Sinusoidal stimulation trains suppress epileptiform spikes induced by 4-AP in the rat hippocampal CA1 region in-vivo

Nonlinear analysis of neuronal firing modulated by sinusoidal stimulation at axons in rat hippocampus

Introduction

Electrical stimulation of the brain has shown promising prospects in treating various brain diseases. Although biphasic pulse stimulation remains the predominant clinical approach, there has been increasing interest in exploring alternative stimulation waveforms, such as sinusoidal stimulation, to improve the effectiveness of brain stimulation and to expand its application to a wider range of brain disorders. Despite this growing attention, the effects of sinusoidal stimulation on neurons, especially on their nonlinear firing characteristics, remains unclear.

Methods

To address the question, 50 Hz sinusoidal stimulation was applied on Schaffer collaterals of the rat hippocampal CA1 region in vivo. Single unit activity of both pyramidal cells and interneurons in the downstream CA1 region was recorded and analyzed. Two fractal indexes, namely the Fano factor and Hurst exponent, were used to evaluate changes in the long-range correlations, a manifestation of nonlinear dynamics, in spike sequences of neuronal firing.

Results

The results demonstrate that sinusoidal electrical stimulation increased the firing rates of both pyramidal cells and interneurons, as well as altered their firing to stimulation-related patterns. Importantly, the sinusoidal stimulation increased, rather than decreased the scaling exponents of both Fano factor and Hurst exponent, indicating an increase in the long-range correlations of both pyramidal cells and interneurons.

Discussion

The results firstly reported that periodic sinusoidal stimulation without long-range correlations can increase the long-range correlations of neurons in the downstream post-synaptic area. These results provide new nonlinear mechanisms of brain sinusoidal stimulation and facilitate the development of new stimulation modes.

Sinusoidal stimulation on afferent fibers can selectively activate different types of neurons in rat hippocampus

Deep brain stimulation (DBS) is a promising therapy for treating various brain disorders. Although narrow electrical pulses have been commonly used by DBS, sinusoidal waveforms have also been investigated to improve the effects of DBS therapy and to save electrical energy. However, the effect of sinusoidal stimulation on neurons is unclear yet. To investigate the modulation of sinusoidal stimulation on different types of neurons in networks, sinusoidal stimulations (50 Hz) with lower-intensity and higher-intensity were applied to the afferent axons (Schaffer collaterals) in rat hippocampal CA1 region. The firing of inhibitory interneurons and excitatory pyramidal cells (the principal neurons of CA1) during the stimulations were detected and were compared with their baseline firing before stimulations. Results showed that sinusoidal stimulation with a lower-intensity (~30 μA) can selectively activate the interneurons thereby suppressing the firing of pyramidal cells in the downstream post-synaptic region. However, sinusoidal stimulation with a higher-intensity (~60 μA) can increase the firing of both types of neurons significantly. Presumably, the two different effects of inhibition and excitation on the principal neurons by different stimulation intensities could be caused by the fact that the firing threshold of interneurons is lower than that of pyramidal cells. The results provide important clues for selective modulation of neuronal activity by brain stimulations thereby developing different stimulation paradigms to treat various brain disorders.

Characterizing Concentration-Dependent Neural Dynamics of 4-Aminopyridine-Induced Epileptiform Activity

Epilepsy remains one of the most common neurological disorders. In patients, it is characterized by unprovoked, spontaneous, and recurrent seizures or ictal events. Typically, inter-ictal events or large bouts of population level activity can be measured between seizures and are generally asymptomatic. Decades of research have focused on understanding the mechanisms leading to the development of seizure-like activity using various pro-convulsive pharmacological agents, including 4-aimnopyridine (4AP).

However, the lack of consistency in the concentrations used for studying 4AP-induced epileptiform activity in animal models may give rise to differences in results and interpretation thereof. Indeed, the range of 4AP concentration in both in vivo and in vitro studies varies from 3 μM to 40 mM. Here, we explored the effects of various 4AP concentrations on the development and characteristics of hippocampal epileptiform activity in acute mouse brain slices of either sex. Using multi-electrode array recordings, we show that 4AP induces hippocampal epileptiform activity for a broad range of concentrations.

The frequency component and the spatiotemporal patterns of the epileptiform activity revealed a dose-dependent response. Finally, in the presence of 4AP, reduction of KCC2 co-transporter activity by KCC2 antagonist VU0240551 prevented the manifestation of the frequency component differences between different concentrations of 4AP. Overall, the study predicts that different concentrations of 4AP can result in the different mechanisms behind hippocampal epileptiform activity, of which some are dependent on the KCC2 co-transporter function.