Interactions between cathodic- and anodic-pulses during high-frequency stimulations with the monophasic-pulses alternating in polarity at axons-experiment and simulation studies.

Background. Electrical neuromodulation therapies commonly utilize high-frequency stimulations (HFS) of biphasic-pulses to treat neurological disorders. The biphasic pulse consists of a leading cathodic-phase to activate neurons and a lagging anodic-phase to balance electrical charges. Because both monophasic cathodic- and anodic-pulses can depolarize neuronal membranes, splitting biphasic-pulses into alternate cathodic- and anodic-pulses could be a feasible strategy to improve stimulation efficiency.Objective. We speculated that neurons in the volume initially activated by both polarity pulses could change to be activated only by anodic-pulses during sustained HFS of alternate monophasic-pulses. To verify the hypothesis, we investigated the interactions of the monophasic pulses during HFS and revealed possible underlying mechanisms.Approach. Different types of pulse stimulations were applied at the alvear fibers (i.e. the axons of CA1 pyramidal neurons) to antidromically activate the neuronal cell bodies in the hippocampal CA1 region of anesthetized ratsin-vivo. Sequences of antidromic HFS (A-HFS) were applied with alternate monophasic-pulses or biphasic-pulses. The pulse frequency in the A-HFS sequences was 50 or 100 Hz. The A-HFS duration was 120 s. The amplitude of antidromically-evoked population spike was measured to evaluate the neuronal firing induced by each pulse. A computational model of axon was used to explore the possible mechanisms of neuronal modulations. The changes of model variables during sustained A-HFS were analyzed.Main results. In rat experiments, with a same pulse intensity, the activation volume of a cathodic-pulse was greater than that of an anodic-pulse. In paired-pulse tests, a preceding cathodic-pulse was able to prevent a following anodic-pulse from activating neurons due to refractory period. This indicated that the activation volume of a cathodic-pulse covered that of an anodic-pulse. However, during sustained A-HFS of alternate monophasic-pulses, the anodic-pulses were able to prevail over the cathodic-pulses in activating neurons in the overlapped activation volume. Model simulation results show the mechanisms of the activation failures of cathodic-pulses. They include the excessive membrane depolarization caused by an accumulation of potassium ions, the obstacle of hyperpolarization in the conduction pathway and the interactions from anodic-pulses.Significance. The study firstly showed the domination of anodic-pulses over cathodic-pulses in their competitions to activate neurons during sustained HFS. The finding provides new clues for designing HFS paradigms to improve the efficiency of neuromodulation therapies.

Adding a single pulse into high-frequency pulse stimulations can substantially alter the following episode of neuronal firing in rat hippocampus.

Background. High-frequency stimulation (HFS) sequences of electrical pulses are commonly utilized in many types of neuromodulation therapies. The temporal pattern of pulse sequences characterized by varying inter-pulse intervals (IPI) has emerged as an adjustable dimension to generate diverse effects of stimulations to meet the needs for developing the therapies.Objective:To explore the hypothesis that a simple manipulation of IPI by inserting a pulse in HFS with a constant IPI can substantially change the neuronal responses.Approach. Antidromic HFS (A-HFS) and orthodromic HFS (O-HFS) sequences were respectively applied at the alveus (the efferent axons) and the Schaffer collaterals (the afferent axons) of hippocampal CA1 region in anesthetized ratsin-vivo. The HFS sequences lasted 120 s with a pulse frequency of 100 Hz and an IPI of 10 ms. In the late steady period (60-120 s) of the HFS, additional pulses were inserted into the original pulse sequences to investigate the alterations of neuronal responses to the changes in IPI. The amplitudes and latencies of antidromic/orthodromic population spikes (APS/OPS) evoked by pulses were measured to evaluate the alterations of the evoked firing of CA1 pyramidal neurons caused by the pulse insertions.Main Results. During the steady period of A-HFS at efferent axons, the evoked APSs were suppressed due to intermittent axonal block. Under this situation, inserting a pulse to shorten an IPI was able to redistribute the following neuronal firing thereby generating an episode of oscillation in the evoked APS sequence including APSs with significantly increased and decreased amplitudes. Also, during the steady period of O-HFS without obvious OPS, a pulse insertion was able to generate a large OPS, indicating a synchronized firing of a large population of post-synaptic neurons induced by a putative redistribution of activations at the afferent axons under O-HFS.Significance. This study firstly showed that under the situation of HFS-induced axonal block, changing an IPI by a single-pulse insertion can substantially redistribute the evoked neuronal responses to increase synchronized firing of neuronal populations during both antidromic and O-HFS with a constant IPI originally. The finding provides a potential way to enhance the HFS action on neuronal networks without losing some other functions of HFS such as generating axonal block.

Activating Interneurons in Local Inhibitory Circuits by High-Frequency Stimulations at the Efferent Fibers of Pyramidal Neurons in Rat Hippocampal CA1 Region.

Stimulation-induced inhibition is one of the important effects of high-frequency stimulation (HFS) utilized by the therapy of deep brain stimulation (DBS) to treat certain neurological diseases such as epilepsy. In order to explore the stimulation sites to induce inhibition, this study investigated the activation effect of HFS of efferent fibers on the local inhibitory interneurons (IN). Antidromic HFS (A-HFS) of 100 Hz pulses was applied for 2 min at the efferent fibers-the alveus (i.e., the axons of pyramidal neurons) in the hippocampal CA1 region of anesthetized rats. Single unit spikes of INs in local feedback inhibitory circuits, as well as antidromically-evoked population spikes (APS) of pyramidal neurons, were recorded simultaneously in the CA1 region upstream of the stimulation site. Results showed that during the late 60 s of A-HFS, with a substantial suppression in APS amplitudes, the mean firing rate of INs was still significantly greater than the baseline level even when the A-HFS was applied with a weak pulse intensity of 0.08 ± 0.05 mA (9 rats). With a strong pulse intensity of 0.33 ± 0.08 mA (10 rats), the mean firing rate of INs was able to keep at a high level till the end of A-HFS. In addition, the mean latency of IN firing was significantly prolonged during the sustained A-HFS, indicating that alterations had been generated in the pathway to activate INs by the stimulations at efferent fibers. The results suggested that HFS at efferent fibers with various stimulation intensities can modulate the firing of local inhibitory neurons. The finding provides new clues for selecting stimulation sites to enhance inhibition in neural circuits by DBS.

An Anodic Phase Can Facilitate Rather Than Weaken a Cathodic Phase to Activate Neurons in Biphasic-Pulse Axonal Stimulations.

Electrical pulses have been promisingly utilized in neural stimulations to treat various diseases. Usually, charge-balanced biphasic pulses are applied in the clinic to eliminate the possible side effects caused by charge accumulations. Because of its reversal action to the preceding cathodic phase, the subsequent anodic phase has been commonly considered to lower the activation efficiency of biphasic pulses. However, an anodic pulse itself can also activate axons with its "virtual cathode" effect. Therefore, we hypothesized that the anodic phase of a biphasic pulse could facilitate neuronal activation in some circumstances. To verify the hypothesis, we compared the activation efficiencies of cathodic pulse, biphasic pulse, and anodic pulse applied in both monopolar and bipolar modes in the axonal stimulation of alveus in rat hippocampal CA1 region in vivo. The antidromically evoked population spikes (APS) were recorded and used to evaluate the amount of integrated firing of pyramidal neurons induced by pulse stimulations. We also used a computational model to investigate the pulse effects on axons at various distances from the stimulation electrode. The experimental results showed that, with a small pulse intensity, a cathodic pulse recruited more neurons to fire than a biphasic pulse. However, the situation was reversed with an increased pulse intensity. In addition, setting an inter-phase gap of 100 µs was able to increase the activation efficiency of a biphasic pulse to exceed a cathodic pulse even with a relatively small pulse intensity. Furthermore, the latency of APS evoked by a cathodic pulse was always longer than that of APS evoked by a biphasic pulse, indicating different initial sites of the neuronal firing evoked by the different types of pulses. The computational results of axon modeling showed that the subsequent anodic phase was able to relieve the hyperpolarization block in the flanking regions generated by the preceding cathodic phase, thereby increasing rather than decreasing the activation efficiency of a biphasic pulse with a relatively great intensity. These results of both rat experiments and computational modeling firstly reveal a facilitation rather than an attenuation effect of the anodic phase on biphasic-pulse stimulations, which provides important information for designing electrical stimulations for neural therapies.

Bifurcations in the Firing of Neuronal Population Caused by a Small Difference in Pulse Parameters During Sustained Stimulations in Rat Hippocampus in Vivo.

OBJECTIVE: The bifurcation of neuronal firing is one of important nonlinear phenomena in the nervous system and is characterized by a significant change in the rate or temporal pattern of neuronal firing on responding to a small disturbance from external inputs. Previous studies have reported firing bifurcations for individual neurons, not for a population of neurons. We hypothesized that the integrated firing of a neuronal population could also show a bifurcation behavior that should be important in certain situations such as deep brain stimulations. The hypothesis was verified by experiments of rat hippocampus in vivo. METHODS: Stimulation sequences of paired-pulses with two different inter-pulse-intervals (IPIs) or with two different pulse intensities were applied on the alveus of hippocampal CA1 region in anaesthetized rats. The amplitude and area of antidromic population spike (APS) were used as indices to evaluate the differences in the responses of neuronal population to the different pulses in stimulations. RESULTS: During sustained paired-pulse stimulations with a high mean pulse frequency such as ∼130 Hz, a small difference of only a few percent in the two IPIs or in the two intensities was able to generate a sequence of evoked APSs with a substantial bifurcation in their amplitudes and areas. CONCLUSION: Small differences in the excitatory inputs can cause nonlinearly enlarged differences in the induced firing of neuronal populations. SIGNIFICANCE: The novel dynamics and bifurcation of neuronal responses to electrical stimulations provide important clues for developing new paradigms to extend neural stimulations to treat more diseases.

Cathodic- and anodic-pulses can alternately activate different sub-populations of neurons during sustained high-frequency stimulation of axons in rat hippocampus.

Objective.Charge-balanced biphasic-pulses are commonly used in neural stimulations to prevent possible damages caused by charge accumulations. The lagging anodic-phases of biphasic-pulses may decrease the activation efficiency of stimulations by counteracting the depolarization effect of the leading cathodic-phases. However, a monophasic anodic-pulse alone can itself activate neurons by depolarizing neuronal membrane through a mechanism of virtual cathode. This study aimed to verify the hypothesis that the anodic-phases/pulses in charge-balanced stimulations could play an activation role during sustained high-frequency stimulations (HFSs).Approach.Two types of antidromic HFS (A-HFS) were applied on the alveus of hippocampal CA1 region of anesthetized rats: monophasic-pulse A-HFS of alternate opposite pulses and biphasic-pulse A-HFS with the same frequency of 100 or 200 Hz. The antidromically-evoked population spike was used as a biomarker to evaluate the activation effects of A-HFS pulses.Main results.Despite a significant difference in the initial abilities of anodic- and cathodic-pulses to activate neurons, an anodic-pulse was able to induce similar amount of neuronal firing as a cathodic-pulse during sustained monophasic-pulse A-HFS. Additionally, the amount of neuronal firing induced by the monophasic-pulse A-HFS was similar to that induced by the biphasic-pulse A-HFS consuming a double amount of electrical energy. Furthermore, the alternate cathodic- and anodic-pulses respectively activated different sub-populations of neurons during steady A-HFS.Significance.The anodic-phases/pulses in charge-balanced HFS at axons can play an activation role in addition to a role of charge balance. The study provides important information for designing charge-balanced stimulations and reveals new mechanisms of neural stimulations.

Adjust Neuronal Reactions to Pulses of High-Frequency Stimulation with Designed Inter-Pulse-Intervals in Rat Hippocampus In Vivo.

Sequences of electrical pulses have been applied in the brain to treat certain disorders. In recent years, altering inter-pulse-interval (IPI) regularly or irregularly in real time has emerged as a promising way to modulate the stimulation effects. However, algorithms to design IPI sequences are lacking. This study proposed a novel strategy to design pulse sequences with varying IPI based on immediate neuronal reactions. Firstly, to establish the correlationship between the neuronal reactions with varying IPIs, high-frequency stimulations with varying IPI in the range of 5-10 ms were applied at the alveus of the hippocampal CA1 region of anesthetized rats in vivo. Antidromically-evoked population spikes (APS) following each IPI were recorded and used as a biomarker to evaluate neuronal reactions to each pulse. A linear mapping model was established to estimate the varied APS amplitudes by the two preceding IPIs. Secondly, the mapping model was used to derive an algorithm for designing an IPI sequence that would be applied for generating a desired neuronal reaction pre-defined by a particular APS distribution. Finally, examples of stimulations with different IPI sequences designed by the algorithm were verified by rat experiments. The results showed that the designed IPI sequences were able to reproduce the desired APS responses of different distributions in the hippocampal stimulations. The novel algorithm of IPI design provides a potential way to obtain various stimulation effects for brain stimulation therapies.

Different effects of monophasic pulses and biphasic pulses applied by a bipolar stimulation electrode in the rat hippocampal CA1 region.

BACKGROUND: Electrical pulse stimulations have been applied in brain for treating certain diseases such as movement disorders. High-frequency stimulations (HFS) of biphasic pulses have been used in clinic stimulations, such as deep brain stimulation (DBS), to minimize the risk of tissue damages caused by the electrical stimulations. However, HFS sequences of monophasic pulses have often been used in animal experiments for studying neuronal responses to the stimulations. It is not clear yet what the differences of the neuronal responses to the HFS of monophasic pulses from the HFS of biphasic pulses are. METHODS: To investigate the neuronal responses to the two types of pulses, orthodromic-HFS (O-HFS) and antidromic-HFS (A-HFS) of biphasic and monophasic pulses (1-min) were delivered by bipolar electrodes, respectively, to the Schaffer collaterals (i.e., afferent fibers) and the alveus fibers (i.e., efferent fibers) of the rat hippocampal CA1 region in vivo. Evoked population spikes of CA1 pyramidal neurons to the HFSs were recorded in the CA1 region. In addition, single pulses of antidromic- and orthodromic-test stimuli were applied before and after HFSs to evaluate the baseline and the recovery of neuronal activity, respectively. RESULTS: Spreading depression (SD) appeared during sequences of 200-Hz monophasic O-HFS with a high incidence (4/5), but did not appear during corresponding 200-Hz biphasic O-HFS (0/6). A preceding burst of population spikes appeared before the SD waveforms. Then, the SD propagated slowly, silenced neuronal firing temporarily and resulted in partial recovery of orthodromically evoked population spikes (OPS) after the end of O-HFS. No SD events appeared during the O-HFS with a lower frequency of 100 Hz of monophasic or biphasic pulses (0/5 and 0/6, respectively), neither during the A-HFS of 200-Hz pulses (0/9). The antidromically evoked population spikes (APS) after 200-Hz biphasic A-HFS recovered to baseline level within ~ 2 min. However, the APS only recovered partially after the 200-Hz A-HFS of monophasic pulses. CONCLUSIONS: The O-HFS with a higher frequency of monophasic pulses can induce the abnormal neuron activity of SD and the A-HFS of monophasic pulses can cause a persisting attenuation of neuronal excitability, indicating neuronal damages caused by monophasic stimulations in brain tissues. The results provide guidance for proper stimulation protocols in clinic and animal experiments.

Suppressing synchronous firing of epileptiform activity by high-frequency stimulation of afferent fibers in rat hippocampus.

AIMS: Deep brain stimulation (DBS) is a promising technology for treating epilepsy. However, the efficacy and underlying mechanisms of the high-frequency stimulation (HFS) utilized by DBS to suppress epilepsy remain uncertain. Previous studies have shown that HFS can desynchronize the firing of neurons. In this study, we investigated whether the desynchronization effects of HFS can suppress epileptiform events. METHODS: HFS trains with seconds of duration (short) and a minute of duration (long) were applied at the afferent fibers (ie, Schaffer collaterals) of the hippocampal CA1 region in anesthetized rats in vivo. The amplitude and the rate of population spikes (PS) appeared in the downstream of stimulation were calculated to evaluate the intensity of synchronized firing of neuronal populations between short and long HFS groups. A test of paired-pulse depression (PPD) was used to assess the alteration of inhibitory neuronal circuits. RESULTS: The sustained stimulation of a 60-s long HFS suppressed the afterdischarges that were induced by a 5-s short HFS to impair the local inhibitions. During the sustained HFS, the mean PS amplitude reduced significantly and the burst firing decreased, while the amount of neuronal firing did not change significantly. The paired-pulse tests showed that with a similar baseline level of small PS2/PS1 ratio indicating a strong PPD, the 5-s HFS increased the PS2/PS1 ratio to a value that was significantly greater than the corresponding ratio during sustained HFS, indicating that the PPD impaired by a short HFS may be restored by a sustained HFS. CONCLUSIONS: The sustained HFS can desynchronize the population firing of epileptiform activity and accelerate a recovery of inhibitions to create a balance between the excitation and the inhibition of local neuronal circuits. The study provides new clues for further understanding the mechanism of DBS and for advancing the clinical application of DBS in treating epilepsy.

Sinusoidal stimulation on afferent fibers modulates the firing pattern of downstream neurons in rat hippocampus.

Electrical stimulation in the brain is an emerging therapy for treating a wide range of neurological disorders. Although electrical pulses are commonly used in the clinic, other electrical waveforms such as sinusoidal-waves have been investigated to improve the therapeutic efficacy, to reduce the risk of tissue damage induced by stimulation, and to decrease the consumption of electrical energy. However, the effects of sinusoidal stimulation on neuronal activity are still unclear. In the present study, we investigated the neuronal responses to the stimulation of 50-Hz sinusoidal-waves applied on the afferent fibers of the neurons in the hippocampal CA1 region of Sprague-Dawley rat in vivo. Results show that the stimulation increased the firing rate of both pyramidal neurons and interneurons in the downstream region of stimulation. Also, the stimulation eliminated the original theta rhythms (2-5 Hz) in the single-unit activity of the two types of neurons and entrained these neurons to fire at the stimulation rhythm. These results provide new clues for the mechanisms of brain stimulation to suppress the pathological rhythms in the neuronal activity, and for the application of sinusoidal waveforms in brain stimulation therapy.

The Appearance Order of Varying Intervals Introduces Extra Modulation Effects on Neuronal Firing Through Non-linear Dynamics of Sodium Channels During High-Frequency Stimulations.

Electrical pulse stimulation in the brain has shown success in treating several brain disorders with constant pulse frequency or constant inter-pulse interval (IPI). Varying IPI may offer a variety of novel stimulation paradigms and may extend the clinical applications. However, a lack of understanding of neuronal responses to varying IPI limits its informed applications. In this study, to investigate the effects of varying IPI, we performed both rat experiments and computational modeling by applying high-frequency stimulation (HFS) to efferent axon fibers of hippocampal pyramidal cells. Antidromically evoked population spikes (PSs) were used to evaluate the neuronal responses to pulse stimulations with different IPI patterns including constant IPI, gradually varying IPI, and randomly varying IPI. All the varying IPI sequences were uniformly distributed in the same interval range of 10 to 5 ms (i.e., 100 to 200 Hz). The experimental results showed that the mean correlation coefficient of PS amplitudes to the lengths of preceding IPI during HFS with random IPI (0.72 ± 0.04, n = 7 rats) was significantly smaller than the corresponding correlation coefficient during HFS with gradual IPI (0.92 ± 0.03, n = 7 rats, P < 0.001, t-test). The PS amplitudes induced by the random IPI covered a wider range, over twice as much as that induced by the gradual IPI, indicating additional effects induced by merely changing the appearance order of IPI. The computational modeling reproduced these experimental results and provided insights into these modulatory effects through the mechanism of non-linear dynamics of sodium channels and potassium accumulation in the narrow peri-axonal space. The simulation results showed that the HFS-induced increase of extracellular potassium ([K+] o ) elevated the membrane potential of axons, delayed the recovery course of sodium channels that were repeatedly activated and inactivated during HFS, and resulted in intermittent neuronal firing. Because of non-linear membrane dynamics, random IPI recruited more neurons to fire together following specific sub-sequences of pulses than gradual IPI, thereby widening the range of PS amplitudes. In conclusion, the study demonstrated novel HFS effects of neuronal modulation induced by merely changing the appearance order of the same group of IPI of pulses, which may inform the development of new stimulation patterns to meet different demands for treating various brain diseases.

Selective modulation of neuronal firing by pulse stimulations with different frequencies in rat hippocampus.

BACKGROUND: Deep brain stimulation (DBS) has a good prospect for treating many brain diseases. Recent studies have shown that axonal activation induced by pulse stimulations may play an important role in DBS therapies through wide projections of axonal fibers. However, it is undetermined whether the downstream neurons are inhibited or excited by axonal stimulation. The present study addressed the question in rat hippocampus by in vivo experiments. METHODS: Pulse stimulations with different frequencies (10-400 Hz) were applied to the Schaffer collateral, the afferent fiber of hippocampal CA1 region in anaesthetized rats. Single-unit spikes of interneurons and pyramidal cells in the downstream region of stimulation were recorded and evaluated. RESULTS: Stimulations with a lower frequency (10 or 20 Hz) did not change the firing rates of interneurons but decreased the firing rates of pyramidal cells (the principal neurons) significantly. The phase-locked firing of interneurons during these stimulations might increase the efficacy of GABAergic inhibitions on the principal neurons. However, stimulations with a higher frequency (100-400 Hz) increased the firing rates of both types of the neurons significantly. In addition, the increases of interneurons' firing were greater than the increases of pyramidal cells. Presumably, increase of direct excitation from afferent impulses together with failure of GABAergic inhibition might result in the increase of pyramidal cells' firing by a higher stimulation frequency. Furthermore, silent periods appeared immediately following the cessation of stimulations, indicating a full control of the neuronal firing by the stimulation pulses during axonal stimulation. Furthermore longer silent periods were associated with higher stimulation frequencies. CONCLUSIONS: Low-frequency (10-20 Hz) and high-frequency (100-400 Hz) stimulations of afferent axonal fibers exerted opposite effects on principal neurons in rat hippocampus CA1. These results provide new information for advancing deep brain stimulation to treat different brain disorders.

Simulation Study of Intermittent Responses of Neuronal Populations to Axonal High-Frequency Stimulation.

Deep brain stimulation (DBS) have shown a promising future for treating various brain disorders. Studies have indicated that the high frequency stimulation (HFS) used in DBS could cause a partial block in axons thereby attenuating the responses of axon fibers to the pulses of HFS. The attenuated response of axons might play a desynchronization role in modulating activity of neuronal populations. To investigate the detail behavior of individual axons under HFS, we created a computational model of neuronal populations including 1250 neurons. Each neuron consisted of a myelinated axon, an axonal initial segment, a soma and dendrites. A 10-s HFS sequence with 100 Hz pulses was applied to the axon layer by a bipolar stimulation electrode. The membrane potentials and the extracellular potassium concentration [K+]o at axons and at somata during the stimulation were investigated. The results showed that the simulation model with a mechanism of potassium accumulation could reproduce the attenuated responses of neuronal populations to persistent axonal HFS in rat experiments. The elevation of [K+]o during HFS resulted in an increase of basic membrane potentials and then generated a depolarization block in the axonal membrane thereby attenuating the responses of neuronal populations. The depolarization block in axons included both complete block (~26%) and intermittent block (~74%), which generated desynchronized firing among axons in fibers and travelled to the cell bodies to induce desynchronized firing in somata. The simulation results may provide important information for revealing the modulation mechanisms of axonal HFS in the therapy of brain stimulation.

Frequency-Dependent Inhibition Induced by Stimulations in Rat Hippocampus.

Deep brain stimulation (DBS) has a good prospect in treating brain diseases such as epilepsy. However, its therapeutic mechanism is undetermined yet. To investigate the inhibitory effects of DBS acting on the downstream neurons in target region, 1-min pulse trains of orthodromic stimulations with various frequencies (10 - 100 Hz) were applied to the Schaffer collaterals of hippocampal CA1 region in anaesthetized rats. Unit spikes of downstream interneurons and pyramidal cells were detected and analyzed. Interneurons activated by the stimulation through mono-synaptic connections were recognized based on short latencies. Their firing was used as an index of stimulation-induced inhibition. Results showed that during stimulations with pulse frequency of 20 Hz, the mean firing rate of mono-synaptically-activated interneurons was significantly higher than the value during 10 Hz stimulations. Afterward, even if the pulse frequency increased to 100 Hz, the mean firing rate of the interneurons did not change significantly. However, only during stimulations with lower frequencies (10, 20 Hz), the firing of pyramidal cells was suppressed by the stimulation-activated interneurons. The inhibitory effects of interneurons were weakened during stimulations with higher frequencies (50 and 100 Hz). De-synchronous firing of interneurons induced by high-frequency stimulation might cause the loss of effective inhibition on the principal neurons. These findings provide new information for advancing the application of DBS.