Supradural inflammatory soup in awake and freely moving rats induces facial allodynia that is blocked by putative immune modulators.

Facial allodynia is a migraine symptom that is generally considered to represent a pivotal point in migraine progression. Treatment before development of facial allodynia tends to be more successful than treatment afterwards. As such, understanding the underlying mechanisms of facial allodynia may lead to a better understanding of the mechanisms underlying migraine. Migraine facial allodynia is modeled by applying inflammatory soup (histamine, bradykinin, serotonin, prostaglandin E2) over the dura. Whether glial and/or immune activation contributes to such pain is unknown. Here we tested if trigeminal nucleus caudalis (Sp5C) glial and/or immune cells are activated following supradural inflammatory soup, and if putative glial/immune inhibitors suppress the consequent facial allodynia. Inflammatory soup was administered via bilateral indwelling supradural catheters in freely moving rats, inducing robust and reliable facial allodynia. Gene expression for microglial/macrophage activation markers, interleukin-1ß, and tumor necrosis factor-α increased following inflammatory soup along with robust expression of facial allodynia. This provided the basis for pursuing studies of the behavioral effects of 3 diverse immunomodulatory drugs on facial allodynia. Pretreatment with either of two compounds broadly used as putative glial/immune inhibitors (minocycline, ibudilast) prevented the development of facial allodynia, as did treatment after supradural inflammatory soup but prior to the expression of facial allodynia. Lastly, the toll-like receptor 4 (TLR4) antagonist (+)-naltrexone likewise blocked development of facial allodynia after supradural inflammatory soup. Taken together, these exploratory data support that activated glia and/or immune cells may drive the development of facial allodynia in response to supradural inflammatory soup in unanesthetized male rats.

Modulatory effects of probenecid on the nitroglycerin-induced changes in the rat caudal trigeminal nucleus.

Four hours after systemic administration of the nitric oxide donor nitroglycerin (10 mg/kg bodyweight, s.c.), the neurons of the rat caudal trigeminal nucleus are activated, the area covered by calcitonin gene-related peptide (CGRP)-immunoreactive fibres is decreased and the neuronal nitric oxide synthase (nNOS)- and the calmodulin-dependent protein kinase II alpha (CamKIIalpha)-immunopositive neurons in the same area are increased. Probenecid is a non-selective inhibitor of multidrug-resistance associated proteins and organic anion transporters thus it can modulate the transport functions in the central nervous system influencing nociception. Accordingly, the aim of the present experiments was to examine the effects of probenecid administration on the nitroglycerin-induced expressions of nNOS, CamKIIalpha and CGRP in the rat caudal trigeminal nucleus. Probenecid (200 mg/kg bodyweight, i.p.) pretreatment proved to mitigate the nitroglycerin-induced changes in expression in the rat caudal trigeminal nucleus. The data suggest that the changes caused by nitroglycerin in the expressions of CGRP, nNOS and CamKIIalpha can be influenced by probenecid modulating the inflammatory functions in the nervous system. These data may be of relevance for the pathogenesis of migraine headache.

Postnatal development of substance P-immunoreaction in the trigeminal caudalis of neonatally capsaicin-treated mice.

The trigeminal subnucleus caudalis (Vc) is a critical relay site for processing nociceptive afferent input from the orofacial area in addition to its modulation by neuroplastic change. Although an administration of capsaicin in neonates induces a selective destruction of substance P (SP)-immunoreactive nerve fibers, little information is available regarding its detailed effects on the Vc, particularly during postnatal development. The present study examined postnatal changes in the distribution of SP in the Vc and trigeminal ganglion (TG) by immunohistochemical techniques in naïve (NV) and neonatally capsaicin-treated (CP) mice, combined with a quantitative analysis. The neonatal mice received a single subcutaneous injection of capsaicin (50 mg/kg) at 48 hours after birth. The neural density of the SP-immunoreaction decreased to approximately a quarter of that in 1-week-old NV mice but increased to three-quarters of that in the NV in the superficial area after postnatal week 2. A double staining with SP and myelin basic protein confirmed the absence of any SP-immunoreaction in the myelinated nerve fibers in both NV and CP mice. The SP-immunoreaction never overlapped with non-peptidergic IB4-labeled neurons in the Vc and TG of either group. Neither the size distribution of SP-positive neurons nor their relative ratio in the TG differed between NV and CP mice at the ages of postnatal weeks 1 and 8. These findings indicate two putative origins for the emergent SP-immunoreaction in the superficial layer of the Vc of the CP mice: the surviving trigeminal neurons with SP against capsaicin treatment and/or intrinsic neurons/interneurons in the Vc without SP under normal conditions.

Substance P receptor-expressing neurons in the medullary and spinal dorsal horns projecting to the nucleus of the solitary tract in the rat.

By using substance P receptor (SPR) immunofluorescence histochemistry combined with fluorescent retrograde labeling, we examined the distribution of the trigeminal and spinal neurons with SPR-like immunoreactivity (-LI) projecting to the nucleus of the solitary tract in the rat. After injection of Fluoro-Gold (FG) into the nucleus of the solitary tract, FG-labeled neurons showing SPR-LI were mainly seen in lamina I of the medullary and spinal dorsal horns, lamina V and the lateral spinal nucleus of the spinal cord. The present results suggest that the trigeminal and spinal neurons with SPR-LI, especially those in lamina I may be involved in the transmission of somatic and/or visceral nociceptive information from the medullary and spinal dorsal horns to the nucleus of the solitary tract.