Dynamics of neuronal firing modulated by high-frequency electrical pulse stimulations at axons in rat hippocampus

Objective. The development of electrical pulse stimulations in brain, including deep brain stimulation, is promising for treating various brain diseases. However, the mechanisms of brain stimulations are not yet fully understood. Previous studies have shown that the commonly used high-frequency stimulation (HFS) can increase the firing of neurons and modulate the pattern of neuronal firing. Because the generation of neuronal firing in brain is a nonlinear process, investigating the characteristics of nonlinear dynamics induced by HFS could be helpful to reveal more mechanisms of brain stimulations. The aim of present study is to investigate the fractal properties in the neuronal firing generated by HFS.Approach. HFS pulse sequences with a constant frequency 100 Hz were applied in the afferent fiber tracts of rat hippocampal CA1 region. Unit spikes of both the pyramidal cells and the interneurons in the downstream area of stimulations were recorded. Two fractal indexes-the Fano factor and Hurst exponent were calculated to evaluate the changes of long-range temporal correlations (LRTCs), a typical characteristic of fractal process, in spike sequences of neuronal firing.Mainresults. Neuronal firing at both baseline and during HFS exhibited LRTCs over multiple time scales. In addition, the LRTCs significantly increased during HFS, which was confirmed by simulation data of both randomly shuffled sequences and surrogate sequences.Conclusion. The purely periodic stimulation of HFS pulses, a non-fractal process without LRTCs, can increase rather than decrease the LRTCs in neuronal firing.Significance. The finding provides new nonlinear mechanisms of brain stimulation and suggests that LRTCs could be a new biomarker to evaluate the nonlinear effects of HFS.

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.

Suppression of Neuronal Firing Following Antidromic High-Frequency Stimulations on the Neuronal Axons in Rat Hippocampal CA1 Region

High-frequency stimulation (HFS) of electrical pulses has been used to treat certain neurological diseases in brain with commonly utilized effects within stimulation periods. Post-stimulation effects after the end of HFS may also have functions but are lack of attention. To investigate the post-stimulation effects of HFS, we performed experiments in the rat hippocampal CA1 region in vivo. Sequences of 1-min antidromic-HFS (A-HFS) were applied at the alveus fibers. To evaluate the excitability of the neurons, separated orthodromic-tests (O-test) of paired pulses were applied at the Schaffer collaterals in the period of baseline, during late period of A-HFS, and following A-HFS. The evoked potentials of A-HFS pulses and O-test pulses were recorded at the stratum pyramidale and the stratum radiatum of CA1 region by an electrode array. The results showed that the antidromic population spikes (APS) evoked by the A-HFS pulses persisted through the entire 1-min period of 100 Hz A-HFS, though the APS amplitudes decreased significantly from the initial value of 9.9 ± 3.3 mV to the end value of 1.6 ± 0.60 mV. However, following the cessation of A-HFS, a silent period without neuronal firing appeared before the firing gradually recovered to the baseline level. The mean lengths of both silent period and recovery period of pyramidal cells (21.9 ± 22.9 and 172.8 ± 91.6 s) were significantly longer than those of interneurons (11.2 ± 8.9 and 45.6 ± 35.9 s). Furthermore, the orthodromic population spikes (OPS) and the field excitatory postsynaptic potentials (fEPSP) evoked by O-tests at ∼15 s following A-HFS decreased significantly, indicating the excitability of pyramidal cells decreased. In addition, when the pulse frequency of A-HFS was increased to 200, 400, and 800 Hz, the suppression of neuronal activity following A-HFS decreased rather than increased. These results indicated that the neurons with axons directly under HFS can generate a post-stimulation suppression of their excitability that may be due to an antidromic invasion of axonal A-HFS to somata and dendrites. The finding provides new clues to utilize post-stimulation effects generated in the intervals to design intermittent stimulations, such as closed-loop or adaptive stimulations.

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.

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.

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.

Small Changes in Inter-Pulse-Intervals Can Cause Synchronized Neuronal Firing During High-Frequency Stimulations in Rat Hippocampus

Deep brain stimulation (DBS) traditionally utilizes electrical pulse sequences with a constant frequency, i.e., constant inter-pulse-interval (IPI), to treat certain brain disorders in clinic. Stimulation sequences with varying frequency have been investigated recently to improve the efficacy of existing DBS therapy and to develop new treatments. However, the effects of such sequences are inconclusive. The present study tests the hypothesis that stimulations with varying IPI can generate neuronal activity markedly different from the activity induced by stimulations with constant IPI. And, the crucial factor causing the distinction is the relative differences in IPI lengths rather than the absolute lengths of IPI nor the average lengths of IPI. In rat experiments in vivo, responses of neuronal populations to applied stimulation sequences were collected during stimulations with both constant IPI (control) and random IPI. The stimulations were applied in the efferent fibers antidromically (in alveus) or in the afferent fibers orthodromically (in Schaffer collaterals) of pyramidal cells, the principal cells of hippocampal CA1 region. Amplitudes and areas of population spike (PS) waveforms were used to evaluate the neuronal responses induced by different stimulation paradigms. During the periods of both antidromic and orthodromic high-frequency stimulation (HFS), the HFS with random IPI induced synchronous neuronal firing with large PS even if the lengths of random IPI were limited to a small range of 5-10 ms, corresponding to a frequency range 100-200 Hz. The large PS events did not appear during control stimulations with a constant frequency at 100, 200, or 130 Hz (i.e., the mean frequency of HFS with random IPI uniformly distributed within 5-10 ms). Presumably, nonlinear dynamics in neuronal responses to random IPI might cause the generation of synchronous firing under the situation without any long pauses in HFS sequences. The results indicate that stimulations with random IPI can generate salient impulses to brain tissues and modulate the synchronization of neuronal activity, thereby providing potential stimulation paradigms for extending DBS therapy in treating more brain diseases, such as disorders of consciousness and vegetative states.

Synchronous Responses of Population Neurons to the Changes of Inter-Pulse-Intervals during Stimulations of Afferent Fibers

Deep brain stimulation (DBS) has been used to treat many brain disorders. Studies have shown that in DBS therapies, high frequency stimulation (HFS) with a constant pulse frequency over ~90 Hz can obtain better efficacy than stimulations with irregular inter-pulse-interval (IPI). The reasons are not clear yet. We hypothesized that irregular IPI might cause synchronous firing in target neurons thereby weakening the DBS efficacy. To test this hypothesis, stimulation trains of orthodromic-HFS (O-HFS) with different IPI were applied on the Schaffer collaterals, i.e., the afferent fiber tracts of the hippocampal CA1 region in anaesthetized rats. The amplitude of evoked population spikes (PS) in the downstream region was used as an electrophysiological index to evaluate the synchronicity of neuronal firing. The results showed that 100 Hz O-HFS with constant IPI induced de-synchronized firing of downstream neurons without PS events, whereas O-HFS with sparse prolonged IPI (20 or 100 ms) or with irregular IPI (1.7 - 50 ms) generated large PS events. Presumably, the longer IPI in O-HFS trains might provide adequate time to allow axons to recover from HFS-induced block and to respond the next coming pulse, synchronously. Therefore, following longer IPI, the population neurons in the target region could receive synchronous impulses from a lot of axonal fibers thereby generating action potentials synchronously. These findings are important for revealing new underlying mechanisms of DBS and for advancing the application of DBS.

High frequency stimulation of afferent fibers generates asynchronous firing in the downstream neurons in hippocampus through partial block of axonal conduction

Deep brain stimulation (DBS) is effective for treating neurological disorders in clinic. However, the therapeutic mechanisms of high-frequency stimulation (HFS) of DBS have not yet been elucidated. Previous studies have suggested that HFS-induced changes in axon conduction could have important contributions to the DBS effects and desiderate further studies. To investigate the effects of prolonged HFS of afferent axons on the firing of downstream neurons, HFS trains of 100 and 200Hz were applied on the Schaffer collaterals of the hippocampal CA1 region in anaesthetized rats. Single unit activity of putative pyramidal cells and interneurons in the downstream region was analyzed during the late periods of prolonged HFS when the axonal conduction was blocked. The results show that the firing rates of both pyramidal cells and interneurons increased rather than decreased during the period of axon block. However, the firing rates were far smaller than the stimulation frequency of HFS. In addition, the firing pattern of pyramidal cells changed from typical bursts during baseline recordings into regular single spikes during HFS periods. Furthermore, the HFS produced asynchronous firing in the downstream neurons in contrast to the synchronous firing induced by single pulses. Presumably, the HFS-induced block of axonal conduction was not complete. During the period of partial block, individual axons could recover intermittently and independently, and drive the downstream neurons to fire in an asynchronous pattern. This axonal mechanism of HFS provides a novel explanation for how DBS could replace an original pattern of neuronal activity by a HFS-modulated asynchronous firing in the target region thereby generating the therapeutic effects of DBS.