Challenges for emerging neurostimulation-based therapies for real-time seizure control.

In step with the worthwhile aim of this special issue, two junior investigators impart their insights on the therapeutic challenges imposed by pharmacoresistant epilepsies and offer viable approaches to improvement of treatment outcomes. Sunderam's comprehensive perspective addresses issues of critical importance for the design of efficacious therapies. Talathi delves into the thorny roles of so-called "interictal" spikes in ictio- and epileptogenesis, roles that are central to understanding the dynamics of these phenomena and implicitly of how to prevent them or abort them. First, however, Osorio and co-workers illustrate the complex behavior of the epileptogenic network and point to the importance of real-time intraindividual adaptation and optimization of therapies for seizures originating from the same epileptogenic network.

A systematic random sampling scheme optimized to detect the proportion of rare synapses in the neuropil.

Synapses can only be morphologically identified by electron microscopy and this is often a very labor-intensive and time-consuming task. When quantitative estimates are required for pathways that contribute a small proportion of synapses to the neuropil, the problems of accurate sampling are particularly severe and the total time required may become prohibitive. Here we present a sampling method devised to count the percentage of rarely occurring synapses in the neuropil using a large sample (approximately 1000 sampling sites), with the strong constraint of doing it in reasonable time. The strategy, which uses the unbiased physical disector technique, resembles that used in particle physics to detect rare events. We validated our method in the primary visual cortex of the cat, where we used biotinylated dextran amine to label thalamic afferents and measured the density of their synapses using the physical disector method. Our results show that we could obtain accurate counts of the labeled synapses, even when they represented only 0.2% of all the synapses in the neuropil.

A new look at embryonic development of the visual system in decapod crustaceans: neuropil formation, neurogenesis, and apoptotic cell death.

In recent years, comparing the structure and development of the central nervous system in crustaceans has provided new insights into the phylogenetic relationships of arthropods. Furthermore, the structural evolution of the compound eyes and optic ganglia of adult arthropods has been discussed, but it was not possible to compare the ontogeny of arthropod visual systems, owing to the lack of data on species other than insects. In the present report, we studied the development of the crustacean visual system by examining neurogenesis, neuropil formation, and apoptotic cell death in embryos of the American lobster, Homarus americanus, the spider crab, Hyas araneus, and the caridean shrimp, Palaemonetes argentinus, and compare these processes with those found in insects. Our results on the patterns of stem cell proliferation provide evidence that in decapod crustaceans and hemimetabolous insects, there exist considerable similarities in the mechanisms by which accretion of the compound eyes and growth of the optic lobes is achieved, suggesting an evolutionary conservation of these mechanisms.

Tracing of a neuronal network in the locust by pressure injection of markers into a synaptic neuropil.

Central neuronal circuits of vertebrates have often been investigated using injection of markers into synaptic neuropils, whereas similar techniques have rarely been applied in invertebrates. In this study we tested several neuroanatomical tracers for their ability to mark central neuronal circuits in insects, using the well described auditory network of the locust, Locusta migratoria. After physiological localization of an auditory neuropil various tracers were pressure injected. Horseradish peroxidase, dextrans (3 and 10 kDa) and especially biocytin and neurobiotin were effectively incorporated by auditory interneurons, which resulted in their extensive labeling. Postsynaptic regions turned out to be the major, if not exclusive sites of uptake of injected markers, which is deduced from two lines of evidence: (i) for labeling of identified auditory neurons it was necessary to apply the tracer to postsynaptic sites of the neuron; (ii) only a few non-auditory neurons were labeled (probably by lesioning axons during electrode impalement). No evidence could be found for an activity dependent uptake. We conclude that pressure injection of certain tracers into synaptic areas can be used to identify central nervous circuits in insects.