Facilitation, augmentation, and potentiation of synaptic transmission at the superior cervical ganglion of the rabbit

The effect of repetitive stimulation on synaptic transmission was studied in the isolated superior cervical ganglion of the rabbit under conditions of reduced quantal content. Excitatory postsynaptic potentials (EPSP) were recorded with the sucrose gap technique to obtain estimates of transmitter release. Four components of increased transmitter release, with time constants of decay similar to those observed at the frog neuromuscular junction at 20 degrees C, were found in the ganglion at 34 degrees C: a first component of facilitation, which decayed with a time constant of 59 +/- 14 ms (mean +/- SD); a second component of facilitation, which decayed with a time constant of 388 +/- 97 ms; augmentation, which decayed with a time constant of 7.2 +/- 1 s; and potentiation, which decayed with a time constant of 88 +/- 25 s. The addition of 0.1-0.2 mM Ba2+ to the Locke solution increased the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation. The addition of 0.2-0.8 mM Sr2+ to the Locke solution appeared to increase the magnitude of the second component of facilitation. Sr2+ had little effect on augmentation or potentiation. These selective effects of Ba2+ and Sr2+ on the components of increased transmitter release in the rabbit ganglion are similar to the effects of these ions at the frog neuromuscular junction. Although the effects of Ba2+ and Sr2+ are similar in the two preparations, the magnitudes of augmentation and the second component of facilitation after a single impulse were about 6-10 times greater in the rabbit ganglion than at the frog neuromuscular junction. These results suggest that the underlying mechanisms in the nerve terminal that give rise to the components of increased transmitter release in the rabbit ganglion and frog neuromuscular junction are similar but not identical.     

Differential effects of Ba2+, Sr2+, and Ca2+ on stimulation-induced changes in transmitter release at the frog neuromuscular junction

Endplate potentials (EPP) were recorded from the frog sartorius neuromuscular junction under conditions of low quantal content to study the effect of Ba2+, Sr2+, and Ca2+ on the changes in evoked transmitter release that occur during and after repetitive stimulation. The addition of 0.1-1 mM Ba2+ or Sr2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with 0.8-1.4 mM Sr2+, led to a greater increase in EPP amplitudes during and immediately after repetitive stimulation. These changes in release were analyzed in terms of the four apparent components of increased transmitter release that have previously been distinguished on the basis of their kinetic properties. The Ba2+-induced increase in EPP amplitudes was associated with an increase in the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation or the first and second components of facilitation. The Sr2+-induced increase in EPP amplitudes was associated with an increase in the magnitude and the time constant of decay of the second component of facilitation. Sr2+ had little effect on potentiation, augmentation, or the first component of facilitation. The selective effects of Ba2+ on augmentation and of Sr2+ on the second component of facilitation were reversible and could be obtained in the presence of the other ion. The addition of 0.1-0.3 mM Ca2+ to the bathing solution had little effect on potentiation, augmentation, or the two components of facilitation. These results provide pharmacological support for the proposal that there are four different components of increased transmitter release associated with repetitive stimulation and suggest that the underlying factors in the nerve terminal that give rise to these components can act somewhat independently of one another.     

Changes in miniature endplate potential frequency during repetitive nerve stimulation in the presence of Ca2+, Ba2+, and Sr2+ at the frog neuromuscular junction

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation- induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation- induced changes in MEPP frequency and EPP amplitude have similar time- courses.     

Role of intracellular Ca2+ in stimulation-induced increases in transmitter release at the frog neuromuscular junction

Under conditions of reduced quantal content, repetitive stimulation of a presynaptic nerve can result in a progressive increase in the amount of transmitter released by that nerve in response to stimulation. At the frog neuromuscular junction, this increase in release has been attributed to four different processes: first and second components of facilitation, augmentation, and potentiation (e.g., Zengel, J. E., and K. L. Magleby. 1982. Journal of General Physiology. 80:583-611). It has been suggested that an increased entry of Ca2+ or an accumulation of intraterminal Ca2+ may be responsible for one or more of these processes. To test this hypothesis, we have examined the role of intracellular Ca2+ in mediating changes in end-plate potential (EPP) amplitude during and after repetitive stimulation at the frog neuromuscular junction. We found that increasing the extracellular Ca2+ concentration or exposing the preparation to carbonyl cyanide m- chlorophenylhydrazone, ionomycin, or cyclopiazonic acid all led to a greater increase in EPP amplitude during conditioning trains of 10-200 impulses applied at a frequency of 20 impulses/s. These experimental manipulations, all of which have been shown to increase intracellular levels of Ca2+, appeared to act by increasing primarily the augmentation component of increased release. The results of this study are consistent with previous suggestions that the different components of increased release represent different mechanisms, and that Ca2+ may be acting at more than one site in the nerve terminal.     

Mechanisms of neurotransmitter release facilitation in strontium solutions

Mechanisms of neurotransmitter release facilitation were studied using electrophysiological recording of end-plate currents (EPC) and nerve ending (NE) responses after substitution of extracellular Ca ions with Sr ions at the frog neuromuscular junction. The solutions with 0.5 mM concentration of Ca ions (calcium solution) or 1 mM concentration of Sr ions (strontium solution) were used where baseline neurotransmitter release (at low-frequency stimulation) is equal. Decay of paired-pulse facilitation of EPC at calcium solutions with increase of interpulse interval from 5 to 500 ms was well described by three-exponential function consisting of early, first and second components. Facilitation at strontium solutions was significantly diminished due mainly to decrease of early and first components. At the same time, EPC facilitation with rhythmic stimulation (10 or 50 imp/s) at strontium solutions was significantly increased. Also more pronounced decrease of NE response 3rd phase, reflecting potassium currents was detected under rhythmic stimulation of 50 imp/s at strontium solutions comparing to calcium solutions. It was concluded that facilitation sites underlying first and early components had lower affinity to Sr ions than to Ca ions. The enhancement of frequency facilitation at strontium solutions is mediated by two mechanisms: more pronounced broadening of NE action potential and increase of bivalent cation influx due to feebly marked activation of Ca(2+)-dependent potassium current by Sr ions, and slower dynamics of Sr(2+) removal from NE axoplasm comparing to Ca(2+).     

Analysis of frequency facilitation in neuromuscular synapses of the sartorius muscle in frogs

Changes in the quantum content of end plate potentials (EPPs) were studied in experiments on the frog sartorius muscle in the course of a gradual increase in stimulation frequency (frequency facilitation). It was shown that the frequency facilitation coefficient does not depend on the initial quantum content of EPPs but is in good agreement with the initial frequency of miniature EPPs and in bad agreement with their amplitude. The frequency facilitation is accompanied by the growth of the statistical binomial parameters P and n. The increase in P parameters is consequent of interference in P changes during the second component of facilitation after the single impulse. The growth of n is accounted for by the summation of the n increase during the first component of facilitation the time course of which is prolonged on repetitive stimulation.     

Facilitation at crayfish neuromuscular junctions

Electrophysical recordings from opener muscle fibers in the crayfishProcambarus clarkii (Fig. 1) show that pre-synaptic facilitation at terminals of the single excitatory axon usually decays in a dual-exponential fashion after a single pulse or after a train of pulses (Figs. 2, 3, 7, 9), as has been reported for frog neuromuscular junctions (Mallart and Martin, 1967) and squid giant synapses (Charlton and Bittner, 1974, 1976). Furthermore, the second component of decay at crayfish synapses is associated with a break in the monotonic decay of the first component, a result which suggests that the decay of facilitation is not due to the simple diffusion of some substance (such as calcium) from specialized release sites.The growth of facilitation at all opener synapses during trains of equalinterval stimuli could not be predicted by assuming that each pulse contributed an equal amount of facilitation which summed linearly with that remaining from all previous stimuli (Figs. 4, 6; Table 2), as reported for synapses in frog and squid. During high frequency stimulation (>40 Hz), those terminals which facilitate dramatically (highF _e synapses) show much greater amounts of facilitation than that predicted by the linear summation model (Figs. 4, 8), whereas other terminals (lowF _e synapses) show much less facilitation than predicted (Fig. 6). The rate of growth of facilitation was often very constant at various stimulus rates in highF _e or mixed type synapses (Figs. 4, 8, 10)-a result not predicted by the linear summation model. Finally, when highF _e synapses were stimulated at different frequencies, the rate of growth of facilitation changed dramatically in a fashion not predictable using linear summation (Mallert and Martin, 1967) or power law (Linder, 1974) models.     

THE OXYGEN CONSUMPTION OF FROG NERVE DURING STIMULATION

1. The resting rate of oxygen consumption of the excised sciatic nerve of the frog is 1.23 c.mm. of oxygen per gm. of nerve per minute. 2. During stimulation with an induction coil with 100 make and 100 break shocks per second there is an excess oxygen consumption amounting on the average to 0.32 c.mm. of oxygen per gm. of nerve per minute of stimulation, or a 26 per cent increase over the resting rate. 3. The magnitude of the excess oxygen consumption in stimulation, in agreement with the all-or-none law, is not markedly influenced by considerable variations in the intensity of stimulation. 4. Increasing the frequency of stimulation from 100 to 200 shocks per second increases the extra oxygen used only 1.12–1.18 times. The same change in frequency of stimulation increases the negative variation 1.15 times and the heat production about 1.25 times (Hill). 5. This parallelism between the excess oxygen and the negative variation argues definitely for some causal connection between the excess oxygen and the nerve impulse itself. 6. Calculation shows that the oxygen tension inside these nerves was not zero.     

Paired-Pulse Facilitation of Transmitter Release at Different Levels of Extracellular Calcium Concentration

High-frequency synaptic activity can cause facilitation of transmitter release due to accumulation of “residual Ca²⁺” at the nerve terminal. However, the mechanism of this phenomenon is still under debate. Here we show that, using extracellular recording from frog cutaneous pectoris muscle, paired-pulse facilitation (PPF) at the frog neuro-muscular junction decays in two or three-exponential manner depending upon the extracellular Ca²⁺ concentration ([Ca²⁺]_e). First, second and “early” PPF components are analyzed and described in this study. Considering the dependence of PPF on [Ca²⁺]_e, existence of several specific high-affinity intra-terminal Ca²⁺-binding sites that underlie the facilitation of transmitter release at the frog neuro-muscular junction is proposed.     

Simultaneous BOLD fMRI and fiber-optic calcium recording in rat neocortex

Functional magnetic resonance imaging (fMRI) based on blood oxygen level-dependent (BOLD) contrast is widely used for probing brain activity, but its relationship to underlying neural activity remains elusive. Here, we combined fMRI with fiber-optic recordings of fluorescent calcium indicator signals to investigate this relationship in rat somatosensory cortex. Electrical forepaw stimulation (1-10 Hz) evoked fast calcium signals of neuronal origin that showed frequency-dependent adaptation. Additionally, slower calcium signals occurred in astrocyte networks, as verified by astrocyte-specific staining and two-photon microscopy. Without apparent glia activation, we could predict BOLD responses well from simultaneously recorded fiber-optic signals, assuming an impulse response function and taking into account neuronal adaptation. In cases with glia activation, we uncovered additional prolonged BOLD signal components. Our findings highlight the complexity of fMRI BOLD signals, involving both neuronal and glial activity. Combined fMRI and fiber-optic recordings should help to clarify cellular mechanisms underlying BOLD signals.     

Nonhomogeneous excitatory synapses of a crab stomach muscle

Neuromuscular synapses of pyloric muscle P1 in the blue crab Callinectes sapidus were examined using electrophysiological and electron microscopic methods. The muscle is innervated by a single excitatory axon of the stomatogastric ganglion. Excitatory postsynaptic potentials show striking facilitation at very low frequencies of stimulation, indicating very slow decay of the facilitation process after a single nerve impulse. Quantal content of transmitter release at a low frequency of stimulation averaged 1.5. Evidence was obtained that not all synapses on a muscle fiber are equivalent. This was particularly evident at the morphological level in serially sectioned nerve terminals. On each nerve terminal examined, a wide range of synapse sizes was found. Synaptic contact areas ranged from less than 0.5 micron2 to almost 10 micron2; the latter value is large compared with those obtained for other crustacean neuromuscular synapses. Most of the smaller synapses lacked the presynaptic dense bodies which are putative release sites for the transmitter substance. The larger synapses all had presynaptic dense bodies, and some showed evidence of splitting apart into smaller subunits. It is postulated that about half the morphologically identified synapses are relatively inactive.     

Birefringence signals in mammalian and frog myocardium. E-C coupling implications

Birefringence signals from mammalian and frog hearts were studied. The period between excitation and the onset of contraction in which optical signals were free of movement artifact was determined by changes in scattered incandescent light and changes in laser diffraction patterns. The birefringence signal preceding contraction was found to behave as a change in retardation and was not contaminated measurably by linear dichroic or isotropic absorption changes. There were two components of the birefringence signal in mammalian heart muscles but only one component in the frog heart. The first component of the birefringence signals in both mammalian and frog hearts had a time course coincident with the action potential upstroke. The second component in mammalian preparations was sensitive to inotropic interventions, such as variation of extracellular Ca2+, stimulation frequency, temperature, and epinephrine, in a manner that correlated with the maximum rate of rise of tension. Caffeine (2-10 mM) not only failed to generate a second component in the frog heart, but also suppressed the second component in the mammalian heart while potentiating twitch tension. The results suggest that the second component of the birefringence signal in the mammalian myocardium is related to Ca2+ release from the sarcoplasmic reticulum.     

Changes in the Asynchronicity of Transmitter Release at the Neuromuscular Junction during Prolonged High-Frequency Stimulation

In experiments on neuromuscular junctions in the frog m. cutaneous-pectoris, changes in the intensity and asynchronicity of transmitter release during high-frequency (10 and 50 sec^-1) rhythmic stimulation of the motor nerve were investigated using extracellular recording. At low extracellular Ca²⁺ concentrations, rhythmic stimulation resulted in a gradual enlargement of the quantum content of end-plate currents (EPC), the so-called facilitation. The latter phenomenon was accompanied by an increase in the average value and variance of synaptic delays of single-quantum EPC, a shift of the main mode of their distribution towards greater values, and an increase in the latency of the nerve ending responses. The above-described changes reduce the magnitude of facilitation in the neuromuscular synapse.     

Differential Responses of Crab Neuromuscular Synapses to Cesium Ion

Excitatory postsynaptic potentials (EPSP's) generated in crab muscle fibers by a single motor axon, differ in amplitude and facilitation. Some EPSP's are large at low frequencies of stimulation and show little facilitation; others are smaller and show pronounced facilitation. When K⁺ is replaced by Cs⁺ in the physiological solution, all EPSP's increase in amplitude, but small EPSP's increase proportionately more than large ones. Quantal content of transmission, determined by external recording at single synaptic regions, undergoes a much larger increase at facilitating synapses. The increase in quantal content of transmission is attributable to prolongation of the nerve terminal action potential in Cs⁺. After 1–2 h of Cs⁺ treatment, defacilitation of synaptic potentials occurs at synapses which initially showed facilitation. This indicates that Cs⁺ treatment drastically increases the fraction of the "immediately available" transmitter store released by each nerve impulse, especially at terminals with facilitating synapses. It is proposed that facilitating synapses normally release less of the "immediately available" store of transmitter than poorly facilitating synapses. Possible reasons for this difference in performance are discussed.     

Inhibitory interaction of receptor units in the eye of Limulus

The inhibition that is exerted mutually among the receptor units (ommatidia) in the lateral eye of Limulus has been analyzed by recording oscillographically the discharge of nerve impulses in single optic nerve fibers. The discharges from two ommatidia were recorded simultaneously by connecting the bundles containing their optic nerve fibers to separate amplifiers and recording systems. Ommatidia were chosen that were separated by no more than a few millimeters in the eye; they were illuminated independently by separate optical systems. The frequency of the maintained discharge of impulses from each of two ommatidia illuminated steadily is lower when both are illuminated together than when each is illuminated by itself. When only two ommatidia are illuminated, the magnitude of the inhibition of each one depends only on the degree of activity of the other; the activity of each, in turn, is the resultant of the excitation from its respective light stimulus and the inhibition exerted on it by the other. When additional receptors are illuminated in the vicinity of an interacting pair too far from one ommatidium to affect it directly, but near enough to the second to inhibit it, the frequency of discharge of the first increases as it is partially released from the inhibition exerted on it by the second (disinhibition). Disinhibition simulates facilitation; it is an example of indirect effects of interaction taking place over greater distances in the eye than are covered by direct inhibitory interconnections. When only two interacting ommatidia are illuminated, the inhibition exerted on each (decrease of its frequency of discharge) is a linear function of the degree of activity (frequency of discharge) of the other. Below a certain frequency (often different for different receptors) no inhibition is exerted by a receptor. Above this threshold, the rate of increase of inhibition of one receptor with increasing frequency of discharge of the other is constant, and may be at least as high as 0.2 impulse inhibited in one receptor per impulse discharged by the other. For a given pair of interacting receptors, the inhibitory coefficients are not always the same in the two directions of action. The responses to steady illumination of two receptor units that inhibit each other mutually are described quantitatively by two simultaneous linear equations that express concisely all the features discussed above. These equations may be extended and their number supplemented to describe the responses of more than two interacting elements.     

Mechanisms of bioelectric activity in electric tissue. I. The response to indirect and direct stimulation of electroplaques of Electrophorus electricus

1. A preparation is described consisting of one or several layers of innervated cells of the electric organ of Electrophorus electricus. 2. Each plaque is multiply innervated and only at its caudal face. The nerve fibers may derive from two or more different nerve trunks. 3. During activity the innervated face becomes negative relative to the non-innervated. 4. The first electrical response of the cell to an increasing neural volley is graded and has the character of a prepotential. At a critical size of the prepotential the cell discharges with an all-or-nothing spike. 5. Both responses have durations of about 2 msec. 6. A neural volley which does not cause the spike discharge facilitates the discharge of the cell by a second subsequent volley in the same nerve (temporal facilitation). 7. The period of facilitation lasts ca. 900 msec. During the first 100 msec., the facilitation is large enough to cause a spike. In the later portion only the prepotential is facilitated. No electrical concomitant has been detected. 8. Neural volleys reaching the plaque from different trunks interact at the cell to produce a period of facilitation lasting only about 2 msec. This interaction is interpreted as spatial summation. 9. In a population of cells, simultaneous stimulation of 2 nerves causes a smaller discharge than the sum of the two isolated responses (occlusion). 10. Cells denervated for 7 weeks or more can be excited directly, but only by a current flow outward through the caudal face. 11. Weak direct stimulation causes a prepotential in the denervated plaque. On increasing the stimulus the prepotential increases to a critical size when a spike develops. The duration of both responses is about 2 msec. 12. The absolutely refractory period of the denervated cell is about 1.5 msec. and relative refractoriness lasts about 15 msec. 13. Direct stimulation causes slight facilitation lasting as long as 200 msec. 14. Repetitive stimulation of the nerve at low frequencies (2 to 3 per second) causes rapid "fatigue" of transmission. The denervated plaque, however, responds for several minutes to repetitive direct stimulation at high frequencies (25 per second).     

Muscle enhancement using closed-loop electrical stimulation: volitional versus induced torque.

In cases of partial deficiency of muscle activation capacity, force augmentation can be achieved by hybrid activation, i.e., by combining electrical stimulation (ES) with volitional activation. In this activation modality the shares of the volitional and induced torques within the overall hybrid torque are unknown. The purpose of this study was to suggest a computational approach to parcel out the volitional and stimulation induced components of joint torque generated during combined voluntary and electrical activation of the Tibialis Anterior muscle (TA). For this purpose, isometric contraction of the TA was studied on 5 healthy subjects, using an activation protocol involving ES alone, volitional activation alone and hybrid activation. Ankle torque and TA EMG were measured. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG filtering and on pre-measured calibration curves of volitional torque versus EMG. The results indicated that for a certain hybrid torque there is a linear decaying relationship between the induced torque and the volitional torque shares. Moreover, based on a defined enhancement ratio, the results indicate that within the range of stimulation intensities, there exist regions of increased facilitation, in which the stimulation efficiency is higher under combined compared to isolated conditions.     

Socially-induced variation in physiological mediators of parental care in a colonial bird

Social facilitation of reproduction occurs in humans and animals, and may represent one of the bases of reproduction in groups. However, its underlying physiological mechanisms remain largely unexplored. Here, we found in a colonial bird, the Adélie penguin (Pygoscelis adeliae), that the number of parental interactions (nest relief ceremonies) performed by breeding individuals on the colony was positively related to prolactin levels in other breeding individuals exposed to these interactions (i.e. focal individuals). As prolactin is typically involved in the expression of parental behaviour in birds, this suggests that parental interactions by conspecifics represent social cues that might increase parental motivation in focal individuals. Moreover, parental interactions were not related to corticosterone levels in focal individuals, suggesting that these social cues were not stressful for penguins. However, social stimulation still had a cost for focal individuals, as it was negatively related to their antioxidant defences (a component of self-maintenance). As social stimulation was also positively related to prolactin levels, this highlights the fact that social stimulation acts on the trade-off between reproduction and self-maintenance. For the first time, the results of the current study shed light on the physiological factors potentially underlying social facilitation of parental care. Importantly, they suggest that, even though social facilitation of parental care may increase breeding performance, it can also negatively affect other fitness components.     

Electrophysiological study of mediator secretion in the frog neuromuscular synapse under a colchicine block of axoplasmic transport

Two weeks after colchicine nerve treatment the evoked transmitter release was blocked in part of the frog sartorius synapses, with spontaneous activity being absent from some of them. In the synapses with evoked and spontaneous transmitter release preserved within this period of time, the magnitudes of the absolute refractory phase of nerve terminals were significantly higher than the control ones, while in part of synapses, the frequency of miniature end plate potentials (MEPP) was considerably increased. Nerve stimulation (5 imp.s-1) led to a rise of the amplitude of evoked potentials and of MEPP frequency followed by irreversible blockade of synaptic activity. It is concluded that substances transported by rapid axonal flow control the level of membrane potential of nerve terminals and are fairly important for presynaptic membrane integrity.