Convergent innervations of mesencephalic trigeminal and vestibular nuclei neurons onto oculomotor and pre-oculomotor neurons-Tract tracing and triple labeling in rats.

In studies of vestibulo-ocular reflex (VOR), the horizontal VOR circuit is much clearer than vertical-torsional VOR. The circuit and mechanism of gravity-related vertical-torsional VOR is probably weak. "Somatosensory vestibular interaction" is a known extra source to facilitate VOR, and cervico-ocular reflex is a representative for torsional VOR compensation. Whereas, how the cervical afferents finally reach the oculomotor system is less documented. Actually, when the head tilts, which generates cervico-ocular reflex, not only the neck muscle is activated, but also the jaw muscle is stretched by gravity dragged mandible and/or tissue-muscle connection between the mandible and clavicle. We have previously identified a projection from the jaw muscle afferent mesencephalic trigeminal nucleus (Vme) neurons to oculomotor nuclei (III/IV) and their premotor neurons in interstitial nucleus of Cajal (INC)-a well-known pre-oculomotor center manipulating vertical-torsional eye movements. We hypothesized that these projections may interact with vestibulo-ocular signals during vertical-torsional VOR, because effects of gravity on jaw muscles and bones has been reported. Thus, we injected different anterograde tracers into the Vme and medial vestibular nucleus (MVN)-the subnuclear area particularly harboring excitatory vestibulo-ocular neurons, and immunostained III/IV motoneurons. Retrograde tracer was injected into the III in the same animals after dual anterograde tracers' injections. Under confocal microscope, we observed the Vme and MVN neuronal endings simultaneously terminated onto the same III/IV motoneurons and the same INC pre-oculomotor neurons. We consider that jaw muscle proprioceptive Vme neurons projecting to the III/IV and INC would sense spindle activity if the jaw muscle is stretched by gravity dragged mandible or connection between mandible and clavicle during head rolling. Therefore, the convergent innervation of the Vme and MVN neurons onto the oculomotor and pre-oculomotor nuclei would be a neuroanatomic substrate for interaction of masticatory proprioception with the vestibulo-ocular signals upon the oculomotor system during vertical-torsional VOR.

Differential Regulation of the Glucocorticoid Receptor in a Rat Model of Inflammatory Pain.

BACKGROUND: Anti-inflammatory corticosteroids are a common treatment for different conditions involving chronic pain and inflammation. Clinically used steroids target the glucocorticoid receptor (GR) for its anti-inflammatory effects. We previously reported that GR in sensory neurons may play central roles in some pain models and that GR immunoreactivity signal in dorsal root ganglia (DRG) decreased after local inflammation of the DRG (a model of low back pain). In the current study, we aimed to determine if similar changes in GR signal also exist in a skin inflammation model, the complete Freund's adjuvant (CFA) model (a model of peripheral inflammatory pain), in which the terminals of the sensory neurons rather than the somata are inflamed. METHODS: A low dose of CFA was injected into the hind paw to establish the peripheral inflammation model in Sprague-Dawley rats of both sexes, as confirmed by measurements of behavior and paw swelling. Immunohistochemical and western blotting techniques were used to determine the expression pattern of the GR in the inflamed hind paw and the DRGs. Plasma corticosterone levels were measured with radioimmunoassay. RESULTS: The immunohistochemical staining revealed that GR is widely expressed in the normal DRG and skin tissues. Paw injection with CFA caused upregulation of the GR in the skin tissue on postinjection day 1, mostly detected in the dermis area. However, paw inflammation significantly reduced the GR signal in the L5 DRG 1 day after the injection. The GR downregulation was still evident 14 days after CFA inflammation. On day 1, western blotting confirmed this downregulation and showed that it could also be observed in the contralateral L5 DRG, as well as in the L2 DRG (a level which does not innervate the paw). Plasma corticosterone levels were elevated in both sexes on day 14 after CFA compared to day 1, suggesting autologous downregulation of the GR by corticosterone may have contributed to the downregulation observed on day 14 but not day 1. CONCLUSIONS: There are distinctive patterns of GR activation under different pain conditions, depending on the anatomical location. The observed downregulation of the GR in sensory neurons may have a significant impact on the use of steroids as treatment in these conditions and on the regulatory effects of endogenous glucocorticoids.

Mineralocorticoid Antagonist Improves Glucocorticoid Receptor Signaling and Dexamethasone Analgesia in an Animal Model of Low Back Pain.

Low back pain, a leading cause of disability, is commonly treated by epidural steroid injections that target the anti-inflammatory glucocorticoid receptor (GR). However, their efficacy has been controversial. All currently used epidural steroids also activate the pro-inflammatory mineralocorticoid receptor (MR) with significant potency. Local inflammation of the dorsal root ganglia (DRG), a rat model of low back pain, was used. This model causes static and dynamic mechanical allodynia, cold allodynia and guarding behavior (a measure of spontaneous pain), and activates the MR, with pro-nociceptive effects. In this study, effects of local Dexamethasone (DEX; a glucocorticoid used in epidural injections), and eplerenone (EPL; a second generation, more selective MR antagonist) applied to the DRG at the time of inflammation were examined. Mechanical and spontaneous pain behaviors were more effectively reduced by the combination of DEX and EPL than by either alone. The combination of steroids was particularly more effective than DEX alone or the model alone (3-fold improvement for mechanical allodynia) at later times (day 14). Immunohistochemical analysis of the GR in the DRG showed that the receptor was expressed in neurons of all size classes, and in non-neuronal cells including satellite glia. The GR immunoreactivity was downregulated by DRG inflammation (48%) starting on day 1, consistent with the reduction of GR (57%) observed by Western blot, when compared to control animals. On day 14, the combination of DEX and EPL resulted in rescue of GR immunoreactivity that was not seen with DEX alone, and was more effective in reducing a marker for satellite glia activation/neuroinflammation. The results suggest that EPL may enhance the effectiveness of clinically used epidural steroid injections, in part by enhancing the availability of the GR. Thus, the glucocorticoid-mineralocorticoid interactions may limit the effectiveness of epidural steroids through the regulation of the GR in the DRG.

Mineralocorticoid Receptor, A Promising Target for Improving Management of Low Back Pain by Epidural Steroid Injections.

AIM OF REVIEW: Low back pain is a major health problem in United States and worldwide. In this review, we aim to show that mineralocorticoid receptor (MR) activation has a critical role in the initiation of immune and inflammatory responses, which in turn can impact the effectiveness of the currently used steroids for epidural injections in low back pain management since most steroids activate MR in addition to the primary target, glucocorticoid receptor (GR). Moreover, we would like to determine some of the benefits of blocking the MR-induced negative effects. Overall, we propose a novel therapeutic approach for low back pain management by using a combination of a MR antagonist and a GR agonist in the epidural injections. METHOD: We will first introduce the societal cost of low back pain and discuss how epidural steroid injections became a popular treatment for this condition. We will then describe several preclinical models used for the study of low back pain conditions and the findings with respect to the role of MR in the development of inflammatory low back pain. RECENT FINDINGS: MR has pro-inflammatory effects in many tissues which can counteract the anti-inflammatory effects induced by GR activation. Blocking MR using the selective MR antagonist eplerenone can reduce pain and sensory neuron excitability in experimental models of low back pain. Moreover, combining the MR antagonist with clinically used steroids is more effective in reducing pain behaviors than using the steroids alone. SUMMARY: MR antagonists are promising candidates to increase the effectiveness of currently used steroids. Since the activation of the MR is evident in preclinical models of low back pain, blocking its deleterious effects can be beneficial in managing inflammatory pain conditions.