Synaptic properties of the lemniscal and paralemniscal pathways to the mouse somatosensory thalamus.

Somatosensory information is thought to arrive in thalamus through two glutamatergic routes called the lemniscal and paralemniscal pathways via the ventral posterior medial (VPm) and posterior medial (POm) nuclei. Here we challenge the view that these pathways functionally represent parallel information routes. Using electrical stimulation and an optogenetic approach in brain slices from the mouse, we investigated the synaptic properties of the lemniscal and paralemniscal input to VPm and POm. Stimulation of the lemniscal pathway produced class 1, or "driver," responses in VPm relay cells, which is consistent with this being an information-bearing channel. However, stimulation of the paralemniscal pathway produced two distinct types of responses in POm relay cells: class 1 (driver) responses in 29% of the cells, and class 2, or "modulator," responses in the rest. Our data suggest that, unlike the lemniscal pathway, the paralemniscal one is not homogenous and that it is primarily modulatory. This finding requires major rethinking regarding the routes of somatosensory information to cortex and suggests that the paralemniscal route is chiefly involved in modulatory functions rather than simply being an information route parallel to the lemniscal channel.

Modulatory effects of activation of metabotropic glutamate receptors on GABAergic circuits in the mouse thalamus.

Metabotropic glutamate receptors (mGluRs) are widely distributed in the central nervous system and modulate the release of neurotransmitters in different ways. We have previously shown that activation of presynaptic group II mGluRs reduces the gain of GABAergic inputs in both primary visual and auditory cortices (V1 and A1). In the present study, we sought to determine whether activation of mGluRs can also affect the inhibitory inputs in thalamus. Using whole cell recordings in a mouse slice preparation, we studied two GABAergic inputs to thalamic relay cells: that of the thalamic reticular nucleus (TRN) to cells of the ventral posteromedial nucleus (VPM) and that of interneurons to cells of the lateral geniculate nucleus (LGN). We found that activation of mGluRs significantly reduced the amplitudes of inhibitory postsynaptic currents (IPSCs) evoked from TRN inputs to VPM cells, and further experiments indicated that this was due to activation of presynaptic group I and group II mGluRs. Similar results were found in the interneuronal inputs to LGN cells. Activation of presynaptic group I (type 1 but not type 5) and group II mGluRs significantly reduced the amplitudes of evoked IPSCs of the axonal inputs to relay cells, and additional experiments were consistent with previous observations that activation of type 5 mGluRs on the dendritic terminals of interneurons enhanced postsynaptic IPSCs. We concluded that group I and II mGluRs may generally reduce the amplitude of evoked GABAergic IPSCs of axonal inputs to thalamic relay cells, operating through presynaptic mechanisms, and this extends our previous findings in cortex.

Properties of the primary somatosensory cortex projection to the primary motor cortex in the mouse.

The primary somatosensory (S1) and primary motor (M1) cortices are reciprocally connected, and their interaction has long been hypothesized to contribute to coordinated motor output. Very little is known, however, about the nature and synaptic properties of the S1 input to M1. Here we wanted to take advantage of a previously developed sensorimotor slice preparation that preserves much of the S1-to-M1 connectivity (Rocco MM, Brumberg JC. J Neurosci Methods 162: 139-147, 2007), as well as available optogenetic methodologies, in order to investigate the synaptic profile of this projection. Our data show that S1 input to pyramidal cells of M1 is highly homogeneous, possesses many features of a "driver" pathway, such as paired-pulse depression and lack of metabotropic glutamate receptor activation, and is mediated through axons that terminate in both small and large synaptic boutons. Our data suggest that S1 provides M1 with afferents that possess synaptic and anatomical characteristics ideal for the delivery of strong inputs that can "drive" postsynaptic M1 cells, thereby potentially affecting their output.

Modulatory effects of activation of metabotropic glutamate receptors on GABAergic circuits in the mouse cortex.

Metabotropic glutamate receptors (mGluRs) have a ubiquitous distribution in the central nervous system and often serve to regulate the release of neurotransmitters. We have previously shown that activation of both presynaptic and postsynaptic mGluRs can affect the gain of glutamatergic inputs in both thalamus and cortex. In the present study, we sought to determine the effect of mGluR activation on GABAergic inputs in cortex. Using whole cell recordings in a mouse slice preparation of either primary visual or auditory cortex (V1 or A1), we tested the effects on mGluRs by applying various agonists to the slice. Two pathways were tested in each area: the GABAergic inputs in layers 2/3 activated from layer 4 and the GABAergic inputs in layer 4 activated from adjacent layer 4. In both of these pathways, we found that activation of mGluRs significantly reduced the amplitude of the evoked inhibitory postsynaptic currents. Because the effects were not blocked by the addition of GDPßS to the recording electrode, and because mGluR agonists did not affect responses to photostimulation of GABA in a low-Ca(2+) and high-Mg(2+) bathing solution, we concluded this reduction was due to activation of presynaptic mGluRs. Furthermore, using specific mGluR agonists, we found that group II mGluRs, but not group I mGluRs, were involved in these modulatory effects. Because similar results were found in both pathways in V1 and A1, a possible cortical pattern for these effects is suggested.

Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse.

Primary somatosensory cortex (S1) receives two distinct classes of thalamocortical input via the lemniscal and paralemniscal pathways, the former via ventral posterior medial nucleus (VPM), and the latter, from the posterior medial nucleus (POm). These projections have been described as parallel thalamocortical pathways. Although the VPM thalamocortical projection has been studied in depth, several details of the POm projection to S1 are unknown. We studied the synaptic properties and anatomical features in the mouse of the projection from POm to all layers of S1 and to layer 4 of secondary somatosensory cortex (S2). Neurons in S1 responded to stimulation of POm with what has been termed Class 2 properties (paired-pulse facilitation, small initial excitatory postsynaptic potentials (EPSPs), a graded activation profile, and a metabotropic receptor component; thought to be modulatory), whereas neurons in layer 4 of S2 responded with Class 1A properties (paired-pulse depression, large initial EPSPs, an all-or-none activation profile, and no metabotropic receptor component, thought to be a main information input). Also, labeling from POm produced small boutons in S1, whereas both small and large boutons were found in S2. Our data suggest that the lemniscal and paralemniscal projections should not be thought of as parallel information pathways to S1 and that the paralemniscal projection may instead provide modulatory inputs to S1.

Functional significance of synaptic terminal size in glutamatergic sensory pathways in thalamus and cortex.

Glutamatergic pathways are a major information-carrying and -processing network of inputs in the brain. There is considerable evidence suggesting that glutamatergic pathways do not represent a homogeneous group and that they can be segregated into at least two broad categories. Class 1 glutamatergic inputs, which are suggested to be the main information carriers, are characterized by a number of unique synaptic and anatomical features, such as the large synaptic boutons with which they often terminate. On the other hand, Class 2 inputs, which are thought to play a modulatory role, are associated, amongst other features, with exclusively small terminal boutons. Here we summarize and briefly discuss these two classes of glutamatergic input and how their unique features, including their terminal bouton size and anatomy, are related to their suggested function.

Synaptic properties of thalamic input to the subgranular layers of primary somatosensory and auditory cortices in the mouse.

The classification of synaptic inputs is an essential part of understanding brain circuitry. In the present study, we examined the synaptic properties of thalamic inputs to pyramidal neurons in layers 5a, 5b, and 6 of primary somatosensory (S1) and auditory (A1) cortices in mouse thalamocortical slices. Stimulation of the ventral posterior medial nucleus and the ventral division of the medial geniculate body resulted in three distinct response classes, two of which have never been described before in thalamocortical projections. Class 1A responses included synaptic depression and all-or-none responses, while Class 1B responses exhibited synaptic depression and graded responses. Class 1C responses are characterized by mixed facilitation and depression as well as graded responses. Activation of metabotropic glutamate receptors was not observed in any of the response classes. We conclude that Class 1 responses can be broken up into three distinct subclasses, and that thalamic inputs to the subgranular layers of cortex may combine with other, intracortical inputs to drive their postsynaptic target cells. We also integrate these results with our recent, analogous study of thalamocortical inputs to granular and supragranular layers (Viaene et al., 2011).

Synaptic properties of thalamic input to layers 2/3 and 4 of primary somatosensory and auditory cortices.

We studied the synaptic profile of thalamic inputs to cells in layers 2/3 and 4 of primary somatosensory (S1) and auditory (A1) cortices using thalamocortical slices from mice age postnatal days 10-18. Stimulation of the ventral posterior medial nucleus (VPM) or ventral division of the medial geniculate body (MGBv) resulted in two distinct classes of responses. The response of all layer 4 cells and a minority of layers 2/3 cells to thalamic stimulation was Class 1, including paired-pulse depression, all-or-none responses, and the absence of a metabotropic component. On the other hand, the majority of neurons in layers 2/3 showed a markedly different, Class 2 response to thalamic stimulation: paired-pulse facilitation, graded responses, and a metabotropic component. The Class 1 and Class 2 response characteristics have been previously seen in inputs to thalamus and have been described as drivers and modulators, respectively. Driver input constitutes a main information bearing pathway and determines the receptive field properties of the postsynaptic neuron, whereas modulator input influences the response properties of the postsynaptic neuron but is not a primary information bearing input. Because these thalamocortical projections have comparable properties to the drivers and modulators in thalamus, we suggest that a driver/modulator distinction may also apply to thalamocortical projections. In addition, our data suggest that thalamus is likely to be more than just a simple relay of information and may be directly modulating cortex.

Synaptic properties of the mammillary and cortical afferents to the anterodorsal thalamic nucleus in the mouse.

Input to sensory thalamic nuclei can be classified as either driver or modulator, based on whether or not the information conveyed determines basic postsynaptic receptive field properties. Here we demonstrate that this distinction can also be applied to inputs received by nonsensory thalamic areas. Using flavoprotein autofluorescence imaging, we developed two slice preparations that contain the afferents to the anterodorsal thalamic nucleus (AD) from the lateral mammillary body and the cortical afferents arriving through the internal capsule, respectively. We examined the synaptic properties of these inputs and found that the mammillothalamic pathway exhibits paired-pulse depression, lack of a metabotropic glutamate component, and an all-or-none response pattern, which are all signatures of a driver pathway. On the other hand, the cortical input exhibits graded paired-pulse facilitation and the capacity to activate metabotropic glutamatergic responses, all features of a modulatory pathway. Our results extend the notion of driving and modulating inputs to the AD, indicating that it is a first-order relay nucleus and suggesting that these criteria may be general to the whole of thalamus.

Operational Characteristics of Institutional Review Boards (IRBs) in the United States.

Background:Federal Law requires approval from an Institutional Review Board prior to conducting human subjects research to ensure ethical distribution of benefits and harms. Notwithstanding this role and almost no prescriptive requirements about design or operation, there is little systematic research describing the key attributes of IRBs, as reported by IRB personnel themselves. Methods: Here, 55 IRB directors completed a survey of 77 questions. The goals of the study were to establish what a typical US IRB "looks like," determine whether IRB characteristics can be summarized by a smaller number of overarching components, determine the best predictors of IRB speed and efficiency, and determine whether IRBs differ by high-level qualitative characteristics such as institution type. The above was explored and tested using the general linear model and principal components analysis, and for the former, dependent variables of interest were, a) the time necessary for an IRB to approve a study, and b) efficiency of the review process for full board and expedited reviews. IVs of interest included multiple IRB characteristics. Results: 1) IRB characteristics can be summarized by four key components; 2) IRB speed and efficiency are most strongly determined by tendency to receive biomedical submissions, especially drug-related; and 3) IRBs do vary by institution type on some key variables. Conclusion: These results are the first step toward establishing national norms and building a working model of US IRBs to which other IRBs can compare themselves.

Early Seizure Activity Accelerates Depletion of Hippocampal Neural Stem Cells and Impairs Spatial Discrimination in an Alzheimer's Disease Model.

Adult hippocampal neurogenesis has been reported to be decreased, increased, or not changed in Alzheimer's disease (AD) patients and related transgenic mouse models. These disparate findings may relate to differences in disease stage, or the presence of seizures, which are associated with AD and can stimulate neurogenesis. In this study, we investigate a transgenic mouse model of AD that exhibits seizures similarly to AD patients and find that neurogenesis is increased in early stages of disease, as spontaneous seizures became evident, but is decreased below control levels as seizures recur. Treatment with the antiseizure drug levetiracetam restores neurogenesis and improves performance in a neurogenesis-associated spatial discrimination task. Our results suggest that seizures stimulate, and later accelerate the depletion of, the hippocampal neural stem cell pool. These results have implications for AD as well as any disorder accompanied by recurrent seizures, such as epilepsy.

Publisher Correction: Stimulation of entorhinal cortex-dentate gyrus circuitry is antidepressive.

In the version of this article originally published, a URL provided in the Methods section was incorrect. The URL had a solidus at the end but should have appeared as http://www.nature.com/authors/policies/image.html. The error has been corrected in the PDF and HTML versions of this article.

Stimulation of entorhinal cortex-dentate gyrus circuitry is antidepressive.

Major depressive disorder (MDD) is considered a 'circuitopathy', and brain stimulation therapies hold promise for ameliorating MDD symptoms, including hippocampal dysfunction. It is unknown whether stimulation of upstream hippocampal circuitry, such as the entorhinal cortex (Ent), is antidepressive, although Ent stimulation improves learning and memory in mice and humans. Here we show that molecular targeting (Ent-specific knockdown of a psychosocial stress-induced protein) and chemogenetic stimulation of Ent neurons induce antidepressive-like effects in mice. Mechanistically, we show that Ent-stimulation-induced antidepressive-like behavior relies on the generation of new hippocampal neurons. Thus, controlled stimulation of Ent hippocampal afferents is antidepressive via increased hippocampal neurogenesis. These findings emphasize the power and potential of Ent glutamatergic afferent stimulation-previously well-known for its ability to influence learning and memory-for MDD treatment.

Epigenetic suppression of hippocampal calbindin-D28k by ΔFosB drives seizure-related cognitive deficits.

The calcium-binding protein calbindin-D28k is critical for hippocampal function and cognition, but its expression is markedly decreased in various neurological disorders associated with epileptiform activity and seizures. In Alzheimer's disease (AD) and epilepsy, both of which are accompanied by recurrent seizures, the severity of cognitive deficits reflects the degree of calbindin reduction in the hippocampal dentate gyrus (DG). However, despite the importance of calbindin in both neuronal physiology and pathology, the regulatory mechanisms that control its expression in the hippocampus are poorly understood. Here we report an epigenetic mechanism through which seizures chronically suppress hippocampal calbindin expression and impair cognition. We demonstrate that ΔFosB, a highly stable transcription factor, is induced in the hippocampus in mouse models of AD and seizures, in which it binds and triggers histone deacetylation at the promoter of the calbindin gene (Calb1) and downregulates Calb1 transcription. Notably, increasing DG calbindin levels, either by direct virus-mediated expression or inhibition of ΔFosB signaling, improves spatial memory in a mouse model of AD. Moreover, levels of ΔFosB and calbindin expression are inversely related in the DG of individuals with temporal lobe epilepsy (TLE) or AD and correlate with performance on the Mini-Mental State Examination (MMSE). We propose that chronic suppression of calbindin by ΔFosB is one mechanism through which intermittent seizures drive persistent cognitive deficits in conditions accompanied by recurrent seizures.

ΔFosB Regulates Gene Expression and Cognitive Dysfunction in a Mouse Model of Alzheimer's Disease.

Alzheimer's disease (AD) is characterized by cognitive decline and 5- to 10-fold increased seizure incidence. How seizures contribute to cognitive decline in AD or other disorders is unclear. We show that spontaneous seizures increase expression of ΔFosB, a highly stable Fos-family transcription factor, in the hippocampus of an AD mouse model. ΔFosB suppressed expression of the immediate early gene c-Fos, which is critical for plasticity and cognition, by binding its promoter and triggering histone deacetylation. Acute histone deacetylase (HDAC) inhibition or inhibition of ΔFosB activity restored c-Fos induction and improved cognition in AD mice. Administration of seizure-inducing agents to nontransgenic mice also resulted in ΔFosB-mediated suppression of c-Fos, suggesting that this mechanism is not confined to AD mice. These results explain observations that c-Fos expression increases after acute neuronal activity but decreases with chronic activity. Moreover, these results indicate a general mechanism by which seizures contribute to persistent cognitive deficits, even during seizure-free periods.

Activation requirements for metabotropic glutamate receptors.

It has been common experimentally to use high frequency, tetanic, stimulation to activate metabotropic glutamate receptors (mGluRs) in cortex and thalamus. To determine what type of stimulation is actually necessary to activate mGluRs we examined the effects of varying stimulation duration and intensity on activating mGluR responses. We used a thalamocortical and an intracortical slice preparation from mice and performed whole cell recordings from neurons in the ventral posterior medial nucleus or in layer 4 of primary somatosensory cortex (S1) while electrically stimulating in layer 6 of S1. Extracellular ionotropic glutamate receptor antagonists and GABAA receptor antagonists were used to isolate Group I or Group II mGluR responses. We observed that high frequency stimulation is not necessary for the activation of either Group I or Group II mGluRs. Either could be activated with as few as 2-3 pulses at stimulation frequencies around 15-20Hz. Additionally, increasing the number of pulses, intensity of stimulation, or stimulation frequency increased amplitude and duration of the mGluR response.

Two populations of corticothalamic and interareal corticocortical cells in the subgranular layers of the mouse primary sensory cortices.

The subgranular layers (layers 5 and 6) of primary sensory cortex provide corticofugal output to thalamus and they also project to the appropriate secondary sensory cortices. Here we injected two combinations of different color retrograde fluorescent markers in the thalamic and cortical targets of these layers from the three primary sensory cortices (somatosensory, auditory, and visual) in mice to examine the degree of overlap between corticothalamic and interareal corticocortical cells in the subgranular layers. We found that, for all three primary sensory cortices, double-labeled cells were extremely rare, indicating that corticothalamic and interareal corticocortical cells in the subgranular layers represent largely independent populations.

Attention to visual, but not tactile, properties of a stimulus results in activation of FOS protein in the visual thalamic reticular nucleus of rats.

Previous reports have suggested that the modality-specific sectors of the thalamic reticular nucleus (TRN) may become selectively activated as a result of attention being drawn to their respective sensory modalities. Here we used a task that required the discrimination of digging bowls on the basis of their visual (the colour of the bowl) or tactile (the external texture of the bowl) characteristics. We trained rats to perform both modality discriminations, ensuring the equity of exposure to both visual and tactile aspects of the stimuli. On the test day, animals had to perform only one of the modality discriminations for a 1-h period prior to being transcardially perfused and their brains removed and processed for Fos immunocytochemistry. We found that animals that performed the visual discrimination prior to sacrifice demonstrated a selective activation of cells in the visual TRN. On the other hand, animals that had performed the tactile discrimination, despite encountering the same stimuli and having received equal visual stimulation as the animals performing the visual discrimination, did not have activation of the visual TRN. This evidence suggests that activation of visual TRN is a function of visual selective attention, and not merely visual stimulation. Surprisingly, the same was not true for somatic TRN, which was not labeled in any animals. It is possible that this lack of a double dissociation is the result of modality-specific differences in the attentional demands of the two discrimination tasks.