Nerve growth factor increases sensitivity to bradykinin, mediated through B2 receptors, in capsaicin-sensitive small neurons cultured from rat dorsal root ganglia

Involvement of Bradykinin Receptor 2 in Nerve Growth Factor Neuroprotective Activity

Neurotrophin nerve growth factor (NGF) has been demonstrated to upregulate the gene expression of bradykinin receptor 2 (B2R) on sensory neurons, thus facilitating nociceptive signals. The aim of the present study is to investigate the involvement of B2R in the NGF mechanism of action in nonsensory neurons in vitro by using rat mixed cortical primary cultures (CNs) and mouse hippocampal slices, and in vivo in Alzheimer’s disease (AD) transgenic mice (5xFAD) chronically treated with NGF. A significant NGF-mediated upregulation of B2R was demonstrated by microarray, Western blot, and immunofluorescence analysis in CNs, indicating microglial cells as the target of this modulation. The B2R involvement in the NGF mechanism of action was also demonstrated by using a selective B2R antagonist which was able to reverse the neuroprotective effect of NGF in CNs, as revealed by viability assay, and the NGF-induced long-term potentiation (LTP) in hippocampal slices. To confirm in vitro observations, B2R upregulation was observed in 5xFAD mouse brain following chronic intranasal NGF treatment. This study demonstrates for the first time that B2R is a key element in the neuroprotective activity and synaptic plasticity mediated by NGF in brain cells.

Nerve Growth Factor Signaling and Its Contribution to Pain

Nerve growth factor (NGF) is a neurotrophic protein essential for the growth, differentiation, and survival of sympathetic and sensory afferent neurons during development. A substantial body of evidence, based on both animal and human studies, demonstrates that NGF plays a pivotal role in modulation of nociception in adulthood. This has spurred development of a variety of novel analgesics that target the NGF signaling pathway. Here, we present a narrative review designed to summarize how NGF receptor activation and downstream signaling alters nociception through direct sensitization of nociceptors at the site of injury and changes in gene expression in the dorsal root ganglion that collectively increase nociceptive signaling from the periphery to the central nervous system. This review illustrates that NGF has a well-known and multifunctional role in nociceptive processing, although the precise signaling pathways downstream of NGF receptor activation that mediate nociception are complex and not completely understood. Additionally, much of the existing knowledge derives from studies performed in animal models and may not accurately represent the human condition. However, available data establish a role for NGF in the modulation of nociception through effects on the release of inflammatory mediators, nociceptive ion channel/receptor activity, nociceptive gene expression, and local neuronal sprouting. The role of NGF in nociception and the generation and/or maintenance of chronic pain has led to it becoming a novel and attractive target of pain therapeutics for the treatment of chronic pain conditions.

Neuronal hyperexcitability and astrocyte activation in spinal dorsal horn of a dermatitis mouse model with cutaneous hypersensitivity

Cleaning products such as soaps, shampoos, and detergents are comprised mainly of surfactants, agents known to cause dermatitis and cutaneous hypersensitivity characterized by itching, stinging, and burning of the skin and scalp. However, the mechanisms underlying surfactant-induced cutaneous hypersensitivity remain unclear. In the present study, we investigated the mechanisms of cutaneous hypersensitivity in mice treated with the detergent sodium dodecyl sulfate (SDS). Repeated SDS application to the skin induced inflammation, xeroderma, and elongation of peripheral nerves into the epidermis. The number of neurons immunopositive for c-Fos, a well known marker of neural activity, was substantially higher (+441%) in spinal dorsal horn (SDH) lamina I-II (but not lamina III-VI) of SDS-treated mice compared to vehicle-treated mice. In vivo extracellular recording revealed enhanced spontaneous (+64%) and non-noxious mechanical stimulation-evoked firing (+139%) of SDH lamina I-II neurons in SDS-treated mice, and stimulation-evoked neuronal firing was sustained (+5333%) even after stimulation. The number of GFAP-positive (activated) astrocytes, but not Iba1-positive microglia, was also elevated (+137%) in SDH lamina I-II of SDS-treated mice compared to vehicle-treated mice. Peripheral nerve elongation and hyperexcitability of afferent or SDH neurons, possible associated with the activation of spinal astrocytes, may underlie cutaneous hypersensitivity induced by surfactants.

Non-myelinated C-fibers, but not myelinated A-fibers, elongate into the epidermis in dry skin with itch

Chronic skin diseases with itch and dry skin show increased peripheral nerve fiber elongation into the epidermis. However, the characteristics of the elongated nerve fibers remain unclear. Therefore, we investigated the characteristics of the elongated nerve fibers using a dry skin mouse model with itch. In this mouse model, prepared via repetitive treatments with an acetone/ether mixture and water, the stratum corneum water content was decreased, whereas spontaneous scratching and epidermal hyperplasia were increased. In addition, the number of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers (C-fibers) was increased in the epidermis of treated mice compared to that in non-treated control mice. However, neurofilament 200-immunoreactive nerve fibers (A-fibers) were not detected in the epidermis of treated mice. These results suggest that the elongated epidermal peripheral nerve fibers comprise SP/CGRP-containing C-fibers but not A-fibers. Thus, these fibers may be involved in the induction of dry skin pruritus.

Nerve growth factor sensitizes adult sympathetic neurons to the proinflammatory peptide bradykinin

Levels of nerve growth factor (NGF) are elevated in inflamed tissues. In sensory neurons, increases in NGF augment neuronal sensitivity (sensitization) to noxious stimuli. Here, we hypothesized that NGF also sensitizes sympathetic neurons to proinflammatory stimuli. We cultured superior cervical ganglion (SCG) neurons from adult male Sprague Dawley rats with or without added NGF and compared their responsiveness to bradykinin, a proinflammatory peptide. The NGF-cultured neurons exhibited significant depolarization, bursts of action potentials, and Ca(2+) elevations after bradykinin application, whereas neurons cultured without NGF showed only slight changes in membrane potential and cytoplasmic Ca(2+) levels. The NGF effect, which requires trkA receptors, takes hours to develop and days to reverse. We addressed the ionic mechanisms underlying this sensitization. NGF did not alter bradykinin-induced M-current inhibition or phosphatidylinositol 4,5-bisphosphate hydrolysis. Maxi-K channel-mediated current evoked by depolarizations was reduced by 50% by culturing neurons in NGF. Application of iberiotoxin or paxilline, blockers of Maxi-K channels, mimicked NGF treatment and sensitized neurons to bradykinin application. A calcium channel blocker also mimicked NGF treatment. We found that NGF reduces Maxi-K channel opening by decreasing the activity of nifedipine-sensitive calcium channels. In conclusion, culture in NGF reduces the activity of L-type calcium channels, and secondarily, the calcium-sensitive activity of Maxi-K channels, rendering sympathetic neurons electrically hyper-responsive to bradykinin.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Roles of kinin B1 and B2 receptors in skin cancer pain produced by orthotopic melanoma inoculation in mice

BACKGROUND

Although bradykinin is a potent algogenic peptide, the roles of this peptide and kinin receptors in cancer pain are unclear.

AIMS

The present study was conducted to clarify whether kinin B(1) and B(2) receptors would be involved in pain using a mouse model of skin cancer pain.

METHODS

B16-BL6 melanoma cells were inoculated into the hind paw of C57BL/6 mice. Licking, an index of spontaneous pain, allodynia and hyperalgesia were observed. Expression of kinin receptor mRNAs was analyzed with reverse transcription and polymerase chain reaction. The contents of kininogen and bradykinin-related peptides were assayed with Western blotting and enzyme immunoassay, respectively.

RESULTS

Melanoma inoculation induced spontaneous licking of the melanoma-bearing paw from day 18 post-inoculation, which was inhibited by local injections of B(1) and B(2) receptor antagonists. Allodynia was briefly attenuated by B(2), but not B(1) antagonist and hyperalgesia was not inhibited by either B(1) or B(2) antagonist. Local injections of B(1) and B(2) receptor agonists increased licking behavior in melanoma-bearing, but not healthy, paw. The expression of kinin B(1), but not B(2), receptor mRNA was markedly increased in the L4/5 dorsal root ganglia on the melanoma-bearing side. Melanoma cells expressed B(1) and B(2) receptors and kininogen. The content of bradykinin and related peptides was increased in the melanoma mass as compared with healthy skin.

CONCLUSIONS

Bradykinin and related peptides released from melanoma cells may cause spontaneous pain and allodynia in the melanoma-bearing paw, in which B(1) and B(2) receptors on primary afferent and melanoma cells may have different roles.

Excitation and sensitization of nociceptors by bradykinin: what do we know?

Bradykinin is an endogenous nonapeptide known to induce pain and hyperalgesia to heat and mechanical stimulation. Correspondingly, it excites nociceptors in various tissues and sensitizes them to heat, whereas sensitizing effect on the mechanical response of nociceptors is not well established. Protein kinase C and TRPV1 contribute to the sensitizing mechanism of bradykinin to heat. In addition, TRPA1 and other ion channels appear to contribute to excitation caused by bradykinin. Finally, prostaglandins sensitize bradykinin-induced excitation in normal tissues by restoring desensitized responses due to the inhibition of protein kinase A.

Activation of the cannabinoid 2 (CB2) receptor inhibits murine mesenteric afferent nerve activity

Abstract Cannabinoid 2 (CB2) receptors have both antinociceptive and antihypersensitivity effects, although the precise mechanisms of action are still unclear. In this study, the modulatory role of CB2 receptors on the mesenteric afferent response to the endogenous immunogenic agent bradykinin (BK) was investigated. Mesenteric afferent recordings were obtained from anaesthetized wild-type and CB2(-/-) mice using conventional extracellular recording techniques. Control responses to BK were obtained in all experiments prior to administration of either CB2 receptor agonist AM1241, or AM1241 plus the CB2 receptor antagonist AM630. Bradykinin consistently evoked activation of mesenteric afferents (n = 32). AM1241 inhibited the BK response in a dose dependent manner. In the presence of AM630 (10 mg kg(-1)), however, AM1241 (10 mg kg(-)1) had no significant effect on the BK response. Moreover, AM1241 had also no significant effect on the BK response in CB2(-/-) mice. Activation of the CB2 receptor inhibits the BK response in mesenteric afferents, demonstrating that the CB2 receptor is an important regulator of neuroimmune function. This may be a mechanism of action for the antinociceptive and antihypersensitive effects of CB2 receptor agonists.

Phospholipase Cbeta 3 mediates the scratching response activated by the histamine H1 receptor on C-fiber nociceptive neurons

Phospholipase Cbeta (PLCbeta) isozymes represent a family of molecules that link G protein-coupled receptors (GPCRs) to an intracellular signaling network. Here, we investigated the function of PLCbeta isozymes in sensory neurons by using mutant mice deficient for specific PLCbeta family members. Expression analysis indicated that PLCbeta3, one of the four isoforms, is predominantly expressed in a subpopulation of C-fiber nociceptors. A subset of these neurons expressed the histamine H1 receptor. Ca(2+) imaging studies revealed that PLCbeta3 specifically mediates histamine-induced calcium responses through the histamine H1 receptor in cultured sensory neurons. In line with this, we found that PLCbeta3(-/-) mice showed significant defects in scratching behavior induced by histamine; histamine-trifluoromethyl-toluidine (HTMT), a selective H1 agonist; and compound 48/80, a mast cell activator. These results demonstrate that PLCbeta3 is required to mediate "itch" sensation in response to histamine acting on the histamine H1 receptor in C-fiber nociceptive neurons.

Desensitization of cold- and menthol-sensitive rat dorsal root ganglion neurones by inflammatory mediators

The interaction between cold sensitivity and inflammation in mammals is not entirely understood. We have used adult rat dorsal root ganglion neurones in primary culture together with calcium microfluorimetry to assess the effects of selected inflammatory mediators on cold responses of cold- and menthol-sensitive (most likely TRPM8-expressing) neurones. We observed a high degree of functional co-expression of TRPM8, the receptors for the inflammatory agents bradykinin, prostaglandin E2 and histamine, and TRPA1 in cultured sensory neurones. Treatment with either bradykinin or prostaglandin E2 led to a reduction in the amplitude of the response to cooling and shifted the threshold temperature to colder values, and we provide evidence for a role of protein kinases C and A, respectively, in mediating these effects. In both cases the effects were mainly restricted to the subgroups of cold- and menthol-sensitive cells which had responded to the application of the inflammatory agents at basal temperature. This desensitization of cold-sensitive neurones may enhance inflammatory pain by removing the analgesic effects of gentle cooling.

The immunosuppressant FK506 activates capsaicin- and bradykinin-sensitive DRG neurons and cutaneous C-fibers

Immunosuppressant drug FK506, which is widely used for the treatment of atopic dermatitis, has multiple actions on the nervous system. In order to elucidate the mechanisms underlying transient burning sensation elicited by topical application of FK506 to the skin of atopic patients, we investigated if FK506 directly activates sensory neurons and fibers, or not. Ca(2+) imaging study on cultured DRG neurons of rats revealed that application of FK506 raised intracellular Ca(2+) levels in a subpopulation of small DRG neurons (3.1% of DRG neurons responsive to high K(+) solution). When DRGs from inflamed rats were used, the incidence increased to 7.4%. FK506 sensitive neurons also responded to a subsequent application of capsaicin (89.5% in normal, and 100% in inflamed rats) and bradykinin (31.6% in normal, and 80.9% in inflamed rats). Single fiber recordings in the skin-nerve preparation confirmed the results of cell culture study, showing that application of FK506 enhanced neuronal discharges of single C-fibers that are responsive to heat and bradykinin. These findings, taken together, indicate that FK506 application on inflamed skin may activate nociceptive C-fibers, which bear bradykinin receptors and capsaicin-sensitive heat transducer of TRP family, TRPV1.

Noxious Cold Ion Channel TRPA1 Is Activated by Pungent Compounds and Bradykinin

Six members of the mammalian transient receptor potential (TRP) ion channels respond to varied temperature thresholds. The natural compounds capsaicin and menthol activate noxious heat-sensitive TRPV1 and cold-sensitive TRPM8, respectively. The burning and cooling perception of capsaicin and menthol demonstrate that these ion channels mediate thermosensation. We show that, in addition to noxious cold, pungent natural compounds present in cinnamon oil, wintergreen oil, clove oil, mustard oil, and ginger all activate TRPA1 (ANKTM1). Bradykinin, an inflammatory peptide acting through its G protein-coupled receptor, also activates TRPA1. We further show that phospholipase C is an important signaling component for TRPA1 activation. Cinnamaldehyde, the most specific TRPA1 activator, excites a subset of sensory neurons highly enriched in cold-sensitive neurons and elicits nociceptive behavior in mice. Collectively, these data demonstrate that TRPA1 activation elicits a painful sensation and provide a potential molecular model for why noxious cold can paradoxically be perceived as burning pain.

Thermal properties of acid-induced depolarization in cultured rat small primary afferent neurons

Tissue ischemia and inflammation result in localized acidosis, and acidic pH can trigger a sensation of pain. Pain is known to be often modified by the tissue temperature. The purpose of this study is to clarify the thermal behavior of nociceptors in response to acidification using intracellular recordings from cultured rat primary afferent neurons. Extracellular acidification induced depolarization of two types, transient and sustained responses. The former (to pH 6.3 and 5.2) was augmented at lower temperature (26, 16 degrees C) and amiloride blocked the response to pH 6.3 at 26 degrees C. On the other hand, the sustained depolarization, which often followed the transient one, in response to pH 6.3 was greater at 36 degrees C and significantly blocked by capsazepine at 36 degrees C, but not at 26 degrees C. The sustained response to pH 5.2 was blocked even at 26 degrees C. These results suggest that the low pH evoked depolarization is temperature-dependent, and the contribution of transient receptor potential V1 (vanilloid receptor 1) to proton-induced response is greater in the physiological body temperature range, while that of the acid-sensing-ion-channel family is greater at room temperature or lower.

Characterization of bradykinin-induced prostaglandin E2 release from cultured rat trigeminal ganglion neurones

Bradykinin and prostaglandins are both local mediators strongly implicated in pain and inflammation. Here, we have investigated the effects of bradykinin on the release of prostaglandin E(2) from cultured neurones derived from adult rat trigeminal ganglia. Bradykinin was a potent inducer of prostaglandin E(2) release, an effect that was likely mediated by bradykinin B(2) receptors, as the bradykinin-induced prostaglandin E(2) release was attenuated by the bradykinin B(2) receptor-selective antagonist, arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-(2 alpha, 3 beta, 7a beta)-octahydro-1H-indole-2-carbonyl-L-arginine (HOE 140), but not by the bradykinin B(1) receptor-selective antagonist, des-Arg(9),[Leu(8)]-bradykinin. Furthermore, bradykinin-induced prostaglandin E(2) release was inhibited following treatment with the phospholipase A(2) inhibitor, arachidonyltrifluoromethyl ketone (AACOCF(3)), the nonselective cyclooxygenase inhibitor, piroxicam, the mitogen-activated protein kinase kinase-1 (MEK1) inhibitor, 2'-amino-3'-methoxyflavone (PD98059), and the protein kinase C inhibitor, bisindolylmaleimide XI (Ro320432). Taken together, these data suggest that bradykinin, acting via bradykinin B(2) receptors, induces prostaglandin E(2) release from trigeminal neurones through the protein kinase C and mitogen-activated protein kinase-dependent activation of phospholipase A(2) and consequent stimulation of cyclooxygenases.

Bradykinin lowers the threshold temperature for heat activation of vanilloid receptor 1

Bradykinin (BK) is an inflammatory mediator that plays a pivotal role in pain and hyperalgesia to heat in inflamed tissues by exciting nociceptors and sensitizing them to heat through activation of protein kinase C (PKC). It has been suggested that the capsaicin receptor (VR1), a nociceptor-specific cation channel sensitive to noxious heat, protons, and capsaicin, is a channel that is modified by BK in these effects. In this study, we examined how BK modulates the activity of VR1. We measured VR1 currents using the patch-clamp technique in human embryonic kidney-derived (HEK293) cells expressing VR1 and B2 BK receptor. We found that BK lowered the threshold temperature for activation of VR1 currents in HEK cells down to well below the physiological body temperature in a concentration-dependent manner through PKC activation. We also demonstrated that in capsaicin-sensitive dorsal root ganglion (DRG) neurons the activation threshold of heat-induced current, which is considered to be VR-1 mediated, was lowered by BK and that this effect was also mediated by PKC. These data further support the supposition that modulation of VR1 is a mechanism for the BK-induced excitation of nociceptors and their sensitization to heat.

Effects of PGE(2) on neurons from rat dorsal root ganglia in intact and adjuvant-inflamed rats: role of NGF on PGE(2)-induced depolarization

The effects of prostaglandin E(2) (PGE(2)) on primary afferent neurons were studied by intracellular recording from small (<30 microm) dorsal root ganglion (DRG) neurons cultured for up to 3 days. PGE(2) (10(-9)-10(-5) M) depolarized 4-10% of neurons cultured with nerve growth factor (NGF) in intact rats. The percentage of neurons depolarized increased in a concentration dependent manner, while the average amplitude of the depolarization did not change with concentration. The threshold to evoke an action potential was decreased by PGE(2) (10(-9)-10(-5) M) with the maximum percentage at 10(-9) M, and this effect was also observed in neurons not depolarized by PGE(2). Whether a neuron was depolarized by PGE(2) was not related with its capsaicin (CAP) sensitivity. In addition, we examined whether NGF influences the PGE(2) response of neurons in adjuvant-inflamed young adult animals. Removal of NGF from culture medium did not change the percentage of neurons depolarized by PGE(2) in intact rats (20 and 18% for neurons cultured without or with NGF for 2-3 days, respectively). Adjuvant induced inflammation increased the percentage of neurons depolarized by PGE(2) to 38%, but this was not reversed by an addition of anti-NGF antibody to the culture medium, suggesting that NGF does not play a substantial role in the increase in sensitivity to be depolarized by PGE(2).

B2 receptor-mediated enhanced bradykinin sensitivity of rat cutaneous C-fiber nociceptors during persistent inflammation

Bradykinin (BK), which has potent algesic and sensitizing effect on nociceptors, is of current interest in understanding the mechanisms of chronic pain. BK response is mediated by B2 receptor in normal conditions; however, findings that B1 receptor blockade alleviated hyperalgesia in inflammation have been highlighting the role of B1 receptor in pathological conditions. It has not yet been clear whether nociceptor activities are modified by B1 receptor agonists or antagonists during inflammation. In addition, previous studies reported the change in BK sensitivity of nociceptors during short-lasting inflammation, and data in persistent inflammation are lacking. Therefore we investigated whether an experimentally induced persistent inflammatory state modulates the BK sensitivity of nociceptors and which receptor subtype plays a more important role in this condition. Complete Freund's adjuvant was injected into the rat-tail and after 2-3 wk, persistent inflammation developed, which was prominent in the ankle joint. Using an in vitro skin-saphenous nerve preparation, single-fiber recordings were made from mechano-heat sensitive C-fiber nociceptors innervating rat hairy hindpaw skin, and their responses were compared with those obtained from C-fibers tested similarly in normal animals. BK at 10(-8) M excited none of the 10 C-fibers in normal animals while it excited 5 of 11 (45%) C-fibers of inflamed animals, and at 10(-6) M BK excited all of the 11 inflamed C-fibers (or 94% of 36 tested C-fibers) but only 4 of 10 (or 45% of 58 tested C-fibers) in normal animals. Thus the concentration-response curves based on the incidence of BK induced excitation, and the total number of impulses evoked in response to BK were significantly shifted to the left. Moreover, an increased percentage of the inflamed C-fibers responded to 10(-6) M BK with bursting or high-frequency discharges. Thirty-percent of inflamed C-fibers had spontaneous activity, and these fibers showed comparatively less tachyphylaxis to consecutive second and third 10(-6) M BK stimulation. A B2 receptor antagonist (D-Arg-[Hyp3, Thi5,8,D-phe7]-BK) completely eliminated BK responses in inflamed rats, while B1 receptor antagonists (B 9958 and Des-Arg9-[Leu8]-BK) had no effect. Selective B1 receptor agonist (Des-Arg10-Kallidin) excited 46% (n = 13) of inflamed C-fibers at 10(-5) M concentration, which is 1,000 times higher than that of BK needed to excite the same percentage of inflamed C-fibers. We conclude that in chronically inflamed tissue, sensitivity of C-fiber nociceptors to BK, which is B2 receptor mediated, is strongly increased and that B1 receptor may not be important to a persistent inflammatory state, at least at the primary afferent level.

Somatostatin sst(2) receptor-mediated inhibition of mesenteric afferent nerves of the jejunum in the anesthetized rat

BACKGROUND & AIMS

Octreotide inhibits visceral sensations in clinical studies, but the site of action and the receptor type(s) involved are unknown. Our aim was to investigate the effects of octreotide, the selective sst(2) receptor agonist (BIM 23027), and the sst(2) antagonist (Cyanamid154806) on the activity of mesenteric afferent fibers innervating the rat jejunum. Their effects were investigated on baseline discharge, mechanosensitivity, and responses to algesic chemicals.

METHODS

Extracellular multiunit recordings of jejunal afferent nerve firing were made in pentobarbitone-anesthetized (60 mg/kg intraperitoneally) male Wistar rats.

RESULTS

Octreotide and BIM23027 (0.001-100 microg/kg intravenously) each evoked a long-lasting inhibition of baseline discharge, which was blocked by cyanamid 154806 (3 mg/kg) and absent in chronically vagotomized animals. Afferent responses to bradykinin were also inhibited by an sst(2) receptor-mediated mechanism but were unaffected by vagotomy. Ramp distentions of the jejunum evoked a biphasic activation of afferent nerve discharge, the low threshold component of which was attenuated in vagotomized animals. Sst(2) receptor agonists significantly inhibited the mechanosensitivity of spinal, but not vagal, afferents.

CONCLUSIONS

These data suggest that activation of somatostatin sst(2) receptors inhibit populations of mesenteric afferents likely to be involved in nociceptive transmission.

Regulation of prostacyclin and prostaglandin E(2) receptor mediated responses in adult rat dorsal root ganglion cells, in vitro

1. Primary cultures of adult rat dorsal root ganglia (DRG) were prepared to examine the properties of prostacyclin (IP) receptors and prostaglandin E(2) (EP) receptors in sensory neurones. 2. IP receptor agonists, cicaprost and iloprost, stimulated adenylyl cyclase activity with EC(50) values of 22 and 28 nM, respectively. Prostaglandin E(1) (PGE(1)) and prostaglandin E(2) (PGE(2)) were 7 fold less potent than cicaprost and iloprost, with PGE(2) displaying a lower maximal response. 3. Adenylyl cyclase activation by iloprost, PGE(1) and PGE(2), but not by forskolin, was highly dependent on DRG cell density. Although the potency of iloprost and PGE(2) for stimulating adenylyl cyclase was unchanged, their maximal responses were significantly increased at low cell density. 4. Both IP and EP(2/4) receptors could be down-regulated by agonist pretreatment, however the presence of cyclo-oxygenase (COX) inhibitors did not prevent this apparent down-regulation of IP and EP(2/4) receptors at high DRG cell densities. 5. Stimulation of adenylyl cyclase by the neuropeptide calcitonin gene-related peptide was also decreased at high DRG cell density, whereas the responses to beta-adrenoceptor agonists were increased at high DRG cell density. 6. Addition of nerve growth factor (NGF), or the addition of anti-neurotrophin antibodies during the 5-day culture of DRG cells, had no effect on IP receptor-mediated responses. 7. These results indicate that G(s)-coupled receptors involved in nociception are regulated in a variable manner in adult rat sensory neurones, and that this cell density-dependent regulation may be agonist-independent for IP and EP(2/4) receptors.

Increase in spontaneous action potentials and sensitivity in response to norepinephrine in dorsal root ganglion neurons of adjuvant inflamed rats

To gain an understanding of the cellular mechanisms of hyperalgesia and spontaneous pain in adjuvant-induced chronic inflammation, we investigated the effects of nerve growth factor (NGF), which is known to increase in inflamed tissues and to cause hyperalgesia, on the spontaneous activities and norepinephrine-induced excitation of dorsal root ganglion (DRG) neurons. Intracellular recordings were obtained from freshly dissociated and cultured DRG neurons (<30 microm) from intact and adjuvant inflamed (AI) rats. Of more than 100 freshly dissociated DRG neurons from the intact rats, none produced spontaneous action potentials, whereas 23% of the neurons from the AI rats did. Spontaneous activities were induced in 34% neurons from intact rats when cultivated for one day with NGF. No neurons from the intact rats responded to norepinephrine (NE), irrespective of whether they were freshly dissociated or cultured with NGF. In contrast, 11% of neurons from the AI rats, both freshly dissociated and cultured without NGF, had a small depolarization in response to NE. The present results suggest that, in AI rats NGF plays an important role in inducing spontaneous activities in DRG neurons, but not in inducing sensitivity to NE.

Endogenous nerve growth factor increases the sensitivity to bradykinin in small dorsal root ganglion neurons of adjuvant inflamed rats

To examine the cellular mechanisms of hyperalgesia observed in an adjuvant-induced chronic inflammation, the role of nerve growth factor (NGF) in the response to bradykinin (BK) in small neurons from dorsal root ganglia (DRG) was studied via intracellular recordings. After 2 days of cultivation in the absence of NGF, the percentage of neurons from adjuvant-inflamed (AI) rats which were depolarized by BK (53%) was significantly higher than that in neurons from intact rats (13%). This higher percentage in AI rat neurons was significantly reduced after culturing with anti-NGF (17%), but was not influenced by the addition of NGF (57%). The present result demonstrated that sensitivity to BK of DRG neurons from AI rats is increased due to the action of endogenous NGF, suggesting that plastic change in primary afferent neurons caused by NGF may be one of the mechanisms involved in hyperalgesia.