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Computation within cortical microcircuits is determined by functional properties of the neurons and their synaptic interactions. While heterogeneity of inhibitory interneurons is well established, the anatomical, physiological, and molecular differentiation of excitatory pyramidal neurons is not fully resolved. To identify functional subtypes within the pyramidal neuron population, we focused on human layer 2-3 cortex which greatly expanded during evolution. We performed multi-neuron patch-clamp recordings in brain slices from the temporal cortex of 22 epilepsy patients. We characterized the electrophysiological properties of up to 80 pyramidal neurons per patient, enabling us to assess inter- and intra-individual functional variability. Hierarchical clustering of the high-dimensional parameter space yielded functionally distinct clusters of pyramidal neurons which were present across individuals. This may represent a generic organizational principle converging with previously described transcriptomic heterogeneity. We further observed substantial heterogeneity in physiological parameters with intra-individual variability being severalfold larger than inter-individual variability. The phenotypic variability within and across pyramidal neuron subtypes has important implications for the computational capacity of the cortical microcircuit.

The medial entorhinal cortex (mEC) shows a high degree of spatial tuning, predominantly grid-cell activity, which is reliant on robust, dynamic inhibition provided by local interneurons (INs). In fact, feedback inhibitory microcircuits involving fast-spiking parvalbumin (PV) basket cells (BCs) are believed to contribute dominantly to the emergence of grid-field firing in principal cells (PrCs). However, the strength of PV BC-mediated inhibition onto PrCs is not uniform in this region, but high in the dorsal and weak in the ventral mEC. This is in good correlation with divergent grid field sizes, but the underlying morphological and physiological mechanisms remain unknown. In this study, we examined PV BCs in layer 2/3 of the mEC characterizing their intrinsic physiology, morphology, and synaptic connectivity in the juvenile rat. We show that while intrinsic physiology and morphology are broadly similar over the dorso-ventral axis, PV BCs form more connections onto local PrCs in the dorsal mEC, independent of target cell type. In turn, the major PrC subtypes, pyramidal (PC) and stellate cells (SC), form connections onto PV BCs with lower, but equal probability. These data thus identify inhibitory connectivity as source of the gradient of inhibition, plausibly explaining divergent grid field formation along this dorso-ventral axis of the mEC. Significance Statement Inhibition by parvalbumin basket cells (PV BCs) is essential for the emergence of grid firing in principal cells (PrCs) in the medial entorhinal cortex (mEC).The strength of PV BC-mediated inhibition decreases along the dorso-ventral axis, in correlation with the grid field size of spatially tuned PrCs. In this study, to identify underlying cellular mechanisms, we combined electrophysiological recordings and neuroanatomical analysis investigating properties and connectivity of PV BCs in layer 2/3 of the rat mEC. While morphological and physiological properties were largely uniform, IN-PrC connectivity was higher in the dorsal than ventral mEC. Thus, our results indentify a difference in PV BC connectivity as source of the inhibitory gradient in the mEC with implications for grid size modulation.

The inhibitory GABAergic system in the brain is involved in the etiology of various psychiatric problems, including autism spectrum disorders (ASD), attention deficit hyperactivity disorder (ADHD) and others. These disorders are influenced not only by genetic but also by environmental factors, such as preterm birth, although the underlying mechanisms are not known. In a translational hyperoxia model, exposing mice pups at P5 to 80% oxygen for 48 h to mimic a steep rise of oxygen exposure caused by preterm birth from in utero into room air, we documented a persistent reduction of cortical mature parvalbumin-expressing interneurons until adulthood. Developmental delay of cortical myelin was observed, together with decreased expression of oligodendroglial glial cell-derived neurotrophic factor (GDNF), a factor involved in interneuronal development. Electrophysiological and morphological properties of remaining interneurons were unaffected. Behavioral deficits were observed for social interaction, learning and attention. These results demonstrate that neonatal oxidative stress can lead to decreased interneuron density and to psychiatric symptoms. The obtained cortical myelin deficit and decreased oligodendroglial GDNF expression indicate that an impaired oligodendroglial-interneuronal interplay contributes to interneuronal damage.

The morphological structure of neurons provides the basis for their functions and is a major focus of contemporary neuroscience studies. Intracellular staining of single cells in acute slices is a well-established approach, offering high-resolution information on neuronal morphology, complementing their physiology. Despite major technical advances, however, a common histological artifact often precludes precise morphological analysis: shrinkage of the sampled tissue after embedding for microscopy. Here, we describe a new approach using a metal spacer, sandwiched between two coverslips to reduce shrinkage of whole-mount slice preparations during embedding with aqueous mounting medium under a coverslip. This approach additionally allows imaging the slices from both sides to obtain better quality images of deeper structures. We demonstrate that the use of this spacer system can efficiently and stably reduce the shrinkage of slices, whereas conventional embedding methods without spacer or with agar spacer cause severe, progressive shrinkage after embedding. We further show that the shrinkage of slices is not uniform and artifacts in morphology and anatomical parameters produced cannot be compensated using linear correction algorithms. Our study, thus, emphasizes the importance of preventing the deformation of slice preparations and offers an effective means for reducing shrinkage and associated artifacts during embedding. Electronic supplementary material The online version of this article (10.1007/s00429-020-02141-3) contains supplementary material, which is available to authorized users.

Alterations of the hyperpolarization activated nonselective cation current (I

miR-128, a brain-enriched microRNA, has been implicated in the control of neurogenesis and synaptogenesis but its potential roles in intervening processes have not been addressed. We show that post-transcriptional mechanisms restrict miR-128 accumulation to post-mitotic neurons during mouse corticogenesis and in adult stem cell niches. Whereas premature miR-128 expression in progenitors for upper layer neurons leads to impaired neuronal migration and inappropriate branching, sponge-mediated inhibition results in overmigration. Within the upper layers, premature miR-128 expression reduces the complexity of dendritic arborization, associated with altered electrophysiological properties. We show that Phf6, a gene mutated in the cognitive disorder Börjeson-Forssman-Lehmann syndrome, is an important regulatory target for miR-128. Restoring PHF6 expression counteracts the deleterious effect of miR-128 on neuronal migration, outgrowth and intrinsic physiological properties. Our results place miR-128 upstream of PHF6 in a pathway vital for cortical lamination as well as for the development of neuronal morphology and intrinsic excitability. DOI: http://dx.doi.org/10.7554/eLife.04263.001, eLife digest The unique capabilities of the mammalian brain depend on the patterns formed by spatial arrangements and connections between millions (sometimes billions) of electrically active cells called neurons, and on the connections between these neurons. During the development of the cortex, the largest part of the brain, neurons are born in stem cell areas that lie deep inside the brain, and these newly made neurons then migrate outwards to their final positions close to the surface of the adult brain. Franzoni et al. have examined how two molecules, a small RNA called miR-128 and a protein called PHF6, control when and how neurons migrate through the cortex and then grow to form connections with other neurons as they mature. Mutations that disrupt PHF6 can cause intellectual disabilities, and one possible reason for this is that PHF6 is needed to ensure that the neurons migrate to the correction location. Franzoni et al. now show that miR-128 can reduce the production of PHF6 and is therefore responsible for controlling when and where PHF6 is active. Studying miR-128 in detail, they show that although an inactive precursor form of miR-128 is present in stem cells and migrating neurons, the active form of miR-128 is only found in neurons that have already reached their final position in the cortex. Franzoni et al. used genetic methods to override the switch that controls when miR-128 becomes active. When the amount of miR-128 was artificially reduced, the neurons migrated too far. Artificially increasing the amount of miR-128 had the opposite effect: both the movement of the neurons and, later, their growth were defective. PHF6 was the key to these effects: if PHF6 levels were kept close to normal, miR-128 could no longer interfere with the movement and growth of the neurons. Further work will be required to better understand how miR-128 is turned off and on, and how PHF6 acts to control neuronal movement and growth. DOI: http://dx.doi.org/10.7554/eLife.04263.002

Norepinephrine acting via β-adrenergic receptors (β-ARs) plays an important role in hippocampal plasticity including the subiculum which is the principal target of CA1 pyramidal cells and which controls information transfer from the hippocampus to other brain regions including the neighboring presubiculum and the entorhinal cortex (EC). Subicular pyramidal cells are classified as regular- (RS) and burst-spiking (BS) cells. Activation of β-ARs at CA1-subiculum synapses induces long-term potentiation (LTP) in burst- but not in RS cells (Wojtowicz et al., 2010). To elucidate seizure-associated disturbances in the norepinephrine-dependent modulation of hippocampal output, we investigated the functional consequences of the β-AR-dependent synaptic plasticity at CA1-subiculum synapses for the transfer of hippocampal output to the parahippocampal region in the pilocarpine model of temporal lobe epilepsy. Using single-cell and multi-channel field recordings in slices, we studied β-AR-mediated changes in the functional connectivity between CA1, the subiculum and its target-structures. We confirm that application of the β-adrenergic agonist isoproterenol induces LTP in subicular BS- but not RS cells. Due to the distinct spatial distribution of RS- and BS cells in the proximo-to-distal axis of the subiculum, in field recordings, LTP was significantly stronger in the distal than in the proximal subiculum. In pilocarpine-treated animals, β-AR-mediated LTP was strongly reduced in the distal subiculum. The attenuated LTP was associated with a disturbed polysynaptic transmission from the CA1, via the subiculum to the presubiculum, but with a preserved transmission to the medial EC. Our findings suggest that synaptic plasticity may influence target-related information flow and that such regulation is disturbed in pilocarpine-treated epileptic rats.

The purpose of this experiment was the demonstration of conditioned stress-induced analgesia in humans. Following a baseline day (day 1), two groups of 10 subjects were each exposed to green light as the conditioned stimulus (CS) and mental arithmetic plus noise as the unconditioned stimulus (US) for 5 days (days 2–6), a third group of 10 subjects was only exposed to the US. On the test day (day 7), pain threshold and pain tolerance were tested, while groups 1 and 3 received the green light CS and group 2 received no CS. Group 1 showed a significantly higher pain threshold and higher pain tolerance on the test day, whereas the two control groups did not have altered pain perception. Heart rate data confirmed successful stress induction. Heart rate and blood pressure were unaltered on the test day, thus making cardiovascular mediation of the conditioning effect unlikely. Conditioned stress analgesia might be useful in the understanding of chronic pain-syndromes.

Epilepsy is a disorder characterized by excessive synchronized neural activity. The hippocampus and surrounding temporal lobe structures appear particularly sensitive to epileptiform activity. Somatostatin (SST)-positive interneurons within the hilar region have been suggested to gate hippocampal activity, and therefore may play a crucial role in the dysregulation of hippocampal activity. In this study, we examined SST interneuron activity in the in vitro 4-aminopyridine (4-AP) model of epilepsy. We employed a multi-disciplinary approach, combining extracellular multi-electrode array (MEA) recordings with patch-clamp recordings and optical imaging using a genetically encoded calcium sensor. We observed that hilar SST interneurons are strongly synchronized during 4-AP-induced local field potentials (LFPs), as assayed by Ca(2+) imaging as well as juxtacellular or intracellular recording. SST interneurons were particularly responsive to GABA-mediated LFPs that occurred in the absence of ionotropic glutamatergic transmission. Our results present evidence that the extensive synchronized activity of SST-expressing interneurons contribute to the generation of GABAergic LFPs in an in vitro model of temporal lobe seizures.