Effect of capsaicin pretreatment on capsaicin-evoked release of immunoreactive somatostatin and substance P from primary sensory neurons

Summary Release of immunoreactive somatostatin (I-SRIF) and immunoreactive substance P(I-SP) was studied from slices prepared from upper dorsal horn (UDH) and lower dorsal plus ventral horn (LDH-VH) of rat spinal cord. Superfusion with capsaicin (10M) led to release of I-SRIF and I-SP from UDH slices but not from LDH-VH slices. The capsaicin-evoked release of I-SP was 6 fold higher than that of I-SRIF. A pulse of 60 mM K⁺ applied after the capsaicin pulse caused release of I-SRIF and I-SP from UDH as well as LDH-VH slices.Pretreatment of rats with capsaicin (125 mg/kg, s.c.) led to a nearly 40% depletion of I-SP in slices from UDH only. Capsaicin-evoked release from these slices was reduced by 81% for I-SRIF and by 79% for I-SP. Release evoked by K⁺ remained unchanged.These results indicate that capsaicin causes release of both I-SRIF and I-SP and that this release is most likely restricted to primary sensory neurons. The marked reduction of the release of I-SP after systemic capsaicin pretreatment may well represent one of the, or even the reason for the insensitivity of capsaicin pretreated rats towards chemogenic pain.     

Decrease of substance P in primary afferent neurones and impairment of neurogenic plasma extravasation by capsaicin

1 Rats were pretreated with capsaicin (50 mg/kg, s.c.) on the 2nd, 10th, or 20th day of life. Three months later immunoreactive substance P (I-SP) was determined in skin, sensory nerves and the central nervous system. Neurogenic plasma extravasation was also examined.2 Pretreatment at the age of 2 or 10 days resulted in a decrease (26 to 69%) of I-SP in skin, saphenous and vagus nerve, dorsal roots, dorsal half of the spinal cord, and medulla oblongata. The I-SP content of the ventral half of the spinal cord, of midbrain, hypothalamus, striatum, cortex, and cerebellum remained unchanged. Neurogenic plasma extravasation was inhibited by more than 80%.3 In contrast to this irreversible effect of capsaicin on newborn rats, pretreatment of 20 day old rats led to reversible depletion of I-SP and to reversible impairment of neurogenic plasma extravasation.4 Capsaicin pretreatment of adult rats caused a marked depletion of I-SP in the skin of the hind paw and an impairment of neurogenic plasma extravasation. A similar decrease of I-SP was seen after chronic denervation of the skin.5 Intra-arterial infusion of substance P (threshold dose 5 x 10(-13) mol/min) or physalaemin induced dose-dependent plasma extravasation. Somatostatin, vasoactive intestinal polypeptide, caerulein and the enkephalin-analogue FK 33-824 were ineffective in doses 100 fold higher.6 The results indicate that the action of capsaicin on substance P neurones is restricted to primary sensory neurones. Since in every case a decreased substance P content of the skin was associated with impaired neurogenic plasma extravasation, it is suggested that release of substance P is involved in neurogenic plasma extravasation.     

Capsaicin and nociception in the rat and mouse

Summary Newborn or adult rats and mice were treated with capsaicin. The effect of systemic or intrathecal treatment on thermonociception, chemonociception, content and release of immunoreactive substance P (I-SP) was investigated.Treatment of two day old rats caused a small, but life-long elevation of the hot plate or tail withdrawal latency. Treatment of adult rats led to a large increase in the reaction time on the hot plate for 4–10 days but the tail withdrawal latency was only slightly elevated for not more than 1–2 days.Mice treated on the 2nd day of life had normal reaction times on the hot plate and a small and inconsistent prolongation of the tail withdrawal latency. In contrast, mice treated on day 7, 10 or as adults had greatly prolonged latencies in both tests for at least 3 months. The changes in latencies were not affected by naloxone or methysergide.Responses to noxious chemical stimuli were moderately inhibited in mice treated on the 2nd day of life, but almost abolished in mice treated on day 7, 10 or as adults.Neonatal capsaicin treatment of rats resulted in a depletion of I-SP in spinal cord and sciatic nerve for 20 months. Capsaicin-evoked release of I-SP from rat spinal cord was reduced by 93% after neonatal treatment, but only by 69% 2 weeks after adult treatment.Treatment of mice on day 2 caused a similar decrease of the I-SP content in spinal cord and of the capsaicin-evoked I-SP release (88%) as treatment on day 4 or 7 although behavioural changes were different. After treatment of adult mice release of I-SP was reduced by 93%.Capsaicin administered intrathecally to rats or mice depleted I-SP in the spinal cord but not in the sciatic nerve. The animals were almost insensitive to noxious heat (tail withdrawal test) and to local application of mustard oil or capsaicin to the hindpaw. Chemosensitivity of the eye, however, remained unchanged.The experiments indicate that systemic or intrathecal capsaicin treatment of rats or mice affects thermo- and chemonociception but species differences were found. It appears, furthermore, that changes in substance P alone cannot explain all the observed behavioral effects after capsaicin treatment.     

Substance P immunoreactive neurons following neonatal administration of capsaicin

Summary Neonatal administration of capsaicin on the days 2, 10 or 20 leads to a long-lasting loss of substance P immunoreactive material in fibers of primary sensory neurons in the spinal cord and medulla oblongata. The degree of depletion examined 6 months after treatment was related to the day of injection. Injections on the second day produced dramatic losses of substance P in fibers of the substantia gelatinosa and the marginal layer of the spinal cord and the spinal nucleus of the trigeminal nerve, although these losses were never complete. The observed depletion of substance P immunoreactive material was homogenous throughout the superficial layers of the dorsal horn and the spinal nucleus of the trigeminal nerve. No changes were observed for the immunoreactivity of Leu-enkephalin in the substantia gelatinosa and the marginal layer of the spinal cord in consecutive sections from the same treated animals. In the medulla oblongata a reduction of substance P immunofluorescent fibers was found in the nucleus tractus solitarii and the spinal nucleus of the trigeminal nerve. Other areas of the central nervous system with a rich innervation of substance P immunoreactive fibers were not affected by capsaicin treatment.     

Time course of capsaicin-induced functional impairments in comparison with changes in neuronal substance P content

1. Changes in the content of substance P (dorsal spinal cord, dorsal roots, dorsal root ganglia, saphenous nerve, skin) and functional changes (neurogenic plasma extravasation, chemosensitivity of the cornea) were measured in the rat from 10 min to 4 days after the s.c. injection of a single dose of 50 mg kg-1 capsaicin. 2. The substance P content in dorsal roots, saphenous nerve and hind paw skin progressively declined to about 60--70% of control 4 days after treatment, whereas that of the dorsal root ganglia rose, after an initial decline, to 140% after 1--4 days. 3. After denervation, impairment of neurogenic plasma extravasation could be observed not earlier than after one day, thus being comparable in time course to the depletion of substance P in the skin and saphenous nerve. 4. Neurogenic plasma extravasation and the chemosensitivity of the cornea were greatly diminished already 10 min after systemic capsaicin treatment, i.e. at a time when the substance P content of the peripheral nerve was still unchanged. These early effects of systemic capsaicin treatment are therefore caused by actions other than depletion of substance P.     

In vitro release of substance P from spinal cord slices by capsaicin congeners

The in vitro release of immunoreactive substance P (I-SP) by capsaicin and three congeners was studied on slices of rat spinal cord upper dorsal horn. Capsaicin and its congeners were all able to stimulate I-SP release, indicating that they act on chemosensitive primary afferents terminating in this region. A positive correlation was found between the I-SP releasing and pain-producing potencies of these compounds. This is in agreement with the concept that primary afferents containing substance P (SP) are involved in the transmission of nociceptive information.     

Effect of FK506 on neurotransmitter content and expression of GAP-43 in neurotoxin-lesioned peripheral sensory and sympathetic neurons

It has previously been shown that the capsaicin-lesioned peptidergic sensory neurons and the 6-hydroxydopamine (6-OHDA)-lesioned sympathetic noradrenergic neurons represent a useful model to study neurotrophin-induced nerve regeneration in the adult rat. The present study was aimed at investigating if the immunosuppressant drug FK506 (tacrolimus) has neuroregeneratory properties in these capsaicin- or 6-OHDA-lesioned peripheral nerves. FK506 was injected in a dose of 0.5 mg/kg/day for 10 days or in a dose of 1.5 mg/kg/day for 7 days. One day after the last FK506 injection neurotransmitter content was investigated in selected tissues. The content of the sensory neuron marker peptide calcitonin gene-related peptide (CGRP) was reduced after the capsaicin treatment in the hind paw skin by 35-40% and in the dorsal lumbar spinal cord by 48%. The treatment with FK506 did not induce a recovery of the CGRP content. Following the 6-OHDA treatment the noradrenaline content was reduced by 50-62% in the hind paw skin and by 73% in the heart atrium. FK506 alone did not increase the noradrenaline levels, whereas an additional local intraplantar treatment with nerve growth factor recovered noradrenaline levels almost completely. The expression of a marker protein for growth processes in cells of sympathetic or sensory ganglia, growth-associated protein-43, was significantly increased by the FK506 treatment. This study demonstrated that despite a stimulatory effect of FK506 on the expression of a growth-associated protein a recovery of the transmitter content is not evident in peripheral small diameter sensory or postganglionic sympathetic neurons of the adult rat.     

Dihydrocapsaicin-induced hypothermia and substance P depletion

Administration of dihydrocapsaicin to rats resulted in a dose-dependent (0.5-10 mg/kg s.c.) hypothermia. Dihydrocapsaicin was approximately 65% more effective in producing hypothermia than capsaicin. Desensitization and cross-tolerance occurred to the hypothermic effects of both capsaicin and dihydrocapsaicin. Repeated administration of either dihydrocapsaicin or capsaicin resulted in chemogenic antinociception but not marked thermal antinociception. In addition, repeated administration of dihydrocapsaicin, like capsaicin, resulted in depletion of substance P from dorsal root ganglia and dorsal spinal cord but not from the hypothalamus, corpus striatum or ventral spinal cord. These data indicate that dihydrocapsaicin, or radiolabelled dihydrocapsaicin, may be a useful tool for investigating the mechanisms by which capsaicin alters thermoregulation and primary afferent neuron function.     

Antinociception produced by capsaicin: spinal or peripheral mechanism?

We have studied the effects of capsaicin, administered at concentrations found to be antinociceptive in behavioural tests, on nociceptive responses evoked both in spinal dorsal horn neurons in vivo and in spinal ventral roots in vitro. In halothane anesthetized rats, C-fibre evoked input produced by transcutaneous electrical stimulation in the peripheral receptive field was recorded from single wide dynamic range neurons located in superficial and deep dorsal horn of the lumbar spinal cord. This input was reduced by systemic administration of capsaicin at an antinociceptive dose (20 mumol/kg s.c.). Intradermal injections of capsaicin localized to the peripheral receptive field produced a transient increase in C-fibre evoked activity followed by a prolonged period of localized insensitivity to C-fibre stimulation. Spinal i.t. administered capsaicin also produced a rapid but reversible attenuation of peripherally evoked C-fibre input. In a neonatal rat spinal cord-tail preparation maintained in vitro, superfusion of the spinal cord with capsaicin (100-500 nM) produced a transient depolarization which was followed by an attenuation of responses to peripheral noxious heat and to spinal administration of substance P. Similar activity was produced by a prolonged superfusion of the spinal cord with substance P (50-200 nM). An HPLC method was used to estimate the concentration of capsaicin in a number of tissues following s.c. administration at an antinociceptive dose. In addition capsaicin concentrations were determined in the spinal cord following an i.t. administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT3 receptor binding sites are on capsaicin-sensitive fibres in the rat spinal cord

Specific binding sites for [3H]zacopride were found in the dorsal part of the rat spinal cord, particularly in the superficial layers of the dorsal horn. These binding sites had the same pharmacological profile as 5-HT3 receptors in membranes from the rat entorhinal cortex or from NG 108-15 neuroblastoma-glioma cells. Administration of capsaicin (50 mg/kg s.c.) to neonatal rats to induce degeneration of unmyelinated primary sensory fibres resulted in a significant decrease in [3H]zacopride specific binding (-50%) in the dorsal zone of the spinal cord of 4 month-old rats. This decrease was as pronounced as the decrease in [3H]bremazocine and [3H]naloxone binding to opiate receptors. These data support the presynaptic location of 5-HT3 receptors, at least in part, on capsaicin-sensitive primary afferent fibres in the rat spinal cord.     

Evidence for the involvement of vanilloid receptor in the antinociception produced by the dialdeydes unsaturated sesquiterpenes polygodial and drimanial in rats

This study investigated whether or not the neonatal treatment of rats with the sesquiterpenes polygodial or drimanial could cause persistent antinociception similar to that induced by capsaicin. Rats were injected subcutaneously 48 h after birth with capsaicin (50 mg/kg), polygodial (150 mg/kg), drimanial (150 mg/kg) or vehicle (1ml/kg). Six to eight weeks later, rats were tested in models of nociception. Treatment of rats with capsaicin, polygodial or drimanial produced significant inhibition of the first phase and, to a lesser extent, the second phase of formalin-induced nociception. A significant reduction in Complete Freund's Adjuvant and capsaicin-induced hyperalgesia was observed in the animals neonatally treated with capsaicin, polygodial or drimanial compared with vehicle-treated rats. Moreover, both sesquiterpenes caused inhibition of plasma extravasation induced by injection of capsaicin. The neonatal treatment with capsaicin, polygodial or drimanial significantly decreased [3H]-resiniferatoxin binding sites in the rat spinal cord, but only capsaicin neonatal treatment significantly reduced the expression of TRPV1 in dorsal root ganglia (DRG) when assessed by Western blot. These results extend our previous findings demonstrating that the neonatal treatment of rats with polygodial or drimanial, similar to that reported for capsaicin, produced persistent antinociception in adult animals associated with TRPV1 down-regulation in the spinal cord, but not TRPV1 expression in DRG.     

Neuropharmacological mechanisms of capsaicin and related substances.

Capsaicin activates poorly myelinated primary afferent neurons, many of which are polymodal nociceptors. Activation is accompanied by membrane depolarization and the opening of a unique, cation-selective, ion channel which can be blocked by the polyvalent dye ruthenium red. The capsaicin-induced activation is mimicked by resiniferatoxin, a potent analogue, and by low pH. Activation is mediated by a specific membrane receptor which can be selectively and competitively antagonized by capsazepine. Repetitive administration of capsaicin produces a desensitization and an inactivation of sensory neurons. Several mechanisms are involved including receptor inactivation, block of voltage activated calcium channels, intracellular accumulation of ions leading to osmotic changes, and activation of proteolytic enzyme processes. Systemic and topical capsaicin produces a reversible antinociceptive and anti-inflammatory action after an initial undesirable algesic effect. Capsaicin analogues, such as olvanil, have similar properties with minimal initial algesic activity. Antinociception produced by capsaicin does not involve neurotoxicity, sensory neuropeptide depletion or activity at peripheral receptors; rather, systemic capsaicin produces antinociception by activating capsaicin receptors on afferent nerve terminals in the spinal cord. Spinal neurotransmission is blocked by a prolonged inactivation of sensory neurotransmitter release. However, local or topical applications of capsaicin block C-fibre conduction and inactive neuropeptide release from peripheral nerve endings. These mechanisms account for localized antinociception and the reduction of neurogenic inflammation, respectively.     

Protection by capsaicin against attenuated endothelium-dependent vasorelaxation due to lysophosphatidylcholine

Previous studies have shown that pretreatment with calcitonin gene-related peptide (CGRP), a principal transmitter in sensory nerves, can protect the endothelial cell. We therefore evaluated whether in vivo capsaicin treatment prevents endothelial damage elicited by lysophosphatidylcholine (LPC) in the rat aorta. Acute treatment or repeated pretreatment with capsaicin resulted in stimulation of neurotransmitter release from sensory nerves or depletion of their transmitter content respectively. Vasodilator responses to acetylcholine (ACh) were examined in the aorta of these animals. Acute application of capsaicin (50 mg/kg) increased the plasma concentration of CGRP-like immunoreactivity (CGRP-LI) concomitantly with a reversal of the inhibition by LPC of endothelium-dependent ACh-induced relaxation in the isolated rat aorta. After repeated pretreatment with capsaicin to deplete sensory nerve neurotransmitter content the effects of capsaicin were absent as shown by the plasma CGRP-LI concentration and the vasodilator response to ACh. The results demonstrate that systemic capsaicin treatment, which evokes the release of CGRP from sensory nerves, protects the endothelial cell. The present study also suggests that CGRP may be an endogenous vascular protective substance. Received: 16 January 1997 / Accepted: 28 April 1997     

Capsaicin-sensitive afferents in the rat urinary bladder activate a spinal sympathetic cardiovascular reflex

Summary In urethane-anesthetized rats with an intact spinal cord, application of capsaicin on the outer surface of the urinary bladder produced a transient bradycardia, hypotension and negative cardiac inotropism which were neither prevented by i. v. atropine (0.5 mg/kg) nor by cervical vagotomy. In acute spinal rats (C2-C3) application of capsaicin (0.2 and 2 pg in 25 pl) on the urinary bladder induced a transient hypertension, tachycardia and positive cardiac inotropism. A second application (30 min later) induced minor cardiovascular effects, expecially with the higher dose, indicating desensitization. All cardiovascular responses to topical capsaicin were abolished by systemic capsaicin desensitization (50 mg/kg s. c., 4 days before). The excitatory cardiovascular response to capsaicin in acute spinal rats was markedly reduced by bilateral section of pelvic but not hypogastric nerves. Further, it was abolished by pretreatment with hexamethonium (20 mg/kg i.v.) or reserpine (5 mg/kg i. p., 2 days before) and reduced, at various extent for the different components, by phentolamine (0.5 mg/kg i. v.) or propranolol (1 mg/kg). In rats with pelvic and hypogastric nerves intact, section of the cord at a level (T12-L1), just above the medullary segments which receive primary afferent input from the bladder (L6-S1), abolished the excitatory cardiovascular response to application of capsaicin on the bladder. In spinal rats (C2-C3) rapid distension of the urinary bladder with saline produced transient tachycardia, hypertension and positive cardiac inotropism similar to that evoked by capsaicin. These responses were not observed in rats systemically pretreated with capsaicin. These findings indicate that certain bladder afferents which are susceptible to capsaicin desensitization in adult rats activate a spinal reflex having excitatory influence on cardiovascular function. This response is apparently mediated by spinal centers located above the site of entry of bladder pelvic afferents into the cord and most likely involves excitation of preganglionic sympathetic neurons in the spinal cord.Send offprint requests to S. Giuliani at the above address     

Capsaicin-sensitive sensory neurons regulate myocardial nitric oxide and cGMP signaling

We studied whether tissue levels of nitric oxide (NO) and cGMP are regulated by sensory nerves in normoxic and ischemic hearts. Wistar rats were treated with capsaicin to deplete neurotransmitters from capsaicin-sensitive sensory nerves. In separate experiments, capsaicin was applied perineurally to both vagus nerves for selective chemodenervation of vagal cardiac afferent nerves. Systemic capsaicin administration significantly decreased basal myocardial NO content assessed by electron spin resonance (ESR) spectroscopy, whereas, local treatment of vagus nerves did not change it. Both systemic and local capsaicin treatment decreased cardiac cGMP content measured by radioimmunoassay. In separate experiments, isolated hearts from control and systemic capsaicin-treated rats were subjected to 30-min global ischemia. NO signal intensity increased 10-fold after ischemia, whereas, cardiac cGMP decreased. Capsaicin pretreatment did not influence ischemic NO or cGMP content. These results suggest a major role for capsaicin-sensitive sensory neurons in the maintenance of basal but not ischemic myocardial NO and cGMP content. Vagal sensory nerves may be involved in the regulation of basal myocardial cGMP but not basal NO level. Consequently, basal NO content in the heart is regulated primarily by spinal afferent nerves.     

Spinal administration of lipoxygenase inhibitors suppresses behavioural and neurochemical manifestations of naloxone-precipitated opioid withdrawal

1. This study investigated the role of spinal lipoxygenase (LOX) products in the induction and expression of opioid physical dependence using behavioural assessment of withdrawal and immunostaining for CGRP and Fos protein expression in the spinal cord. 2. Administration of escalating doses (5-50 mg kg-1; i.p.) of morphine for 5 days markedly elevated CGRP-like immunoreactivity in the dorsal horn of the rat spinal cord. Naloxone (2 mg kg-1; i.p.) challenge precipitated a robust withdrawal syndrome that depleted CGRP-like immunoreactivity and increased the number of Fos-like immunoreactive neurons in the dorsal horn. 3. Intrathecal administration of NDGA (10, 20 microg), a nonselective LOX inhibitor, AA-861 (1.5, 3 microg), a 5-LOX selective inhibitor, or baicalein (1.4, 2.8 microg), a 12-LOX selective inhibitor, concurrently with systemic morphine for 5 days or as a single injection immediately preceding naloxone challenge, blocked the depletion of CGRP-like immunoreactivity, prevented increase in the number of Fos-like immunoreactive neurons in the dorsal horn, and significantly attenuated the morphine withdrawal syndrome. 4. The results of this study suggest that activity of LOX products, at the spinal level, contributes to the expression of opioid physical dependence, and that this activity may be expressed through increased sensory neuropeptide release.     

Capsaicin-induced local effector responses,autonomic reflexes and sensory neuropeptide depletion in the pig

Summary Systemic capsaicin pretreatment (total cumulative dose 50 mg/kg administered s.c. over 2 h) was performed in pigs under pentobarbitone anaesthesia and the effects on sensory and sympatho-adrenal mechanisms were examined acutely and 2 days after treatment. During pretreatment with capsaicin, pronounced sensory and sympatho-adrenal activation were noticed. This resulted in a several-fold increase in the systemic arterial plasma levels of calcitonin gene-related peptide (CGRP), neurokinin A (NKA), noradrenaline (NA), adrenaline (Adr) and neuropeptide Y (NPY), and a slight increase (39%) in plasma cortisol. Simultaneously, there was marked tachycardia, an increase in blood pressure, total skin erythema and some bronchoconstriction, all lasting for about 30 min. Upon repeated injections tachyphylaxis was observed. 2 days after capsaicin pretreatment, basal plasma levels of the neuropeptides, catecholamines and cortisol as well as basal cardiovascular and pulmonary parameters were similar in control and capsaicin-treated pigs.The tissue content of CGRP and NKA was reduced by 50–65% in the airways and by 80–90% in the skin 2 days after capsaicin pretreatment. In contrast, the CGRP content was unchanged or increased (by 195%) in the nodose and spinal ganglia, respectively. The corresponding tissue levels of vasoactive intestinal polypeptide (VIP) and NPY were basically unchanged in capsaicintreated pigs. A bolus injection of capsaicin (1 mg/kg i.v.) in control animals resulted in a marked increase in plasma catecholamines and NPY, concomitant with elevation in blood pressure and heart rate. These effects were preceded by an initial bradycardia and decrease in blood pressure. Moreover, capsaicin i.v. produced a clear-cut increase in plasma CGRP and NKA levels together with vasodilatation in the nasal, laryngeal, bronchial and skin circulations and a slight fall in dynamic lung compliance. In capsaicin-treated pigs there was no release of catecholamines, NPY, CGRP or NKA upon i.v. capsaicin challenge. Furthermore, the increase in blood pressure and heart rate and the vasodilatation in the bronchial and skin circulation were markedly reduced. However, the initial bradycardia and the vasodilatation in the nasal and laryngeal circulations after capsaicin i.v. injection persisted in capsaicin-treated animals, and the fall in dynamic compliance was inverted to an increase together with a fall in pulmonary resistance. Local capsaicin aerosol (10 mg) challenge in the airways of control pigs induced a marked vasodilatation, and this response was reduced by 55–85% at all three levels of the airways in capsaicin-treated pigs. Local skin injections with capsaicin (50g) in control pigs produced a marked vasodilatation, but no plasma protein (Evans blue) extravasation. This vasodilatory response was abolished by capsaicin pretreatment.We conclude that systemic capsaicin pretreatment of the anaesthetized pig may be used as a model to show depletion of the content of sensory neuropeptides in peripheral sensory nerve endings of large animals. Furthermore, the functional importance of capsaicin-sensitive sensory nerves in local axon reflexes and sympathoadrenal activation may be studied using this model, although capsaicin-induced effects presumably of parasympathetic origin persist. Send offprint requests to K. Alving at the above address     

Capsaicin in adult frogs: Effects on nociceptive responses to cutaneous stimuli and on nervous tissue concentrations of immunoreactive substance P,somatostatin and cholecystokinin

Summary Adult frogs (Rana esculenta) were given subcutaneous injections of 10, 20, 30, 50 and 100 mg/kg capsaicin in sequential order over 5 days, or the vehicle only. The nociceptive thresholds to electrical, thermal and chemical stimuli were measured before, and 1, 5 and 24 h after each injection. Capsaicin was followed by a dose-related reduction of nociceptive responses to all stimuli, but these effects lasted for only 1–5 h after the given injection. Water/acetic extracts of undivided brains and spinal cords were prepared at the corresponding time periods for the radioimmunoassay of peptides. Spinal cord concentrations of immunoreactive substance P were essentially unaffected by capsaicin, while those of immunoreactive somatostatin were significantly increased after the second for fourth injections (20, 30 and 50 mg/kg) of capsaicin. Brain extracts showed an increase of somatostatin and substance P concentrations after the dose of 50 mg/kg. In an additional experiment, immunoreactive substance P, somatostatin and cholecystokinin were measured in tissue samples taken at 2 and 10 min, and 1, 5 and 24 h after a single dose of either 50 mg/kg capsaicin or the vehicle. The only signficant effect of capsaicin was an increase of immunoreactive somatostatin concentration in brain homogenates at 5 h, while the vehicle in itself elicited major variations of all three peptides in spinal cord and/or brain. These results indicate that capsaicin reduces the nociceptive responses to cutaneous stimuli in adult frogs. This effect is transient, and bears no clear relationship to the variations of spinal cord nor of brain concentrations of immunoreactive substance P, somatostatin and cholecystokinin. In the present experimental conditions, the effects of the vehicle injection to neuropeptides far exceeded those of capsaicin itself.     

Inhibition of SNL-induced upregulation of CGRP and NPY in the spinal cord and dorsal root ganglia by the 5-HT(2A) receptor antagonist ketanserin in rats

Our previous study has demonstrated that topical and systemic administration of the 5-HT_2A receptor antagonist ketanserin attenuates neuropathic pain. To explore the mechanisms involved, we examined whether ketanserin reversed the plasticity changes associated with calcitonin gene-related peptides (CGRP) and neuropeptide Y (NPY) which may reflect distinct mechanisms: involvement and compensatory protection. Behavioral responses to thermal and tactile stimuli after spinal nerve ligation (SNL) at L5 demonstrated neuropathic pain and its attenuation in the vehicle- and ketanserin-treated groups, respectively. SNL surgery induced an increase in CGRP and NPY immunoreactivity (IR) in laminae I-II of the spinal cord. L5 SNL produced an expression of NPY-IR in large, medium and small diameter neurons in dorsal root ganglion (DRG) only at L5, but not adjacent L4 and L6. Daily injection of ketanserin (0.3 mg/kg, s.c.) for two weeks suppressed the increase in CGRP-IR and NPY-IR in the spinal cord or DRG. The present study demonstrated that: (1) the expression of CGRP was enhanced in the spinal dorsal horn and NPY was expressed in the DRG containing injured neurons, but not in the adjacent DRG containing intact neurons, following L5 SNL; (2) the maladaptive changes in CGRP and NPY expression in the spinal cord and DRG mediated the bioactivity of 5-HT/5-HT_2A receptors in neuropathic pain and (3) the blockade of 5-HT_2A receptors by ketanserin reversed the evoked upregulation of both CGRP and NPY in the spinal cord and DRG contributing to the inhibition of neuropathic pain.     

Capsaicin-sensitive nerves modulate resting blood flow and vascular tone in rat gut

Summary Acute and chronic treatments with capsaicin were used to evaluate the role of afferent neurons in the regulation of intestinal blood flow. Experiments were performed on anesthetized rats, in which mean intestinal blood flow was determined with a pulsed Doppler flowmeter mean systemic arterial pressure was determined with a transducer, and intenstinal vascular conductance (C) was calculated from these measurements. Acute administration of periarterial capsaicin (0.5 mg) induced biphasic intestinal vascular responses. An early hyperemic response occurred with a maximal increase in blood flow of 31 % at 5 min, followed by a decrease in blood flow of 17% at 30 min. Arterial pressure was decreased by the application of capsaicin, initially by 10%. There was an early increase of 49% in conductance, followed by a 15% decrease, compared with control values. When 1 or 4 mg capsaicin was instilled into the lumen of the jejunum there was a response pattern similar to that observed after periarterial application of capsaicin. Intrajejunal capsaicin (4 mg) increased blood flow by 51%, followed by a decrease of 16%. Mean mesenteric artery conductance was increased by 32% initially and subsequently was decreased by 21%, in response to acute intrajejunal administration of capsaicin. Both mean blood flow and conductance were increased (44% and 76%, respectively) in adult rats chronically pretreated with capsaicin (170 mg total dose) when compared with vehicle-treated controls.However, in rats pretreated neonatally with capsaicin (50 mg/kg) and allowed to mature, basal flood flow was lower than in control animals but C was not different from control littermates. These findings suggest that the hyperemic effect of acute administration of capsaicin may be related to the stimulation of afferent sensory nerves and release of vasodilatory peptides. The late vasoconstrictor effect could be due to neuronal depletion of vasodilator peptides in perivascular nerves. However, changes observed in the intestinal circulation after chronic pretreatment with capsaicin in the adult rat, in contrast to the observations in the neonatally treated rat, cannot be explained entirely by simple depletion of peptides from the sensory nerves. We conclude that capsaicin-sensitive, afferent nerves in the small intestine modulate the resting vascular tone in rat gut. Send offprint requests to O. D. Hottenstein at the above address