Cyfip1 haploinsufficiency does not alter GABA_A receptor δ-subunit expression and tonic inhibition in dentate gyrus PV⁺ interneurons and granule cells.

Simon Trent, Jeremy Hall, William M Connelly & Adam C. Errington*

(2019) eNeuro 6(3) ENEURO.0364-18.2019 1-20.

Copy number variation at chromosomal region 15q11.2 is linked to increased risk of neurodevelopmental disorders including autism and schizophrenia. A significant gene at this locus is cytoplasmic fragile X mental retardation protein (FMRP) interacting protein 1 (CYFIP1). CYFIP1 protein interacts with FMRP, whose monogenic absence causes Fragile X syndrome. FMRP knock-out has been shown to reduce tonic GABAergic inhibition by interacting with the δ-subunit of the GABA_A receptor. Using in situ hybridization, qPCR, western blot techniques and patch clamp electrophysiology in brain slices from a Cyfip1 haploinsufficient mouse, we examined δ-subunit mediated tonic inhibition in the dentate gyrus. In wild-type mice, dentate gyrus granule cells (DGGC) responded to the δ-subunit selective agonist THIP with significantly increased tonic currents. In heterozygous mice, no significant difference was observed in THIP evoked currents in DGGC. Phasic GABAergic inhibition in DGGC was also unaltered with no difference in properties of spontaneous inhibitory post synaptic currents (IPSCs). Additionally, we demonstrate that dentate gyrus granule cell layer PV⁺-interneurons (PV⁺-IN) have functional δ-subunit mediated tonic GABAergic currents which, unlike DGGC, are also modulated by the α₁ selective drug zolpidem. Similar to DGGC, both IPSCs and THIP-evoked currents in PV⁺-IN were not different between Cyfip1 heterozygous and wild-type mice. Supporting our electrophysiological data, we found no significant change in δ-subunit mRNA expression or protein level and no change in α₁/α₄ subunit mRNA expression. Thus, Cyfip1 haploinsufficiency, mimicking human 15q11.2 microdeletion syndrome, does not alter hippocampal phasic or tonic GABAergic inhibition, substantially differing from the FMRP knock-out mouse model.

Human Pluripotent Stem Cell-Derived Striatal Interneurons: Differentiation and Maturation In Vitro and in the Rat Brain

Noakes Z, Keefe F, Tamburini C, Kelly CM, Santos MC, Dunnett SB, Errington AC, Li M

(2019) Stem Cell Reports In Press https://doi.org/10.1016/j.stemcr.2018.12.014 

Our new paper on the properties of stem cell derived striatial interneurons done in collaboration with Professor Meng Li (NMHRI, Cardiff).

Striatal interneurons are born in the medial and caudal ganglionic eminences (MGE and CGE) and play an important role in human striatal function and dysfunction in Huntington’s disease and dystonia. MGE/CGE-like neural progenitors have been generated from human pluripotent stem cells (hPSCs) for studying cortical interneuron development and cell therapy for epilepsy and other neurodevelopmental disorders. Here, we report the capacity of hPSC-derived MGE/CGE-like progenitors to differentiate into functional striatal interneurons. In vitro, these hPSC neuronal derivatives expressed cortical and striatal interneuron markers at the mRNA and protein level and displayed maturing electrophysiological properties. Following transplantation into neonatal rat striatum, progenitors differentiated into striatal interneuron subtypes and were consistently found in the nearby septum and hippocampus. These findings highlight the potential for hPSC-derived striatal interneurons as an invaluable tool in modeling striatal development and function in vitro or as a source of cells for regenerative medicine.

Dual function of thalamic low-vigilance state oscillations: rhythm-regulation and plasticity.

Vincenzo Crunelli, Magor L Lorincz, William M Connelly, Francois David, Stuart W Hughes, Regis Lambert, Nathalie Leresche and Adam C Errington

(2018) Nature Reviews Neuroscience 19(2):107-118. 

During inattentive wakefulness and non-rapid eye movement (NREM) sleep, the neocortex and thalamus cooperatively engage in rhythmic activities that are exquisitely reflected in the electroencephalogram as distinctive rhythms spanning a range of frequencies from <1 Hz slow waves to 13 Hz alpha waves. In the thalamus, these diverse activities emerge through the interaction of cell-intrinsic mechanisms and local and long-range synaptic inputs. One crucial feature, however, unifies thalamic oscillations of different frequencies: repetitive burst firing driven by voltage-dependent Ca²⁺ spikes. Recent evidence reveals that thalamic Ca²⁺ spikes are inextricably linked to global somatodendritic Ca²⁺ transients and are essential for several forms of thalamic plasticity. Thus, we propose herein that alongside their rhythm-regulation function, thalamic oscillations of low-vigilance states have a plasticity function that, through modifications of synaptic strength and cellular excitability in local neuronal assemblies, can shape ongoing oscillations during inattention and NREM sleep and may potentially reconfigure thalamic networks for faithful information processing during attentive wakefulness.

Variable action potential backpropagation during tonic firing and low-threshold spike bursts in thalamocortical but not thalamic reticular nucleus neurons.

William M. Connelly, Vincenzo Crunelli and Adam C. Errington

(2017) The Journal of Neuroscience, 37(21) 5319-5333.

In neurons throughout the brain it has been shown that backpropagation of action potentials into the dendritic tree is important for a number of physiological processes in neurons, particularly plasticity processes. In both thalamocortical (TC) and thalamic reticular nucleus (TRN) neurons the full extent of action potential backpropagation has remained unclear despite calcium imaging and computational modelling studies. In this study we have used 2-photon targeted dendritic recordings to directly measure the backpropagation of action potentials into TC and TRN neuron dendrites. In these neurons we find that action potentials are initiated in the region close to the soma, most likely the axon initial segment, and are able to backpropagate into the dendritic tree. However, compared with other cell types backpropagation into the dendritic tree in not particularly efficient in TC and TRN neurons. We also show that in TC neurons the extent of action potential backpropagation varies between low-threshold spike bursts and tonic firing but this does not occur in TRN neurons where bAPs are unaffected by firing mode. These data support earlier findings using 2-photon Ca2+-imaging and demonstrate that there is a clear difference in dendritic signalling in thalamic neurons during tonic firing, which is prevalent during wakefulness, and low-threshold spike bursts that occur during lower vigilance states.

We have recently held our Division of Psychological Medicine and Clinical Neurosciences senior staff retreat at the beautiful Gregynog Hall in Powys, mid-Wales. Two days of stimulating discussions set out the future plans for continuing the excellent and world leading work done by Cardiff researchers in neuropsychiatric genetics and genomics and clinical neurosciences for the next decade and beyond. A big thank you to all of the speakers and organisers for a great meeting.

We have added our multi-compartmental Neuron model of a dLGN thalamocortical neuron to the Model DB repository. This model is based on our dendritic recordings from thalamocortical neurons and accurately reproduces the firing behaviour and synaptic integration properties of thalamocortical neurons. The model was used in our publications in The Journal of Neuroscience (Connelly et al., 2015, 2016).

Passive synaptic normalization and input synchrony-dependent amplification of cortical feedback in thalamocortical neuron dendrites.

William M Connelly, Vincenzo Crunelli & Adam C Errington                                               

(2016) The Journal of Neuroscience, 36(13) 3735-3
754.

The numerically dominant synaptic input to sensory thalamic nuclei including the dorsal lateral geniculate nucleus (dLGN), does not, as might be expected, come from sensory sources (e.g retinal ganglion cells) but, in fact, comes from cortical feedback. Up to 50% of synapses on thalamocortical (TC) neurons are corticothalamic (CT) inputs from layer VI cortical pyramidal neurons. These synapses produce very small EPSPs at the soma of TC neurons and have consequently traditionally been considered as ‘modulators’ of TC neuron activity. One aspect of our understanding of CT feedback to TC neurons has, however, remained unknown. Specifically, how TC neurons integrate these CT numerous inputs and the effects that these inputs produce locally in dendrites. We have published a paper in The Journal of Neuroscience that has addressed some of these questions and provides new insight into the role of CT feedback in TC neurons.

By using paired somatodendritic patch clamp recordings, patterned two photon glutamate uncaging and computational modelling we have demonstrated in vitro that individual quantal sized CT synapses produce large EPSPs in dendrites that are strongly attenuated as they propagate to the soma. However, because of the dendritic properties (see also Connelly et al., 2015) of TC neurons, CT EPSPs are ‘passively normalized’ such that, unlike synaptic potentials in dendrites of other neurons, the amplitude and shape of CT EPSPs is almost entirely unaffected by the location of the synapse within the dendritic tree. Thus, CT feedback seems to be a synaptic democracy with all inputs having an equal effect on TC neuron membrane potential.

Our new data also shows that TC neuron dendrites have mechanisms, involving T-type Ca2+ channels and NMDA receptors, that can amplify CT feedback in a manner dependent upon the temporal pattern of the input. We suggest that these mechanisms might significantly increase the range of responses that CT feedback can produce in TC neurons to exert their modulatory effects. In fact, we find that under certain conditions CT feedback might be sufficient to act as a ‘conditional driver’ of TC neuron firing.

Follow the link above to read the paper in full (freely available).