The chief inhibitory neurons of the mammalian brain, GABAergic neurons, are comprised of a myriad of diverse neuronal subtypes. To facilitate the study of these neurons, transgenic mice were generated that express enhanced green fluorescent protein (EGFP) in subpopulations of GABAergic neurons. In one of the resulting transgenic lines, called GIN (GFP-expressing Inhibitory Neurons), EGFP was found to be expressed in a subpopulation of somatostatin-containing GABAergic interneurons in the hippocampus and neocortex. In both live and fixed brain preparations from these mice, detailed microanatomical features of EGFP-expressing interneurons were readily observed. In stratum oriens of the hippocampus, EGFPexpressing interneurons were comprised almost exclusively of oriens/alveus interneurons with lacunosum-moleculare axon arborization (O-LM cells). In the neocortex, the somata of EGFP-expressing interneurons were largely restricted to layers II-IV and upper layer V.In hippocampal area CA1, two previously uncharacterized subtypes of interneurons were identified using the GIN mice: stratum pyramidale interneurons with lacunosum-moleculare axon arborization (P-LM cells) and stratum radiatum interneurons with lacunosum-moleculare axon arborization (R-LM cells). These newly identified interneuronal subtypes appeared to be closely related to O-LM cell, as they selectively innervate stratum lacunosum-moleculare. Whole-cell patch-clamp recordings revealed that these cells were fast-spiking and showed virtually no spike frequency accommodation. The microanatomical features of these cells suggest that they function primarily as "input-biasing" neurons, in that synaptic volleys in stratum radiatum would lead to their activation, which in turn would result in selective suppression of excitatory input from the entorhinal cortex onto CA1 pyramidal cells.

SUMMARY Inhibitory neurons are critical for proper brain function, and their dysfunction is implicated in several disorders, including autism, schizophrenia, and Rett syndrome. These neurons are heterogeneous, and it is unclear which subtypes contribute to specific neurological phenotypes. We deleted Mecp2, the mouse homolog of the gene that causes Rett syndrome, from the two most populous subtypes, parvalbumin-positive (PV+) and somatostatin-positive (SOM+) neurons. Loss of MeCP2 partially impairs the affected neuron, allowing us to assess the function of each subtype without profound disruption of neuronal circuitry. We found that mice lacking MeCP2 in either PV+ or SOM+ neurons have distinct, non-overlapping neurological features: mice lacking MeCP2 in PV+ neurons developed motor, sensory, memory, and social deficits, whereas those lacking MeCP2 in SOM+ neurons exhibited seizures and stereotypies. Our findings indicate that PV+ and SOM+ neurons contribute complementary aspects of the Rett phenotype and may have modular roles in regulating specific behaviors.

1. Long-lasting potentiation of synaptic transmission was studied in the CA1 region of guinea-pig hippocampal slices maintained in vitro. 2. Stimulating pulses were delivered alternately to two independent afferent pathways, stratum radiatum and stratum oriens. The presynaptic volleys and field e.p.s.p.s. were recorded from the same two layers, while an electrode in the pyramidal cell body layer recorded the population spike or in other experiments the extra- or intracellular potentials from a single pyramidal cell. 3. A short tetanus to either of the two input pathways produced a long-lasting enhancement of the field e.p.s.p. as well as an increased size and a reduced latency of the population spike. This long-lasting potentiation was observed for up to 110 min after tetanization. Extracellular unit recordings showed that this potentiation is accompanied by an increased probability of firing and a reduced firing latency. Intracellular recordings showed an increased e.p.s.p., through the increase was smaller and less regular than for the extracellular field e.p.s.p. 4. No corresponding changes were seen in the field potential responses to stimulation of the untetanized input path, or in the intracellularly measured soma membrane potential, resistance, or excitability. The latter two properties were measured by intracellular injection of current pulses. It is concluded that long-lasting potentiation is specific to the pathway which has received the tetanization. 5. Following tetanization there was also a short-lasting (usually 2-4 min) depression, most often seen for the control pathway but sometimes visible on the tetanized side as well, superimposed on the potentiation. It is concluded that the short-lasting depression is not confined to any particular pathway but is a generalized (unspecific) phenomenon.

Copy number variations have been frequently associated with developmental delay, intellectual disability, and autism spectrum disorders1. MECP2 duplication syndrome is one of the most common genomic rearrangements in males2 and is characterized by autism, intellectual disability, motor dysfunction, anxiety, epilepsy, recurrent respiratory tract infections, and early death3–5. The broad range of deficits caused by methyl-CpG-binding protein 2 (MeCP2) overexpression poses a daunting challenge to traditional biochemical pathway-based therapeutic approaches. Accordingly, we sought strategies that directly target MeCP2 and are amenable to translation into clinical therapy. The first question, however, was whether the neurological dysfunction is reversible after symptoms set in. Reversal of phenotypes in adult symptomatic mice has been demonstrated in some models of monogenic loss-of-function neurological disorders6–8, including loss of MeCP2 in Rett syndrome9, indicating that, at least in some cases, the neuroanatomy may remain sufficiently intact so that correction of the molecular dysfunction underlying these disorders can restore healthy physiology. Given the absence of neurodegeneration in MECP2 duplication syndrome, we hypothesized that restoration of normal MeCP2 levels in MECP2 duplication adult mice would rescue their phenotype. Therefore, we first generated and characterized a conditional Mecp2-overexpressing mouse model and showed that correction of MeCP2 levels largely reversed the behavioral, molecular, and electrophysiological deficits. Next, we sought a translational strategy to reduce MeCP2 and turned to antisense oligonucleotides (ASOs). ASOs are small modified nucleic acids that can selectively hybridize with mRNA transcribed from a target gene and silence it10,11, and have been successfully used to correct deficits in different mouse models12–18. We found that ASO treatment induced a broad phenotypic rescue in adult symptomatic transgenic MECP2 duplication mice (MECP2-TG)19,20, and corrected MECP2 levels in lymphoblastoid cells from MECP2 duplication patients in a dose-dependent manner.

To explore the anatomical substrates for network hyperexcitability in adult rats that become chronically epileptic after recurrent seizures in infancy, the dendritic and axonal arbors of biocytin-filled hippocampal pyramidal cells were reconstructed. On postnatal day 10, tetanus toxin was unilaterally injected into the hippocampus and produced brief but recurrent seizures for 1 week. Later, hippocampal slices taken from these rats exhibited synchronized network bursts in area CA3C. Both the apical and basilar dendritic arbors of CA3C pyramidal cells were markedly abnormal in these epileptic rats. There was a considerable reduction in the density of dendrite spines, although the extent of this loss could vary among dendritic segments. Spine density on terminal segments of the basilar and apical dendrites was reduced on average by 35 and 20%, respectively. In addition, the diameters of these same dendritic segments were markedly reduced. Dendritic spine loss has previously been suggested to indicate a partial deafferentation of epileptic neurons, but this interpretation is difficult to reconcile with the critical role recurrent excitatory synaptic transmission plays in the generation of synchronized network burst. In this study, axonal arbors of CA3C pyramidal cells exhibited normal branching patterns, branching complexity, and varicosity density. This suggests that if deafferentation occurs, synapses other than recurrent excitatory ones are lost. The morphological abnormalities reported here would be expected to significantly alter electrical signaling within dendrites that may contribute importantly to seizures and other behavioral sequelae of early-onset epilepsy.

Mutations in genes encoding synaptic proteins cause many neurodevelopmental disorders, with the majority affecting postsynaptic apparatuses and much fewer in presynaptic proteins. Syntaxin-binding protein 1 (STXBP1, also known as MUNC18-1) is an essential component of the presynaptic neurotransmitter release machinery. De novo heterozygous pathogenic variants in STXBP1 are among the most frequent causes of neurodevelopmental disorders including intellectual disabilities and epilepsies. These disorders, collectively referred to as STXBP1 encephalopathy, encompass a broad spectrum of neurologic and psychiatric features, but the pathogenesis remains elusive. Here we modeled STXBP1 encephalopathy in mice and found that Stxbp1 haploinsufficiency caused cognitive, psychiatric, and motor dysfunctions, as well as cortical hyperexcitability and seizures. Furthermore, Stxbp1 haploinsufficiency reduced cortical inhibitory neurotransmission via distinct mechanisms from parvalbumin-expressing and somatostatin-expressing interneurons. These results demonstrate that Stxbp1 haploinsufficient mice recapitulate cardinal features of STXBP1 encephalopathy and indicate that GABAergic synaptic dysfunction is likely a crucial contributor to disease pathogenesis.

Studies of neurons from human epilepsy tissue and comparable animal models of focal epilepsy have consistently reported a marked decrease in dendritic spine density on hippocampal and neocortical pyramidal cells. Spine loss is often accompanied by focal varicose swellings or beading of dendritic segments. An ongoing excitotoxic injury of dendrites (dendrotoxicity), produced by excessive release of glutamate during seizures, is often assumed to produce these abnormalities. Indeed, application of glutamate receptor agonists to dendrites can produce both spine loss and beading. However, the cellular mechanisms underlying the two processes appear to be different. One recent study suggests NMDA‐induced spine loss is produced by Ca2+‐mediated alterations of the spine cytoskeleton. In contrast, dendritic beading is not dependent on extracellular Ca2+; instead, it appears to be produced by the movement of Na+ and Cl− intracellularly and an obligate movement of water to maintain osmolarity. A decrease in dendritic spine density was recently reported in a model of recurrent focal seizures in early life. Unlike results from other models, dendritic beading was not observed, and other signs of neuronal injury and death were absent. Thus, additional mechanisms to those of excitotoxicity may produce dendritic spine loss in epileptic tissue. A hypothesis is presented that spine loss can be a product of a partial deafferentation of pyramidal cells, resulting from an activity‐dependent pruning of neuronal connectivity induced by recurring seizures. The dendritic abnormalities observed in epilepsy are commonly suggested to be a product and not a cause of epilepsy. However, anatomical remodeling may be accompanied by alterations in molecular expression and targeting of both voltage‐ and ligand‐gated channels in dendrites. It is conceivable that such changes could contribute to the neuronal hyperexcitability of epilepsy. Hippocampus 2000;10:617–625. © 2000 Wiley‐Liss, Inc.

We conclude that mTOR kinase hyperactivation is a molecular mechanism underlying the development of cytomegalic neurons. This finding may lead to the development of novel therapeutic approaches for childhood epilepsy associated with cortical dysplasia.