Role of spinal serotonin1 receptor subtypes in thermally and mechanically elicited nociceptive reflexes

Chronic administration of 5-HT1A receptor agonist relieves depression and depression-induced hypoalgesia

Previous studies have shown that depressed patients as well as animal models of depression exhibit decreased sensitivity to evoked pain stimuli, and serotonin is indicated to be involved in depression-induced hypoalgesia. The purpose of this study was to investigate the potential role of 5-HT1A receptor in the depression-induced hypoalgesia. Acute or chronic administration of 5-HT1A receptor agonist, 8-OH-DPAT, was performed in olfactory bulbectomy (OB) and sham-operated rats. The depression-like behavior and pain thresholds were measured using open-field test and radiant heat thermal pain test, respectively. We found that acute administration of 8-OH-DPAT increased locomotor activity and pain thresholds in the sham rats but had no effect on the OB rats. In contrast, chronic administration of 8-OH-DPAT reduced locomotor activity and pain thresholds and restored them to normal level. Increased pain thresholds were also observed in the sham rats after the chronic administration. These results demonstrated that chronic administration of 8-OH-DPAT reversed the depression-induced decrease in pain sensitivity in rats, suggesting that 5-HT1A receptor may play a role in the depression-associated hypoalgesia.

Effect of buspirone on thermal sensory and pain thresholds in human volunteers

Background

Buspirone is a partial 5-HT_1A receptor agonist. Animal studies have shown that modulation of serotoninergic transmission at the 5-HT_1A receptor can induce analgesia in acute pain models. However, no studies have been published so far on the effects of serotonin receptor agonists on pain perception in humans.

Methods

The effects of buspirone (30 mg p.o.) on thermal sensory and pain thresholds were investigated in twelve female volunteers (26 ± 2 yrs) in a prospective, randomized, double-blind, double-dummy, placebo-controlled study with morphine (10 mg i.v.) as positive control.

Results

Morphine significantly increased the heat pain detection threshold (ΔT: placebo 1.0°C and 1.3°C, p < 0.05) at 60 minutes. Buspirone caused mild sedation in six participants at 60 minutes, but was without effect on any of the measured parameters.

Conclusion

Buspirone in the maximal recommended dose was without significant effect on thermal pain. However, as it is only a partial agonist at the 5-HT_1A receptor and also acts on other receptor types, the negative results of the present study do not rule out a possible analgesic effect of more specific 5-HT_1A receptor agonists.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Effects of m-CPP in altering neuronal function: blocking depolarization in invertebrate motor and sensory neurons but exciting rat dorsal horn neurons

The compound m-chlorophenylpiperazine (m-CPP) is used clinically to manipulate serotonergic function, though its precise mechanisms of actions are not well understood. m-CPP alters synaptic transmission and neuronal function in vertebrates by non-selective agonistic actions on 5-HT(1) and 5-HT(2) receptors. In this study, we demonstrated that m-CPP did not appear to act through a 5-HT receptor in depressing neuronal function in the invertebrates (crayfish and Drosophila). Instead, m-CPP likely decreased sodium influx through voltage-gated sodium channels present in motor and primary sensory neurons. Intracellular axonal recordings showed that m-CPP reduced the amplitude of the action potentials in crayfish motor neurons. Quantal analysis of excitatory postsynaptic currents, recorded at neuromuscular junctions (NMJ) of crayfish and Drosophila, indicated a reduction in the number of presynaptic vesicular events, which produced a decrease in mean quantal content. m-CPP also decreased activity in primary sensory neurons in the crayfish. In contrast, serotonin produces an increase in synaptic strength at the crayfish NMJ and an increase in activity of sensory neurons; it produces no effect at the Drosophila NMJ. In the rat spinal cord, m-CPP enhances the occurrence of spontaneous excitatory postsynaptic potentials with no alteration in evoked currents.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.

Rectal antinociceptive properties of alverine citrate are linked to antagonism at the 5-HT1A receptor subtype

Serotonin (5-HT) is considered as a major mediator causing hyperalgesia and is involved in inflammatory reactions and irritable bowel syndrome. Alverine citrate may possess visceral antinociceptive properties in a rat model of rectal distension-induced abdominal contractions. This study was designed to evaluate the pharmacological properties of alverine citrate in a rat model of rectal hyperalgesia induced by 5-HTP (5-HT precursor) and by a selective 5-HT1A agonist (8-OH-DPAT) and to compare this activity with a reference 5-HT1A antagonist (WAY 100635). At 4 h after their administration, 5-HTP and 8-OH-DPAT increased the number of abdominal contractions in response to rectal distension at the lowest volume of distension (0.4 mL). When injected intraperitoneally before 8-OH-DPAT and 5-HTP, WAY 100635 (1 mg kg(-1)) blocked their nociceptive effect, but also reduced the response to the highest volume of distension (1.6 mL). Similarly, when injected intraperitoneally, alverine citrate (20 mg kg(-1)) suppressed the effect of 5-HTP, but not that of 8-OH-DPAT. However, when injected intracerebroventricularly (75 microg/rat) alverine citrate reduced 8-OH-DPAT-induced enhancement of rectal distension-induced abdominal contractions. In-vitro binding studies revealed that alverine citrate had a high affinity for 5-HT1A receptors and a weak affinity for 5-HT3 and 5-HT4 subtypes. These results suggest that 5-HTP-induced rectal hypersensitivity involves 5-TH1A receptors and that alverine citrate acts as a selective antagonist at the 5-HT1A receptor subtype to block both 5-HTP and 8-OH-DPAT-induced rectal hypersensitivity.

Effect of the serotonin agonist 8-OH-DPAT on the sensorimotor system of the rat

8-Hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT, 2 mg/kg) is used to induce perseverative behavior in rats in a T-maze as a model for obsessive-compulsive disorder (OCD). Using the open-field test, radiant heat test, and the test with von Frey filaments, we examined whether alterations in sensorimotor functioning could contribute to the perseverative tendencies in this model by measuring differences in left versus right hind paw reactions after 8-OH-DPAT administration (2 mg/kg, sc). Also, the effect of repeated 8-OH-DPAT administration on sensorimotor functioning was tested every third day. 8-OH-DPAT administration induced a significantly decreased sensorimotor performance in the open-field test, an increased threshold for noxious thermal stimulation (increased withdrawal latency, WL, and decreased elevation time, ET) in the radiant heat test, and a decreased nociceptive threshold for mechanical stimulation in the test with von Frey filaments. All changes in sensorimotor functioning were similar for left and right hind paws suggesting that, these changes as measured with the tests in the present study, are not likely to contribute to the perseverative behavior of rats in a T-maze. Further, repeated administration of 8-OH-DPAT had no effect in the radiant heat test and the test with the Frey filaments, but produced a tolerance effect in the open-field test.

Serotonin receptor subtypes involved in the spinal antinociceptive effect of 5-HT in rats

The present study was designed to investigate which subtypes of spinal 5-HT receptors are involved in 5-HT-induced antinociception using the mechanical pain test. Serotonin and various selective antagonists or agonists for 5-HT receptor subtypes (5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3) and 5-HT(4)) were administered intrathecally (i.t.) in rats. The i.t. injection of 5-HT (1 microg) produced significant antinociceptive effects using the paw pressure test. Pretreatment with the 5-HT(2C) receptor antagonist mesulergine (1 and 10 microg) and the 5-HT(3) receptor antagonist tropisetron (1 and 10 microg) reversed totally the antinociception induced by 5-HT. Furthermore, at a dose of 10 microg, both the 5-HT(2A) receptor antagonist ketanserin and the 5-HT(1B) receptor antagonist penbutolol, but neither the 5-HT(1A) receptor antagonist WAY 100635 nor the 5-HT(4) receptor antagonist GR113808, attenuated the antinociceptive effect induced by 5-HT. In addition, an i.t. injection of the 5-HT(3) agonist mCPBG induced significant antinociceptive effects whereas the 5-HT(2) agonist DOI did not produce analgesia. These results suggest that although the precise degree of the involvement of spinal serotonergic 5-HT(3) receptors remains to be elucidated due to some differences in the effect of agonists or antagonists, these receptors seem to play a role in the antinociceptive effect of 5-HT against a mechanical acute noxious stimulus. The involvement of 5-HT(2C) is more questionable due to the observed discrepancies between the effects of the used agonist and antagonist. 5-HT(1A) and 5-HT(4) receptors do not seem to be involved. In addition, a possible functional interaction between spinal serotonergic receptors may exist.

Acetylsalicylic acid potentiates the antinociceptive effect of morphine in the rat: involvement of the central serotonergic system

Acetylsalicylic acid and morphine are the most widely distributed and most frequently used drugs in the relief of pain, but their analgesic activity has adverse side-effects. Mixtures containing these two drugs are frequently used to relieve mild to moderate pain despite the paucity of relevant experimental evidence so far published. We set out to study the possible antinociceptive effect of a combination of subactive doses of the two drugs in rats. A combination of low doses of acetylsalicylic acid (50 mg/kg i.p.) and morphine (3 mg/kg s.c.) was administered and the pain threshold was evaluated in the hot-plate and formalin tests, and 5-HT2 receptor binding capacity, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were measured in the cortex and pontine areas of the brain. The combination of acetylsalicylic acid and morphine had an analgesic effect in both tests that was associated with an increase in 5-HT levels and a decrease in 5-HT2 receptors in the cortex. These effects were either completely abolished or partially prevented by i.p. pretreatment with naloxone (1 mg/kg i.p.). Our results demonstrate that subactive doses of acetylsalicylic acid and morphine can exert analgesic and biochemical effects when given in combination in the rat and suggest an involvement of serotonergic and opiatergic systems.

Mast cell degranulation induces delayed rectal allodynia in rats: role of histamine and 5-HT

Visceral hypersensitivity is a common feature of functional bowel disorders, where an increased number of mast cells have often been described. Thus, we investigated the effect of an experimental mast cell degranulation induced by BrX-537A on somatic (tail heating) and visceral (rectal distension) sensitivity in rats and the involvement of histamine and/or serotonin on this last response. After BrX-537A administration, the latency of tail withdrawal reflex was shortened within the 2- to 8-hr period. Moreover, BrX-537A reduced the distension volume threshold from 0.8 ml to 0.4 ml inducing allodynia, from 6 to 12 hr after its administration. This effect was suppressed by doxantrazole (mast cell stabilizing agent) and WAY 100635 (5-HT1A receptor antagonist), and reproduced by 5-HTP (5-HT precursor) and 8-OH-DPAT (5-HT1A receptor agonist). However, neither granisetron (5-HT3 receptor antagonist) nor H1, H2, or H3 histamine receptor antagonists modified the BrX-537A-induced allodynia. Consequently, mast cell degranulation initiates a delayed somatic and visceral allodynia, with the participation of serotonin, through 5-HT1A receptor activation, on the visceral response.

A dual effect of 5-HT1B receptor stimulation on nociceptive dorsal horn neurones in rats

In this study the modulatory effects of 5-HT1B receptor activation on wide dynamic range neurones in the spinal cord were studied. Extracellular single unit recordings of dorsal horn neurones were performed in intact urethane-anaesthetized female Sprague-Dawley rats, and the receptive field distally on one hind paw was electrically stimulated with needle electrodes applied to the skin. The 5-HT1B receptor agonist, CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one), the 5-HT1A/B receptor antagonist cyanopindolol, and the 5-HT1A receptor antagonist WAY100635 (N-[2-[4-(2-methoxypheny])-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride), were applied directly onto the spinal cord, and single unit responses were counted separately for A beta-, A delta-, C-fibre responses and post-discharge according to the latencies. A dual effect of CP-93,129 was observed: 50 nmol CP-93,129 caused a clear inhibition of the A delta-fibre responses, whereas 50 and 150 nmol CP-93,129 produced a dose-dependent increase in post-discharge without affecting A beta- and C-fibre responses. Application of 50 nmol cyanopindolol or 50 nmol WAY 100635 alone did not affect neither the neuronal A-fibre nor the C-fibre responses, but when 50 nmol cyanopindolol was coadministered with 50 nmol CP-93,129 the effect of CP-93,129 alone was blocked: the A delta-fibre response was not inhibited and the post-discharge was not increased. In contrast, 50 nmol WAY100635 did not block the effect of 50 nmol CP-93,129 when the two drugs were coadministered. These results suggest that stimulation of the 5-HT1B receptors may have both pro- and antinociceptive effects on wide dynamic range neurones in the dorsal horn after repeated electrical stimulation.

Effect of intrathecal serotonin on nociception in rats: influence of the pain test used

The involvement of serotonin (5-HT) in the modulation of nociceptive impulse in the spinal cord has been widely studied. However, its activity, considering the nature of noxious stimuli and the type of 5-HT receptors involved, merits to be further elucidated. The present behavioural study was performed to compare the dose-antinociceptive effect relationship of 5-HT in rats, after intrathecal (i.t.) injection (10 microliters/rat), using mechanical (paw pressure), thermal (tail immersion and tail-flick) and chemical (formalin) pain tests. In rats submitted to the paw pressure test, 5-HT was found to possess a dose-dependent antinociceptive activity (0.01, 0.1, 1, 10 and 20 micrograms/rat) when vocalization threshold was assessed as a pain parameter. A peak effect occurred 5 min after the injection and the effect was maintained for 45 min. The lowest active dose was 0.1 microgram (maximum increase in vocalization thresholds, 23 +/- 3%) and a plateau was observed for 10 micrograms and 20 micrograms (maximum increase in vocalization thresholds, 72 +/- 7% and 71 +/- 6%, respectively). When paw withdrawal was assessed, 5-HT induced a weak hyperalgesic effect for the highest dose (60 micrograms), while other doses were ineffective. In the tail-immersion (warmth and cold) and tail-flick tests, different doses (0.01, 0.1, 1, 10, 30, 60 and 100 micrograms/rat) were studied. In the two immersion tests, only the highest doses (60 micrograms and 100 micrograms) significantly increased the withdrawal thresholds from 5 to 45 min after the injection. The maximum effect was observed at 5 min (23 +/- 4% and 21 +/- 6% for 60 micrograms; 27 +/- 3% and 30 +/- 6% for 100 micrograms in the warmth and cold immersion test, respectively). In the tail-flick test, the doses of 30, 60 and 100 micrograms/rat dose-dependently and significantly increased the withdrawal thresholds from 5 to 45 min after the injection, with a maximum effect at 5 min (30 +/- 5% for 30 micrograms; 37 +/- 6% for 60 micrograms; and 45 +/- 4% for 100 micrograms). In the formalin test, 5-HT (10, 25, 50, 75 and 100 micrograms/rat) produced dose-related antinociception. The nociceptive response (licking of the injected paw) was significantly reduced from 25 micrograms (-59 +/- 11%) in the early phase, whereas the lowest active dose in the late phase was 50 micrograms (-46 +/- 17%). For both phases, a total inhibition was obtained with 100 micrograms. It is concluded that the effect of 5-HT on pain tests may differ according to the applied stimulus and the parameter assessed; unspecific effects of 5-HT may modify motor reactions to noxious stimuli. Mechanical test (assessment of vocalization) was the most sensitive to 5-HT. These observations are of importance in order to further study the pharmacological mechanisms involved in 5-HT spinally induced antinociception.

Exogenous gaseous superoxide potentiates the antinociceptive effect of opioid analgesic agents

The study examined the potentiation of the antinociceptive action of opioid analgesics produced by gaseous superoxide (GS) in the rat hind paw withdrawal test (PWT) and by GS or hydrogen peroxide (HP) in the formalin test. In the PWT, inhalation of GS for 50 minutes before i.p. injection of threshold doses of morphine (0.5 mg/kg) and trimeperidine (1.0 mg/kg) increased the threshold of nociception (TN) by a maximum of 43.0% (p < 0.05) and 113.4% (p < 0.01) respectively. The GS/trimeperidine-dependent increase in TN showed two peaks, the second of which could be suppressed by nialamide. Naloxone abolished the GS/ morphine-dependent increased in the TN. In the formalin test, a significant antinociceptive effect developed after GS inhalation or HP administration (intranasally, 2 x 5 microliters of 2 x 10(-5) mol/l solution in saline) in combination with low doses of Omnopon (0.06-0.75 mg/kg). These results suggest that both GS and HP potentiate the antinociceptive effects of opioid analgesics.

The analgesic effect of clonixine is not mediated by 5-HT3 subtype receptors

1. The analgesic effect of clonixinate of L-lysine (Clx) in the nociceptive C-fiber reflex in rat and in the writhing test in mice is reported. 2. Clx was administered by three routes, i.v., i.t. and i.c.v., inducing a dose-dependent antinociception. 3. The antinociceptive effect of Clx was 40-45% with respect to the control integration values in the nociceptive C-fiber reflex method. 4. The writhing test yielded ED50 values (mg/kg) of 12.0 +/- 1.3 (i.p.), 1.8 +/- 0.2 (i.t.) and 0.9 +/- 0.1 (i.c.v.) for Clx administration. 5. Ondansetron was not able to antagonize the antinociception response of Clx in the algesiometric tests used. 6. Chlorophenilbiguanide did not produce any significative change in the analgesic effect of Clx in the nociceptive C-fiber reflex method. 7. It is suggested that the mechanism of action of the central analgesia of Clx is not mediated by 5-HT3 subtype receptors.

Mechanisms underlying antinociception provoked by heterosegmental noxious stimulation in the rat tail-flick test

Physiological studies were conducted to examine the effects of noxious stimulation of one hindpaw or one forepaw on the latency of the withdrawal reflex in the tail-flick test in lightly anesthetized spinally intact or transected rats. Male Sprague-Dawley rats were anesthetized with an intraperitoneal injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were taken in the tail-flick test, the effects of various noxious stimuli applied to remote body regions were assessed. The noxious stimuli included unilateral or bilateral hindpaw or unilateral forepaw thermal (immersion in water at 55 degrees C for 90 s), unilateral or bilateral chemical (subcutaneous hindpaw injection of 50 microliters of 5% formalin) and unilateral or bilateral mechanical (pinch with clamp exerting a force of 14.75 or 27 N) stimulation. Bilateral chemical and thermal, and unilateral thermal stimulation induced an antinociceptive response, consisting of an increase in tail-flick latency, peaking at 30 s after stimulation. Recovery to baseline levels occurred over the next 3-6 min. The antinociceptive effect of noxious thermal stimulation was attenuated or absent in chronically spinalized animals (T6/7) following hindpaw or forepaw immersion, respectively. Noxious mechanical stimulation had no effect on tail-flick latency. The data provide evidence that a noxious thermal or chemical stimulus produces a heterosegmental antinociceptive effect which is mediated in part via a supraspinal mechanism and in part via a local spinal mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of 5-HT1 agonists and antagonists on nociceptive processing in the rat spinal cord: results from behavioural and electrophysiological studies

We have developed a technique which allows drugs to be microinjected intrathecally intrathecally in anaesthetised rats whilst single unit recordings are made from dorsal horn neurones. Using this technique together with recordings of tail flick latency (TFL) elicited from lightly anaesthetised rats we have found that the specific 5-HT1a agonist 8-OH DPAT (15, 150, 300 nmol) increases nociceptive responses recorded from single dorsal horn neurones and decreases TFL. The non-specific 5-HT1b agonist TFMPP (300 nmol) and the general 5-HT1 agonist 5-CT (0.3, 3.0, 30 nmol) both decreased nociceptive responses and has inconsistent effects on TFL. Intrathecally applied 5-HT (130, 260 nmol) generally reduced nociceptive neuronal responses and increased TFL. In a minority of experiments, however, 5-HT increased nociceptive responses and it is suggested that this effect is associated with activation of 5-HT1a receptors. Activity at 5-HT1b receptors has the effect of suppressing or reducing responsiveness. The increased responsiveness of dorsal horn neurones to noxious stimulation associated with activity at 5-HT1a receptors may be associated either with increases in receptive field size, promotion of spinal nocifensive reflexes or the facilitation of the rostral transmission to specific brainstem sites.

Serotonin and pain: evidence that activation of 5-HT1A receptors does not elicit antinociception against noxious thermal, mechanical and chemical stimuli in mice

In this study, we examined whether activation of 5-HT1A receptors elicits antinociception in response to acute noxious chemical, thermal and mechanical stimuli in mice. In the writhing test, both agonists (e.g., 8-OH-DPAT, S 14671 and WY 50,324) and partial agonists (e.g., buspirone and gepirone) elicited a pronounced antinociception. However, antagonists (e.g., (-)-alprenolol and WAY 100,135) also induced antinociception and, at lower (inactive) doses, failed to modify the action of agonists. In addition, the separation between doses required for induction of antinociception as compared to those required for induction of ataxia (in the rotarod test) was variable and low for both agonists (median: 1.9) and partial agonists (median: 1.3), although it was somewhat greater for antagonists (> or = 3.3). In the hot-plate test, only certain agonists (e.g., 8-OH-DPAT) and partial agonists (e.g., gepirone) elicited antinociception and their actions were not attenuated by 5-HT1A antagonists which, themselves, were inactive in this paradigm. The 5-HT1C/2 antagonist, ritanserin, the 5-HT3 antagonist, ondansetron, the dopamine D2 receptor antagonist, raclopride, and the alpha 1-adrenoceptor antagonist, prazosin, were also ineffective in modifying the antinociception evoked by 5-HT1A agonists and partial agonists in the hot-plate test. In contrast, their actions were strongly attenuated by the alpha 2-adrenoceptor antagonist, idazoxan. In the tail-flick tests to noxious heat and noxious pressure, 5-HT1A receptor agonists, partial agonists and antagonists generally failed to induce antinociception. Moreover, modulation of stimulus intensity (from very weak to very intense) did not reveal any influence upon the latency to respond. In conclusion, in the writhing test, the data provide no evidence for a specific antinociceptive effect of the activation of 5-HT1A receptors. Further, in the hot-plate test, for those 5-HT1A agonists and partial agonists which induce antinociception, alpha 2-adrenoceptors rather than 5-HT1A receptors are implicated in their actions. Finally, in reflexive tests, irrespective of stimulus quality or intensity, 5-HT1A agonists and partial agonists do not mediate antinociception. These data suggest that the activation of 5-HT1A receptors does not, under these conditions of acute noxious stimulation, elicit antinociception.

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)