Nicotinic acetylcholine receptor antagonist effect of fluoxetine in rat hippocampal slices

We compared the effect of mecamylamine and fluoxetine on the hippocampal noradrenaline (NA) release evoked by nicotine in vitro. Nicotine (100 μM) increased the basal release of [³H]NA from rat hippocampal slices. This effect was blocked by the potent nicotinic antagonist mecamylamine in a dose-dependent manner (IC₅₀=0.19 μM). The selective serotonin reuptake inhibitor (SSRI) fluoxetine also antagonised the response to nicotine in a dose-dependent manner with a similar strength (IC₅₀=0.57 μM). Our data indicate that fluoxetine has nicotinic acetylcholine receptor antagonist effect in the central nervous system. The possible clinical significance of this finding is discussed.     

The influence of Bay K 8644 treatment on (±)-epibatidine-induced analgesia

The purpose of this investigation was to determine if analogous to (−)-nicotine's analgesic effect, the analgesic effect of the recently characterized potent nicotinic acetylcholine receptor (nAChR) agonist (±)-epibatidine was altered in response to treatment with the calcium channel agonist (±)-Bay K 8644. In addition, the effects of the enantiomers, (+)-Bay K 8644, reported to be a calcium channel antagonist, and (−)-Bay K 8644, reported to be a calcium channel agonist were examined. (±)-Bay K 8644 (2.8 μmol/kg; i.p.) produced a large analgesic response in mice in the hot-plate paradigm that rapidly dissipated by 30 min after treatment. This analgesic effect of (±)-Bay K 8644 was not prevented by pre-treatment with the nicotinic antagonist mecamylamine (5 μmol/kg; i.p.). Treatment with non-analgesic doses of the calcium channel agonists (±)- and (−)-Bay K 8644 (1.4 μmol/kg; i.p.) significantly potentiated the analgesic effect of (±)-epibatidine (0.05 μmol/kg; i.p.). Potentiation of (±)-epibatidine's analgesic effect occurred when the agonists were administered prior to (±)-epibatidine or after (±)-epibatidine as long as analgesia testing was conducted 15 to 30 min after Bay K 8644 treatment. Pre-treatment with the calcium channel antagonist (+)-Bay K 8644 was found to attenuate (±)-epibatidine-induced analgesia. When given after (±)-epibatidine, (+)-Bay K 8644 had no effect on (±)-epibatidine's analgesic effect. These data provide additional in vivo evidence that altering calcium dynamics can modulate neuronal nAChR function.     

The antihyperalgesic activity of a selective P2X7 receptor antagonist, A-839977, is lost in IL-1αβ knockout mice

The pro-inflammatory cytokine interleukin-1β (IL-1β) has been implicated in both inflammatory processes and nociceptive neurotransmission. Activation of P2X7 receptors is the mechanism by which ATP stimulates the rapid maturation and release of IL-1β from macrophages and microglial cells. Recently, selective P2X7 receptor antagonists have been shown to reduce inflammatory and neuropathic pain in animal models. However, the mechanisms underlying these analgesic effects are unknown. The present studies characterize the pharmacology and antinociceptive effects of a structurally novel P2X7 antagonist. A-839977 potently (IC₅₀ = 20–150 nM) blocked BzATP-evoked calcium influx at recombinant human, rat and mouse P2X7 receptors. A-839977 also potently blocked agonist-evoked YO-PRO uptake and IL-1β release from differentiated human THP-1 cells. Systemic administration of A-839977 dose-dependently reduced thermal hyperalgesia produced by intraplantar administration of complete Freund's adjuvant (CFA) (ED₅₀ = 100 μmol/kg, i.p.) in rats. A-839977 also produced robust antihyperalgesia in the CFA model of inflammatory pain in wild-type mice (ED₅₀ = 40 μmol/kg, i.p.), but the antihyperalgesic effects of A-839977 were completely absent in IL-1αβ knockout mice. These data demonstrate that selective blockade of P2X7 receptors in vivo produces significant antinociception in animal models of inflammatory pain and suggest that the antihyperalgesic effects of P2X7 receptor blockade in an inflammatory pain model in mice are mediated by blocking the release of IL-1β.     

Effects of nicotine on cortical high voltage spindles in rats

Cholinergic systems have been shown to modulate 6–10 Hz immobility-related cortical spike wave discharges (high voltage spindles - HVS) in rats. This study reports that activation of central nicotinic receptors inhibits HVS identified from cortical EEG recordings. Nicotine (0.19–1.9 μmol/kg i.p.) significantly reduced the summed duration of HVS bursts during 20 min of waking immobility. The nicotinic antagonist mecamylamine (5.0 μmol/kg i.p.) blocked the effect of nicotine (0.62 μmol/kg i.p.) without itself significantly affecting HVS. At higher doses, mecamylamine (15.0 and 25.0 μmol/kg i.p.) increased HVS activity. Dimethylphenylpiperazinium (0.62–6.2 μmol/kg i.p.), a nicotinic agonist which does not cross the blood-brrrier, did not affect HVS, consistent with the idea that the effect of nicotine on HVS is due to an action in the central nervous system. Cotinine, the major metabolite of nicotine, did not affect HVS at doses similar to or higher than those tested for nicotine. Cotinine also did not block the effect of nicotine, indicating that this metabolite does not interfere with the modulatory effect of nicotine on HVS. These results suggest a role for nicotinic regulation of the neuronal substrates involved in the generation of HVS.     

Inhibition by neurosurugatoxin of nicotine-induced antinociception

We have found a selective inhibition of nicotine-induced antinociception in mice by neusurugation (NSTX, 0.4–3.8 nmol/kg), a neurotoxin with a high affinity for ganglionic nicotinic receptors (ED₅₀ = 0.65nmol/kg). The toxin also reduced specific [³H]nicotine binding in mouse brain membranes (IC₅₀ = 95nM). The anti-nicotiniv acitivity of NSTX was markedly greater than that of mecamylamine and pempidine. These data indicate that NSTX may block functional nicotinic cholinoceptors possibly in the central nervous system.     

Depression by morphine-6-glucuronide of nociceptive activity in rat thalamus neurons: comparison with morphine

To assess the contribution of the active metabolite of morphine, morphine-6-glucuronide (M6G), to the analgesic effect of systemically administered morphine, experiments were carried out on rats under urethane anesthesia in which nociceptive activity was evoked by electrical stimulation of afferent C fibers in the sural nerve and recorded from single neurons in the ventrobasal complex of the thalamus. Intravenous (i.v.) injections of morphine completely blocked the activity at doses of 500 and 1000 μg/kg, the ED,, being 44 μg/kg. M6G administered by i.v. injection reduced the evoked nociceptive activity only by about 40% at 80 and 160 μg/kg, the ED₅₀ being 6 μg/kg. After intrathecal (i.t.) injection, morphine produced maximum depression of 55% of the control activity at 20 μg the ED₅₀ is 18 μg. M6G injected i.t. produced maximum depression of 40% at doses ranging from 0.2 to 10 μg. The ED₅₀ of M6G i.t. is below 0.2 μg. The effects of morphine and M6G were reversed by naloxone (200 μg/kg i.v.). The results show that M6G is more potent than morphine, regardless of the route of administration, while morphine is more effective when injected i.v. Due to the low efficacy of M6G, it seems unlikely that this glucuronide contributes substantially to the analgesic effect of morphine when renal function is normal. The results also make evident that the maximum effect of morphine results from an action at spinal and supraspinal sites.     

N-n-propyl-norapomorphine: An extremely potent stimulant of dopamine autoreceptors

N-n-propyl-norapomorphine (NPA) is 10–20 times more potent than apomorphine in producing hypomotility and inhibiting both striatal and limbic dopamine (DA) synthesis and the firing rate of nigral dopaminergic cells in rats. The threshold subcutaneous doses of NPA and apomorphine to significantly inhibit motor activity or DA synthesis are 1.25 and 25 μg/kg, respectively. The intravenous ED₅₀ of NPA to inhibit dopaminergic firing is 0.36 μg/kg and that for apomorphine is 9.1 μg/kg. The above effects of NPA, as those of apomorphine, are antagonized by haloperidol and, stereospecifically by (−)sulpiride.     

Antinociceptive effects of the stereoisomers of nicotine given intrathecally in spinal rats

Summary Spinalized rats received an intrathecal injection of either (–)-nicotine or (+)-nicotine in order to study the stereoselectivity of antinociception. Pain threshold was measured using the tail-flick test. Both stereoisomers had antinociceptive effects, but (–)-nicotine was up to 970 times more potent, depending on test conditions. The antinociceptive action of (–)-nicotine was antagonized by mecamylamine and yohimbine but not by naloxone and atropine. The findings show that spinal mechanisms are highly stereoselective toward nicotine, and suggest that primarily nicotinergic andalpha-adrenergic receptors are involved in its central antinociceptive effects.     

Pharmacological consequences of nicotinergic plus serotonergic manipulations

The present study investigates the effects of concurrent manipulations of nicotinic cholinergic receptors (nicotinic cholinergic agonist: nicotine 0.03, 0.1, 0.3 mg/kg, nicotinic cholinergic antagonist: mecamylamine 7.5 mg/kg) and serotonin neurons (p-chlorophenylalanine (PCPA), 400/kg mg on each of 3 days) on spatial navigation (water maze, WM) and passive avoidance (PA) performance. Nicotine did not affect PA performance but at the highest dose slightly impaired WM performance. PCPA did not affect WM navigation or PA performance in saline or nicotine-treated rats. Nicotine restored WM and PA performance defect in mecamylamine pretreated rats. PCPA aggravated the WM defect and decreased the WM performance-improving effect of nicotine in mecamylamine pretreated rats. PCPA did not aggravate the PA performance defect of mecamylamine but completely blocked the PA performance-improving effect of nicotine in mecamylamine pretreated rats. These results suggest that serotonergic and nicotinergic cholinergic systems jointly modulate performance in WM and PA tests.     

Differential effects of dopamine agonists on evoked dopamine release from slices of striatum and nucleus accumbens in rats

The effects of dopamine receptor agonists on electrically evoked dopamine release from slices of nucleus accumbens were compared with the effects on release from striatal slices in rats. Apomorphine, which has equal potency at the dopamine D₂ and D₃ receptors, reduced the evoked dopamine release from both regions to the same extent (ED₅₀, 0.42 μM for nucleus accumbens; ED₅₀, 0.46 μM for striatum). Quinpirole of 7-[³H]hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT), which are much more potent at the D₃ receptor than at the D₂ receptor, reduced the evoked dopamine release from the nucleus accumbens (ED₅₀, 0.12 μM for quinpirole; 0.02 μM for 7-OHDPAT) much more than the release from the striatum (ED₅₀, 1.6 μM for quinpirole; 0.55 μM for 7-OHDPAT). These results suggest that the contribution of D₃ receptors in nucleus accumbens to regulate dopamine release from dopamine nerve terminals is much greater than that in striatum.     

κ-opioid receptor mediated antinociception in rats is dependent on the functional state of dihydropyridine-sensitive calcium channels

The modulatory effect of the dihydropyridine Ca²⁺ channel antagonist nimodipine on the analgesic action of the κ-opioid receptor agonist U-69,593 was analyzed using the tail-flick test in rats. The antinociceptive effect of U-69,593 (0.25–4 mg/kg) was antagonized by L-type Ca²⁺ channel blockade with nimodipine (200 μg/kg, i.p.), the ED₅₀ being increased from 1.4 to 7.3 mg/kg. On the contrary, when an increase in the density of these channels was induced by means of chronic and simultaneous treatment with nimodipine (1 μg/h, 7 days) and sufentanil (2 μg/h, 8 days), the analgesic effect of U-69,593 was potentiated by 5-fold. Our results suggest a functional coupling between κ-opioid receptors and L-type Ca²⁺ channels in nociception.     

Antinociceptive effect of ketanserin in mice: involvement of supraspinal 5-HT2 receptors in nociceptive transmission

The involvement of 5-HT₂ receptors in pain transmission was investigated in mice. Subcutaneous administration of the selective 5-HT₂ receptor antagonist ketanserin produced dose-dependent antinociception in the hot-plate and acetic acid-induced writhing tests withED₅₀ values (95% confidence limit) of 1.51 (1.13–1.89) and 0.62 (0.10–1.40) mg/kg, respectively, but was without any significant effect on the tail-flick test. Pretreatment with the catecholamine depletors 6-hydroxydopamine (2.5 μg, i.c.v.) orα-methyl-p-tyrosine (200 mg/kg, s.c.), or the serotonin synthesis inhibitorp-chlorophenylalanine methylester (200 mg/kg, s.c.), resulted in a significant decrease in the antinociceptive effect of ketanserin. Likewise, intrathecal (i.t.) administration of 1 μg/mouse of idazoxan (anα₂-antagonist), methysergide (mixed 5-HT₁, and 5-HT₂ antagonist) or ketanserin also reversed the antinociceptive effect of s.c. administered ketanserin. The results of this work indicate that 5-HT₂ receptors located supraspinally may inhibit descending nociceptive neurotransmission. In addition, these studies suggest that 5-HT₂ receptors located at the spinal level modulate nociception.     

Profile of Anticonvulsant Activity and Minimal Toxicity of Methyl 4-[(p-Chlorophenyl)amino]-6-Methyl-2-Oxo-Cyclohex-3-En-l-Oate and Some Prototype Antiepileptic Drugs in Mice and Rats

The anticonvulsant and toxic properties of methyl 4-[(p-chlorophenyl)amino]-6-methyl-2-oxo-cyclohex-3-en-l-oate (ADD 196022), were compared with those of phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA). These compounds were evaluated in mice and rats using well-standardized anticonvulsant testing procedures. Results indicate that ADD 196022 is a very potent anticonvulsant in the maximal electroshock seizure (MES) model. The compound was effective in nontoxic doses after intraperitoneal (i.p.) administration in mice and oral administration in rats. In mice, i.p. administration of ADD 196022 resulted in an ED₅₀ value of 26.2 mg/kg as compared with a value of 6.48 mg/kg for PHT in the same assay. ADD 196022 was more potent that PHT in the oral rat model, having an ED₅₀ value of 5.79 mg/kg as compared to 23.2 mg/kg for PHT. ADD 196022 was ineffective in nontoxic doses against all other seizure models evaluated and thus has a pharmacologic profile similar to that of PHT.     

Interaction between 5-HT1A and nicotinic cholinergic receptors in the regulation of water maze navigation behavior

The interaction between serotonin (5-HT)_1A and nicotinic cholinergic reptors in the regulation of spatial navigation behavior in the Morris water maze (WM) test was studied. Pretraining intraperitoneal (i.p.) injections of a combination of subthreshold doses of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (a 5-HT_1A receptor agonist) at 30 μg/kg and mecamylamine (a nicotinic cholinergic receptor antagonist) at 2500 μg/kg greatly impaired WM navigation to a hidden platform and slightly, but not statistically significantly, impaired WM navigation to a visible platform. Post-training i.p. injections of this combination had no effect on WM navigation performance. Serotonin depletion induced byp-chlorophenylalanine (PCPA) increased the performance impairing action of pretraining injected combination of 8-OH-DPAT 30 μg/kg and mecamylamine 2500 μg/kg. In trained rats combined injections of 8-OH-DPAT 30 μg/kg and mecamylamine 2500 μg/kg given pretraining had no effect on the navigation to a hidden platform located in a familiar or in a novel position. Pretraining trial injected combination of hexamethonium 2000 μg/kg (a peripherally acting nicotinic antagonist) and 8-OH-DPAT 30 μg/kg had no effect on navigation. These data suggest that a combined treatment with a 5-HT_1a receptor agonist and a nicotinic cholinergic receptor antagonist more severely impair non-mnemonic acquisition performance processes than consolidation and retrieval processes.     

Functional central nicotinic acetylcholine receptor antagonism by systemic administration of Tinuvin 770 (BTMPS)

Tinuvin 770 (BTMPS) is a non-competitive, use-dependent antagonist of nicotinic acetylcholine receptors (nAChRs). The drug is highly lipid soluble and as such it has the potential to act within the brain. Presently the ganglionic blocking drug mecamylamine is used almost exclusively to block central nAChRs upon peripheral administration. These experiments were designed to confirm the nAChR antagonism effectiveness of BTMPS in both peripheral (ganglionic stimulation) and central (locomotor activity and thermal nociceptive sensitivity) nicotinic system in vivo. BTMPS inhibited the expression of the pressor response produced by i.v. injection of the ganglionic stimulant DMPP in anesthetized rats. The inhibition dose-response profile appeared to be biphasic with the maximal inhibition occurring after administration of the 0.48 mg/kg dose of BTMPS. In rats acclimated to the test apparatus, nicotine increased different measures of locomotor activity, particularly at the 0.75 mg/kg dose. BTMPS pretreatment significantly inhibited the nicotine-induced increase in motor behaviors, again with a biphasic dose-response relationship. Lastly, nicotine elicited an antinociceptive response in rats (hot plate test). BTMPS almost completely blocked the antinociceptive responses to 1 and 1.5 mg/kg nicotine. On its own, BTMPS failed to decrease blood pressure and to decrease the nociceptive threshold. The drug also generally failed to alter locomotor activity. The use-dependent aspect of BTMPS-induced inhibition of nAChRs was evident in the drug's greater effectiveness in the presence of the highest doses of nicotine. Therefore, BTMPS can be considered as an alternative to or as a confirmatory drug for mecamylamine when inhibition of central nicotinic receptors is required.     

Influence of Central Nicotinic Receptors on Arachidonylcyclopropylamide (ACPA)-Induced Antinociception in Mice

The nicotinic cholinergic receptors have been reported to be involved in several actions of cannabinoids (e.g., bradycardia, hypothermia). However, the influence of central cholinergic system on cannabinoids antinociceptive effect has not been reported. This study investigated the possible part played by nicotinic cholinergic modulator drugs on the antinociceptive effect of central administration of arachidonylcyclopropylamide (ACPA) in mice. The antinociceptive effects of intracerebroventricular (i.c.v.) administration of ACPA using the formalin test have been studied in mice. The effects of nicotine or mecamylamine (a nicotinic cholinergic antagonist) on ACPA analgesia are also studied. i.c.v. administration of ACPA (0.004-1 μg/mice) induced antinociceptive effect in mice. i.c.v. administration of nicotine (0.1 or 0.5 μg/mice) or mecamylamine (2 μg/mice) potentiated or antagonized ACPA antinociceptive effects, respectively. It is concluded that ACPA-induced analgesia is influenced by central nicotinic cholinergic activity.     

Influence of central nicotinic receptors on arachidonylcyclopropylamide (ACPA)-induced antinociception in mice

The nicotinic cholinergic receptors have been reported to be involved in several actions of cannabinoids (e.g., bradycardia, hypothermia). However, the influence of central cholinergic system on cannabinoids antinociceptive effect has not been reported. This study investigated the possible part played by nicotinic cholinergic modulator drugs on the antinociceptive effect of central administration of arachidonylcyclopropylamide (ACPA) in mice. The antinociceptive effects of intracerebroventricular (i.c.v.) administration of ACPA using the formalin test have been studied in mice. The effects of nicotine or mecamylamine (a nicotinic cholinergic antagonist) on ACPA analgesia are also studied. i.c.v. administration of ACPA (0.004-1 microg/mice) induced antinociceptive effect in mice. i.c.v. administration of nicotine (0.1 or 0.5 microg/mice) or mecamylamine (2 microg/mice) potentiated or antagonized ACPA antinociceptive effects, respectively. It is concluded that ACPA-induced analgesia is influenced by central nicotinic cholinergic activity.     

The Riluzole Derivative 2-Amino-6-trifluoromethylthio-benzothiazole (SKA-19), a Mixed KCa2 Activator and NaV Blocker,is a Potent Novel Anticonvulsant

Inhibitors of voltage-gated sodium channels (Na_v) have been used as anticonvulsants since the 1940s, while potassium channel activators have only been investigated more recently. We here describe the discovery of 2-amino-6-trifluoromethylthio-benzothiazole (SKA-19), a thioanalog of riluzole, as a potent, novel anticonvulsant, which combines the two mechanisms. SKA-19 is a use-dependent Na_V channel blocker and an activator of small-conductance Ca²⁺-activated K⁺ channels. SKA-19 reduces action potential firing and increases medium afterhyperpolarization in CA1 pyramidal neurons in hippocampal slices. SKA-19 is orally bioavailable and shows activity in a broad range of rodent seizure models. SKA-19 protects against maximal electroshock-induced seizures in both rats (ED₅₀ 1.6 mg/kg i.p.; 2.3 mg/kg p.o.) and mice (ED₅₀ 4.3 mg/kg p.o.), and is also effective in the 6-Hz model in mice (ED₅₀ 12.2 mg/kg), Frings audiogenic seizure-susceptible mice (ED₅₀ 2.2 mg/kg), and the hippocampal kindled rat model of complex partial seizures (ED₅₀ 5.5 mg/kg). Toxicity tests for abnormal neurological status revealed a therapeutic index (TD₅₀/ED₅₀) of 6–9 following intraperitoneal and of 33 following oral administration. SKA-19 further reduced acute pain in the formalin pain model and raised allodynic threshold in a sciatic nerve ligation model. The anticonvulsant profile of SKA-19 is comparable to riluzole, which similarly affects Na_V and KCa2 channels, except that SKA-19 has a ~4-fold greater duration of action owing to more prolonged brain levels. Based on these findings we propose that compounds combining KCa2 channel-activating and Na_v channel-blocking activity exert broad-spectrum anticonvulsant and analgesic effects.

Electronic supplementary material

The online version of this article (doi:10.1007/s13311-014-0305-y) contains supplementary material, which is available to authorized users.     

Effect of α2 adrenergic agents upon central etorphine antinociception in the cat

Systemic (s.c.) administration of α₂ agonists clonidine (25–100 μg/kg) or guanfacine (50–400 μg/kg) elicited antinociception as assessed by the cat tail-flick model and potentiated in a dose-dependent manner the antinociceptive effect of etorphine (2.5 μg) administered directly into the periaqueductal gray. Conversely, systemic yohimbine (1 mg/kg) attenuated the effects of central etorphine, and diminished potentiation of etorphine by theα₂ agonists. Prior microinjection of clonidine (5μg) or guanfacine (5 μg) into the locus coeruleus (LC) reduced the intensity of central etorphine antinociception whereas central yohimbine (20 μg) pretreatment increased peak antinociceptive activity and prolonged the duration of etorphine. Thus, systemicα₂ agonists are inherently antinociceptive and potentiate central narcotic antinociception; however, the site of interaction betweenα₂ agonists and opiates does not appear to be the LC inasmuch asα₂ agonists attenuate the antinociceptive effect of etorphine when administered directly into the LC. A spinal site of action is suggested based upon known LC-spinal projections and our experimental observations.     

The peripheral nociceptive actions of intravenously administered 5-HT in the rat requires dual activation of both 5-HT2 and 5-HT3 receptor subtypes

In 16-week-old Sprague-Dawley rats lightly anesthetized with pentobarbital, 5-HT (3–96 μg/kg, i.v.;n = 6) produced distinct pseudaffective responses and a dose-dependent (slope= 17.2 ± 6.8s/log₁₀dose) inhibition of the tail-flick (TF) reflex (ED₅₀ = 32.6 ± 9.2 μg/kg). In the same rats, a 1:1 combination of α-methyl 5-HT (a 5-HT₂ receptor selective agonist) and 2-methyl 5-HT (a 5-HT₃ receptor selective agonist) (3–192 μg/kg, i.v.), produced the same profile of pseudaffective responses and also resulted in a dose-dependent (slope= 34.0± 7.0s/log₂dose) inhibition of the TF reflex (ED₅₀ = 88.4 ± 20.5 μg/kg). In contrast, administration of α-methyl 5-HT (3–192 μg/kg, i.v.) or 2-methyl 5-HT (3–192 μg/kg, i.v.) alone did not produce any pseudaffective responses or any change in TF latency from baseline. In conscious 16-week-old male Sprague-Dawley rats, administration of 5-HT (48 μg/kg, i.v.;n = 5), or a 1:1 combination of α-methyl 5-HT and 2-methyl 5-HT (total dose= 120 μg/kg, i.v.;mn = 5), resulted in a passive avoidance behavior assessed in a step-down paradigm (slopes= 139.7 ± 58.2and154.9 ± 63.9s/trial, respectively), and the same profile of distinct pseudaffective responses exhibited by the lightly pentobarbital-anesthetized rats. However, administration of either α-methyl 5-HT (96 μg/kg, i.v.;n = 4) or 2-methyl 5-HT (96 μg/kg, i.v.;n = 4), while producing significant 5-HT receptor-mediated cardiovascular responses, produced a learned behavior not different from saline (0.25 ml, i.v.;n = 6) (slopes= 7.6 ± 2.5, 6.3 ± 1.8and7.4 ± 3.6s/trial, respectively). These results are consistent with the hypothesis that the peripheral nociceptive responses to i.v. 5-HT requires dual activation of 5-HT₂ and 5-HT₃ receptor subtypes.