The effect of kainic acid on hippocampal dendritic spine motility at the early and late stages of brain development

Detecting Early Cognitive Decline in Alzheimer's Disease with Brain Synaptic Structural and Functional Evaluation

Early cognitive decline in patients with Alzheimer's (AD) is associated with quantifiable structural and functional connectivity changes in the brain. AD dysregulation of Aβ and tau metabolism progressively disrupt normal synaptic function, leading to loss of synapses, decreased hippocampal synaptic density and early hippocampal atrophy. Advances in brain imaging techniques in living patients have enabled the transition from clinical signs and symptoms-based AD diagnosis to biomarkers-based diagnosis, with functional brain imaging techniques, quantitative EEG, and body fluids sampling. The hippocampus has a central role in semantic and episodic memory processing. This cognitive function is critically dependent on normal intrahippocampal connections and normal hippocampal functional connectivity with many cortical regions, including the perirhinal and the entorhinal cortex, parahippocampal cortex, association regions in the temporal and parietal lobes, and prefrontal cortex. Therefore, decreased hippocampal synaptic density is reflected in the altered functional connectivity of intrinsic brain networks (aka large-scale networks), including the parietal memory, default mode, and salience networks. This narrative review discusses recent critical issues related to detecting AD-associated early cognitive decline with brain synaptic structural and functional markers in high-risk or neuropsychologically diagnosed patients with subjective cognitive impairment or mild cognitive impairment.

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

Methamphetamine Learning Induces Persistent and Selective Nonmuscle Myosin II-Dependent Spine Motility in the Basolateral Amygdala

Nonmuscle myosin II inhibition (NMIIi) in the basolateral amygdala (BLA), but not dorsal hippocampus (CA1), selectively disrupts memories associated with methamphetamine (METH) days after learning, without retrieval. However, the molecular mechanisms underlying this selective vulnerability remain poorly understood. A known function of NMII is to transiently activate synaptic actin dynamics with learning. Therefore, we hypothesized that METH-associated learning perpetuates NMII-driven actin dynamics in synapses, leading to an extended window of vulnerability for memory disruption. We used time-lapse two-photon imaging of dendritic spine motility in acutely prepared brain slices from female and male mice following METH-associated learning as a readout of actin-myosin dynamics. Spine motility was persistently increased in the BLA, but not in CA1. Consistent with the memory disrupting effect of intra-BLA NMII inhibition, METH-induced changes to BLA spine dynamics were reversed by a single systemic injection of an NMII inhibitor. Intra-CA1 NMII inhibition, on the other hand, did not disrupt METH-associated memory. Thus, we report identification of a previously unknown ability for spine actin dynamics to persist days after stimulation and that this is under the control of NMII. Further, these perpetual NMII-driven spine actin dynamics in BLA neurons may contribute to the unique susceptibility of METH-associated memories.SIGNIFICANCE STATEMENT There are no Food and Drug Administration-approved pharmacotherapies to prevent relapse to the use of stimulants, such as methamphetamine (METH). Environmental cues become associated with drug use, such that the memories can elicit strong motivation to seek the drug during abstinence. We previously reported that the storage of METH-associated memories is uniquely vulnerable to immediate, retrieval-independent, and lasting disruption by direct actin depolymerization or by inhibiting the actin driver nonmuscle myosin II (NMII) in the BLA or systemically. Here we report a potential structural mechanism responsible for the unique vulnerability of METH-associated memories and METH-seeking behavior to NMII inhibition within the BLA.

Ultrastructural changes to rat hippocampus in pentylenetetrazol- and kainic acid-induced status epilepticus: A study using electron microscopy

A pentylenetetrazol (PTZ)-induced status epilepticus model in rats was used in the study. The brains were studied one month after treatment. Ultrastructural observations using electron microscopy performed on the neurons, glial cells, and synapses, in the hippocampal CA1 region of epileptic brains, demonstrated the following major changes over normal control brain tissue. (i) There is ultrastructural alterations in some neurons, glial cells and synapses in the hippocampal CA1 region. (ii) The destruction of cellular organelles and peripheral, partial or even total chromatolysis in some pyramidal cells and in interneurons are observed. Several astrocytes are proliferated or activated. Presynaptic terminals with granular vesicles and degenerated presynaptic profiles are rarely observed. (iii) The alterations observed are found to be dependent on the frequency of seizure activities following the PTZ treatment. It was observed that if seizure episodes are frequent and severe, the ultrastructure of hippocampal area is significantly changed. Interestingly, the ultrastructure of CA1 area is found to be only moderately altered if seizure episodes following the status epilepticus are rare and more superficial; (iv) alterations in mitochondria and dendrites are among the most common ultrastructural changes seen, suggesting cell stress and changes to cellular metabolism. These morphological changes, observed in brain neurons in status epilepticus, are a reflection of epileptic pathophysiology. Further studies at the chemical and molecular level of neurotransmitter release, such as at the level of porosomes (secretory portals) at the presynaptic membrane, will further reveal molecular details of these changes.

A new tool for the quantitative analysis of dendritic filopodial motility

Dendritic filopodia are tiny and highly motile protrusions formed along the dendrites of neurons. During the search for future presynaptic partners, their shape and size change dynamically, with a direct impact on the formation, stabilization and maintenance of synaptic connections both in vivo and in vitro. In order to reveal molecular players regulating synapse formation, quantitative analysis of dendritic filopodia motility is needed. Defining the length or the tips of these protrusions manually, however, is time consuming, limiting the extent of studies as well as their statistical power. Additionally, area detection based on defining a single intensity threshold can lead to significant errors throughout the image series, as these small structures often have low contrast in fluorescent images. To overcome these problems, the open access Dendritic Filopodia Motility Analyzer, a semi-automated ImageJ/Fiji plugin was created. Our method calculates the displacement of the centre of mass (CoM) within a selected region based on the weighted intensity values of structure forming pixels, selected by upper and lower intensity thresholds. Using synthetic and real biological samples, we prove that the displacement of the weighted CoM reliably characterizes the motility of dendritic protrusions. Additionally, guidelines to define optimal parameters of live cell recordings from dendritic protrusions are provided. © 2014 International Society for Advancement of Cytometry.