Effects Of Neonatal Capsaicin Treatment Research Articles

Effect of neonatal capsaicin treatment on orthodontic tooth movement in male Sprague-Dawley rats

In this study, we examined the effect of neonatal administration of capsaicin on the magnitude of orthodontic tooth movement in rats. Twelve timed pregnant Sprague-Dawley rats were randomized between the capsaicin group and the vehicle group. The pups received treatment with either capsaicin or vehicle on day 2 of life. Capsaicin treatment has been shown to produce a selective destruction of fine myelinated and unmyelinated Aδ and C sensory nerve fibers, causing an inhibition of the effects from neurogenic inflammation. Tooth-movement experiments began at 12 weeks of age. A mesial tipping force was applied to the maxillary first molar by using a 3-mm length of Sentalloy closed-coil spring (Dentsply GAC Intl, Bohemia, NY) activated from a bonded molar cleat to the maxillary incisors; this appliance delivers a constant tipping force of 50 g. Diastema measurements between the first and second molars were made at 2 and 4 weeks after appliance placement. Measurements were made indirectly from stone models by using a charge-coupled device microscope camera and Optimas 5.2 measurement software (Media Cybernetics, Bethesda, Md). Two-way repeated-measures analysis of variance (ANOVA) was used to analyze the differences between the groups. The capsaicin-treated rats and the controls did not differ in the amount of tooth movement at the collected time points (P>0.05). Similarly, the magnitude of change of tooth movement from 2 to 4 weeks did not differ between the groups (P>0.05). An increase in average diastema size was observed between 2 and 4 weeks after appliance activation in both treatment groups (P<0.0001). These results suggest that neonatal capsaicin desensitization in the rat does not affect the rate of orthodontic tooth movement after the application of a 50-g tipping force to the maxillary first molar. This might be due in part to the development of compensatory mechanisms in the chronically desensitized rat. Further studies are necessary to determine the reproducibility and histologic characteristics of this treatment.

Long‐Term Effects of Neonatal Capsaicin Treatment on Intraepidermal Nerve Fibers and Keratinocyte Proliferation in Rat Glabrous Skin

Innervation is required to preserve several aspects of skin homeostasis. Previous studies in rodents have shown that sciatic nerve transection leads to epidermal thinning and reduced keratinocyte proliferation. As the sciatic nerve is composed of sensory and motor axons, it is not clear whether skin alterations reflect motor or sensory disturbances. In this study, we used neonatal capsaicin treatment to evaluate whether sensory chemical denervation affects keratinocyte proliferation at 1, 3, and 6 months of age. Using design-based stereological methods, we estimated the total length of intraepidermal nerve fibers (IENF) that were of peptidergic type and the number of bromodeoxyuridine-labeled (BrdU(+) ) nuclei in the hind paw glabrous epidermis of control and capsaicin-treated rats. We found that the treatment decreased the total fiber length of IENF immunoreactive for both protein gene product 9.5 (PGP(+) ) and of IENF immunoreactive for calcitonin gene-related peptide (CGRP(+) ). The length of PGP(+) fibers decreased by 83%, 81%, and 77% and that of CGRP(+) fibers decreased by 48%, 58%, and 58% at 1, 3, and 6 months, respectively. Double-immunofluorescence staining for neural beta III tubulin and CGRP revealed that the majority of the remaining fibers in the epidermis after capsaicin treatment were of peptidergic type. The number of BrdU(+) nuclei was similar in both groups. Our findings suggest that IENF present after capsaicin treatment are sufficient to maintain epidermal replacement.

Receptor changes in brain tissue of rats treated as neonates with capsaicin

Capsaicin, the hot chemical in chillies, administered to neonatal rats, causes destruction of polymodal nociceptive primary afferent neurons by acting on TRPV1 receptors causing intrinsic somatosensory deprivation. Although the effects of neonatal capsaicin treatment in the periphery have been extensively investigated, less is known about the brain networks to which the capsaicin sensory neurons are relayed. In the present study the effect of neonatal capsaicin treatment on brain receptors that have been shown to interact with TRPV1 was examined. Wistar rats were treated on neonatal day 2 with capsaicin and at 15–16 weeks of age, brains were processed to measure levels of muscarinic M 1/M 2 and M 2/M 4, serotonin 5HT 2A, cannabinoid CB 1, dopamine D 1, D 2 receptors and dopamine transporter. Overall increases in levels of muscarinic M 1/M 4 ( F = 8.219, df = 1, p = 0.005), muscarinic M 2/M 4 ( F = 99.759, df = 1, p < 0.0001), serotonin 5HT 2A ( F = 28.892, df = 1, p < 0.0001), dopamine D 1 ( F = 8.726, df = 1, p = 0.008) and cannabinoid CB 1 ( F = 25.084, df = 1, p < 0.0001) receptors were found in the brains of capsaicin-treated rats, although significant regional changes occurred only in muscarinic M 2/M 4 and serotonin 5HT 2A receptors. The results of the present study suggest that neonatal intrinsic somatosensory deprivation may have a significant impact on substrates at the central nervous system that manifest as changes in central cholinergic, monaminergic and cannabinoid systems in the adult animal.

The effect of neonatal capsaicin treatment on the CGRP-immunoreaction in the trigeminal subnucleus caudalis of mice

The calcitonin-gene related peptide (CGRP) is a primary afferent neurotransmitter in the trigeminal system. Although a neonatal administration of capsaicin eliminates substance P (SP)-mediated nociceptive responses to induce a permanent functional reduction in C-fibers, little information is available regarding changes in CGRP-immunoreaction in mice undergoing neonatal capsaicin treatment (CP mice). This study examined postnatal changes in the distribution of CGRP-immunoreaction in the trigeminal subnucleus caudalis and trigeminal ganglion of CP mice by immunohistochemical technique and a quantitative analysis. Immunohistochemistry for CGRP in the subnucleus caudalis (Vc) demonstrated two dense distributions of neurons in the CP mice as well as naïve mice: in the marginal layer and the region 400-600 mum deep. The quantitative analysis revealed no significant difference in the density of CGRP immunoreaction between naïve and CP mice 1-8 weeks of age. In the trigeminal ganglion of both groups, the size distribution of CGRPpositive neurons displayed a distribution pattern with one peak in 200-300 mum(2) at week 1 and with two peaks in 200-300 mum(2) and 600-700 mum(2) at week 8 but no significant difference in neural density existed between these regions. When double staining in the naïve mice with CGRP or SP and VR1, a capsaicin receptor, was done, many trigeminal ganglion neurons co-expressed SP- and VR1-immunoreactions, but rarely exhibited CGRP/VR1-co-localization. Taken together with previous data, these current observations suggest that CGRP containing afferent neurons possibly performs differing roles in nociceptive afferent input transmission within the Vc from SP-containing neurons in mice.

Different Roles of Capsaicin-sensitive and H1 Histamine Receptor-expressing Sensory Neurones in Itch of Mosquito Allergy in Mice

Although mosquito allergy induces the release of histamine, the itch-related response, scratching, is not effectively suppressed by blockade of H1 histamine receptors. To address this question, we examined the effects of neonatal capsaicin treatment on allergic reactions and H1 histamine receptor-expressing sensory neurones in mice. Neonatal capsaicin treatment almost completely abolished allergy-associated scratching, without effects on plasma extravasation or increase in serum concentrations of immunoglobulins E and G1. An injection of edema contents from an animal exhibiting allergic reaction elicited scratching in naive animals, suggesting the production of pruritogen(s) by allergic reaction; this production was not suppressed by neonatal capsaicin treatment. This treatment markedly decreased the number of sensory neurones immunoreactive for TRPV1 capsaicin receptor, with little effect on sensory neurones immunoreactive for neurofilament 200, a marker of myelinated A-fibre neurones. In addition, there was a trend towards a reduction in numbers of sensory neurones immunoreactive for H1 histamine receptor. The results suggest that capsaicin-sensitive sensory neurones that lack H1 histamine receptors play a key role in signalling of allergic itch.

Effects of neonatal capsaicin treatment in the neutrophil production, and expression of preprotachykinin-I and tachykinin receptors in the rat bone marrow

Bone marrow is richly innervated with both myelinated and non-myelinated nerve fibers, but the role of this innervation on hemopoiesis is poorly understood. Therefore, the aim of this study was to investigate the role of C-fibers on hematopoiesis. Wistar rats were neonatally injected with either capsaicin or its vehicle, and used at adult ages (8–10 weeks). In capsaicin-pretreated rats, the levels of substance P (SP) in bone marrow fluid were markedly reduced in comparison with the vehicle group (13.1±4.5pg/ml versus 47.3±5.5pg/ml, p<0.05). In bone marrow, the number of total leukocytes was 28% higher (p<0.05) in capsaicin-pretreated group, and this accompanied by a higher number of neutrophils, particularly of the immature forms. The mononuclear cell and eosinophils counts did not differ significantly among vehicle and capsaicin groups. In peripheral blood, the number of circulating neutrophils in the capsaicin group increased by 53.8% (p<0.05), whereas the number of mononuclear cells did not change significantly among groups. Eosinophils were virtually absent in the circulating blood in both groups. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that both preprotachykinin (PPT)-I mRNA and the tachykinin neurokinin (NK)-1 mRNA expression in bone marrow cells significantly increased in capsaicin group, whereas the NK-2 mRNA expression was unchanged after capsaicin pretreatment. In conclusion, our data show that chronic neuropeptide depletion enhance the neutrophil proliferation and differentiation in the rat bone marrow by mechanisms involving upregulation of PPT-I gene and NK-1 receptors.

Neonatal Capsaicin Treatment Affects Rat Thymocyte Proliferation and Cell Death by Modulating Substance P and Neurokinin-1 Receptor Expression

Herein we provide evidence that substance P (SP) and its neurokinin-1 receptor (NK-1R) expressed on thymocytes counteract thymus depletion induced by neonatal capsaicin (CPS) treatment by affecting thymocyte proliferation and apoptotic death. SP administration reversed the CPS-mediated inhibitory effects on the total thymocyte number and subset distribution, namely CD4+ and CD4– CD8– cells, through its interaction with NK-1R as shown by concomitant NK-1R (SR140333) antagonist administration. SP-induced enhancement of thymus cellularity parallels its ability of inhibiting the thymocyte apoptotic program. Indeed, exogenously administered SP completely nullified CPS-induced apoptosis, and SR140333 abrogated the SP-mediated protective effect. SP administration also stimulated concanavalin A (Con A)-induced thymocyte proliferation of CPS-treated rats, completely reversing the CPS-induced inhibition. The SP-mediated stimulation of Con A-induced thymocyte proliferation was NK-1R dependent as shown by concomitant administration of SP and SR140333 to CPS-treated rats. Our results also demonstrate that CPS treatment induces a marked decrease of thymocyte PPT-A mRNA level and endogenous SP content as evaluated by quantitative RT-PCR, in situ hybridization and cytofluorimetric analysis. By contrast, NK-1R mRNA levels were increased in thymocytes from CPS-treated rats. Exogenous SP administration augmented PPT-A, SP and NK-1R thymocyte expression in CPS-treated rats, and this enhancement was antagonized by SR140333 administration. Overall, our results strongly suggest that the immunomodulatory effects of neonatal CPS treatment on rat thymocyte functions are dependent on vanilloid-mediated regulation of SP and NK-1R functional expression by neuronal and immune cells.

Selective C-fiber deafferentation of the spinal dorsal horn prevents lesion-induced transganglionic transport of choleragenoid to the substantia gelatinosa in the rat

The effect of neonatal capsaicin treatment, producing selective elimination of almost all unmyelinated C-fiber sensory axons, was studied on lesion-induced transganglionic labelling of the substantia gelatinosa of the spinal cord by choleragenoid. In both control and capsaicin-pretreated rats, the injection of choleragenoid-horseradish peroxidase conjugate into the intact sciatic nerves resulted in intense labelling only of the deeper layers of the spinal dorsal horn. In the control but not the capsaicin-pretreated rats, the injection of the tracer into sciatic nerves transected 2 weeks previously produced an intense homogeneous labelling of the substantia gelatinosa. It is concluded that the uptake and axonal transport of choleragenoid by capsaicin-sensitive C-fiber afferents may be accounted for by the lesion-induced transganglionic labelling of the substantia gelatinosa, rather than by A-fiber sprouting.

Effect of neonatal capsaicin treatment on neural activity in the medullary dorsal horn of neonatal rats evoked by electrical stimulation to the trigeminal afferents: an optical, electrophysiological, and quantitative study

To elucidate which glutamate receptors, NMDA or non-NMDA, have the main role in synaptic transmission via unmyelinated afferents in the trigeminal subnucleus caudalis (the medullary dorsal horn), and to examine the early functional effects of neonatal capsaicin treatment to the subnucleus caudalis, optical recording, field potential recording, and quantitative study using electron micrographs were employed. A medulla oblongata isolated from a rat 5–7 days old was sectioned horizontally 400-μm thick or parasagittally and stained with a voltage-sensitive dye, RH482 or RH795. Single-pulse stimulation with high intensity to the trigeminal afferents evoked optical responses mainly in the subnucleus caudalis. The optical signals were composed of two phases, a fast component followed by a long-lasting component. The spatiotemporal properties of the optical signals were well correlated to those of the field potentials recorded simultaneously. The fast component was eliminated by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 10 μM), while the long-lasting component was not. The latter increased in amplitude under a condition of low Mg 2+ but was significantly reduced by dl-2-amino-5-phosphonovaleric acid (AP5; 30 μM). Neonatal capsaicin treatment also reduced the long-lasting component markedly. In addition, the decreases in the ratio of unmyelinated axons to myelinated axons and in the ratio of unmyelinated axons to Schwann cell subunits of trigeminal nerve roots both showed significant differences ( P<0.05, Student’s t-test) between the control group and the neonatal capsaicin treatment group. This line of evidence indirectly suggests that synaptic transmission via unmyelinated afferents in the subnucleus caudalis is mediated substantially by NMDA glutamate receptors and documented that neonatal capsaicin treatment induced a functional alteration of the neural transmission in the subnucleus caudalis as well as a morphological alteration of primary afferents within several days after the treatment.

Serotonin-induced population primary afferent depolarisation in vitro: the effects of neonatal capsaicin treatment

The effect of 5-hydroxytryptamine (5-HT) on population primary afferent depolarisation (PAD) has been studied using in vitro spinal cord preparations from normal and capsaicin pre-treated (neonatal subcutaneous injection; 75 mg kg −1) rats aged 10–14 days. In preparations from untreated rats, the concentration–response curve for PAD in response to 0.1–100 μM 5-HT was bell-shaped but in the capsaicin pre-treated group, a non-saturating 5-HT-induced PAD concentration–response curve was generated. Quantitatively, the mean PAD responses to 0.1–10 μM 5-HT were of a greater amplitude in the control group compared to the capsaicin pre-treated group ( p≤0.05). For the highest 5-HT concentration of 100 μM, PAD values were significantly greater in the capsaicin pre-treated group ( p≤0.05). These data indicate that control of sensory afferent polarity may involve two 5-HT receptor types and that nociceptive and non-nociceptive afferents may be targets for released 5-HT.

Neonatal capsaicin treatment abolishes formalin-induced spinal PGE2 release.

We investigated the effect of neonatal capsaicin treatment on formalin-evoked pain behavior and spinal levels of nociceptive neuromodulators using in vivo intrathecal microdialysis in conscious adult rats and age-matched controls. Capsaicin-treated rats displayed thermal hypoalgesia and a significant decrease in tissue content of calcitonin gene-related peptide. Paw swelling, flinching and release of spinal prostaglandin E2 induced by injection of formalin into the hindpaw were also reduced in capsaicin-treated rats compared with controls, whereas glutamate, aspartate and taurine release was unaffected. These data suggest that formalin-induced inflammation, pain behavior and spinal prostaglandin E2 release are mediated by mechanisms sensitive to neonatal capsaicin while the formalin-evoked release of amino acids in the spinal cord is not.

A quantitative investigation of the effects of neonatal capsaicin treatment on vagal afferent neurons in the rat.

The sensory cells of the nodose and jugular ganglia of the rat have been quantitatively evaluated in longitudinal paraffin sections. The right vagal ganglia contain significantly more neurons than the left, particularly neurons with somata having sectioned areas 200-400 microm2 and longest diameters 15-25 microm. Such neurons appear to be homogenously distributed because sections of the right vagal ganglia did not show specific areas of neuronal density compared with those of the left. Neonatal capsaicin treatment reduced the number of neurons in both the left and right ganglia to about 30% of controls. Capsaicin destroyed neurons with sectioned areas of 100-600 microm2 and longest diameters of 15-35 microm, but had no statistically significant effects on larger neurons.

Effects of neonatal capsaicin treatment on descending modulation of spinal nociception from the rostral, medial medulla in adult rat

Stimulation-produced modulation from the rostral, medial medulla (RMM) on the spinal nociceptive tail-flick (TF) reflex and on lumbar spinal dorsal horn neuron reponses to noxious cutaneous stimulti was studied in adult rats treated as neonates with capsaicin or vehicle. In vehicle-treated rats ( n = 7), both descending facilitatory and inhibitory influences on the TF reflex were produced from the RMM. At 11/23 sites in the RMM, electrical stimulation produced biphasic modulatory effects. Electrical stimulation facilitated the spinal nociceptive TF reflex at low intensities (5–25 μA) and inhibited the TF reflex at greater intensities (50–200 μA). The mean threshold intensity of stimulation to inhibit the TF reflex (cut-off time= 7.0s) was 66 μA ( n = 11). At 11 of 23 sites, electrical stimulation only inhibited the TF reflex; the mean threshold intensity of stimulation to inhibit the TF reflex was 50 μA ( n = 11). At one stimulation site, electrical stimulation only facilitated the TF reflex at the intensities tested (5–100 μA). In capsaicin-treated rats ( n = 6), the proportion of sites from which electrical stimulation only inhibited the TF reflex was significantly less (3/27sites= 11%) than in vehicle-treated rats (11/23= 48%). The threshold intensity of stimulation to inhibit the TF reflex from these three sites was 50 μA. The number of sites RMM fronm which electrical stimulation only facilitated the TF reflex was significantly greater in capaicin-treated rats (15/27= 56%) than in vehicle-treated rats (1/23= 4%). Neither the number of sites in RMM from which electrical stimulation produced biphasic modulatory effects on the TF reflex (48% and 33%, respectively) nor the intensities of stimulation or magnitudes of facilitation or inhibition of the TF reflex significantly differed between vehicle- and capsaicin-treated rats. In electrophysiological experiments, all units studied responded to non-noxious and noxious intensities of mechanical stimulation applied to the glabrous skin of the plantar surface of the ipsilateral hind foot and also to noxious heating of the skin (50°C). The number of sites where electrical stimulation produced only facilitatory effects on responses of spinal dorsal horn neurons to noxious stimulation (thermal or mechanical) of the skin was significantly increased from 13% of the total sites in vehicle-treated rats to 40% in capsaicin-treated rats. The number of sites where stimulation only produced inhibitory effects on responses of spinal dorsal horn neurons to noxious stimulation was decreased from 47% of the total sites in vehicle-treated rats to 35% in capsaicin-treated rats. The intensity of stimulation for producing inhibitory effects and the magnitude of inhibitory effects produced were not significantly different between vehicle- and capsaicin-treated rats. Immunocytochemical evaluation of the lumbar spinal dorsal horn verified that there were substantial reductions in substance-P and calcitonin gene-related peptide-like immunoreactivity, confirming the effectiveness of neonatal capsaicin treatment. The present study, in support of anatomical studies documenting central effects of capsaicin, suggests that neonatal capsaicin treatment also has significant effects on bulbospinal systems important to the modulation of spinal nociceptive transmission. Specifically, capsaicin treatment of neonates leads to a reduction in inhibitory and an increase in facilitatory influences on spinal nociception descending from the caudal brainstem.

Maturational changes in sympathetic and sensory innervation of the rat uterus: Effects of neonatal capsaicin treatment

The plasticity of the sympathetic and sensory innervation of the rat uterus was examined, before and after puberty, in controls and in animals where primary sensory nerves had been destroyed by neonatal capsaicin treatment. Immunohistochemical and histochemical methods were used in association with nerve density measurements and biochemical assays. The main findings were as follows: 1.(1) Puberty was associated with a marked increase in the weight of the uterine horn, uterine cervix and parametrial tissue. This was unaffected by capsaicin treatment.2.(2) The sympathetic innervation of the uterine horn and parametrial tissue was reduced following puberty as revealed by a decrease in the density of noradrenaline-containing nerves and a marked decrease in the tissue concentration of noradrenaline. Sympathetic nerves supplying the uterine cervix and the blood vessels of the uterus appeared to be unaffected by puberty.3.(3) In contrast, the sensory supply of the uterus by substance P and calcitonin gene-related peptide-containing nerves increased in parallel with uterine growth during puberty resulting in no change in nerve density and only a slight reduction in peptide concentration.4.(4) Neonatal capsaicin treatment caused a long-lasting depletion of substance P- and calcitonin gene-related peptide-containing nerves. In the uterine horn and parametrial tissue, capsaicin-resistant calcitonin gene-related peptide, but not substance P, still increased with tissue weight during puberty, indeed, in the uterine horn, the relative increase was greater than in controls.5.(5) Sensory denervation resulted in an increase in the non-vascular sympathetic supply of the uterus, although there was a regional variation in the time course of the response. Perivascular sympathetic nerves were unaffected by capsaicin treatment. The pattern of change in non-vascular noradrenaline-containing nerves associated with puberty was similar in nature to controls. Thus, there is considerable plasticity in the innervation of the uterus both during puberty and following sensory denervation. A complex pattern of change occurs with differential responses in vascular and nonvascular nerves and in different regions of the uterus. Such differences may be due in part to the different origins of individual nerve populations and/or to their relative sensitivities to sex hormones.

Quantification of axotomy‐induced alteration of neuropeptide mRNAs in dorsal root ganglion neurons with special reference to neuropeptide Y mRNA and the effects of neonatal capsaicin treatment

Alteration in mRNA expression in dorsal root ganglia (DRG) neurons encoding 5 neuropeptides was quantitatively compared in normal rats and in those neonatally treated with capsaicin, a selective neurotoxin which destroys a subpopulation of DRG neurons with unmyelinated axons. Adult rats received a unilateral transection of the sciatic nerve and were killed 7 days later. Oligonucleotide probes specific for the genes encoding neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), galanin (GAL), somatostatin (SOM), and calcitonin gene-related peptide (CGRP) were used for in situ hybridization and RNA blot analysis. Following the nerve cut, RNA blot analysis demonstrated a dramatic induction of NPY, VIP, and GAL mRNA levels from the undetectable constitutive level of expression. Conversely, CGRP and SOM mRNAs, which are constitutively expressed, were reduced 55% and 70%, respectively, following the nerve cut. A unimodal size distribution for neurons expressing NPY mRNA was determined, with a mean cross-sectional area of 1700 microns2 representing 24.4% of DRG neurons ipsilateral to the nerve cut. Neurons expressing VIP mRNA were mainly small sized, with a cross-sectional area of approximately 700 microns2, while those expressing GAL mRNA were both small (approximately 700 microns2) and medium (approximately 1,300 microns2) sized. The percentages of neurons expressing VIP or GAL mRNA were 19.9% and 33.7%, respectively. In neonatal capsaicin-treated rats, there was a 10% reduction in neurons expressing NPY mRNA, a 37% reduction for VIP, and a 27% for GAL mRNA compared to vehicle-treated rats after nerve cut. Capsaicin-sensitive neurons comprised 37% of CGRP neurons and 83% of SOM neurons. These observations suggest that NPY is primarily induced in myelinated primary afferent neurons, while VIP and GAL mRNA induction occurs in a mixed population, a sizeable percentage of which has unmyelinated axons. Additionally, SOM mRNA expression is associated mainly with unmyelinated primary afferents.

Effect of neonatal capsaicin treatment on neurogenic pulmonary edema from fluid-percussion brain injury in the adult rat

The frequent occurrence of acute death from pulmonary failure in experimental head injury studies on Sprague-Dawley rats prompted an investigation into the manner in which acute neurogenic pulmonary edema develops in these animals as a result of an applied fluid pressure pulse to the cerebral hemispheres. Studies were performed in adult animals using histamine H1 and H2 blocking agents, or in adult animals treated as neonates with capsaicin to destroy unmyelinated C-fibers. Recordings were made of either the pulmonary arterial or the right ventricular pressure, and the left atrial and femoral arterial pressures before, during, and after injury to provide a record of the hemodynamic response throughout the development of neurogenic pulmonary edema. Head injury triggered the almost immediate development of pressure transients with and without neurogenic pulmonary edema. All rats, regardless of treatment, reacted with nearly identical systemic arterial pressure responses; however, the pulmonary responses followed a time course that was independent of systemic arterial pressure changes. Acute neurogenic pulmonary edema was always associated with a substantial increase in pulmonary arterial and left atrial pressures; conversely, pressure increases of similar magnitude were not always associated with edema. Histamine H1 and H2 blockers significantly reduced the pulmonary pressure surges only in rats free of neurogenic pulmonary edema. All capsaicin-treated rats showed suppressed pulmonary pressure responses, normal lung water content, elevated lung surface tension, and significantly reduced levels of immunoreactive substance P in the spinal cord and vagus nerve. While the pressures cannot clarify how edema influences the observed hemodynamics, they do not support the view that edema is the direct consequence of pulmonary hypertension. It is proposed that neurogenic pulmonary edema is a functional disturbance provoked by adverse stimuli from outside the lungs and that in the rat the primary afferent fiber is essential to the production of this entity.

The effect of neonatal capsaicin on the development of bronchial hyperresponsiveness in allergic rabbits

We have investigated the effects of neonatal capsaicin treatment of rabbits on the development of bronchial hyperresponsiveness following allergen exposure from birth. In vivo airways responsiveness was assessd as the concentrations of histamine to cause a 35% decrease in compliance (PC 35) and a 50% increase in resistance (PC 50). Rabbits treated with vehicle capsaicin (10% ethanol, 10% tween 80 and 80% saline) and then immunised with Altenaria tenius (40 000 protein nitrogen units (PNU)/ml + AL(OH) 3 + saline in a ratio 2:1:1) were more responsive to aerosol histamine in vivo at three months of age when compared to sham-immunised (AL(OH) 3 + saline 1:3) or saline-treated rabbits for compliance measurements (P < 0.05). However, immunised rabbits although not significantly different to sham-immunised (P > 0.05) were significantly more responsive than saline-treated rabbits for resistance measurements (P < 0.05). Neonatal capsaicin treatment significantly attenuated the increased responsiveness seen in immunised rabbits for both compliance and resistance measurements (P < 0.05). The bronchial hyperresponsiveness is unlikely to be due to cellular infiltration per se as cell numbers assessed by bronchoalveolar lavage were not significantly different between groups (P > 0.05). Exogenous in vitro functional bronchial responses to capsaicin were increased in vehicle-immunised rabbits, an effect attenuated by neonatal capsaicin treatment (P < 0.05) whereas responses to methacholine, histamine or electrical field stimulation remained similar between groups (P > 0.05). Bronchial levels of calcitonin gene-related peptide- and substance P-like immunoreactivity were unaffected by any treatment (P > 0.05). In conclusion, our present results suggest that the bronchial hyperresponsiveness to histamine at three months of age induced by antigen exposure from birth may involve capsaicin-sensitive nerves. The mechanisms by which capsaicin attenuates allergen-induced bronchial hyperresponsiveness appears not to be via depletion of sensory neuropeptides although total tissue content may not reflect local sensory nerve ending concentrations of these peptides.

Long-term effects of neonatal capsaicin on C-fiber excitability and dorsal horn C-input processing in the rat

The effects of neonatal capsaicin treatment (50 mg/kg, SC, at day 2 of postnatal life) on C-fiber excitability and dorsal horn C-input processing were studied through recording of a C-fiber-evoked spinal flexor reflex in 55–60-day-old rats, anesthetized with urethane (1.1 g/ kg, IP) and spinalized at Th9–10. Neonatal capsaicin resulted in decreased C-fiber excitability, as revealed by increased chronaxie values determined in a strength-duration paradigm. Besides, capsaicin-treated rats exhibited a reduced potentiation of the C-reflex discharge in response to repetitive stimulation. The results indicate that capsaicin given to rats early in life leads to both functional disturbances of surviving C-fibers and altered temporal synaptic processing of the C-input in the spinal cord.

The effect of neonatal capsaicin treatment on gustatory behavior in the albino rat

Small-diameter fibers present in gustatory peripheral nerves have historically been suspected of relaying information about the bitter quality of a taste stimulus. Neonatally injected capsaicin irreversibly destroys a proportion of unmyelinated C- and some Aδ-fibers. Consummatory responses to increasing concentrations of quinine and other chemical solutions following neonatal capsaicin injection were compared to those of untreated and vehicle-injected control Sabra albino rats. Capsaicin-treated rats significantly increased their withdrawal thresholds to noxious, CO 2 laser-generated heat pulses verifying treatment effectiveness. Furthermore, neonatal capsaicin treatment diminished sensitivity to pungent capsaicin solutions in mature rats. However, there were no group differences in quinine intake, suggesting that the full array of unmyelinated fibers associated with taste buds is not essential for the transmission of bitter taste. Capsaicin-treated animals showed a significant reduction in intake of normally highly preferred sodium chloride and sucrose concentrations. These results were probably not due to loss of peripheral unmyelinated afferent fibers per se, but rather to secondary central changes.

Distribution of neurokinin B in rat spinal cord and peripheral tissues: Comparison with neurokinin A and substance P and effects of neonatal capsaicin treatment

In the present study, highly specific radioimmunoassays were developed and used to measure neurokinin B, neurokinin A and substance P in the rat spinal cord and various peripheral tissues. The results are as follows. (1) Neurokinin B and neurokinin A were distributed all along the rostrocaudal axis of the spinal cord, as is substance P, and were more concentrated in the dorsal than in the ventral region. (2) Substance P was more abundant in the central and peripheral nervous tissues than neurokinin A, while in certain peripheral organs, neurokinin A was more abundant than substance P. In the spinal cord, neurokinin B concentrations were lower than those of the other two tachykinins. (3) In contrast to neurokinin A and substance P, neurokinin B was not detected in any of the peripheral tissues examined. (4) Capsaicin treatment reduced by half neurokinin A and substance P concentrations in the dorsal region of the spinal cord, the dorsal root ganglia and the sciatic nerve, but was without effect on neurokinin B concentrations in the spinal cord. Neurokinin A, like substance P, may therefore have an important function in the transmission of sensory information, particularly in nociceptive transmission from the periphery to the spinal cord and in peripheral neurogenic inflammation. In contrast, since neurokinin B was not found in the sensory neurons, it is not likely to have these functions, but may perhaps control them.