Sites of renal pain processing in the rat spinal cord. A c-fos study using a percutaneous method to perform ureteral obstruction

A Role for Translational Regulation by S6 Kinase and a Downstream Target in Inflammatory Pain

BACKGROUND AND PURPOSE

Translational controls pervade neurobiology. Nociceptors play an integral role in the detection and propagation of pain signals. Nociceptors can undergo persistent changes in their intrinsic excitability. Pharmacologic disruption of nascent protein synthesis diminishes acute and chronic forms of pain-associated behaviors. Yet, the targets of translational controls that facilitate plasticity in nociceptors are unclear.

EXPERIMENTAL APPROACH

We used ribosome profiling to probe the translational landscape in DRG neurons after treatment of the inflammatory mediators NGF and IL-6. We validated the expression dynamics of c-Fos using immunoblotting and immunohistochemistry. Given that inflammation is known to stimulate mTOR signaling, we reasoned that downstream factors (e.g., ribosomal protein S6 kinase 1, S6K1) might control c-Fos levels. We utilized small-molecule inhibitors of S6K1 (DG2) or c-Fos (T-5224) to probe their effects on nociceptor activity in vitro using multi-electrode arrays (MEAs) and pain behavior in vivo using a hyperalgesic priming model.

KEY RESULTS

We demonstrate that c-Fos is expressed in sensory neurons. Inflammatory mediators that promote pain in both humans and rodents promote c-Fos translation. We demonstrate that the mTOR effector S6K1 is essential for c-Fos biosynthesis. Inhibition of S6K1 or c-Fos with small molecules diminish mechanical and thermal hypersensitivity in response to inflammatory cues. Additionally, both inhibitors reduce evoked nociceptor activity.

CONCLUSION

Our data reveal a novel role of S6K1 in modulating rapid response to inflammatory mediators, with c-Fos being one key downstream target. Targeting the S6 kinase pathway or c-Fos is an exciting new avenue for pain-modulating compounds.

Nerve injury-induced tactile allodynia is present in the absence of FOS labeling in retrogradely labeled post-synaptic dorsal column neurons

The dorsal column pathway consists of direct projections from primary afferents and of ascending fibers of the post-synaptic dorsal column (PSDC) cells. This pathway mediates touch but may also mediate allodynia after nerve injury. The role of PSDC neurons in nerve injury-induced mechanical allodynia is unknown. Repetitive gentle, tactile stimulus or noxious pinch was applied to the ipsilateral hindpaw of rats with spinal nerve ligation (SNL) or sham surgery that had previously received tetramethylrhodamine dextran in the ipsilateral n. gracilis. Both touch and noxious stimuli produced marked increases in FOS expression in other cells throughout all laminae of the ipsilateral dorsal horn after nerve injury. However, virtually none of the identified PSDC cells expressed FOS immunofluorescence in response to repetitive touch or pinch in either the nerve-injured or sham groups. In contrast, labeled PSDC cells expressed FOS in response to ureter ligation and labeled spinothalamic tract (STT) cells expressed FOS in response to noxious pinch. Identified PSDC neurons from either sham-operated or SNL rats did not express immunoreactivity to substance P, CGRP, NPY, PKCY, MOR, the NK1 and the NPY-Y1 receptor. Retrogradely labeled DRG cells of nerve injured rats were large diameter neurons, which expressed NPY, but no detectable CGRP or substance P. Spinal nerve injury sensitizes neurons in the spinal dorsal horn to repetitive light touch but PSDC neurons apparently do not participate in touch-evoked allodynia. Sensitization of these non-PSDC neurons may result in activation of projections integral to the spinal/supraspinal processing of enhanced pain states and of descending facilitation, thus priming the central nervous system to interpret tactile stimuli as being aversive.

Fos expression in spinothalamic and postsynaptic dorsal column neurons following noxious visceral and cutaneous stimuli

The spinothalamic tract (STT) has been classically viewed as the major ascending pathway for pain transmission while the dorsal column (DC) was thought to be involved primarily in signaling innocuous stimuli. Recent clinical studies have shown that limited midline myelotomy, which transects fibers in the DC, offers good pain relief in patients with visceral cancer pain. Experimental studies provided evidence that a DC lesion decreases the activation of thalamic neurons by visceral stimuli and suggested that this effect is due to transection of the axons of postsynaptic dorsal column (PSDC) neurons. In our study, Fos protein expression in retrogradely labeled STT and PSDC neurons in the lumbosacral enlargement in rats was used as an anatomical marker of enhanced activation to compare the role of these neurons in cutaneous and visceral pain. The noxious stimuli used were intradermal injection of capsaicin and distention of the ureter. Retrogradely labeled PSDC neurons were found in laminae III-IV and in the vicinity of the central canal. STT neurons were located in laminae I, III-VII and X. Ureter distention evoked Fos expression in PSDC and STT neurons located in all laminae in which retrogradely labeled cells were found, with the maximum in the L(2) spinal segment. The Fos-positive PSDC neurons represented a significantly higher percentage of the retrogradely labeled PSDC neurons (19.3+/-2.3% SEM) than of the STT Fos-positive neurons (13.2+/-1.5% SEM). Intradermal capsaicin injection also evoked Fos expression in both PSDC and STT neurons, but with no significant difference between these two, when expressed as a percentage of the retrogradely labeled cells (11.6+/-2.9% SEM, 10.8+/-1.1% SEM). These results show that both PSDC and STT neurons are activated by cutaneous and visceral noxious stimuli. Their particular role in transmission and modulation of painful stimuli needs to be investigated further.

Vanilloid receptor 1 expression in the rat urinary tract

Previous findings have shown that the capsaicin sensitivity of sensory fibres is due to the expression of a specific membrane protein, the vanilloid receptor type 1 (VR1). In the present work we studied the distribution, morphology and the neurochemical content of nerve fibres expressing this receptor in the rat urinary tract. Immunolabelling was performed against the VR1 and the positive fibres were examined by light and electron microscopy. Colocalisation of VR1 and substance P or calcitonin gene-related peptide immunoreactivities, and isolectin B4 binding, was evaluated under the confocal microscope. In addition, the effect of intravesical administration of resiniferatoxin, an ultra-potent vanilloid receptor agonist, in the receptor expression in the bladder was also studied. Numerous VR1-immunoreactive fibres were found in the mucosa and muscular layer of the entire urinary tract except the kidney. In the bladder, most fibres were also substance P- or calcitonin gene-related peptide-immunoreactive but did not bind isolectin B4. Under the electron microscope VR1 immunoreactivity was confined to unmyelinated axons and varicosities containing small clear and large dense-core synaptic vesicles. They occurred beneath or among epithelial cells or closely apposed to smooth muscle cells. Intravesical resiniferatoxin decreased VR1 immunoreactivity transiently. These data indicate that primary sensory fibres expressing VR1 are extremely abundant in the rat urinary tract and that, in contrast to the skin, they belong almost exclusively to the peptide-containing sub-population of primary afferents. As capsaicin-sensitive bladder afferents are involved in nociception and reflex micturition control, the numerous free terminal nerve endings expressing VR1 in the mucosa seem more adequate to accomplish the former function. However, the close apposition between VR1-expressing fibres and smooth muscle cells suggests that they may also encode the tonus of the muscular layer.

Effects of baclofen on colon inflammation-induced Fos, CGRP and SP expression in spinal cord and brainstem

The present study demonstrates sites of expression for Fos protein in the brainstem and lumbosacral spinal cord of rats subjected to mustard oil irritation of the colon. The protective effect of baclofen, a selective GABA(B) receptor agonist, on the induced Fos protein increases was determined. Mustard oil injected into the lumen of the colon produces an acute site-specific inflammation. Immunocytochemical localization of Fos protein in neuronal nuclei was evident after 1 h, was greatest at 2 h and was still evident but declining at 8 h. In the spinal cord the majority of Fos labeled neurons were localized in the superficial laminae of lumbar (L6) cord with more found in the sacral (S1) cord. Some labeled neurons were also found in the deeper spinal laminae, intermediolateral nucleus and around lamina X. Brainstem sites expressing Fos included the nucleus of the solitary tract in the medulla, parabrachial, locus coeruleus, pontine and caudal dorsal raphe nuclei and periaqueductal gray. Weak Fos protein labeling existed in a few cells in vehicle control animals. Systemic administration of the GABA(B) receptor agonist, baclofen (10 mg/kg, i.p.), significantly reduced Fos expression in the spinal cord after mustard oil treatment but significantly increased the relative number of nuclei labeled in the nucleus of the solitary tract. Baclofen also significantly decreases dorsal horn CGRP immunoreactivity relative to the increased levels seen after inflammation of the colon. The SP content increases observed after inflammation of the colon were not altered by baclofen. These data suggest that: (1) neurons in regions important for nociceptive transmission, descending inhibitory control and autonomic control are activated by noxious stimulation of the colon, and (2) baclofen specifically reduces Fos expression in the superficial dorsal horn of the spinal cord induced by nociceptive afferent input.

Visceral nociception

Visceral pain is of great concern to the medical community because it remains particularly resistant to current clinical treatments. A serendipitous and initially unexplainable clinical finding that a punctate midline dorsal column lesion is effective in eliminating visceral pain, however, has initiated a resurgence of interest in the study of the basic mechanisms of visceral nociception. Clinical and anatomic findings have determined that visceral pain either of thoracic or pelvic origin can be relieved by carefully placed lesions directed at the lateral edge or the medial edge of the gracile fasciculus, respectively. Studies are demonstrating that visceral pain is quite unique from cutaneous pain.