Dendritic spine density in multisensory versus primary sensory cortex

Neocortical pyramidal neurons with axons emerging from dendrites are frequent in non-primates, but rare in monkey and human

The canonical view of neuronal function is that inputs are received by dendrites and somata, become integrated in the somatodendritic compartment and upon reaching a sufficient threshold, generate axonal output with axons emerging from the cell body. The latter is not necessarily the case. Instead, axons may originate from dendrites. The terms ‘axon carrying dendrite’ (AcD) and ‘AcD neurons’ have been coined to describe this feature. In rodent hippocampus, AcD cells are shown to be functionally ‘privileged’, since inputs here can circumvent somatic integration and lead to immediate action potential initiation in the axon. Here, we report on the diversity of axon origins in neocortical pyramidal cells of rodent, ungulate, carnivore, and primate. Detection methods were Thy-1-EGFP labeling in mouse, retrograde biocytin tracing in rat, cat, ferret, and macaque, SMI-32/βIV-spectrin immunofluorescence in pig, cat, and macaque, and Golgi staining in macaque and human. We found that in non-primate mammals, 10–21% of pyramidal cells of layers II–VI had an AcD. In marked contrast, in macaque and human, this proportion was lower and was particularly low for supragranular neurons. A comparison of six cortical areas (being sensory, association, and limbic in nature) in three macaques yielded percentages of AcD cells which varied by a factor of 2 between the areas and between the individuals. Unexpectedly, pyramidal cells in the white matter of postnatal cat and aged human cortex exhibit AcDs to much higher percentages. In addition, interneurons assessed in developing cat and adult human cortex had AcDs at type-specific proportions and for some types at much higher percentages than pyramidal cells. Our findings expand the current knowledge regarding the distribution and proportion of AcD cells in neocortex of non-primate taxa, which strikingly differ from primates where these cells are mainly found in deeper layers and white matter.

Sex differences in dendritic spine density and morphology in auditory and visual cortices in adolescence and adulthood

Dendritic spines are small protrusions on dendrites that endow neurons with the ability to receive and transform synaptic input. Dendritic spine number and morphology are altered as a consequence of synaptic plasticity and circuit refinement during adolescence. Dendritic spine density (DSD) is significantly different based on sex in subcortical brain regions associated with the generation of sex-specific behaviors. It is largely unknown if sex differences in DSD exist in auditory and visual brain regions and if there are sex-specific changes in DSD in these regions that occur during adolescent development. We analyzed dendritic spines in 4-week-old (P28) and 12-week-old (P84) male and female mice and found that DSD is lower in female mice due in part to fewer short stubby, long stubby and short mushroom spines. We found striking layer-specific patterns including a significant age by layer interaction and significantly decreased DSD in layer 4 from P28 to P84. Together these data support the possibility of developmental sex differences in DSD in visual and auditory regions and provide evidence of layer-specific refinement of DSD over adolescent brain development.

What is a multisensory cortex? A laminar, connectional, and functional study of a ferret temporal cortical multisensory area

Now that examples of multisensory neurons have been observed across the neocortex, this has led to some confusion about the features that actually designate a region as "multisensory." While the documentation of multisensory effects within many different cortical areas is clear, often little information is available about their proportions or net functional effects. To assess the compositional and functional features that contribute to the multisensory nature of a region, the present investigation used multichannel neuronal recording and tract tracing methods to examine the ferret temporal region: the lateral rostral suprasylvian sulcal area. Here, auditory-tactile multisensory neurons were predominant and constituted the majority of neurons across all cortical layers whose responses dominated the net spiking activity of the area. These results were then compared with a literature review of cortical multisensory data and were found to closely resemble multisensory features of other, higher-order sensory areas. Collectively, these observations argue that multisensory processing presents itself in hierarchical and area-specific ways, from regions that exhibit few multisensory features to those whose composition and processes are dominated by multisensory activity. It seems logical that the former exhibit some multisensory features (among many others), while the latter are legitimately designated as "multisensory."

Early Sensory Loss Alters the Dendritic Branching and Spine Density of Supragranular Pyramidal Neurons in Rodent Primary Sensory Cortices

Multisensory integration in primary auditory (A1), visual (V1), and somatosensory cortex (S1) is substantially mediated by their direct interconnections and by thalamic inputs across the sensory modalities. We have previously shown in rodents (Mongolian gerbils) that during postnatal development, the anatomical and functional strengths of these crossmodal and also of sensory matched connections are determined by early auditory, somatosensory, and visual experience. Because supragranular layer III pyramidal neurons are major targets of corticocortical and thalamocortical connections, we investigated in this follow-up study how the loss of early sensory experience changes their dendritic morphology. Gerbils were sensory deprived early in development by either bilateral sciatic nerve transection at postnatal day (P) 5, ototoxic inner hair cell damage at P10, or eye enucleation at P10. Sholl and branch order analyses of Golgi-stained layer III pyramidal neurons at P28, which demarcates the end of the sensory critical period in this species, revealed that visual and somatosensory deprivation leads to a general increase of apical and basal dendritic branching in A1, V1, and S1. In contrast, dendritic branching, particularly of apical dendrites, decreased in all three areas following auditory deprivation. Generally, the number of spines, and consequently spine density, along the apical and basal dendrites decreased in both sensory deprived and non-deprived cortical areas. Therefore, we conclude that the loss of early sensory experience induces a refinement of corticocortical crossmodal and other cortical and thalamic connections by pruning of dendritic spines at the end of the critical period. Based on present and previous own results and on findings from the literature, we propose a scenario for multisensory development following early sensory loss.

Synaptic distribution and plasticity in primary auditory cortex (A1) exhibits laminar and cell-specific changes in the deaf

The processing sequence through primary auditory cortex (A1) is impaired by deafness as evidenced by reduced neuronal activation in A1 of cochlear-implanted deaf cats. Such a loss of neuronal excitation should be manifest as changes in excitatory synaptic number and/or size, for which the post-synaptic correlate is the dendritic spine. Therefore, the present study sought evidence for this functional disruption using Golgi-Cox/light microscopic techniques that examined spine-bearing neurons and their dendritic spine features across all laminae in A1 of early-deaf (ototoxic lesion <1 month; raised into adulthood >16 months) and hearing cats. Surprisingly, in the early-deaf significant increases in spine density and size were observed in the supragranular layers, while significant reductions in spine density were observed for spiny non-pyramidal, but not pyramidal, neurons in the granular layer. No changes in dendritic spine density consistent with loss of excitatory inputs were seen for infragranular neurons. These results indicate that long-term early-deafness induces plastic changes in the excitatory circuitry of A1 that are laminar and cell-specific. An additional finding was that, unlike the expected abundance of stellate neurons that characterize the granular layer of other primary sensory cortices, pyramidal neurons predominate within layer 4 of A1. Collectively, these observations are important for understanding how neuronal connectional configurations contribute to region-specific processing capabilities in normal brains as well as those with altered sensory experiences.

Synaptic Basis for Cross-modal Plasticity: Enhanced Supragranular Dendritic Spine Density in Anterior Ectosylvian Auditory Cortex of the Early Deaf Cat

In the cat, the auditory field of the anterior ectosylvian sulcus (FAES) is sensitive to auditory cues and its deactivation leads to orienting deficits toward acoustic, but not visual, stimuli. However, in early deaf cats, FAES activity shifts to the visual modality and its deactivation blocks orienting toward visual stimuli. Thus, as in other auditory cortices, hearing loss leads to cross-modal plasticity in the FAES. However, the synaptic basis for cross-modal plasticity is unknown. Therefore, the present study examined the effect of early deafness on the density, distribution, and size of dendritic spines in the FAES. Young cats were ototoxically deafened and raised until adulthood when they (and hearing controls) were euthanized, the cortex stained using Golgi-Cox, and FAES neurons examined using light microscopy. FAES dendritic spine density averaged 0.85 spines/μm in hearing animals, but was significantly higher (0.95 spines/μm) in the early deaf. Size distributions and increased spine density were evident specifically on apical dendrites of supragranular neurons. In separate tracer experiments, cross-modal cortical projections were shown to terminate predominantly within the supragranular layers of the FAES. This distributional correspondence between projection terminals and dendritic spine changes indicates that cross-modal plasticity is synaptically based within the supragranular layers of the early deaf FAES.

The neocortex of cetartiodactyls. II. Neuronal morphology of the visual and motor cortices in the giraffe (Giraffa camelopardalis)

The present quantitative study extends our investigation of cetartiodactyls by exploring the neuronal morphology in the giraffe (Giraffa camelopardalis) neocortex. Here, we investigate giraffe primary visual and motor cortices from perfusion-fixed brains of three subadults stained with a modified rapid Golgi technique. Neurons (n = 244) were quantified on a computer-assisted microscopy system. Qualitatively, the giraffe neocortex contained an array of complex spiny neurons that included both "typical" pyramidal neuron morphology and "atypical" spiny neurons in terms of morphology and/or orientation. In general, the neocortex exhibited a vertical columnar organization of apical dendrites. Although there was no significant quantitative difference in dendritic complexity for pyramidal neurons between primary visual (n = 78) and motor cortices (n = 65), there was a significant difference in dendritic spine density (motor cortex > visual cortex). The morphology of aspiny neurons in giraffes appeared to be similar to that of other eutherian mammals. For cross-species comparison of neuron morphology, giraffe pyramidal neurons were compared to those quantified with the same methodology in African elephants and some cetaceans (e.g., bottlenose dolphin, minke whale, humpback whale). Across species, the giraffe (and cetaceans) exhibited less widely bifurcating apical dendrites compared to elephants. Quantitative dendritic measures revealed that the elephant and humpback whale had more extensive dendrites than giraffes, whereas the minke whale and bottlenose dolphin had less extensive dendritic arbors. Spine measures were highest in the giraffe, perhaps due to the high quality, perfusion fixation. The neuronal morphology in giraffe neocortex is thus generally consistent with what is known about other cetartiodactyls.

Dendritic morphology of pyramidal neurons in the chimpanzee neocortex: regional specializations and comparison to humans

The primate cerebral cortex is characterized by regional variation in the structure of pyramidal neurons, with more complex dendritic arbors and greater spine density observed in prefrontal compared with sensory and motor cortices. Although there are several investigations in humans and other primates, virtually nothing is known about regional variation in the morphology of pyramidal neurons in the cerebral cortex of great apes, humans' closest living relatives. The current study uses the rapid Golgi stain to quantify the dendritic structure of layer III pyramidal neurons in 4 areas of the chimpanzee cerebral cortex: Primary somatosensory (area 3b), primary motor (area 4), prestriate visual (area 18), and prefrontal (area 10) cortex. Consistent with previous studies in humans and macaque monkeys, pyramidal neurons in the prefrontal cortex of chimpanzees exhibit greater dendritic complexity than those in other cortical regions, suggesting that prefrontal cortical evolution in primates is characterized by increased potential for integrative connectivity. Compared with chimpanzees, the pyramidal neurons of humans had significantly longer and more branched dendritic arbors in all cortical regions.

Laminar and connectional organization of a multisensory cortex

The transformation of sensory signals as they pass through cortical circuits has been revealed almost exclusively through studies of the primary sensory cortices, for which principles of laminar organization, local connectivity, and parallel processing have been elucidated. In contrast, almost nothing is known about the circuitry or laminar features of multisensory processing in higher order, multisensory cortex. Therefore, using the ferret higher order multisensory rostral posterior parietal (PPr) cortex, the present investigation employed a combination of multichannel recording and neuroanatomical techniques to elucidate the laminar basis of multisensory cortical processing. The proportion of multisensory neurons, the share of neurons showing multisensory integration, and the magnitude of multisensory integration were all found to differ by layer in a way that matched the functional or connectional characteristics of the PPr. Specifically, the supragranular layers (L2/3) demonstrated among the highest proportions of multisensory neurons and the highest incidence of multisensory response enhancement, while also receiving the highest levels of extrinsic inputs, exhibiting the highest dendritic spine densities, and providing a major source of local connectivity. In contrast, layer 6 showed the highest proportion of unisensory neurons while receiving the fewest external and local projections and exhibiting the lowest dendritic spine densities. Coupled with a lack of input from principal thalamic nuclei and a minimal layer 4, these observations indicate that this higher level multisensory cortex shows functional and organizational modifications from the well-known patterns identified for primary sensory cortical regions.