Measurement of network activity in acute slices of the healthy and parkinsonian striatum on microelectrode arrays

Gunneweg, Freddy (2015) Measurement of network activity in acute slices of the healthy and parkinsonian striatum on microelectrode arrays.

Abstract:In order to measure neuronal activity in a controlled setting, microelectrode arrays (MEA) provide a multifunctional platform for various tissue preparations. Their capability to measure from many sites in the tissue simultaneously and with high temporal resolution makes them very suitable to measure activity from a network of interconnected neurons, for instance in cultured networks or in acute slices. Such analysis is especially relevant for research on Parkinson’s disease (PD). It has been shown that due to the lack of dopamine in the basal ganglia, network activity in this brain area is severely affected, exhibiting excessive synchronization and oscillations. Although the exact mechanism behind this behaviour remains unclear, the striatum has been implied to play a major role in emergence of this pathological activity, because it is the input structure of the basal ganglia and the main target of dopamine. In investigations into how the striatum contributes to pathological oscillations, the MEA platform may provide a valuable tool for analysis of network activity. The striatum mainly consists of medium spiny neurons (MSN), which are typically hyperpolarized but can be excited by coherent input from the cortex. The circuitry of the striatum is almost completely inhibitory, and is orchestrated by a small group of ’fast spiking interneurons’ (FSI). These interconnected cells provide strong feedforward inhibition onto MSN’s. Computational studies have shown in parkinsonian conditions, this network can nevertheless exhibit abnormal synchrony and oscillations as a result of compromised processing of cortical inputx. In in-vivo and in-vitro experiments, striatal MSN’s are typically silent, but exhibit relatively depolarized states that enable action potential initiation. However, such experiments are either too complicated or too artificial. Acute slices may provide a more realistic yet stable simulation of the in-situ network and structure to investigate these patterns. In the present study, slices of the healthy and parkinsonian striatum are investigated using MEA recordings. To this end, a test group of 7 rats was injected unilaterally with 6-OHDA, lesioning the substantia nigra to simulate dopamine denervation. Parkinsonian slices are expected to show higher firing rates and occurence of bursts. However, in contrast with previous literature, no significant difference in firing patterns could be observed between control and lesioned animals. During recordings, it was found that striatal tissue did not readily show spontaneous activity. Technical issues were assumed to be responsible, specifically the potential damage caused during slice preparation and the manual pressing of the slice onto the MEA chip. However, following protocol optimization, activity was still very sparse, presumably as a result of defects in the glutamatergic input from the cortex. Electrical stimulation and application of L-glutamine to the medium were generally not successful to provide sufficient excitation for long-term activity. In order to provide a positive control for these measurements and to exclude technical issues, additional recordings were performed in the hippocampus, a brain region which is often used in acute slices and is typically much more active. Here, following recordings in the striatum, the hippocampus of the same slice was recorded, yielding much higher success rates. Spontaneous activity can be picked up immediately and could be measured for much longer durations. Additionally, in contrast with striatal recordings, activity was increased significantly following glutamine application, indicating that pharmacological stimuli are efficacious. The results presented here show that in acute slices, cortical input is insufficient to excite striatal MSN’s, although it was previously reported that individual corticostriatal connections were kept intact. This provides further proof for the hypothesis that striatal cells act as filters for cortical signals, firing only when sufficient correlated input is received. Since the striatum is inherently inactive, it is difficult to determine during experiments if a slice is alive or dead, because external factors can easily cause cell damage and result in cell silence as well. A more powerful depolarizing stimulation should be applied to ensure that slices are viable. Following confirmation of slice survival, network activity should be recorded during simulating of cortical input, for example by electrical stimulation. However, this will likely entrain the network into an unrealistic state of activity, effectively masking the effects under investigation. It can be concluded that although extracellular recordings on acute slices may be a promising method to simulate in-vivo tissue, this may not readily be applied to striatal tissue. However, recent findings in literature have suggested that following 24 hours of incubation, acute slices are able to restore their natural firing rhythms. Besides the practical benefits of recording a day after slice preparation, this would also solve the effective silence in future studies on striatal network activity.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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