Transmission Of Nociceptive Stimuli Research Articles

Estrogen receptor-α expression in nociceptive-responsive neurons in the medullary dorsal horn of the female rat

Estrogens exert a substantial influence on the transmission of nociceptive stimuli and the susceptibility to pain disorders as made evident by studies in both animals and human subjects. The estrogen receptor (ER) seems to be of crucial importance to the cellular mechanisms underlying such an influence. However, it has not been clarified whether nociceptive neurons activated by pain express ERs. In this study, a noxious injection of formalin was given into the lower lip of female rats, thereby activating nociceptive neurons in the trigeminal subnucleus caudalis as demonstrated by immunohistochemical labeling of Fos. Using a dual-label immunohistochemistry protocol ERα-containing cells were visualized in the same sections. In the superficial layers of the medullary dorsal horn, 12% of ERα-labeled cells, mainly located in lamina II, also expressed noxious-induced Fos. These findings show that nociceptive-responsive neurons in the medullary dorsal horn express ERα, thus providing a possible morphological basis for the hypothesis that estrogens directly regulate pain transmission at this level.

Fatty acid amide hydrolase inhibition enhances the anti-allodynic actions of endocannabinoids in a model of acute pain adapted for the mouse

Cannabinoid ligands have been shown to be anti-nociceptive in animal models of acute and chronic pain by acting at the two known cannabinoid receptors, cannabinoid-1 receptor (CB-1) and cannabinoid-2 receptor (CB-2). A major concern with the use of cannabinoids for pain relief is that they activate receptors at sites other than those involved in the transmission of nociceptive stimuli. An alternative approach is to target the naturally occurring endocannabinoids, such as anandamide (AEA), 2-arachidonylglycerol (2-AG) and N-arachidonylglycine (N-AG). However in vivo results obtained with these compounds appear to be weak, most probably due to their rapid degradation and subsequent short half-life. The predominant enzyme responsible for the hydrolysis of anandamide (and some other endocannabinoids) in the brain is fatty acid amide hydrolase (FAAH). Recently, the α-ketoheterocycle OL135 has been synthesized and shown to be a highly potent and selective inhibitor of FAAH with efficacy in pain models in vivo. In the present study, we have adapted the mild thermal injury (MTI) model of acute pain for the mouse and pharmacologically characterized this model by showing significant reversal of the tactile allodynia by morphine (3, 5 and 10 mg kg−1 s.c.), gabapentin (100 and 300 mg kg−1 i.p.), ibuprofen (100 mg kg−1 i.p.) and OL135 (10, 30 and 100 mg kg−1 i.p.). Furthermore we have demonstrated, using this model, that a subtherapeutic dose of OL135 can enable the endocannabinoids AEA and 2-AG, but not N-AG to be active at doses where they are otherwise nonanalgesic (20 mg kg−1 i.p.). The implications of this model in the study of pain in mice, and the therapeutic potential of FAAH inhibition to provide analgesia without the undesirable side effects of direct agonism of cannabinoid receptors are discussed.

Effect of Bradykinin Receptor Antagonists on Vincristine- and Streptozotocin-Induced Hyperalgesia in a Rat Model of Chemotherapy-Induced and Diabetic Neuropathy

The role of bradykinin receptor blockade in the development of neuropathies caused by diabetes mellitus and vincristine was examined. The effects of a potent and selective B₁ receptor antagonist (des-Arg¹⁰-HOE 140) as well as a specific antagonist of B₂ receptors (HOE 140) were investigated. Both agents significantly decreased hyperalgesia caused otherwise by vincristine. In a diabetic neuropathy model, both agents almost completely suppressed hyperalgesia in the first 10 days of the study. However, from day 11 after administration of streptozotocin, the action of des-Arg¹⁰-HOE 140 was significantly weaker than that of HOE 140. The results of the study suggest involvement of both B₁ and B₂ receptors in transmission of nociceptive stimuli in the vincristine-induced as well as diabetic neuropathy model.

Anatomical distribution and biochemical characterization of the novel RFamide peptide 26RFa in the human hypothalamus and spinal cord

26RFa is a novel RFamide peptide originally isolated in the amphibian brain. The 26RFa precursor has been subsequently characterized in various mammalian species but, until now, the anatomical distribution and the molecular forms of 26RFa produced in the CNS of mammals, in particular in human, are unknown. In the present study, we have investigated the localization and the biochemical characteristics of 26RFa-like immunoreactivity (LI) in two regions of the human CNS--the hypothalamus and the spinal cord. Immunohistochemical labeling using specific antibodies against human 26RFa and in situ hybridization histochemistry revealed that in the human hypothalamus 26RFa-expressing neurons are located in the paraventricular and ventromedial nuclei. In the spinal cord, 26RFa-expressing neurons were observed in the dorsal and lateral horns. Characterization of 26RFa-related peptides showed that two distinct molecular forms of 26RFa are present in the human hypothalamus and spinal cord, i.e. 26RFa and an N-terminally elongated form of 43 amino acids designated 43RFa. These data provide the first evidence that 26RFa and 43RFa are actually produced in the human CNS. The distribution of 26RF-LI suggests that 26RFa and/or 43RFa may modulate feeding, sexual behavior and transmission of nociceptive stimuli.

Ziconotide: A new option for refractory pain

Ziconotide has been introduced as a new nonopioid treatment for chronic pain. Structurally, it is a peptide, the synthetic analog of the omega-conotoxin, derived from the marine snail, Conus magus. N-type voltage-sensitive calcium channels play a role in the transmission of nociceptive stimuli and also are involved in the release of neurotransmitters important in pain transmission. Ziconotide's therapeutic benefit derives from its potent and selective blockade of neuronal-type voltage-sensitive calcium channels. Blockade of the channels results in suppression of abnormal ectopic discharges from the injury site or the dorsal root ganglia, possibly resulting in decreased neuroplasticity, and decreased synaptic transmission that leads to the generation of chronic pain syndromes. The advantage of ziconotide is that tolerance does not occur, while disadvantages associated with ziconotide are the need for intrathecal administration and significant neurotoxicites associated with its use. When tested in clinical trials, ziconotide has been shown to have synergistic or additive value to the effect of morphine. Ziconotide, formerly known also as SNX- 111, represents a new class of agents, the N-type calcium channel blockers. These may represent another option for patients with refractory pain and refractory pain syndromes.

Voltage Gated Calcium Channels as Targets for Analgesics

Management of pain is an essential aspect of medicine; however, current therapies are frequently insufficient owing to severe side effects or limited effectiveness. Therefore, the discovery of new analgesics is needed, especially to treat the proportion of painful patients poorly improved by available analgesics. The transmission of nociceptive stimuli in primary afferent neurons critically depends on a peculiar repertoire of various types of ion channels such as a number of TRP channels, persistent sodium channels, inwardly rectifying potassium channels and voltage-gated calcium channels that either detect noxious stimuli, or regulate cellular excitability and synaptic transmission. Moreover, some of these channels are redistributed and upregulated in pathological states leading to abnormal detection or transmission of harmful stimuli, and consequently lead to states of chronic pain. Hence, these channels are considered key targets for the development of analgesics. The nervous system expresses multiple types of calcium channels with specialized roles in neurophysiology. Here, we review the role of these channels and their accessory subunits in nociceptive signaling, and their potential as targets for development of innovative analgesics.

Isolation, characterization, and distribution of a novel neuropeptide, Rana RFamide (R-RFa), in the brain of the European green frog Rana esculenta.

A novel neuropeptide of the RFamide peptide family was isolated in pure form from a frog (Rana esculenta) brain extract by using reversed-phase high performance liquid chromatography in combination with a radioimmunoassay for mammalian neuropeptide FF (NPFF). The primary structure of the peptide was established as Ser-Leu-Lys- Pro-Ala-Ala-Asn-Leu-Pro-Leu- Arg-Phe-NH(2). The sequence of this neuropeptide, designated Rana RFamide (R-RFa), exhibits substantial similarities with those of avian LPLRFamide, gonadotropin-inhibitory hormone, and human RFRP-1. The distribution of R-RFa was investigated in the frog central nervous system by using an antiserum directed against bovine NPFF. In the brain, immunoreactive cell bodies were primarily located in the hypothalamus, i.e., the anterior preoptic area, the suprachiasmatic nucleus, and the dorsal and ventral hypothalamic nuclei. The most abundant population of R-RFa-containing neurons was found in the periependymal region of the suprachiasmatic nucleus. R-RFa- containing fibers were widely distributed throughout the brain from the olfactory bulb to the brainstem, and were particularly abundant in the external layer of the median eminence. In the spinal cord, scattered immunoreactive neurons were found in the gray matter. R-RFa-positive processes were found in all regions of the spinal cord, but they were more abundant in the dorsal horn. This study provides the first characterization of a member of the RFamide peptide family in amphibians. The occurrence of this novel neuropeptide in the hypothalamus and median eminence and in the dorsal region of the spinal cord suggests that, in frog, R-RFa may exert neuroendocrine activities and/or may be involved in the transmission of nociceptive stimuli.

Calcium-binding protein-immunoreactive projection neurons in the caudal subnucleus of the spinal trigeminal nucleus of the rat.

It has been reported that calcium-binding proteins are good markers for different sets of neurons in various brain regions. We examined expression of the main calcium-binding proteins in projection neurons in the rat medullary dorsal horn (MDH) by combining immunofluorescence histochemistry for calbindin D28k (CB), calretinin (CR) and parvalbumin (PV) with the retrograde tract-tracing method. A fluorescence tracer, tetramethylrhodamine-dextran amine (TMR-DA), was injected into the parabrachial, thalamic or hypothalamic region. After such injections, a number of PV-, CR-, and/or CB-immunoreactive MDH neurons were labeled retrogradely with TMR-DA. Triple-immunofluorescence histochemistry further revealed that a number of CB-, CR-, or PV-immunoreactive TMR-DA-labeled MDH neurons showed immunoreactivity for substance P receptor (NK1), and that they expressed immunoreactivity for c-fos protein in the rats which were injected with formalin into the lips. Thus, it was indicated that some of CB-, CR-, or PV-containing projection neurons in the MDH might be involved in the transmission of nociceptive stimuli.

Central versus peripheral site of action of the tachykinin NK1-antagonist RP 67580 in inhibiting chemonociception.

1. Many studies indicate an involvement of substance P in the transmission of nociceptive stimuli, without, however, presenting any conclusive evidence as to its exact site and mode of action. The present experiments tested the involvement of substance P in the mediation of chemical nociception using the non-peptidic specific tachykinin NK1-receptor antagonist, RP 67580 (2-[1-imino-2-(2-methoxyphenyl-ethyl]-7,7diphenyl-4-perhydroiso indolone (3aR, 7aR)). 2. Mean arterial pressure (MAP) and intragastric pressure (IGP) were measured in anaesthetized rats. The reflex changes of these parameters in response to i.p. or s.c. injections of hydrochloric acid or capsaicin were taken to indicate nociception. 3. Intravenous administration of RP 67580 up to 5 mg kg-1 had little influence on the reflex changes in MAP or IGP in response to hydrochloric acid or capsaicin. In contrast, the sensitization of rats to i.p. capsaicin by preinjection of prostaglandin E2 was significantly reduced by 1 mg kg-1 RP 67580. 4. Intrathecal injection of 5 micrograms RP 67580 inhibited the reflex changes of MAP and IGP in response to i.p. or s.c. capsaicin whereas the inactive enantiomer RP 68651 was ineffective. 5. The results indicate that spinal NK1-receptors are involved in the acute transmission of chemically induced pain, while such receptors in the periphery take part in the sensitization by prostaglandin E2. The rather minor ability of i.v. RP 67580 to inhibit the acute nociceptive reflex is attributed to an insufficient penetration of the blood-brain-barrier.

Electron microscopic localization of nerve growth factor receptor (p75)-immunoreactivity in pars caudalis / medullary dorsal horn of the cat

Previous studies have demonstrated the presence of nerve growth factor receptor [NGFr(p75)]-immunoreactivity (IR) in the spinal trigeminal nucleus of both 8–10 week-old kittens and mature cats. Most of the NGFr(p75)-IR is lost following retrogasserian rhizotomy, indicating that the majority of the NGFr(p75)-IR within the spinal trigeminal nucleus is of trigeminal primary afferent origin. Here, we examined the ultrastructural localization of NGFr(p75)-IR within lamina II outer of pars caudalis/medullary dorsal horn in the mature cat. Lamina II outer represents a location where dense NGFr(p75)-IR is seen with the light microscope. The NGFr(p75)-IR identified with the electron microscope was located within small thinly myelinated and unmyelinated axons and within axon terminals. The terminals with NGFr(p75)-IR typically formed asymmetric synaptic specializations onto dendritic profiles and at times were postsynaptic to other axon terminals at symmetric synaptic specializations. The terminals with NGFr(p75)-IR were either simple (associated with a single profile) or more complex, such as those that typically formed the central element in synaptic glomeruli. The NGFr(p75)-IR in terminals was especially prominent on microtubules and the plasmalemma and these findings are consistent with proposed roles for NGFr(p75) in axoplasmic/neuronal transport and as a membrane protein, respectively. The profiles with NGFr(p75)-IR seen with the electron microscope indicate a primary afferent origin and show some similarities when compared to other markers of primary afferent fibers such as calcitonin gene-related peptide. In addition, a possible role for NGFr(p75) in the transmission of nociceptive stimuli is also discussed.