32 research outputs found
Histaminergic modulation of striatal development
No full textInadequate synthesis of the neurotransmitter histamine in the brain has been suggested to be one of causes of Tourette’s syndrome and several psychiatric diseases. Previous findings suggested that dysfunction of the corticostriatal circuits are associated with such mental disorders. Indeed, recent research has reported a hypofunction of histamine in patients with Tourette’s syndrome and obsessive-compulsive disorder in human and animal studies. However, the functional role of histamine in young developing animal brains is poorly known. Firstly, this study investigated whether and to what extent histamine plays a significant role in the corticostriatal circuit in the developing mouse brain. Secondly, the study investigated whether and to what extent histamine affects neurogenesis in the striatum in the embryonic mouse brain. Towards the first aim, the study recruited three postnatal young age period mice (postnatal (P)3-6 days, P9-12 days and P21+ days) and applied extracelluar field recordings of the corticostriatal transmission. As a result, the study found that histamine negatively modulates the corticostriatal synaptic transmission in all of young age period mice, acting at the H3 receptors. In addition, histamine facilitated long-term potentiation of the corticostriatal synapses in the P9-12 period, but blocked it in the P21+ period, whereas the baseline condition showed long-term facilitation in the P21+, not the P9-12 periods. Collective analysis of this long-term plasticity experiment revealed that the dorsolateral striatum (DLS) is more likely to be facilitated than the dorsomedial striatum (DMS). Histamine’s negative modulation on the corticostriatal synaptic transmission is found in developing age periods, which was found in only adult mice brains in past studies. Also histamine’s opposite effect on the corticostriatal long-term plasticity in the P9-12 and P21+ periods implies that histamine may cause significant change in the dynamics of synaptic properties in particular developing age periods. For the second aim, the study injected α-fluoromethylhistidine (α-FMH) into the ventricle of embryonic mouse brain at embryonic day (E)12.5 and E15.5. The study then collected their brains 2-3 days after the application of α-FMH to compare the size of the striatum and proliferative zones between control and α-FMH conditions. Finally, a reduced size of striatum was observed in E15.5 brains (medial section) treated with α-FMH than those in control conditions. However, the study could not make further conclusions due to the small number of samples
Investigating translaminar signaling from layer 5 Pyramidal subpopulations to layer 2/3 in Somatosensory Cortex
No full textInformation processing in the mammalian cortex is achieved via a precise arrangement of synaptic connections between different neuronal subtypes. The excitatory neurons of the cortex are arranged in a laminar manner across six layers and form specific intralaminar and translaminar connections with one another. In addition, these excitatory neurons interact with a series of inhibitory interneuron subtypes, such that the relative levels and timing of excitatory and inhibitory synaptic transmission determine how information is processed by the cortical network. This thesis examines the translaminar signaling from layer 5 (L5) to layer 2/3 (L2/3) of cortex. Considered the principal output layer, L5 is populated by intratelencephalic (IT) and pyramidal tract (PT) pyramidal neurons, which form long-range projections to other cortical and subcortical regions, respectively. These long-range axons of L5 pyramidal neurons also branch locally and form ascending collaterals that extend to more superficial layers, including L2/3. An unanswered question in the field, is what sorts of translaminar signals do these local collaterals deliver.
I have used the mouse somatosensory cortex in order to address this question. In the first series of experiments, transgenic mice expressing the optogenetic activator, channelrhodopsin-2 (ChR2), were used to stimulate a non-specific population of L5 pyramidal neurons. This was shown to elicit both monosynaptic excitation and disynaptic inhibition within L2/3, and to modulate the action potential activity of L2/3 pyramidal neurons in vitro and in vivo. I then used retrograde labelling strategies in order to distinguish between IT and PT L5 pyramidal neurons, revealing that the long-range target of the L5 neuron is associated with morphological differences in its local ascending collaterals. To investigate whether the functional consequences, I used a retrograde viral system to deliver ChR2 to each of the projection-defined L5 subtypes. Patch clamp recordings revealed that IT L5 neurons evoked robust excitation but little inhibition within L2/3. In contrast, PT L5 neurons elicited significant levels of synaptic inhibition, which was shown to result from the selective targeting of L2/3 parvalbumin-expressing interneurons. Finally, I was able to demonstrate that this differential recruitment of excitation and inhibition by the L5 subtypes results in opposing modulatory effects upon L2/3 pyramidal neuron activity in vitro and during sensory-driven input in vivo.
The work supports growing evidence that L5 plays a significant role in modulating ongoing local cortical activity. It also suggests that projection-defined L5 subpopulations provide opposing translaminar signals to L2/3, which could be important for sensory processing, sensory-motor integration and synaptic plasticity
Combining whole-cell patch-clamp recordings with single-cell RNA sequencing
No full textTo understand how the brain functions we need to understand the properties of its constituent cells. Whole-cell patch-clamp recordings of neurons have enabled studies of their intrinsic electrical properties as well as their synaptic connectivity within neural circuits. Recent technological advances have now made it possible to combine this with a sampling of their transcriptional profile. Here we provide a detailed description how to combine whole-cell patch-clamp recordings of neurons in brain slices followed by extraction of their cytoplasm suitable for single-cell RNA sequencing and analysis
Perisomatic-targeting interneurons control the initiation of hippocampal population bursts
No full textReplay of spike sequences can be seen during sharp wave – ripple population burst activity in the hippocampus. It is thought that this activity, which occurs during rest and sleep, is involved in memory consolidation. The cellular mechanisms underlying the initiation of these replay events are not well understood. To investigate this, a hippocampal slice model, showing spontaneous sharp wave – ripple activity, and a combination of planar multi-electrode array recordings and whole-cell patch-clamp recordings of anatomically identified hippocampal neurons were used. Firstly, the spatial and temporal profile of sharp waves in vitro was analysed in detail. Sharp waves were generated by changing subpopulations of pyramidal neurons in the CA3 region and had characteristics similar to those found in vivo. Secondly, four major receptor types present in hippocampal CA3, namely NMDA, AMPA, GABAA and GABAB receptors, were investigated for their involvement in sharp wave generation. Surprisingly, not only AMPA receptor-mediated events, but also phasic GABAA receptor-mediated inhibition, were necessary for sharp wave generation. Thirdly, single perisomatic-targeting interneurons were activated. This experiment showed that induced spiking activity of an individual perisomatic-targeting interneuron can both suppress and subsequently enhance local sharp wave generation. Spiking activity of other neuron types (i.e. pyramidal neurons, dendritic-targeting interneurons and interneuron-selective interneurons) had no significant effect on sharp wave incidence. Finally, it is suggested that this post-inhibitory enhancement of sharp wave generation can be mediated by a transient increase in the ratio of excitation to inhibition in the local network. In conclusion, these results suggest a new role for perisomatic-targeting interneurons in controlling the local initiation of sharp waves by selectively suppressing and subsequently enhancing recruitment of a subpopulation of pyramidal neurons. These results further imply that interneurons may play an integral part in the local information processing that takes place in the hippocampal network
The role of neural progenitor diversity in striatal neural circuit formation and function
No full textThe striatum transforms excitatory inputs from across the cortex and thalamus into inhibitory outputs that shape basal ganglia activity and control motor and cognitive behaviours. GABAergic spiny projection neurons (SPNs) are the major neuronal cell type within the striatum and are typically classified into broad populations according to their output targets and location within spatial and neurochemical domains. However, how the cellular and circuit diversity within these populations is established developmentally is largely unknown. As SPNs are born from heterogeneous pools of embryonic neural progenitors, this thesis investigates whether distinct developmental origins can manifest as differences between adult SPNs.
In utero labelling was used to distinguish a molecularly and morphologically distinct apical intermediate progenitor (aIP) pool from other progenitors (OPs) and aIP and OP-derived SPNs were compared with a range of techniques in the postnatal striatum. Firstly, aIP-derived SPNs are shown to have greater dendritic complexity than OP-derived SPNs while sharing similar intrinsic electrical properties. Next, using “patch-seq”, aIP-derived SPNs are found to be enriched for a specific SPN subtype, providing a new link between progenitor diversity and the reported transcriptional heterogeneity of the striatum. Neural circuit mapping approaches then uncover that aIP-derived SPNs receive a weaker excitatory drive from the parafascicular nucleus of the thalamus, showing that progenitor origin can bias the fine-scale synaptic connectivity of striatal SPNs. Finally, single-cell RNA-seq during striatal neural circuit assembly identifies gene regulatory network activity in developing SPNs that reflects their progenitor origin and likely drives their functional maturation. Taken together, these results demonstrate that cellular and circuit complexity in the postnatal striatum can arise from distinct progenitor pools and establish progenitor origin as a novel functional division of SPNs
Histamine, neuroinflammation and neurodevelopment: a review
No full textThe biogenic amine, histamine, has been shown to critically modulate inflammatory processes as well as the properties of neurons and synapses in the brain, and is also implicated in the emergence of neurodevelopmental disorders. Indeed, a reduction in the synthesis of this neuromodulator has been associated with the disorders Tourette’s syndrome and obsessive-compulsive disorder, with evidence that this may be through the disruption of the corticostriatal circuitry during development. Furthermore, neuroinflammation has been associated with alterations in brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there are suggestions that histamine deficiency may leave the developing brain more vulnerable to proinflammatory insults. While most studies have focused on neuronal sources of histamine it remains unclear to what extent other (non-neuronal) sources of histamine, e.g., from mast cells and other sources, can impact brain development. The few studies that have started exploring this in vitro, and more limited in vivo, would indicate that non-neuronal released histamine and other preformed mediators can influence microglial-mediated neuroinflammation which can impact brain development. In this Review we will summarize the state of the field with regard to non-neuronal sources of histamine and its impact on both neuroinflammation and brain development in key neural circuits that underpin neurodevelopmental disorders. We will also discuss whether histamine receptor modulators have been efficacious in the treatment of neurodevelopmental disorders in both preclinical and clinical studies. This could represent an important area of future research as early modulation of histamine from neuronal as well as non-neuronal sources may provide novel therapeutic targets in these disorders
Histamine and the striatum.
No full textThe neuromodulator histamine is released throughout the brain during periods of wakefulness. Combined with an abundant expression of histamine receptors, this suggests potential widespread histaminergic control of neural circuit activity. However, the effect of histamine on many of these circuits is unknown. In this review we will discuss recent evidence for histaminergic modulation of the basal ganglia circuitry, and specifically its main input nucleus; the striatum. Furthermore, we will discuss recent findings of histaminergic dysfunction in several basal ganglia disorders, including in Parkinson's disease and most prominently, in Tourette's syndrome, which has led to a resurgence of interest in this neuromodulator. Combined, these recent observations not only suggest a central role for histamine in modulating basal ganglia activity and behaviour, but also as a possible target in treating basal ganglia disorders
Investigating predictive coding as a principle of function in sensory systems
No full textAccording to the theory of predictive coding, the brain does not generate sensations simply by ’sponging up’ bottom-up information arriving from the senses. Instead, cortical circuits are fundamentally wired to actively make predictions on the basis of experience, communicate these in a top-down fashion to areas with access to less global information, and prop- agate the residual errors that ensue when predictions are compared with reality (Friston, 2005). As these error signals feed forward through the processing hierarchy, they allow each successive processing stage to refine its predictions, allowing future stimulation to be better predicted and, in some ways, better ’understood’ by the brain’s internal model of the world. The critical predictive and error signals are thought to stem from anatomically distinct neural populations, with predictions represented both in the deep and superficial cortical layers, and errors localised to superficial sites (Kanai et al., 2015). Evidence that the superficial and deep layers are, to some degree, specialised for bottom-up and top-down communications, affiliated with activity in the γ (30-100 Hz) and α-β (8-25 Hz) frequency bands respectively (Bastos et al., 2015; Michalareas et al., 2016) has also raised the possibility that predictions and errors are encoded in different spectral channels. The goal of this thesis work was to examine the hypothesised anatomical and functional affiliation of predictive coding variables with distinct cortical layers and their oscillatory signatures. This question was pursued on a range of scales - from the behaving animal and isolated in vitro brain circuit, to the single cell.
In the first chapter, I piloted two behavioural paradigms in which mice made 2-Alternative Forced Choice judgements about visual stimuli while auditory cues predicting stimulus orientation were used to incentivise sensory predictions. The goal was to eventually conduct translaminar multi-electrode recordings to test whether neuronal activity patterns in the mouse visual cortex carry decodable predictive representations, and whether these are localised to any cortical layer or frequency band. I found that, while mice successfully acquired the first, stimulus location discrimination task, their behaviour showed no evidence of using auditory cues to produce orientation specific stimulus predictions. Furthermore, mice failed to achieve above-chance performance on the second, same/different orientation judgement task. Thus, neither task was appropriate for exploring predictive coding in the visual system of the behaving mouse.
In the second chapter, I conducted two series of experiments, combining local field potential recordings with optogenetics, to explore whether the proposed oscillatory signatures of predictive coding can be found in a local circuit of mouse primary somatosensory cortex (S1) in vitro. Slices of S1 expressing the photosensitive channelrhodopsin under the CaMKII promoter received stimulus patterns which followed particular statistical norms. On a minority of trials, norms were violated with stimuli which either deviated from the expected stimulation amplitude (Exp 1) or locus of stimulation within the barrel column (Exp 2). Both experiments provided no evidence that power or peak frequency in the γ-band - the alleged carrier of prediction error signals - was sensitive to stimulus (un)predictability. Furthermore, I obtained no evidence that power in the α-band represented sensory predictions, on an acute or long-term temporal basis. α-power did not differentiate between conditions with diverging likely futures, and failed to scale with increasing stimulation history which goes hand-in-hand with accrual of sensory expectation (van Ede et al., 2010; van Pelt et al., 2016).
In the third chapter, I combined patch-clamp electrophysiology and computational modelling to explore how single cell computation is affected by the γ rhythm - a critical information carrier in the predictive coding scheme. Previous research has shown that γ-modulated excitation boosts the input-output slopes of cortical pyramidal cells over and above what can be achieved with arrhythmic excitation (Sohal et al., 2009). However, it is still unclear whether the same enhanced response properties can be achieved with γ-rhythmic inhibition, which is critical for inducing network γ oscillations (Mann et al., 2005; Traub et al., 1996; Whittington et al., 1995), and whether the γ-rhythm is unique in this respect. This question was explored by examining spike patterns of putative pyramidal cells in mouse S1 in response to excitatory current pulses of increasing amplitudes, while simulated rhythmic inhibitory post-synaptic potentials (IPSGs) were administered with varying degrees of γ-power, using dynamic clamp. The experiment revealed that, with increasing γ-power in the IPSG, pyramidal cells achieved higher maximal spike rates, as well as exhibiting higher input-output slopes and spike timing fidelity. A further experiment was conducted to compare the effects of varying the frequency of inhibition, finding that γ-frequency inhibition (40 Hz) appeared to be optimal in its ability to boost the input-output properties of pyramidal cells. A computational model of the single cell was able to replicate these effects by simulating conditions of low M-type K+ channel conductance and high flux through voltage-gated Ca2+ channels, shedding some light on the potential underpinnings of γ’s unique effects on single-cell computation
Of sounds, photons and maps: in vivo optical characterisation of the auditory thalamocortical system
No full textAs information ascends up the sensory streams, the maps of receptor surface might be faithfully relayed from one brain structure to another, degraded or lost altogether. Such transformations can inform about the type of circuit computation carried out in each region. In vivo high-resolution imaging methods, like two-photon microscopy, are useful for characterising the functional architecture of neuronal circuits down to the micro-scale. To better understand the rules governing thalamocortical connectivity and the origin of cortical maps, I used in vivo two-photon calcium imaging to characterise the properties of thalamic axons innervating different layers and subfields of mouse auditory cortex. Although topographically organised at a global level, I found the frequency selectivity of individual thalamocortical axons to be surprisingly heterogeneous, even in the middle layers (L3b/4) of the primary cortical areas where the thalamic input is dominated by the lemniscal projection. Subsequently, I employed a dual-colour imaging approach to explore the spectral transformations taking place between the thalamocortical projection and granular and supragranular layers of auditory cortex. Some differences in local spectral properties between neurons in L4 and L2/3 were observed in awake, passively listening mice, confirming previous observations on the anaesthetised preparation. Finally, I present a demonstration of a novel fibre-based high-resolution fluorescence imaging method to optically investigate neuronal circuits in deep brain regions, with minimal invasiveness. In short, my work provides some new insights on the functional micro-organisation of the auditory thalamocortical system and constitutes a classical example of the power of optical methods for the study of neuronal circuits in vivo.</p
From Progenitors to Progeny: Shaping Striatal Circuit Development and Function
No full textUnderstanding how neurons of the striatum are formed and integrate into complex synaptic circuits is essential to provide insight into striatal function in health and disease. In this review, we summarize our current understanding of the development of striatal neurons and associated circuits with a focus on their embryonic origin. Specifically, we address the role of distinct types of embryonic progenitors, found in the proliferative zones of the ganglionic eminences in the ventral telencephalon, in the generation of diverse striatal interneurons and projection neurons. Indeed, recent evidence would suggest that embryonic progenitor origin dictates key characteristics of postnatal cells, including their neurochemical content, their location within striatum, and their long-range synaptic inputs. We also integrate recent observations regarding embryonic progenitors in cortical and other regions and discuss how this might inform future research on the ganglionic eminences. Last, we examine how embryonic progenitor dysfunction can alter striatal formation, as exemplified in Huntington's disease and autism spectrum disorder, and how increased understanding of embryonic progenitors can have significant implications for future research directions and the development of improved therapeutic options. SIGNIFICANCE STATEMENT This review highlights recently defined novel roles for embryonic progenitor cells in shaping the functional properties of both projection neurons and interneurons of the striatum. It outlines the developmental mechanisms that guide neuronal development from progenitors in the embryonic ganglionic eminences to progeny in the striatum. Where questions remain open, we integrate observations from cortex and other regions to present possible avenues for future research. Last, we provide a progenitor-centric perspective onto both Huntington's disease and autism spectrum disorder. We suggest that future investigations and manipulations of embryonic progenitor cells in both research and clinical settings will likely require careful consideration of their great intrinsic diversity and neurogenic potential
