Simultaneous Recordings of Cortical Local Field Potentials and Electrocorticograms in Response to Nociceptive Laser Stimuli from Freely Moving Rats

Summary

We developed a technique that simultaneously records both electrocorticography and local field potentials in response to nociceptive laser stimuli from freely moving rats. This technique helps establish a direct relationship of electrocortical signals at the mesoscopic and macroscopic levels, which facilitates the investigation of nociceptive information processing in the brain.

Abstract

Electrocortical responses, elicited by laser heat pulses that selectively activate nociceptive free nerve endings, are widely used in many animal and human studies to investigate the cortical processing of nociceptive information. These laser-evoked brain potentials (LEPs) consist of several transient responses that are time-locked to the onset of laser stimuli. However, the functional properties of the LEP responses are still largely unknown, due to the lack of a sampling technique that can simultaneously record neural activities at the surface of the cortex (i.e., electrocorticogram [ECoG] and scalp electroencephalogram [scalp EEG]) and inside the brain (i.e., local field potential [LFP]). To address this issue, we present here an animal protocol using freely moving rats. This protocol is composed of three main procedures: (1) animal preparation and surgical procedures, (2) a simultaneous recording of ECoG and LFP in response to nociceptive laser stimuli, and (3) data analysis and feature extraction. Specifically, with the help of a 3D-printed protective shell, both ECoG and LFP electrodes implanted on the rat's skull were securely held together. During data collection, laser pulses were delivered on the rat's forepaws through gaps in the bottom of the chamber when the animal was in spontaneous stillness. Ongoing white noise was played to avoid the activation of the auditory system by the laser-generated ultrasounds. As a consequence, only nociceptive responses were selectively recorded. Using the standard analytical procedures (e.g., band-pass filtering, epoch extraction, and baseline correction) to extract stimulus-related brain responses, we obtained results showing that LEPs with a high signal-to-noise ratio were simultaneously recorded from ECoG and LFP electrodes. This methodology makes the simultaneous recording of ECoG and LFP activities possible, which provides a bridge of electrocortical signals at the mesoscopic and macroscopic levels, thereby facilitating the investigation of nociceptive information processing in the brain.

Introduction

EEG is a technique to record electrical potentials and oscillatory brain activities generated by the synchronized activities of thousands of neurons in the brain. It is popularly used in many basic studies and clinical applications^1,2. For instance, EEG responses to intense laser heat pulses (i.e., LEPs) are widely adopted to investigate the peripheral and central processing of nociceptive sensory input^3,4,5. In humans, LEPs mainly consist of three distinct deflections: the early component (N1) that is somato....

Protocol

Adult male Sprague-Dawley rats (weighing 400 - 450 g) were used in the experiment. All surgical and experimental procedures followed the Guide for Care and Use of Laboratory Animals of the National Institutes of Health. The procedures were approved by the Research Ethics Committee at the Institute of Psychology, Chinese Academy of Sciences.

1. Electrode Implantation

1. Anesthetize the rat in a chamber with 5% isoflurane and an air flow rate of 1 L/min before the surgery.
2. Using a stereotaxic apparatus, fix the head of the rat with its nose placed into the anesthetic mask. Administrate isoflurane via the anesthetic....

Results

In the representative experiment, the electrophysiological data from five rats were recorded. The laser stimuli were delivered to the right forepaw of each rat for 20 times with >40 s interstimulus intervals. The laser-evoked brain responses were recorded using both ECoG screws and depth wires, and the depth wires were implanted in bilateral primary somatosensory cortices (S1) and primary motor cortices (M1).

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Discussion

In the present study, we described a technique to concurrently record ECoGs and cortical LFP responses elicited by nociceptive laser stimuli from freely moving rats. The results showed that LEP responses could be clearly detected after the onset of laser stimuli in both ECoG and LFP signals. The simultaneous recording of ECoG and cortical LFP signals will enable scientists to investigate their relationship for better understanding the contribution of neuronal activities to the LEP components.

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Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Male Sprague-Drawley rats	Vital River
Isoflurane	RWD Life Science
Small animal isoflurane anaesthetic system	RWD Life Science		Including the anesthesia gas mask for rats
Stereotaxic apparatus	RWD Life Science
The apparatus with combined ECoG and LFP electrodes			The apparatus is home-made, which assembles the ECoG and depth wire electrodes to a connector module
3D-printed protective shell			The texture of shell is polylactic, and the shell is home-made and contains three parts: a base, a wall and a cap. The wall is covered by copper tapers to construct as a Faraday cage
Tungsten wires (diameter: 50 mm)	California Fine Wires Company		The electrodes for cortical LFP recording
Stainless steel screws (diameter: 0.6 mm)			The electrodes for ECoG recording
Electric cranial drill	RWD Life Science
Drill bit (diameter: 0.5 mm)	RWD Life Science		The drill is used for drilling the holes of ECoG screws
Drill bit (diameter: 0.2 mm)	RWD Life Science		The drill is used for drilling the holes of depth wires
Dental arylic powder	SNC dental
Dental arylic liquid	SNC dental
Paraffin	Fisher Scientific		The mixture is used for seal the craniotomy to ensure the following movement of micro-wire arrays
Mineral Oil	Fisher Scientific
Electrocoagulator	Bovie medical Corporation
RHD2132 Amplifier Boards	Intan Technologies		A 32-channel headstage
RHD2000 systerm	Intan Technologies		The data acquisition systerm
Infrared neodymium yttrium aluminum perovskite (Nd:YAP) laser generator	Electronical Engineering
Matlab R2016b	The MathWorks