Delineating mechanisms underlying parvalbumin neuron impairment in different neurological and neurodegenerative disorders: the emerging role of mitochondrial dysfunction

Given the current paucity of effective treatments in many neurological disorders, delineating pathophysiological mechanisms among the major psychiatric and neurodegenerative diseases may fuel the development of novel, potent treatments that target shared pathways. Recent evidence suggests that various pathological processes, including bioenergetic failure in mitochondria, can perturb the function of fast-spiking, parvalbumin-positive neurons (PV+). These inhibitory neurons critically influence local circuit regulation, the generation of neuronal network oscillations and complex brain functioning. Here, we survey PV+ cell vulnerability in the major neuropsychiatric, and neurodegenerative diseases and review associated cellular and molecular pathophysiological alterations purported to underlie disease aetiology.

AAV8 vector induced gliosis following neuronal transgene expression

Introduction

Expression of light sensitive ion channels by selected neurons has been achieved by viral mediated transduction with gene constructs, but for this to have therapeutic uses, for instance in treating epilepsy, any adverse effects of viral infection on the cerebral cortex needs to be evaluated. Here, we assessed the impact of adeno-associated virus 8 (AAV8) carrying DNA code for a soma targeting light activated chloride channel/FusionRed (FR) construct under the CKIIa promoter.

Methods

Viral constructs were harvested from transfected HEK293 cells in vitro and purified. To test functionality of the opsin, cultured rodent neurons were transduced and the light response of transduced neurons was assayed using whole-cell patch-clamp recordings. In vivo expression was confirmed by immunofluorescence for FR. Unilateral intracranial injections of the viral construct were made into the mouse neocortex and non-invasive fluorescence imaging of FR expression made over 1-4 weeks post-injection using an IVIS Spectrum system. Sections were also prepared from injected mouse cortex for immunofluorescence staining of FR, alongside glial and neuronal marker proteins.

Results

In vitro, cortical neurons were successfully transduced, showing appropriate physiological responses to light stimulation. Following injections in vivo, transduction was progressively established around a focal injection site over a 4-week period with spread of transduction proportional to the concentration of virus introduced. Elevated GFAP immunoreactivity, a marker for reactive astrocytes, was detected near injection sites associated with, and proportional to, local FR expression. Similarly, we observed reactive microglia around FR expressing cells. However, we found that the numbers of NeuN+ neurons were conserved close to the injection site, indicating that there was little or no neuronal loss. In control mice, injected with saline only, astrocytosis and microgliosis was limited to the immediate vicinity of the injection site. Injections of opsin negative viral constructs resulted in comparable levels of astrocytic reaction as seen with opsin positive constructs.

Discussion

We conclude that introduction of an AAV8 vector transducing expression of a transgene under a neuron specific promotor evokes a mild inflammatory reaction in cortical tissue without causing extensive short-term neuronal loss. The expression of an opsin in addition to a fluorescent protein does not significantly increase neuroinflammation.

A novel mouse model of mitochondrial disease exhibits juvenile-onset severe neurological impairment due to parvalbumin cell mitochondrial dysfunction

Mitochondrial diseases comprise a common group of neurometabolic disorders resulting from OXPHOS defects, that may manifest with neurological impairments, for which there are currently no disease-modifying therapies. Previous studies suggest inhibitory interneuron susceptibility to mitochondrial impairment, especially of parvalbumin-expressing interneurons (PV+). We have developed a mouse model of mitochondrial dysfunction specifically in PV+ cells via conditional Tfam knockout, that exhibited a juvenile-onset progressive phenotype characterised by cognitive deficits, anxiety-like behaviour, head-nodding, stargazing, ataxia, and reduced lifespan. A brain region-dependent decrease of OXPHOS complexes I and IV in PV+ neurons was detected, with Purkinje neurons being most affected. We validated these findings in a neuropathological study of patients with pathogenic mtDNA and POLG variants showing PV+ interneuron loss and deficiencies in complexes I and IV. This mouse model offers a drug screening platform to propel the discovery of therapeutics to treat severe neurological impairment due to mitochondrial dysfunction.

Expression of the schizophrenia associated gene FEZ1 in the early developing fetal human forebrain

Introduction

The protein fasciculation and elongation zeta-1 (FEZ1) is involved in axon outgrowth but potentially interacts with various proteins with roles ranging from intracellular transport to transcription regulation. Gene association and other studies have identified FEZ1 as being directly, or indirectly, implicated in schizophrenia susceptibility. To explore potential roles in normal early human forebrain neurodevelopment, we mapped FEZ1 expression by region and cell type.

Methods

All tissues were provided with maternal consent and ethical approval by the Human Developmental Biology Resource. RNAseq data were obtained from previously published sources. Thin paraffin sections from 8 to 21 post-conceptional weeks (PCW) samples were used for RNAScope in situ hybridization and immunohistochemistry against FEZ1 mRNA and protein, and other marker proteins.

Results

Tissue RNAseq revealed that FEZ1 is highly expressed in the human cerebral cortex between 7.5-17 PCW and single cell RNAseq at 17-18 PCW confirmed its expression in all neuroectoderm derived cells. The highest levels were found in more mature glutamatergic neurons, the lowest in GABAergic neurons and dividing progenitors. In the thalamus, single cell RNAseq similarly confirmed expression in multiple cell types. In cerebral cortex sections at 8-10 PCW, strong expression of mRNA and protein appeared confined to post-mitotic neurons, with low expression seen in progenitor zones. Protein expression was observed in some axon tracts by 16-19 PCW. However, in sub-cortical regions, FEZ1 was highly expressed in progenitor zones at early developmental stages, showing lower expression in post-mitotic cells.

Discussion

FEZ1 has different expression patterns and potentially diverse functions in discrete forebrain regions during prenatal human development.

Clinical and Non-Clinical Cardiovascular Disease Associated Pathologies in Parkinson's Disease

Despite considerable breakthroughs in Parkinson's disease (PD) research, understanding of non-motor symptoms (NMS) in PD remains limited. The lack of basic level models that can properly recapitulate PD NMS either in vivo or in vitro complicates matters. Even so, recent research advances have identified cardiovascular NMS as being underestimated in PD. Considering that a cardiovascular phenotype reflects sympathetic autonomic dysregulation, cardiovascular symptoms of PD can play a pivotal role in understanding the pathogenesis of PD. In this study, we have reviewed clinical and non-clinical published papers with four key parameters: cardiovascular disease risks, electrocardiograms (ECG), neurocardiac lesions in PD, and fundamental electrophysiological studies that can be linked to the heart. We have highlighted the points and limitations that the reviewed articles have in common. ECG and pathological reports suggested that PD patients may undergo alterations in neurocardiac regulation. The pathological evidence also suggested that the hearts of PD patients were involved in alpha-synucleinopathy. Finally, there is to date little research available that addresses the electrophysiology of in vitro Parkinson's disease models. For future reference, research that can integrate cardiac electrophysiology and pathological alterations is required.

Closed-loop optogenetic control of the dynamics of neural activity in non-human primates

Electrical neurostimulation is effective in the treatment of neurological disorders, but associated recording artefacts generally limit its applications to open-loop stimuli. Real-time and continuous closed-loop control of brain activity can, however, be achieved by pairing concurrent electrical recordings and optogenetics. Here we show that closed-loop optogenetic stimulation with excitatory opsins enables the precise manipulation of neural dynamics in brain slices from transgenic mice and in anaesthetized non-human primates. The approach generates oscillations in quiescent tissue, enhances or suppresses endogenous patterns in active tissue and modulates seizure-like bursts elicited by the convulsant 4-aminopyridine. A nonlinear model of the phase-dependent effects of optical stimulation reproduced the modulation of cycles of local-field potentials associated with seizure oscillations, as evidenced by the systematic changes in the variability and entropy of the phase-space trajectories of seizures, which correlated with changes in their duration and intensity. We also show that closed-loop optogenetic neurostimulation could be delivered using intracortical optrodes incorporating light-emitting diodes. Closed-loop optogenetic approaches may be translatable to therapeutic applications in humans.

Anti-Cholestatic Therapy with Obeticholic Acid Improves Short-Term Memory in Bile Duct-Ligated Mice

Patients with cholestatic liver disease, including those with primary biliary cholangitis, can experience symptoms of impaired cognition or brain fog. This phenomenon remains unexplained and is currently untreatable. Bile duct ligation (BDL) is an established rodent model of cholestasis. In addition to liver changes, BDL animals develop cognitive symptoms early in the disease process (before development of cirrhosis and/or liver failure). The cellular mechanisms underpinning these cognitive symptoms are poorly understood. Herein, the study explored the neurocognitive symptom manifestations, and tested potential therapies, in BDL mice, and used human neuronal cell cultures to explore translatability to humans. BDL animals exhibited short-term memory loss and showed reduced astrocyte coverage of the blood-brain barrier, destabilized hippocampal network activity, and neuronal senescence. Ursodeoxycholic acid (first-line therapy for most human cholestatic diseases) did not reverse symptomatic or mechanistic aspects. In contrast, obeticholic acid (OCA), a farnesoid X receptor agonist and second-line anti-cholestatic agent, normalized memory function, suppressed blood-brain barrier changes, prevented hippocampal network deficits, and reversed neuronal senescence. Co-culture of human neuronal cells with either BDL or human cholestatic patient serum induced cellular senescence and increased mitochondrial respiration, changes that were limited again by OCA. These findings provide new insights into the mechanism of cognitive symptoms in BDL animals, suggesting that OCA therapy or farnesoid X receptor agonism could be used to limit cholestasis-induced neuronal senescence.

A Closed-Loop Optogenetic Platform

Neuromodulation is an established treatment for numerous neurological conditions, but to expand the therapeutic scope there is a need to improve the spatial, temporal and cell-type specificity of stimulation. Optogenetics is a promising area of current research, enabling optical stimulation of genetically-defined cell types without interfering with concurrent electrical recording for closed-loop control of neural activity. We are developing an open-source system to provide a platform for closed-loop optogenetic neuromodulation, incorporating custom integrated circuitry for recording and stimulation, real-time closed-loop algorithms running on a microcontroller and experimental control via a PC interface. We include commercial components to validate performance, with the ultimate aim of translating this approach to humans. In the meantime our system is flexible and expandable for use in a variety of preclinical neuroscientific applications. The platform consists of a Controlling Abnormal Network Dynamics using Optogenetics (CANDO) Control System (CS) that interfaces with up to four CANDO headstages responsible for electrical recording and optical stimulation through custom CANDO LED optrodes. Control of the hardware, inbuilt algorithms and data acquisition is enabled via the CANDO GUI (Graphical User Interface). Here we describe the design and implementation of this system, and demonstrate how it can be used to modulate neuronal oscillations in vitro and in vivo.

Increased hippocampal excitability in miR-324-null mice

MicroRNAs are non-coding RNAs that act to downregulate the expression of target genes by translational repression and degradation of messenger RNA molecules. Individual microRNAs have the ability to specifically target a wide array of gene transcripts, therefore allowing each microRNA to play key roles in multiple biological pathways. miR-324 is a microRNA predicted to target thousands of RNA transcripts and is expressed far more highly in the brain than in any other tissue, suggesting that it may play a role in one or multiple neurological pathways. Here we present data from the first global miR-324-null mice, in which increased excitability and interictal discharges were identified in vitro in the hippocampus. RNA sequencing was used to identify differentially expressed genes in miR-324-null mice which may contribute to this increased hippocampal excitability, and 3'UTR luciferase assays and western blotting revealed that two of these, Suox and Cd300lf, are novel direct targets of miR-324. Characterisation of microRNAs that produce an effect on neurological activity, such as miR-324, and identification of the pathways they regulate will allow a better understanding of the processes involved in normal neurological function and in turn may present novel pharmaceutical targets in treating neurological disease.

Hippocampal network hyperexcitability in young transgenic mice expressing human mutant alpha-synuclein

Abnormal excitability in cortical networks has been reported in patients and animal models of Alzheimer's disease (AD), and other neurodegenerative conditions. Whether hyperexcitability is a core feature of alpha(α)-synucleinopathies, including dementia with Lewy bodies (DLB) is unclear. To assess this, we used two murine models of DLB that express either human mutant α-synuclein (α-syn) the hA30P, or human wild-type α-syn (hWT-α-syn) mice. We observed network hyperexcitability in vitro in young (2–5 months), pre-symptomatic transgenic α-syn mice. Interictal discharges (IIDs) were seen in the extracellular local field potential (LFP) in the hippocampus in hA30P and hWT-α-syn mice following kainate application, while only gamma frequency oscillations occurred in control mice. In addition, the concentration of the GABA_A receptor antagonist (gabazine) needed to evoke IIDs was lower in slices from hA30P mice compared to control mice. hA30P mice also showed increased locomotor activity in the open field test compared to control mice. Intracellular recordings from CA3 pyramidal cells showed a more depolarised resting membrane potential in hA30P mice. Quadruple immunohistochemistry for human α-syn, and the mitochondrial markers, porin and the complex IV enzyme cytochrome c oxidase subunit 1 (COX1) in parvalbumin (PV+)-expressing interneurons showed that 25% of PV+ cells contained human α-syn in hA30P mice. While there was no change in PV expression, COX1 expression was significantly increased in PV+ cells in hA30P mice, perhaps reflecting a compensatory change to support PV+ interneuron activity. Our findings suggest that hippocampal network hyperexcitability may be an important early consequence of α-syn-mediated impairment of neuronal/synaptic function, which occurs without any overt loss of PV interneurons. The therapeutic benefit of targeting network excitability early in the disease stage should be explored with respect to α-synucleinopathies such as DLB.

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Young transgenic α-syn mice exhibit network hyperexcitability in the hippocampus in vitro.

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Young transgenic α-syn mice have increased locomotor activity in an open field test.

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Hippocampal pyramidal cells are more depolarised in young transgenic α-syn mice.

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Increased mitochondrial cytochrome c oxidase (complex IV) function in PV+ interneurons in young transgenic a-syn mice.

Anti-inflammatory treatment rescues memory deficits during aging in nfkb1-/- mice

Chronic inflammation is a common feature of many age-related conditions including neurodegenerative diseases such as Alzheimer's disease. Cellular senescence is a state of irreversible cell-cycle arrest, thought to contribute to neurodegenerative diseases partially via induction of a chronic pro-inflammatory phenotype. In this study, we used a mouse model of genetically enhanced NF-κB activity (nfκb1-/- ), characterized by low-grade chronic inflammation and premature aging, to investigate the impact of inflammaging on cognitive decline. We found that during aging, nfkb1-/- mice show an early onset of memory loss, combined with enhanced neuroinflammation and increased frequency of senescent cells in the hippocampus and cerebellum. Electrophysiological measurements in the hippocampus of nfkb1-/- mice in vitro revealed deficits in gamma frequency oscillations, which could explain the decline in memory capacity. Importantly, treatment with the nonsteroidal anti-inflammatory drug (NASID) ibuprofen reduced neuroinflammation and senescent cell burden resulting in significant improvements in cognitive function and gamma frequency oscillations. These data support the hypothesis that chronic inflammation is a causal factor in the cognitive decline observed during aging.

Early Disruption of Cortical Sleep-Related Oscillations in a Mouse Model of Dementia With Lewy Bodies (DLB) Expressing Human Mutant (A30P) Alpha-Synuclein

Changes in sleep behavior and sleep-related cortical activity have been reported in conditions associated with abnormal alpha-synuclein (α-syn) expression, in particular Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). Notably, changes can occur in patients years before the onset of cognitive decline. Sleep-related network oscillations play a key role in memory function, but how abnormal α-syn impacts the generation of such activity is currently unclear. To determine whether early changes in sleep-related network activity could also be observed, prior to any previously reported cognitive dysfunction, we used mice that over-express human mutant α-syn (A30P). Recordings in vivo were performed under urethane anesthesia in the medial prefrontal cortex (mPFC) and CA1 region of the hippocampus in young male (2.5 – 4 months old) A30P and age-matched wild type (WT) mice. We found that the slow oscillation (SO) < 1 Hz frequency was significantly faster in both the mPFC and hippocampus in A30P mice, and Up-state-associated fast oscillations at beta (20 – 30 Hz) and gamma (30 – 80 Hz) frequencies were delayed relative to the onset of the Up-state. Spindle (8 – 15 Hz) activity in the mPFC was also altered in A30P mice, as spindles were shorter in duration and had reduced density compared to WT. These changes demonstrate that dysregulation of sleep-related oscillations occurs in young A30P mice long before the onset of cognitive dysfunction. Our data suggest that, as seen in patients, changes in sleep-related oscillations are an early consequence of abnormal α-syn aggregation in A30P mice.

Electrical stimulation of the ventral tegmental area evokes sleep-like state transitions under urethane anaesthesia in the rat medial prefrontal cortex via dopamine D1 -like receptors

The role of dopamine in regulating sleep-state transitions during, both natural sleep and under anaesthesia, is still unclear. Recording in vivo in the rat mPFC under urethane anaesthesia, we observed predominantly slow wave activity (SWA) of <1 Hz in the local field potential interrupted by occasional spontaneous transitions to a low-amplitude-fast (LAF) pattern of activity. During periods of SWA, transitions to LAF activity could be rapidly and consistently evoked by electrical stimulation of the ventral tegmental area (VTA). Spontaneous LAF activity, and that evoked by stimulation of the VTA, consisted of fast oscillations similar to those seen in the rapid eye movement (REM)-like sleep state. Spontaneous and VTA stimulation-evoked LAF activity occurred simultaneously along the dorsoventral extent of all mPFC subregions. Evoked LAF activity depended on VTA stimulation current and could be elicited using either regular (25-50 Hz) or burst stimulation patterns and was reproducible upon repeated stimulation. Simultaneous extracellular single-unit recordings showed that during SWA, presumed pyramidal cells fired phasically and almost exclusively on the Up state, while during both spontaneous and VTA-evoked LAF activity, they fired tonically. The transition to LAF activity evoked by VTA stimulation depended on dopamine D1 -like receptor activation as it was almost completely blocked by systemic administration of the D1 -like receptor antagonist SCH23390. Overall, our data demonstrate that activation of dopamine D1 -like receptors in the mPFC is important for regulating sleep-like state transitions.

Impaired Fast Network Oscillations and Mitochondrial Dysfunction in a Mouse Model of Alpha-synucleinopathy (A30P)

Intracellular accumulation of alpha-synuclein (α-syn) is a key pathological process evident in Lewy body dementias (LBDs), including Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB). LBD results in marked cognitive impairments and changes in cortical networks. To assess the impact of abnormal α-syn expression on cortical network oscillations relevant to cognitive function, we studied changes in fast beta/gamma network oscillations in the hippocampus in a mouse line that over-expresses human mutant α-syn (A30P). We found an age-dependent reduction in the power of the gamma (20-80 Hz) frequency oscillations in slices taken from mice aged 9-16 months (9+A30P), that was not present in either young 2-6 months old (2+A30P) mice, or in control mice at either age. The mitochondrial blockers potassium cyanide and rotenone both reduced network oscillations in a concentration-dependent manner in aged A30P mice and aged control mice but slices from A30P mice showed a greater reduction in the oscillations. Histochemical analysis showed an age-dependent reduction in cytochrome c oxidase (COX) activity, suggesting a mitochondrial dysfunction in the 9+A30P group. A deficit in COX IV expression was confirmed by immunohistochemistry. Overall, our data demonstrate an age-dependent impairment in mitochondrial function and gamma frequency activity associated with the abnormal expression of α-syn. These findings provide mechanistic insights into the consequences of over-expression of α-syn which might contribute to cognitive decline.

Dorsal vs. ventral differences in fast Up-state-associated oscillations in the medial prefrontal cortex of the urethane-anesthetized rat

We demonstrate, in the urethane-anesthetized rat, that within the medial prefrontal cortex (mPFC) there are clear subregional differences in the fast network oscillations associated with the slow oscillation Up-state. These differences, particularly between the dorsal and ventral subregions of the mPFC, may reflect the different functions and connectivity of these subregions.

Cortical slow oscillations (0.1–1 Hz), which may play a role in memory consolidation, are a hallmark of non-rapid eye movement (NREM) sleep and also occur under anesthesia. During slow oscillations the neuronal network generates faster oscillations on the active Up-states and these nested oscillations are particularly prominent in the PFC. In rodents the medial prefrontal cortex (mPFC) consists of several subregions: anterior cingulate cortex (ACC), prelimbic (PrL), infralimbic (IL), and dorsal peduncular cortices (DP). Although each region has a distinct anatomy and function, it is not known whether slow or fast network oscillations differ between subregions in vivo. We have simultaneously recorded slow and fast network oscillations in all four subregions of the rodent mPFC under urethane anesthesia. Slow oscillations were synchronous between the mPFC subregions, and across the hemispheres, with no consistent amplitude difference between subregions. Delta (2–4 Hz) activity showed only small differences between subregions. However, oscillations in the spindle (6–15 Hz)-, beta (20–30 Hz), gamma (30–80 Hz)-, and high-gamma (80–150 Hz)-frequency bands were consistently larger in the dorsal regions (ACC and PrL) compared with ventral regions (IL and DP). In dorsal regions the peak power of spindle, beta, and gamma activity occurred early after onset of the Up-state. In the ventral regions, especially the DP, the oscillatory power in the spindle-, beta-, and gamma-frequency ranges peaked later in the Up-state. These results suggest variations in fast network oscillations within the mPFC that may reflect the different functions and connectivity of these subregions.

NEW & NOTEWORTHY We demonstrate, in the urethane-anesthetized rat, that within the medial prefrontal cortex (mPFC) there are clear subregional differences in the fast network oscillations associated with the slow oscillation Up-state. These differences, particularly between the dorsal and ventral subregions of the mPFC, may reflect the different functions and connectivity of these subregions.

Subregional differences in the generation of fast network oscillations in the rat medial prefrontal cortex (mPFC) in vitro

KEY POINTS

Fast network oscillations in the beta (20-30 Hz) frequency range can be evoked with combined activation of muscarinic and kainate receptors in different subregions of the medial prefrontal cortex (mPFC). Subregional differences were observed as the oscillations in the dorsal prelimbic cortex (PrL) were smaller in magnitude than those in the ventral dorsopeduncular (DP) region, and these differences persisted in trimmed slices containing only PrL and DP regions. Oscillations in both regions were dependent upon GABAA and AMPA receptor activation but NMDA receptor blockade decreased oscillations only in the DP region. Subregional differences in neuronal properties of the presumed pyramidal cells were found between PrL and DP, with many more cells in DP firing rhythmically compared to the PrL region. Presumed inhibitory synaptic potentials (IPSPs) recorded from principal cells were more rhythmic and coherent, and significantly larger in amplitude, in the DP region; the data suggest that variation in the patterns of activity between subregions may reflect distinct functional roles.

ABSTRACT

Fast network oscillations in the beta (20-30 Hz) and low gamma (30-80 Hz) range underlie higher cognitive functions associated with the medial prefrontal cortex (mPFC) including attention and working memory. Using a combination of kainate (KA, 200 nm) and the cholinergic agonist carbachol (Cb, 10 μm) fast network oscillations, in the beta frequency range, were evoked in the rat mPFC in vitro. Oscillations were elicited in the prelimbic (PrL), infralimbic (IL) and the dorsopeduncular (DP) cortex, with the largest oscillations observed in DP cortex. Oscillations in both the PrL and DP were dependent, with slightly different sensitivities, on γ-aminobutyric acid (GABA)A , α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, but only oscillations in the DP were significantly reduced by N-methyl-d-aspartate (NMDA) receptor blockade. Intracellular recordings showed that 9/20 regular spiking (RS) cells in the PrL exhibited a notable cAMP-dependent hyperpolarisation activated current (Ih ) in contrast to 16/17 in the DP cortex. Extracellular single unit recordings showed that the majority of cells in the PrL, and DP regions had interspike firing frequencies (IFFs) at beta (20-30 Hz) frequencies and fired at the peak negativity of the field oscillation. Recordings in DP revealed presumed inhibitory postsynaptic potentials (IPSPs) that were larger in amplitude and more rhythmic than those in the PrL region. Our data suggest that each PFC subregion may be capable of generating distinct patterns of network activity with different cell types involved. Variation in the properties of oscillations evoked in the PrL and DP probably reflects the distinct functional roles of these different PFC regions.

Multiple origins of the cortical γ rhythm

Gamma rhythms (30-80 Hz) are a near-ubiquitous feature of neuronal population activity in mammalian cortices. Their dynamic properties permit the synchronization of neuronal responses to sensory input within spatially distributed networks, transient formation of local neuronal "cell assemblies," and coherent response patterns essential for intercortical regional communication. Each of these phenomena form part of a working hypothesis for cognitive function in cortex. All forms of physiological gamma rhythm are inhibition based, being characterized by rhythmic trains of inhibitory postsynaptic potentials in populations of principal neurons. It is these repeating periods of relative enhancement and attenuation of the responsivity of major cell groups in cortex that provides a temporal structure shared across many millions of neurons. However, when considering the origins of these repeating trains of inhibitory events considerable divergence is seen depending on cortical region studied and mode of activation of gamma rhythm generating networks. Here, we review the evidence for involvement of multiple subtypes of interneuron and focus on different modes of activation of these cells. We conclude that most massively parallel brain regions have different mechanisms of gamma rhythm generation, that different mechanisms have distinct functional correlates, and that switching between different local modes of gamma generation may be an effective way to direct cortical communication streams. Finally, we suggest that developmental disruption of the endophenotype for certain subsets of gamma-generating interneuron may underlie cognitive deficit in psychiatric illness.

NMDA receptor-dependent switching between different gamma rhythm-generating microcircuits in entorhinal cortex

Local circuits in the medial entorhinal cortex (mEC) and hippocampus generate gamma frequency population rhythms independently. Temporal interaction between these areas at gamma frequencies is implicated in memory-a phenomenon linked to activity of NMDA-subtype glutamate receptors. While blockade of NMDA receptors does not affect frequency of gamma rhythms in hippocampus, it exposes a second, lower frequency (25-35 Hz) gamma rhythm in mEC. In experiment and model, NMDA receptor-dependent mEC gamma rhythms were mediated by basket interneurons, but NMDA receptor-independent gamma rhythms were mediated by a novel interneuron subtype-the goblet cell. This cell was distinct from basket cells in morphology, intrinsic membrane properties and synaptic inputs. The two different gamma frequencies matched the different intrinsic frequencies in hippocampal areas CA3 and CA1, suggesting that NMDA receptor activation may control the nature of temporal interactions between mEC and hippocampus, thus influencing the pathway for information transfer between the two regions.

Impairment of hippocampal gamma (γ)-frequency oscillations in vitro in mice overexpressing human amyloid precursor protein (APP)

Alzheimer's disease is associated with a dramatic decline in cognitive performance including hippocampal-dependent memory. We have investigated one feature of hippocampal activity related to memory, the gamma (30-80 Hz)-frequency rhythm. Hippocampal slices from mice overexpressing the human amyloid precursor protein (APP)(SWE) mutation (TAS10) were compared at 8 and 16 months of age with wild-type littermates. In slices obtained from TAS10 mice aged 8 months the gamma-frequency activity evoked with bath application of 200 nm kainate was significantly (P < 0.05; n = 8 slices, five animals) impaired (area power, 5956 +/- 2487 microV(2)) compared to slices from wild-type animals (area power, 18 256 +/- 7880 microV(2)). At 16 months of age there was no longer a significant difference (P > 0.05; n = 11 slices from five animals) between slices from TAS10 and wild-type control mice as the wild-type mice now exhibited a marked age-dependent reduction in gamma-frequency activity (TAS10 area power, 5751 +/- 1573 microV(2); wild-type area power = 5379 +/- 1454 microV(2)). Although no dense-core plaques were evident at 8 months there was detectable amyloid labelling in the TAS10 mice which might account for the deficits in gamma activity observed at this age. Dense plaques were clearly evident in the TAS10, but not wild-type, mice at 16 months of age but no further reductions in gamma-frequency activity were seen in the TAS10 mice. These data suggest that deficits in network function in Alzheimer's disease occur early and are not directly correlated to amyloid load.

A beta2-frequency (20-30 Hz) oscillation in nonsynaptic networks of somatosensory cortex

Beta2 frequency (20-30 Hz) oscillations appear over somatosensory and motor cortices in vivo during motor preparation and can be coherent with muscle electrical activity. We describe a beta2 frequency oscillation occurring in vitro in networks of layer V pyramidal cells, the cells of origin of the corticospinal tract. This beta2 oscillation depends on gap junctional coupling, but it survives a cut through layer 4 and, hence, does not depend on apical dendritic electrogenesis. It also survives a blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors or a blockade of GABA(A) receptors that is sufficient to suppress gamma (30-70 Hz) oscillations in superficial cortical layers. The oscillation period is determined by the M type of K+ current.

Region-specific reduction in entorhinal gamma oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness

Psychiatric illnesses, particularly schizophrenia, are associated with disrupted markers for interneuronal function and interneuron-mediated brain rhythms such as gamma frequency oscillations. Here we investigate a possible link between these two observations in the entorhinal cortex and hippocampus by using a genetic and an acute model of psychiatric illness. Lysophosphatidic acid 1 receptor-deficient (LPA1-deficient) mice show psychomotor-gating deficits and neurochemical changes resembling those seen in postmortem schizophrenia studies. Similar deficits are seen acutely with antagonism of the NMDA subtype of glutamate receptor. Neither model induced any change in power or frequency of gamma rhythms generated by kainate in hippocampal slices. In contrast, a dramatic decrease in the power of gamma oscillations was seen in superficial, but not deep, medial entorhinal cortex layers in both models. Immunolabeling for GABA, parvalbumin, and calretinin in medial entorhinal cortex from LPA1-deficient mice showed an approximately 40% reduction in total GABA- and parvalbumin-containing neurons, but no change in the number of calretinin-positive neurons. This deficit was specific for layer II (LII). No change in the number of neurons positive for these markers was seen in the hippocampus. Acute NMDA receptor blockade, which selectively reduces synaptic drive to LII entorhinal interneurons, also disrupted gamma rhythms in a similar manner in superficial entorhinal cortex, but not in hippocampus. These data demonstrate an area-specific deficit in gamma rhythmogenesis in animal models of psychiatric illness and suggest that loss, or reduction in function, of interneurons having a large NMDA receptor expression may underlie the network dysfunction that is seen.

Tuning the network: modulation of neuronal microcircuits in the spinal cord and hippocampus

Adaptation of an organism to its changing environment ultimately depends on the modification of neuronal activity. The dynamic interaction between cellular components within neuronal networks relies on fast synaptic interaction via ionotropic receptors. However, neuronal networks are also subject to modulation mediated by various metabotropic G-protein-coupled receptors that modify synaptic and neuronal function. Modulation increases the functional complexity of a network, because the same cellular components can produce different outputs depending on the behavioural state of the animal. This review, which is part of the TINS Microcircuits Special Feature, provides an overview of neuromodulation in two neuronal circuits that both produce oscillatory activity but differ fundamentally in function. Hippocampal circuits are compared with the spinal networks generating locomotion, with a view to exploring common principles of neuromodulatory activity.

Synaptic pathways in neural microcircuits

The functions performed by different neural microcircuits depend on the anatomical and physiological properties of the various synaptic pathways connecting neurons. Neural microcircuits across various species and brain regions are similar in terms of their repertoire of neurotransmitters, their synaptic kinetics, their short-term and long-term plasticity, and the target-specificity of their synaptic connections. However, microcircuits can be fundamentally different in terms of the precise recurrent design used to achieve a specific functionality. In this review, which is part of the TINS Microcircuits Special Feature, we compare the connectivity designs in spinal, hippocampal, neocortical and cerebellar microcircuits, and discuss the different computational challenges that each microcircuit faces.

Microcircuits in action – from CPGs to neocortex

To understand the interface between global brain function and molecular neuroscience--that is, the microcircuit level--a major challenge. Such understanding is prerequisite if we are to account for neural function in cellular terms. Very few vertebrate microcircuits are yet understood because their analysis is demanding technically. In this review of the TINS Microcircuits Special Feature, we attempt to shed light on the problem by comparing the operation of four types of microcircuit, to identify common molecular and cellular components. Central pattern generator (CPG) networks underlying rhythmic movements and hippocampal microcircuits that generate gamma and theta rhythms are compared with the neocortical microcircuits used in cognitive tasks and a cerebellar network. The long-term goal is to identify the components of a molecular and synaptic tool kit for the design of different microcircuits.

Oscillatory activity within rat substantia gelatinosa in vitro: a role for chemical and electrical neurotransmission

Although rhythmic behaviour of mammalian spinal ventral horn networks has been extensively studied little is known about oscillogenesis in the spinal dorsal horn. The aims of this in vitro study were to record and determine the underlying mechanisms of potassium-evoked network field oscillations in the substantia gelatinosa of the neonatal rat dorsal horn, a lamina involved in nociceptive processing. Transient pressure ejection of a potassium solution evoked reproducible rhythmic activity in discrete areas of the substantia gelatinosa which lasted for 5-15 s with a single prominent peak in the 4-12 Hz frequency band (7.7 +/- 0.1 Hz, n = 60). Oscillations of similar frequency and amplitude were also observed in isolated dorsal horn quadrants. Application of CNQX (10 microm) reduced peak power amplitude and integrated power area (from 4 to 12 Hz) of the power spectrum, whereas D-AP5 (50 microm) had no effect on the potassium-evoked rhythm. Bicuculline (30 microm) or strychnine (10 microm) reduced the power amplitude and area. On combination of bicuculline (30 microm) and strychnine (10 microm) the reductions in power amplitude and area were not significantly different (P > 0.05) when compared with application of either drug alone. The gap junction blockers carbenoxolone (100 microm) or octanol (1 mM) significantly reduced power amplitude and area. Although TTX (1 microm) or a calcium-free perfusate both caused reductions in the power amplitude and area, potassium-evoked rhythmic activity persisted. However, this persistent rhythm was further reduced on combination of calcium-free perfusate with octanol (1 mM) and was abolished using a cocktail of drugs. Blockade of the potassium delayed rectifier current by tetraethylammonium (5 mM) or the hyperpolarization-activated current (I(h)) by ZD7288 (10 microm) disrupted the synchronization of the potassium-induced oscillation. The frequency of potassium-induced rhythms was unaffected by any of the drugs tested. These novel findings demonstrate that transient pressure ejection of potassium evokes oscillatory activity in the substantia gelatinosa in vitro. This rhythm is partly dependent upon various receptors (AMPA/kainate, GABA(A) and glycine), ion channels (potassium delayed rectifier and I(h)) and gap junctions. Oscillatory behaviour in the substantia gelatinosa could potentially play a role in the processing of nociceptive signals.

Single-column thalamocortical network model exhibiting gamma oscillations, sleep spindles, and epileptogenic bursts

To better understand population phenomena in thalamocortical neuronal ensembles, we have constructed a preliminary network model with 3,560 multicompartment neurons (containing soma, branching dendrites, and a portion of axon). Types of neurons included superficial pyramids (with regular spiking [RS] and fast rhythmic bursting [FRB] firing behaviors); RS spiny stellates; fast spiking (FS) interneurons, with basket-type and axoaxonic types of connectivity, and located in superficial and deep cortical layers; low threshold spiking (LTS) interneurons, which contacted principal cell dendrites; deep pyramids, which could have RS or intrinsic bursting (IB) firing behaviors, and endowed either with nontufted apical dendrites or with long tufted apical dendrites; thalamocortical relay (TCR) cells; and nucleus reticularis (nRT) cells. To the extent possible, both electrophysiology and synaptic connectivity were based on published data, although many arbitrary choices were necessary. In addition to synaptic connectivity (by AMPA/kainate, NMDA, and GABA(A) receptors), we also included electrical coupling between dendrites of interneurons, nRT cells, and TCR cells, and--in various combinations--electrical coupling between the proximal axons of certain cortical principal neurons. Our network model replicates several observed population phenomena, including 1) persistent gamma oscillations; 2) thalamocortical sleep spindles; 3) series of synchronized population bursts, resembling electrographic seizures; 4) isolated double population bursts with superimposed very fast oscillations (>100 Hz, "VFO"); 5) spike-wave, polyspike-wave, and fast runs (about 10 Hz). We show that epileptiform bursts, including double and multiple bursts, containing VFO occur in rat auditory cortex in vitro, in the presence of kainate, when both GABA(A) and GABA(B) receptors are blocked. Electrical coupling between axons appears necessary (as reported previously) for persistent gamma and additionally plays a role in the detailed shaping of epileptogenic events. The degree of recurrent synaptic excitation between spiny stellate cells, and their tendency to fire throughout multiple bursts, also appears critical in shaping epileptogenic events.

Persistent gamma oscillations in superficial layers of rat auditory neocortex: experiment and model

Persistent in vitro gamma oscillations, induced by bath application of carbachol and kainate (amongst other drugs), were discovered by Eberhard Buhl and collaborators in 1998. The oscillations are robust, in that they can continue for hours; but the oscillations are also intricate in their mechanisms: they depend upon phasic synaptic excitation and inhibition, upon electrical coupling between interneurones and between pyramidal neurones, and--at least in neocortex--they depend upon complex intrinsic properties of some of the neurones.

Cellular mechanisms of neuronal population oscillations in the hippocampus in vitro

A variety of population oscillations, at frequencies approximately 5 Hz up to 200 Hz and above, can be induced in hippocampal slices either by (a) manipulation of the ionic environment, or (b) by stimulation of metabotropic receptors; brief oscillations can even occur spontaneously. In this review, we consider in vitro theta (4-12 Hz), gamma/beta (15-70 Hz), and very fast oscillations (VFO) (>70 Hz). Many in vitro oscillations are gated by synaptic inhibition but are influenced by electrical coupling as well; one type depends solely on electrical coupling. For some oscillations dependent upon inhibition, the detailed firing patterns of interneurons can influence long-range synchronization. Two sorts of electrical coupling are important in modulating or generating various in vitro oscillations: (a) between interneurons, primarily between dendrites; and (b) between axons of pyramidal neurons. VFO can exist in isolation or can act as generators of gamma frequency oscillations. Oscillations at gamma frequencies and below probably create conditions under which synaptic plasticity can occur, between selected neurons-even those separated by significant axonal conduction delays.

A role for fast rhythmic bursting neurons in cortical gamma oscillations in vitro

Basic cellular and network mechanisms underlying gamma frequency oscillations (30-80 Hz) have been well characterized in the hippocampus and associated structures. In these regions, gamma rhythms are seen as an emergent property of networks of principal cells and fast-spiking interneurons. In contrast, in the neocortex a number of elegant studies have shown that specific types of principal neuron exist that are capable of generating powerful gamma frequency outputs on the basis of their intrinsic conductances alone. These fast rhythmic bursting (FRB) neurons (sometimes referred to as "chattering" cells) are activated by sensory stimuli and generate multiple action potentials per gamma period. Here, we demonstrate that FRB neurons may function by providing a large-scale input to an axon plexus consisting of gap-junctionally connected axons from both FRB neurons and their anatomically similar counterparts regular spiking neurons. The resulting network gamma oscillation shares all of the properties of gamma oscillations generated in the hippocampus but with the additional critical dependence on multiple spiking in FRB cells.

The role of electrical signaling via gap junctions in the generation of fast network oscillations

In recent years, several key studies have shed new light on the roles of electrical signaling via gap junctions between neurons in the adult brain. In particular, it is now clear that electrical signaling is important, if not essential, for the generation of a wide variety of different network interactions which may underlie rhythmic activity, of cognitive relevance, seen in EEG recordings. Two types of such rhythmic activity observed in the hippocampus both in vivo and in vitro are gamma frequency (30-80Hz) oscillations and ultrafast (>80Hz) "ripple" oscillations. Several lines of work, discussed here, show that gap junction-mediated signaling plays a central role in the generation of both these types of network activity. Recent work also now suggests that a number of different, anatomically discrete, gap junction-mediated networks may exist which could both function and be modulated independently.

GABA-enhanced collective behavior in neuronal axons underlies persistent gamma-frequency oscillations

Gamma (30-80 Hz) oscillations occur in mammalian electroencephalogram in a manner that indicates cognitive relevance. In vitro models of gamma oscillations demonstrate two forms of oscillation: one occurring transiently and driven by discrete afferent input and the second occurring persistently in response to activation of excitatory metabotropic receptors. The mechanism underlying persistent gamma oscillations has been suggested to involve gap-junctional communication between axons of principal neurons, but the precise relationship between this neuronal activity and the gamma oscillation has remained elusive. Here we demonstrate that gamma oscillations coexist with high-frequency oscillations (>90 Hz). High-frequency oscillations can be generated in the axonal plexus even when it is physically isolated from pyramidal cell bodies. They were enhanced in networks by nonsomatic gamma-aminobutyric acid type A (GABA(A)) receptor activation, were modulated by perisomatic GABAA receptor-mediated synaptic input to principal cells, and provided the phasic input to interneurons required to generate persistent gamma-frequency oscillations. The data suggest that high-frequency oscillations occurred as a consequence of random activity within the axonal plexus. Interneurons provide a mechanism by which this random activity is both amplified and organized into a coherent network rhythm.

Sharp wave-like activity in the hippocampus in vitro in mice lacking the gap junction protein connexin 36

Bath application of kainate (100-300 nM) induced a persistent gamma-frequency (30-80 Hz) oscillation that could be recorded in stratum radiatum of the CA3 region in vitro. We have previously described that in knockout mice lacking the gap junction protein connexin 36 (Cx36KO), gamma-frequency oscillations are reduced but still present. We now demonstrate that in the Cx36KO mice, but not in wild-type (WT), large population field excitatory postsynaptic potentials, or sharp wave-burst discharges, also occurred during the on-going gamma-frequency oscillation. These spontaneous burst discharges were not seen in WT mice. Burst discharges in the Cx36KO mice occurred with a mean frequency of 0.23 +/- 0.11 Hz and were accompanied by a series of fast (approximately 60-115 Hz) population spikes or "ripple" oscillations in many recordings. Intracellular recordings from CA3 pyramidal cells showed that the burst discharges consisted of a depolarizing response and presumed coupling potentials (spikelets) could occasionally be seen either before or during the burst discharge. The burst discharges occurring in Cx36KO mice were sensitive to gap junctions blockers as they were fully abolished by carbenoxolone (200 microM). In control mice we made several attempts to replicate this pattern of sharp wave activity/ripples occurring with the on-going kainate-evoked gamma-frequency oscillation by manipulating synaptic and electrical signaling. Partial disruption of inhibition, in control slices, by bath application of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline (1-4 microM) completely abolished all gamma-frequency activity before any burst discharges occurred. Increasing the number of open gap junctions in control slices by using trimethylamine (TMA; 2-10 mM), in conjunction with kainate, failed to elicit any sharp wave bursts or fast ripples. However, bath application of the potassium channel blocker 4-aminopyridine (4-AP; 20-80 microM) produced a pattern of activity in control mice (13/16 slices), consisting of burst discharges occurring in conjunction with kainate-evoked gamma-frequency oscillations, that was similar to that seen in Cx36KO mice. In a few cases (n = 9) the burst discharges were accompanied by fast ripple oscillations. Carbenoxolone also fully blocked the 4-AP-evoked burst discharges (n = 5). Our results show that disruption of electrical signaling in the interneuronal network can, in the presence of kainate, lead to the spontaneous generation of sharp wave/ripple activity similar to that observed in vivo. This suggests a complex role for electrically coupled interneurons in the generation of hippocampal network activity.

Contrasting roles of axonal (pyramidal cell) and dendritic (interneuron) electrical coupling in the generation of neuronal network oscillations

Electrical coupling between pyramidal cell axons, and between interneuron dendrites, have both been described in the hippocampus. What are the functional roles of the two types of coupling? Interneuron gap junctions enhance synchrony of gamma oscillations (25-70 Hz) in isolated interneuron networks and also in networks containing both interneurons and principal cells, as shown in mice with a knockout of the neuronal (primarily interneuronal) connexin36. We have recently shown that pharmacological gap junction blockade abolishes kainate-induced gamma oscillations in connexin36 knockout mice; without such gap junction blockade, gamma oscillations do occur in the knockout mice, albeit at reduced power compared with wild-type mice. As interneuronal dendritic electrical coupling is almost absent in the knockout mice, these pharmacological data indicate a role of axonal electrical coupling in generating the gamma oscillations. We construct a network model of an experimental gamma oscillation, known to be regulated by both types of electrical coupling. In our model, axonal electrical coupling is required for the gamma oscillation to occur at all; interneuron dendritic gap junctions exert a modulatory effect.

A model of atropine-resistant theta oscillations in rat hippocampal area CA1

Theta frequency oscillations are a predominant feature of rhythmic activity in the hippocampus. We demonstrate that hippocampal area CA1 generates atropine-resistant theta population oscillations in response to metabotropic glutamate receptor activation under conditions of reduced AMPA receptor activation. This activity occurred in the absence of inputs from area CA3 and extra-ammonic areas. Field theta oscillations were co-expressed with pyramidal distal apical dendritic burst spiking and were temporally related to trains of IPSPs with slow kinetics. Pyramidal somatic responses showed theta oscillations consisted of compound inhibitory synaptic potentials with initial IPSPs with slow kinetics followed by trains of smaller, faster IPSPs. Pharmacological modulation of IPSPs altered the theta oscillation suggesting an inhibitory network origin. Somatic IPSPs, dendritic burst firing and stratum pyramidale interneuron activity were all temporally correlated with spiking in stratum oriens interneurons demonstrating intrinsic theta-frequency oscillations. Disruption of spiking in these interneurons was accompanied by a loss of both field theta and theta frequency IPSP trains. We suggest that population theta oscillations can be generated as a consequence of intrinsic theta frequency spiking activity in a subset of stratum oriens interneurons controlling electrogenesis in pyramidal cell apical dendrites.

Fast network oscillations induced by potassium transients in the rat hippocampus in vitro

Brief pressure ejection of solutions containing potassium, caesium or rubidium ions into stratum radiatum of the CA1 or CA3 regions of the hippocampal slice evoked a fast network oscillation. The activity evoked lasted approximately 3-25 s with the predominant frequency component being in the gamma frequency range (30-80 Hz), although beta frequency (15-30 Hz) and ultrafast (> 80 Hz) components could also be seen. The gamma frequency component of the oscillation remained constant, even when large changes in power occurred, and was synchronous across the CA1 region. Measurements with potassium ion-sensitive electrodes revealed that the network oscillation was accompanied by increases (0.5 to 2.0 mM) in the extracellular potassium concentration [K+]o. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 microM) had no significant effect but the alpha-amino-3-hydroxy-5-methyl-4-isooxazolepropionic acid (AMPA)/kainate receptor antagonist 2,3,-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide disodium (NBQX; 20 microM) caused a significant reduction (86.7 +/- 4.5 %) in the power in the gamma frequency range. Residual rhythmic activity, presumably arising in the interneuronal network, was blocked by the GABA(A) receptor antagonist bicuculline. The putative gap junction blocker octanol caused a decrease in the power of the gamma frequency component of 75.5 +/- 5.6 %, while carbenoxolone produced a reduction of only 14 +/- 42 %. These experiments demonstrate that a modest increase in exogenous [K+]o in the hippocampus in vitro is sufficient to evoke a fast network oscillation, which is an emergent property of the synaptically and electrically interconnected neuronal network.

Fast network oscillations in the rat dentate gyrus in vitro

The dentate gyrus is a prominent source of gamma frequency activity in the hippocampal formation in vivo. Here we show that transient epochs of gamma frequency network activity (67 +/- 12 Hz) can be generated in the dentate gyrus of rat hippocampal slices, following brief pressure ejections of a high-molarity potassium solution onto the molecular layer. Oscillatory activity remains synchronized over distances >300 microm and is accompanied by a modest rise in [K(+)](o). Gamma frequency oscillations were abolished by a GABA(A) receptor antagonist demonstrating their dependence on rhythmic inhibition. However, in many cases, higher frequency oscillations (>80 Hz) remained in the absence of synaptic transmission, thus demonstrating that nonsynaptic factors may underlie fast oscillatory activity.