Heterogeneity in Kv2 Channel Expression Shapes Action Potential Characteristics and Firing Patterns in CA1 versus CA2 Hippocampal Pyramidal Neurons.

The CA1 region of the hippocampus plays a critical role in spatial and contextual memory, and has well-established circuitry, function and plasticity. In contrast, the properties of the flanking CA2 pyramidal neurons (PNs), important for social memory, and lacking CA1-like plasticity, remain relatively understudied. In particular, little is known regarding the expression of voltage-gated K+ (Kv) channels and the contribution of these channels to the distinct properties of intrinsic excitability, action potential (AP) waveform, firing patterns and neurotransmission between CA1 and CA2 PNs. In the present study, we used multiplex fluorescence immunolabeling of mouse brain sections, and whole-cell recordings in acute mouse brain slices, to define the role of heterogeneous expression of Kv2 family Kv channels in CA1 versus CA2 pyramidal cell excitability. Our results show that the somatodendritic delayed rectifier Kv channel subunits Kv2.1, Kv2.2, and their auxiliary subunit AMIGO-1 have region-specific differences in expression in PNs, with the highest expression levels in CA1, a sharp decrease at the CA1-CA2 boundary, and significantly reduced levels in CA2 neurons. PNs in CA1 exhibit a robust contribution of Guangxitoxin-1E-sensitive Kv2-based delayed rectifier current to AP shape and after-hyperpolarization potential (AHP) relative to that seen in CA2 PNs. Our results indicate that robust Kv2 channel expression confers a distinct pattern of intrinsic excitability to CA1 PNs, potentially contributing to their different roles in hippocampal network function

Dysregulation of Sodium Channels in a Rat Model of Absence Epilepsy

Absence epilepsy is a generalized form of epilepsy where spike-wave discharges (SWDs) involve both hemispheres of the brain and thereby alter consciousness. Recent evidence by Meeren et al (2002) in the WAG/Rij rat model of absence epilepsy points to a cortical focus of SWDs before rapid generalization of the SWDs. This focus belongs in the peri-oral area of the somatosensory cortex, and it was found to consistently lead SWDs in other cortical and subcortical areas. With this recent finding, it seems plausible that a defect lies in this focal region of the cortex, leading to SWD in the WAG/Rij model. It is likely that an alteration of one or more ion channels leads to seizure generation in this rat model, as ion channels are what produce the hyperexcitability of seizures. In this study, our laboratory performed three consecutive days of scalp EEG recordings on WAG/Rij animals at different ages and compared this to control rat EEGs. As has been found before, we saw an increase in time spent in SWDs as the WAG/Rij animals aged. After completing EEG recordings, the animals were sacrificed and quantitative PCR and immunocytochemistry was performed on six regions of the cortex. In comparison to control animals, WAG/Rij rats had an increase in sodium channel subunits Nav1.1 and Nav1.6 in the region corresponding to the seizure focus identified by Meeren et al. In addition, as WAG/Rij rats aged, the amount of Nav1.1 and Nav1.6 also steadily increased in the peri-oral region of the somatosensory cortex. These findings suggest that specific sodium channelopathies may initiate SWD generation in this rodent model. The results of our study have many implications. Perhaps many, if not all, forms of human absence epilepsy are rooted in ion channelopathies which could be limited to specific regions of the brain. If this is so, and if the specific channelopathies are identified, it is also possible that very targeted therapies could be devised either medically or surgically to treat both benign and refractory absence epilepsies. Future studies are needed to determine whether the sodium channel dysregulation found in this rodent model is the cause or effect of SWDs and whether other channelopathies or dysregulation of channels exists. Our lab is currently looking at what effects ethosuximide, an anti-absence drug, has on sodium channel composition in the cortex of the WAG/Rij rat

TMEM16B regulates anxiety-related behavior and GABAergic neuronal signaling in the central lateral amygdala.

TMEM16B (ANO2) is the Ca2+-activated chloride channel expressed in multiple brain regions, including the amygdala. Here we report that Ano2 knockout mice exhibit impaired anxiety-related behaviors and context-independent fear memory, thus implicating TMEM16B in anxiety modulation. We found that TMEM16B is expressed in somatostatin-positive (SOM+) GABAergic neurons of the central lateral amygdala (CeL), and its activity modulates action potential duration and inhibitory postsynaptic current (IPSC). We further provide evidence for TMEM16B actions not only in the soma but also in the presynaptic nerve terminals of GABAergic neurons. Our study reveals an intriguing role for TMEM16B in context-independent but not context-dependent fear memory, and supports the notion that dysfunction of the amygdala contributes to anxiety-related behaviors

TRPV1 channels are critical brain inflammation detectors and neuropathic pain biomarkers in mice

The capsaicin receptor TRPV1 has been widely characterized in the sensory system as a key component of pain and inflammation. A large amount of evidence shows that TRPV1 is also functional in the brain although its role is still debated. Here we report that TRPV1 is highly expressed in microglial cells rather than neurons of the anterior cingulate cortex and other brain areas. We found that stimulation of microglial TRPV1 controls cortical microglia activation per se and indirectly enhances glutamatergic transmission in neurons by promoting extracellular microglial microvesicles shedding. Conversely, in the cortex of mice suffering from neuropathic pain, TRPV1 is also present in neurons affecting their intrinsic electrical properties and synaptic strength. Altogether, these findings identify brain TRPV1 as potential detector of harmful stimuli and a key player of microglia to neuron communication

Changes in the Morphology of Hypoglossal Motor Neurons in the Brainstem of Developing Rats

The autonomic brainstem generates and modifies breathing rhythm by integrating inputs from chemo- and mechanosensors in the viscera while coordinating descending outputs from higher CNS structures. Hypoglossal motoneurons (XII MNs) receive inputs from respiratory premotor neurons in the medulla. Previous studies in rodents have demonstrated significant changes in breathing control during the first three weeks of life, with a sensitive period at 10 to 13 days post-birth (P10–P13) characterized by pronounced changes in neurotransmitters, receptors, excitation-inhibition balance, and breathing. However, age-dependent morphological changes of XII MNs during the first three weeks post-birth and especially during this sensitive period, have not been thoroughly studied. In this study, we comprehensively characterized and quantified the postnatal morphological changes in rat XII MNs. We hypothesized that morphological changes occur in XII MN morphology and arbor complexity corresponding to the functionally-defined sensitive period observed at P10–P13. To test this hypothesis, we used innovative statistical approaches to quantify and compare developmental changes in Golgi-Cox stained XII MNs at nine postnatal ages between P1–P21. Additionally, we performed 3D reconstructions of the neurons importing these geometries into the modeling environment NEURON to simulate the biophysical properties of XII MNs. Soma size increased ~40% from P1 to P21, with no significant change in shape. However, dendritic arborization increased in extent and complexity with branching of neurons significantly increasing from P1 through P13, with the greatest increase at P10–P13 based on the Sholl method. Three age groups 1) P1–P5, 2) P7–P12, and 3) P13–P21 were found as possible windows of development. We also found that at specific ages certain parameters such as soma size and dendritic complexity were non-normally distributed. I found support for differences in the density of selected voltage-gated ion channels with age and correlations between passive electrophysiological properties and morphology. Although a direct relationship was not found between morphology and the active properties, I did find support for an indirect relationship. Our detailed characterization of XII MN morphological development establishes a foundation for the study and elucidation of morphological changes caused by maternal and perinatal conditions using a rigorous approach

Genetically targeted anatomical and behavioral characterization of the cornu ammonis 2 (CA2) subfield of the mouse hippocampus

The hippocampus is critical for storing declarative memory, our repository of knowledge of who, what, where, and when. Mnemonic information is processed and encoded in the hippocampus through several parallel routes, most notably the trisynaptic pathway, in which information proceeds from entorhinal cortex (EC) to dentate gyrus (DG) to CA3 and then to CA1, the main hippocampal output. Absent from this pathway is the CA2 subfield, a relatively small area interposed between CA3 and CA1 that has recently been shown to mediate a powerful disynaptic circuit linking EC input with CA1 output. Usually ignored or grouped together with CA3, CA2 has generally escaped exploration presumably due to its relatively small size and somewhat ill-defined borders. A few studies have proposed an important role for the CA2 subfield of the hippocampus, however, the relevance of this subfield in a behaving animal has not been explored. The function of a particular brain region may be inferred by examining the effects of a lesion of that area. Indeed, the hippocampus's role in learning and memory was elucidated following the bilateral medial temporal lobe ablation of Henry Molaison (patient H.M.). Similarly, a lesion of CA2 could be used to infer its role in learning, memory, and disease. Due to the relatively small size of CA2, physical or chemical lesions are not precise enough to ablate this region without collateral damage. To overcome this limitation, I generated a CA2-specific transgenic mouse line to enable genetic targeting of this subfield. I used this mouse line to map CA2 connectivity and explore its behavioral role. Using monosynaptic rabies tracing, CA2 axon tracing, and electrophysiology, I confirmed the disynaptic pathway and presence of septal and subcortical inputs to CA2. Genetically targeted inactivation of CA2 caused a remarkably profound loss of social memory, with no change in sociability. This impairment was not the result of a general loss of hippocampal function as CA2-inactivation did not impact performance on several other hippocampal-dependent tasks, including spatial and contextual memory. These behavioral and anatomical results thus reveal CA2 as a hub of sociocognitive processing and implicate its dysfunction in social endophenotypes of psychiatric diseases such as schizophrenia and autism

Development of Manganese-Enhanced Magnetic Resonance Imaging (MEMRI) Methods to Study Pathophysiology Underlying Neurodegenerative Diseases in Murine Models

Manganese-enhanced magnetic resonance imaging (MEMRI) opens the great opportunity to study complex paradigms of central nervous system (CNS) in freely behaving animals and reveals new pathophysiological information that might be otherwise difficult to gain. Due to advantageous chemical and biological properties of manganese (Mn2+), MEMRI has been successfully applied in the studies of several neurological diseases using translational animal models to assess comprehensive information about neuronal activity, morphology, neuronal tracts, and rate of axonal transport. Although previous studies highlight the potential of MEMRI for brain imaging, the limitations concerning the use of Mn2+ in living animals and applications of MEMRI in neuroscience research are in their infancy. Therefore, development of MEMRI methods for experimental studies remains essential for diagnostic findings, development of therapeutic as well as pharmacological intervention strategies. Our lab has been dedicating to develop novel MEMRI methods to study the pathophysiology underlying neurodegenerative diseases in murine models. In the first study, we investigated the cellular mechanism of MEMRI signal change during neuroinflammation in mice. The roles of neural cells (glia and neurons) in MEMRI signal enhancement were delineated, and ability of MEMRI to detect glial (astrocyte and microglia) and neuronal activation was demonstrated in mice treated with inflammatory inducing agents. In vitro work demonstrated that cytokine-induced glial activation facilitates neuronal uptake of Mn2+,and that glial Mn2+ content was not associated with glial activation. The in vivo work confirmed that MEMRI signal enhancement in the CNS is induced by astrocytic activation by stimulating neuronal Mn2+ uptake. In conclusion, our results supported the notion that MEMRI reflects neuronal excitotoxicity and impairment that can occur through a range of insults that include neuroinflammation. In the second study, we evaluated the efficacy of MEMRI in diagnosing the complexities of neuropathology in an ananimal model of a neurodegenerative disease, neuroAIDS. This study demonstrated that MEMRI reflects brain region specific HIV-1-induced neuropathology in virus-infected NOD/scid-IL-2Rγcnull humanized mice. Altered MEMRI signal intensity was observed in affected brain regions. These included, but were not limited to, the hippocampus, amygdala, thalamus, globus pallidus, caudoputamen, substantia nigra and cerebellum. MEMRI signal was coordinated with levels of HIV-1 infection, neuroinflammation (astro- and micro- gliosis), and neuronal injury. Following the application of MEMRI to assess HIV-1 induced neuropathology in immune deficient mice humanized with lymphoid progenitor cells, our successful collaboration with Dr. Sajja BR (Department of Radiology, UNMC, Omaha, NE) led to the generation of a MEMRI-based NOD/scid-IL-2Rγcnull (NSG) mouse brain atlas. Mouse brain MRI atlases allow longitudinal quantitative analyses of neuroanatomical volumes and imaging metrics. As NSG mice allow human cell transplantation to study human disease, these animals are used to assess brain morphology. MEMRI provided sufficient contrast permitting 41 brain structures to be manually labeled on average brain of 19 mice using alignment algorithm. The developed atlas is now made available to researchers through Neuroimaging Informatics Tools and Resources Clearinghouse (NITRC) website (https://www.nitrc.org/projects/memribrainatlas/). Finally, we evaluated the efficacy of N-acetylated-para-aminosalicylic acid (AcPAS) to accelerate Mn2+ elimination from rodent brain, enabling repeated use of MEMRI to follow the CNS longitudinally in weeks or months as well as inhibiting the confounding effects of residual Mn2+ from preceding administrations on imaging results. Two-week treatment with AcPAS (200 mg/kg/dose × 3 daily) accelerated the decline of Mn2+ induced enhancement in MRI. This study demonstrated that AcPAS could enhance MEMRI utility in evaluating brain biology in small animals