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

Postnatal Maturation of Glutamatergic Inputs onto Rat Jaw-closing and Jaw-opening Motoneurons

Motoneurons that innervate the jaw-closing and jaw-opening muscles play a critical role in oro-facial behaviors, including mastication, suckling, and swallowing. These motoneurons can alter their physiological properties through the postnatal period during which feeding behavior shifts from suckling to mastication; however, the functional synaptic properties of developmental changes in these neurons remain unknown. Thus, we explored the postnatal changes in glutamatergic synaptic transmission onto the motoneurons that innervate the jaw-closing and jaw-opening musculatures during early postnatal development in rats. We measured miniature excitatory postsynaptic currents (mEPSCs) mediated by non-NMDA receptors (non-NMDA mEPSCs) and NMDA receptors in the masseter and digastric motoneurons. The amplitude, frequency, and rise time of non-NMDA mEPSCs remained unchanged among postnatal day (P)2-5, P9-12, and P14-17 age groups in masseter motoneurons, whereas the decay time dramatically decreased with age. The properties of the NMDA mEPSCs were more predominant at P2-5 masseter motoneurons, followed by reduction as neurons matured. The decay time of NMDA mEPSCs of masseter motoneurons also shortened remarkably across development. Furthermore, the proportion of NMDA/non-NMDA EPSCs induced in response to the electrical stimulation of the supratrigeminal region was quite high in P2-5 masseter motoneurons, and then decreased toward P14-17. In contrast to masseter motoneurons, digastric motoneurons showed unchanged properties in non-NMDA and NMDA EPSCs throughout postnatal development. Our results suggest that the developmental patterns of non-NMDA and NMDA receptor-mediated inputs vary among jaw-closing and jaw-opening motoneurons, possibly related to distinct roles of respective motoneurons in postnatal development of feeding behavior.

Developmental changes in GABAergic and glycinergic synaptic transmission to rat motoneurons innervating jaw-closing and jaw-opening muscles

Trigeminal motoneurons (MNs) innervating the jaw-closing and jaw-opening muscles receive numerous inhibitory synaptic inputs from GABAergic and glycinergic neurons, which are essential for oromotor functions, such as the orofacial reflex, suckling, and mastication. The properties of the GABAergic and glycinergic inputs of these MNs undergo developmental alterations during the period in which their feeding behavior proceeds from suckling to mastication; however, the detailed characteristics of the developmental patterns of GABAergic and glycinergic transmission in these neurons remain to be elucidated. This study was conducted to investigate developmental changes in miniature inhibitory postsynaptic currents (mIPSCs) in masseter (jaw-closing) and digastric (jaw-opening) MNs using brainstem slice preparations obtained from Wistar rats on postnatal day (P)2-5, P9-12, and P14-17. The frequency and amplitude of glycinergic mIPSCs substantially increased with age in both the masseter and digastric MNs. The rise time and decay time of glycinergic mIPSCs in both MNs decreased during development. In contrast, the frequency of GABAergic components in masseter MNs was higher at P2-5 than at P14-17, whereas that in the digastric MNs remained unchanged throughout the postnatal period. The proportion of currents mediated by GABA-glycine co-transmission was higher at P2-5, and then it decreased with age in both MNs. These results suggest that characteristics related to the development of inhibitory synaptic inputs differ between jaw-closing and jaw-opening MNs and between GABAergic and glycinergic currents. These distinct developmental characteristics may contribute to the development of feeding behaviors.

From periodontal mechanoreceptors to chewing motor control: A systematic review

PURPOSE

This critical review summarizes the current knowledge of the structural and functional characteristics of periodontal mechanoreceptors, and understands their role in the signal pathways and functional motor control.

METHOD

A systematic review of the literature was conducted. Original articles were searched through Pubmed, Cochrane Central database and Embase until january 2016.

RESULT

1466 articles were identified through database searching and screened by reviewing the abstracts. 160 full-text were assessed for eligibility, and after 109 exclusion, 51 articles were included in the review process. Studies selected by the review process were mainly divided in studies on animal and studies on humans. Morphological, histological, molecular and electrophysiological studies investigating the periodontal mechanoreceptors in animals and in humans were included, evaluated and described.

CONCLUSION

Our knowledge of the periodontal mechanoreceptors, let us conclude that they are very refined neural receptors, deeply involved in the activation and coordination of the masticatory muscles during function. Strictly linked to the rigid structure of the teeth, they determine all the functional physiological and pathological processes of the stomatognathic system. The knowledge of their complex features is fundamental for all dental professionists. Further investigations are of utmost importance for guiding the technological advances in the respect of the neural control in the dental field.

Innervation of tracheal parasympathetic ganglia by esophageal cholinergic neurons: evidence from anatomic and functional studies in guinea pigs

In the present study, we describe a subset of nerve fibers, characterized by their immunoreactivity for the calcium-binding protein calretinin, that are densely and selectively associated with cholinergic postganglionic neurons in the guinea pig tracheal ganglia. Retrograde neuronal tracing with cholera toxin B, combined with immunohistochemical analyses, showed that these nerve fibers do not originate from sensory neurons in the nodose, jugular, or dorsal root ganglia or from motor neurons in the nucleus ambiguus, dorsal motor nucleus of the vagus nerve, spinal cord, stellate ganglia, or superior cervical ganglia. Calretinin-immunoreactive nerve fibers disappeared from tracheal segments after 48 h in organotypic culture, indicating that the fibers were of extrinsic origin. However, calretinin-positive nerve fibers persisted in tracheal ganglia when tracheae were cocultured with the adjacent esophagus intact. Immunohistochemical analysis of the esophagus revealed a population of cholinergic neurons in the esophageal myenteric plexus that coexpressed calretinin. In functional studies, electrical stimulation of the esophagus in vitro evoked measurable contractions of the trachea. These contractions were not altered by prior organotypic culture of the trachea and esophagus to remove the extrinsic innervation to the airways but were significantly (P < 0.05) inhibited by the ganglionic blocker hexamethonium or by physical disruption of the tissue connecting the trachea and esophagus. These data suggest that a subset of esophageal neurons, characterized by the expression of calretinin and acetylcholine, provide a previously unrecognized excitatory input to tracheal cholinergic ganglia in guinea pigs.

Differential cell-specific location of Cav-1 and Ca(2+)-ATPase in terminal Schwann cells and mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisor

Caveolae are involved in clathrin-independent endocytosis, transcytosis, signal transduction, and tumor suppression - all of which depend on their main constituent protein caveolin families. The periodontal Ruffini ending has been reported to develop a caveola-like structure on the cell membrane of both the axon terminals and Schwann sheaths, suggesting the existence of an axon-Schwann cell interaction in the periodontal Ruffini endings. However, little information is available concerning the functional significance of these caveolae. The present study was undertaken to examine the immunolocalization of caveolin-1, -3 (Cav-1, Cav-3) and Ca(2+)-ATPase in the periodontal Ruffini endings of the rat incisor. Decalcified sections of the upper jaws were processed for immunocytochemistry at the levels of light and electron microscopy. Some immunostained sections were treated with histochemistry for nonspecific cholinesterase (nChE) activity. Observations showed the periodontal Ruffini endings were immunopositive for Cav-1, but not Cav-3. Immunoreactive products for Cav-1 were confined to caveola-like structures in the cell membranes of the cytoplasmic extensions and cell bodies of the terminal Schwann cells associated with the periodontal Ruffini endings. However, the axonal membranes of the terminals did not express any Cav-1 immunoreaction. Double staining with Ca(2+)-ATPase and either protein gene product 9.5 (PGP 9.5) or S-100 protein disclosed the co-localization of immunoreactions in the axonal branches of the periodontal Ruffini endings, but not in the terminal Schwann cells. As Ca(2+) plays an important role in mechanotransduction, these characteristic immunolocalizations show Cav-1/Ca(2+)-ATPase might be involved in the quick elimination of intracellular Ca(2+) in mechanotransduction.

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1–P4 neonatal rats and inhibitory in P9–P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

Requirement of proper occlusal force for morphological maturation of neural components of periodontal Ruffini endings of the rat incisor

The present study examined the effect of reduced occlusal force on morphological maturation of periodontal Ruffini endings, primary mechanoreceptors in the periodontal ligament, of the rat incisor. The reduction of occlusal force was induced by grinding the cutting edges of unilateral incisors of the rat from postnatal day 14 (PN14d), when periodontal Ruffini endings are immature. Under normal development, the axon terminals of Ruffini endings gradually ramified with the passage of time, and showed ruffled outlines having numerous dot-like structures around PN28d. When the mechanical stimulation was reduced, appearance of dot-like structures at the axon terminals delayed. Quantitative analysis elucidated that the percentages of immunoreactive areas for protein gene product 9.5, a marker protein of neural elements, at ground side were significantly smaller than those at non-ground side 14 days following the initial grinding. The distribution and morphology of terminal Schwann cells was not apparently affected. The present results indicate that the proper mechanical stimulation to the ligament contributes to the morphological maturation of the periodontal Ruffini endings.

Histochemical evaluation for the biological effect of menatetrenone on metaphyseal trabeculae of ovariectomized rats

To evaluate the biological effects of vitamin K2 (menatetrenone, MK-4) on ovariectomy (OVX)-induced bone loss, we have examined histological alterations of femoral metaphyses of sham-operated (sham group), ovariectomized (OVX group), and MK-4 dietary-supplemented OVX (MK-4 group; 50 mg/kg per day) female Fischer rats 1, 2, 5, and 8 weeks after OVX. In the first week, rats of the OVX and MK-4 groups showed discontinuous trabeculae compared with sham-operated rats. At 2 weeks after OVX, the OVX rats revealed many large tartrate resistant acid phosphatase (TRAP)-positive osteoclasts, while osteoclasts in the MK-4-treated rats were similar in size to those of the sham group. At 5 weeks, the OVX and MK-4 groups revealed fragmented trabeculae in femoral metaphyses. The cartilage matrix was partially exposed due to stimulated bone resorption in the OVX group, but not in the MK-4 group. After 8 weeks, the OVX rats had little metaphyseal trabeculae, whereas the MK-4-treated rats had maintained short trabeculae. Despite the presence of intense alkaline phosphatase-positive osteoblasts on trabeculae in the MK-4 group, TRAP-positive osteoclasts were flattened without developing ruffled borders. Therefore, MK-4 appeared to lessen the increase in osteoclastic bone resorption induced by OVX, as well as to maintain the accelerated osteoblastic activity. It is of importance to identify the target cells for MK-4 in bone. Autoradiography localized [3H]-labeled MK-4 mainly in osteoblasts and adjacent bone matrices, but not in osteoclasts, indicating that MK-4 targets osteoblasts. Thus, MK-4 appears to target osteoblasts, consequently inhibiting bone loss induced by ovariectomy.

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.

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 periodontal Ruffini endings in brain derived neurotrophic factor (BDNF) deficient mice

Innervation and terminal morphology in the lingual periodontal ligament of the incisor were investigated in brain derived neurotrophic factor (BDNF) heterozygous mice and littermate wild-type mice (aged two months) using immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker. In addition, computer-assisted quantitative analysis was performed for a comparison of neuronal density in the periodontal ligament between heterozygous and wild-type mice. In wild-type mice, the periodontal ligament was found to be richly innervated by the mechanoreceptive Ruffini endings and nociceptive free nerve endings in the alveolus-related part of the periodontal ligament. The periodontal Ruffini endings in the wild-type mice incisor ligament were classified into two types: type I with ruffled outlines, and type II with a smooth outline. BDNF heterozygous mice showed malformations of the type I Ruffini endings which included fewer nerve fibers and fewer ramifications than those in wild-type mice as well as smooth outlines of the axon terminals. Quantitative analysis under a confocal microscope showed a roughly 18% reduction in neuronal density in the periodontal ligament of the heterozygous mice. These findings suggest that the development and maturation of the periodontal Ruffini endings require BDNF.

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 development of terminal Schwann cells associated with periodontal Ruffini endings in the rat incisor ligament

The postnatal development of the terminal Schwann cell, an analogue of the lamellar cell in cutaneous sensory receptors, was examined by histochemistry for non-specific cholinesterase and immunohistochemistry for S-100 protein in the periodontal Ruffini endings of the rat incisor. Double immunohistochemistry for S-100 protein and protein gene product 9.5 (PGP 9.5) was also performed to examine the relationship between terminal Schwann cells and axons. Histochemistry for non-specific cholinesterase was able to demonstrate the age-related development of the terminal Schwann cells; the morphology and distribution of the developing terminal Schwann cells became almost identical to those in adults during postnatal days 15-18. Axons showing PGP 9.5-like immunoreactivity elongated and expanded after arrangement of terminal Schwann cells in the alveolus-related part. This suggests that the terminal Schwann cell is important in the development and maturation of the periodontal Ruffini endings.