Stephen Brickley | Imperial College London (original) (raw)

Papers by Stephen Brickley

eLife

The ubiquitous presence of inhibitory interneurons in the thalamus of primates contrasts with the... more The ubiquitous presence of inhibitory interneurons in the thalamus of primates contrasts with the sparsity of interneurons reported in mice. Here, we identify a larger than expected complexity and distribution of interneurons across the mouse thalamus, where all thalamic interneurons can be traced back to two developmental programmes: one specified in the midbrain and the other in the forebrain. Interneurons migrate to functionally distinct thalamocortical nuclei depending on their origin: the abundant, midbrain-derived class populates the first and higher order sensory thalamus while the rarer, forebrain-generated class is restricted to some higher order associative regions. We also observe that markers for the midbrain-born class are abundantly expressed throughout the thalamus of the New World monkey marmoset. These data therefore reveal that, despite the broad variability in interneuron density across mammalian species, the blueprint of the ontogenetic organisation of thalamic i...

The Journal of Physiology

Cell reports, Jan 24, 2018

We have applied optogenetics and mGRASP, a light microscopy technique that labels synaptic contac... more We have applied optogenetics and mGRASP, a light microscopy technique that labels synaptic contacts, to map the number and strength of defined corticocollicular (CC) connections. Using mGRASP, we show that CC projections form small, medium, and large synapses, and both the number and the distribution of synapse size vary among the IC regions. Using optogenetics, we show that low-frequency stimulation of CC axons expressing channelrhodopsin produces prolonged elevations of the CC miniature EPSC (mEPSC) rate. Functional analysis of CC mEPSCs reveals small-, medium-, and large-amplitude events that mirror the synaptic distributions observed with mGRASP. Our results reveal that descending ipsilateral projections dominate CC feedback via an increased number of large synaptic contacts, especially onto the soma of IC neurons. This study highlights the feasibility of combining microscopy (i.e., mGRASP) and optogenetics to reveal synaptic weighting of defined projections at the level of sing...

Scientific reports, Jan 13, 2017

The relatively simple and compact morphology of cerebellar granule cells (CGCs) has led to the vi... more The relatively simple and compact morphology of cerebellar granule cells (CGCs) has led to the view that heterogeneity in CGC shape has negligible impact upon the integration of mossy fibre (MF) information. Following electrophysiological recording, 3D models were constructed from high-resolution imaging data to identify morphological features that could influence the coding of MF input patterns by adult CGCs. Quantification of MF and CGC morphology provided evidence that CGCs could be connected to the multiple rosettes that arise from a single MF input. Predictions from our computational models propose that MF inputs could be more densely encoded within the CGC layer than previous models suggest. Moreover, those MF signals arriving onto the dendrite closest to the axon will generate greater CGC excitation. However, the impact of this morphological variability on MF input selectivity will be attenuated by high levels of CGC inhibition providing further flexibility to the MF → CGC pa...

Frontiers in cellular neuroscience, 2017

Cell-type specific differences in the kinetics of inhibitory postsynaptic conductance changes (IP... more Cell-type specific differences in the kinetics of inhibitory postsynaptic conductance changes (IPSCs) are believed to impact upon network dynamics throughout the brain. Much attention has focused on how GABAA receptor (GABAAR) α and β subunit diversity will influence IPSC kinetics, but less is known about the influence of the γ subunit. We have examined whether GABAAR γ subunit heterogeneity influences IPSC properties in the thalamus. The γ2 subunit gene was deleted from GABAARs selectively in the dorsal lateral geniculate nucleus (dLGN). The removal of the γ2 subunit from the dLGN reduced the overall spontaneous IPSC (sIPSC) frequency across all relay cells and produced an absence of IPSCs in a subset of relay neurons. The remaining slower IPSCs were both insensitive to diazepam and zinc indicating the absence of the γ2 subunit. Because these slower IPSCs were potentiated by methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), we propose these IPSCs involve γ1 subunit-con...

In Journal of Physiology, 2003

J Neurosci, 2009

The GABA(A) receptor has been identified as the single most important target for the intravenous ... more The GABA(A) receptor has been identified as the single most important target for the intravenous anesthetic propofol. How effects at this receptor are then translated into a loss of consciousness, however, remains a mystery. One possibility is that anesthetics act on natural sleep pathways. Here, we test this hypothesis by exploring the anesthetic sensitivities of GABAergic synaptic currents in three specific brain nuclei that are known to be involved in sleep. Using whole-cell electrophysiology, we have recorded GABAergic IPSCs from the tuberomammillary nucleus (TMN), the perifornical area (Pef), and the locus ceruleus (LC) in brain slices from both wild-type mice and mice that carry a specific mutation in the GABA(A) receptor beta(3) subunit (N265M), which greatly reduces their sensitivity to propofol, but not to the neurosteroid alphaxalone. We find that this in vivo pattern of anesthetic sensitivity is mirrored in the hypothalamic TMN and Pef nuclei, consistent with their role as direct anesthetic targets. In contrast, anesthetic sensitivity in the LC was unaffected by the beta(3)N265M mutation, ruling out this nucleus as a major target for propofol. In support of the hypothesis that orexinergic neurons in the Pef are involved in propofol anesthesia, we further show that these neurons are selectively inhibited by GABAergic drugs in vivo during anesthesia, and that a modulation in the activity of Pef neurons alone can affect loss of righting reflex. Overall, our results support the idea that GABAergic anesthetics such as propofol exert their effects, at least in part, by modulating hypothalamic sleep pathways.

In Journal of Physiology, 1995

In Fens Forum Abstracts, 2002

F1000 - Post-publication peer review of the biomedical literature, 2011

Inhibition is a physiological process that decreases the probability of a neuron generating an ac... more Inhibition is a physiological process that decreases the probability of a neuron generating an action potential. The two main mechanisms that have been proposed for inhibition are hyperpolarization and shunting. shunting results from increased membrane conductance, and it reduces the neuron-firing probability. Here we show that ambient GABA, the main inhibitory neurotransmitter in the brain, can excite adult hippocampal interneurons. In these cells, the GABA A current reversal potential is depolarizing, making baseline tonic GABA A conductance excitatory. Increasing the tonic conductance enhances shunting-mediated inhibition, which eventually overpowers the excitation. such a biphasic change in interneuron firing leads to corresponding changes in the GABA A-mediated synaptic signalling. The described phenomenon suggests that the excitatory or inhibitory actions of the current are set not only by the reversal potential, but also by the conductance.

The ability of neurons, such as cerebellar granule neurons (CGNs), to fire action potentials (APs... more The ability of neurons, such as cerebellar granule neurons (CGNs), to fire action potentials (APs) at high frequencies during sustained depolarization is usually explained in relation to the functional properties of voltage-gated ion channels. Two-pore domain potassium (K 2P) channels are considered to simply hyperpolarize the resting membrane potential (RMP) by increasing the potassium permeability of the membrane. However, we find that CGNs lacking the TASK-3 type K 2P channel exhibit marked accommodation of action potential firing. The accommodation phenotype was not associated with any change in the functional properties of the underlying voltage-gated sodium channels, nor could it be explained by the more depolarized RMP that resulted from TASK-3 channel deletion. A functional rescue, involving the introduction of a nonlinear leak conductance with a dynamic current clamp, was able to restore wild-type firing properties to adult TASK-3 knockout CGNs. Thus, in addition to the accepted role of TASK-3 channels in limiting neuronal excitability, by increasing the resting potassium conductance TASK-3 channels also increase excitability by supporting high-frequency firing once AP threshold is reached.

The Journal of Neuroscience : The Official Journal of the Society for Neuroscience

Many neurons express a multiplicity of GABAA receptor subunit isoforms. Despite having only a sin... more Many neurons express a multiplicity of GABAA receptor subunit isoforms. Despite having only a single source of inhibitory input, the cerebellar granule cell displays, at various stages of development, more than 10 different GABAA subunit types. This subunit diversity would be expected to result in significant receptor heterogeneity, yet the functional consequences of such heterogeneity remain poorly understood. Here we have used single-channel properties to characterize GABAA receptor types in the synaptic and extrasynaptic membrane of granule cells. In the presence of high concentrations of GABA, which induced receptor desensitization, extrasynaptic receptors in outside-out patches from the soma entered long-lived closed states interrupted by infrequent clusters of openings. Each cluster of openings, which is assumed to result from the repeated activation of a single channel, was to one of three main conductance states (28, 17, or 12 pS), the relative frequency of which differed be...

The Journal of neuroscience : the official journal of the Society for Neuroscience, Jan 9, 2003

The timing of action potentials is an important determinant of information coding in the brain. T... more The timing of action potentials is an important determinant of information coding in the brain. The shape of the EPSP has a key influence on the temporal precision of spike generation. Here we use dynamic clamp recording and passive neuronal models to study how developmental changes in synaptic conductance waveform and intrinsic membrane properties combine to affect the EPSP and action potential generation in cerebellar granule cells. We recorded EPSCs at newly formed and mature mossy fiber-granule cell synapses. Both quantal and evoked currents showed a marked speeding of the AMPA receptor-mediated component. We also found evidence for age- and activity-dependent changes in the involvement of NMDA receptors. Although AMPA and NMDA receptors contributed to quantal EPSCs at immature synapses, multiquantal release was required to activate NMDA receptors at mature synapses, suggesting a developmental redistribution of NMDA receptors. These changes in the synaptic conductance waveform r...

Frontiers in neural circuits, 2013

We have made use of the δ subunit-selective allosteric modulator DS2 (4-chloro-N-[2-(2-thienyl)im... more We have made use of the δ subunit-selective allosteric modulator DS2 (4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide) to assay the contribution of δ-GABAARs to tonic and phasic conductance changes in the cerebellum, thalamus and neocortex. In cerebellar granule cells, an enhancement of the tonic conductance was observed for DS2 and the orthosteric agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol). As expected, DS2 did not alter the properties of GABAA receptor-mediated inhibitory postsynaptic synaptic conductances (IPSCs) supporting a purely extrasynaptic role for δ-GABAARs in cerebellar granule cells. DS2 also enhanced the tonic conductance recorded from thalamic relay neurons of the visual thalamus with no alteration in IPSC properties. However, in addition to enhancing the tonic conductance DS2 also slowed the decay of IPSCs recorded from layer II/III neocortical neurons. A slowing of the IPSC decay also occurred in the presence of the voltage-gated so...

The Journal of neuroscience : the official journal of the Society for Neuroscience, Jan 15, 2003

NMDA receptors (NMDARs) are thought to be tetrameric assemblies composed of NR1 and at least one ... more NMDA receptors (NMDARs) are thought to be tetrameric assemblies composed of NR1 and at least one type of NR2 subunit. The identity of the NR2 subunit (NR2A, -B, -C, -D) is critical in determining many of the functional properties of the receptor, such as channel conductance and deactivation time. Further diversity may arise from coassembly of more than one type of NR2 subunit, if the resulting triheteromeric assembly (NR1 plus two types of NR2) displays distinct functional properties. We have used gene-ablated mice (NR2D -/-) to examine the effects of the NR2D subunit on NMDAR channels and NMDAR EPSCs in cerebellar Golgi cells. These cells are thought to express both NR2B and NR2D subunits, a combination that occurs widely in the developing nervous system. Our experiments provide direct evidence that the low conductance NMDAR channels in Golgi cells arise from diheteromeric NR1/NR2D assemblies. To investigate whether a functionally distinct triheteromeric assembly was also expressed...

The Journal of neuroscience : the official journal of the Society for Neuroscience, Jan 15, 1999

The Journal of physiology, Jan 15, 1996

1. To investigate the origin and functional significance of a recently described tonic GABAA rece... more 1. To investigate the origin and functional significance of a recently described tonic GABAA receptor-mediated conductance in cerebellar granule cells we have made recordings from cells in cerebellar slices from rats of different ages (postnatal days P4 to P28). 2. During development there was a dramatic change in the properties of GABA-mediated synaptic transmission. The contribution to GABAA receptor-mediated charge transfer from the tonic conductance (GGABA), relative to that resulting from discrete spontaneous postsynaptic currents (sPSCs), was increased from 5% at P7 to 99% at P21. GGABA was reduced by bicuculline, tetrodotoxin and by lowering extracellular Ca2+, and was initially present only in those cells which exhibited sPSCs. 3. At P7 sPSCs were depolarizing, occasionally triggering a single action potential. By P18 the GABA reversal potential was shifted close to the resting potential and GGABA produced a shunting inhibition. Removal of GGABA by bicuculline increased gran...

Pflügers Archiv - European Journal of Physiology, 2014

In this study, we explored the possibility that two-pore domain potassium (K 2P) channels are suf... more In this study, we explored the possibility that two-pore domain potassium (K 2P) channels are sufficient to support action potential (AP) generation in the absence of conventional voltage-gated potassium (K V) channels. Hodgkin-Huxley parameters were used to mimic the presence of voltage-gated sodium (Na V) channels in HEK-293 cells. Recombinant expression of either TREK-1 or TASK-3 channels was then used to generate a hyperpolarised resting membrane potential (RMP) leading to the characteristic non-linear current-voltage relationship expected of a K 2P-mediated conductance. During conductance simulation experiments, both TASK-3 and TREK-1 channels were able to repolarise the membrane once AP threshold was reached, and at physiologically relevant current densities, this K 2P-mediated conductance supported sustained AP firing. Moreover, the magnitude of the conductance correlated with the speed of the AP rise in a manner predicted from our computational studies. We discuss the physiological impact of axonal K 2P channels and speculate on the possible clinical relevance of K 2P channel modulation when considering the actions of general and local anaesthetics.

Nature Neuroscience, 2015

Do sedatives engage natural sleep pathways? It is usually assumed that anesthetic-induced sedatio... more Do sedatives engage natural sleep pathways? It is usually assumed that anesthetic-induced sedation and loss of righting reflex (LORR) arise by influencing the same circuitry to lesser or greater extents. For the a2 adrenergic receptor agonist dexmedetomidine, we found that sedation and LORR were in fact distinct states, requiring different brain areas: the preoptic hypothalamic area and locus coeruleus (LC), respectively. Selective knockdown of a2A adrenergic receptors from the LC abolished dexmedetomidine-induced LORR, but not sedation. Instead, we found that dexmedetomidine-induced sedation resembled the deep recovery sleep that follows sleep deprivation. We used TetTag pharmacogenetics in mice to functionally mark neurons activated in the preoptic hypothalamus during dexmedetomidine-induced sedation or recovery sleep. The neuronal ensembles could then be selectively reactivated. In both cases, non-rapid eye movement sleep, with the accompanying drop in body temperature, was recapitulated. Thus, a2 adrenergic receptor-induced sedation and recovery sleep share hypothalamic circuitry sufficient for producing these behavioral states.