The mutability of low-affinity nerve growth factor receptor (p75NGFR) expression in the rat cuneate nucleus following perinatal injury and adult deafferentations: comparisons with cytochrome oxidase

Wiring the ocular surface: A focus on the comparative anatomy and molecular regulation of sensory innervation of the cornea

The cornea is richly innervated with sensory nerves that function to detect and clear harmful debris from the surface of the eye, promote growth and survival of the corneal epithelium and hasten wound healing following ocular disease or trauma. Given their importance to eye health, the neuroanatomy of the cornea has for many years been a source of intense investigation. Resultantly, complete nerve architecture maps exist for adult human and many animal models and these maps reveal few major differences across species. Interestingly, recent work has revealed considerable variation across species in how sensory nerves are acquired during developmental innervation of the cornea. Highlighting such species-distinct key differences, but also similarities, this review provides a full, comparative anatomy analysis of sensory innervation of the cornea for all species studied to date. Further, this article comprehensively describes the molecules that have been shown to guide and direct nerves toward, into and through developing corneal tissue as the final architectural pattern of the cornea's neuroanatomy is established. Such knowledge is useful for researchers and clinicians seeking to better understand the anatomical and molecular basis of corneal nerve pathologies and to hasten neuro-regeneration following infection, trauma or surgery that damage the ocular surface and its corneal nerves.

Forelimb amputation-induced reorganization in the cuneate nucleus (CN) is not reflected in large-scale reorganization in rat forepaw barrel subfield cortex (FBS)

We examined reorganization in cuneate nucleus (CN) in juvenile rat following forelimb amputation (n=34) and in intact controls (n=5) to determine whether CN forms a substrate for large-scale reorganization in forepaw barrel subfield (FBS) cortex. New input from the shoulder first appears in the FBS 4 weeks after amputation, and by 6 weeks, the new shoulder input comes to occupy most of the FBS. Electrophysiological recording was used to map CN in controls and in forelimb amputees during the first 12 weeks following deafferentation and at 26 and 30 weeks post-amputation. Mapping was confined to a location 300 μm anterior to the obex where a medial-to-lateral row of electrode penetrations traversed through a complete complement of cytochrome-oxidase stained clusters (called barrelettes) that are associated with the representation of the glabrous forepaw digits and pads and adjacent non-cluster zones that are associated with the representation of the wrist, arm, and shoulder. Following amputation, non-cluster zones became occupied with new input from the body/chest and head/neck, while the cluster zone remained largely devoid of new input except at the border. A regression analysis comparing controls and amputees over the first 12 weeks post-amputation found significant differences for the total area of new input from the body/chest and head/neck in the non-cluster zones, while no significant differences were found for any new input into the cluster zone. When the averaged areas of a body-part representation were re-examined as a percentage of the averaged zonal area, a non-significant increase in new input from the body was observed within the cluster zone during post-amputation weeks 2-3 that returned to baseline in the subsequent weeks. In contrast, significant differences in averaged area of body-part representations for body/chest and head/neck were found in non-cluster zones over the first 12 weeks post-amputation. The present findings suggest that reorganization occurs only within the non-cluster zones whereby new input from the body/chest and head/neck moves in and occupies the deafferented territory immediately after amputation. Additionally, the lack of significant differences in new shoulder input in either cluster or non-cluster zones over the first 12 weeks after amputation suggests that CN provides an unlikely substrate for large-scale reorganization in the FBS.

Functional and structural organization of the forelimb representation in cuneate nucleus in rat

We examined the physiological representation of the forelimb in the cuneate nucleus (CN) of forelimb-intact young adult rats (n=38) as the first part in a series of studies aimed at understanding the possible role that CN plays in delayed cortical reorganization that follows forelimb amputation. Metabolic labeling with cytochrome oxidase (CO) and electrophysiological mapping were used to examine the relationship between the structural and functional organization of CN. CN is a cylinder-shaped structure that lies bilaterally in the brainstem and extends nearly 4mm in the rostrocaudal direction. The forelimb is represented along the rostrocaudal extent. CN contains three zones; the rostral and caudal zones receive input largely from deep muscle and joint receptors and a middle zone, in the vicinity of the obex, receives input primarily from cutaneous receptors in the skin. The middle zone is somatotopically organized with the glabrous digits represented centrally, bordered on the medial side by ulnar wrist, ulnar forearm, and posterior upper arm representations; on the lateral side by radial wrist, radial forearm, and anterior upper arm representations; and on dorsal side by the dorsal digits and dorsal hand. The middle zone also contains well-defined CO-filled glomerular structures, called barrelettes, which are located within a homogenously stained field. The barrelettes are associated with the representation of the glabrous digits, with D5 represented most dorsal followed sequentially in a ventral-to-lateral direction by the representation of D4, D3, D2, and D1. The digit representations are topographically organized with the distal digit surface represented laterally with respect to the more medially lying proximal digit surface. The digit and palmar pads are also represented by barrelettes located on the medial side of CN. In contrast, the dorsal digit surfaces are represented dorsally and the dorsal hand is represented directly beneath the cuneate fasciculus, in a region devoid of barrelettes. The representations of the ulnar and radial wrist, forearm, and upper arm also lie within the homogeneously stained field in CN. The forelimb representation is bordered on the medial side by representation of trunk and hindlimb, and on the lateral side by representation of shoulder, ear, and head. While the present findings support and extend previous electrophysiological and anatomical studies of CN in the rat, they also provide a detailed physiological description of the functional organization of CN that is necessary for subsequent understanding of the functional reorganization of CN that may result following forelimb amputation.

Neurotrophin receptor (p75) in the trigeminal thalamus of the rat: development, response to injury, transient vibrissa-related patterning, and retrograde transport

We report on the transient, patterned expression of p75 in the ventrobasal (VB) thalamus, the major thalamic relay for somatosensation. We immunostained the brains of developing rats ranging in age from embryonic day (E) 14.5 to postnatal day (PD) 15 with an antibody against p75. To compare p75 expression with the developing synaptic organization within VB, we also immunolocalized the synaptic-vesicle-associated protein, synaptophysin (SYN), on alternate sections. p75-immunoreactivity (IR) was dense and uniform in the ventroposterior medial nucleus (VPM) in the late embryonic and early postnatal periods (E 16.5 to PD 3). In contrast, from PD 4-10, p75-IR in the VPM was patterned, reminiscent of cytochrome-oxidase-stained barreloids, a characteristic feature of the VB in rodents. By PD 14, p75-IR in the VPM was no longer detectable. The ventroposterior lateral nucleus (VPL), in contrast, exhibited no p75-IR. No p75-IR was detected in the ventroposterior lateral nucleus (VPL) at any developmental stage in which VPM could be distinguished from VPL. Light, but clearly patterned SYN-IR, first detectable on PD 2-3, increased in intensity in both VPL and VPM through PD 15. Sectioning the infraorbital nerve on PD 0 resulted in blurred patterns of p75- and SYN-IR within VPM in PD 7-9 rat pups. Removing large portions of the somatosensory cortex on PD 0 resulted in subsequent greatly reduced p75- and SYN-IR within VB. To specify the source of the p75-IR terminals, we stereotaxically injected into the VPM of PD 4-5 rats a monoclonal antibody to p75. One to 2 days later, IR of retrogradely transported p75 antibodies could be traced within axons and cell bodies of neurons associated with the trigeminothalamic pathway through the caudal diencephalon and mesencephalon; labelling was confined to the contralateral trigeminal principal sensory nucleus. The observed, transiently patterned p75-IR in VPM the early postpartum period suggests a role for p75 in synaptogenesis and pattern formation.

Distribution of the low-affinity neurotrophin receptor (p75) in the developing trigeminal brainstem complex in the rat

The low-affinity neurotrophin receptor (p75) binds all members of the neurotrophin family. In the rat, during the first week postpartum, dense p75-immunoreactivity (IR) is present throughout all components of the trigeminal brainstem complex (TBC), largely associated with primary sensory afferents. Within subnucleus caudalis (SpC) of the TBC, intense p75-IR is present in all laminae at birth. During the second and third postnatal weeks, p75-IR in SpC gradually fades within the deeper laminae, becoming generally restricted in the adult to laminae I and II. Similar declines in p75-IR intensity occur in the subnucleus oralis (SpO); in the SpO in the adult, p75-IR is confined to the dorsalmost portion of SpO. In subnucleus interpolaris, an emerging, vibrissa-related pattern of p75-IR is detectable on PD0 (first 24 hr postpartum), which becomes fully differentiated during PD4-PD7. However, this pattern gradually disappears during the third postnatal week. Ventrally in the nucleus principalis (PrV), a pattern of p75-IR that mirrors the topographical arrangement of the vibrissae is detectable by PD0-PD1, is fully differentiated by the end of the first postnatal week, and persists into adulthood. Perinatal unilateral sectioning of the infraorbital nerve on PD0-PD1, but not as late as PD4, disrupts p75-IR patterning in the adult PrV. Although p75 appears to be associated with primary afferent pattern formation, to determine whether it is essential, we examined mutant mice unable to form functional p75. In the TBC of these knockout mice, examined as adults, patterns of cytochrome oxidase staining (which parallel those of p75-IR) appeared to be normal. In summary, during early development, p75 is widely expressed in the TBC during periods of active synaptogenesis and pattern formation, whereas in the adult, its expression is restricted to association with populations of primary sensory afferents. However, the absence of functional p75 in genetically altered mice does not appear to prevent primary afferent pattern formation.

Synaptophysin immunoreactivity and distributions of calcium-binding proteins highlight the functional organization of the rat's dorsal column nuclei

The mammalian dorsal column nuclei (DCN) are principally composed of the cuneate (CN) and gracile (GN) nuclei. Data presented here support previously published anatomical and functional evidence that the longitudinal organization of the CN and GN reflect the complex role of the DCN in somatosensory processing. The CN is organized longitudinally into three parts. Within the middle portion of this nucleus, primary afferent projections and cuneothalamic cells are concentrated. Although traditional cytoarchitectonic analyses had failed to detect this tripartite organization in rats, we found evidence for it, with a functional middle region, extending approximately 0.2-0.9 mm caudal to the obex, characterized by precise somatotopy of primary afferent terminations and corresponding somatotopy of cytochrome oxidase (CO) blotches. Additional evidence is presented here consistent with a functionally distinct middle region within the rat's CN: (1) patches of dense synaptophysin (a synaptic-vesical-associated protein)-immunoreactivity (SYN-IR) are limited to the middle CN region, coincident with the dense CO blotches; (2) neurons immunoreactive for the calcium-binding proteins calbindin-D28 (CB), calretinin (CR) and parvalbumin (PV) are concentrated in the middle CN region. Furthermore, in adult rats subjected to perinatal forepaw removal, (1) the patterns of SYN-IR in the middle region of the CN are disrupted, as had previously been shown for the patterns of CO blotches; (2) in contrast, however, distributions of CN cells with PV-, CB- and CR-IR are unaffected. Evidence for a tripartite division in the GN is also presented, based on the distributions of cells with PV-, CB- and CR-IR.