Postoperative Recovery and Stability of Chronic Intracranial Multielectrode Electroencephalography Recording in Rats

Summary

This protocol outlines a standardized framework for post-surgical evaluation after chronic multi-electrode EEG implantation in rats, including sequential monitoring of signal quality, behavioral recovery, and neuroinflammatory markers to determine appropriate initiation of stable long-term recordings.

Abstract

Intracranial multi-electrode implantation enables simultaneous acquisition of neural signals across brain regions but introduces post-surgical perturbations that can affect data quality and experimental timing. This protocol describes a standardized workflow for chronic multi-site intracranial EEG electrode implantation and structured post-implantation assessment in Sprague–Dawley rats. Tungsten wire electrodes are stereotactically implanted at three predefined intracranial sites (anterior cingulate cortex, hippocampus CA1, and entorhinal cortex) using a surgical strategy designed to minimize tissue disruption.

Following implantation, the protocol specifies longitudinal evaluation across five domains: electrode localization accuracy, local field potential integrity, pain-related behavior, feeding behavior, and neuroinflammatory markers. Electrophysiological and behavioral assessments are conducted at predefined postoperative time points (baseline and postoperative days 1, 4, 7, 10, and 13) to support consistent monitoring of early post-surgical effects (n = 3). Histological analyses are used to characterize local tissue responses at the end of the study (n = 3 per time point).

Operational criteria and assessment intervals are provided to guide decisions regarding experimental readiness and initiation of long-term recordings. This protocol provides a structured approach to post-implantation monitoring and informs the appropriate timing of downstream neurophysiological experiments.

Introduction

Intracranial electroencephalography (EEG) enables high-resolution measurement of neural activity by recording signals directly from brain tissue^1,2, providing advantages in spatial specificity and signal fidelity over scalp EEG approaches³. Multiple-region intracranial recordings offer insights into network-level interactions, which are widely used to investigate neural dynamics underlying cognition and disease and are commonly implemented in rodent models for mechanistic and longitudinal studies^4,5.

Tungsten wire electrodes are commonly employed for chronic recordings because of their mechanical stability and biocompatibility^6,7,8. However, implantation requires invasive surgery that can temporarily disrupt neural physiology, tissue integrity, and animal behavior^9,10,11. Challenges such as acute surgical stress, local inflammation, glial responses, and changes in feeding or nociceptive behavior can influence signal quality and experimental interpretation during the early post-implantation period^12,13,14. However, this recovery period remains largely empirical, with reported recovery windows ranging from 5 to 7 days^15,16. Prolonged delays, conversely, may introduce additional confounds related to tissue remodeling or electrode displacement¹⁷.

Despite the widespread use of chronic intracranial EEG, there is no standardized framework for post-implantation monitoring that integrates electrophysiological, behavioral, inflammatory, and histological assessments to inform experimental timing. Therefore, the present protocol addresses this gap by outlining a structured approach for evaluating post-implantation status in a chronic intracranial multi-electrode implantation model in anesthetized Sprague–Dawley (SD) rats (6 weeks old). Using tungsten wire electrodes implanted in the anterior cingulate cortex (Cg1), hippocampus CA1, and entorhinal cortex (EC), the protocol specifies longitudinal assessment of electrode stability, EEG signal characteristics, behavioral indicators, and inflammatory markers over a two-week period. Although assessments are performed under general anesthesia for practical convenience, this approach does not affect the primary aim of establishing a framework for electrode implantation and stability. This protocol is intended to guide post-surgical monitoring and support consistent determination of appropriate time points for initiating downstream neurophysiological studies.

Protocol

All animal procedures were performed in accordance with institutional guidelines and approved by the Institutional Animal Care and Use Committee of Capital Medical University (Approval ID: AEEI-2024-391).

1. Animal preparation

1. Obtain 6-week-old male SD rats (body weight 220–230 g). House the animals in a specific pathogen-free environment (24 ± 2 °C, 50 ± 10% humidity, 12 h light/dark cycle). Provide food and water ad libitum. Acclimate the rats for 7 days prior to surgery.
2. Following electrode implantation, house rats individually under the same environmental conditions to prevent damage to the headstage.
   NOTE: The study flowchart and the corresponding measurements are presented in Figure 1A. Reagents and equipment are listed in the Table of Materials.

2. Multielectrode implantation surgery

1. Anesthetize rats with isoflurane (3–4% induction, 2% maintenance) delivered with oxygen at 2 L/min.
2. Place the rat in a stereotaxic frame and maintain body temperature at 37 °C using a thermostatically controlled heating pad. Apply erythromycin ophthalmic ointment to prevent corneal drying.
3. Shave the scalp and disinfect with 0.5% iodophor solution.
4. Perform a midline scalp incision (1 × 2 cm) to expose the skull and gently remove the periosteum using sterile cotton swabs.
5. Skull preparation and drilling
   1. Identify bregma and lambda and level the skull in both anterior–posterior and medial–lateral planes.
   2. Drill burr holes (≤2 mm diameter) at target coordinates using a low-speed dental drill with intermittent saline irrigation.
6. Electrode preparation and insertion
   1. Use polyimide-insulated tungsten wire electrodes (shank diameter: 300 µm) with an exposed tip length of 4–8 mm (flush-cut end). Measure electrode impedance (|Z|) at 1 kHz; acceptable values range from 7–10 kΩ with a phase angle ranging from 1°–2°.
   2. Slowly insert electrodes into the following regions: Cg1 (anterior cingulate cortex): AP +1.2 mm, ML −0.2 mm, DV −2.4 mm; CA1 (hippocampus): AP −3.3 mm, ML −1.8 mm, DV −3.0 mm; EC (entorhinal cortex): AP −6.7 mm, ML −4.0 mm, DV −8.0 mm.
   3. Implant one stainless-steel skull screw over the contralateral CA1 region and use it as the ground electrode.
7. Electrode fixation and closure
   1. Verify correct electrode depth and targeting using stereotaxic coordinates during implantation; confirm electrode localization post hoc by histological identification of electrode tracks.
   2. Secure electrodes and screws using dental resin (cure 30–60 s per layer).
   3. Connect electrode leads to a headstage connector and suture the surrounding skin.
8. Record baseline EEG activity for 30 min at the corresponding time point (Figure 1A) while maintaining anesthesia.

3. Postoperative care and handling

1. Allow rats to recover on a warming pad until fully ambulatory.
2. House rats under controlled temperature and humidity with soft bedding and ad libitum access to food and water to mitigate stress.
3. Inspect incision sites, electrode stability, and headstage integrity daily.
4. House animals individually, provide daily wound care (0.5% iodophor solution), and monitor welfare without pharmacological agents to avoid confounding effects, in line with previous studies^6,18.

4. Longitudinal EEG recording

1. Perform EEG recordings on days 1, 4, 7, 10, and 13 post implantation.
2. Anesthetize rats with isoflurane (3–4% induction, 1.5% maintenance).
3. Stabilize recording cables to minimize motion artifacts and electromagnetic noise.
4. Record EEG for 30 min starting at 9:00 a.m. to control for circadian variability.
5. Use the following acquisition parameters: sampling rate: 1,000 Hz; hardware band-pass filter: 0.5–40 Hz; gain: 8–10. Perform additional filtering and artifact rejection offline.
6. Acceptable EEG signals show stable baselines, minimal noise or drift, and physiologically plausible amplitude and frequency content; exclude recordings not meeting these criteria.

5. EEG signal analysis

1. Analyze EEG signals using the referenced software.
2. Visually inspect signals and remove segments with motion or electrical artifacts.
3. Compute power spectral density (PSD) using FFT (pwelch function).
4. Quantify oscillatory activity by calculating the area under the PSD curve (AUCpsd) for predefined frequency bands.
5. Calculate the signal-to-noise ratio (SNR) as the ratio of power in the target signal band (0.5–40 Hz) to the background noise band (40–80 Hz).
   NOTE: Stable signals exhibit consistent PSD profiles across consecutive recording days.

6. Pain assessment

1. Assess pain using the Rat Grimace Scale^19,20 one day before surgery (baseline) and on all recording days.
2. Use an observation chamber (25 cm × 15 cm × 15 cm) and position the camera in the upper left corner at a 45° angle.
3. Capture facial images, randomize image order, and score orbital tightening, nose/cheek flattening, whisker changes, and ear position (0–2 scale).
4. Sum scores to generate a total pain score per animal per time point.

7. Feeding behavior and body weight

1. Weigh rats and measure food intake daily. Calculate food intake by subtracting the remaining chow from preweighed food after 24 h.
2. Record rat behavior continuously for 24 h using a video recording system. Analyze feeding behavior offline by experimenters blinded to experimental conditions.
3. Monitor feeding continuously. Define a feeding bout as ≥1 event lasting 5 s–15 min²¹.
4. Calculate body weight-adjusted intake (g × 100/g body weight); mean intake per meal (g/number of meal events); and intake rate (g/total feeding duration in min).

8. Neuroinflammation assessment

1. Euthanize rats using an overdose of isoflurane followed by decapitation.
2. Collect brain tissue from implanted hemispheres at baseline and postoperative days 1, 4, 7, 10, and 13 (n = 3 per time point).
3. Western blotting
   1. Extract total protein from brain tissue and quantify protein concentration using a bicinchoninic acid (BCA) assay. Adjust all samples to a final concentration of 2 µg/µL and denature at 100 °C for 5 min.
   2. Separate proteins by 10% SDS-PAGE at 120 V for 90 min, loading 20 µg of protein per lane.
   3. Transfer proteins onto a PVDF membrane at 300 mA for 1 h.
   4. Block membranes and incubate overnight at 4 °C with primary antibodies against tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) (1:1,000).
   5. Incubate with HRP-conjugated secondary antibody (1:20,000, 2 h, room temperature).
   6. Visualize bands using ECL and image with a chemiluminescence imaging system using the automatic exposure mode.
   7. Perform densitometry using ImageJ.

9. Safety and waste disposal

1. Handle isoflurane using an active scavenging system.
2. Dispose of sharps and biological tissues in designated biohazard containers.
3. Dispose of chemical reagents according to institutional safety regulations.

Results

All 21 SD rats tolerated intracranial electrode implantation and completed the study protocol. Longitudinal measures (EEG, body weight, feeding behavior, and pain scores) were collected from the same animals across time points (n = 3), while neuroinflammatory assessments were terminal measures at each time point (n = 3 rats per day: baseline, 1, 4, 7, 10, 13). Data were assessed for normality using Shapiro-Wilk tests. Normally distributed data are expressed as mean ± SEM and compared using one-way or repeated-measures AN...

Discussion

Chronic intracranial electrode implantation for EEG monitoring in rodents remains a cornerstone technique for investigating neural dynamics at high temporal resolution. Establishing an appropriate postoperative monitoring and recovery window is critical for ensuring both signal reliability and animal welfare. This study provides a structured framework for evaluating electrophysiological stability, behavioral outcomes, and inflammatory responses following multi-electrode implantation in SD rats, offering practical guidanc...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Active isofluorane scavening system	RDW	R510-31-6
Animals	Charles River
BCA assay kit	Solarbio	PC0020
ChemiDoc Imaging System	Bio-Rad	17001401
Dental resin	Vertex	XG473P03
ECL detection reagent	Solarbio	PE0010
Erythromycin eye ointment	Renhe	1.00E+11
Goat anti-rabbit secondary antibody	Proteintech	SA00001-2
GraphPad Prism 9.0	GraphPad Software		statistical analysis software
Headstage amplifier	BIOPAC Systems Inc	EEG100C
Heating pad	YuYan technology	S-100
ImageJ	National Institutes of Health		image analysis software
IL-6 polyclonal antibody	Proteintech	21865-1-AP
Iodophore	Cleanboom	430-DFXDY100
Isoflurane	RWD	R510–31
Neuro-recording device with NeuroExplorer (v 5.012)	Plexon	OPX-A1600-128-16C	electrophysiological data analysis software
PVDF membrane	Cytiva	RPN303F
SDS-PAGE System	Bio-Rad	1658004
Stereotaxic apparatus	ZhongShi science technology	ZS-PDC
TNF-αpolyclonal antibody	Proteintech	17590-1-AP
Transfer Apparatus	Bio-Rad	1703937
Tungsten wire	A-M Systems	785500
Video recording system	Basler	acA1920
β-actin monoclonal antibody	Proteintech	2D4H5