A bundled microwire array for long-term chronic single-unit recording in deep brain regions of behaving rats
Highlights
► A bundled microwire array for chronic recording in the deep brain of behaving rats. ► The recording stability was assessed using spike shape and receptive field. ► Stable trigeminal ganglion and thalamic single-units were recorded up to 80 days.
Introduction
Monitoring the change of activity of the same neuron for an extended period of time is needed for probing many physiological and pathological questions. For example, Herry et al. (2008) recorded the same amygdala neurons for 11 days in series of behavior tasks to test the role of single amygdaloid neuron in context-specific conditioning and extinction. In the field of long-term memory and chronic diseases, an even longer monitoring time is required.
Many types of electrodes, such as movable electrodes (Eliades and Wang, 2008, Haiss et al., 2010, Jackson and Fetz, 2007, Wilson et al., 2003, Yamamoto and Wilson, 2008, Yang et al., 2010), tetrodes (Tolias et al., 2007), microarrays (Nicolelis et al., 2003), microelectrode bundles (Herry et al., 2008, Kubie, 1984, Nicolelis et al., 1997, Szymusiak et al., 1998), and silicone based probes (Suner et al., 2005, Vetter et al., 2004), have been successfully used to obtain good unit recordings chronically in rodents and primates. In the primate cortex, good quality neuronal recording could be extended to 1.5 years using microarray electrodes (Nicolelis et al., 2003) and silicone based probes (Suner et al., 2005). However, high density array electrodes have limitedly used in deeper brain regions due to their tendency of causing severe brain damages. In addition, it is very difficult for the conventional linear microwire arrays to reach deep brain targets accurately and thus is of little practical use.
Bundled microwires, either singularly or in stereode/tetrode configurations, are better choices for making recordings in the nuclei of the deeper brain, and these were successfully used to record the nuclei in many regions, such as the amygdala (Chang et al., 2005, Herry et al., 2008), hypothalamus (Szymusiak et al., 1998), hippocampus (Kubie, 1984, Thompson and Best, 1990), and lateral thalamus (Nicolelis et al., 1997). It has been demonstrated in rabbits and primates that the temporal recording stability of bundle electrodes in visual cortex (Porada et al., 2000) and hippocampus (Thompson and Best, 1990) was over a year. However, one difficulty is the recording of trigeminal ganglion (TG) units. TG is located deep under the brain (∼9 mm from the brain surface in a rat) while being surrounded by thick dura. Therefore, long thin microwires (∼1 cm long), even when bundled together, cannot have sufficient rigidity to penetrate the layers of dura to reach it. Several studies successfully recorded the single-unit activity of TG using single-channel metal electrode with a wider shank (125 or 250 μm, to maintain the rigidity) and a sharp tip (to penetrate more easily) (Bermejo et al., 2004, Khatri et al., 2009, Leiser and Moxon, 2007). The drawback of the above-mentioned method is using the wider shank as well as the limited channel number; accordingly only one or two electrodes can be implanted.
Ideally, it is preferable to have more channels while minimizing tissue damage within a certain brain region. We noted that a tungsten microwire maintains strong rigidity when it is shorter than 1.5 mm. In addition, at this length, if the adjacent microwires are separated by a minimal inter-electrode distance of 200 μm, they can resist the adhesion force of water to remain separated in the implantation process. Based on the above characteristics, we developed a multi-channel electrode set for multiple single-unit recording chronically in the deep brain region.
Section snippets
Electrode construction
A glass slide with a piece of two-sided adhesive tape stuck on it was used as the fabrication jig. A 12-mm stainless steel needle with a sharp tip was cut to serve as a guide tube (30- and 29-guage for TG and thalamic recording respectively). Three or four circular notches were made with an electronic cutter at the blunt end, and a 15-mm copper wire (with a diameter of about 150 μm) was tightly wound two or three times on the notches and soldered onto the needle to serve as the ground wire. The
Results
Both TG and thalamic single-unit recordings were successfully obtained from 6 rats. The recorded noises were effectively attenuated by the needle-grounding with local reference method as shown in Fig. 2. These included smaller peak-to-peak background noise, and fewer and smaller far-field noise from animal movements.
Behavioral evaluation of the rats showed that escape thresholds of the ipsilateral and contralateral sides of the whisker pad significantly dropped to 22 ± 8.7 and 20.2 ± 8.8 g
Discussion
Chronically tracking neuronal signals in the same electrode over a period of 1.5 years has been reported using silicone-base microelectrode in primate motor cortex (Suner et al., 2005). In rodents, high quality cortical single-units could be recorded for over 127 days (Vetter et al., 2004). It is unclear, however, whether the same single-units were followed. Recently, the prevailing technique for chronic recording in the awake primates and rodents is the movable electrode (microarray or
Acknowledgement
This study was supported by a grant (NSC98-2313-B-197-003-MY3) from the National Science Council, Taiwan.
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