Scaling the primate lateral geniculate nucleus: niche and neurodevelopment in the regulation of magnocellular and parvocellular cell number and nucleus volume

Trade-Offs in the Sensory Brain between Diurnal and Nocturnal Rodents

INTRODUCTION

Transitions in temporal niche have occurred many times over the course of mammalian evolution. These are associated with changes in sensory stimuli available to animals, particularly with visual cues, because levels of light are so much higher during the day than at night. This relationship between temporal niche and available sensory stimuli elicits the expectation that evolutionary transitions between diurnal and nocturnal lifestyles will be accompanied by modifications of sensory systems that optimize the ability of animals to receive, process, and react to important stimuli in the environment.

METHODS

This study examines the influence of temporal niche on investment in sensory brain tissue of 13 rodent species (five diurnal; eight nocturnal). Animals were euthanized and the brains immediately frozen on dry ice; olfactory bulbs were subsequently dissected and weighed, and the remaining brain was weighed, sectioned, and stained. Stereo Investigator was used to calculate volumes of four sensory regions that function in processing visual (lateral geniculate nucleus, superior colliculus) and auditory (medial geniculate nucleus, inferior colliculus) information. A phylogenetic framework was used to assess the influence of temporal niche on the relative sizes of these brain structures and of olfactory bulb weights.

RESULTS

Compared to nocturnal species, diurnal species had larger visual regions, whereas nocturnal species had larger olfactory bulbs than their diurnal counterparts. Of the two auditory structures examined, one (medial geniculate nucleus) was larger in diurnal species, while the other (inferior colliculus) did not differ significantly with temporal niche.

CONCLUSION

Our results indicate a possible indirect association between temporal niche and auditory investment and suggest probable trade-offs of investment between olfactory and visual areas of the brain, with diurnal species investing more in processing visual information and nocturnal species investing more in processing olfactory information.

Neuronal density and expression of calcium-binding proteins across the layers of the superior colliculus in the common marmoset (Callithrix jacchus)

The superior colliculus (SC) is a layered midbrain structure with functions that include polysensory and sensorimotor integration. Here, we describe the distribution of different immunohistochemically identified classes of neurons in the SC of adult marmoset monkeys (Callithrix jacchus). Neuronal nuclei (NeuN) staining was used to determine the overall neuronal density in the different SC layers. In addition, we studied the distribution of neurons expressing different calcium-binding proteins (calbindin [CB], parvalbumin [PV] and calretinin [CR]). Our results indicate that neuronal density in the SC decreases from superficial to deep layers. Although the neuronal density within the same layer varies little across the mediolateral axis, it tends to be lower at rostral levels, compared to caudal levels. Cells expressing different calcium-binding proteins display differential gradients of density according to depth. Both CB- and CR-expressing neurons show markedly higher densities in the stratum griseum superficiale (SGS), compared to the stratum opticum and intermediate and deep layers. However, CR-expressing neurons are twice as common as CB-expressing neurons outside the SGS. The distribution of PV-expressing cells follows a shallow density gradient from superficial to deep layers. When normalized relative to total neuronal density, the proportion of CR-expressing neurons increases between the superficial and intermediate layers, whereas that of CB-expressing neurons declines toward the deep layers. The proportion of PV-expressing neurons remains constant across layers. Our data provide layer-specific and accurate estimates of neuronal density, which may be important for the generation of biophysical models of how the primate SC transforms sensory inputs into motor signals.

Generic Homo sapiens and Unique Mus musculus: Establishing the Typicality of the Modeled and the Model Species

The question of how complex human abilities evolved, such as language or face recognition, has been pursued by means of multiple strategies. Highly specialized non-human species have been examined analytically for formal similarities, close phylogenetic relatives have been examined for continuity, and simpler species have been analyzed for the broadest view of functional organization. All these strategies require empirical evidence of what is variable and predictable in both the modeled and the model species. Turning to humans, allometric analyses of the evolution of brain mass and brain components often return the interesting, but disappointing answer that volumetric organization of the human brain is highly predictable seen in its phylogenetic context. Reconciling this insight with unique human behavior, or any species-typical behavior, represents a serious challenge. Allometric analyses of the order and duration of mammalian neural development show that, while basic neural development in humans is allometrically predictable, conforming to adult neural architecture, some life history features deviate, notably that weaning is unusually early. Finally, unusual deviations in the retina and central auditory system in the laboratory mouse, which is widely assumed to be “generic,” as well as severe deviations from expected brain allometry in some mouse strains, underline the need for a deeper understanding of phylogenetic variability even in those systems believed to be best understood.

Neuronal degeneration in the dorsal lateral geniculate nucleus following lesions of primary visual cortex: comparison of young adult and geriatric marmoset monkeys

Neuronal loss in the lateral geniculate nucleus (LGN) is a consequence of lesions of the primary visual cortex (V1). Despite the importance of this phenomenon in understanding the residual capacities of the primate visual system following V1 damage, few quantitative studies are available, and the effect of age at the time of lesion remains unknown. We compared the volume, neuronal number, and neuronal density in the LGN, 6-21 months after unilateral V1 lesions in marmoset monkeys. Stereological sampling techniques and neuronal nuclei (NeuN) staining were used to assess the effects of similar-sized lesions in adult (2-4 years) and geriatric (10-14 years) animals. We found that lesions involving the opercular and caudal calcarine parts of V1 caused robust loss of neurons in topographically corresponding regions of the ipsilateral LGN (lesion projection zones), concomitant with a substantial reduction in the volume of this nucleus. Neuronal density was markedly reduced in the lesion projection zones, relative to the corresponding regions of the contralateral LGN, or the LGN in non-lesioned animals. Moreover, the percentage decrease in neuronal density within the lesion projection zones was significantly greater in the geriatric group, compared with the adult groups. The volume and neuronal density in the contralateral LGN of lesioned adult and geriatric marmosets were similar to those in non-lesioned animals. These results show that the primate LGN becomes more vulnerable to degeneration with advancing age. However, even in geriatric primates there is a population of LGN neurons which survives degeneration, and which could play a role in blindsight.

Taxonomic Separation of Hippocampal Networks: Principal Cell Populations and Adult Neurogenesis

While many differences in hippocampal anatomy have been described between species, it is typically not clear if they are specific to a particular species and related to functional requirements or if they are shared by species of larger taxonomic units. Without such information, it is difficult to infer how anatomical differences may impact on hippocampal function, because multiple taxonomic levels need to be considered to associate behavioral and anatomical changes. To provide information on anatomical changes within and across taxonomic ranks, we present a quantitative assessment of hippocampal principal cell populations in 20 species or strain groups, with emphasis on rodents, the taxonomic group that provides most animals used in laboratory research. Of special interest is the importance of adult hippocampal neurogenesis (AHN) in species-specific adaptations relative to other cell populations. Correspondence analysis of cell numbers shows that across taxonomic units, phylogenetically related species cluster together, sharing similar proportions of principal cell populations. CA3 and hilus are strong separators that place rodent species into a tight cluster based on their relatively large CA3 and small hilus while non-rodent species (including humans and non-human primates) are placed on the opposite side of the spectrum. Hilus and CA3 are also separators within rodents, with a very large CA3 and rather small hilar cell populations separating mole-rats from other rodents that, in turn, are separated from each other by smaller changes in the proportions of CA1 and granule cells. When adult neurogenesis is included, the relatively small populations of young neurons, proliferating cells and hilar neurons become main drivers of taxonomic separation within rodents. The observations provide challenges to the computational modeling of hippocampal function, suggest differences in the organization of hippocampal information streams in rodent and non-rodent species, and support emerging concepts of functional and structural interactions between CA3 and the dentate gyrus.

Evolution of cytoarchitectural landscapes in the mammalian isocortex: Sirenians (Trichechus manatus) in comparison with other mammals

The isocortex of several primates and rodents shows a systematic increase in the number of neurons per unit of cortical surface area from its rostrolateral to caudomedial border. The steepness of the gradient in neuronal number and density is positively correlated with cortical volume. The relative duration of neurogenesis along the same rostrocaudal gradient predicts a substantial fraction of this variation in neuron number and laminar position, which is produced principally from layers II-IV neurons. However, virtually all of our quantitative knowledge about total and laminar variation in cortical neuron numbers and neurogenesis comes from rodents and primates, leaving whole taxonomic groups and many intermediate-sized brains unexplored. Thus, the ubiquity in mammals of the covariation of longer cortical neurogenesis and increased cortical neuron number deriving from cortical layers II-IV is undetermined. To begin to address this gap, we examined the isocortex of the manatee using the optical disector method in sectioned tissue, and also assembled partial data from published reports of the domestic cat brain. The manatee isocortex has relatively fewer neurons per total volume, and fewer II-IV neurons than primates with equivalently sized brains. The gradient in number of neurons from the rostral to the caudal pole is intermediate between primates and rodents, and, like those species, is observed only in the upper cortical layers. The cat isocortex (Felis domesticus) shows a similar structure. Key species for further tests of the origin, ubiquity, and significance of this organizational feature are discussed.

Mapping arealisation of the visual cortex of non-primate species: lessons for development and evolution

The integration of the visual stimulus takes place at the level of the neocortex, organized in anatomically distinct and functionally unique areas. Primates, including humans, are heavily dependent on vision, with approximately 50% of their neocortical surface dedicated to visual processing and possess many more visual areas than any other mammal, making them the model of choice to study visual cortical arealisation. However, in order to identify the mechanisms responsible for patterning the developing neocortex, specifying area identity as well as elucidate events that have enabled the evolution of the complex primate visual cortex, it is essential to gain access to the cortical maps of alternative species. To this end, species including the mouse have driven the identification of cellular markers, which possess an area-specific expression profile, the development of new tools to label connections and technological advance in imaging techniques enabling monitoring of cortical activity in a behaving animal. In this review we present non-primate species that have contributed to elucidating the evolution and development of the visual cortex. We describe the current understanding of the mechanisms supporting the establishment of areal borders during development, mainly gained in the mouse thanks to the availability of genetically modified lines but also the limitations of the mouse model and the need for alternate species.

Allocating structure to function: the strong links between neuroplasticity and natural selection

A central question in brain evolution is how species-typical behaviors, and the neural function-structure mappings supporting them, can be acquired and inherited. Advocates of brain modularity, in its different incarnations across scientific subfields, argue that natural selection must target domain-dedicated, separately modifiable neural subsystems, resulting in genetically-specified functional modules. In such modular systems, specification of neuron number and functional connectivity are necessarily linked. Mounting evidence, however, from allometric, developmental, comparative, systems-physiological, neuroimaging and neurological studies suggests that brain elements are used and reused in multiple functional systems. This variable allocation can be seen in short-term neuromodulation, in neuroplasticity over the lifespan and in response to damage. We argue that the same processes are evident in brain evolution. Natural selection must preserve behavioral functions that may co-locate in variable amounts with other functions. In genetics, the uses and problems of pleiotropy, the re-use of genes in multiple networks have been much discussed, but this issue has been sidestepped in neural systems by the invocation of modules. Here we highlight the interaction between evolutionary and developmental mechanisms to produce distributed and overlapping functional architectures in the brain. These adaptive mechanisms must be robust to perturbations that might disrupt critical information processing and action selection, but must also recognize useful new sources of information arising from internal genetic or environmental variability, when those appear. These contrasting properties of "robustness" and "evolvability" have been discussed for the basic organization of body plan and fundamental cell physiology. Here we extend them to the evolution and development, "evo-devo," of brain structure.