Comprehensive immunofluorescence and lectin binding analysis of intervibrissal fur innervation in the mystacial pad of the rat

Innervation of the digit on the forepaw of the raccoon

The innervation of the digits on the raccoon forepaw was examined by using immunochemistry for protein gene product 9.5, calcitonin-gene related peptide, substance P, neuropeptide-Y, tyrosine hydroxylase, and neurofilament protein. The larger-caliber axons in the ventral glabrous skin terminate as Pacinian corpuscles deep in the dermis, small corpuscles and Merkel endings around the base of dermal papillae, and Merkel endings on rete pegs in dermal papillae. Extensive fine-caliber innervation terminates in the epidermis and on the microvasculature. The innervation is more dense in the distal than in the proximal volar pads. Pacinian endings are also concentrated in the transverse crease separating the distal and proximal pads. In the dorsal hairy skin, hair follicles are well innervated with piloneural complexes. Merkel innervation is located under slight epidermal elevations and in some large Merkel rete pegs located at the apex of transverse skin folds just proximal to the claw. No cutaneous Ruffini corpuscles were found anywhere on the digit. The claw is affiliated with dense medial and lateral beds of Pacinian endings, bouquets of highly branched Ruffini-like endings at the transition from the distal phalanx and unmyelinated innervation in the skin around the perimeter. Encapsulated endings are located at the lateral edge of the articular surface of the distal phalanx. Extensive fine-caliber innervation is affiliated with sweat glands and with the vasculature and is especially dense at presumptive arteriovenous sphincters. Virtually all of the sweat gland and vascular innervation is peptidergic, whereas most of the unmyelinated epidermal innervation is nonpeptidergic.

Directional specificity and patterning of sensory axons in trigeminal ganglion-whisker pad cocultures

In the rodent trigeminal pathway, trigeminal axons invade the developing whisker pad from a caudal to rostral direction. We investigated directional specificity of embryonic day (E) 15 rat trigeminal axons within this peripheral target field using explant cocultures. E15 trigeminal axons readily grow into the same age whisker pad explants and form follicle-related patterns along a caudal to rostral direction. They also can grow into this target from its lateral aspects. In contrast, they are unable to invade the whisker pad from the rostral (nasal) pole. We did not find any correlation between the distribution of extracellular matrix molecules and trigeminal axon growth preferences. We also examined age-related changes in trigeminal axon responsiveness to directional cues. E19 trigeminal axons readily grew into E15 whisker pad explants from either the caudal or the rostral pole. These results suggest the presence of growth permissive and repulsive cues that guide sensory axons in the whisker pad. Furthermore, trigeminal axons lose their responsiveness to growth inhibitory cues at later stages of development.

Hair cycle-dependent changes in adrenergic skin innervation, and hair growth modulation by adrenergic drugs

Skin nerves may exert "trophic" functions during hair follicle development, growth, and/or cycling. Here, we demonstrate hair cycle-related plasticity in the sympathetic innervation of skin and hair follicle in C57BL/6 mice. Compared with telogen skin, the number of nerve fibers containing norepinephrine or immunoreactive for tyrosine hydroxylase increased during the early growth phase of the hair cycle (anagen) in dermis and subcutis. The number of these fibers declined again during late anagen. beta2-adrenoreceptor-positive keratinocytes were transiently detectable in the noncycling hair follicle epithelium, especially in the isthmus and bulge region, but only during early anagen. In early anagen skin organ culture, the beta2-adrenoreceptor agonist isoproterenol promoted hair cycle progression from anagen III to anagen IV. The observed hair cycle-dependent changes in adrenergic skin innervation on the one hand, and hair growth modulation by isoproterenol, accompanied by changes in beta2-adrenoreceptor expression of selected regions of the hair follicle epithelium on the other, further support the concept that bi-directional interactions between the hair follicle and its innervation play a part in hair growth control. This invites one to systematically explore the neuropharmacologic manipulation of follicular neuroepithelial interactions as a novel therapeutic strategy for managing hair growth disorders.

A rapid and dynamic regulation of GDNF-family ligands and receptors correlate with the developmental dependency of cutaneous sensory innervation

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) are members of the transforming growth factor-beta family and have been shown to elicit neurotrophic effects upon several classes of neurons including dopaminergic neurons, motoneurons, parasympathetic, sympathetic as well as primary sensory neurons. However, there is little information available on their roles in cutaneous innervation. Herein, we have studied the regulation of gdnf, ntn and the GDNF family receptors and examined their role in the development of facial cutaneous innervation in GDNF mutant mice. A dynamic spatial and temporal regulation of gdnf, ntn and their ligand binding receptors within the follicle-sinus complex correlate with development of distinct subclasses of sensory nerve endings. Furthermore, development of NGF-dependent myelinated mechanoreceptors, i.e. reticular and transverse lanceolate endings also require GDNF during ending formation and maintenance. In addition, ligand and receptor association seems to be intricately linked to a local Schwann cell-axon interaction essential for sensory terminal formation. Our results suggests that functionally specified nerve endings depend on different GDNF family members and that in contrast to neurotrophins, this family of neurotrophic factors may be acting at local sites of terminal Schwann cell-axon growth cone interactions and that they collaborate with neurotrophins by supporting the same populations of neurons but at different times in development.

BDNF overexpression induces differential increases among subsets of sympathetic innervation in murine back skin

Besides their recognized dependence on nerve growth factor (NGF) during development, the dependence of mature sympathetic ganglion neurons on other neurotrophins is still unclear. Here, we have investigated the sympathetic innervation of back skin in mice overexpressing brain-derived neurotrophic factor (BDNF) under the alpha-myosin heavy-chain promoter, as well as in BDNF knockout (-/-) mice. Compared with wild-type controls, the dorsal skin of BDNF overexpressing mice displayed a significantly enhanced number of adrenergic, tyrosine hydroxylase-immunoreactive (IR) nerve fibres, while cholinergic or peptidergic sensory nerve fibres appeared unaltered. The adrenergic hyperinnervation in dorsal skin of BDNF overexpressing mice was most pronounced in the arrector pili muscle of hair follicles, while no increase of tyrosine hydroxylase-or neuropeptide Y-IR fibres associated with subcutaneous blood vessels was found. Instead, back skin of BDNF knockout (-/-) mice contained significantly fewer tyrosine hydroxylase-IR dermal nerve fibres than wild-type animals. This suggests that BDNF plays an important role in the control of different subsets of adrenergic innervation in murine back skin, and indicates that paravertebral sympathetic ganglia display a previously unrecognized differential BDNF-dependence in vivo.

Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons

Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

Differential dependency of cutaneous mechanoreceptors on neurotrophins, trk receptors, and P75 LNGFR

The impact of null mutations of the genes for the NGF family of neurotrophins and their receptors was examined among the wide variety of medium to large caliber myelinated mechanoreceptors which have a highly specific predictable organization in the mystacial pad of mice. Immunofluorescence with anti-protein gene product 9.5, anti-200-kDa neurofilament protein (RT97), and anti-calcitonin gene-related product was used to label innervation in mystacial pads from mice with homozygous null mutations for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), the three tyrosine kinase receptors (trkA, trkB, trkC), and the low-affinity nerve growth factor receptor p75. Specimens were sacrificed at birth and at 1, 2, and 4 weeks for each type of mutation as well as at 11 weeks and 1 year for p75 and trkC mutations, respectively. Our results demonstrate several major concepts about the role of neurotrophins in the development of cutaneous mechanoreceptors that are supplied by medium to large caliber myelinated afferents. First, each of the high-affinity tyrosine kinase receptors, trkA, trkB, and trkC, as well as the low-affinity p75 receptor has an impact on at least one type of mechanoreceptor. Second, consistent with the various affinities for particular trk receptors, the elimination of NGF, BDNF, and NT-3 has an impact comparable to or more complex than the absence of their most specific high-affinity receptors: trkA, trkB, and trkC, respectively. These complexities include potential NT-3 signaling through trkA and trkB to support some neuronal survival. Third, most types of afferents are dependent on a different combination of neurotrophins and receptors for their survival: reticular and transverse lanceolate afferents are dependent upon NT-3, NGF, and trkA; Ruffini afferents upon BDNF and trkB; longitudinal lanceolate afferents upon NGF, trkA, BDNF, and trkB; and Merkel afferents on NGF, trkA, NT-3, trkC, and p75. NT-4 has no obvious detrimental impact on the mechanoreceptor development in the presence of BDNF. Fourth, NT-4 and BDNF signaling through trkB may suppress Merkel innervation and NT-3 signaling through trkC may suppress Ruffini innervation. Finally, regardless of the neurotrophin/receptor dependency for afferent survival and neurite outgrowth, NT-3 has an impact on the formation of all the sensory endings. In the context of these findings, indications of competitive and suppressive interactions that appear to regulate the balance of innervation density among the various sets of innervation were evident.