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

Abstract

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.

Uncovering specific changes in network wiring underlying the primate cerebrotype

“…The study supplements our previous analysis (Hamodeh et al 2014 ) and now includes the data of five adult (8–14 years) male monkeys (Macaca mulatta) weighing 8–12 kg. Handling and care of the animals was approved by the regional authorities (Regierungspraesidium) and complied fully with the guidelines of the European Community (EUVD 86/609/EEC).…”

“…Histological processing was carried out as described previously (Hamodeh et al 2014 ). In brief, tissue was mounted and sectioned to serial transversal slices of either 60 µm for the rats and, in the case of the monkey brains, 40 or 50 µm for immunohistochemistry and 60 µm for cell counting.…”

“…Images were acquired with a laser scanning confocal microscope (LSM 510, Carl Zeiss, Jena, Germany) using the Argon laser (wavelength 488 nm) as described elsewhere (Hamodeh et al 2010 , 2014 ). Image stacks were taken for each probe using a pinhole diameter equal to 1 Airy unit.…”

“…The following parameters were extracted from our fiber segmentation and reconstruction analysis (Hamodeh et al 2010 , 2014 ): dendritic and PCax diameter ( D dia ) and dendritic and PCax length density (Dl dens ). The average dendritic length per neuron (Dl neu ) was obtained by normalizing the dendritic length density by the neuron count density.…”

“…We also systematically evaluated the tissue penetration of the antibody within each probe by calculating the fluorescence intensity in relation to the depth of the section. From this evaluation, and as described in detail previously (Hamodeh et al 2010 , 2014 ), we limited the region of fiber analysis to the stack of optical sections whose maximal intensity was 95.4% of that ascertained within the total probe. We also accounted for slice shrinkage by checking slice thickness after staining and embedding.…”