Phosphorylated Histone 3 at Serine 10 Identifies Activated Spinal Neurons and Contributes to the Development of Tissue Injury-Associated Pain

Transcriptional changes in superficial spinal dorsal horn neurons (SSDHN) are essential in the development and maintenance of prolonged pain. Epigenetic mechanisms including post-translational modifications in histones are pivotal in regulating transcription. Here, we report that phosphorylation of serine 10 (S10) in histone 3 (H3) specifically occurs in a group of rat SSDHN following the activation of nociceptive primary sensory neurons by burn injury, capsaicin application or sustained electrical activation of nociceptive primary sensory nerve fibres. In contrast, brief thermal or mechanical nociceptive stimuli, which fail to induce tissue injury or inflammation, do not produce the same effect. Blocking N-methyl-D-aspartate receptors or activation of extracellular signal-regulated kinases 1 and 2, or blocking or deleting the mitogen- and stress-activated kinases 1 and 2 (MSK1/2), which phosphorylate S10 in H3, inhibit up-regulation in phosphorylated S10 in H3 (p-S10H3) as well as fos transcription, a down-stream effect of p-S10H3. Deleting MSK1/2 also inhibits the development of carrageenan-induced inflammatory heat hyperalgesia in mice. We propose that p-S10H3 is a novel marker for nociceptive processing in SSDHN with high relevance to transcriptional changes and the development of prolonged pain.

Characterisation of cannabinoid 1 receptor expression in the perikarya, and peripheral and spinal processes of primary sensory neurons

The cannabinoid 1 (CB1) receptor is expressed by a sub-population of primary sensory neurons. However, data on the neurochemical identity of the CB1 receptor-expressing cells, and CB1 receptor expression by the peripheral and central terminals of these neurons are inconsistent and limited. We characterised CB1 receptor expression in dorsal root ganglia (DRG) and spinal cord at the lumbar 4-5 level, as well as in the urinary bladder and glabrous skin of the hindpaw. About 1/3 of DRG neurons exhibited immunopositivity for the CB1 receptor, the majority of which showed positivity for the nociceptive markers calcitonin gene-related peptide (CGRP) or/and Griffonia (bandeiraea) simplicifolia IB4 isolectin-binding. Virtually all CB1 receptor-immunostained fibres showed immunopositivity for CGRP in the skin, while very few did in the urinary bladder. No CB1 receptor-immunopositive nerve fibres were IB4 positive in either peripheral tissue. Spinal laminae I and II-outer showed the highest density of CB1 receptor-immunopositive punctae, the majority of which showed positivity for CGRP or/and IB4 binding. These data indicate that a major sub-population of nociceptive primary sensory neurons expresses CB1 receptors that are transported to both peripheral and central terminals of these cells. Therefore, the present data suggest that manipulation of endogenous CB1 receptor agonist levels in these areas may significantly reduce nociceptive input into the spinal cord.

Dendrodendritic connections between the cochlear efferent neurons in guinea pig

The outer hair cells of organ of Corti are innervated by the efferent neurons of medial olivocochlear neurons (MOC) of the brainstem which modify the cochlear auditory processing and sensitivity. Most of the MOC neurons are excited by a dominant ear and only a small portion of them is excited by both ears resulting in a binaural facilitation. The functional role of the feedback system between the organ of Corti and the cochlear efferent neurons is the protection of the ear from acoustic injury. The rapid impulse propagation in the bilateral olivocochlear system is suggestive of an electrotonic interaction between the bilateral olivocochlear neurons. The morphological background of the MOC pathway is not yet completely characterized. Therefore, we have labeled the bilateral cochlear nerves with different neuronal tracers in guinea pigs. In the anesthetized animals the cochlear nerves were exposed in the basal part of the modiolus and labeled simultaneously with different retrograde fluorescent tracers. By using confocal laser scanning microscope we could detect close appositions between the dendrites of the neurons of bilateral MOC. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. These connections might serve as one of the underlying mechanisms of the binaural facilitation mediated by the olivocochlear system.

Severe burn injury induces a characteristic activation of extracellular signal-regulated kinase 1/2 in spinal dorsal horn neurons

We have studied scalding-type burn injury-induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the spinal dorsal horn, which is a recognised marker for spinal nociceptive processing. At 5min after severe scalding injury to mouse hind-paw, a substantial number of phosphorylated ERK1/2 (pERK1/2) immunopositive neurons were found in the ipsilateral dorsal horn. At 1h post-injury, the number of pERK1/2-labelled neurons remained substantially the same. However, at 3h post-injury, a further increase in the number of labelled neurons was found on the ipsilateral side, while a remarkable increase in the number of labelled neurons on the contralateral side resulted in there being no significant difference between the extent of the labelling on both sides. By 6h post-injury, the number of labelled neurons was reduced on both sides without there being significant difference between the two sides. A similar pattern of severe scalding injury-induced activation of ERK1/2 in spinal dorsal horn neurons over the same time-course was found in mice which lacked the transient receptor potential type 1 receptor (TRPV1) except that the extent to which ERK1/2 was activated in the ipsilateral dorsal horn at 5 min post-injury was significantly greater in wild-type animals when compared to TRPV1 null animals. This difference in activation of ERK1/2 in spinal dorsal horn neurons was abolished within 1h after injury, demonstrating that TRPV1 is not essential for the maintenance of ongoing spinal nociceptive processing in inflammatory pain conditions in mouse resulting from at least certain types of severe burn injury.

Modification of innervation pattern by fluoroquinolone treatment in the rat salivary glands

Fluoroquinolone antibiotics (FQAs) are widely used in dental and medical therapy. Despite their known severe adverse actions on the central and peripheral nervous system, little attention has been directed toward the potential toxic side effects of these compounds on the oral tissues. As the saliva secretion is controlled by the nervous system and neuropeptides, the neurotoxic effect of pefloxacin (PEF), a representative member of FQAs, was studied in rats in the present work. Previously, we demonstrated a significant weight loss of parotid gland tissue, a marked decrease in 3H-thymidine incorporation, a decreased volume of saliva and amylase activity of the glandular tissue in response to PEF. Animals received intraperitoneal injection of PEF (20 mg/100 g body weight daily) for 3 and 7 days. Normal histology, and neurofilament 200, substance P (SP) and calcitonin gene-related polypeptide (CGRP) containing nerve fibers were detected with immunohistochemical methods. A marked decrease of the weights in salivary glands and the acinar diameters were measured. Similarly, a strong and significant decrease of the number of SP and CGRP containing nerve fibers were detected. These findings suggest that the impaired morphology and innervation pattern of salivary glands is related to the neurotoxic adverse effect of FQA treatment.

Neurochemical characterization of insulin receptor-expressing primary sensory neurons in wild-type and vanilloid type 1 transient receptor potential receptor knockout mice

The insulin receptor (IR) is expressed by a subpopulation of primary sensory neurons (PSN), including a proportion of cells expressing the nociceptive transducer vanilloid type 1 transient receptor potential receptor (TRPV1). Recent data suggest functional links between the IR and other receptors, including TRPV1, which could be involved in the development of PSN malfunctions in pathological insulin secretion. Here we used combined immunohistochemical labelling on sections from L4-5 dorsal root ganglia of wild-type (WT) and TRPV1 knockout (KO) mice to examine the neurochemical properties of IR-expressing PSN and the possible effect of deletion of TRPV1 on those characteristics. We found that antibodies raised against the high-molecular-weight neurofilament (NF-200) and the neurofilament protein peripherin distinguished between small and large neurons. We also found that the IR was expressed predominantly by the small peripherin-immunopositive cells both in the WT and in the KO animals. IR expression, however, did not show any preference between the major subpopulations of the small cells, the calcitonin gene-related peptide (CGRP)-expressing and Bandeiraea simplicifolia isolectin B4 (IB4)-binding neurons, either in the WT or in the KO mice. Nevertheless, a significant proportion of the IR-expressing cells also expressed TRPV1. Comparison of the staining pattern of these markers showed no difference between WT and KO animals. These findings indicate that the majority of the IR-expressing PSN are small neurons, which are considered as nociceptive cells. Furthermore, these data show that deletion of the TRPV1 gene does not induce any additional changes in neurochemical phenotype of nociceptive PSN.

Insulin induces cobalt uptake in a subpopulation of rat cultured primary sensory neurons

Previous findings show that both the vanilloid receptor 1 and the insulin receptor are expressed on small primary sensory neurons. As insulin evokes activity in second messengers which could induce opening of the vanilloid receptor 1, we examined, by using the cobalt-uptake technique, whether or not insulin can activate cultured rat primary sensory neurons through activating the vanilloid receptor 1. Capsaicin (50, 100 and 500 nm) induced concentration-dependent labelling in primary sensory neurons. Preincubation of cells in insulin (10 micromoles) for 10 min followed by a 2-min wash did not produce significant change in the capsaicin-induced labelling. Coapplication of insulin (10 micromoles) with capsaicin, however, potentiated the 50 and 100 nm capsaicin-evoked staining. Insulin itself also produced cobalt labelling in a concentration-dependent manner. The size-frequency distributions of neurons showing capsaicin- or insulin-induced cobalt accumulation were similar. The insulin-induced cobalt labelling was significantly reduced by the tyrosine kinase inhibitor, tyrphostin AG1024, the vanilloid receptor 1 antagonists, ruthenium red and capsazepine, the protein kinase inhibitor, staurosporine and the phospholipase C inhibitor neomycin. Double immunostaining of cultured primary sensory neurons and sections from dorsal root ganglia revealed that about one-third of the cells coexpress the insulin receptor and vanilloid receptor 1. These findings suggest that insulin activates a subpopulation of primary sensory neurons, probably through phosphorylation- and/or phosphatidylinositol(4,5)biphosphate hydrolysis-evoked activation of the vanilloid receptor 1. Although the insulin-induced activation of vanilloid receptor 1 seems to be a short-lived effect in vitro, in vivo it might play a role in the development of burning pain sensation in hyperinsulinism.

Immunohistochemical localization of neurokinin-l receptor in the lumbar spinal cord of young rats: morphology and distribution

The type and distribution of neurokinin-1 (NK-1) receptor-expressing neurones were studied in young (14-day-old) rats' lumbar spinal cord using pre-embedding immunohistochemistry. The heaviest immunoreactivity was observed in the middle part and lateral fourth of lamina I where the great majority of immunoreactive perikarya represented fusiform and multipolar cells. In lamina II the middle and medial part showed moderate immunoreactivity, most of the cells resembled stalked cells. In lamina III the labelled perikarya were evenly distributed, while those in lamina IV accumulated mainly in the lateral part. In both laminae most of the labelled neurones represented central cells, the rest of them belonged to the antenna-type cells with long dorsally directed dendrites penetrating the superficial laminae. The immunoreactivity in laminae V-VII was uniform and relatively weak. In lamina VIII the immunopositive perikarya were encountered only rarely while in lamina IX virtually all motoneurones showed weak immunoreactivity. Lamina X contained small, multipolar and fusiform labelled perikarya. In conclusion, we found that the general appearance of the NK-1 receptor immunostaining and the major type of NK-I receptor-expressing neurones were similar to that found previously in adult spinal cord. Using the same method as Brown and colleagues the number of labelled NK- 1 receptor immunoreactive cells was similar in young and adult animals except lamina I where the number of immunoreactive neurones was twice that in adults.

Primary afferent terminals in the nucleus of the solitary tract of the frog: an electron microscopic study

In the frog solitarius nucleus, primary afferent terminals of the facial and glossopharyngeal-vagal nerves were identified with cobalt labelling and electron microscopy. The labelled terminals were grouped in two main categories, one with small (1-2 micron) and pale terminals, and another with large (3-5 micron) and dark terminals. The small terminals greatly outnumbered the large ones. In addition many terminals intermediate in size and staining reactions were found. All kinds of labelled boutons contained medium-size clear synaptic vesicles, among which dense-core vesicles of the smaller type frequently occurred. The labelled primary afferent terminals established axo-dendritic contacts of the asymmetric type. Close to these contact sites they were themselves very frequently contacted by a profile interpreted as presynaptic in relation to them. Such profiles contained spherical, pleomorphic (including dense-core) or flattened vesicles; a fourth kind was interpreted as presynaptic dendrites. It is concluded that viscerosensory fibres, as opposed to somatosensory fibres, predominantly generate small and lightly stained terminals. It is likely that the effect of synaptic transmission at the solitarius tract terminals is modulated in a very versatile manner by the various presynaptic profiles converging on these terminals.

The termination of cutaneous nerves in the dorsal horn of the spinal cord in normal and in skin-rotated frogs

Following transganglionic transport of cobaltic lysine applied to the cut end of cutaneous nerves, the central terminal arbours were filled to such an extent that it was possible to visualize the dermatomal projection upon the dorsal horn in unoperated and in skin-rotated frogs. Sensory nerves of the trunk skin terminate in an oval-shaped area which probably corresponds to laminae 1-3 (or 1-4) in the mammalian spinal cord. One type of terminating collateral had a restricted terminal field either in the medial, or in the lateral, part of the oval-shaped area. Another type ended with a large terminal arbour covering almost the total extent of the oval shaped area. These terminal arbours were found for both the dorsal and the ventral cutaneous nerves. Dorsal cutaneous nerves projected preferentially to the lateral part, while ventral cutaneous nerves projected to the medial part, but both nerves sent a significant contingent of collaterals to the whole extent of the oval-shaped area. A rearrangement of the central projection patterns of the dorsal cutaneous nerves, consistently mediating misdirected responses, was not observed in skin-rotated frogs. Collaterals of trunk skin nerves terminated exclusively in the thoracic segments; they did not enter the limb-innervating (brachial and lumbosacral) segments of the spinal cord. The results corroborate the crossed pattern of sensory input in the dermatomal projection. The large, extensively overlapping, areas of termination of individual skin nerves suggest that wiping responses mediated by these nerves should be regarded as complex behaviour released from a preprogrammed centre rather than a reflex arc in which the presence specificity would be determined by wiring specificity.

Peripheral reinnervation patterns and dorsal root ganglion cell topography in skin-grafted Rana pipiens frogs

Behavioral responses to mechanical stimulation of the skin were observed in unoperated, 180 degrees skin-rotated and sham-grafted Rana pipiens frogs before and after crushing various cutaneous nerve bundles. In the two control groups, wiping responses directed towards the animal's dorsum were mediated solely via dorsomedial (DM) and dorsolateral (DL) nerve trunks. In skin-rotated frogs, DM and DL nerve trunks were responsible for almost all of the misdirected responses elicited from dorsal cutaneous areas. Six frogs (2 sham- and 4 skin-rotated) possessed some areas of plical skin from which responses could only have been mediated via more ventrally located nerve trunks. However, such dorsal expansions of ventral receptive fields never included the midline skin areas from which misdirected responses had been elicited in the intact animal. The topographic arrangement of cells within the DRG appears to be weakly polarized in cobalt-stained sections, but with large individual variations in size and observed number of neuronal profiles. For the skin-rotated group the ratio of cobalt-filled profiles in the dorsal as compared with the ventral half of the ganglion was 1.6, which did not differ significantly from the control value of 1.9. Taken as a whole, the results of the present study indicate that selective (re)innervation of displaced skin by the original nerve fibers cannot explain the origin of misdirected responses in Rana pipiens.

Peripheral reinnervation patterns and dorsal root ganglion topography in skin-grafted frogs: a behavioral and histological examination

Cutaneous respective fields for the first ventrolateral (VL1) and dorsomedial nerve trunks were mapped out behaviourally in normal and in skin-grafted Discoglossus pictus frogs. Ventrally directed wiping responses were obtained from stimulation of dorsally located belly skin grafts in all experimental animals. These misdirected responses were abolished completely by cutting either the dorsomedial nerve trunk or both the dorsomedial and the VL1 nerve trunks. The topography of cutaneous neurons of the dorsal root ganglion innervating dorsal skin areas was demonstrated by back-filling the dorsomedial nerve trunk with cobalt. In both normal and skin-grafted frogs, twice as many cobalt-filled neurons were observed in the dorsal half of the ganglion as in the ventral half although the absolute number of filled cells in different regions was the same in both groups. It was concluded that misdirected responses following skin rotation cannot be explained on the basis of a re-establishment of the original peripheral nerve connections.