Longitudinal variations of brain functional connectivity: A case report study based on a mouse model of epilepsy

Brain Functional Connectivity (FC) quantifies statistical dependencies between areas of the brain. FC has been widely used to address altered function of brain circuits in control conditions compared to different pathological states, including epilepsy, a major neurological disorder. However, FC also has the as yet unexplored potential to help us understand the pathological transformation of the brain circuitry. Our hypothesis is that FC can differentiate global brain interactions across a time-scale of days. To this end, we present a case report study based on a mouse model for epilepsy and analyze longitudinal intracranial electroencephalography data of epilepsy to calculate FC changes from the initial insult (status epilepticus) and over the latent period, when epileptogenic networks emerge, and at chronic epilepsy, when unprovoked seizures occur as spontaneous events. We found that the overall network FC at low frequency bands decreased immediately after status epilepticus was provoked, and increased monotonously later on during the latent period. Overall, our results demonstrate the capacity of FC to address longitudinal variations of brain connectivity across the establishment of pathological states.

Two-photon imaging in living brain slices

Two-photon excitation laser scanning microscopy (TPLSM) has become the tool of choice for high-resolution fluorescence imaging in intact neural tissues. Compared with other optical techniques, TPLSM allows high-resolution imaging and efficient detection of fluorescence signal with minimal photobleaching and phototoxicity. The advantages of TPLSM are especially pronounced in highly scattering environments such as the brain slice. Here we describe our approaches to imaging various aspects of synaptic function in living brain slices. To combine several imaging modes together with patch-clamp electrophysiological recordings we found it advantageous to custom-build an upright microscope. Our design goals were primarily experimental convenience and efficient collection of fluorescence. We describe our TPLSM imaging system and its performance in detail. We present dynamic measurements of neuronal morphology of neurons expressing green fluorescent protein (GFP) and GFP fusion proteins as well as functional imaging of calcium dynamics in individual dendritic spines. Although our microscope is a custom instrument, its key advantages can be easily implemented as a modification of commercial laser scanning microscopes.

Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity

Activity shapes the structure of neurons and their circuits. Two-photon imaging of CA1 neurons expressing enhanced green fluorescent protein in developing hippocampal slices from rat brains was used to characterize dendritic morphogenesis in response to synaptic activity. High-frequency focal synaptic stimulation induced a period (longer than 30 minutes) of enhanced growth of small filopodia-like protrusions (typically less than 5 micrometers long). Synaptically evoked growth was long-lasting and localized to dendritic regions close (less than 50 micrometers) to the stimulating electrode and was prevented by blockade of N-methyl-D-aspartate receptors. Thus, synaptic activation can produce rapid input-specific changes in dendritic structure. Such persistent structural changes could contribute to the development of neural circuitry.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part I: trans-Golgi network-derived organelles undergo regulated exocytosis

Exocytosis is a widely observed cellular mechanism for delivering transmembrane proteins to the cell surface and releasing signaling molecules into the extracellular space. Calcium-evoked exocytosis, traditionally thought to be restricted to presynaptic specializations in neurons, has been described recently in many cells. Here, calcium-evoked dendritic exocytosis (CEDE) is visualized in living cultured hippocampal neurons. Organelles that undergo CEDE are in somata, dendrites, and perisynaptic regions, identified by using immunocytochemistry and correlative light and electron microscopy. CEDE is regulated developmentally: neurons <9 d in vitro do not show CEDE. In addition, CEDE is blocked by tetanus toxin, an inhibitor of regulated exocytosis, and nocodazole, an inhibitor of microtubule polymerization. Organelles that undergo CEDE often are found on the base of spines, putative sites of synaptic plasticity. CEDE therefore could be involved in structural and functional modification of spines and could play a role in synaptic plasticity, where it might involve changes in receptor/channel density, release of active compounds having effect on pre- and postsynaptic function, and/or growth of synaptic structures.

Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. Part II: mediation by calcium/calmodulin-dependent protein kinase II

Calcium-evoked dendritic exocytosis (CEDE), demonstrated in cultured hippocampal neurons, is a novel mechanism that could play a role in synaptic plasticity. A number of forms of neuronal plasticity are thought to be mediated by calcium/calmodulin-dependent protein kinase II (CaMKII). Here, we investigate the role of CaMKII in CEDE. We find that the developmental time course of CEDE parallels the expression of alphaCaMKII, a dominant subunit of CaMKII. An inhibitor of this enzyme, KN-62, blocks CEDE. Furthermore, 7 d in vitro neurons (which normally do not express alphaCaMKII nor show CEDE) can undergo CEDE when infected with a recombinant virus producing alphaCaMKII. Expression of a constitutively active CaMKII produces dendritic exocytosis in the absence of calcium stimulus, and this exocytosis is blocked by nocodazole, an inhibitor of microtubule polymerization that also blocks CEDE. These results indicate that CEDE is mediated by the activation of CaMKII, consistent with the view that CEDE plays a role in synaptic plasticity.

Differential spatiotemporal expression of K+ channel polypeptides in rat hippocampal neurons developing in situ and in vitro

Hippocampal neurons are highly plastic in their excitable properties, both during development and in the adult brain. As voltage-sensitive K+ channels are major determinants of membrane excitability, one mechanism for generating plasticity is through regulation of K+ channel activity. To gain insights into the regulation of K+ channels in the hippocampus, we have analyzed the spatiotemporal expression patterns of five K+ channel polypeptides in rat hippocampal neurons developing in situ and in vitro. Delayed rectifier-type channels (Kv1.5, Kv2.1, and Kv2.2) are expressed on all neuronal somata and proximal dendrites, while A-type channels (Kv1.4 and Kv4.2) are present distally on distinct subpopulations of neurons. The development of these patterns in situ is monotonic; that is, while the time and spatial development varies among the channels, each K+ channel subtype initially appears in its adult pattern, suggesting that the mechanisms underlying spatial patterning operate through development. Immunoblots confirm the differential temporal expression of K+ channels in the developing hippocampus, and demonstrate developmentally regulated changes in the microheterogeneity of some K+ channel polypeptide species. Temporal expression patterns of all five K+ channels observed in situ are retained in vitro, while certain aspects of cellular and subcellular localization are altered for some of the K+ channel polypeptides studied. Similarities in K+ channel polypeptide expression in situ and in vitro indicate that the same regulatory mechanisms are controlling spatiotemporal patterning in both situations. However, differences between levels of expression for all subtypes studied except Kv2.1 indicate additional mechanisms operating in situ but absent in vitro that are important in determining polypeptide abundance.