Protein gene product (PGP) 9.5 immunoreactivity in nerve fibres and pinealocytes of guinea-pig pineal gland: interrelationship with tyrosine- hydroxylase- and neuropeptide-Y-immunoreactive nerve fibres

Morphological Methods to Evaluate Peripheral Nerve Fiber Regeneration: A Comprehensive Review

Regeneration of damaged peripheral nerves remains one of the main challenges of neurosurgery and regenerative medicine, a nerve functionality is rarely restored, especially after severe injuries. Researchers are constantly looking for innovative strategies for tackling this problem, with the development of advanced tissue-engineered nerve conduits and new pharmacological and physical interventions, with the aim of improving patients' life quality. Different evaluation methods can be used to study the effectiveness of a new treatment, including functional tests, morphological assessment of regenerated nerve fibers and biomolecular analyses of key factors necessary for good regeneration. The number and diversity of protocols and methods, as well as the availability of innovative technologies which are used to assess nerve regeneration after experimental interventions, often makes it difficult to compare results obtained in different labs. The purpose of the current review is to describe the main morphological approaches used to evaluate the degree of nerve fiber regeneration in terms of their usefulness and limitations.

Expression of 5-HT3 receptors and TTX resistant sodium channels (Na(V)1.8) on muscle nerve fibers in pain-free humans and patients with chronic myofascial temporomandibular disorders

BACKGROUND

Previous studies have shown that 5-HT3-antagonists reduce muscle pain, but there are no studies that have investigated the expression of 5-HT3-receptors in human muscles. Also, tetrodotoxin resistant voltage gated sodium-channels (NaV) are involved in peripheral sensitization and found in trigeminal ganglion neurons innervating the rat masseter muscle. This study aimed to investigate the frequency of nerve fibers that express 5-HT3A-receptors alone and in combination with NaV1.8 sodium-channels in human muscles and to compare it between healthy pain-free men and women, the pain-free masseter and tibialis anterior muscles, and patients with myofascial temporomandibular disorders (TMD) and pain-free controls.

METHODS

Three microbiopsies were obtained from the most bulky part of the tibialis and masseter muscles of seven and six healthy men and seven and six age-matched healthy women, respectively, while traditional open biopsies were obtained from the most painful spot of the masseter of five female patients and from a similar region of the masseter muscle of five healthy, age-matched women. The biopsies were processed by routine immunohistochemical methods. The biopsy sections were incubated with monoclonal antibodies against the specific axonal marker PGP 9.5, and polyclonal antibodies against the 5-HT3A-receptors and NaV1.8 sodium-channels.

RESULTS

A similar percentage of nerve fibers in the healthy masseter (85.2%) and tibialis (88.7%) muscles expressed 5-HT3A-receptors. The expression of NaV1.8 by 5-HT3A positive nerve fibers associated with connective tissue was significantly higher than nerve fibers associated with myocytes (P < .001). In the patients, significantly more fibers per section were found with an average of 3.8 ± 3 fibers per section in the masseter muscle compared to 2.7 ± 0.2 in the healthy controls (P = .024). Further, the frequency of nerve fibers that co-expressed NaV1.8 and 5-HT3A receptors was significantly higher in patients (42.6%) compared to healthy controls (12.0%) (P < .001).

CONCLUSIONS

This study showed that the 5-HT3A-receptor is highly expressed in human masseter and tibialis muscles and that there are more nerve fibers that express 5-HT3A-receptors in the masseter of women with myofascial TMD compared to healthy women. These findings indicate that 5-HT3-receptors might be up-regulated in myofascial TMD and could serve as potential biomarkers of chronic muscle pain.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

Central GABAergic innervation of the mammalian pineal gland: a light and electron microscopic immunocytochemical investigation in rodent and nonrodent species

Light and electron microscopic immunocytochemical observations were made to demonstrate central pinealopetal fibers immunoreactive for gamma-aminobutyric acid (GABA) and synapses between their terminals and pinealocytes in the pineal gland of four rodent (Wistar-King rat; mouse; Syrian hamster, Mesocricetus auratus; Hartley strain guinea pig) and one nonrodent (tree shrew, Tupaia glis) species. GABA-immunoreactive myelinated and unmyelinated fibers and endings were found in the parenchyma of the pineal gland of all the animals examined. In the rodent species, GABAergic fibers were mainly found in the intermediate and proximal portions of the pineal gland and were nearly or entirely absent in the distal portion of the gland. Abundant GABAergic fibers were evenly distributed throughout the gland of the tree shrew. In all the animals, the habenular and posterior commissures contained abundant GABA-positive fibers, and some of them were followed to the pineal gland. GABA-positive endings made synaptic contact with pinealocytes, occasionally in mice and guinea pigs, and frequently in tree shrews; no synapses were observed in Syrian hamsters and rats. In the pineal gland of all the animals, GABA-immunoreactive cell bodies were not detected, and sympathetic fibers were not immunoreactive for GABA. These data indicate that GABAergic fibers are main pinealopetal projections from the brain. In view of the difference in the distribution of these fibers, central GABAergic innervation may play a more significant role in nonrodents than in rodents. The frequent occurrence of GABAergic synapses on pinealocytes in the tree shrew suggests that GABA released at these synapses directly controls activity of pinealocytes of this animal.

Peptidergic peripheral nervous systems in the mammalian pineal gland

The distribution and density of tyrosine hydroxylase (TH) and neuropeptide Y (NPY)-immunoreactive, sympathetic fibers and calcitonin gene-related peptide (CGRP)-, substance P (SP)-, and vasoactive intestinal polypeptide (VIP)-immunoreactive, non-sympathetic fibers in the pineal gland, the effects of superior cervical ganglionectomy (SCGX) on these fibers, and the location of their terminals in the pineal gland were compared between rodents and non-rodents. A dense network of TH/NPY-positive fibers is present all over the pineal gland. A less dense network of CGRP/SP- or VIP-positive fibers occurs in the whole pineal gland of non-rodents, but these fibers are usually confined to the superficial pineal gland in rodents. After SCGX, some TH/NPY-fibers remain only in the deep pineal gland in rodents, whereas considerable numbers of these fibers persist throughout the gland in non-rodents. Thus, the remaining fibers, probably originating from the brain, may be more numerous in non-rodents. Since CGRP-, SP- or VIP-immunoreactive fibers in the pineal capsule can be traced to those in the gland, and since these fibers are ensheathed by Schwann cells, it is concluded that these fibers belong to the peripheral nervous system. However, the existence of SP-positive central fibers cannot be denied in some species. In the superficial pineal gland of rodents, sympathetic terminals are mostly localized in perivascular spaces, whereas the parenchymal innervation by sympathetic fibers in the pineal gland is more dense in non-rodents than in rodents. Synapses between sympathetic nerve terminals and pinealocytes occur occasionally in non-rodents, but only rarely in the superficial pineal gland of rodents. The occurrence of the synapses may depend on the frequency of intraparenchymal sympathetic terminals.

Cholinergic neurons and terminal fields revealed by immunohistochemistry for the vesicular acetylcholine transporter. I. Central nervous system

Antibodies directed against the C-terminus of the rat vesicular acetylcholine transporter mark expression of this specifically cholinergic protein in perinuclear regions of the soma and on secretory vesicles concentrated within cholinergic nerve terminals. In the central nervous system, the vesicular acetylcholine transporter terminal fields of the major putative cholinergic pathways in cortex, hippocampus, thalamus, amygdala, olfactory cortex and interpeduncular nucleus were examined and characterized. The existence of an intrinsic cholinergic innervation of cerebral cortex was confirmed by both in situ hybridization histochemistry and immunohistochemistry for the rat vesicular acetylcholine transporter and choline acetyltransferase. Cholinergic interneurons of the olfactory tubercle and Islands of Calleja, and the major intrinsic cholinergic innervation of striatum were fully characterized at the light microscopic level with vesicular acetylcholine transporter immunohistochemistry. Cholinergic staining was much more extensive for the vesicular acetylcholine transporter than for choline acetyltransferase in all these regions, due to visualization of cholinergic nerve terminals not easily seen with immunohistochemistry for choline acetyltransferase in paraffin-embedded sections. Cholinergic innervation of the median eminence of the hypothalamus, previously observed with vesicular acetylcholine transporter immunohistochemistry, was confirmed by the presence of vesicular acetylcholine transporter immunoreactivity in extracts of median eminence by western blotting. Cholinergic projections to cerebellum, pineal gland, and to the substantia nigra were documented by vesicular acetylcholine transporter-positive punctate staining in these structures. Additional novel localizations of putative cholinergic terminals to the subependymal zone surrounding the lateral ventricles, and putative cholinergic cell bodies in the sensory mesencephalic trigeminal nucleus, a primary sensory afferent ganglion located in the brainstem, are documented here. The cholinergic phenotype of neurons of the sensory mesencephalic trigeminal nucleus was confirmed by choline acetyltransferase immunohistochemistry. A feature of cholinergic neurons of the central nervous system revealed clearly with vesicular acetylcholine transporter immunohistochemistry in paraffin-embedded sections is the termination of cholinergic neurons on cholinergic cell bodies. These are most prominent on motor neurons of the spinal cord, less prominent but present in some brainstem motor nuclei, and apparently absent from projection neurons of the telencephalon and brainstem, as well as from the preganglionic vesicular acetylcholine transporter-positive sympathetic and parasympathetic neurons visualized in the intermediolateral and intermediomedial columns of the spinal cord. In addition to the large puncta decorating motor neuronal perikarya and dendrites in the ventral horn, vesicular acetylcholine transporter-positive terminal fields are distributed in lamina X surrounding the central canal, where additional small vesicular acetylcholine transporter-positive cell bodies are located, and in the superficial layers of the dorsal horn. Components of the central cholinergic nervous system whose existence has been controversial have been confirmed, and the existence of new components documented, with immunohistochemistry for the vesicular acetylcholine transporter. Quantitative visualization of terminal fields of known cholinergic systems by staining for vesicular acetylcholine transporter will expand the possibilities for documenting changes in synaptic patency accompanying physiological and pathophysiological changes in these systems.

Impairment of neural nitric oxide-mediated relaxation after antigen exposure in guinea pig airways in vitro

Nitric oxide (NO), a neurotransmitter of inhibitory nonadrenergic noncholinergic (iNANC) nerves in airways, is a radical with a short half-life, and its function may be modified by airway inflammation. To test this hypothesis, we examined whether airway allergic inflammation affects iNANC responses mediated by NO in guinea pigs in vitro. Animals sensitized with ovalbumin (OA) were challenged with 0.03% OA (OA group) or saline (saline group) by inhalation on 3 consecutive days. On the day after the final challenge, iNANC responses elicited by electrical field stimulation (2 to 16 Hz) or relaxation responses to 3-morpholinosydnonimine (SIN-1), 10(-8) to 10(-4) M, were obtained in the tracheal strips precontracted by histamine (3 x 10(-6) M) in the presence of atropine and propranolol (both 10(-6) M). The INANC responses of the OA group were significantly attenuated compared with those of the saline group (p < 0.05), and the inhibitory effect of a NO synthase (NOS) inhibitor, Nm-nitro-L-arginine methyl ester, on the INANC responses was abolished in the OA group. SIN-1-induced tracheal smooth muscle relaxation was also significantly affected by antigen exposure (p < 0.05), the effect of which disappeared in the presence of a NO scavenger, carboxy PTIO (3 x 10(-6) M). The impairment of the INANC responses after antigen exposure was significantly restored by superoxide dismutase (1,000 U/ml), especially at lower frequencies. Histochemical demonstration of NADPH-diaphorase-positive nerves representing neural NOS density was not different between the two groups. These results suggest that allergic airway inflammation impairs neural NO-induced relaxation, presumably by inhibiting the access of neural NO to the airway smooth muscle.

Differential immunocytochemical localization of calretinin in the pineal gland of three mammalian species

Calcium plays an important role for signal transduction in the mammalian pineal organ. The regulation of the intracellular concentration of free calcium probably involves calcium-binding proteins of the calmodulin superfamily. In the present study, we have investigated the expression of calretinin, one member of this superfamily, in the pineal organ of hamsters, gerbils and guinea-pigs by means of immunochemical and immunocytochemical analyses with a calretinin-specific antiserum. In immunoblots this antibody recognized a single protein band of approximately 29 kDa in the brain and pineal organ of all three mammalian species. Immunocytochemical investigations of serial semithin sections of plastic-embedded pineals revealed the constant occurrence of variable numbers of calretinin-positive cells throughout all glands. In order to identify the immunopositive cells precisely, adjacent sections were exposed to antibodies against various marker proteins of pineal cell types, i.e., synaptophysin, neuron-specific enolase, protein gene product 9.5, S-antigen, vimentin and S-100. By this approach, calretinin could be localized to vimentin-positive cells in the gerbil which are generally considered as interstitial glial cells. Likewise, calretinin-positive cells in the guinea-pig probably correspond to interstitial cells, taking into account their morphology and the lack of calretinin immunoreactivity in pinealocytes. The unusual expression of calretinin in astrocyte-like cells further supports the notion that pineal glial cells are endowed with peculiar properties. In contrast to gerbil and guinea-pig, a subpopulation of pinealocytes displayed calretinin immunoreactivity in the hamster. This finding adds to the hypothesis that in pinealocytes of some species calretinin plays a role in calcium-mediated signal transduction which eventually is linked to melatonin synthesis. Our results demonstrate that calretinin is a regular constituent of pineal glands in three mammalian species, but that its cellular localisation shows interspecific variation. This variation suggests that the protein is involved in diverse calcium-mediated functions in the mammalian pineal gland.

Morphological study of the submucosal and mucosal plexuses of the sheep forestomach

Submucosal and mucosal nerve plexuses in the ovine forestomach were examined by immunohistochemical staining for protein gene-product 9.5 (PGP 9.5) and for S-100 protein (S-100), using whole-mount specimens that had been prepared by treatment with KOH. Nerve fibers of various sizes and glial cells (i.e., Schwann cells and satellite cells) were stained with antibodies against PGP 9.5 and S-100 respectively. The network of the submucosal plexus in the rumen is irregular and some nerve bundles in the plexus cross over other bundles. Some of the nerve bundles penetrate the ruminal papillae. The submucosal plexus in the reticulum consists of a network in the reticular wall and the reticular folds. The submucosal plexus in the omasum is also divided into two segments; namely, the sublaminar and the intralaminar plexuses. Most of the submucosal ganglion cells are unipolar and smooth-surfaced, being located singly or in small groups. A few perikarya were detected in the ruminal papillae. The number of perikarya per unit surface area is greater in the caudal portion of the omasal lamina (19.32 +/- 8.62 per cm2). In the mucosal plexuses, a well-developed network of beaded fibers with PGP 9.5-like immunoreactivity and a glial framework of S-100 like immunoreactivity was observed, in particular in the ruminal, reticular and omasal papillae. The intrapapillary nervous networks are interconnected by thin bundles of nerves in the interpapillary region. The present results suggest that some of the mucosal functions are intrinsically regulated by the submucosal and mucosal plexuses in the ovine forestomach.

Nitrergic innervation and nitrergic cells in arteriovenous anastomoses

Nitrergic innervation and nitrergic epithelioid cells were studied in arteriovenous anastomoses of the tongue, ear, eye, and glomus organ of the finger in different species (rat, rabbit, dog, and man), by means of immunohistochemistry for nitric oxide synthase and enzyme histochemistry utilizing the catalytic activity of this enzyme (the NADPH-diaphorase reaction). Nitrergic perivascular fibers of the tongue were concentrated along the arterial tree and were maximal at the arteriovenous anastomoses in all species. Generally, fewer fibers were located around comparable segments of the episcleral eye vasculature. Only a few nitrergic fibers were found in the canine and rabbit ear, and in the glomus organ of the human finger; however, epithelioid cells in the tunica media of arteriovenous anastomoses of these organs were NADPH-diaphorase-positive and were moderately immunoreactive for nitric oxide synthase. In the epithelioid cells, the reaction product of the NADPH-diaphorase could also be demonstrated by transmission electron microscopy. The epithelioid cells were negative for the panneural and neuroendocrine marker PGP 9.5 confirming the myocytotic nature of these nitrergic cells. Thus, nitric oxide might play a role in mediating the vessel tone of arteriovenous anastomoses via nitrergic nerves or epithelioid cells.

Current methodologies for the study of pineal morphophysiology

Light and electron microscopes, with or without the use of immunohistochemical techniques, have been the instruments of choice for study of the pineal complex even up to recent times. Other morphological technologies have become available during the past decade that, if applied to current questions concerning pineal morphophysiology, could add considerably to our understanding of this complex system. Those technologies discussed include confocal scanning laser microscopy (in conjunction with other techniques including immunohistochemistry and three-dimensional reconstruction), tissue culture methodologies, carbocyanine dyes (i.e., DiI), in situ hybridization, and application of microinjection methodologies. It is suggested that these technologies will be necessary for morphophysiologists to not only collaborate with molecular biologists and biochemists who study the pineal complex, but to corroborate the molecular and biochemical results of our colleagues.