetitive stimulation of a dorsal root. The results show that the post-tetanic membrane hyperpolarization occurred at a time when the intracellular [Na+] reached its maximal value, intracellular [K+] had its lowest level and extracellular [K+] was still elevated. The hyperpolarization was blocked by ouabain and reduced by Li+. These data support the previous suggestion that an electrogenic Na+/K+ pump mode may be the mechanism underlying the post-tetanic membrane hyperpolarization.

The ionic dependence of black widow spider venom action at the stretch receptor neuron and neuromuscular junction of crustaceans

The effects of black widow spider venom (BWSV) on the crayfish stretch receptor and the lobster neuromuscular junction were examined. In crayfish stretch receptor neurons, BWSV caused a slight hyperpolarization followed by a large depolarization. The venom-induced depolarization of the stretch receptor was caused by an increase in membrane conductance to Na+ and Ca2+. Black widow spider venom also caused an increase in the frequency of miniature inhibitory postsynaptic potentials recorded in the stretch receptor. The ability of BWSV to increase the frequency of miniature excitatory postsynaptic potentials (MEPSPs) at the lobster neuromuscular junction was dependent on the divalent cation composition of the bathing medium. Ringer solutions containing Ca2+ supported the greatest venom-induced increase in MEPSP frequency, Mg2+ and Mn2+ supported a moderate increase in MEPSP frequency, while Co2+ and Zn2+ blocked this venom effect entirely. Black widow spider venom did not block axonal conduction in lobster walking leg axons or in the axon of the crayfish stretch receptor. The results suggest that in crustaceans, BWSV interacts specifically with membrane of the soma-dendritic region of the stretch receptor and with nerve terminal membrane, causing an increase in Na+ and Ca2+ conductance.

A model accounting for effects of vibratory amplitude on responses of cutaneous mechanoreceptors in macaque monkey

1. A mechanoreceptor model, developed in the preceding paper (Freeman & Johnson, 1982), was used to study the effects of vibratory intensity and frequency on the responses of slowly adapting, rapidly adapting and Pacinian afferents in monkey hairless skin. As in the previous paper almost all of the response properties studied here were accounted for by the equivalent circuit model; changes in membrane time constant and amplitude sensitivity accounted for the differences between the three mechanoreceptive fibre types.2. The stimulus-response function of primary concern was the relationship between impulse rate and vibratory amplitude. This relationship had the same general form in each of the three fibre types. Amplitudes, I, less than I(0) produced no impulse on any stimulus cycles. Amplitudes greater than I(1) produced one impulse on every cycle. As I rose from I(0) to I(1) the impulse rate rose monotonically from 0 to 1 impulse/cycle. For each fibre type the form of this ramp depended on the stimulus frequency.3. At stimulus frequencies low in the frequency range of each fibre type the (I(0), I(1)) ramp tended to be steep and sigmoidal in shape. Two or more impulses occurred on some cycles and none on others.4. At intermediate frequencies the (I(0), I(1)) ramps became linear with at most one impulse on each cycle. A short plateau appeared at 0.5 impulses/cycle (i.e. there was a range of intensities yielding one impulse on alternate cycles). All of these response properties at low and intermediate frequencies were explained by the model.5. At higher frequencies the (I(0), I(1)) ramps became shallower and developed discontinuities in slope at impulse rates of 0.5 impulses/cycle. At stimulus frequencies greater than 20 Hz for SAs and RAs, the upper segment of the (I(0), I(1)) slope became steeper. For frequencies greater than 80 Hz, the upper segments of the Pacinian (I(0), I(1)) slopes were shallower than the lower segments. These effects suggested transient periods of hyperexcitability following each action potential, and reductions in sensitivity due to high impulse rates, respectively.6. The model's membrane time constant was adjusted to match the observed reduction in the (I(0), I(1)) slope with increasing stimulus frequency. The time constants required for least-squares fitting were 58, 29 and 4.2 msec for slowly adapting, rapidly adapting and Pacinian afferents, respectively; these values are of the same order as those obtained in the preceding paper.7. Receptor sensitivity varied across the frequency spectrum, slow adaptors being most sensitive at low frequencies, rapidly adapting units at mid-range, and Pacinians at the high frequencies. According to the model, the high frequency roll-off in a receptor's tuning curve is due to the current integrating properties of receptor membrane, and the low frequency roll-off is due to a high pass filter, presumably mechanical, situated in the tissues between the stimulus probe and receptor membrane.8. Impulse phase advances with increasing stimulus intensity in both receptor and model. The ability of the model to fit both the rate-intensity function and phase advance functions in individual receptors is demonstrated.

Post-tetanic depolarization in sympathetic neurones of the guinea-pig

1. Repetitive intracellular stimulation at a frequency of 5-30 Hz for 1-10 s evoked in neurones of the isolated inferior mesenteric and superior cervical ganglia of the guinea-pig three types of post-spike membrane potential changes: (i) hyperpolarization, (ii) hyperpolarization followed by a slow depolarization, and (iii) a second hyperpolarization following the initial two responses.2. The initial post-spike hyperpolarization had a mean duration of 2.0 s and was often associated with a fall in membrane resistance; it could be elicited in every sympathetic neurone studied. This response was termed the post-tetanic hyperpolarization (PTH).3. The slow depolarization which could be induced only in a portion of neurones had a mean amplitude and duration of 2.2 mV and 27.5 s, respectively; it was termed the post-tetanic depolarization (PTD).4. PTD was associated with a fall in membrane resistance, augmented by membrane hyperpolarization, and reduced by depolarization; its mean extrapolated equilibrium potential was -38 mV.5. PTD was not blocked by nicotinic and muscarinic antagonists, or alpha-and beta-adrenergic receptor antagonists, whereas it was suppressed by adrenaline, noradrenaline, Co(2+) and a low Ca(2+) solution.6. The amplitude of the single spike after-hyperpolarization in normal Krebs solution as well as in high K(+) solution was increased during PTD; furthermore, conditioning hyperpolarization to the level of E(K) increased the amplitude of PTD in normal Krebs as well as in high K(+) solution.7. PTD with similar amplitude, time course and membrane characteristics could be evoked in a portion of neurones of the rabbit superior cervical ganglia; however, PTD was not detected in neurones of the rat superior cervical ganglia.8. Decentralization of the guinea-pig and rabbit superior cervical ganglia for 14 d did not alter the number of neurones in which PTD could be elicited, its amplitude, or its time course.9. Our results suggest that a chemical substance(s) is responsible for the generation of PTD; it may be released from the soma and/or dendrites and acts in an auto-receptive manner on the cells in question. The nature and origin of the second hyperpolarization remain to be clarified.

Ammonium action on post-synaptic inhibition in crayfish neurones: implications for the mechanism of chloride extrusion

1. The reversal potential of the Cl(-)-dependent, inhibitory post-synaptic potential (E(i.p.s.p.)) was measured in the isolated crayfish stretch receptor neurone using two intracellular micro-electrodes. The difference between E(i.p.s.p.) and the resting membrane potential (E(m)), the i.p.s.p. driving force, was reversibly decreased by addition of NH(3)/NH(4) (+), and the mechanism of this decrease was investigated.2. The NH(3)/NH(4) (+)-induced decrease in i.p.s.p. driving force was dose-dependent with an onset at about 0.2 mM. E(i.p.s.p.) always remained more negative than E(m) or, when the neurone was spontaneously firing, the threshold potential. E(m) and resting membrane resistance (R(m)) also decreased in a dose-dependent fashion. Synaptic conductance (g(s)) increased with low doses, but decreased on application of 20 mM-NH(3)/NH(4) (+). All the effects were fully reversible on return to normal Ringer solution.3. Intracellular acidification (substitution of 50% Cl(-) by acetate compared with isethionate) considerably reduced the i.p.s.p. driving force. Simultaneous application of NH(3)/NH(4) (+) and acetate-substituted Ringer solution caused a similar decrease in the driving force to application of the same concentration of NH(3)/NH(4) (+) under normal conditions. Increasing the extracellular pH at which a given concentration of NH(3)/NH(4) (+) was applied caused a smaller decline in the i.p.s.p. driving force. These results suggest that intracellular acidification decreases the i.p.s.p. driving force and that the NH(3)/NH(4) (+)-induced decline is caused by an action of the ammonium ion.4. Elevation of extracellular K(+) (K(+) (0)) decreased the i.p.s.p. driving force, E(m) and R(m), and increased g(s). Reduction of K(+) (0) had the converse effects on all parameters.5. Application of Rb(+) or Cs(+) mimicked the effects of NH(3)/NH(4) (+). Substitution of K(+) (0) by Rb(+), Cs(+) or NH(3)/NH(4) (+) opposed or even reversed the increase in i.p.s.p. driving force when Na(+) was used as the substitute. The effectiveness of the various cations in decreasing the driving force was in the following order: Rb(+) > NH(4) (+) > K(+) > Cs(+).6. Inhibition of the Na pump by ouabain or K(+)-free Ringer solution caused a gradual reduction in the i.p.s.p. driving force. Since the driving force also decreased when the Na(+) gradient probably was increased (elevated K(+) (0)), this suggests a dependence on the K(+) gradient rather than the Na(+) gradient or the Na pump itself.7. Frusemide (6 x 10(-4) M) reversibly decreased the i.p.s.p. driving force and E(m), and increased g(s). R(m) was not significantly affected. Application of frusemide in the presence of 5 mM-Rb(+) and vice versa, caused a further reduction in the driving force. The recovery of the driving force on removal of either agent was slowed by the presence of the other.8. Application of 4,4-diisothiocyanostilbene-2,2-disulphonic acid (DIDS; 10(-4) M) caused spontaneous firing and reduced E(i.p.s.p.) to the threshold potential. R(m) and g(s) increased. The effects were slowly reversible on removal of the drug.9. It is proposed that the i.p.s.p. driving force is maintained by a K(+)-Cl(-) co-transport mechanism, driven by the K(+) gradient. The K(+) site exhibits the binding selectivity: Rb(+) > NH(4) (+) > K(+) > Cs(+) and the mechanism is inhibited partially by frusemide and completely by DIDS.

Ultrastructure of the crayfish stretch receptor in relation to its function

The crayfish slow-adapting abdominal stretch receptor was fixed under the relaxed or stretched condition. During this procedure action potentials of the sensory neuron were recorded by a suction electrode. The receptor organ consists of a receptor muscle and a sensory neurons with its dendrites embedded in the connective tissue zone in the receptor muscle. From the cell body of the neuron, several "primary dendrites" arise, branch successively into "dendritic branches", and finally terminate as "dendritic tips," which are cylindrical processes of fairly uniform diameter. In contrast to the primary dendrites and the dendritic branches, the dendritic tips have neither mitochondria nor sheaths and are embedded in the connective tissue zone or apposed to the receptor muscle with a gap of about 15 nm. Microtubules and smooth ER are seen in all parts of the dendrites. When the receptor is stretched and then fixed with 1.6% glutaraldehyde in 0.12 M phosphate buffer (total osmolarity of this solution is isosmotic with the physiological solution), dendritic tips became more parallel to the long axis of the receptor muscle and showed marked deformation consisting of alternate regions of swelling and shrinkage, resulting in a bead-like appearance. When fixed with 1.6% glutaraldehyde in 0.2 M phosphate buffer (total osmolarity of this solution is hyperosmotic), the dendritic tips showed less tendency toward such deformation. These results suggest that the dendritic tip membrane is susceptible to stretch and might be the region where the generator potential is produced.

On the action of the anticonvulsant 5,5-diphenylhydantoin and the convulsant picrotoxin in crayfish stretch receptor

1. The effects of the anticonvulsant drug 5,5-diphenylhydantoin (DPH) and the convulsant drug picrotoxin (PTX) on various membrane properties and GABA-ergic inhibition were investigated in the slowly adapting neurone of the isolated crayfish stretch receptor. The soma was penetrated with two micro-electrodes to allow accurate determination of membrane conductances. 2. Neither DPH nor PTX at 10(-4) M had any significant effect on parameters of the anti- or orthodromic action potential or on the amplitude and duration of post-tetanic hyperpolarization. This suggests that the pharmacological properties of the two drugs are unlikely to be mediated by effects on cationic movements in this preparation. 3. DPH increased the amplitude and duration of the inhibitory post-synaptic potential (i.p.s.p.) within the range 10(-9) to 10(-4) M. The response to ionophoretically applied GABA was similarly prolonged. 4. PTX decreased the amplitude of the i.p.s.p. and prolonged its rising phase within the range 10(-8) to 10(-4) M. The response to ionophoretically applied GABA was similarly depressed. 5. A slow component of fluctuations in the resting potential was accentuated by DPH at 10(-4) M and eliminated by PTX 10(-4) M. This may reflect effects on the random opening and closing of inhibitory channels. 6. We conclude that the action of both drugs is post-synaptic and suggest that DPH decreases the probability of closing, and PTX the probability of opening, of the transmitter-activated channels. 7. The lack of any structural similarity between the two drugs suggests that they modify post-synaptic inhibition at separate sites. These sites appear to be interdependent since analysis of the shift in the DPH dose-response curve by PTX and vice versa, showed neither truly non-competitive nor competitive interaction.