Regeneration of periodontal primary afferents of the rat incisor following injury of the inferior alveolar nerve with special reference to neuropeptide Y-like immunoreactive primary afferents

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

Rapamycin Accelerates Axon Regeneration Through Schwann Cell-mediated Autophagy Following Inferior Alveolar Nerve Transection in Rats

Sensory disturbance in the orofacial region owing to trigeminal nerve injury is caused by dental treatment or accident. Commercially available therapeutics are ineffective for the treatment of sensory disturbance. Additionally, the therapeutic effects of rapamycin, an allosteric inhibitor of mammalian target of rapamycin (mTOR), which negatively regulates autophagy, on the sensory disturbance are not fully investigated. Thus, we investigated the therapeutic effects of rapamycin on the sensory disturbance in the mandibular region caused by inferior alveolar nerve (IAN) transection (IANX) in rats. The expression levels of the phosphorylated p70S6K, a downstream molecule of mTOR, in the proximal and distal stumps of the transected IAN were significantly reduced by rapamycin administration to the injured site. Conversely, the increments of both Beclin 1 and microtubule-associated protein-1 light chain 3-II protein levels in the proximal and distal stumps of the transected IAN was induced by rapamycin administration. Immunohistochemical analyses revealed that Beclin 1 was located in Schwann cells in the proximal stump of the IAN. Accumulation of myelin protein zero and myelin basic protein in the proximal and distal stumps of the IAN was significantly reduced by rapamycin administration. Rapamycin administration facilitated axon regeneration after IANX and increased the number of brain-derived neurotrophic factor positive neurons in the trigeminal ganglion. Thus, recovery from sensory disturbance in the lower lip caused by IANX was markedly facilitated by rapamycin. These findings suggest that rapamycin administration is a promising treatment for the sensory disturbance caused by IANX.

Modified technique for the exposure of the inferior alveolar nerve in rats

The inferior alveolar nerve (IAN), a branch of the mandibular division of the trigeminal nerve, is a major component of the neurovascular bundle along with the inferior alveolar artery and vein. In rats, when exposed using an external oral approach while remaining intact, it can serve as an important tool to study the different effects of neuromediators and assess the role of different groups of nerve fibers. This paper describes a new technique to expose this nerve giving some experimental results to support its usefulness.

Modulation of Trigeminal Spinal Subnucleus Caudalis Neuronal Activity Following Regeneration of Transected Inferior Alveolar Nerve in Rats

Modulation of trigeminal spinal subnucleus caudalis neuronal activity following regeneration of transected inferior alveolar nerve in rats. To clarify the neuronal mechanisms of abnormal pain in the face innervated by the regenerated inferior alveolar nerve (IAN), nocifensive behavior, trigeminal ganglion neuronal labeling following Fluorogold (FG) injection into the mental skin, and trigeminal spinal subnucleus caudalis (Vc) neuronal properties were examined in rats with IAN transection. The mechanical escape threshold was significantly higher at 3 days and lower at 14 days after IAN transection, whereas head withdrawal latency to heat was significantly longer at 3, 14, and 60 days after IAN transection. The number of FG-labeled ganglion neurons was significantly reduced at 3 days after IAN transection but increased at 14 and 60 days. The number of wide dynamic range (WDR) neurons with background (BG) activity was significantly higher at 14 and 60 days after IAN transection compared with naïve rats, and the number of WDR and low-threshold mechanoreceptive (LTM) neurons with irregularly bursting BG activity was increased at these two time points. Mechanically evoked responses were significantly larger in WDR and LTM neurons 14 days after IAN transection compared with naïve rats. Heat- and cold-evoked responses in WDR neurons were significantly lower at 14 days after transection compared with naïve rats. Mechanoreceptive fields were also significantly larger in WDR and LTM neurons at 14 and 60 days after IAN transection. These findings suggest that these alterations may be involved in the development of mechanical allodynia in the cutaneous region innervated by the regenerated IAN.

Rapid hit to lead evaluation of pyrazolo[3,4-d]pyrimidin-4-one as selective and orally bioavailable mGluR1 antagonists

Our HTS effort yielded a preferential mGluR1 pyrimidinone antagonist 1 with lead-like characteristics. Rapid hit to lead (HTL) study identified compounds with improved functional activity and selectivity such as 1b with little improvements in ADME properties. Addition of an aminosulfonyl group on the N-1 aromatic ring led to 2f, a compound with similar in vitro biochemical profiles as those of 1b but drastically improved in vitro ADME properties. These improvements were paralleled by rat PK study characterized by low clearance and quantitative bioavailability. Compound 2f represented a true lead-like molecule that is amenable for further lead optimization (LO) evaluation.

Glutamate and metabotropic glutamate receptors associated with innervation of the uterine cervix during pregnancy: Receptor antagonism inhibits c-fos expression in rat lumbosacral spinal cord at parturition

Dorsal root ganglia (DRG) neurons connect the spinal cord and uterine cervix, and are activated at parturition with subsequent stimulation of secondary neurons in the spinal dorsal horn and autonomic areas. Neuropeptide neurotransmitters and receptors have been studied in these areas, but amino acid transmitters, e.g., glutamate, an excitatory neurotransmitter involved in sensory and nociceptive processing, have not been characterized. To determine if glutamate is involved in innervation of the cervix, rats were examined for markers of glutamatergic neurons in the L6-S1 spinal cord, DRG and cervix. Metabotropic glutamate receptors mGluR5 in the spinal dorsal horn and their expression over pregnancy were examined in pregnant rats and pregnant rats treated continuously with an antagonist of mGluR5, 2-methyl-6-(phenylethynyl) pyridine (MPEP). Rats were allowed to deliver pups to determine if the antagonist altered the expression of an early response gene protein, Fos, in the L6-S1 cord. Immunohistochemistry showed glutamate- and vesicular glutamate transporter1 (VGluT1)-positive fibers in the cervix, glutamate- and VGluT1-expressing neurons in the DRG, some of which also exhibited retrograde tracer from cervical injections, and VGluT1 and mGluR5 immunoreactivities in the L6-S1 spinal dorsal horns. Expression of mGluR5 receptors increased over pregnancy. Fos-positive neurons were present among mGluR5-immunoreactivity in the spinal dorsal horn. Parturition-induced Fos-positive neurons in the spinal cords were abundant in control rats, but were reduced by 70% in MPEP-treated animals. These results suggest that glutamate is likely involved in the transmission of sensory signals, possibly pain, from the cervix to the spinal cord at parturition.

Perivascular nerve damage in the cerebral circulation following traumatic brain injury

Traumatic brain injury (TBI) causes cerebral vascular dysfunction. Most have assumed that it was the result of endothelial and/or smooth muscle alteration. No consideration, however, has been given to the possibility that the forces of injury may also damage the perivascular nerve network, thereby contributing to the observed abnormalities. To test this premise, we subjected rats to impact acceleration. At 6 h, 24 h and 7 days post-TBI, cerebral basal arteries were removed and processed with antibody targeting protein gene product 9.5 (PGP-9.5), with parallel assessments of 5-hydroxytryptamine (5-HT) accumulation in the perivascular nerves. Additionally, Fluoro-Jade was also used as a marker of axonal degeneration. The perivascular nerve network revealed no abnormality in sham animals. However, by 6 h post injury, Fluoro-Jade reactivity appeared in the perivascular regions, with the number of fibers increasing with time. By 24 h post injury, a significant reduction in the perivascular 5-HT accumulation occurred, together with a reduction in PGP-9.5 fiber staining. At 7 days, a recovery of the PGP-9.5 immunoreactivity occurred, however, it did not reach a control-like distribution. These studies suggest that neurogenic damage occurs following TBI and may be a contributor to some of the associated vascular abnormalities.

Regeneration of periodontal Ruffini endings of rat lower incisors following nerve cross-anastomosis with mental nerve

The present study utilized protein gene product 9.5 (PGP 9.5) and S-100 protein immunohistochemistry to examine if Ruffini endings, the primary mechanoreceptors in periodontal ligaments, can regenerate following nerve cross-anastomosis with an inappropriate nerve. Normally, axon terminals of periodontal Ruffini endings are extensively ramified, and terminal Schwann cells, identified by their S-100 immunoreactivity, are associated with axon terminals. Schwann cells are restricted to the alveolus-related part (ARP), but not tooth-related part (TRP) or the shear zone at the border between the ARP and the TRP of the lingual periodontal ligament of the lower incisor. When the central portion of the mental nerve (MN) was connected with the peripheral portion of the inferior alveolar nerve (IAN), regenerating MN fibers invaded the IAN around postoperative day 5 (PO 5). During the postoperative period, numerous S-100-immunoreactive (IR) cells, presumably terminal Schwann cells, began to migrate to the shear zone and the TRP. PGP 9.5-IR elements reappeared at PO 7 and gradually increased in number. Around PO 28, the terminal portion of the regenerating Ruffini endings appeared dendritic, but less expanded, and the rearrangement of terminal Schwann cells was noted. Regenerated periodontal Ruffini endings were slightly smaller in number. The number of trigeminal ganglion neurons sending peripheral processes beyond the site of injury was smaller compared to those of normal MN, but their cross-sectional areas were almost comparable. Expressions of calbindin D28k and calretinin, normally localized in axonal elements in Ruffini endings, were first detected around PO 56. The present results show that parts of periodontal Ruffini endings can regenerate following nerve cross-anastomosis with mental nerve.

The involvement of brain-derived neurotrophic factor (BDNF) in the regeneration of periodontal Ruffini endings following transection of the inferior alveolar nerve

The present study employed immunohistochemistry for protein gene product 9.5 (PGP 9.5) to examine the regeneration process of Ruffini endings, the primary mechanoreceptor in the periodontal ligament, in heterozygous mice with targeted disruption of the brain-derived neurotrophic factor (BDNF) gene and their littermates, following transection of the inferior alveolar nerve. When immunostained for PGP 9.5, periodontal Ruffini endings appeared densely distributed in the periodontal ligament of the heterozygous mice, but the density of the positively stained nerve fibers in the ligament was 20% lower than that in the control littermates. At 3 days after surgery, the PGP 9.5-positive neural elements had disappeared; they began to appear in the periodontal ligament of both animals at 7 days. However, the recovery pattern of the PGP 9.5-positive nerves differed between heterozygous and wild type mice, typical periodontal Ruffini endings morphologically identical to those in the control group appeared in the wild-type mice at 7 days, whereas such Ruffini endings were detectable in the heterozygous mice at 28 days, though much smaller in number. On day 28, when PGP 9.5-positive nerves were largely regenerated in wild type mice, their distribution was much less dense in the ligament of the heterozygous mice than in the non-treated heterozygous mice. The density of PGP 9.5-positive nerve fibers was significantly lower in the heterozygous mice than in wild type mice at any stage examined. These data showing that a reduced expression of BDNF causes delayed regeneration of the periodontal Ruffini endings suggest the involvement of BDNF in the regeneration process of these mechanoreceptors.

Ultrastuctural immunocytochemistry of calcium-binding proteins in rapidly-adapting avian mechanoreceptors

The localization of the calcium-binding proteins (CaBPs) calbindin-D28K (CB) and parvalbumin (PV) in avian rapidly-adapting Herbst and Grandry sensory corpuscles was studied with the use of immunocytochemistry and monoclonal antibodies. Strongest immunostaining was detected in cells of the capsule in both receptor types. Staining was more pronounced in the vicinity of endoplasmic reticulum and mitochondria in the perinuclear regions, whereas staining was distinct in pinocytotic vesicles in peripheral cytoplasmic lamellae. Fibroblasts and macrophages in the subcapsular space of Herbst receptors also showed strong immunostaining in organelles in perinuclear regions. Modified Schwann cells in both receptor types revealed moderately-expressed immunostaining, which was more pronounced in perinuclear regions. The various parts of the receptor nerve fibers showed weak to strong staining. The physiological roles of the investigated CaBPs may be associated with cytoplasmic calcium ion (Ca++) storage, which is necessary for either active metabolism in the immunostained structures and/or their transfer to sensory axonal regions where Ca++ channels are present.

Regeneration of periodontal Ruffini endings in adults and neonates

We reviewed the regeneration of periodontal Ruffini endings, primary mechanoreceptors in the periodontal ligament, following injury to the inferior alveolar nerve (IAN) in adult and neonatal rats. Morphologically, mature Ruffini endings are characterized by an extensive arborization of axonal terminals and association with specialized Schwann cells, called lamellar or terminal Schwann cells. Following injury to IAN in the adult, the periodontal Ruffini endings of the rat lower incisor ligament regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells migrate into regions where they are never found under normal conditions. The development of periodontal Ruffini endings of the rat incisor is closely associated with the eruption of the teeth; the morphology and distribution of the terminal Schwann cells became almost identical to those in adults during postnatal days 15-18 (PN 15-18d) when the first molars appear in the oral cavity, while the axonal elements showed extensive ramification around PN 28d when the functional occlusion commences. When the IAN was injured in neonates, the regeneration of periodontal Ruffini endings was delayed compared with the adults. The migration of terminal Schwann cells is also observed following IAN injury, after which the distribution of terminal Schwann cells became almost identical to that of the adults, i.e., PN 14d. Since the interaction between axon and Schwann cell is important during regeneration and development, further studies are required to elucidate its molecular mechanism during the regeneration as well as the development of the periodontal Ruffini endings.

The Role of Neurotransmitters in Neurite Outgrowth and Synapse Formation

Besides a well-established role in neuronal communication in the adult central nervous system, neurotransmitters have diverse tasks in the embryonic brain, ranging from early developmental functions in morphogenesis /13/, to later functions in target selection and synapse formation /87/. For example, growth cones of developing neurons are known to release transmitters /26,36,88,110,115/ and respond to transmitters released from other neurons /35,44,59, 61,70/. Moreover, depletion of transmitters during embryonic development results in developmental deficits of the brain /21,48,84,109/, suggesting that transmitters have crucial roles as morphogens and/or neurotrophic factors. Although recently the idea of neurotransmitters being important for neural development has been challenged /99/, there is a vast amount of literature that seems to support the hypothesis that neurotransmitter release in the developing central nervous system is crucial for proper brain development. In this review we focus on the roles that neurotransmitters play in neurite outgrowth, target selection and synapse formation, with particular emphasis on the effects of the transmitters serotonin and dopamine.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Peripheral group I metabotropic glutamate receptors modulate nociception in mice

The metabotropic glutamate receptors (mGluRs) are found throughout the central nervous system, where they modulate neuronal excitability and synaptic transmission. Here we report the presence of phospholipase C-coupled group I mGluRs (mGluR1 and mGluR5) outside the central nervous system on peripheral unmyelinated sensory afferents. Given their localization on predominantly nociceptive afferents, we investigated whether these receptors modulate nociceptive signaling, and found that agonist-induced activation of peripheral group I mGluRs leads to increased sensitivity to noxious heat, a phenomenon termed thermal hyperalgesia. Furthermore, group I mGluR antagonists not only prevent, but also attenuate established formalin-induced pain. Taken together, these results suggest that peripheral mGluRs mediate a component of hyperalgesia and may be therapeutically targeted to prevent and treat inflammatory pain.

Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Effects of neonatal injury of the inferior alveolar nerve on the development and regeneration of periodontal nerve fibers in the rat incisor

Our previous study showed that the migration of terminal Schwann cells occurred in the periodontal ligament of the rat lower incisor following transection of the inferior alveolar nerve (IAN) in the adult animals [Y. Atsumi, K. Matsumoto, M. Sakuda, T. Maeda, K. Kurisu, S. Wakisaka, Altered distribution of Schwann cells in the periodontal ligament of the rat incisor following resection of the inferior alveolar nerve: An immunohistochemical study on S-100 proteins, Brain Res. 849 (1999) 187-195]. The aim of the present study was to investigate the effects of neonatal transection of the IAN on the regeneration of axon elements and Schwann cells in the periodontal ligament of the rat lower incisor. Following transection of IAN at post-natal day 5 (PN 5d), when the numbers of both axon elements and the terminal Schwann cells were very small, regenerating nerve fibers appeared between post-injured days 7 (PO 7d) and PO 14d, and increased in number thereafter gradually. Although the terminal morphologies of regenerated Ruffini endings became identical to those of the adult animals by PO 54d, the number of regenerated PGP 9.5-IR nerve fibers did not recover the adult levels even by PO 56d. A small number of Schwann cells migrated into the shear zone, the border between the alveolus-related part (ARP) and the tooth-related part (TRP), but did not enter into the TRP. Following transection of the IAN at PN 14d or PN 28d, when clusters of apparent terminal Schwann cells could be recognized, axon regeneration started around PO 5d. Individual axon terminals of the regenerating Ruffini endings ramified and became identical to those of the adult animals around PO 28d, but the number of regenerated Ruffini endings was smaller than that of the adult animals. Similar to the adult animals, the migration of Schwann cells into the shear zone and TRP occurred, and disappeared prior to the completion of the axonal regeneration. The present results indicate that the migration of the Schwann cells into TRP during the regeneration of the periodontal nerve fibers following nerve injury to the IAN depends on the maturation of the terminal Schwann cells of the periodontal Ruffini endings, not on post-operative time.

Morphological and cytochemical characteristics of periodontal Ruffini ending under normal and regeneration processes

Current knowledge on the Ruffini endings, primary mechanoreceptors in the periodontal ligament is reviewed with special reference to their cytochemical features and regeneration process. Morphologically, they are characterized by extensive ramifications of expanded axonal terminals and an association with specialized Schwann cells, called lamellar or terminal Schwann cells, which are categorized, based on their histochemical properties, as non-myelin-forming Schwann cells. Following nerve injury, the periodontal Ruffini endings of the rat incisor ligament can regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells associated with the periodontal Ruffini endings migrate into regions where they are never found under normal conditions. Also during regeneration, alterations in the expression level of various bioactive substances occur in both axonal and Schwann cell elements in the periodontal Ruffini endings. Neuropeptide Y, which is not detected in intact periodontal Ruffini endings, is transiently expressed in their regenerating axons. Growth-associated protein-43 (GAP-43) is expressed transiently in both axonal and Schwann cell elements during regeneration, while this protein is localized in the Schwann sheath of periodontal Ruffini endings under normal conditions. The expression of calbindin D28k and calretinin, both belonging to the buffering type of calcium-binding proteins, was delayed in periodontal Ruffini endings, compared to their morphological regeneration. As the importance of axon-Schwann cell interactions has been proposed, further investigations are needed to elucidate their molecular mechanism particularly the contribution of growth factors during the regeneration as well as development of the periodontal Ruffini endings.

The Ruffini ending as the primary mechanoreceptor in the periodontal ligament: its morphology, cytochemical features, regeneration, and development

The periodontal ligament receives a rich sensory nerve supply and contains many nociceptors and mechanoreceptors. Although its various kinds of mechanoreceptors have been reported in the past, only recently have studies revealed that the Ruffini endings--categorized as low-threshold, slowly adapting, type II mechanoreceptors--are the primary mechanoreceptors in the periodontal ligament. The periodontal Ruffini endings display dendritic ramifications with expanded terminal buttons and, furthermore, are ultrastructurally characterized by expanded axon terminals filled with many mitochondria and by an association with terminal or lamellar Schwann cells. The axon terminals of the periodontal Ruffini endings have finger-like projections called axonal spines or microspikes, which extend into the surrounding tissue to detect the deformation of collagen fibers. The functional basis of the periodontal Ruffini endings has been analyzed by histochemical techniques. Histochemically, the axon terminals are reactive for cytochrome oxidase activity, and the terminal Schwann cells have both non-specific cholinesterase and acid phosphatase activity. On the other hand, many investigations have suggested that the Ruffini endings have a high potential for neuroplasticity. For example, immunoreactivity for p75-NGFR (low-affinity nerve growth factor receptor) and GAP-43 (growth-associated protein-43), both of which play important roles in nerve regeneration/development processes, have been reported in the periodontal Ruffini endings, even in adult animals (though these proteins are usually repressed or down-regulated in mature neurons). Furthermore, in experimental studies on nerve injury to the inferior alveolar nerve, the degeneration of Ruffini endings takes place immediately after nerve injury, with regeneration beginning from 3 to 5 days later, and the distribution and terminal morphology returning to almost normal at around 14 days. During regeneration, some regenerating Ruffini endings expressed neuropeptide Y, which is rarely observed in normal animals. On the other hand, the periodontal Ruffini endings show stage-specific configurations which are closely related to tooth eruption and the addition of occlusal forces to the tooth during postnatal development, suggesting that mechanical stimuli due to tooth eruption and occlusion are a prerequisite for the differentiation and maturation of the periodontal Ruffini endings. Further investigations are needed to clarify the involvement of growth factors in the molecular mechanisms of the development and regeneration processes of the Ruffini endings.

Effects of different types of injury to the inferior alveolar nerve on the behavior of Schwann cells during the regeneration of periodontal nerve fibers of rat incisor

The present study reports on different regeneration patterns of axons and Schwann cells in the periodontal ligament of the rat incisor using immunohistochemistry of protein gene product 9.5 (PGP 9.5) and S-100 protein. Three kinds of injury (transection, crush and segmental resection) were applied to the inferior alveolar nerve. In normal animals, PGP 9.5- and S-100-immunoreactivities were detected in the axons and Schwann cell elements of periodontal Ruffini endings, respectively. They were restricted to the alveolus-related part, occurring only rarely in the tooth-related part and in the shear zone (the border between the alveolus-related and tooth-related parts). Both transection and segmental resection caused the complete disappearance of PGP 9.5-immunoreactive nerve fibers in the periodontal ligament, while a small number of them could be found following the crush injury. Regenerating PGP 9.5-reactive nerve fibers appeared at 5 days and 21 days following the transection and segmental resection, respectively. The regeneration of periodontal nerve fibers completed in a period of 21-28 days and 14-21 days following the transection and crush, respectively, but was not completed even at 56 days following the segmental resection. The behavior of Schwann cells during regeneration was similar after the different nerve injuries; spindle-shaped S-100-immunoreactive cells, presumably Schwann cells, appeared in the shear zone and the tooth-related part. These cells disappeared 5-7 days prior to the completion of the regeneration of axonal elements of the periodontal ligament following the transection and crush. Following the segmental resection, in contrast, spindle-shaped S-100-positive cells disappeared from the tooth-related part at 42 days, although the axonal regeneration of periodontal Ruffini endings proceeded even until 56 days. We thus conclude that the duration of the migration of Schwann cells depends on the state of the regeneration of axons.

Altered distribution of Schwann cells in the periodontal ligament of the rat incisor following resection of the inferior alveolar nerve: an immunohistochemical study on S-100 proteins

The present study employed immunohistochemistry for the detection of S-100 proteins to reveal the alteration in the distribution of Schwann cells in the periodontal ligament of the rat incisor following resection of the inferior alveolar nerve (IAN). In normal animals, S-100-immunostaining demonstrated the profiles of Ruffini endings, primary mechanoreceptors in the periodontal ligament, in the alveolus-related part of the ligament. Under the electron microscope, S-100-like immunoreactivity (-LI) was observed in the cytoplasm of the terminal Schwann cell elements and in some axon profiles of the Ruffini endings. During the regeneration, S-100-like immunoreactive (-IR) terminal Schwann cells in the alveolus-related part of the ligament gradually decreased in number. In contrast, S-100-LI was found in the spindle-shaped cells at the shear zone (the border between alveolus-related and tooth-related parts) and in the tooth-related part, where S-100-LI was rarely detected in normal animals. Immunoelectron microscopic observations revealed that some S-100-IR spindle-shaped cells contained fibrous long spacing (FLS) fibers, suggesting that they were Schwann cells. Some regenerating axons were observed at the shear zone, but were rarely found in the tooth-related part. With the progress of the regeneration of the periodontal Ruffini endings, S-100-IR terminal Schwann cells became rearranged in the alveolus-related part by 42-56 days post injury, whereas the S-100-IR spindle-shaped Schwann cells in the shear zone and tooth-related part disappeared when the regeneration was complete.

Course and composition of the nerves that supply the mandibular teeth of the rat

The rodent dentition has become an important model for investigations of interactions between dental tissues and peripheral neurons. Although experimental nerve injury has been widely used for such studies, there is uncertainty about the courses of nerve fibers supplying the mandibular teeth. In order to clarify this, we used a mixture of monoclonal antibodies against neurofilament proteins to enhance demonstration of nerve fibers so that small nerves could be readily traced in serial frozen sections of mandibles of Sprague Dawley rats ranging in age from embryonic day (E) 18 to postnatal day (P) 90. The 1st molar and anterior portion of the 2nd molar were innervated by small nerves that emerged as distinct branches of the IAN trunk at or near the mandibular foramen. In contrast, the nerve supply to the 3rd molar and posterior part of the 2nd molar was a branch of the lingual nerve that bypassed the mandibular canal altogether. The IAN trunk split into the mental nerve and a large branch to the incisor about 2 mm anterior to the mandibular foramen. Thick branches of the incisor nerve descended into the incisor socket to form a dense plexus of nerve fiber bundles extending along the length of the incisor periodontium. The sparse pulpal innervation of the incisor was provided by a few thin fascicles that emerged from the caudal portion of the periodontal plexus to enter the incisor apex. The dental branches of the IAN and lingual nerve seen in the adult were well established and readily identifiable at age E18 even though their targets were limited to the follicles of the developing teeth. These studies show that the trigeminal branches that supply the mandibular teeth can be identified at a wide range of ages as distinct nerves at a considerable distance proximal to their targets. This detailed information on the courses taken by the dental nerves can provide an anatomical basis for increased precision in characterization and perturbation of neural pathways from the molars and incisor.

Postnatal expression of calretinin-immunoreactivity in periodontal Ruffini endings in the rat incisor: a comparison with protein gene product 9.5 (PGP 9.5)-immunoreactivity

The postnatal expression of immunoreactivity for calretinin, one of the calcium binding proteins, and for protein gene product 9.5 (PGP 9.5), a general neuronal marker, was investigated in mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisor. Age-related changes in the expression of these two proteins in periodontal nerves were further quantified with a computerized image analysis. At 1 day after birth, a few PGP 9.5-immunoreactive nerve fibers and a still smaller number of calretinin-positive fibers were found in the periodontal ligament: they were thin and beaded in appearance and no specialized nerve terminals were recognized. Tree-like terminals, reminiscent of immature Ruffini endings, were recognizable in 4-day-old rats by PGP 9.5-immunohistochemistry, while calretinin-immunostaining failed to reveal these specialized endings. At postnatal 7-11 days when PGP 9.5-immunostaining could demonstrate typical Ruffini endings, calretinin-immunopositive nerve fibers merely tapered off without forming the Ruffini type endings. A small number of Ruffini endings showing calretinin-immunoreactivity began to occur in the periodontal ligament at 24-26 days after birth when the occlusion of the first molars had been established. At the functional occlusion stage (60-80 days after birth), the Ruffini endings showing calretinin-immunoreactivity drastically increased in number and density, but less so than those positive for PGP 9.5-immunoreaction. The delayed expression of calretinin suggests that the function of the periodontal Ruffini endings is established after the completion of terminal formation because Ca2+, which binds to calcium binding proteins including calretinin with high affinity, plays an important role in mechano-electric transduction.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Calretinin-like immunoreactivity in the regenerating periodontal ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Regeneration of calretinin (CR)-like immunoreactive (IR) nerve fibers was investigated in the periodontal ligament of the rat lower incisor following resection of the inferior alveolar nerve (IAN). In addition, the degeneration and regeneration processes of periodontal nerve fibers were examined by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker. In normal animals, the periodontal nerve fibers showing PGP 9.5-like immunoreactivity (LI) formed either periodontal Ruffini endings with expanded arborization and thin free nerve endings in the alveolar half of the ligament. Thick CR-IR nerve fibers also appeared in a dendritic fashion in the same region, but thin CR-IR nerve fibers were rarely observed. Five days following resection of the IAN, a major population of PGP 9.5-IR and all CR-IR nerve fibers disappeared except for some thin PGP 9.5-IR nerves in the periodontal ligament. Regenerated PGP 9.5-IR nerve fibers appeared around 7 days following resection, in contrast to a very small number of regenerated CR-IR nerve fibers. Around 14-21 days following resection, the number and terminal morphology of regenerated PGP 9.5-IR nerve fibers were comparable to those observed in normal animals, but the number of regenerated CR-IR nerve fibers was still smaller than that of normal animals. The number of regenerated CR-IR nerve fibers increased to return to normal by 56 days following injury. The delay of expression of CR-LI in the regenerated periodontal Ruffini endings suggests that functional recovery of periodontal Ruffini endings occurred after the completion of the regeneration of periodontal nerve fibers.

Growth-associated protein-43 (GAP-43) in the regenerating periodontal Ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Alterations in the levels of growth-associated protein 43 (GAP-43)-like immunoreactivity (-LI) were examined in the lingual periodontal ligament of the rat incisor following two types of injury (resection and crush) to the inferior alveolar nerve (IAN). In normal animals, GAP-43-like immunoreactive (IR) structures were observed as tree-like ramifications in the alveolar half of the lingual periodontal ligament of incisors. Under immunoelectron microscopy, GAP-43-LI appeared in the Schwann sheaths associated with periodontal Ruffini endings; neither cell bodies of the terminal Schwann cells nor axonal profiles showed GAP-43-LI. During regeneration of the periodontal Ruffini endings following resection of the IAN, GAP-43-LI appeared in the cytoplasm of the terminal Schwann cell bodies and axoplasm of the terminals. The distribution of GAP-43-LI in the Ruffini endings returned to almost normal levels on days 28 and 56 following the injury. The changes in the distribution of GAP-43-LI following the crush injury were similar to those following resection; however, expression of GAP-43-LI was slightly higher for the entire experimental period compared with the resection. The transient expression of GAP-43 in the terminal Schwann cells and axonal profiles of the periodontal Ruffini endings following nerve injury suggests that GAP-43 is closely associated with axon-Schwann cells interactions during regeneration.