Sex Differences and Olfactory Bulb Alterations Contribute to Limbic Circuit Dysfunction and Behavioral Disturbances in Pilocarpine Model of Epilepsy

The olfactory bulb at the sensory and circuit level transmits information to the limbic and cortical systems for behavioral outputs, and disruption of such circuits induces behavioral disturbances in rodents. Previously, data from our laboratory showed the occurrence of behavioral disturbances in Wistar rats submitted to the pilocarpine model of epilepsy (PME) and that these alterations were sex related. Here we deepen our findings that sex-linked differences are present in PME and that male epileptic rats exhibit profound recurrent seizure patterns, namely seizure duration, severity, and distribution along the light/dark cycle different from that observed in epileptic female rats. Further, using isotropic fractionator we observed significant alterations in the number of neuronal and non-neuronal cells of the olfactory bulb, amygdala, and hippocampus following 3 months of spontaneous recurrent seizures in epileptic male and female rats. Altogether, our study suggests that neuronal and non-neuronal cell death in olfactory bulb may interfere with sex-related differential recurrent seizure patterns, limbic circuit dysfunction, and behavioral disturbances in PME. Lastly, the pilocarpine epilepsy model provides an evidence-based tool to study mechanisms of behavioral disturbances in epileptogenesis that may provide future therapeutic insights in our quest to improve the life of people with epilepsy.

Allometric analysis of brain cell number in Hymenoptera suggests ant brains diverge from general trends

Many comparative neurobiological studies seek to connect sensory or behavioural attributes across taxa with differences in their brain composition. Recent studies in vertebrates suggest cell number and density may be better correlated with behavioural ability than brain mass or volume, but few estimates of such figures exist for insects. Here, we use the isotropic fractionator (IF) method to estimate total brain cell numbers for 32 species of Hymenoptera spanning seven subfamilies. We find estimates from using this method are comparable to traditional, whole-brain cell counts of two species and to published estimates from established stereological methods. We present allometric scaling relationships between body and brain mass, brain mass and nuclei number, and body mass and cell density and find that ants stand out from bees and wasps as having particularly small brains by measures of mass and cell number. We find that Hymenoptera follow the general trend of smaller animals having proportionally larger brains. Smaller Hymenoptera also feature higher brain cell densities than the larger ones, as is the case in most vertebrates, but in contrast with primates, in which neuron density remains rather constant across changes in brain mass. Overall, our findings establish the IF as a useful method for comparative studies of brain size evolution in insects.

Peeking Inside the Lizard Brain: Neuron Numbers in Anolis and Its Implications for Cognitive Performance and Vertebrate Brain Evolution

Studies of vertebrate brain evolution have mainly focused on measures of brain size, particularly relative mass and its allometric scaling across lineages, commonly with the goal of identifying the substrates that underly differences in cognition. However, recent studies on birds and mammals have demonstrated that brain size is an imperfect proxy for neuronal parameters that underly function, such as the number of neurons that make up a given brain region. Here we present estimates of neuron numbers and density in two species of lizard, Anolis cristatellus and A. evermanni, representing the first such data from squamate species, and explore its implications for differences in cognitive performance and vertebrate brain evolution. The isotropic fractionator protocol outlined in this article is optimized for the unique challenges that arise when using this technique with lineages having nucleated erythrocytes and relatively small brains. The number and density of neurons and other cells we find in Anolis for the telencephalon, cerebellum, and the rest of the brain (ROB) follow similar patterns as published data from other vertebrate species. Anolis cristatellus and A. evermanni exhibited differences in their performance in a motor task frequently used to evaluate behavioral flexibility, which was not mirrored by differences in the number, density, or proportion of neurons in either the cerebellum, telencephalon, or ROB. However, the brain of A. evermanni had a significantly higher number of nonneurons and a higher nonneuron to neuron ratio across the whole brain, which could contribute to the observed differences in problem solving between A. cristatellus and A. evermanni. Although limited to two species, our findings suggest that neuron number and density in lizard brains scale similarly to endothermic vertebrates in contrast to the differences observed in brain to body mass relationships. Data from a wider range of species are necessary before we can fully understand vertebrate brain evolution at the neuronal level.

The reliability of the isotropic fractionator method for counting total cells and neurons

BackgroundThe Isotropic Fractionator (IF) is a method used to determine the cellular composition of nervous tissue. It has been mostly applied to assess variation across species, where differences are expected to be large enough not to be masked by methodological error. However, understanding the sources of variation in the method is important if the goal is to detect smaller differences, for example, in same-species comparisons. Comparisons between different mice strains suggest that the IF is consistent enough to detected these differences. Nevertheless, the internal validity of the method has not yet been examined directly.MethodIn this study, we evaluate the reliability of the IF method for the determination of cellular and neuronal numbers. We performed repeated cell counts of the same material by different experimenters to quantify different sources of variation.ResultsIn total cell counts, we observed that for the cerebral cortex most of the variance was at the counter level. For the cerebellum, most of the variance is attributed to the sample itself. As for neurons, random error along with the immunological staining correspond to most of the variation, both in the cerebral cortex and in the cerebellum. Test-retest reliability coefficients were relatively high, especially for cell counts.ConclusionsAlthough biases between counters and random variation in staining could be problematic when aggregating data from different sources, we offer practical suggestions to improve the reliability of the method. While small, this study is a most needed step towards more precise measurement of the brain’s cellular composition.HighlightsMost variance in cell counts was between counters (η = 0.58) for cerebral cortices.For cerebella, most of the variance was attributed to the samples (η = 0.49).Variance in immunocytochemical counts was mostly residual/random (η > 0.8).Test-retest reliability was high (same counter, same sample).Practical suggestions are offered to improve the reliability of the method.

Birds have primate-like numbers of neurons in the forebrain

Some birds achieve primate-like levels of cognition, even though their brains tend to be much smaller in absolute size. This poses a fundamental problem in comparative and computational neuroscience, because small brains are expected to have a lower information-processing capacity. Using the isotropic fractionator to determine numbers of neurons in specific brain regions, here we show that the brains of parrots and songbirds contain on average twice as many neurons as primate brains of the same mass, indicating that avian brains have higher neuron packing densities than mammalian brains. Additionally, corvids and parrots have much higher proportions of brain neurons located in the pallial telencephalon compared with primates or other mammals and birds. Thus, large-brained parrots and corvids have forebrain neuron counts equal to or greater than primates with much larger brains. We suggest that the large numbers of neurons concentrated in high densities in the telencephalon substantially contribute to the neural basis of avian intelligence.