Co-ordinated neuronal activity is intrinsically linked with behaviour and malfunction of neuronal coordination results in psychiatric and neurological disorders. Timing is crucial for neuronal integration including events lasting from milliseconds up to several seconds. Much of the neuronal activity is rhythmic in the brain, as rhythmicity facilitates local and global interactions and enables the representation of temporal sequences.
Current project: Synaptic circuits of identified neurons in the human cerebral cortex - ERC 694988 INHIBITHUMAN
The aims of the project are to define neuronal cell types, synaptic plasticity and the effects induced by key neuroactive drugs in living human cerebral cortex explored in vitro.
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 limited. Cell-type specific therapies may be facilitated by defining synaptic circuits with pharmacological validation of drug action on identified neurons.
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 many 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 from one to three neurons at the same time using electrodes that allow the visualisation of the recorded cells. 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. The results have demonstrated the value to define neuronal cell types, synaptic interactions and drug sensitive sites in the human neocortex, which is based on the in vitro recording of a sample of uniquely diverse GABAergic neurons and pyramidal cells.
- 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 demonstrated that tonic GABA-A receptor mediated currents, which regulate neuronal excitability in the cortex, are cell type specific in GABAergic human cortical neurons.
- We have demonstrated novel combinations of molecules involved in GABAergic neuronal signalling such as somatostatin, VIP, VGLUT3 and calcium binding proteins in single identified GABAergic neurons.
- We have shown that GABAergic double bouquet cells act preferentially on dendritic spines through GABA-A receptors and their GABAergic input is increased by activating group III metabotropic glutamate receptors.
- We have shown that human GABAergic parvalbumin-positive dendrite-targeting cells act mainly on dendritic shafts and less on somata of postsynaptic neurons and their GABAergic input is reduced by activating group III metabotropic glutamate receptors.
- We have electrically recorded and visualised the only known human sample of GABAergic cannabinoid receptor-1 expressing GABAergic interneurons and defined some of their signalling molecules.
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 have revealed novel mechanisms of action mediated by key receptors (e.g., glutamate metabotropic receptors, dopamine receptors) and document novel synaptic plasticity mechanisms. The results advance fundamental knowledge of the mechanisms and neuron types of the human cerebral cortex.
Recent publications:
- Lukacs IP, Francavilla R, Field M, Hunter E, Howarth M, Horie S, Plaha P, Stacey R, Livermore L, Ansorge O, Tamas G, Somogyi P (2023) Differential effects of group III metabotropic glutamate receptors on spontaneous inhibitory synaptic currents in spine-innervating double bouquet and parvalbumin-expressing dendrite-targeting GABAergic interneurons in human neocortex. Cerebral Cort, 33:2101-2142. https://doi.org/10.1093/cercor/bhac195
- Field M, Lukacs IP, Hunter E, Stacey R, Plaha P, Livermore L, Ansorge O, Somogyi P (2021) Tonic GABAA receptor mediated currents of human cortical GABAergic interneurons vary amongst cell types. J Neurosci 41:9702-9719. https://doi.org/10.1523/JNEUROSCI.0175-21.2021
- Louth EL, Jørgensen RL, Sørensen JCH, Capogna M (2021) Dopaminergic neuromodulation of spike timing dependent plasticity in mature adult rodent and human cortical neurons Front Cell Neurosci 15:135. https://www.frontiersin.org/articles/10.3389/fncel.2021.668980/full
- Bocchio M, Lukacs IP, Stacey R, Plaha P, Apostolopoulos V, Livermore L, Sen A, Ansorge O, Martin J. Gillies MJ, Somogyi P, Capogna M (2019) Group II metabotropic glutamate receptors mediate presynaptic inhibition of excitatory transmission in pyramidal neurons of the human cerebral cortex. Front Cellular Neurosci, 12:508. https://doi.org/10.3389/fncel.2018.00508
- Rhythmicity provides flexibility by resetting the frequency, amplitude and phase of population activity for encoding and delivering information in support of behavioural needs. Importantly, firing of single and groups of neurons can change systematically relative to the population rhythm and reflect temporal coding of information.
- One of the most studied and widespread cortical oscillations is rhythmic slow activity in the theta frequency range, typically 4-12 Hz in the temporal cortex of rodents. Theta oscillations modulate higher frequency gamma (30-120 Hz) oscillations associated with cognitive processes in both rodents and humans. Such cross-frequency coupling is a basic principle of brain function as changes in coupling strength indicate dynamic changes in neuronal network activity underlying behaviour, cognitive states such as navigation, decision-making and memory performance.
Key Research Areas:
- To define how specific cell types contribute to the timing of neuronal population activity in the CA3 area of the hippocampus.
- To establish the identity of GABAergic neurons in the CA1 and CA3 areas and determine their in vivo firing patterns relative to theta, gamma and 100-200 Hz network oscillations and pyramidal cell activity.
- To test the hypothesis that disinhibition, i.e. the phasic inhibition, or deactivation of key GABAergic neurons in the CA3 area is a condition of the synchronous pyramidal cell discharge in CA3 assembly activity that generates sharp waves in the CA1 area.
- To explain how behaviour specific changes in interneuron firing contribute to pyramidal assembly activity.
- To explain the role of identified neurons in well-defined and behaviourally significant cortical network events.
Longer-term Perspectives:
We explore how distinct neuronal types contribute to behaviour, and how the network mechanisms governing neuronal activity relate to both normal and abnormal brain function. We hypothesise that temporal coordination of neuronal assemblies is regulated by a temporal redistribution of inhibition over principal cell subcellular domains.
We reveal how the firing patterns of different GABAergic, cholinergic and glutamatergic neurons relate to network oscillations during different behavioural states such as movement and sleep and how their connectivity to other cells can provide a mechanism for these underlying network oscillations.
Research Techniques:
- In vivo extracellular recordings from single neurons followed by juxtacellular labelling in drug-free freely moving rats and head-restrained mice during behaviour
- Spike train analysis and coupling to local field potential oscillations for single cells and multi-unit activity
- Investigation of synaptic mechanisms governing delta, theta, gamma, and ripple oscillations
- Characterisation of identified hippocampal place cells
- Cell type identification of recorded neurons from both the basal forebrain, hippocampal formation, and neocortex
- Analysis of axonal arbours and their postsynaptic targets (GABAergic interneurons, GABAergic and cholinergic projection neurons, principle cells)
- Mapping subcortical inputs to specific types of cortical interneuron with anterograde and retrograde tracers
- Immunohistochemical characterisation and HRP-based diaminobenzidine processing of high-quality post-mortem human and rodent basal forebrain and cerebral cortical samples for light and electron microscopy
- Investigation of synaptic mechanisms governing delta, theta, gamma, and ripple oscillations
- Confocal and electron microscopic analysis of identified neural circuits, synapses and subcellular distribution of ion channels and receptors
Our team
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Peter P Somogyi
Emeritus Professor of Neurobiology
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Linda Katona
Academic Visitor - Research Fellow, University College Cork
