NADPH-diaphorase activity in nerves and Schwann cells in the periodontal ligament of rat incisor teeth

Distribution of NADPH Diaphorase-Exhibiting Primary Afferent Neurons in the Trigeminal Ganglion and Mesencephalic Trigeminal Nucleus of the Rabbit

1. Nitric oxide (NO) is highly reactive gaseous molecule to which many physiological and pathological functions have been attributed in the central (CNS) and peripheral (PNS) nervous system. The present investigation was undertaken to map the distribution pattern of the enzyme responsible for the synthesis of NO, nitric oxide synthase (NOS), and especially its neuronal isoform (nNOS) in the population of primary afferent neurons of the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN) of the rabbit. 2. In order to identify neuronal structures expressing nNOS we applied histochemistry to its specific histochemical marker nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd). 3. We found noticeable amount of NADPHd-exhibiting primary afferent neurons in TG of the rabbit under physiological conditions. The intensity of the histochemical reaction was highly variable reaching the maximum in the subpopulation of small-to-medium-sized neurons. The large-sized neurons were only weakly stained or actually did not posses any NADPHd-activity. In addition, NADPHd-positive nerve fibers were detected between clusters of the ganglionic cells and in the peripheral branches of the trigeminal nerve (TN). NADPHd-exhibiting MTN neurons were noticed in the whole rostrocaudal extent of the nucleus even though some differences were found concerning the ratio of NADPHd-positive versus NADPHd-negative cell bodies. Similarly, we observed striking diversity in the intensity of NADPHd histochemical reaction in the subpopulations of small-, medium-, and large-sized MTN neurons. 4. The predominant localization of NADPHd in the subpopulation of small-to-medium-sized TG neurons which are generally considered to be nociceptive suggests that NO probably takes part in the modulation of nociceptive inputs from the head and face. Furthermore, we tentatively assume that NADPHd-exhibiting MTN neurons probably participate in transmission and modulation of the proprioceptive impulses from muscle spindles of the masticatory muscles and mechanoreceptors of the periodontal ligaments and thus provide sensory feedback of the masticatory reflex arc.

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.

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.

Cyclic tension force activates nitric oxide production in cultured human periodontal ligament cells

BACKGROUND

Nitric oxide (NO) is involved in a number of physiological and pathophysiological processes. The aim of this study was to examine the ability of human periodontal ligament (PDL) cells to produce NO and whether mechanical forces could induce NO production in the PDL cells.

METHODS

Human PDL cells were seeded onto flexible bottoms of a culture plate and subjected to cyclic tension forces. NO production was evaluated by measuring concentration of NO2- and NO3- (NO2-/NO3-), the oxidized products of NO, in the culture medium. We employed reverse transcription polymerase chain reaction (RT-PCR) methods to detect NO synthase mRNA in the PDL cells. NO synthase immunoreactivity was also evaluated in both stimulated and unstimulated PDL cells.

RESULTS

In unstimulated PDL cell culture, NO2-/NO3- increased to 140% of the initial value in 12 hours. In contrast, NO2-/NO3- showed a 3-fold increase when the cells had been subjected to cyclic tension forces for 12 hours. The increase in NO production was blocked by NG-monomethyl-L-arginine (5 x 10(-4) M), an inhibitor of NO synthase. Endothelial NO synthase (ecNOS) mRNA was expressed in both stimulated and unstimulated PDL cells, whereas inducible NO synthase (iNOS) mRNA was detected in neither culturing condition. We found strong ecNOS but not iNOS immunoreactivity in the stimulated PDL cells.

CONCLUSIONS

These results suggest that human PDL cells produce NO by ecNOS and that the production is enhanced by stimulating the cells with cyclic tension forces. Mechanically stimulated PDL cells may modulate the function of periodontium by the upregulated NO production.

Immunohistochemical detection of heme oxygenase-2 in the periodontal Ruffini ending of the rat incisor

The present study was carried out to examine the occurrence of heme oxygenase-2 (HO-2) in the periodontal ligament of the rat incisor. HO-2-like immunoreactive (-IR) structures showed dendritic profiles, resembling the Ruffini endings, in the alveolar half of the ligament of rat incisor. Neither thin nerve fibers nor perivascular nerve fibers displayed HO-2-like immunoreactivity (-LI). No non-neural elements exhibited HO-2-LI. Electron microscopy revealed that immunoreactions were diffusely observed in the axon terminals of the Ruffini endings, but neither terminal Schwann cells nor Schwann sheaths contained immunoreactions for HO-2. Both most neurons in the trigeminal ganglion and trigeminal mesencephalic nucleus showed HO-2-LI. The presence of HO-2 in the periodontal Ruffini endings and its absence in the periodontal thin nerve fibers suggest the involvement of carbon monoxide produced by HO-2 in mechanoreception in the periodontal ligament.