Use of bipolar parallel electrodes for well-controlled microstimulation in a mouse hippocampal brain slice

Deep-brain electrical microstimulation is an effective tool to explore functional characteristics of somatosensory neurons in the rat brain

In neurophysiology researches, peripheral stimulation is used along with recordings of neural activities to study the processing of somatosensory signals in the brain. However, limited precision of peripheral stimulation makes it difficult to activate the neuron with millisecond resolution and study its functional properties in this scale. Also, tissue/receptor damage that could occur in some experiments often limits the amount of responses that can be recorded and hence reduces data reproducibility. To overcome these limitations, electrical microstimulation (ES) of the brain could be used to directly and more precisely evoke neural responses. For this purpose, a deep-brain ES protocol for rat somatosensory relay neurons was developed in this study. Three male Wistar rats were used in the experiment. The ES was applied to the thalamic region responsive to hindpaw tactile stimulation (TS) via a theta glass microelectrode. The resulting ES-evoked cortical responses showed action potentials and thalamocortical relay latencies very similar to those evoked by TS. This result shows that the developed deep-brain ES protocol is an effective tool to bypass peripheral tissue for in vivo functional analysis of specific types of somatosensory neurons. This protocol could be readily applied in researches of nociception and other somatosensory systems to allow more extensive exploration of the neural functional networks.

Contribution of NMDA receptor-mediated component to the EPSP in mouse Schaffer collateral synapses under single pulse stimulation protocol

The degree to which NMDA receptors contribute to hippocampal CA(1) stratum radiatum excitatory postsynaptic potentials (EPSP) is a matter of debate. This experiment was designed to resolve the issue by documenting and positively identifying the elements of the NMDA dependent component in the extracellularly recorded stratum radiatum CA(1) field potential under low stimulation conditions and in the presence of physiologic levels of Mg(2+). We show that EPSP generation consists of activation of both AMPA and NMDA receptor channels, which mediate distinct components of the recorded field potential. We propose that the EPSP is a combination of two waves rather than one, which sometimes has been attributed to the exclusive activation of AMPA channels. Our data suggest that the three recorded peaks signify different events. The first peak reflects the presynaptic volley while the other two represent the actual EPSP. The first peak of the EPSP is determined mainly by flow of ions through AMPA channels. The second peak most likely is determined by the concurrence of two phenomena: ionic flow through NMDA channels and the source corresponding to the sink generated at the cell bodies in the pyramidal layer. The NMDA dependent component was recorded when Mg(2+) was present in physiological concentrations. The presynaptic volley and second peak do not saturate over a 10-fold increase of the stimulation charge and their amplitudes are highly correlated. The first peak amplitude rapidly saturates. The sensitivity of the recorded signals is different, the first peak being the most sensitive (1.25-0.26 mV/nC). Isolation of NMDA dependent components under physiological conditions when using a single pulse low stimulation protocol would allow more precise investigations of the NMDA dependent forms of synaptic plasticity.

Dipole distance for minimum threshold current to stimulate unmyelinated axons with microelectrodes

Excitation thresholds for long nerve or muscle fibers with two point sources parallel to the fiber axis depend on the dipole length. The aim of this study was to find the optimal interelectrode distance for the minimum stimulation current. For a specific electrode-fiber distance (z_el) dipole length is constrained by the energy efficacy of the electrodes requiring small interelectrode distances, and by rather low stimulation currents requiring large dipole distances. Far-field values for optimal dipole distance (approximately 1.4 *z_el) can be explained by the superposition of the positive parts of the activating functions for the monopolar elements of the dipole. A current redistribution effect in a target fiber close to the electrodes shifts the dipole length for threshold stimulation from the theoretical optimal activating function approach value towards greater dipole distances. Spike initiations in straight fibers and retinal ganglion cell axons are investigated.

A new fast electrophysiological response measured extracellularly in a mouse hippocampal brain slice

In a hippocampal mouse brain slice the CA1 stratum radiatum field potential evoked by electrical stimulation of the Schaffer collaterals was used to analyze the very early responses. Parallel bipolar electrodes were utilized to lower the voltage induced by the stimulating current in the recording electrode, and to keep the induced voltage within the linear range of the headstage preamplifier and the recording amplifiers. When the stimulating and recording electrodes are in saline in the same plane and same depth, the shapes of the recorded voltage and stimulating current are similar. The shape of the recorded voltage significantly changes when the recording electrode is in the stratum radiatum of CA1. The signal recorded in the first 2 ms, before the presynaptic volley, indicates the existence of a fast response shown as a positive deflection. For times slightly longer than the stimulating pulse width, the fast response overlaps with the voltage induced by the discharging of the polarization charge accumulated in the double layer capacitance at the interface between metal and saline. For times longer than about 1.4 ms the fast response and presynaptic volley are superimposed. The fast response is about 0.5 mV when the stimulating charge is 15-25 nC. The demonstration that the fast response disappears upon bath perfusion of tetrodotoxin is convincing support for its axonal origin. We postulate that this fast response represents the current source corresponding to the Schaffer collaterals excitatory sink. An undistorted recording of the first 2 ms of the sweep gives useful information about the fast neuronal activity measured extracellularly, and cannot be regarded as a simple stimulus artifact.