Population based quantification of dendrites: evidence for the lack of microtubule-associate protein 2a,b in Purkinje cell spiny dendrites

Excitatory and inhibitory receptors utilize distinct post- and trans-synaptic mechanisms in vivo

Ionotropic neurotransmitter receptors at postsynapses mediate fast synaptic transmission upon binding of the neurotransmitter. Post- and trans-synaptic mechanisms through cytosolic, membrane, and secreted proteins have been proposed to localize neurotransmitter receptors at postsynapses. However, it remains unknown which mechanism is crucial to maintain neurotransmitter receptors at postsynapses. In this study, we ablated excitatory or inhibitory neurons in adult mouse brains in a cell-autonomous manner. Unexpectedly, we found that excitatory AMPA receptors remain at the postsynaptic density upon ablation of excitatory presynaptic terminals. In contrast, inhibitory GABA_A receptors required inhibitory presynaptic terminals for their postsynaptic localization. Consistent with this finding, ectopic expression at excitatory presynapses of neurexin-3 alpha, a putative trans-synaptic interactor with the native GABA_A receptor complex, could recruit GABA_A receptors to contacted postsynaptic sites. These results establish distinct mechanisms for the maintenance of excitatory and inhibitory postsynaptic receptors in the mature mammalian brain.

Uncovering specific changes in network wiring underlying the primate cerebrotype

Regular scaling of brain networks during evolution has been proposed to be the major process leading to enlarged brains. Alternative views, however, suggest that deviations from regular scaling were crucial to the evolution of the primate brain and the emergence of different cerebrotypes. Here, we examined the scaling within the major link between the cerebellum and the cerebral cortex by studying the deep cerebellar nuclei (DCN). We compared the major axonal and dendritic wiring in the DCN of rodents and monkeys in search of regular scaling. We were able to confirm regular scaling within the density of neurons, the general dendritic length per neuron and the Purkinje cell axon length. However, we also observed specific modification of the scaling rules within the primates’ largest and phylogenetically newest DCN, the dentate nucleus (LN/dentate). Our analysis shows a deviation from regular scaling in the predicted dendritic length per neuron in the LN/dentate. This reduction in the dendritic length is also associated with a smaller dendritic region-of-influence of these neurons. We also detected specific changes in the dendritic diameter distribution, supporting the theory that there is a shift in the neuronal population of the LN/dentate towards neurons that exhibit spatially restricted, clustered branching trees. The smaller dendritic fields would enable a larger number of network modules to be accommodated in the primate LN/dentate and would provide an explanation for the unique folded structure of the primate LN/dentate. Our results show that, in some brain regions, connectivity maximization (i.e., an increase of dendritic fields) is not the sole optimum and that increases in the number of network modules may be important for the emergence of a divergent primate cerebrotype.

Intracerebral Inoculation of Mouse-Passaged Saffold Virus Type 3 Affects Cerebellar Development in Neonatal Mice

Saffold virus (SAFV), a human cardiovirus, is occasionally detected in infants with neurological disorders, including meningitis and cerebellitis. We recently reported that SAFV type 3 isolates infect cerebellar glial cells, but not large neurons, in mice. However, the impact of this infection remained unclear. Here, we determined the neuropathogenesis of SAFV type 3 in the cerebella of neonatal ddY mice by using SAFV passaged in the cerebella of neonatal BALB/c mice. The virus titer in the cerebellum increased following the inoculation of each of five passaged strains. The fifth passaged strain harbored amino acid substitutions in the VP2 (H160R and Q239R) and VP3 (K62M) capsid proteins. Molecular modeling of the capsid proteins suggested that the VP2-H160R and VP3-K62M mutations alter the structural dynamics of the receptor binding surface via the formation of a novel hydrophobic interaction between the VP2 puff B and VP3 knob regions. Compared with the original strain, the passaged strain showed altered growth characteristics in human-derived astroglial cell lines and greater replication in the brains of neonatal mice. In addition, the passaged strain was more neurovirulent than the original strain, while both strains infected astroglial and neural progenitor cells in the mouse brain. Intracerebral inoculation of either the original or the passaged strain affected brain Purkinje cell dendrites, and a high titer of the passaged strain induced cerebellar hypoplasia in neonatal mice. Thus, infection by mouse-passaged SAFV affected cerebellar development in neonatal mice. This animal model contributes to the understanding of the neuropathogenicity of SAFV infections in infants. IMPORTANCE Saffold virus (SAFV) is a candidate neuropathogenic agent in infants and children, but the neuropathogenicity of the virus has not been fully elucidated. Recently, we evaluated the pathogenicity of two clinical SAFV isolates in mice. Similar to other neurotropic picornaviruses, these isolates showed mild infectivity of glial and neural progenitor cells, but not of large neurons, in the cerebellum. However, the outcome of this viral infection in the cerebellum has not been clarified. Here, we examined the tropism of SAFV in the cerebellum. We obtained an in vivo-passaged strain from the cerebella of neonatal mice and examined its genome and its neurovirulence in the neonatal mouse brain. The passaged virus showed high infectivity and neurovirulence in the brain, especially the cerebellum, and affected cerebellar development. This unique neonatal mouse model will be helpful for elucidating the neuropathogenesis of SAFV infections occurring early in life.

Systematic analysis of neuronal wiring of the rodent deep cerebellar nuclei reveals differences reflecting adaptations at the neuronal circuit and internuclear levels

A common view of the architecture of different brain regions is that, despite their heterogeneity, they have optimized their wiring schemes to make maximal use of space. Based on experimental findings, computational models have delineated how about two-thirds of the neuropil is filled out with dendrites and axons optimizing cable costs and conduction time while keeping the connectivity at the highest level. However, whether this assumption can be generalized to all brain regions has not yet been tested. Here we quantified and charted the components of the neuropil in the four deep cerebellar nuclei (DCN) of the rat's brain. We segmented and traced the neuropil stained with one of two antibodies, one antibody against dendritic microtubule-associated proteins (MAP2a,b) and the second against the Purkinje cell axons (PCP2). We compared fiber length density, average fiber diameter, and volume fraction within different components of the DCN in a random, systematic fashion. We observed differences in dendritic and axonal fiber length density, average fiber diameters, and volume fraction within the four different nuclei that make up the DCN. We observe a relative increase in the length density of dendrites and Purkinje cell axons in two of the DCN, namely, the posterior interposed nucleus and the lateral nucleus. Furthermore, the DCN have a surprisingly low volume fraction of their dendritic length density, which we propose is related to their special circuitry. In summary, our results show previously unappreciated functional adaptations among these nuclei.

From cerebellar texture to movement optimization

The cerebellum is a major site for supervised procedural learning and appears to be crucial for optimizing sensorimotor performance. However, the site and origin of the supervising signal are still elusive. Furthermore, its relationship with the prominent neuronal circuitry remains puzzling. In this paper, I will review the relevant information and seek to synthesize a working hypothesis that explains the unique cerebellar structure. The aim of this review was to link the distinctive functions of the cerebellum, as derived from cerebellar lesion studies, with potential elementary computations, as observed by a bottom-up approach from the cerebellar microcircuitry. The parallel fiber geometry is ideal for performing millisecond computations that extract instructive signals. In this scenario, the higher time derivatives of kinematics such as acceleration and/or jerk that occur during motor performance are detected via a tidal wave mechanism and are used (with appropriate gating) as the instructive signal to guide motor smoothing. The advantage of such a mechanism is that movements are optimized by reducing "jerkiness" which, in turn, lowers their energy requirements.

Comparative morphology of dendritic arbors in populations of Purkinje cells in mouse sulcus and apex

Foliation divides the mammalian cerebellum into structurally distinct subdivisions, including the concave sulcus and the convex apex. Purkinje cell (PC) dendritic morphology varies between subdivisions and changes significantly ontogenetically. Since dendritic morphology both enables and limits sensory-motor circuit function, it is important to understand how neuronal architectures differ between brain regions. This study employed quantitative confocal microcopy to reconstruct dendritic arbors of cerebellar PCs expressing green fluorescent protein and compared arbor morphology between PCs of sulcus and apex in young and old mice. Arbors were digitized from high z-resolution (0.25 µm) image stacks using an adaptation of Neurolucida's (MBF Bioscience) continuous contour tracing tool, designed for drawing neuronal somata. Reconstructed morphologies reveal that dendritic arbors of sulcus and apex exhibit profound differences. In sulcus, 72% of the young PC population possesses two primary dendrites, whereas in apex, only 28% do. Spatial constraints in the young sulcus cause significantly more dendritic arbor overlap than in young apex, a distinction that disappears in adulthood. However, adult sulcus PC arbors develop a greater number of branch crossings. These results suggest developmental neuronal plasticity that enables cerebellar PCs to attain correct functional adult architecture under different spatial constraints.

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.