Noradrenergic-serotonergic interactions and nociception in the rat

Bee Venom Acupuncture Effects on Pain and Its Mechanisms: An Updated Review

Bee venom (BV) is a complex natural toxin that contains various pharmaceutical compounds. Bee venom acupuncture (BVA), involving a BV injection into a certain acupuncture point, has been utilized to relieve a range of pain conditions. Regardless of whether pain is caused by disease or injury, if not effectively treated, pain can exert a detrimental effect on all aspects of life. In the past decade, many researchers have investigated the anti-nociceptive effects of BVA through clinical use and experimental evaluation. This report reviews the existing knowledge on the analgesic effects of BVA, focusing on musculoskeletal pain, inflammatory pain and neuropathic pain, and its analgesic mechanisms. Although further clinical trials are needed to clinical application of experimental results, this review will contribute to the standardization and generalization of BVA.

Long-Lasting and Additive Analgesic Effects of Combined Treatment of Bee Venom Acupuncture and Venlafaxine on Paclitaxel-Induced Allodynia in Mice

Paclitaxel, a primary chemotherapeutic agent used to treat numerous solid malignancies, is commonly associated with debilitating peripheral neuropathy. However, a satisfactory gold-standard monotherapy for this neuropathic pain is not currently available. A combination strategy of two or more medications with different properties may achieve more beneficial effects than monotherapy. Thus, we investigated the analgesic efficacies and spinal mechanisms of the combination strategy, including bee venom acupuncture (BVA) and venlafaxine (VLX) against paclitaxel-induced allodynia in mice. Four intraperitoneal infusions of paclitaxel on alternating days (2 mg/kg/day) induced cold and mechanical allodynia for at least 1 week as assessed using acetone and the von Frey hair test, respectively. Co-treatment of BVA (1.0 mg/kg, s.c., ST36) with VLX (40 mg/kg, i.p.) at the medium dose produced a longer-lasting and additive effect than each monotherapy at the highest dose (BVA, 2.5 mg/kg; VLX, 60 mg/kg). Spinal pre-administration of idazoxan (α₂-adrenergic receptor antagonist, 10 μg), methysergide (mixed 5-HT₁/5-HT₂ receptor antagonist, 10 μg), or MDL-72222 (5-HT₃ receptor antagonist, 10 μg) abolished this analgesia. These results suggest that the combination therapy with BVA and VLX produces long-lasting and additive analgesic effects on paclitaxel-induced allodynia, via the spinal noradrenergic and serotonergic mechanism, providing a promising clinical strategy.

Nociceptive input after peripheral nerve injury results in cognitive impairment and alterations in primary afferent physiology in rats

Pain alters cognitive performance through centrally mediated effects in the brain. In this study, we hypothesized that persistent activation of peripheral nociceptors after injury would lead to the development of a chronic pain state that impairs attention-related behavior and results in changes in peripheral neuron phenotypes. Attentional performance was measured in rats using the 5-choice serial reaction time titration variant to determine the initial impact of partial L5 spinal nerve ligation and the effect of persistent nociceptor activation on the resolution of injury. The changes in peripheral neuronal sensibilities and phenotypes were determined in sensory afferents using electrophysiologic signatures and receptive field properties from dorsal root ganglion recordings. Partial spinal nerve injury impaired attentional performance, and this was further impaired in a graded fashion by nociceptive input through an engineered surface. Impairment in attention persisted for only up to 4 days initially, followed by a second phase 7 to 10 weeks after injury in animals exposed to nociceptive input. In animals with prolonged impairment in behavior, the mechanonociceptors displayed a persistent hypersensitivity marked by decreased threshold, increased activity to a given stimulus, and spontaneous activity. Nerve injury disrupts attentional performance acutely and is worsened with peripheral mechanonociceptor activation. Acute impairment resolves, but persistent nociceptive activation produces re-emergence of impairment in the attention-related task associated with electrophysiological abnormalities in peripheral nociceptors. This is consistent with the development of a chronic pain state marked by cognitive impairment and related to persistently abnormal peripheral input.

Role of brainstem serotonin in analgesia produced by low-intensity exercise on neuropathic pain after sciatic nerve injury in mice

Physical exercise is a low-cost, safe, and efficient intervention for the reduction of neuropathic chronic pain in humans. However, the underlying mechanisms for how exercise reduces neuropathic pain are not yet well understood. Central monoaminergic systems play a critical role in endogenous analgesia leading us to hypothesize that the analgesic effect of low-intensity exercise occurs through activation of monoaminergic neurotransmission in descending inhibitory systems. To test this hypothesis, we induced peripheral nerve injury (PNI) by crushing the sciatic nerve. The exercise intervention consisted of low-intensity treadmill running for 2 weeks immediately after injury. Animals with PNI showed an increase in pain-like behaviors that were reduced by treadmill running. Reduction of serotonin (5-hydroxytryptamine) synthesis using the tryptophan hydroxylase inhibitor para-chlorophenylalanine methyl ester prevented the analgesic effect of exercise. However, blockade catecholamine synthesis with the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine had no effect. In parallel, 2 weeks of exercise increased brainstem levels of the 5-HT and its metabolites (5-hydroxyindoleacetic acid), decreased expression of the serotonin transporter, and increased expression of 5-HT receptors (5HT-1B, 2A, 2C). Finally, PNI-induced increase in inflammatory cytokines, tumor necrosis factor-alpha, and interleukin-1 beta, in the brainstem, was reversed by 2 weeks of exercise. These findings provide new evidence indicating that low-intensity aerobic treadmill exercise suppresses pain-like behaviors in animals with neuropathic pain by enhancing brainstem 5-HT neurotransmission. These data provide a rationale for the analgesia produced by exercise to provide an alternative approach to the treatment of chronic neuropathic pain.

The critical role of spinal 5-HT7 receptors in opioid and non-opioid type stress-induced analgesia

The opioid and non-opioid types of stress-induced analgesia have been well defined. One of the non-opioid type involve the endocannabinoid system. We previously reported that the spinal serotonin 7 receptor (5-HT7) blockers inhibit both morphine and cannabinoid-induced analgesia, thus we hypothesized that descending serotonergic pathways-spinal 5-HT7 receptor loop might contribute to stress-induced analgesia. Stress-induced analgesia was induced with warm (32°C) or cold (20°C) water swim stress in male Balb-C mice. The effects of intrathecal injection of a selective 5-HT7 receptor antagonist, SB 269970, of the denervation of serotonergic neurons by intrathecal administration of 5,7-dihydroxytryptamine (5,7-DHT) and of lesions of the dorsolateral funiculus on opioid and non-opioid type stress-induced analgesia were evaluated with the tail-flick and hot plate tests. The expression of 5-HT7 receptors mRNA in the dorsal lumbar region of spinal cord were analyzed by RT-PCR following spinal serotonin depletion or dorsolateral funiculus lesion. The effects of the selective 5-HT7 receptor agonists LP 44 and AS 19 were tested on nociception. Intrathecal SB 269970 blocked both opioid and non-opioid type stress-induced analgesia. Dorsolateral funiculus lesion or denervation of the spinal serotonergic neurons resulted in a marked decrease in 5-HT7 receptor expression in the dorsal lumbar spinal cord, accompanied by inhibition of opioid and non-opioid type stress-induced analgesia. However, the systemic or intrathecal LP 44 and AS 19 alone did not produce analgesia in unstressed mice. These results indicate that descending serotonergic pathways and the spinal 5-HT7 receptor loop play a crucial role in mediating both opioid and non-opioid type stress-induced analgesia.

Noradrenergic-Dopaminergic Interactions Due to DSP-4-MPTP Neurotoxin Treatments: Iron Connection

The investigations of noradrenergic lesions and dopaminergic lesions have established particular profiles of functional deficits and accompanying alterations of biomarkers in brain regions and circuits. In the present account, the focus of these lesions is directed toward the effects upon dopaminergic neurotransmission and expression that are associated with the movement disorders and psychosis-like behavior. In this context, it was established that noradrenergic denervation, through administration of the selective noradrenaline (NA) neurotoxin, DSP-4, should be performed prior to the depletion of dopamine (DA) with the selective neurotoxin, MPTP. Employing this regime, it was shown that (i) following DSP-4 (50 mg/kg) pretreatment of C57/Bl6 mice, both the functional and neurochemical (DA loss) effects of MPTP (2 × 20 and 2 × 40 mg/kg) were markedly exacerbated, and (ii) following postnatal iron (Fe(2+), 7.5 mg/kg, on postnatal days 19-12), pretreatment with DSP-4 followed by the lower 2 × 20 mg/kg MPTP dose induced even greater losses of motor behavior and striatal DA. As yet, the combination of NA-DA depletions, and even more so Fe(2+)-NA-DA depletion, has been considered to present a movement disorder aspect although studies exploring cognitive domains are lacking. With intrusion of iron overload into this formula, the likelihood of neuropsychiatric disorder, as well, unfolds.

Neurotropin® relieves oxaliplatin-induced neuropathy via Gi protein-coupled receptors in the monoaminergic descending pain inhibitory system

AIMS

Oxaliplatin is a key drug in the treatment of colorectal cancer, but it causes acute and chronic peripheral neuropathies. We previously reported that repeated administration of neurotropin prevents oxaliplatin-induced mechanical allodynia by inhibiting axonal degeneration in rats. In the present study, we investigated the analgesic effect of a single administration of neurotropin on oxaliplatin-induced neuropathy in rats.

MAIN METHODS

Oxaliplatin (4mg/kg) was administered intraperitoneally twice a week for 4weeks. Cold hyperalgesia was assessed using the acetone test and mechanical allodynia was evaluated using the von Frey test.

KEY FINDINGS

Repeated injection of oxaliplatin induced cold hyperalgesia on day 5 and mechanical allodynia on day 28. A single administration of neurotropin transiently relieved both pain behaviors. The analgesic effect of neurotropin was inhibited by pretreatment with 5-HT1A, 5-HT2, 5-HT3, and α2 receptor antagonists and by monoamine depletion. Moreover, the analgesic effect of neurotropin was abolished by intrathecal injection of pertussis toxin, a Gi protein inhibitor.

SIGNIFICANCE

These results suggest that neurotropin is effective in relieving oxaliplatin-induced neuropathy, and that Gi protein-coupled receptors in the monoaminergic descending pain inhibitory system may be involved in the analgesic effect of neurotropin. Neurotropin may have clinical potential for the treatment of oxaliplatin-induced neuropathies.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

An increase in spinal cord noradrenaline is a major contributor to the antihyperalgesic effect of antidepressants after peripheral nerve injury in the rat

Antidepressants are often used for the treatment of neuropathic pain. Clinical studies suggest that the efficacy of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) for neuropathic pain is greater than that of selective 5-HT reuptake inhibitors (SSRIs). In the present study, we determined the efficacy and mechanisms involved in the antihyperalgesic effects of milnacipran, an SNRI, compared with paroxetine, an SSRI, and maprotiline, a selective NA reuptake inhibitor, using a rat model of neuropathic pain. Male Sprague-Dawley rats underwent spinal nerve ligation (SNL), and the withdrawal threshold to paw pressure was measured. Intraperitoneal injection of milnacipran (3-30mg/kg) produced a dose-dependent antihyperalgesic effect. The effect was reversed by intrathecal injection of the α(2)-adrenoceptor antagonist idazoxan (30μg), but not by various 5-HT receptor antagonists. Paroxetine produced an antihyperalgesic effect only at the highest dose tested (10mg/kg). This effect was reversed by intrathecal injection of both idazoxan and ondansetron (30μg), a 5-HT3 receptor antagonist. Maprotiline produced an antihyperalgesic effect (10 and 30mg/kg), and the effect was reversed by intrathecal idazoxan. In microdialysis studies, NA and 5-HT concentrations in the spinal dorsal horn were increased after injection of either milnacipran or paroxetine, and only NA was increased after maprotiline. Furthermore, the NA content in the spinal cord of SNL rats was greater than that in normal animals. These findings suggest that an increase in NA in the spinal cord plays an important role in the antihyperalgesic effects of not only NA reuptake inhibitors but also SSRIs.

Advances in perioperative pain management: use of medications with dual analgesic mechanisms, tramadol & tapentadol

Recovery from ambulatory surgical procedures can be limited by postoperative pain. Inadequate analgesia may delay or prevent patient discharge and can result in readmission. More frequently, postoperative pain produces discomfort and interrupts sleep, contributing to postoperative fatigue. The development of effective analgesic regimens for the management of postoperative pain is a priority especially in patients with impaired cardiorespiratory, hepatic, or renal function. Tramadol and tapentadol hydrochloride are novel in that their analgesic actions occur at multiple sites. Both agents are reported to be mu-opioid receptor agonists and monoamine-reuptake inhibitors. In contrast to pure opioid agonists, both drugs are believed to have lower risks of respiratory depression, tolerance, and dependence. The Food and Drug Administration has approved both drugs for the treatment of moderate-to-severe acute pain in adults. This article provides an evidence-based account of the role of tramadol and tapentadol in modern clinical practice.

Treatment consideration and manifest complexity in comorbid neuropsychiatric disorders

Psychiatric disorders may co-occur in the same individual. These include, for example, substance abuse or obsessive-compulsive disorder with schizophrenia, and movement disorders or epilepsy with affective dysfunctional states. Medications may produce iatrogenic effects, for example cognitive impairments that co-occur with the residual symptoms of the primary disorder being treated. The observation of comorbid disorders in some cases may reflect diagnostic overlap. Impulsivity, impulsiveness or impulsive behaviour is implicated in a range of diagnostic conditions including substance abuse, affective disorder and obsessive-compulsive disorder. These observations suggest a need to re-evaluate established diagnostic criteria and disorder definitions, focusing instead on symptoms and symptom-profiles.

Functional consequences of iron overload in catecholaminergic interactions: the Youdim factor

The influence of postnatal iron overload upon implications of the functional and interactive role of dopaminergic and noradrenergic pathways that contribute to the expressions of movement disorder and psychotic behaviours in mice was studied in a series of experiments. (1) Postnatal iron overload at doses of 7.5 mg/kg (administered on Days 10-12 post partum) and above, invariably induced a behavioural syndrome consisting of an initial (1st 20-40 min of a 60-min test session) hypoactivity followed by a later (final 20 min of a 60-min test session) hyperactivity, when the mice were tested at adult ages (age 60 days or more). (2) Following postnatal iron overload, subchronic treatment with the neuroleptic compounds, clozapine and haloperidol, dose-dependently reversed the initial hypoactivity and later hyperactivity induced by the metal. Furthermore, DA D(2) receptor supersensitivity (as assessed using the apomorphine-induced behaviour test) was directly and positively correlated with iron concentrations in the basal ganglia. (3) Brain noradrenaline (NA) denervation, using the selective NA neurotoxin, DSP4, prior to administration of the selective DA neurotoxin, MPTP, exacerbated both the functional (hypokinesia) and neurochemical (DA depletion) effects of the latter neurotoxin. Treatment with L-Dopa restored motor activity only in the animals that had not undergone NA denervation. These findings suggest an essential neonatal iron overload, termed "the Youdim factor", directing a DA-NA interactive component in co-morbid disorders of nigrostriatal-limbic brain regions.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Postnatal iron overload destroys NA-DA functional interactions

C57/BL6 mice were administered either postnatal iron (Fe(2+) 7.5 mg/kg, on postnatal days 10-12) or vehicle, followed by administration of either DSP4 (50 mg/kg, s.c., 30 min after injection of zimeldine, 20 mg/kg, s.c.) or vehicle (saline) at 63 days of age. Three weeks later, iron/vehicle treated, DSP4/vehicle treated mice were injected with either a low dose of MPTP (2 x 20 mg/kg, with a 24-hr interval between injections) or vehicle. Behaviour testing took place a further three weeks (spontaneous behaviour and L-Dopa induced) and two weeks (clonidine-L-Dopa induced) later. Postnatal iron administration exacerbated the bradykinesia induced by MPTP and virtually abolished all spontaneous motor activity in NA-denervated mice that were MPTP-treated. Postnatal iron administration reduced markedly the restoration of motor activity by suprathreshold L-Dopa (20 mg/kg) following a 60-min habituation to the test chambers. Pretreatment with DSP4 effectively eliminated the restorative effect of L-Dopa in the MPTP mice. The synergistic effects of co-administration of clinidine (1 mg/kg) with a subthreshold dose of L-Dopa (5 mg/kg) in elevating the motor activity of MPTP mice were reduced markedly by postnatal iron administration, as well as by pretreatment with DSP4. NA-denervation by DSP4, after postnatal iron treatment, totally abolished the activity-elevating effects of the alpha-adrenoceptor agonist + DA-precursor combination in MPTP mice, and virtually eliminated these effects in saline (non-MPTP) mice. Postnatal iron administration caused enduring higher levels of total iron content in all the groups with an increased level in mice treated with DSP4 followed by MPTP. These divergent findings confirm the direct influence of NA innervation upon dopaminergic functional expression and indicate a permanent vulnerability both in the noradrenergic and dopaminergic pathways following the postnatal infliction of an iron overload.

Central serotonergic and noradrenergic receptors in functional dyspepsia

Functional dyspepsia is a symptom complex characterised by upper abdominal discomfort or pain, early satiety, motor abnormalities, abdominal bloating and nausea in the absence of organic disease. The central nervous system plays an important role in the conducting and processing of visceral signals. Alterations in brain processing of pain, perception and affective responses may be key factors in the pathogenesis of functional dyspepsia. Central serotonergic and noradrenergic receptor systems are involved in the processing of motor, sensory and secretory activities of the gastrointestinal tract. Visceral hypersensitivity is currently regarded as the mechanism responsible for both motor alterations and abdominal pain in functional dyspepsia. Some studies suggest that there are alterations in central serotonergic and noradrenergic systems which may partially explain some of the symptoms of functional dyspepsia. Alterations in the autonomic nervous system may be implicated in the motor abnormalities and increases in visceral sensitivity in these patients. Noradrenaline is the main neurotransmitter in the sympathetic nervous system and again alterations in the functioning of this system may lead to changes in motor function. Functional dyspepsia causes considerable burden on the patient and society. The pathophysiology of functional dyspepsia is not fully understood but alterations in central processing by the serotonergic and noradrenergic systems may provide plausible explanations for at least some of the symptoms and offer possible treatment targets for the future.

Central Hypersensitivity in Chronic Pain: Mechanisms and Clinical Implications

The available literature consistently shows increased pain sensitivity after sensory stimulation of healthy tissues in patients who have various chronic pain conditions. This indicates a state of hypersensitivity of the CNS that amplifies the nociceptive input arising from damaged tissues. Experimental data indicate that central hypersensitivity is probably induced primarily by nociceptive input arising from a diseased tissue. In patients, imbalance of descending modulatory systems connected with psychologic distress may play a role. There is experimental support in animal studies for the persistence of central hypersensitivity after complete resolution of tissue damage. This is particularly true for neuropathic pain conditions, whereby potentially irreversible plasticity changes of the CNS have been documented in animal studies. Whether such changes are present in musculoskeletal pain states is at present uncertain. Despite the likely importance of central hypersensitivity in the pathophysiology of chronic pain, this mechanism should not be used to justify the lack of understanding on the anatomic origin of the pain complaints in several pain syndromes, which is mostly due to limitations of the available diagnostic tools. Treatment strategies for central hypersensitivity in patients have been investigated mostly in neuropathic pain states. Possible therapy modalities for central hypersensitivity in chronic pain of musculoskeletal origin are largely unexplored. The limited evidence available and everyday practice show, at best, modest efficacy of the available treatment modalities for central hypersensitivity. The gap between basic knowledge and clinical benefits remains large and should stimulate further intensive research.

Influence of noradrenaline denervation on MPTP-induced deficits in mice

C57/BL6 mice were administered either DSP4 (50 mg/kg, s.c., 30 min after injection of zimeldine, 20 mg/kg, s.c.) or vehicle (saline) at 63 days of age. Three weeks later, one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered MPTP (2 x 40 mg/kg, s.c., 24 hours between injections; the High dose groups), one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered MPTP (2 x 20 mg/kg, s.c., 24 hours between injections; the Low dose groups), and one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered vehicle. Three weeks later, all six groups were tested in motor activity test chambers, followed by injections of L-Dopa (20 mg/kg, s.c.), and then tested over a further 360 min in the activity test chambers. It was found that pretreatment with the selective NA neurotoxin, DSP4, deteriorated markedly the dose-dependent motor activity deficits observed in the vehicle pretreated MPTP treated mice. These 'ultra-deficits' in the spontaneous motor behaviour of MPTP-treated mice were observed over all three parameters: locomotion, rearing and total activity, and were restricted to the 1(st) and 2(nd) 20-min periods. Administration of L-Dopa (20 mg/kg) following the 60-min testing of spontaneous behaviour restored the motor activity of Vehicle + MPTP treated mice (neither the Vehicle + MPTP-Low nor the Vehicle + MPTP-High groups differed from the Vehicle-Vehicle group, here) but failed to do so in the DSP4 pretreated mice. Here, a dose-dependent deficit of L-Dopa-induced motor activity (over all three parameters) was obtained thereby offering further evidence of an 'ultra-deficit' of function due to previous denervation of the NA terminals. The present findings support the notion that severe damage to the locus coeruleus noradrenergic system, through systemic DSP4, disrupts the facilitatory influence on the nigrostriatal DA system, and interferes with the ability of the nigrostriatal pathway to compensate for or recover from marked injury, MPTP treatment.

Sex differences in tail-flick latency of non-stressed and stressed rats

The present study was conducted to assess whether there are sex differences between male and female subjects in their response to noxious stimuli under non-stressed and stressed conditions. Tail-flick latency assay was used as an experimental tool on 12 adult male and 12 adult female Sprague-Dawley rats before immobilization (i.e., non-stressed condition) and after 30, 120, 240, and 360 min of continuous immobilization (i.e., stressed conditions). It was found that the non-stressed female rats exhibited significantly longer response latency to noxious thermal stimuli than the non-stressed male rats. Stressed conditions caused by immobilization of the animal inside a ventilated restrainer significantly prolonged the tail-flick latency thresholds of both sexes. Female and male rats did not develop any adaptation after 120, 240, and 360 min of immobilization compared to their 30 min of immobilization, as demonstrated by the tail-flick assay. Moreover, the difference in the tail-flick latency between male and female rats was reduced as the duration of immobilization in the restrainer lengthened. At 30 min of immobilization, the significant difference between the sexes reduced to p < .05 compared to p < .01 before immobilization, and to statistically non-significant differences after 120, 240, and 360 min of immobilization. Results from this study suggested that female rats had longer tail-flick latency than male rats in non-stressed environment, and the difference in tail-flick latency after immobilization between the sexes became less significant with time. In addition, 360 min of immobilization was not enough to adapt to the restrainer, as shown by the tail-flick assay.

Serotonin reuptake inhibitors attenuate morphine withdrawal syndrome in neonatal rats passively exposed to morphine

Previous investigations had shown that inhibitor of serotonin reuptake transporter (SERT) could attenuate morphine withdrawal syndrome in adult animals. In the present study, we determined whether postnatal injection of serotonin reuptake inhibitors, fluoxetine, clomipramine, or citalopram, is able to attenuate the expression of the naloxone-precipitated morphine withdrawal syndrome in 5-day-old neonatal Sprauge-Dawley rats born to dams rat that received morphine injection since a week before mating till 5 days after delivery. Withdrawal syndrome of morphine, manifested as frequent abdominal stretching and yawning, was generated by injection of naloxone on postnatal day 5. Pre-injection with fluoxetine, clomipramine, or citalopram, significantly attenuated the naloxone-precipitated syndrome in a dose-dependent manner without apparent side effect. The rank order of inhibitory potency is citalopram=clomipramine>fluoxetine. This result suggests that inhibitor of SERT may be of potential in treating neonatal morphine withdrawal syndrome.

Evidence, Mechanisms, and Clinical Implications of Central Hypersensitivity in Chronic Pain After Whiplash Injury

OBJECTIVES

To provide insights into the mechanisms underlying central hypersensitivity, review the evidence on central hypersensitivity in chronic pain after whiplash injury, highlight reflections on the clinical relevance of central hypersensitivity, and offer a perspective of treatment of central hypersensitivity.

METHODS

A review of animal and human studies focusing on the mechanisms of postinjury central sensitization, an analysis of psychophysical investigations on central hypersensitivity in patients with chronic pain after whiplash injury, and a review of possible treatment modalities.

RESULTS

Animal data show that tissue damage produces plasticity changes at different neuronal structures that are responsible for amplification of nociception and exaggerated pain responses. Some of these changes are potentially irreversible. There is consistent psychophysical evidence for hypersensitivity of the central nervous system to sensory stimulation in chronic pain after whiplash injury. Tissue damage, detected or not by the available diagnostic methods, is probably the main determinant of central hypersensitivity. Psychologic distress could contribute to central hypersensitivity via imbalance of supraspinal and descending modulatory mechanisms. Although specific treatment strategies are limited, they are largely unexplored.

IMPLICATIONS

Central hypersensitivity may explain exaggerated pain in the presence of minimal nociceptive input arising from minimally damaged tissues. This could account for pain and disability in the absence of objective signs of tissue damage in patients with whiplash. Central hypersensitivity may provide a common neurobiological framework for the integration of peripheral and supraspinal mechanisms in the pathophysiology of chronic pain after whiplash. Therapy studies are needed.

Loss of amitriptyline analgesia in alpha 2A-adrenoceptor deficient mice

Tricyclic antidepressants have analgesic and sedative effects in addition to their antidepressive properties. We tested the acute analgesic and locomotor inhibitory effects of the tricyclic antidepressant amitriptyline and the alpha(2)-adrenoceptor agonist clonidine in wild-type control and in alpha(2A)-adrenoceptor knockout mice in hot-plate and tail-flick tests. Amitriptyline-induced analgesia was lost in alpha(2A)-adrenoceptor knockout mice. The locomotor inhibitory effect of amitriptyline was reduced, but not fully abolished in alpha(2A)-adrenoceptor knockout mice. Similar results were obtained with clonidine. We conclude that alpha(2A)-adrenoceptors appear to have a significant role in amitriptyline-induced acute analgesia in mice, and that alpha(2A)-adrenoceptors also participate in the sedative effects of amitriptyline.

Effects of neurotoxic destruction of descending noradrenergic pathways on cannabinoid antinociception in models of acute and tonic nociception

The effects of neurotoxic destruction of catecholaminergic projections to the spinal cord on cannabinoid antinociception were examined in models of acute and tonic nociception. High performance liquid chromatography was used to quantify monoamine levels in sham-operated and lesioned rats. Intrathecal administration of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) induced a selective depletion of norepinephrine (by approximately 85% of control) in rat lumbar spinal cord without altering levels of dopamine or serotonin. By contrast, brain levels of monoamines did not differ in sham-operated and lesioned rats. Pain behavior was similar in sham-operated and lesioned rats receiving vehicle in models of both acute and tonic nociception. The cannabinoid agonist WIN55,212-2 (5 or 10 mg/kg, i.p.) produced antinociception in the tail-flick test in sham-operated rats. The antinociceptive effect of WIN55,212-2 was attenuated relative to control conditions in rats depleted of spinal norepinephrine. WIN55,212-2 suppressed tonic pain behavior in the formalin test in sham-operated rats during phase 2 (15-60 min post formalin) of nociceptive responding. By contrast, in lesioned rats, WIN55,212-2 suppressed pain behavior during phase 1 (0-9.9 min) and phase 2A (10-39.9 min), but not during phase 2B (40-60 min). The cannabinoid agonist suppressed formalin-evoked Fos protein expression, a marker of neuronal activity, in the lumbar dorsal horn of sham-operated rats, but no suppression was observed in lesioned rats. The number of formalin-evoked Fos-like immunoreactive (FLI) cells was greater in lamina I and II of lesioned rats relative to sham-operated rats. These data indicate that the suppressive effect of the cannabinoid on formalin-evoked Fos protein expression in the superficial dorsal horn was attenuated following destruction of descending noradrenergic pathways. Our data are consistent with the hypothesis that cannabinoids produce antinociception, in part, by modulating descending noradrenergic systems and support a differential involvement of noradrenergic projections to the spinal cord in cannabinoid modulation of acute versus tonic nociception.

Antinociceptive effects of methysergide in various pain models

Several studies have demonstrated that the nonselective opioid receptor antagonist naloxone produces a paradoxical antinociception in the formalin test. The opioid system is related to the serotonergic system for producing antinociception at the spinal level. Here we also asked whether systemic (i.p.) and intrathecal (i.t.) administrations of a nonselective serotonergic antagonist, methysergide, might produce paradoxical antinociception similar to naloxone in the mouse formalin test. A diluted formalin solution was injected into the mouse plantar region of the hind paw and the duration of licking responses was measured at periods of 0-5 min (1st phase) and 20-40 min (2nd phase) after formalin injection. Methysergide administered i.p. and i.t. showed an attenuated licking duration only in the 2nd phase. The effect observed in the 2nd phase was reversed in the 5,7-dihydroxytriptamine, but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine pretreated group of mice, suggesting that descending serotonergic, but not noradrenergic, systems are involved in the methysergide antinociception. To further investigate the mechanism by which methysergide inhibited the nociceptive behaviors induced by formalin, the antinociceptive effect of methysergide was also tested in substance P (i.t.) and excitatory amino acids (i.t.), such as glutamate, N-methyl-D-aspartic acid, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainic acid, which are major components in the formalin-induced nociceptive transmission in the spinal cord pain models. The duration of nociceptive behaviors shown in these models was significantly shortened by i.p. and i.t. administration of methysergide. These results suggest that methysergide also produces a paradoxical antinociception in various pain models including the formalin test, similar to the results of naloxone.

Possible antinociceptive mechanisms of opioid receptor antagonists in the mouse formalin test

It has been reported that opioid receptor antagonist can induce antinociception in several nociceptive tests. In the intraplantar formalin pain model, however, opioid antagonist-induced antinociception, as well as its underlying mechanism, has not been well characterized. Therefore, in the mouse formalin test, we attempted to characterize the site of action and the possible opioid receptor subtypes. We found that naltrexone (a nonselective opioid antagonist) injected intraperitoneally (i.p., 1-20 mg/kg), intrathecally (i.t., 0.1-10 microg) and intracerebroventricularly (i.c.v., 0.1-10 microg) phase. Administration of beta-funaltrexamine (beta-FNA, 10-40 mg/kg i.p., 1.25-5 microg it or i.c.v.), naltrindole (1-10 mg/kg i.p., 1.25-5 microg it or i.c.v.) and nor-binaltorphimine (nor-BNI, 1-10 mg/kg i.p., 10-40 microg it or i.c.v.), which are selective mu-, delta- and kappa-opioid antagonists, respectively, also produced antinociception during the second phase. Additionally, we examined the involvement of the descending monoaminergic systems in the naltrexone-induced antinociception in the formalin test. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, a serotonergic neurotoxin, 20 microg i.t.), but not N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, a noradrenergic neurotoxin, 20 microg i.t.), reversed the naltrexone-induced antinociception during the second phase. Our results suggest that blockade of supraspinally or spinally located opioid receptors may play roles in the regulation of antinociception during the tonic painful stage. In addition, opioid receptors localized at the neuroterminal of the descending serotonergic, but not noradrenergic, inhibitory system in the spinal cord appear to be involved in opioid antagonist-induced antinociception during the second tonic phase of the formalin test.

On the antinociceptive effect of fluoxetine, a selective serotonin reuptake inhibitor

Antidepressant drugs are reported to be used as co-analgesics in clinical management of migraine and neuropathic pain. The mechanism through which they alleviate pain remains unknown. The present study explores the possible mechanism of a selective serotonin reuptake inhibitor (SSRI) fluoxetine-induced antinociception in animals. Acetic acid-induced writhing, hot plate and tail-flick test were used to assess fluoxetine-induced antinociception. Fluoxetine (5-20 mg kg(-1), i.p.) produced a significant and dose-dependent antinociceptive effect against acetic acid-induced writhing in mice. Fluoxetine (20 mg kg(-1)) also exhibited antinociceptive effect in tail flick as well as hot plate assays. Further, i.c.v. administration of fluoxetine showed significant antinociception against writhing test in rats. However, fluoxetine (1 microg/10 microl/rat, i.c.v.) did not exhibit any antinociceptive effect in serotonin-depleted animals. Further, pindolol (10 mg kg(-1), i.p.) enhanced fluoxetine-induced antinociceptive effect. The antinociceptive effect of fluoxetine was sensitive to blockade by naloxone (5 mg kg(-1), i.p.) and naltrexone (5 mg kg(-1), i.p.). These data suggest that fluoxetine-induced antinociception involves both central opioid and the serotoninergic pathways.

Reduction in excessive muscle tone by selective depletion of serotonin in intercollicularly decerebrated rats

Intercollicular decerebration in animals induces sustained facilitation of muscle tone of the limbs and this animal model has been used to assess centrally acting muscle relaxants. We have examined the involvement of central and spinal cord serotonergic pathways in the onset of excessive muscle tone in an intercollicularly decerebrated rat. Descending serotonergic pathways are known to modulate, directly or indirectly, the excitability of spinal cord motoneurons and it is inferred that serotonin (5-HT) plays an important role in locomotion. Alteration of muscle tone has been investigated in 5-HT-depleted rats with a neurotoxin, 5, 7-dihydroxytryptamine (5,7-DHT) after pretreatment with desipramine. Intracerebroventricular (i.c.v.) administration of 5,7-DHT reduced 5-HT content in the forebrain to 50.5% and that in the spinal cord to 10.5%, while intrathecal (i.t.) administration of 5,7-DHT decreased 5-HT content in the spinal cord to 8.9% without causing any change in the forebrain. In contrast, noradrenaline or dopamine content was not affected by the neurotoxin in both tissues. These treatments significantly attenuated the muscle tone in the animal models. Moreover, the measurement of 5-HT and 5-hydroxyindoleacetic acid content in intact rats after decerebration showed that facilitation of the 5-HT turnover in the spinal cord, but not in the forebrain, was enhanced compared with sham-operated rats. These findings suggest that the descending serotonergic pathways are essential to induce excessive muscle tone in the intercollicular decerebrated rats and that 5-HT antagonists might be candidates for centrally acting muscle relaxants.

NMDA and AMPA receptors evoke transmitter release from noradrenergic axon terminals in the rat spinal cord

N-methyl-D-aspartate (NMDA) stimulated release of [3H]noradrenaline (NA) from prelabelled rat spinal cord slices. The release was partially insensitive to tetrodotoxin (TTX) and was inhibited by the NMDA antagonist MK-801. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) also evoked release of [3H]NA, which was enhanced by blocking AMPA receptor desensitization with cyclothiazide. AMPA-evoked release was inhibited by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX) but was not affected by TTX. NMDA and AMPA showed synergistic effects, indicating co-existence of NMDA and AMPA receptors on noradrenergic terminals. Kainate evoked [3H]NA release only at high concentrations and the release was not potentiated by blocking kainate receptor desensitization with concanavalin A. Thus, the results indicate that there are stimulatory presynaptic NMDA and AMPA receptors on noradrenergic axon terminals in the spinal cord and that they interact synergistically to evoke release of [3H]NA.

Anti-opioid efficacy of neuropeptide FF in morphine-tolerant mice

The modulatory effects of 1DMe (d-Tyr-Leu-(NMe)Phe-Gln-Pro-Gln-Arg-Phe-NH2), an agonist of Neuropeptide FF (NPFF) receptors, on opioid antinociceptive activity have been compared in naive and tolerant mice in the tail-flick and the hot-plate tests. In naive mice, 1DMe alone had no effect on pain threshold but decreased dose-dependently (3-22 nmol) the analgesic activity of morphine in both tests. In tolerant mice, injections of 60-fold lower doses of 1DMe (0.05-0.5 nmol) reverse morphine-induced analgesia in the tail-flick test but this anti-opioid effect was no longer observed with the highest doses of 1DMe tested (3-22 nmol). In the hot-plate test, the anti-opioid action of 1DMe was not detected, whatever doses tested. Neither the NPFF-like immunoreactivity content of spinal cord and of olfactory bulbs, nor the density of NPFF receptors in olfactory bulbs, were altered. These results indicate that a chronic morphine treatment modifies the pharmacological properties of NPFF but the type of pain test is crucial in determining NPFF effects.

Influence of flupirtine on a G-protein coupled inwardly rectifying potassium current in hippocampal neurones

1. Previous studies have shown that flupirtine, a centrally acting, non-opioid analgesic agent, also exhibits neuroprotective activity in focal cerebral ischaemia in mice and reduces apoptosis induced by NMDA, gp 120 of HIV, prior protein fragment or lead acetate as well as necrosis induced by glutamate or NMDA in cell culture. To study the potential mechanism of the neuroprotective action of flupirtine, we investigated whether flupirtine is able to modulate potassium or NMDA-induced currents in rat cultured hippocampal neurones by use of the whole-cell configuration of the patch-clamp technique. 2. We demonstrated that 1 microM flupirtine activated an inwardly rectifying potassium current (K(ir)) in hippocampal neurones (deltaI=-39+/-18 pA at -130 mV; n=10). This effect was dose-dependent (EC50=0.6 microM). The reversal potential for K(ir) was in agreement with the potassium equilibrium potential predicted from the Nernst equation showing that K(ir) was predominantly carried by K+. Furthermore, the induced current was blocked completely by Ba2+ (1 mM), an effect typical for K(ir). 3. The activation of K(ir) by flupirtine was largely prevented by pretreatment of the cells with pertussis toxin (PTX) indicating the involvement of a PTX-sensitive G-protein in the transduction mechanism (deltaI=-3+/-6 pA at -130 mV; n=8). Inclusion of cyclic AMP in the intracellular solution completely abolished the activation of K(ir) (n=7). 4. The selective alpha2-adrenoceptor antagonist SKF-86466 (10 microM), the selective 5-HT1A antagonist NAN 190 as well as the selective GABA(B) antagonist 2-hydroxysaclofen (10 microM) failed to block the flupirtine effect on the inward rectifier. 5. Flupirtine (1 microM) could not change the current induced by 50 microM NMDA. 6. These results show that in cultured hippocampal neurones flupirtine activates an inwardly rectifying potassium current and that a PTX-sensitive G-protein is involved in the transduction mechanism.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Evaluation of anti-nociceptive effects of neuronal nicotinic acetylcholine receptor (NAChR) ligands in the rat tail-flick assay

In the present investigation, anti-nociceptive effects of neuronal nicotinic acetylcholine receptor (NAChR) ligands, (+)- and (-)-nicotine, cytisine, methylcarbamylcholine (MCC), dimethylphenylpiperazinium iodide (DMPP), and (+/-)-epibatidine were evaluated in the rat tail-flick assay both after subcutaneous (s.c.) and intracerebroventricular (i.c.v.) administration. The pharmacology of the tail-flick response to NAChR ligands after s.c. and i.c.v. routes was similar. Epibatidine was the most potent ligand examined with a longer duration of action than any other agonist. (-)-Nicotine was more active than (+)-nicotine indicating stereospecificity. ICV administration studies indicated an apparent partial agonist activity for (+)-nicotine in the tail-flick response. Tail-flick responses to NAChR agonists are independent of opioid and muscarinic pathways and appear to be mediated both by central and peripheral NAChR recognition sites. Central administration of MCC activates both NAChR and muscarinic anti-nociceptive mechanisms. Studies employing the alpha-adrenergic receptor alkylating agent, phenoxybenzamine or the noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), suggested that the NAChR-noradrenergic and NAChR-serotoninergic interactions play an important role in the tail-flick response. Studies employing a selective alpha-bungarotoxin-sensitive NAChR receptor antagonist, methyllycaconitine (MLA), suggested a minimal role for these receptors in the tail-flick response. The biochemical studies also indicated that a sub-population of NAChR receptors are located pre-synaptically on noradrenergic and/or serotoninergic pathways in the hippocampus.

Serotonin modulates retinotectal and corticotectal convergence in the superior colliculus

A dense serotonin (5-HT)-containing projection to the superficial layers of the superior colliculus (SC) has been demonstrated in diverse mammalian species, but how 5-HT may affect visual signals within these laminae is largely unknown. This study undertook to investigate the distribution of 2 types of 5-HT receptors in the SC and to ascertain their physiological effects on transmission of visual signals to the SC from the retinotectal and corticotectual pathways. Autoradiography of tissue sections exposed to [3H]-8-OH-DPAT (8-hydroxy-dipropylaminotetraline) or to [125I]cyanopindolol plus isoproterenol showed that 5-HT1A and 5-HT1B receptors, respectively, were present in the superficial SC layers. In unilaterally enucleated animals, binding of ligand to 5-HT1B receptors was greatly reduced on the deafferented (contralateral) side, which is consistent with the possibility that these receptors are located on preterminal axons. Binding to 5-HT1A receptors was unaltered by enucleation. In recordings of superficial layer neurons from SC slices, application of 5-HT during blockade of 5-HT1A receptors with spiperone reduced the amplitude of EPSPs evoked by stimulation of the optic tract. The 5-HT concentration for a 50% reduction in EPSP amplitude was 6 microM. Under these conditions, there were no significant alterations in either membrane potential or input resistance concurrent with 5-HT mediated reduction in EPSPs. During extracellular in vivo recordings, 5-HT, applied by iontophoresis or micropressure or by endogenous release produced by electrical stimulation of the dorsal raphé nucleus, strongly suppressed visual activity in SC neurons. The effectiveness of 5-HT application was significantly stronger on responses evoked by electrical stimulation of the optic chiasm (an average response decrement of 92.2%) than on these evoked in the same neurons by stimulation of visual cortex (an average response reduction of 32.3%). These results support the following conclusions. The 5-HT1B receptors are located preferentially on optic axon terminals and exert presynaptic inhibition of retinotectal inputs. Secondly, 5-HT1A receptors probably have a postsynaptic localization and may affect activity of SC neurons irrespective of the source of input. The combined effect of 5-HT at both subtypes would bias SC visual activity toward information received from the corticotectal pathway.

Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception

The effects of 5-hydroxytryptamine (5-HT) and ligands selective for particular 5-HT receptor subtypes on the transmission of nociceptive information in the spinal cord are complex. In these studies, we have focused on their interactions with two endogenous mediators of pain suppression, noradrenaline (NA) and adenosine. Spinal antinociception by 5-HT is blocked by alpha-adrenoreceptor antagonists and depletion of endogenous NA by 6-hydroxydopamine, while it is potentiated by blockade of NA reuptake with desipramine. These observations provide evidence for a 5-HT receptor-mediated increase in the release of NA from the spinal cord. This action appears to be due to activation of a 5-HT1-like receptor as it is mimicked by some 5-HT1 receptor ligands (mCPP, TFMPP and 5-Me-O-DMT), but not by DOI (5-HT2) or 2-Me-5-HT (5-HT3). An additional component of 5-HT action is via release of adenosine. Antinociception by 5-HT is blocked by the adenosine receptor antagonist 8-phenyltheophylline, and 5-HT has been shown to release adenosine from the spinal cord in in vitro and in vivo paradigms. Methylxanthine-sensitive antinociception is seen with some 5-HT1 receptor ligands (CGS 12066B, mCPP), but not with others or with DOI or 2-Me-5-HT. Further characterization of the 5-HT receptor subtype involved in adenosine release will require the use of additional approaches.

Intrathecal 5-methoxy-N,N-dimethyltryptamine in mice modulates 5-HT1 and 5-HT3 receptors

The antinociceptive effects of intrathecally administered 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), a potent 5-HT receptor agonist, were studied in three behavioral tests in mice: the tail-flick test and the intrathecal substance P and N-methyl-D-aspartic acid (NMDA) assays. Intrathecal administration of 5-MeO-DMT (4.6-92 nmol/mouse) produced a significant prolongation of the tail-flick latency. This action was blocked by 5-HT3 and gamma-aminobutyric acidA (GABAA) receptor antagonists but not by 5-HT2, 5-HT1A, 5-HT1B or 5-HT1S receptor antagonists. Binding studies indicated that 5-MeO-DMT had very low affinity for 5-HT3 receptors. 5-MeO-DMT inhibited biting behavior while increasing scratching behavior induced by intrathecally administered substance P. The inhibition of biting behavior was antagonized by intrathecal co-administration of 5-HT1B and GABAA receptor antagonists while 5-HT1A, 5-HT1S, 5-HT2 and 5-HT3 receptor antagonists had no effect. 5-MeO-DMT-enhanced scratching behavior was inhibited by all the antagonists used except ketanserin and bicuculline, suggesting the involvement of 5-HT1A, 5-HT1B, 5-HT1S, 5-HT3 and GABAA receptors. NMDA-induced biting behavior was inhibited by 5-MeO-DMT pretreatment; this action was antagonized by 5-HT1B, 5-HT3 and GABAA receptor antagonists. The involvement of these receptors in 5-MeO-DMT action suggests that it may promote release of 5-HT (5-hydroxytryptamine, serotonin).

Intrathecal coadministration of serotonin and morphine differentially modulates the tail-flick reflex of intact and spinal rats

In a previous study, we found that the antinociceptive effect of IT-administered morphine on the tail-flick (TF) reflex of rats was potentiated within 1 day after spinal transection. This suggested that the analgesic effect of spinal morphine in the intact animal was tonically suppressed, presumably by the release of a transmitter(s) from descending supraspinal pathway(s), and that the potency of IT morphine was increased because these inputs were removed by spinalization. Because spinally projecting serotonin [5-hydroxytryptamine (5-HT)] fibers are known to be involved in modulating nociception at this site, the present studies examined the possibility that 5-HT might be the proposed "antiopiate" at the spinal cord. Separate groups of intact and spinal rats were pretested on the TF and then injected IT with either morphine (intact: 0.25-5.0 micrograms, spinal: 0.0312-0.5 microgram) or 5-HT (1-200 micrograms), or combinations of these two agents, in a single solution. All rats were then retested 15 min later and the difference in latency was used to compare the effect of these treatments. The results confirmed that the antinociceptive effect of IT morphine was significantly increased by spinalization, whereas the antinociceptive effect of 5-HT was essentially abolished. In intact rats, morphine-induced analgesia was potentiated by a low (10 micrograms) dose of 5-HT but not by higher doses. However, in the spinal rat morphine-induced antinociception was antagonized by the same (10 micrograms) dose. The data suggest that IT 5-HT promotes antinociception in intact rats but acts pro-nociceptively in spinal rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous noradrenaline activates alpha 2-adrenoceptors on serotonergic nerve endings in human and rat neocortex

Slices from human neocortex preincubated with [3H]serotonin ([3H]5-HT) were superfused and stimulated electrically to investigate whether the alpha 2-adrenoceptors on serotonergic terminals can be stimulated by endogenous noradrenaline (NA) released from neighboring noradrenergic fibers. The stimulation-evoked 3H overflow, representing action potential-induced, exocytotic release of 5-HT, was depressed by the NA uptake blocker (+)-oxaprotiline. Rauwolscine (a mixed alpha 2-adrenoceptor antagonist/5-HT autoreceptor agonist) or phentolamine [a combined alpha-adrenoceptor/5-HT autoreceptor antagonist; the latter drug in the presence of (+)-oxaprotiline] enhanced the release when the 5-HT autoreceptors had previously been blocked by metitepine. Under hypothermia the release of 5-HT was found to be decreased and that of NA to be increased; under these conditions idazoxan (an alpha 2-adrenoceptor antagonist) enhanced the release of 5-HT. In neocortex slices from rats (+)-oxaprotiline similarly depressed the release of 5-HT (measured with the same methods) as in human tissue. When rats were pretreated with 6-hydroxydopamine, the inhibitory effect of exogenous NA on 5-HT release was increased, and in slices from rats pretreated with desipramine, it was decreased. In conclusion, alpha 2-heteroreceptors can be activated by endogenous NA released from neighboring noradrenergic fibers. Because regulatory processes analogous to those in rats probably occur in humans as well, an up- or down-regulation of alpha 2-heteroreceptors in depressed patients with a (pathological) decrease or a (therapeutic) enhancement of the noradrenergic neurotransmission may also be assumed to occur.

The serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine facilitates noradrenaline release from rat spinal cord slices and inhibits monoamine oxidase activity

1. The influences of the purported serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (MeODMT) on noradrenaline release and metabolism were investigated in a rat spinal cord release model and a monoamine oxidase (MAO) assay. 2. MeODMT inhibited the basal outflow of tritium from rat spinal cord slices preincubated with [3H]noradrenaline and enhanced the electrically-evoked overflow. 3. Effects on basal outflow were not observed, when monoamine oxidase (MAO) was inhibited by pargyline. Effects on the evoked overflow were not observed in the presence of metitepine or phentolamine. 4. Preferential inhibition by MeODMT of MAO A-type enzyme activity was found in a direct assay. 5. The results provide evidence for two different effects by which MeODMT reinforces noradrenergic neurotransmission in the rat spinal cord: facilitation of stimulation-evoked noradrenaline release and inhibition of noradrenaline metabolism by MAO inhibition.

Noradrenergic mediation of spinal antinociception by 5-hydroxytryptamine: characterization of receptor subtypes

The present study examined the involvement of spinal noradrenergic mechanisms in spinal antinociception by the 5-hydroxy-tryptamine (5-HT) receptor-selective agonists CGS 12066B (5-HT1B; 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline), TFMPP (5-HT1C; M-trifluoromethylphenyl-piperazine) and DOI (5-HT2; 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) using the rat hot plate test. Effects of alpha-adrenoreceptor antagonists (phentolamine, yohimbine), the adrenergic neurotoxin 6-hydroxydopamine, and the selective noradrenergic uptake blocker desipramine were determined. CGS 12066B, TFMPP and DOI produced dose-related antinociception. The antinociceptive effect of each agent was reduced by pretreatment with both phentolamine and yohimbine (15-60 micrograms). Pretreatment with 6-hydroxydopamine (100 micrograms, intrathecal) for 7-10 days, which reduced spinal cord levels of noradrenaline by 87%, inhibited the action of TFMPP (and 5-HT), but not CGS 12066B or DOI. Pretreatment with desipramine (25 mg/kg, systemic) potentiated the action of TFMPP but not CGS 12066B or DOI (or 2-methyl-5-HT). These results suggest that antinociception by TFMPP is dependent on release of endogenous noradrenaline from the spinal cord, while that produced by CGS 12066B and DOI is not. As TFMPP exhibits a close similarity to 5-HT in these experiments, the 5-HT receptor subtype being activated to induce noradrenaline release may either be a 5-HT1C or a 5-HT1S subtype. Other mechanisms account for the observed blockade of the action of CGS 12066B and DOI by alpha-adrenoreceptor antagonists.

Effect of 5-HT receptor and adrenoceptor antagonists on micturition in conscious cats

Micturition was induced in awake cats by infusing saline into the bladder at a physiological filling rate. Methysergide, a serotonergic antagonist given intrathecally, decreased the volume at which micturition occurred. Phentolamine, a non-specific alpha-adrenoceptor antagonist, also decreased volume threshold. Prazosin, an alpha 1-adrenoceptor antagonist, was without effect on micturition. These results imply that 5-HT receptors and alpha 2-adrenoceptors may be inhibitory to micturition at a spinal level.

Desipramine potentiates spinal antinociception by 5-hydroxytryptamine, morphine and adenosine

The effects of pretreatment with desipramine, a selective noradrenaline (NA) uptake blocker, on spinal antinociception by 5-hydroxytryptamine (5-HT), morphine and an adenosine analog (NECA) in the rat hot-plate test were examined to determine if endogenous NA is involved in the spinal action of these agents. Desipramine, 25 mg/kg, had no significant intrinsic effect in the hot-plate test but potentiated spinal antinociception by NA and 5-HT. Potentiation was more prominent at higher doses of NA and 5-HT. Desipramine also enhanced the action of morphine and NECA, but, in these instances, the greatest enhancement occurred at lower doses. These results, in conjunction with others, suggest that 5-HT releases NA from the spinal cord while morphine and NECA interact synergistically with endogenously released NA.

Functional interrelationship of serotonin and norepinephrine: cortisol response to MCPP and DMI in patients with panic disorder, patients with depression, and normal control subjects

The relationship between norepinephrine (NE) and serotonin (5-hydroxytryptamine; 5HT) functioning was explored in a neuroendocrine challenge paradigm. Ten normal control subjects, 17 patients with major depression, and 22 patients with panic disorder volunteered to participate in this study. Each subject received a challenge with meta-chlorophenylpiperazine (MCPP; 0.25 mg/kg, p.o.), a 5HT agonist, and desmethylimipramine (DMI; 75 mg, i.m.), an indirect NE agonist, in randomized order. The peak-minus-baseline cortisol response to MCPP was used as an indicator of 5HT function, and cortisol response at 75 minutes-minus-baseline to DMI was used as an indicator of NE function. The cortisol responses to DMI and MCPP were found to be highly negatively correlated in the total sample, in particular in the patients with major depression and panic disorder. This finding suggests that the functions (or dysfunctions) of the NE and 5HT systems may not be separate as is usually believed, and that the NE and 5HT disturbances observed in major depression and panic disorder may not be independent. Rather, there may be a joint disturbance of NE-5HT in these disorders.

Clinical interactions with alpha-2-adrenergic agonists in anesthetic practice

With the continued use of alpha-2-adrenergic agonists in anesthetic practice, careful attention should be given to the potential for drug interactions. Based on a review of the basic and applied pharmacology of this class of compound, we have made recommendations for the safe and efficacious use of alpha-2-adrenergic agonists in the clinical setting.

Noradrenergic and purinergic involvement in spinal antinociception by 5-hydroxytryptamine and 2-methyl-5-hydroxytryptamine

The adrenergic involvement in spinal antinociception by 5-hydroxytryptamine (5-HT) and 2-methyl-5-hydroxytryptamine (2-Me-5-HT) was examined using the alpha-adrenoceptor antagonists phentolamine, yohimbine and prazosin, and the neurotoxin 6-hydroxydopamine. Intrathecal pretreatment with phentolamine and yohimbine (7.5-30 micrograms), but not prazosin (30 micrograms), reduced the action of 5-HT and 2-Me-5-HT in both the tail flick and hot plate tests. Pretreatment with 6-hydroxydopamine (100 micrograms) reduced (5-HT) or increased (2-Me-5-HT) antinociception in the hot plate test, while tail flick responses were largely unaffected. In other experiments, 8-phenyltheophylline, an adenosine receptor antagonist, reduced the action of 5-HT, but not 2-Me-5-HT, in both tests. These results indicate that (a) antinociception by both 5-HT and 2-Me-5-HT involves some form of interaction with spinal alpha 2-adrenoceptors, but the nature of the interaction for these two agents is different because only 5-HT is dependent on endogenous noradrenaline, (b) release of adenosine from the spinal cord contributes to spinal antinociception by 5-HT but not by 2-Me-5-HT.

Intrathecal coadministration of clonidine with serotonin receptor agonists produces supra-additive visceral antinociception in the rat

The intrathecal (i.t.) coadministration of sub-antinociceptive doses of clonidine, an alpha 2-adrenoceptor agonist, with DOI or RU-24969 (5-HT2 or 5-HT1B receptor agonists, respectively) produced dose-dependent supra-additive antinociceptive effects in a model of visceral pain. The enhanced attenuation of responses to noxious colorectal distension produced by the coadministration of these drugs is evidenced by significant leftward shifts in the dose-response curves as compared to those of each drug alone and by isobolographic analysis. The supra-additive antinociceptive effects produced following the i.t. coadministration of clonidine with RU-24969 were antagonized by i.t. pretreatment with phentolamine; the coadministration of phentolamine with methysergide produced no greater antagonism of effects. The supra-additive antinociceptive effects produced by i.t. coadministration of clonidine with DOI were antagonized by i.t. pretreatment with methysergide; the coadministration of methysergide with yohimbine produced no greater antagonism of effects. These data suggest that receptors acted upon by descending bulbospinal neurons interact to modulate the rostrad transmission of visceral nociceptive transmission.

Central analgesic mechanisms: a review of opioid receptor physiopharmacology and related antinociceptive systems

Clinical applications of these principles, based on the increased understanding of central analgetic mechanisms, are already being undertaken. Not only does the use of intrathecal and epidural opioids have the potential to decrease pain and related morbidity after surgical procedures, but there is at least one study that demonstrates a significant reduction in both major morbidity and mortality in high-risk surgical patients in whom epidural anesthesia and analgesia were used. These principles are also useful for the management of patients undergoing cardiac surgery. Currently, high-dose narcotic anesthesia is the technique of choice for such patients because of the greater hemodynamic stability this anesthetic technique provides. However, breakthrough hypertension and tachycardia still occur, and prolonged postoperative ventilation is a necessary consequence due to the high doses of narcotics that are required. In one study of patients undergoing coronary artery surgery, preoperative administration of clonidine, 5 micrograms/kg, orally, was demonstrated to decrease fentanyl requirements by 45% (110 to 61 micrograms/kg) while producing a similar degree of hemodynamic stability as seen with high-dose fentanyl. Extubation times were not compared, but the significantly lower dosage of fentanyl in the clonidine-treated group would be expected to lead to an earlier extubation. Whether similar potentiation of narcotic effects would be seen with dexmedetomidine, which may also prevent narcotic-induced rigidity, has not been determined, but the clinical application of such synergistic and complementary agents is another consequence of the greater understanding of central analgesic mechanisms, and augurs well for the future.