Large-amplitude 5-HT1A receptor activation: a new mechanism of profound,central analgesia

We report the discovery of F 13640 and evidence suggesting this agent to produce powerful, broad-spectrum analgesia by novel molecular and neuroadaptative mechanisms. F 13640 stimulates G(alphaomicron) protein coupling to 5-HT(1A) receptors to an extent unprecedented by selective, non-native 5-HT(1A) ligands. Fifteen minutes after its injection in normal rats, F 13640 (0.01-2.5 mg/kg) decreases the vocalization threshold to paw pressure; 15 min upon injection in rats that are exposed to formalin-induced tonic nociception, F 13640 inhibits pain behavior. The initial hyperalgesia induced by 0.63 mg/kg F 13640 was followed, 8 hrs later, by paradoxical hypo-algesia; 5 mg/kg of morphine produces the opposite effects (i.e., hypo-algesia followed by hyper-algesia). Repeated F 13640 injections cause an increase in the basal vocalization threshold and a reduction of F 13640-produced hyperalgesia; in these conditions, morphine causes basal hyperalgesia and antinociceptive tolerance. Continuous two-week infusion of F 13640 (0.63 mg/day) exerts little effect on the threshold in normal rats, but markedly reduces analgesic self-administration in arthritic rats. F 13640 infusion also decreases allodynic responses to tactile and thermal stimulations in rats sustaining spinal cord or sciatic nerve injury. In these models of chronic nociceptive and neuropathic pain, the analgesia afforded by F 13640 consistently surpasses that of morphine (5 mg/day), imipramine (2.5 mg/day), ketamine (20 mg/day) and gabapentin (10 mg/day). Very-high-efficacy 5-HT(1A) receptor activation constitutes a novel mechanism of central analgesia that grows rather than decays with chronicity, that is amplified by nociceptive stimulation, and that may uniquely relieve persistent nociceptive and neuropathic pains.     

The novel analgesic, F 13640, produces intra- and postoperative analgesia in a rat model of surgical pain

F 13640 is a newly discovered high-efficacy 5-HT(1A) receptor agonist that produces exceptional analgesia in animal models of tonic and chronic, nociceptive and neuropathic pains by novel molecular and neuroadaptive mechanisms. Here we examined the effects of F 13640 and remifentanil (0.63 mg/kg with either compound) when injected i.p. either before or 15 min after rats underwent orthopedic surgery. Surgery consisted of the drilling of a hole in the calcaneus bone and of an incision of the skin, fascia and plantar muscle of one foot. During surgery, the concentration of volatile isoflurane was progressively incremented depending on the animal's response to surgical maneuvers. Other experiments examined the dose-dependent effects of F 13640 (0.04 to 0.63 mg/kg) on surgical pain as well as on the Minimum Alveolar Concentration of isoflurane. Both F 13640 and remifentanil markedly reduced the intra-operative isoflurane requirement. F 13640 also reduced measures of postoperative pain (i.e., paw elevation and flexion). With these postoperative measures, remifentanil produced short-lived analgesia followed by hyperalgesia. F 13640 significantly reduced both surgical pain and the isoflurane Minimum Alveolar Concentration from 0.16 mg/kg onward. F 13640 produced powerful intra- and postoperative analgesia in rats undergoing orthopedic surgery. Unlike the opioid, remifentanil, F 13640 caused no hyperalgesia with ongoing postoperative pain, and should remain effective with protracted postoperative use.     

Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.     

Tolerance and inverse tolerance to the hyperalgesic and analgesic actions,respectively, of the novel analgesic,F 13640

5-HT(1A) receptor activation by the very-high-efficacy, selective 5-HT(1A) receptor agonist F 13640 [(3-Chloro-4-fluoro-phenyl)-[4-fluoro-4-([(5-methyl-pyridin-2-ylmethyl)-amino]-methyl)piperidin-1-yl]-methanone] was recently discovered to constitute a novel central mechanism of broad-spectrum analgesia that, remarkably, grows rather than decays with chronicity. However, in rodents not exposed to nociception, F 13640 induces its analgesic effect only after having initially induced hyperalgesia. Numerical simulations implementing a signal transduction theory here show that the progressive increase in the intensity of nociceptive stimulation which F 13640 presumably mimics should eventually produce a large analgesic effect without initially causing marked pain. In vivo studies examined the effects of progressively increasing doses of F 13640 on the threshold of mechanically induced vocalization and, also, on the 5-HT syndrome in rats. The infusion of increasing (0.04-0.63 mg/rat/day) doses of F 13640 over a 5-week period induced a large analgesia preceded by a hyperalgesic effect that was small and comparable to that induced by initial exposure to a low, 0.04 mg/rat/day dose. Furthermore, increasing the dose of F 13640 induced tachyphylaxis to the 5-HT syndrome. Producing the mirror opposite of morphine's neuroadaptive actions, F 13640 causes an analgesia that becomes more powerful with chronic administration, and this at the expense of the initial hyperalgesia which it may also produce.     

Curative-like analgesia in a neuropathic pain model: parametric analysis of the dose and the duration of treatment with a high-efficacy 5-HT(1A) receptor agonist

High-efficacy activation of central 5-HT(1A) receptors by means of the recently discovered, selective 5-HT(1A) receptor ligand, F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]methyl}piperidin-1-yl]methanone, fumaric acid salt] causes an unprecedented, broad-spectrum analgesia in rat models of acute and chronic pain of nociceptive and neuropathic origin; it also is effective in conditions where opioids either are ineffective, induce analgesic tolerance, or elicit persistent hyperalgesia/allodynia. Inversely mirroring morphine's actions, F 13640's ("curative-like") analgesic effects persist after the discontinuation of treatment. Here, we examined the relationships, if any, between the dose and the duration of F 13640 treatment on the one hand, and the duration of persistent analgesia on the other. Rats received unilateral infraorbital nerve injury and developed allodynia - as assessed by an increased response to von Frey filament stimulation - within 24 days; thereafter, using osmotic pumps, rats were subcutaneously infused with F 13640 in two experiments. In one, a one-week infusion was instituted at 0.04-10-mg/day doses; in a second experiment, a 0.63-mg/day dose was implemented for a duration ranging from 1 to 56 days. These 250- and 56-fold variations of the dose and duration of treatment caused post-treatment, persistent analgesia for about 10 and 40 days, respectively. At least as much as dose, the duration of F 13640 treatment determines F 13640-induced persistent analgesia. Neuroadaptive modulations at pre- and postsynaptic, brain and spinal cord 5-HT(1A) receptors may be involved in the dynamical, dose- and time-dependent, pre-treatment rise and post-treatment decay of the analgesia induced by high-efficacy 5-HT(1A) receptor activation.     

5-HT(1A) receptor activation: new molecular and neuroadaptive mechanisms of pain relief

Guided by an understanding of signal transduction in pain-processing systems, high-efficacy 5-hydroxytryptamine (5HT)1A receptor activation, by means of F-13640, has been discovered as a new molecular mechanism of pain relief in laboratory animals, inducing two neuroadaptive phenomena. Firstly, this activation cooperates with nociceptive stimulation, paradoxically causing analgesia, and secondly, inverse tolerance develops so that the resulting analgesia grows rather than decays. As an apparent result of these novel neuroadaptive mechanisms, F-13640 exerts an analgesic action in rat models of acute, tonic and chronic nociceptive pain that is rivaled only by large doses of high-efficacy mu-opioid receptor agonists. In models of neuropathic allodynia of peripheral or central origin, chronic F-13640 administration causes an analgesia that surpasses that observed with morphine or other agents exemplifying other central nervous system drug mechanisms of pain relief (e.g., ketamine, imipramine and gabapentin). Indeed, F-13640 produces long-lasting, preemptive and, most remarkably, curative-like actions in neuropathic allodynia. Although awaiting proof-of-concept evidence in humans, high-efficacy 5-HT(1A) receptor activation may uniquely challenge the opioids for pain therapy.     

High-efficacy 5-HT1A receptor activation causes a curative-like action on allodynia in rats with spinal cord injury

The selective, high-efficacy 5-HT(1A) receptor agonist, (3-chloro-4-fluoro-phenyl)-[4-fluoro-4-[[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl]piperidin-1-yl]-methanone (F 13640) has been reported to produce long-term analgesia in rodent models of chronic nociceptive and neuropathic pain; it also preempts allodynia following spinal cord injury. Here, rats underwent spinal cord injury, fully developed allodynia, and were infused with saline or 0.63 mg/day of F 13640 for 56 days. Infusion was then discontinued, and further assessments of allodynia (vocalization threshold to von Frey filament stimulation, responses to brush and cold) were conducted for another 70 days. F 13640-induced analgesia persisted during this post-treatment period. The data offer initial evidence that high-efficacy 5-HT(1A) receptor activation produces an unprecedented curative-like action on pathological pain.     

In the formalin model of tonic nociceptive pain, 8-OH-DPAT produces 5-HT1A receptor-mediated, behaviorally specific analgesia

The experiments examined antinociceptive and intrinsic behavioral effects induced by the prototypical 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino] tetralin) in rats. 8-OH-DPAT (0.01-2.5 mg/kg, subcutaneous (s.c.)) reduced both the paw licking and paw elevation induced by (2.5%) formalin injection into the plantar surface of the right hindpaw; it also produced forepaw treading. All of these effects were completely blocked by pretreatment with WAY 100635 (N-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (0.16 mg/kg, s.c.); prazosin (0.63 mg/kg, s.c.) inhibited forepaw treading, but not 8-OH-DPAT's action on paw elevation and paw licking. Repeated injection of 8-OH-DPAT (0.63 mg/kg, s.c.) twice daily for 4 days, markedly reduced 8-OH-DPAT's ability to produce forepaw treading, but exerted only little and inconsistent effects on its paw licking and paw elevation-inhibiting action. The data indicate that 8-OH-DPAT exerts an analgesic action in the formalin model of tonic nociceptive pain; this action is mediated by 5-HT(1A) receptors, and is not confounded by the productive sign (i.e., forepaw treading) of the 5-HT syndrome which 8-OH-DPAT also induces.     

The very-high-efficacy 5-HT1A receptor agonist, F 13640, preempts the development of allodynia-like behaviors in rats with spinal cord injury

Central neuropathic pain after spinal cord injury (SCI) presents a challenging clinical problem with limited treatment options. [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-([(5-methyl-pyridin-2-ylmethyl)-amino]-methyl)piperidin-1-yl]]-methadone (F 13640) is a recently discovered very-high-efficacy, selective 5-HT1A receptor agonist that produces a remarkably powerful, central analgesia through unprecedented neuroadaptive mechanisms. In a rat model of spinal cord injury pain, we previously found that chronic infusion of F 13640 alleviated pain-like behaviors. Here, we report that infusion of 0.63 mg/day of F 13640 for 8 weeks starting 24 h before the induction of injury significantly attenuates the development of chronic allodynia-like behavior in rats sustaining a photochemically-induced, ischaemic injury of the dorsal laminae of the L3-L5 segments of the spinal cord. Importantly, the preemptive effect of F 13640 persisted for 2 months after treatment was discontinued. The data warrant the study of the possible effects of the early administration of F 13640 in patients sustaining spinal cord injury.     

Region-specific changes in 5-HT1A agonist-induced Extracellular signal-Regulated Kinases 1/2 phosphorylation in rat brain: a quantitative ELISA study

Brain serotonin 5-HT(1A) receptor, a traditional target for the treatment of mood disorders, modulates intracellular signalling pathways, such as the Extracellular signal-Regulated Kinases 1/2 (ERK1/2) pathway. The present studies are the first to determine levels of phospho-ERK1/2 (pERK1/2) in brain using a quantitative Enzyme Linked-Immuno-Sorbent Assay. We examined pERK1/2 levels in rat brain following administration of (+)8-OH-DPAT, buspirone as well as of the more selective, high-efficacy 5-HT(1A) agonists F13640 and F13714. Intraperitoneal injection of these compounds increased pERK1/2 in prefrontal cortex and hypothalamus, with a maximum at 5-15min and a significant effect lasting until 30-60min post-injection. However, these compounds reduced hippocampal pERK1/2 with a maximum effect at 30min, persisting until 60min post-injection. In hippocampus, F13640, F13714 and buspirone inhibited pERK1/2 in a dose-dependent manner as of 0.04, 0.04 and 2.5mg/kg, respectively. Given these low doses, this response is likely related to activation of sensitive presynaptic 5-HT(1A) receptors in the raphe nucleus. 4- and 16-fold higher doses of these compounds were necessary to stimulate pERK1/2 in prefrontal cortex and hypothalamus, respectively, via direct 5-HT(1A) receptor activation. In contrast, (+)8-OH-DPAT was active at similar doses (0.63mg/kg) in these different regions. Pretreatment with the 5-HT(1A) antagonist, WAY100635, completely blocked the effects of these compounds, with the exception of buspirone-induced pERK1/2 increases in hypothalamus. Thus, 5-HT(1A) agonist-induced changes in pERK1/2 in rat brain are time- and dose-dependent and region-specific. Furthermore, F13640, F13714, buspirone, but not (+)8-OH-DPAT, exert their effects via preferential activation of presynaptic 5-HT(1A) receptors.     

Differential analgesic actions of serotonin 5-HT3 receptor antagonists in the mouse

The present study examined the analgesic effects of the serotonin 5-HT3 receptor antagonists ICS-205-930, MDL-7222 and GR-38032F in mice using acute thermal, mechanical and chemical pain tests. Subcutaneous administration (1-10 mg/kg) of these agents did not produce analgesia in either the thermal or mechanical pain tests. However, ICS-205-930, MDL-72222 and GR-38032F all produced dose-dependent analgesia in the chemical pain test, that was not altered by systemic naloxone administration (1 mg/kg, s.c.). Intracerebroventricular administration of these drugs (0.1-10 micrograms) was ineffective in producing analgesia in acute thermal, mechanical and chemical pain tests. These results suggest that peripheral 5-HT3 receptors play a role in chemical, but not thermal or mechanical nociceptive mechanisms.     

Different mechanisms underlie the analgesic actions of paracetamol and dipyrone in a rat model of inflammatory pain

BACKGROUND AND PURPOSE: The analgesics, paracetamol and dipyrone are weak inhibitors of the cyclooxygenase isoforms 1 or 2 (COX-1, COX-2) but more potent on COX-3. Both are also weak anti-inflammatory agents, relative to their analgesic and antipyretic activities. In a model of inflammatory pain mediated by prostaglandins, both compounds were analgesic. We have analysed this shared effect further in order to elucidate the underlying mechanisms.EXPERIMENTAL APPROACH: Inflammation was induced in one hind paw of rats by intraplantar injection of 250 microg lambda-carrageenan (CG) and the contralateral paw injected with saline. Nociceptive thresholds to mechanical stimulation were measured immediately before and for 6 h after, injection of CG. The analgesics were s.c. or locally (intraplantar) injected either 30 min before or 2 h after CG. In some groups, naltrexone was injected (s.c. or intraplantar), 1 h before CG. KEY RESULTS: Pretreatment with paracetamol or dipyrone (60-360 mg kg(-1)) reversed hyperalgesia induced by CG and increased nociceptive threshold in the inflamed paw above the basal level (hypoalgesia). Paracetamol, but not dipyrone, also raised nociceptive thresholds in the non-inflamed paw. Subcutaneous, but not local, administration of naltrexone, a specific opioid antagonist, reversed the hypoalgesia induced by paracetamol, but similar naltrexone treatment had no effect on dipyrone-induced analgesia.CONCLUSIONS AND IMPLICATIONS: Although both paracetamol and dipyrone are inhibitors of COX isoforms and thus of prostaglandin biosynthesis and were analgesic in our model, their analgesic actions were functionally and mechanistically different. Satisfactory mechanisms of action for these analgesics still remain to be established.     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Influence of doxepin used in preemptive analgesia on the nociception in the perioperative period. Experimental and clinical study.

The aim of the present research was to assess in experimental and clinical study the influence of doxepin administered intraperitoneally (ip) as preemptive analgesia on the nociception in the perioperative period. The pain thresholds for mechanical stimuli were measured in rats. The objective of clinical investigation was to assess the influence of preemptive administration of doxepin on postoperative pain intensity, analgesic requirement in the early postoperative period as well as an assessment of the quality of postoperative analgesia by the patient. Doxepin injected ip (3-30 mg/kg) dose-dependently increased the pain threshold for mechanical stimuli measured in paw pressure test in rats. Doxepin injected 30 min before formalin significantly increased the nociceptive threshold in the paw pressure test. In contrast, doxepin injected 240 min before formalin or 10 min after formalin did not change the nociceptive threshold. Morphine administered subcutaneously (sc) at a dose of 1 mg/kg increased the pain threshold measured in the paw pressure test 55 min after formalin treatment. Injection of 10 mg/kg of doxepin 30 min before formalin further enhanced the response after morphine administration. The results of the clinical study demonstrated that the patients who were administered doxepin preemptively showed significantly lower pethidine requirement in order to achieve a similar level of postoperative analgesia. The results of the research under discussion confirm the theoretical assumptions that there is a possibility to modify the nociception process in the perioperative period through preemptive analgesia using a drug that modifies the activity of the descending antinociceptive system.     

Role of 5-HT and NA in spinal dopaminergic analgesia

Spinal rats and rats with an intact neuraxis given the dopamine (DA) agonist R-apomorphine (0.31-1.75 mumol/kg) in the lumbar subarachnoid space by intrathecal injection were tested 5 and 10 min later for spinal analgesia (increased tail-flick response latency). Apomorphine produced analgesia in spinal rats but not in rats with an intact neuraxis. However, pretreatment of intact rats with the serotonergic (5-HT) receptor antagonist methysergide, the noradrenergic (NA) receptor antagonist phentolamine or the two antagonists together led to a dose-dependent analgesia following apomorphine. Intact rats pretreated with the monoamine depleting drug reserpine, the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) or the NA synthesis inhibitor FLA 63 also showed analgesia to apomorphine. On the other hand, pretreatment with the catecholamine depleting agent alpha-methyl-p-tyrosine (AMPT), the beta-adrenergic receptor blocker propranolol or the opioid receptor antagonist naloxone failed to produce DA analgesia. The present findings suggest that both 5-HT and NA descending fiber systems exert tonic inhibitory effects on spinal DA nociceptive processes.     

Evaluation of the anti-inflammatory and anti-nociceptive effects of different antidepressants in the rat.

The present study was designed to compare the anti-inflammatory and anti-nociceptive effects of different classes of antidepressant drugs on the carrageenan paw oedema and tail-electric stimulation assays in the rat. Drugs were intraperitoneally administered 30 min prior to carrageenan or nociceptive testing. The non-selective noradrenaline (NA) and serotonin (5-HT) reuptake inhibitors imipramine, amitriptyline and clomipramine displayed anti-inflammatory activity in the carrageenan model of paw inflammation. The maximal degree of oedema inhibitions seen with these agents were 28.8, 41.5 and 46.8% for 5, 10 and 20 mg kg(-1) amitriptyline, 26.2, 38.2 and 51.4% for 3.75, 7.5 and 15 mg kg(-1) imipramine and 51.2 and 54.1% for 16 and 32 mg kg(-1) clomipramine, respectively. The heterocyclic agent trazodone significantly inhibited paw oedema by 46 and 41% at 1 and 2h after dosing at the highest dose (40 mg kg(-1)) examined. Fluoxetine, a selective 5-HT reuptake inhibitor (SSRI) caused dose-related reduction of paw oedema, with 20.7% inhibition at the dose of 10 mg kg(-1). In contrast, sertraline, another SSRI caused dose-dependent enhancement of paw oedema. All antidepressant drugs in the study showed anti-nociceptive properties in the tail-electric stimulation assay with amitriptyline and trazodone being the most effective in this respect. Taken together, data in the present study confirm anti-inflammatory and anti-nociceptive effect for some antidepressant drugs and indicate that SSRIs differently affects inflammation.     

Differential effects of substance P on serotonin-modulated spinal nociceptive reflexes

Recent immunohistochemical studies indicate the presence of a bulbospinal substance P (SP) system, as well as a bulbospinal serotonin (5-HT) system, involved in spinal pain transmission. Although electrophysiological studies indicate that SP may modulate the effects of 5-HT on post-synaptic spinal nociceptive neurons, the functional relationship between SP and 5-HT on “pain behavior” remains obscure. To bridge this gap between mechanism and behavior, the purpose of the present study was to determine specific postsynaptic behavioral effects of SP and 5-HT on local spinal nociceptive reflexes in spinally transected animals. Administration of the 5-HT agonists 5-methoxydi-methyltryptamine (5-MeODMT) (0, 0.5, 1.5, 2.0 mg/kg) and quipazine (0, 5, 10, 20 mg/kg) 2 days after transection significantly expanded the receptive field (RF) areas of three spinal reflexes, as previously reported. Intrathecal administration of SP alone (0, 0.25, 2.5, 7.5 ng) also resulted in hyperalgesia, indicated by a significant expansion of the RF areas of all three nociceptive reflexes. However, administration of SP, in animals pretreated with 5-HT agonists, decreased the 5-HT-induced expansion of RF size. Therefore, SP had opposite effects on spinal nociceptive reflexes depending on whether or not the animal was pretreated with 5-HT agonists, i.e., hyperalgesia in the absence of 5-HT agonists, and analgesia in the presence of 5-HT agonists. The two effects of SP on local spinal reflexes may be related to the anatomical organization of the two spinal SP systems: 1) SP released from primary afferents facilitates nociceptive reflexes, and 2) SP associated with the descending bulbospinal system interacts with the descending bulbospinal 5-HT system and inhibits nociceptive reflexes. The present results help explain contradictory literature regarding the effect of SP on spinal nociceptive reflexes.     

Effects of antipsychotics and reference monoaminergic ligands on marble burying behavior in mice

Antipsychotics constitute efficacious augmenting agents in the treatment of anxiety disorders, including refractory obsessive-compulsive disorder. We examined the effects of 36 compounds, including typical, atypical and novel antipsychotics with dual dopamine D2/5-hydroxytryptamine 1A (D2/5-HT1A) actions on marble burying behavior in mice, a putative preclinical test for anxiety disorders. One hour after drug administration, male NMRI mice were placed individually in cages containing 20 marbles, and the total number of marbles buried after 30 min was counted. The selective serotonin reuptake inhibitors, citalopram (2.5-40 mg/kg), fluoxetine (2.5-10 mg/kg) and the benzodiazepine diazepam (2.5-10 mg/kg), reduced the number of buried marbles. The atypical antipsychotic, clozapine (0.16-10 mg/kg), but not its congener olanzapine, was effective in this test. Haloperidol, a typical antipsychotic, also reduced the number of buried marbles, albeit not in a dose-dependent manner. The atypical risperidone was partially active (0.16-0.63 mg/kg), as was the benzamide derivative, amisulpride, albeit at high (10-40 mg/kg) doses. Among the 'third-generation' antipsychotics possessing combined D2/5-HT1A properties, bifeprunox was active at 0.0025 mg/kg, whereas SLV313 and aripiprazole were active only at the highest doses (2.5 and 10 mg/kg, respectively). SSR181507, F15063 and the antidyskinetic agent, sarizotan, were without any effect. Among a series of receptor subtype-selective ligands, only the 5-HT1A agonist, (+)-8-OH-DPAT (0.63-2.5 mg/kg) and the 5-HT2A/2B/2C antagonist, ritanserin (0.63-2.5 mg/kg) were active. Among novel antipsychotics with dual D2/5-HT1A properties, only bifeprunox was able to potently reduce the number of buried marbles. Inhibition of marble burying behavior may result from the interplay of several receptor systems, including 5-HT2 receptor blockade, dopamine D2 partial agonism and serotonin 5-HT1A agonism.     

侧脑室或鞘内注射烟碱对恩氟烷催眠和镇痛作用的影响

目的初步分析恩氟烷的催眠和镇痛作用与神经元烟碱受体之间的关系。方法催醒实验:小鼠ip恩氟烷2.2mL.kg-1,翻正反射消失1min后,分别脑室注射烟碱10,20和40μg(5μL),记录翻正反射恢复时间(即睡眠时间)。镇痛实验:①甲醛实验:小鼠ip恩氟烷0.5mL.kg-1,5min后分别鞘内注射烟碱5,10和15μg(5μL),再5min后于足底皮下注射2%甲醛溶液20μL,记录60min内小鼠舔被注射足的累积时间。②热板实验:给药方法同甲醛实验,于注射烟碱后5,10,15,20和25min记录小鼠足部接触热板至开始添后足的时间作为后足痛阈。结果脑室注射烟碱能明显减少恩氟烷催眠小鼠的睡眠时间;鞘内注射烟碱不能拮抗甲醛实验中恩氟烷的镇痛作用,但可拮抗热板实验中恩氟烷的镇痛作用。结论神经元烟碱受体可能是恩氟烷催眠作用的重要靶位之一;也可能是恩氟烷对热刺激镇痛作用的重要靶位之一,而非对化学、炎性刺激镇痛作用的靶位。     

Different role of 5-HT1A and 5-HT2 receptors in spinal cord in the control of nociceptive responsiveness.

The effects of the 5-hydroxytryptamine type-2 (5-HT2) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1A agonist (+)-8-hydroxy-2-(di-n-propylamino)-tetralin [(+)-8-OH-DPAT] on nociceptive responsiveness were compared in mice. Intrathecal administration of DOI (5-20 micrograms) produced a dose-dependent behavioural syndrome, consisting of biting or licking, directed towards the caudal part of the body and reciprocal hindlimb scratching. However, (+)-8-OH-DPAT (5-20 micrograms) did not produce the biting and scratching behaviour. The response to DOI (20 micrograms) was reversed by treatment with the substance P receptor antagonist, [D-Arg1, D-Trp7,9, Leu11]-SP (Spantide) (5 micrograms). The tail-flick reflex was markedly depressed 5-20 min after administration of (+)-8-OH-DPAT; DOI did not change the tail-flick reflex after 5 min but significantly inhibited the reflex response 10-20 min after injection. The data show that stimulation of 5-HT2 receptors, but not 5-HT1A receptors, induced a behavioural syndrome, which may reflect activation of nociceptive pathways. The tail-flick reflex was more markedly inhibited by stimulation of 5-HT1A than 5-HT2 receptors. Accordingly, 5-HT2 and 5-HT1A receptors seem to have a different function in the modulation of nociceptive responsiveness in the mouse.