Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - INHIBITHUMAN (Dis-inhibitory circuits in the human cerebral cortex)

Teaser

The aims of the project is to define nerve cell types (neurons), synaptic plasticity, consisting of neuron-to-neuron change of communication strength, and effects induced by key neuroactive drugs in slices of living human cerebral cortex. Although brain diseases are very...

Summary

The aims of the project is to define nerve cell types (neurons), synaptic plasticity, consisting of neuron-to-neuron change of communication strength, and effects induced by key neuroactive drugs in slices of living human cerebral cortex.

Although brain diseases are very widespread, discoveries of new therapies are developing slowly. A crucial factor hampering progress is the lack of good animal models for human brain diseases leading to failure in clinical translation. Furthermore, neuronal centres of the human brain consist of many distinct cell types, but our knowledge on their identity, responses to drugs and roles in the neuronal circuits is still very limited. Cell-type specific therapies may fail if heterogeneity of structure and function across species is neglected.

Our programme aims to improve this situation. We use surgically resected human cortical tissue that is normally removed and discarded in order to gain access to deeper brain areas during neurosurgery. We obtain small cortical samples and keep them alive as oxygenated thin slices of for several hours. This allows us to investigate the physiological interactions and roles of specific neuron types, their synaptic plasticity and their responses to neuroactive drugs. The experiments consist of electrophysiological recordings, involving the detection if voltage potentials of few millivolts or the detection of picoampere magnitude membrane currents, by insertion of a thin recording device into each neuron, from one to three neurons at the same time. We investigate synaptic plasticity and the effects mediated by clinically relevant pharmacological agents (e.g., cognitive enhancers). After the electrophysiological recordings, the slices containing the recorded cells, are fixed, re-sectioned and subjected to immunohistochemical reactions to identify key molecules expressed by specific neuron types. Using this approach, the programme provides exciting discoveries on human cortical neurons that use gamma-butyric-acid (GABA) as a molecule in their synaptic communication and pharmacological responses.

Work performed

The interim results as of June 2019 demonstrate the strengths of our approach to define neuronal cell types, synaptic interactions and drug sensitive sites in the human neocortex.

• We have successfully implemented dual electrical recording of synaptically coupled neurons in pairs of connections where one or both cells are GABAergic interneurons.
• We have successfully implemented a spike timing dependent synaptic plasticity protocol and defined the modulatory action of dopamine.
• We have established that a metabotropic glutamate receptor agonist developed for clinical applications acts presynaptically on glutamatergic inputs to pyramidal cells.
• We have demonstrated novel combinations of signalling molecules in single identified GABAergic neurons.
• We have recorded and visualised the largest known sample of GABAergic double bouquet cells, characterised the pharmacological properties of their input synapses and identified some of their postsynaptic target cells.
• We have electrically recorded and visualised the only known human sample of GABAergic cannabinoid receptor-1 expressing GABAergic interneurons, and defined their signalling molecules.

Final results

Overall, the results derived from the project define novel neuronal types in the human cerebral cortex and clarify the roles and place some of the known types. We disclose novel mechanisms of action mediated by key receptors (e.g., glutamate metabotropic receptors) and document novel synaptic plasticity mechanisms that could be involved in diseases (e.g., synaptic re-modelling after stroke) or therapeutic intervention (e.g., transcranial magnetic stimulation).

The appreciation that the adult human brain is more plastic than previously imagined is growing and is becoming clear to stakeholders, such as the directors of pharma industry, the media, and the lay public. However, crucial mechanistic evidence for neuroplasticity at human cortical synapses is still lacking, as only few rigorous papers has been published on this topic so far. We expect that the results obtained by the end of the project will significantly advance fundamental knowledge of the mechanisms and neuron types of the human cerebral cortex involved in synaptic plasticity. We hypothesise that contributing to this vision has immense scientific and clinical value.