Assessment of gliosis around moveable implants in the brain

Abstract

Repositioning microelectrodes post-implantation is emerging as a promising approach to achieve long-term reliability in single neuronal recordings. The main goal of this study was to (a) assess glial reaction in response to movement of microelectrodes in the brain post-implantation and (b) determine an optimal window of time post-implantation when movement of microelectrodes within the brain would result in minimal glial reaction. Eleven Sprague-Dawley rats were implanted with two microelectrodes each that could be moved in vivo post-implantation. Three cohorts were investigated: (1) microelectrode moved at day 2 (n = 4 animals), (2) microelectrode moved at day 14 (n = 5 animals) and (3) microelectrode moved at day 28 (n = 2 animals). Histological evaluation was performed in cohorts 1-3 at four-week post-movement (30 days, 42 days and 56 days post-implantation, respectively). In addition, five control animals were implanted with microelectrodes that were not moved. Control animals were implanted for (1) 30 days (n = 1), (2) 42 days (n = 2) and (3) 56 days (n = 2) prior to histological evaluation. Quantitative assessment of glial fibrillary acidic protein (GFAP) around the tip of the microelectrodes demonstrated that GFAP levels were similar around microelectrodes moved at day 2 when compared to the 30-day controls. However, GFAP expression levels around microelectrode tips that moved at day 14 and day 28 were significantly less than those around control microelectrodes implanted for 42 and 56 days, respectively. Therefore, we conclude that moving microelectrodes after implantation is a viable strategy that does not result in any additional damage to brain tissue. Further, moving the microelectrode downwards after 14 days of implantation may actually reduce the levels of GFAP expression around the tips of the microelectrodes in the long term.

Figure 1

Illustration of the top, middle and bottom sections of the implanted microelectrode in a coronal section of a rat brain.

Figure 2

Equivalent electrical circuit model used to model the impedance. R_ex is the extracellular resistance, C_dl is the double layer capacitance, R_ct is the charge transfer resistance, and Q is the constant phase element.

Figure 3

Immunohistological images from the implant site for the movement at day-2 cohort (columns 1–6) and the 30-day control (columns 7–8). Each column is a separate implant; each row is the top (a), middle (b), or bottom (c) sections of the implant site. Scale bars are 150 μm. Arrows indicate implant sites.

Figure 4

Graphical representation of GFAP pixel intensities (mean ± std dev over two or three brain sections at each depth) around the implant site for the movement at day-2 cohort and 30-day control animals. The pixel intensities are directly related to the level of GFAP expression in the tissue, with higher pixel intensities indicating higher levels of GFAP and a more significant glial scar. There is statistically no difference between the 30-day control tissue samples and the movement at day-2 tissue samples at the top, middle or bottom of the implant.

Figure 5

Immunohistological images from the implant site for the movement at day-14 cohort (columns 1–6) and the 42-day control (columns 7–9). Each column is one separate implant; each row is the top (a), middle (b), or bottom (c) sections of the implant site. Scale bars are 150 μm. Arrows indicate implant sites.

Figure 6

Graphical representation of the GFAP pixel intensities (mean ± std dev over two or three brain sections at each depth) around the implant site for the movement at day-14 cohort and 42-day control animals. The pixel intensities are directly related to the amount of GFAP in the tissue, with higher pixel intensities indicating higher levels of GFAP and a more significant glial scar. There is no significant difference in the levels of GFAP in the top and middle sections. However, among the bottom sections there is a significant difference in GFAP levels between the movement at day-14 cohort and the 42-day control sections.

Figure 7

Immunohistological images from the implant site for the movement at day-28 cohort (columns 1–3) and the 56-day control (columns 4–6). Each column is one separate implant; each row is the top (a), middle (b) or bottom (c) sections of the implant site. Scale bars are 150 μm. Arrows indicate implant sites (two separate implant sites in each tissue section of the first column).

Figure 8

Graphical representation of the GFAP pixel intensities (mean ± std dev over two or three brain sections at each depth) around the implant site for the movement at day-28 cohort compared to 56-day control animals. The pixel intensities are directly related to the amount of GFAP in the tissue, higher pixel intensities indicate higher levels of GFAP and a more significant glial scar. There is no significant difference in the levels of GFAP in the top and middle sections. However, in the bottom sections there is a significant difference between the GFAP expression levels of the 56-day controls and the 28-day movement cohort.

Figure 9

Graphical representation of the GFAP pixel intensities (mean ± std dev) only at the electrode tip (labeled ‘bottom’). GFAP expression is lower in the brain tissue when implant is moved at days 14 or 28 when compared to the case when implant is moved at day 2. Implant movement at or after 14-day post-implantation results in lower GFAP expression at the tip of the electrode as assessed 28-day post-movement.

Figure 10

Summary of averaged (over all the animals in each cohort) pixel intensities (mean ± std dev) at each tissue depth for all of the movement animals and control animals. Each symbol represents an average of all of the pixel values within that tissue area.