Development of a miniature microdrive recording system for multisite multichannel recording from rodent brains

Introduction Assessing the electrical performance of cell populations can reveal their functions only partially. To discover the underlying mechanisms it is important to examine how they communicate with other brain regions, which can be achieved by implanting multiple recording devices to all possible cooperating areas. Multisite implantations are limited by available space in small animals, thus the need for a miniaturised yet reliable microdrive that allows for increased implantation counts in a single animal is of utmost importance. Here we describe a miniature microdrive unit that can be used in various experimental settings. Methodology Based on a previously used design, but by substituting parts with miniature modelling components, we managed to build a reduced size microdrive which is reliable, biocompatible and customisable. The new device is fully compatible with existing recording solutions but it can be implanted in increased amounts into a single animal, where connector size becomes the only technical limitation. Discussion The theoretical number of implantThe theoretical number of implantable microdrives is 10 in mice (up to 320 recording channels), 24 in rats (up to 768 recording channels), and is now primarily limited by the connector size. The low profile also allows the use of wireless telemetry to replace the cable and provide unrestricted exploration for the animal as well as interacting animals concurrently, opening the floor to social– behavioural studies. Conclusion We successfully modified the concept of the original microdrive and constructed a miniature device that allows the mechanical manipulation of electrode depth in the brain in chronic experiments without the need to anaesthetise the animal. By using the new microdrive, cortical and deep brain structures can be targeted. Although due to the reduced size handling and implantation can be more challenging, the increased number of implantable units makes up for the difficulties and allows the simultaneous examination of multiple brain regions with high accuracy and reliability. Introduction An approach to assessing the realtime performance of an area or cell population is the registration of local bioelectrical activity generated by interacting neurons. Although the generating mechanism of the electrophysiological signals still has not yet been revealed entirely, it is considered to reflect the summed electrical activity of several thousand neurons, situated in close vicinity. Due to signal attenuation the activity of deeper brain structures cannot be recorded from the head surface, hence the need for invasive recording techniques enabling to reach deeper cortical layers or subcortical areas. With respect to gradual alterations in brain bioelectrical signals it is believed that subthreshold postsynaptic potential changes on the dendrites of pyramidal cells are responsible for the signal generation, as well as fast, all-ornothing-like action potentials (units), which are also integral part of the cerebral bioelectrical activity, though their magnitude in the total mass is significantly smaller. Therefore, the development of sophisticated recording equipment capable of registering and separating brain activity is of utmost importance. Deeper brain structures often with nuclear nature require the use of stereotrode or tetrode electrodes, the advantages of which have been widely acknowledged in the separation of unit activity1,2. These techniques allow for the assessment of electrical activity in subcortical structures, but they do not provide information about the network structure of concurrent or cooperating regions in the brain when used on their own. Simultaneous observation of multiple, anatomically related structures is essential for understanding how cerebral networks function under various conditions, as synchronisation and de-synchronisation of spatially distant areas play vital roles in learning and memory processes, including behaviour, and numerous other mechanisms of the nervous system3. Our aim was to provide a novel tool for accessing multiple brain structures at the same time and further increase the number of implantable devices by miniaturising microdrives that enable the fine tuning of electrode positioning in the brain. Our new microdrive design reduces size without compromising reliability and usability, while supporting the simultaneous examination of electrical activity in spatially distant or close regions. * Corresponding author Email: [email protected] Neuronal Networks Group, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, B15 2TT, UK