The effects of axotomy on neurons and satellite glial cells in mouse trigeminal ganglion

Cellular analysis of S100Beta and fibroblast growth factor-2 in the dorsal root ganglia and sciatic nerve of rodents. focus on paracrine actions of activated satellite cells after axotomy

The role of satellite cells, a type of peripheral glia, in the paracrine mechanisms related to neuronal maintenance and plasticity in the dorsal root ganglia (DRG) needs to be further investigated. This study employed immunohistochemistry and image analysis to investigate basic fibroblast growth factor (bFGF, FGF-2) and S100Beta immunoreactivities in the DRG and sciatic nerve of the rat and mouse. Well-characterized antibodies against bovine (residues 1-24) and rat (residues 1-23) FGF-2 were employed. Furthermore, the state of satellite cell reaction and changes in the FGF-2/S100Beta immunoreactivity were analyzed after axotomy of rat sciatic nerve. Scattered neurons and the majority of the satellite cells of the rat DRG and also Schwann cells of the rat sciatic nerve stained for S100Beta. In the mouse, strong S100Beta was encountered in the majority of sensory neurons and Schwann cells. Moderate FGF-2 (residues 1-24) immunoreactivity was found in scattered small size neurons of the rat DRG. A strong FGF-2 (residues 1-23) immunoreactivity was achieved in the satellite cells of rat DRG. Both FGF-2 antisera showed strong labeling in the mouse DRG sensory neurons. Activated satellite cells of the axotomized DRG possessed increased amount of FGF-2 and S100Beta immunoreactivity as demonstrated by quantitative image analysis. The proximal stump of the lesioned rat sciatic nerve showed increased FGF-2 (residues 1-24 and 1-23) in the Schwann cells, myelin sheaths, and neuronal fibers, without changes in the level of S100Beta immunoreactivity. Results suggested a possible interaction between FGF-2 and S100Beta in activated satellite cells of the DRG, which might trigger paracrine actions in the axotomized sensory neurons.

Purinoceptor-mediated calcium signaling in primary neuron-glia trigeminal cultures

Receptors for extracellular nucleotides (the P2X-calcium channels and the phospholipase C-coupled P2Y receptors) play key roles in pain signaling, but little is known on their function in trigeminal ganglia, whose hyperactivation leads to the development of migraine pain. Here we characterize calcium signaling via P2X(3) and P2Y receptors in primary mouse neuron-glia trigeminal cultures. Comparison with intact ganglion showed that, in dissociated cultures, sensory neurons retain, at least in part, their physical relationships with satellite glia. RT-PCR indicated expression of P2X(2)/P2X(3) (confirmed by immunocytochemistry) and of all cloned P2Y receptors. Single-cell calcium imaging with subtype-selective P2-agonists/antagonists revealed presence of functional neuronal P2X(3), as well as of ADP-sensitive P2Y(1,12,13) and UTP-activated P2Y(2)/P2Y(4) receptors on both neurons and glia. Calcium responses were much higher in glia, that also responded to UDP, suggesting functional P2Y(6) receptors. To study whether trigeminal ganglia P2 receptors are modulated upon treatment with pro-inflammatory agents, cultures were acutely (up to 3 min) or chronically (24 h) exposed to bradykinin. This resulted in potentiation of algogenic P2X(3) receptor-mediated calcium responses followed by their down-regulation at 24 h. At this exposure time, P2Y receptors responses in satellite glia were instead upregulated, suggesting a complex modulation of P2 receptors in pain signaling.

"Listening" and "talking" to neurons: implications of immune activation for pain control and increasing the efficacy of opioids

It is recently become clear that activated immune cells and immune-like glial cells can dramatically alter neuronal function. By increasing neuronal excitability, these non-neuronal cells are now implicated in the creation and maintenance of pathological pain, such as occurs in response to peripheral nerve injury. Such effects are exerted at multiple sites along the pain pathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition, activated glial cells are now recognized as disrupting the pain suppressive effects of opioid drugs and contributing to opioid tolerance and opioid dependence/withdrawal. While this review focuses on regulation of pain and opioid actions, such immune-neuronal interactions are broad in their implications. Such changes in neuronal function would be expected to occur wherever immune-derived substances come in close contact with neurons.

Primary sensory neuron addition in the adult rat trigeminal ganglion: evidence for neural crest glio-neuronal precursor maturation

It is debated whether primary sensory neurons of the dorsal root ganglia increase the number in adult animals and, if so, whether the increase is attributable to postnatal neurogenesis or maturation of dormant, postmitotic precursors. Similar studies are lacking in the trigeminal ganglion (TG). Here we demonstrate by stereological methods that the number of neurons in the TG of adult male rats nearly doubles between the third and eighth months of age. The increase is mainly attributable to the addition of small, B-type neurons, with a smaller contribution of large, A-neurons. We looked for possible proliferative or maturation mechanisms that could explain this dramatic postnatal expansion in neuron number, using bromodeoxyuridine (BrdU) labeling, immunocytochemistry for neural precursor cell antigens, retrograde tracing identification of peripherally projecting neurons, and in vitro isolation of precursor cells from adult TG explant cultures. Cell proliferation identified months after an extended BrdU administration was sparse and essentially corresponded to glial cells. No BrdU-labeled cell took up the peripherally injected tracer, and only a negligible number coexpressed BrdU and the pan-neuronal tracer neuron-specific enolase. In contrast, a population of cells not recognizable as mature neurons in the TG and neighboring nerve expressed neuronal precursor antigens, and neural crest glioneuronal precursor cells were successfully isolated from adult TG explants. Our data suggest that a protracted maturation process persists in the TG that can be responsible for the neuronal addition found in the adult rat.

Satellite glial cells in sensory ganglia: their possible contribution to inflammatory pain

Neurons in dorsal root ganglia (DRG) are surrounded by an envelope of satellite glial cells (SGCs). Little is known about SGC physiology and their interactions with neurons. In this work, we investigated changes in mouse DRG neurons and SGC following the induction of inflammation in the hind paw by the injection of complete Freund's adjuvant (CFA). The electrophysiological properties of neurons were characterized by intracellular electrodes. Changes in coupling mediated by gap junctions between SGCs were monitored using intracellular injection of the fluorescent dye Lucifer yellow. Pain was assessed with von Frey hairs. We found that two weeks after CFA injection there was a 38% decrease in the threshold for firing an action potential in DRG neurons, consistent with neuronal hyperexcitability. Injection of Lucifer yellow into SGCs revealed that, compared with controls, coupling by gap junctions among SGCs surrounding adjacent neurons increased 2.7-, 3.2-, and 2.5-fold one week, two weeks, and one month, respectively, after CFA injection. In SGCs enveloping neurons that project into the inflamed paw this effect was more enhanced (5.4-fold). Interneuronal coupling was augmented by up to 7% after CFA injection. Pain threshold in the injected paw decreased by 13%, 16%, and 11% compared with controls at one week, two weeks, and one month, respectively, after CFA injection. Intraperitoneal injection of the gap junction blocker carbenoxolone prevented the inflammation-induced decrease in pain threshold. The results show that augmented glial coupling is one of the major events occurring in DRG following inflammation. The elevation in pain threshold after carbenoxolone administration provides indirect support for the idea that augmented intercellular coupling might contribute to chronic pain.

Enhanced excitability of nociceptive trigeminal ganglion neurons by satellite glial cytokine following peripheral inflammation

Peripheral nerve injury activates satellite cells to produce interleukin 1beta (IL-1beta) which mediates inflammation and hyperalgesia. This study investigated the hypothesis that activation of satellite glial cells modulates the excitability of trigeminal ganglion (TRG) neurons via IL-1beta following inflammation. Inflammation was induced by injection of complete Freund's adjuvant (CFA) into the whisker pad area. The threshold for escape from mechanical stimulation applied to the whisker pad in inflamed rats was significantly lower than that in control. Two days post-CFA injection, the mean percentage of TRG neurons encircled by glial fibrillary acidic protein (GFAP)-/IL-1beta-immunoreactive cells was significantly increased compared to controls. GFAP and IL-1beta immunoreactivities were coexpressed in the same cells. Fluorogold (FG) labeling identified the site of inflammation. The number of FG-labeled IL-receptor type I (IL-1RI) TRG neurons in inflamed rats was significantly greater than in controls. In FG-labeled small TRG neurons, the size of IL-1beta (1 nM) induced-depolarization in inflamed rats was larger than in controls. IL-1beta application significantly increased firing rates evoked by depolarizing pulses in the neurons of inflamed rats, compared to controls. The response to IL-1beta was abolished by treatment with the IL-1RI antagonist. These results suggest that activation of satellite glial cells modulates the excitability of small-diameter TRG neurons via IL-1beta following inflammation, and that the upregulation of IL-1RI in the soma may contribute to the mechanism underlying inflammatory hyperalgesia. Therefore IL-1beta blockers are potential therapeutic agents for prevention of trigeminal hyperalgesia.

Aging is associated with an increase in dye coupling and in gap junction number in satellite glial cells of murine dorsal root ganglia

Glial cells in both central and peripheral nervous systems are connected by gap junctions, which allow electrical and metabolic coupling between them. In spite of the great current interest in aging of the nervous system, the effect of aging on glial cell coupling received little attention. We examined coupling between satellite glial cells in murine dorsal root ganglia using the dye coupling technique and electron microscopy. We studied mice at ages of postnatal 90-730 days. Dye coupling incidence between satellite glial cells associated with a single neuron increased from 24.2% at postnatal day 90 to 50.5% at postnatal day 730. Dye coupling between satellite glial cells that are in contact with two or more neurons increased from 2.7% at postnatal day 90 to 18.6% at postnatal day 730 (P<0.05). Examination of the ganglia with the electron microscope showed that the number of gap junctions per 100 microm2 of surface area of satellite glial cells increased from 0.22 at postnatal day 90 to 1.56 at postnatal day 730 (P<0.01). The mean length of individual gap junctions did not change with age. These results provide strong evidence for an increase of functional coupling between satellite glial cells during life. This increase is apparently due to an increase in the total area of the system of gap junctions connecting these cells.

Morphological and electrophysiological changes in mouse dorsal root ganglia after partial colonic obstruction

There is evidence that sensitization of neurons in dorsal root ganglia (DRG) may contribute to pain induced by intestinal injury. We hypothesized that obstruction-induced pain is related to changes in DRG neurons and satellite glial cells (SGCs). In this study, partial colonic obstruction was induced by ligation. The neurons projecting to the colon were traced by an injection of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate into the colon wall. The electrophysiological properties of DRG neurons were determined using intracellular electrodes. Dye coupling was examined with an intracellular injection of Lucifer yellow (LY). Morphological changes in the colon and DRG were examined. Pain was assessed with von Frey hairs. Partial colonic obstruction caused the following changes. First, coupling between SGCs enveloping different neurons increased 18-fold when LY was injected into SGCs near neurons projecting to the colon. Second, neurons were not coupled to other neurons or SGCs. Third, the firing threshold of neurons projecting to the colon decreased by more than 40% (P < 0.01), and the resting potential was more positive by 4-6 mV (P < 0.05). Finally, the number of neurons displaying spontaneous spikes increased eightfold, and the number of neurons with subthreshold voltage oscillations increased over threefold. These changes are consistent with augmented neuronal excitability. The pain threshold to abdominal stimulation decreased by 70.2%. Inflammatory responses were found in the colon wall. We conclude that obstruction increased neuronal excitability, which is likely to be a major factor in the pain behavior observed. The augmented dye coupling between glial cells may contribute to the neuronal hyperexcitability.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

Dye coupling among satellite glial cells in mammalian dorsal root ganglia

Dorsal root ganglia (DRG) are key elements in sensory signaling under physiological and pathological conditions. Little is known about electrical coupling among cells in these ganglia. In this study, we injected the fluorescent dye Lucifer yellow (LY) into single cells to examine dye coupling in DRG. We found no dye coupling between neurons or between neurons and their attendant satellite glial cells (SGCs). In mouse DRG, we observed that in 26.2% of the cases SGCs that surround a given neuron were dye coupled. In only 3.2% of the cases SGCs that make envelopes around different neurons were coupled. The data from mouse ganglia were very similar to those from rat and guinea pig DRG. The results obtained by injection of the tracer biocytin were very similar to those observed with LY. The coupling incidence within the envelopes increased 3.1-fold by high extracellular pH (8.0), but coupling between envelopes was not affected. Acidic pH (6.8) reduced the coupling. High extracellular K+ (9.4 mM) increased the coupling 2.4-fold and 4.7-fold within and between envelopes, respectively. Low extracellular Ca2+ (0.5, 1.0 mM) partly reversed the effect of high K+ on coupling. The results showed that SGCs in mammalian sensory ganglia are connected by gap junctions. This coupling is very sensitive to changes in pH, and can therefore be modulated under various physiological and pathological conditions. The dependence of the coupling on extracellular K+ and Ca2+ suggests that the permeability of gap junctions can be altered by physiological and pharmacological stimuli.