Harvard researchers have made a significant advancement in neuronal recording by mapping and cataloguing over 70,000 synaptic connections from approximately 2,000 rat neurons. This breakthrough, published in Nature Biomedical Engineering , brings scientists closer to creating a detailed synaptic connection map of the brain. Higher-order brain functions are thought to arise from the complex connections between neurons, which occur at contact points called synapses. While electron microscopy provides visual maps of these connections, it lacks information on connection strengths, which are crucial for understanding network function.
The gold standard for intracellular recording has been the patch-clamp electrode, which can detect faint synaptic signals with high sensitivity, revealing both the existence and strength of connections. However, traditional methods have struggled to scale this technique to record from more than a few neurons simultaneously. To address this limitation, the research team led by Donhee Ham, the John A. and Elizabeth S. Armstrong Professor of Engineering and Applied Sciences at Harvard SEAS, developed a silicon chip equipped with an array of 4,096 microhole electrodes. This innovation enabled massively parallel intracellular recording of rat neurons cultured on the chip.
The researchers extracted over 70,000 synaptic connections from the data, far surpassing the roughly 300 connections achieved with their earlier nanoneedle electrode array. The new design improved upon the previous technology, which featured vertical nanoneedles that allowed neurons to wrap around them for intracellular recording. Although effective, the nanoneedle approach was limited in scalability and fabrication complexity.
In contrast, the microhole electrode array offers superior coupling to neuron interiors and is easier to fabricate. By applying gentle current injections through the electrodes, the team successfully opened up cells for intracellular recording. On average, 90% of the electrodes (over 3,600 out of 4,096) were coupled intracellularly to neurons, enabling the extraction of a vast number of synaptic connections. The quality of the recordings was also enhanced, allowing the researchers to categorize each synaptic connection based on its characteristics and strength.
The integrated electronics within the silicon chip played a critical role in the process, providing precise currents to achieve intracellular access while simultaneously recording signals. Postdoctoral researcher Jun Wang noted that the microhole design closely resembles the patch-clamp electrode, with the added advantage of being simpler to produce. Co-lead author Woo-Bin Jung emphasized the importance of the chip's electronics in facilitating both the intracellular access and signal recording.
One of the major challenges following the successful recording was analyzing the massive dataset. The team has made substantial progress in deriving insights about synaptic connections from the data. They are now exploring designs that could be used in live brains, moving closer to real-time mapping of neural activity.
This work builds on the team's 2020 breakthrough with the nanoneedle electrode array, which already represented a leap forward in synaptic connection mapping. The new microhole electrode array significantly advances this field, offering unprecedented scalability and data quality. Paper co-authors include Rona S. Gertner and Hongkun Park, and the research was supported by the Samsung Advanced Institute of Technology of Samsung Electronics. This development represents a major step toward understanding the intricate workings of the brain's neural networks.
