Central visual system of the naked mole-rat (Heterocephalus glaber)

Distal axonopathy with structural persistence in glaucomatous neurodegeneration

An early hallmark of neuronal degeneration is distal transport loss and axon pathology. Glaucoma involves the degeneration of retinal ganglion cell (RGC) neurons and their axons in the optic nerve. Here we show that, like other neurodegenerations, distal axon injury appears early in mouse glaucoma. Where RGC axons terminate in the superior colliculus, reduction of active transport follows a retinotopic pattern resembling glaucomatous vision loss. Like glaucoma, susceptibility to transport deficits increases with age and is not necessarily associated with elevated ocular pressure. Transport deficits progress distal-to-proximal, appearing in the colliculus first followed by more proximal secondary targets and then the optic tract. Transport persists through the optic nerve head before finally failing in the retina. Although axon degeneration also progresses distal-to-proximal, myelinated RGC axons and their presynaptic terminals persist in the colliculus well after transport fails. Thus, distal transport loss is predegenerative and may represent a therapeutic target.

The visual system in subterranean African mole-rats (Rodentia, Bathyergidae): retina, subcortical visual nuclei and primary visual cortex

We have studied the visual system of subterranean mole-rats of the rodent family Bathyergidae, for which light and vision seem of little importance. The eye diameter varies between 3.5mm in Bathyergus suillus and 1.3mm in Heterocephalus glaber. The small superficial eyes have features typical of sighted animals (clear optics, well-developed pupil and well-organized retina) and appear suited for proper image formation. The retinae are rod-dominated but possess rather high cone proportions of about 10%. The total number of retinal ganglion cells and optic nerve fibres ranges between 6000 in Bathyergus suillus and 2100 in Heliophobius argenteocinereus. Visual acuity (estimated from counts of peak ganglion cell density and axial length of the eye) is low, ranging between 0.3 and 0.5 cycles/degree. The retina projects to all the visual structures described in surface-dwelling sighted rodents. The suprachiasmatic nucleus is large and receives bilateral retinal input. All other visual nuclei are reduced in size and receive almost exclusively contralateral retinal projections of varying magnitude. The primary visual cortex is small and, in comparison to other rodents, displaced laterally. In conclusion, the African mole-rats possess relatively well-developed functional visual subsystems involved in photoperiodicity, form and brightness discrimination. In contrast, visual subsystems involved in coordination of visuomotor reflexes are severely reduced. This pattern suggests the retention of basic visual capabilities. Residual vision may enable subterranean mammals to localize breaches in the burrows that let in light thus providing a cue to enable mole-rats to reseal such entry points and to prevent entry of predators.

Touching on somatosensory specializations in mammals

Specialized species often reveal general principles of brain organization and provide systems for analysis of sensory function. Subterranean species dependent on touch have particularly large somatosensory areas with modular cortical representations of sensory surfaces. Some species have added cortical areas to processing networks, have developed tactile foveas and have superior colliculi primarily devoted to somatosensation rather than vision. Recent studies reveal surprisingly large cortical representations of oral structures in primates and mole-rats. Cortical modules represent a range of different sensory surfaces in rodents, star-nosed moles and primates, indicating that similar developmental mechanisms operate in diverse species. Finally, manipulation of patterning genes in mice suggests evolutionary mechanisms for producing the specialized corticies of subterranean species.

A stereotaxic atlas of the brain of the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Rodentia, Bathyergidae: Heterocephalus glaber) is a strictly subterranean eusocial mammal. These rodents show a suite of morphological and physiological adaptations, including brain specializations, to this underground milieu that they have inhabited since the early Miocene. Recently, naked mole-rats have received considerable attention as the longest living rodent known, and some of these brain specializations may be potentially important to their exceptional longevity. To serve as a basis for future brain studies, we have constructed a stereotaxic atlas of the brain of this species, labeling all major brain structures.