Effects of Long-Lasting Afferent Activation on Segmental Reflex Responses in Albino Rats,and Modifications of These Effects by Thyroxine and 4-Aminopyridine

In albino rats, we studied the effects of long-lasting tetanization of the dorsal roots of the L ₅ (homosynaptic activation) and L ₄ (heterosynaptic activation) segments on reflex discharges in the L ₅ ventral root evoked by single stimulation of the dorsal root of the same segment. Tetanization trains consisted of 5,000 stimuli applied with frequencies of 10, 50, 100, or 300 sec^–1, and their effects were tested during 10 min. There were no long-term post-tetanic potentiation (PTP) of monosynaptic responses when low frequencies of homosynaptic tetanization (10 and 50 sec^–1) were used. In the case of higher frequencies, PTP was rather clear and long-lasting. Under conditions of heterosynaptic activation, there was no PTP. Facilitation of polysynaptic responses developed at all the frequencies of homosynaptic tetanization used; when heterosynaptic tetanization was applied, such facilitation (although weaker) was also observed. In rats treated with agents increasing the excitability of spinal neuronal systems, such as thyroxine and 4-aminopyridine, tetanization of the studied inputs evoked long-term depression (LTD) of both mono- and polysynaptic components of the reflex discharges instead of PTP. Probable mechanisms of postsynaptic changes in the segmental reflex responses are discussed.     

Post-tetanic potentiation of response in monosynaptic reflex pathways of the spinal cord

Following tetanic afferent stimulation of a monosynaptic reflex pathway, the transmission through that pathway of isolated reflex volleys is enhanced for some minutes. Post-tetanic potentiation is comparable in the monosynaptic reflex arcs of flexor and extensor muscles. The facilitator and inhibitor actions of monosynaptic reflex afferent fibers, as well as the transmitter action, are potentiated following tetanization. Little post-tetanic change attends reflex transmission through plurisynaptic reflex arcs. Various tests for excitability change made independently of the tetanized afferent fibers reveal none or a slight depression. Hence the potentiating influence of a tetanus is limited to subsequent action on the part of the recently tetanized fibers themselves. Increase in the size of the individual impulses comprising an afferent volley such as might occur during positive after-potential, would accommodate the requirement for a limited process and provide for increased synaptic action. The proposed association between post-tetanic potentiation and positive after-potential (i.e. hyperpolarization) is supported by the following lines of evidence:- 1. Changes in intensity and duration of potentiation with change in frequency and duration of tetanic stimulation are characteristic of, and parallel to, the changes of positive after-potential in similar circumstances. 2. Afferent impulses are increased following a tetanus, and in a fashion that parallels the course of monosynaptic reflex potentiation. Post-tetanic potentiation, as here described, and after-discharge, whatever may be its mechanism, are unrelated phenomena.     

Facilitation of reactions of cortical neurons to cortical and peripheral stimulation after tetanization of the brain surface

The electrical activity of neurons of the sensomotor cortex of an unanesthetized rabbit was investigated. Conditioning tetanization of the cortex was carried out through surface electrodes located close to the site of the lead. Test stimuli were supplied through electrodes 2.5–12 mm more caudally on the cortical surface. In addition, peripheral test stimuli were applied. Impulse reactions to previously ineffective stimuli develop after conditioning tetanization and prolonged (up to 1 min) intensification of exciting postsynaptic potentials (EPSP) to cortical and peripheral test stimuli is observed. Facilitation of the reactions is especially clear during tetanization superthreshold for evoking epileptiform afterdischarges. It continued after the conclusion of these discharges and could also be observed during tetanization subthreshold for evoking afterdischarges. The time course of the facilitation was similar to the time course of the post-tetanic intensification of reactions of single stimuli applied with the electrodes used for tetanization. An analysis of the changes in intracellular activity makes it possible to assume that the mechanism of post-tetanic potentiation (PTP) lies at the basis of the described facilitation, which is considered as the "cellular analog" of the dominant focus which develops as a result of tetanization of the cortical surface.Institute of the Brain, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 3–12, January–February, 1971.     

Effects of tetanic stimulation on monosynaptic and dorsal root reflexes in kittens.

The effects of tetanic stimulation of peripheral afferents were examined on monosynaptic reflexes and dorsal root reflexes in kittens of various ages. Concomitantly recorded monosynaptic and dorsal root reflexes resulting from the stimulation of muscle nerves showed similar post-tetanic changes, namely, predominantly post-tetanic depression in neonates and post-tetanic potentiation in older kittens or adults. However, the changes in post-tetanic responses expressed as a percentage of control in dorsal root reflexes were much smaller than those in monosynaptic reflexes. When dorsal root reflexes originating from muscle and cutaneous afferents were compared, dorsal root reflexes from the latter behaved quite differently. For all ages, post-tetanic effects on dorsal root reflexes arising from cutaneous afferents were either insignificant or very small. The possible mechanisms underlying differences in post-tetanic effects from muscle and cutaneous afferents in adults and neonates are discussed.     

Motor Nerve Terminals As the Site of Initial Functional Changes After Denervation

For the cat soleus nerve-muscle system, motor nerve section 48 hr prior to in situ experiment causes certain characteristic transmission losses. Responses to repetitive stimulation are sharply altered: The capacity to transmit iterative stimulation is severely reduced; post-tetanic potentiation and the post-tetanic repetition of soleus nerve terminals responsible for it are also greatly impaired; a phenomenon of post-tetanic depression was frequently observed. However, function of the extramuscular axons appears normal and single impulse transmission is usually not seriously affected. The loss of reactivity to repetitive stimulation has been traced to soleus motor nerve terminals. In view of these data and the known absence of denervation hypersensitivity at this time, the earliest functional failure may be said to occur in the unmyelinated terminals. This subacutely denervated preparation therefore offers a simple means of evaluating motor nerve terminal responsiveness.     

Post-tetanic changes in intracellular response to direct cortical stimulation

Changes in responses to single stimulations of the cortical surface after tetanization (frequency 50/sec, duration 1–10 sec) were studied in sensorimotor cortical neurons of an unanesthetized rabbit on intracellular and "quasiintracellular" recordings. After tetanization insufficient to generate epileptiform after-discharges, an increase was observed in the amplitude and duration of exciting postsynaptic potentials (EPSP) induced by a single test stimulus. This increase is considered as post-tetanic potentiation (PTP). Its duration did not exceed 1 min. The amplitude of inhibitory postsynaptic potentials (IPSP) showed a considerably smaller increase or did not change or even decreased. The PTP increased with an increase in the strength and duration of the tetanization, reaching especially high values during tetanization sufficiently intensive to evoke epileptiform after-discharges. In this case the response to a single test stimulus was identical to an epileptiform intracellular discharge. The data obtained confirm the important role of PTP of the exciting synapses in the generation of epileptiform after-discharges. A simple model of a neuron network with exciting and inhibiting feedbacks which accounts for the generation of epileptiform activity is examined.Institute of the Brain, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 6, pp. 601–610, November–December, 1970.     

In vivo imaging of vesicle motion and release at the Drosophila neuromuscular junction

Recently, it has become possible to directly detect changes in neuropeptide vesicle dynamics in nerve terminals in vivo and to measure the release of neuropeptides induced experimentally or evoked by normal behavior. These results were obtained with the use of transgenic fruit flies that express a neuropeptide tagged with green fluorescent protein. Here, we describe how vesicle movement and neuropeptide release can be studied in the larval Drosophila neuromuscular junction using fluorescence microscopy. Analysis methods are described for quantifying movement based on time lapse and fluorescence recovery after photobleaching data. Specific approaches that can be applied to nerve terminals include single particle tracking, correlation and Fourier analysis. Utilization of these methods led to the first detection of vesicle mobilization in nerve terminals and the discoveries of activity-dependent capture of transiting vesicles and post-tetanic potentiation of neuropeptide release. Overall, this protocol can be carried out in an hour with ready Drosophila.     

Role of Ca(2+) channels in short-term synaptic plasticity

Repetitive nerve activity induces various forms of short-term synaptic plasticity that have important computational roles in neuronal networks. Several forms of short-term plasticity are caused largely by changes in transmitter release, but the mechanisms that underlie these changes in the release process have been difficult to address. Recent studies of a giant synapse - the calyx of Held - have shed new light on this issue. Recordings of Ca(2+) currents or Ca(2+) concentrations at nerve terminals reveal that regulation of presynaptic Ca(2+) channels has a significant role in three important forms of short-term plasticity: short-term depression, facilitation and post-tetanic potentiation.     

Prolonged primary afferent induced alterations in dorsal horn neurones, an intracellular analysis in vivo and in vitro

1.) Peripheral tissues injury produces long lasting sensory and motor disturbances in man that present as the post-injury hypersensitivity syndrome with a reduction in the threshold required to elicit either pain or the flexion withdrawal reflex and an exaggeration of the normal response to suprathreshold stimuli. 2.) Two mechanisms contribute to these changes; sensitization of the peripheral terminals of high threshold primary afferents and an increase in the excitability of the spinal cord; a phenomenon known as central sensitization. 3.) Central sensitization has previously been shown by our laboratory to be the consequence of activity in unmyelinated primary afferents. Brief (20 s) C-fibre strength conditioning stimuli have the capacity to produce both a prolonged heterosynaptic facilitation of the flexion reflex and an alteration in the response properties of dorsal horn neurones, that long outlast the conditioning stimulus. 4.) In the adult decerebrate-spinal rat preparation we have, using intracellular recordings of dorsal horn neurones, examined the time course of the central effects of different types of orthodromic inputs. The hemisected spinal cord preparation isolated from 12-14 day rat pups has been used to see whether prolonged alterations in dorsal horn properties induced by orthodromic inputs can be studied in vitro. 5.) Single stimuli applied to a cutaneous nerve at graded strengths to successively recruit A beta, A delta and C-afferents produce, in the majority of neurones recorded in the deep dorsal horn in vivo, a series of post synaptic potentials that last from between ten and several hundred milliseconds. 6.) Repeated low frequency stimulation of C but not A-afferent fibres results in a pattern of progressive response increment or windup in a proportion of dorsal horn neurones. In some of the neurones the windup is associated with a depolarization that outlasts the stimulus period for tens of seconds. 7.) Application of the chemical irritant mustard oil to the skin activates chemosensitive C-afferent fibres for 1-3 minutes. Such a conditioning stimulus results however in an expansion in the size and an alteration in the response properties of the receptive fields of dorsal horn neurones that lasts for tens of minutes. 8.) In dorsal horn neurones recorded intracellularly in the isolated hemisected spinal cord, both intrinsic membrane properties and the orthodromic responses to primary afferent input can be studied. Repeated stimulation of a dorsal root produces in some neurones a prolonged heterosynaptic facilitation with both an augmentation of the response to the conditioning root (homosynaptic potentiation) and to adjacent test roots (heterosynaptic potentiation).(ABSTRACT TRUNCATED AT 400 WORDS)     

EphrinB2 induces pelvic-urethra reflex potentiation via Src kinase-dependent tyrosine phosphorylation of NR2B

Recently, the role of EphB receptor (EphBR) tyrosine kinase and their ephrinB ligands in pain-related neural plasticity at the spinal cord level have been identified. To test whether Src-family tyrosine kinase-dependent glutamatergic N-methyl-d-aspartate receptor NR2B subunit phosphorylation underlies lumbosacral spinal EphBR activation to mediate pelvic-urethra reflex potentiation, we recorded external urethra sphincter electromyogram reflex activity and analyzed protein expression in the lumbosacral (L(6)-S(2)) dorsal horn in response to intrathecal ephrinB2 injections. When compared with vehicle solution, exogenous ephrinB2 (5 μg/rat it)-induced reflex potentiation, in associated with phosphorylation of EphB1/2, Src-family kinase, NR2B Y1336 and Y1472 tyrosine residues. Both intrathecal EphB1 and EphB2 immunoglobulin fusion protein (both 10 μg/rat it) prevented ephrinB2-dependent reflex potentiation, as well as protein phosphorylation. Pretreatment with PP2 (50 μM, 10 μl it), an Src-family kinase antagonist, reversed the reflex potentiation, as well as Src kinase and NR2B phosphorylation. Together, these results suggest the ephrinB2-dependent EphBR activation, which subsequently provokes Src kinase-mediated N-methyl-d-aspartate receptor NR2B phosphorylation in the lumbosacral dorsal horn, is crucial for the induction of spinal reflex potentiation contributing to the development of visceral pain and/or hyperalgesia in the pelvic area.     

Transmitter potentiality of homonymous and heteronymous monosynaptic reflex connections of individual motoneurons

An assemblage of 110 individual tricipital motoneurons has been examined with the aim of determining those factors that predispose certain motoneurons to heteronymous response in post-tetanic potentiation. Motoneurons that respond most readily to homonymous volleys are not those that respond most readily to post-tetanically potentiated heteronymous volleys. Hence differences in presynaptic organization rather than differences in mean postsynaptic threshold determine differences in readiness for response. Every motoneuron exhibits a distinct asymmetry in transmitter potentiality of homonymous and heteronymous monosynaptic reflex connections. The range of transmitter potentialities is wide and that of heteronymous connections to some motoneurons is greater than that of homonymous connections to some other motoneurons.     

Attenuation of phrenic motor discharge by phrenic nerve afferents

Short latency phrenic motor responses to phrenic nerve stimulation were studied in anesthetized, paralyzed cats. Electrical stimulation (0.2 ms, 0.01-10 mA, 2 Hz) of the right C5 phrenic rootlet during inspiration consistently elicited a transient reduction in the phrenic motor discharge. This attenuation occurred bilaterally with an onset latency of 8-12 ms and a duration of 8-30 ms. Section of the ipsilateral C4-C6 dorsal roots abolished the response to stimulation, thereby confirming the involvement of phrenic nerve afferent activity. Stimulation of the left C5 phrenic rootlet or the right thoracic phrenic nerve usually elicited similar inhibitory responses. The difference in onset latency of responses to cervical vs. thoracic phrenic nerve stimulation indicates activation of group III afferents with a peripheral conduction velocity of approximately 10 m/s. A much shorter latency response (5 ms) was evoked ipsilaterally by thoracic phrenic nerve stimulation. Section of either the C5 or C6 dorsal root altered the ipsilateral response so that it resembled the longer latency contralateral response. The low-stimulus threshold and short latency for the ipsilateral response to thoracic phrenic nerve stimulation suggest that it involves larger diameter fibers. Decerebration, decerebellation, and transection of the dorsal columns at C2 do not abolish the inhibitory phrenic-to-phrenic reflex.     

Characteristics of tetanic and post-tetanic potentiation in the septohippocampal and hippocampal commissural systems in the acute rabbit

The frequency characteristics of tetanic and post-tetanic potentiation of the septohippocampal and hippocampal commissural systems were studied in the acute rabbit preparation. Glass micropipettes were employed to stimulate the medial septal (MSR) and contralateral CA1 (cCA1) regions. Extracellular postsynaptic potentials were recorded in the stratum radiatum and stratum oriens layers of dorsal CA1. Low frequencies of stimulation (2–12 Hz) and brief stimulus trains (7 or 16 stimuli) ensured that only short-term effects appeared in the data. With MSR and cCA1 stimulation, tetanic potentiation became pronounced at 4 Hz, and plateaued at 6–8 Hz. Thus potentiation was found to be pronounced within the range of the rabbit hippocampal theta rhythm. No differences were found in the characteristics of potentiation evoked by stimulation of MSR and cCA1. Post-tetanic potentiation lasting 6–12 sec was found. Again, potentiation characteristics did not depend on stimulus site, suggesting a common mechanism for the pathways studied. A two-factor mechanism was proposed to account for the post-tetanic potentiation data.     

A common rule governs the synaptic locus of both short-term and long-term potentiation

BACKGROUND: At synapses between neurons in the brain, transmitter molecules are released from presynaptic terminals in multi-molecular packets called quanta. Excitatory synapses in the CA1 region of the hippocampus show a long-lasting increase in strength known as long-term potentiation (LTP), which may be important for some kinds of learning and memory. LTP can involve an increase in the number of quanta released, or in the size of the response each quantum produces in the postsynaptic cell, or both, depending on the initial condition of the synapse. These synapses also show two forms of brief potentiation: post-tetanic potentiation (PTP), which lasts for a minute or less and involves only modifications at the presynaptic terminal, and short-term potentiation (STP), which lasts rather longer. The significance of STP, the mechanisms whereby it is produced and its relationship to other forms of potentiation are poorly understood. We have studied STP electrophysiologically using slices of the rat hippocampus maintained in vitro. RESULTS: We found that STP, like LTP, can involve increases in either the number of quanta released, or their postsynaptic effect, or both. The rule governing the relative contribution from these two mechanisms appears to be the same as operates during LTP. Both the presynaptic and postsynaptic changes can develop equally rapidly and so must involve fast-acting messenger systems. CONCLUSIONS: STP seems to be a separate phenomenon from PTP, but appears closely related to LTP. The rapidity of its onset may require a reappraisal of current understanding of the messenger systems involved in bringing about changes in synaptic strength.     

Duration of trace processes in the neocortex of rabbits

Changes of pyramidal tract (PT) response after short tetanization, similar to natural stimulation conditions, were analysed in unanaesthetized and nonimmobilized rabbits. PT response recording revealed a long-term (1 h and more) potentiation of monosynaptic neocortical reactions. Predominant better expressed and more preserved increase of synaptic (N) component provides evidence to the conjecture that the basic mechanism of the long-term potentiation consists in the rise of efficiency of excitatory synaptic connections. Less protracted and differently directed changes of D-component permit to consider that excitability change of neurones may be only an additional mechanism of the long-term potentiation. Such features of neocortical long-term potentiation were revealed as its low-frequency depression (at test stimuli repetition) and its spontaneous restoration after depression.     

Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Posttetanic changes in the afferent and efferent activity of monosynaptic reflex in kittens

Both the afferent volleys from the dorsal root and the monosynaptic reflex discharges from the corresponding ventral root were recorded with hook electrodes during stimulation of the nerves innervating the triceps surae muscles. The effects of conditioning high frequency tetanus on the magnitudes of these afferents and reflex volleys were examined in kittens of postnatal age 1-90 days and in adult cats. In young kittens under barbiturate anaesthesia, large-amplitude monosynaptic reflex discharge can be evoked without prior conditioning. The amplitude of this reflex discharge decreased with increasing age of the animal. Application of conditioning tetanic stimuli to the muscle nerves resulted in posttetanic depression followed by posttetanic potentiation of the monosynaptic reflex. The magnitude of posttetanic depression was much higher than that of potentiation in the first postnatal week. As the age increased, the magnitude of depression decreased while the magnitude of potentiation increased. The afferent volley showed a considerable posttetanic potentiation in older kittens and cats. No significant potentiation or depression was observed in the younger animals. Possible mechanisms contributing to posttetanic depression and potentiation are discussed.     

Transmission in fractionated monosynaptic spinal reflex systems

A study has been made of conditions that support monosynaptic reflex transmission from afferent fibers of one part of a synergic muscle mass to motoneurons of another part. Heteronymous response so called can be brought on by prior tetanization of the afferent pathway and by asphyxiation to a critical stage. The response is facilitated by cooling and may appear in the cold preparation without need for prior tetanization. By appropriate asymmetrical subdivision of a monosynaptic reflex system an afferent inflow can be obtained that is sufficiently powerful to secure heteronymous transmission without the need for prior tetanization or cooling. Each junction between a monosynaptic afferent fiber and a motoneuron possesses some degree of potentiality for transmitting. Transmitter potentiality of an afferent fiber at its several junctions with motoneurons varies widely. Reasons are advanced for supposing the variation to be graded rather than stepwise, and quantitative rather than qualitative.     

Role of substance P in post-tetanic potentiation in myenteric plexus-longitudinal muscle preparations; the effect of substance P-antagonist

A role of substance P in post-tetanic potentiation in the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum was investigated by means of substance P-antagonist, (D-Pro2, D-Trp7,9)-substance P. After the addition of substance P-antagonist (10(-7)-10(-5) mol x l-1) a dose-dependent reduction of post-tetanic potentiation of neurogenic twitches was observed. Post-tetanic potentiation was actually abolished in the presence of 10(-5) mol x l-1 substance P-antagonist. It has been shown previously by our group that post-tetanic potentiation is reduced in preparations desensitized by substance P; this, together with our present results supported the hypothesis that the release of substance P-like compounds during tetanic stimulation played a key role in the mechanism of post-tetanic potentiation.     

Effect of phencyclidine on post-tetanic twitch tension of the mouse diaphragm preparation

The effects of phencyclidine(PCP) on the post-tetanic potentiation(PTP) of twitch tension were studied on the isolated mouse phrenic nerve diaphragm preparation. Phencyclidine increased directly elicited twitch tension while it decreased post-tetanic potentiation of the indirectly elicited twitch tension. The maximal depression effect of the PTP was found after higher frequencies and longer durations of stimulation. After repetitive stimulation, the amplitude of endplate potential was potentiated. Phencyclidine decreased the post-tetanic potentiation of the amplitude of endplate potential while the quantal content of the endplate potential was not affected. 4-Aminopyridine increased both directly and indirectly elicited twitch tension while it did not inhibit the post-tetanic potentiation of the twitch tension. It is concluded that phencyclidine suppressed the post-tetanic potentiation of the indirectly elicited twitch tension. The depressant effect may be mainly due to its effect on the acetylcholine receptor-ionic channel complex of the motor endplate.