Pregabalin may represent a novel class of anxiolytic agents with a broad spectrum of activity.

The present study examines the effect of pregabalin (previously S-Isobutylgaba and CI-1008) in two distinct rat models of anxiety. Pregabalin binds with high affinity and selectivity to the alpha(2)delta subunit of voltage dependent calcium channels (VDCC). Its corresponding R-enantiomer (R-isobutylgaba) is approximately 10 fold weaker. Pregabalin dose-dependently induced anxiolytic-like effects in both the rat conflict test and elevated X-maze with respective minimum effective doses (MED) of 3 and 10 mg kg(-1). In contrast, R-isobutylgaba only showed activity at the highest dose of 100 mg kg(-1) in the conflict test. These data indicate that pregabalin may possess clinical utility as a novel anxiolytic agent and demonstrates the importance of the alpha(2)delta subunit of VDCC in the mediation of anxiety related behaviours.

Enadoline, a selective kappa-opioid receptor agonist shows potent antihyperalgesic and antiallodynic actions in a rat model of surgical pain.

Enadoline is a highly selective and potent kappa-opioid receptor agonist. This report describes and compares the activities of enadoline and morphine in a rat model of postoperative pain. A 1 cm incision through the muscle and skin of the plantar surface of the right hind paw induced thermal hyperalgesia as well as static and dynamic allodynia lasting at least 2 days. Postoperative testing was carried out using the plantar test for thermal hyperalgesia, von Frey hairs for static allodynia and light stroking with a cotton bud for dynamic allodynia. A single i.v. dose of enadoline 15 min before surgery dose-dependently (1-100 microg/kg) blocked the development of thermal hyperalgesia as well as static and dynamic allodynia for over 24 h with respective MEDs of < or = 1, 10 and 10 microg/kg. The administration of enadoline (100 microg/kg, i.v.), 1 h after surgery, completely blocked the maintenance of the hyperalgesic and allodynic responses, but its duration of action was much shorter (2 h) than when administered before surgery. Previous studies have shown that administration of morphine (1-6 mg/kg, s.c.) 0.5 h before surgery can prevent the development of thermal hyperalgesia with a MED of < or =1 mg/kg, but it has little effect on static allodynia. In the present study similar administration of morphine (1-3 mg/kg), unlike enadoline, had no effect on the development of dynamic allodynia. Morphine dose-dependently (1-6 mg/kg, s.c.) potentiated isoflurane-induced sleeping time and respiratory depression in the rat. However, whilst enadoline also (1-1000 microg/kg, i.v.) potentiated isoflurane-induced sleeping time, it did not cause respiratory depression. It is suggested that enadoline may possess therapeutic potential as a pre-emptive antihyperalgesic and antiallodynic agent.

Identification of novel ligands for the gabapentin binding site on the alpha2delta subunit of a calcium channel and their evaluation as anticonvulsant agents.

As part of a program to investigate the structure-activity relationships of Gabapentin (Neurontin), a number of alkylated analogues were synthesized and evaluated in vitro for binding to the Gabapentin binding site located on the alpha2delta subunit of a calcium channel. A number of other bridged and heterocyclic analogues are also reported along with their in vitro data. Two compounds showing higher affinity than Gabapentin were selected for evaluation in an animal model of epilepsy. One of these compounds, cis-(1S,3R)-(1-(aminomethyl)-3-methylcyclohexyl)acetic acid hydrochloride (19), was shown to be effective in this model with a profile similar to that of Gabapentin itself.

Evaluation of PD 154075, a tachykinin NK1 receptor antagonist, in a rat model of postoperative pain.

PD 154075 ([(2-benzofuran)-CH2OCO]-(R)-alpha-MeTrp-(S)-NHCH(CH3) Ph) is a selective tachykinin NK1 receptor antagonist. Its effect on development and maintenance of thermal and mechanical hypersensitivity was examined in a rat model of surgical pain. When administered 30 min before surgery, PD 154075 dose-dependently (3-100 mg/kg, s.c.) prevented the development of thermal and mechanical hypersensitivity with respective minimum effective doses of 10 and 30 mg/kg. These antihypersensitivity effects lasted for 72 h. In contrast, the administration of PD 154075 (30 mg/kg, s.c.) after surgery had little or no effect on these nociceptive responses. PD 154075 antagonised thermal hypersensitivity induced by intrathecal administration of substance P, over the same dose range that blocked surgical hypersensitivity. However, it only partially blocked the thermal hypersensitivity induced by the selective NK2 receptor agonist [betaAla8]neurokinin A-(4-10). Morphine dose-dependently (1-6 mg/kg, s.c.) lengthened isoflurane and pentobarbitone-induced sleeping time in the rat. In contrast, PD 154075 (3-100 mg/kg, s.c.) did not interact with these anaesthetics. It is suggested that tachykinin NK1 receptor antagonists, such as PD 154075, may possess therapeutic potential as pre-emptive antihypersensitive agents.

Gabapentin (neurontin) and S-(+)-3-isobutylgaba represent a novel class of selective antihyperalgesic agents.

1. Gabapentin (neurontin) is a novel antiepileptic agent that binds to the alpha 2 delta subunit of voltage-dependent calcium channels. The only other compound known to possess affinity for this recognition site is the (S)-(+)-enantiomer of 3-isobutylgaba. However, the corresponding (R)-(-)-enantiomer is 10 fold weaker. The present study evaluates the activity of gabapentin and the two enantiomers of 3-isobutylgaba in formalin and carrageenan-induced inflammatory pain models. 2. In the rat formalin test, S-(+)-3-isobutylgaba (1-100 mg kg-1) and gabapentin (10-300 mg kg-1) dose-dependently inhibited the late phase of the nociceptive response with respective minimum effective doses (MED) of 10 and 30 mg kg-1, s.c. This antihyperalgesic action of gabapentin was insensitive to naloxone (0.1-10.0 mg kg-1, s.c.). In contrast, the R-(-)-enantiomer of 3-isobutylgaba (1-100 mg kg-1) produced a modest inhibition of the late phase at the highest dose of 100 mg kg-1. However, none of the compounds showed any effect during the early phase of the response. 3. The s.c. administration of either S-(+)-3-isobutylgaba (1-30 mg kg-1) or gabapentin (10-100 mg kg-1), after the development of peak carrageenan-induced thermal hyperalgesia, dose-dependently antagonized the maintenance of this response with MED of 3 and 30 mg kg-1, respectively. Similar administration of the two compounds also blocked maintenance of carrageenan-induced mechanical hyperalgesia with MED of 3 and 10 mg kg-1, respectively. In contrast, R-(-)-3-isobutylgaba failed to show any effect in the two hyperalgesia models. 4. The intrathecal administration of gabapentin dose-dependently (1-100 micrograms/animal) blocked carrageenan-induced mechanical hyperalgesia. In contrast, administration of similar doses of gabapentin into the inflamed paw was ineffective at blocking this response. 5. Unlike morphine, the repeated administration of gabapentin (100 mg kg-1 at start and culminating to 400 mg kg-1) over 6 days did not lead to the induction of tolerance to its antihyperalgesic action in the formalin test. Furthermore, the morphine tolerance did not cross generalize to gabapentin. The s.c. administration of gabapentin (10-300 mg kg-1), R-(-) (3-100 mg kg-1) or S-(+)-3-isobutylgaba (3-100 mg kg-1) failed to inhibit gastrointestinal motility, as measured by the charcoal meal test in the rat. Moreover, the three compounds (1-100 mg kg-1, s.c.) did not generalize to the morphine discriminative stimulus. Gabapentin (30-300 mg kg-1) and S-(+)-isobutylgaba (1-100 mg kg-1) showed sedative/ataxic properties only at the highest dose tested in the rota-rod apparatus. 6. Gabapentin (30-300 mg kg-1, s.c.) failed to show an antinociceptive action in transient pain models. It is concluded that gabapentin represents a novel class of antihyperalgesic agents.

The antiepileptic agent gabapentin (Neurontin) possesses anxiolytic-like and antinociceptive actions that are reversed by D-serine.

This report describes the activity of the antiepileptic agent gabapentin (Neurontin) in animal models predictive of anxiolysis and analgesia. Gabapentin displayed anxiolytic-like action in the rat conflict test, the mouse light/dark box and the rat elevated X-maze with respective minimum effective doses (MEDs) of 3, 10 and 30 mg/kg. Furthermore, gabapentin also induced behavioural changes suggestive of anxiolysis in the marmoset human threat test with a MED of 30 mg/kg. In the rat formalin test of tonic nociception, gabapentin dose-dependently (30-300 mg/kg) and selectively blocked the late phase with a MED of 100 mg/kg. However, it failed to block carrageenan-induced paw oedema. The intracerebroventricular (ICV) administration of the glycine/NMDA receptor agonist D-Serine, dose-dependently (10-100 micrograms/animal) reversed the antinociceptive action of gabapentin (200 mg/kg, SC). D-Serine (30 micrograms/animal, ICV) also reversed the anxiolytic-like effects (in the light/dark box and the rat elevated X-maze) of gabapentin (30 mg/kg). In contrast, L-Serine (100 micrograms, ICV) failed to block the antinociceptive action of gabapentin. The antinociceptive action of (+)-HA-966 (25 mg/kg, SC), a partial agonist at the glycine/NMDA receptor, was reversed by D-Serine (100 micrograms/animal, ICV). However, D-Serine (100 micrograms/animal, ICV) failed to affect the antinociceptive action of a competitive NMDA receptor antagonist CGS 19755 (3 mg/kg, SC). Gabapentin has negligible affinity for the strychnine insensitive [3H]glycine binding site. This indicates that the interaction between gabapentin and D-Serine may not involve the NMDA receptor complex. Gabapentin may represent a novel type of anxiolytic and analgesic agent.

Modulation of the in vivo actions of morphine by the mixed CCKA/B receptor antagonist PD 142898.

The ability of a mixed CCKA/B receptor antagonist PD 142898 (benzenebutanic acid, beta-[[3-(1 H-indol-3-yl)-2-methyl-2-[[[(2-methyl- cyclohexyl)oxy]carbonyl]amino]-1-oxopropyl]amino]-[1 S-[1 alpha [S*(R*)]-2 beta]]) to modulate the antinociceptive, positive reinforcing and gastrointestinal actions of morphine was investigated in the rat. PD 142898 antagonised the development and maintenance of morphine (2.0 mg/kg, s.c.) induced conditioned place preference at 0.1 mg/kg, i.p. However, it potentiated the antinociceptive action of a subthreshold dose of morphine in the radiant tail flick model at doses of 0.001 and 0.01 mg/kg, s.c. Furthermore, PD 142898 (0.0001-1.0 mg/kg, s.c.) also potentiated the antinociceptive action of morphine (1.0 mg/kg, s.c.) against the late phase of formalin response associated with inflammation at the dose of 0.001-1.0 mg/kg. PD 142898 (0.001 mg/kg, s.c.) blocked the development of tolerance to morphine in the formalin test. It failed (0.001-1.0 mg/kg, i.p.) to modulate the inhibitory action of morphine (5.0 mg/kg, s.c.) on gastrointestinal transit as measured using the charcoal meal test. It is argued that the effect of PD 142898 in the conditioned place preference test involves antagonism of CCKA receptors, whilst the potentiation of the antinociceptive action of morphine is mediated via blockade of CCKB receptors. These results suggest that the mixed CCKA/B receptor antagonist may potentiate the analgesic action of morphine, block the development of tolerance without a concomitant increase in constipation and may also reduce the abuse potential of the opiate.

Effect of CCK receptor antagonists on the antinociceptive, reinforcing and gut motility properties of morphine.

1. The ability of a selective CCKA receptor antagonist PD 140548 and a selective CCKB receptor antagonist CI-988 (formerly PD 134308) to modulate the various in vivo properties of morphine was investigated in the rat. 2. PD 140548 dose-dependently (0.001-1.0 mg kg-1, i.p.) antagonised the development of conditioned place preference to morphine (2.0 mg kg-1, s.c.). In contrast, CI-988 (0.01-1.0 mg kg-1, i.p.) did not affect this morphine-induced behaviour. Neither of the CCK receptor antagonists blocked or generalised to the morphine (3.0 mg kg-1, i.p.) discriminative stimulus. 3. CI-988 (0.001-10.0 mg kg-1, s.c.) at doses of 0.05 and 0.1 mg kg-1 (s.c.), potentiated the antinociceptive action of a threshold dose of morphine (5.0 mg kg-1, i.p.) in a radiant heat model of acute nociception, the rat tail flick test. Furthermore, at 0.01 mg kg-1 it potentiated the antinociceptive action of morphine (3.0 mg kg-1) during the acute phase of the rat paw formalin test. And at doses of 0.01 and 0.1 mg kg-1 it also potentiated the antinociceptive action of morphine (1.0 mg kg-1) during the tonic phase of the formalin test. However, in both models, higher doses of CI-988 were ineffective. In contrast, PD 140548 (0.001-10 mg kg-1, s.c.) was only active at a dose of 1.0 mg kg-1 (s.c.) and only in the tonic phase of the formalin test. Neither CI-988 nor PD 140548 possessed any intrinsic antinociceptive action in either of the tests. Chronic treatment with CI-988 (0.01 mg kg-1, s.c.) prevented the development of tolerance to morphine antinociception (4 mg kg-1, s.c.) following a 6 day period of twice daily injections of morphine escalating from 1 to 16 mg kg-1 (i.p.). 4. Morphine dose-dependently (1-10 mg kg-1, s.c.) reduced the distance travelled by a charcoal meal in the rat intestine. Neither PD 140548 (0.01-1.0 mg kg-1, i.p.) nor CI-988 (0.01-1.0 mg kg-1, i.p.) potentiated or suppressed this inhibitory action of morphine. 5. In conclusion, the results of the present study indicate that CCKA and CCKB receptors modulate different properties of morphine. Thus, whilst a selective CCKA receptor antagonist blocked the rewarding properties of morphine, a selective CCKB receptor antagonist potentiated the antinociceptive action. However, neither compound displayed a potential for modulating the influence of morphine on gastro-intestinal motility. It is suggested that these findings may have important implications for development of CCK receptor antagonists as analgesic adjuncts to the therapeutic use of morphine.

The behavioural properties of CI-988, a selective cholecystokininB receptor antagonist.

1. The behavioural effects of a selective cholecystokininB (CCKB) receptor antagonist CI-988 were investigated in rodents. 2. In three rodent tests of anxiety (rat elevated X-maze, rat social interaction test and mouse light/dark box) CI-988 over the dose range 0.001-10.0 mg kg-1, (i.p.) produced an anxiolytic-like action. The magnitude of this effect was similar to that of chlordiazepoxide (CDP). In contrast, the selective CCKA receptor antagonist, devazepide, was inactive. CI-988 also showed anxiolytic-like action in the rat conflict test but the magnitude of this effect was about 2.5 fold less than that of CDP. 3. Central but not peripheral administration of the selective CCKB receptor agonist, pentagastrin, like FG 7142, produced an anxiogenic-like action. 4. The pentagastrin-induced anxiety was dose-dependently antagonized by CI-988, whereas devazepide was inactive. However, ten times higher doses of CI-988 were required to block a similar action of FG 7142. 5. In contrast to CDP, CI-988 up to 3000 fold higher doses than those inducing anxiolysis was inactive in tests measuring sedation and ataxia. It also failed to antagonize pentylenetetrazol-induced tonic seizures. Furthermore, CI-988 did not interact with alcohol or barbiturates. Thus, CI-988 appears to be an anxioselective compound. 6. The anxiolytic-like action of CDP in the rat elevated X-maze was dose-dependently antagonized by flumazenil. In contrast, the benzodiazepine receptor antagonist failed to block a similar effect of CI-988. 7. Thus, CI-988 shows anxiolytic-like activity in several animal models of anxiety. The anxiolytic-like effect of CI-988 involves a novel mechanism of action, that is likely to be mediated by selective antagonism of the brain CCKB receptor. It is suggested that CI-988 should have a better side-effect profile in man than the benzodiazepines.

Evidence that a proconvulsant action of lithium is mediated by inhibition of myo-inositol phosphatase in mouse brain.

Lithium inhibits myo-inositol mono- and polyphosphatase activity in brain at concentrations similar to those optimal for the treatment of manic depressive psychosis. A consequence of this inhibition is the possibility that the availability of myo-inositol for the regeneration of polyphosphoinositides involved in cellular signalling mechanisms may be reduced. While there are no good models of manic depressive disorders in rodents, lithium is known to alter their behavioural responsiveness to a number of neurotransmitter receptor agonists, but the role of the phosphatidylinositol second messenger system in these effects is unknown. Consistent with the myo-inositol depletion hypothesis, when injected directly into the CNS, myo-inositol, but not its biologically inactive epimer, scyllo-inositol or D-mannitol, has been found to reverse a proconvulsant action of lithium in mice given the muscarinic receptor agonist, pilocarpine.

A slow intravenous infusion of N-methyl-DL-aspartate as a seizure model in the mouse.

A seizure model involving slow i.v. infusion of the excitatory amino acid N-methyl-DL-aspartate (NMDLA) in the mouse is described. It allows determination of the threshold doses of NMDLA required to elicit clonic and tonic seizures in individual mice. The NMDA receptor antagonists MK-801, CPP, ifenprodil and 7-chlorokynurenic acid (7-CLK), and diazepam dose-dependently increased the dose of NMDLA required to elicit a tonic seizure. CPP, 7-CLK and diazepam also increased the dose of NMDLA inducing clonic seizures. In contrast, ifenprodil at doses which antagonised tonic seizures had no effect on clonic seizures. The glycine and polyamine modulatory site agonists, D-serine and spermidine respectively, dose-dependently reduced the dose of NMDLA required to induce clonic and tonic seizures. The NMDLA infusion model appears to be more sensitive than the classical bolus injection test and can detect both anticonvulsant and proconvulsant actions mediated by the NMDA receptor complex.

Modulation of seizure susceptibility in the mouse by the strychnine-insensitive glycine recognition site of the NMDA receptor/ion channel complex.

1. In order to determine whether the strychnine-insensitive glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex is fully activated in vivo, the ability of the selective glycine receptor agonist, D-serine, to modulate seizure susceptibility in the mouse has been examined. 2. D-Serine (10-200 micrograms per mouse, i.c.v.) dose-dependently increased the potency of NMDLA in inducing seizures in Swiss Webster mice by approximately 3 fold. L-Serine was without significant effect. 3. The potency of pentylenetetrazol in inducing seizures was also enhanced by D-, but not L-serine, although the magnitude of the shift (1.6 fold) was considerably less than for NMDLA. 4. Similar doses of D-serine were also able to block the anticonvulsant effect of the non-selective glycine receptor antagonist, kynurenic acid, against seizures induced by NMDLA, but were without effect on the anticonvulsant effect of the competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). 5. D-Serine completely antagonized the protective effect of the selective glycine receptor antagonist, 7-chlorokynurenic acid, against sound-induced seizures in DBA/2 mice, but was less effective in this model against the less selective antagonist, kynurenic acid. 6 The results indicate that in vivo, NMDA receptors are not maximally potentiated by endogenous glycine and suggest an important involvement of the glycine modulatory site on the NMDA receptor/ion channel complex in the pathophysiology of epilepsy.

Enantiomers of HA-966 (3-amino-1-hydroxypyrrolid-2-one) exhibit distinct central nervous system effects: (+)-HA-966 is a selective glycine/N-methyl-D-aspartate receptor antagonist, but (-)-HA-966 is a potent gamma-butyrolactone-like sedative.

The antagonist effect of (+/-)-3-amino-1-hydroxypyrrolid-2-one (HA-966) at the N-methyl-D-aspartate (NMDA) receptor occurs through a selective interaction with the glycine modulatory site within the receptor complex. When the enantiomers of (+/-)-HA-966 were resolved, the (R)-(+)-enantiomer was found to be a selective glycine/NMDA receptor antagonist, a property that accounts for its anticonvulsant activity in vivo. In contrast, the (S)-(-)-enantiomer was only weakly active as an NMDA-receptor antagonist, but nevertheless it possessed a marked sedative and muscle relaxant action in vivo. In radioligand binding experiments, (+)-HA-966 inhibited strychnine-insensitive [3H]glycine binding to rat cerebral cortex synaptic membranes with an IC50 of 12.5 microM, whereas (-)-HA-966 had an IC50 value of 339 microM. In electrophysiological experiments, (+)-HA-966 selectively antagonized NMDA receptor responses in rat cortical slices, whereas the (-)-enantiomer was much weaker. On cultured cortical neurones (+)-HA-966 inhibited glycine-potentiated NMDA responses with an IC50 = 13 microM compared with (-)-HA-966, which has an IC50 = 708 microM. In agreement with findings with racemic HA-966, even high concentrations of (+)-HA-966 did not completely inhibit NMDA responses, suggesting that (+)-HA-966 is a low-efficacy partial agonist. (+)-HA-966 produced parallel shifts to the right of the glycine concentration curve for potentiation of NMDA responses, resulting in an estimated pKb = 5.6. In mice, (+)-HA-966 antagonized sound and N-methyl-DL-aspartic acid (NMDLA)-induced seizures with ED50 values of 52.6 mg/kg of body weight (i.p.) and 900 mg/kg (i.v.), respectively. The coadministration of D-serine dose-dependently (10-100 micrograms into the cerebral ventricles per mouse) antagonized the anticonvulsant effect of a submaximal dose of (+)-HA-966 (100 micrograms administered directly into the cerebral ventricles) against NMDLA-induced seizures. The sedative/ataxic effect of racemic HA-966 was mainly attributable to the (-)-enantiomer, which was greater than 25-fold more potent than the (+)-enantiomer. It is suggested that, as in the case of the sedative gamma-butyrolactone, disruption of striatal dopaminergic mechanisms may be responsible for this action.

The behavioural effects of MK-801: a comparison with antagonists acting non-competitively and competitively at the NMDA receptor.

The selective non-competitive NMDA receptor antagonist, MK-801, potently blocked convulsions induced in the mouse by N-methyl-DL-aspartic acid (NMDLA) with an i.v. ED50 dose of 0.2 mg/kg. Similar doses of MK-801 were also effective in blocking seizures induced by pentylenetetrazol (PTZ), electroshock and by sound in audiogenic seizure-prone animals. Other less selective non-competitive NMDA receptor antagonists including phencyclidine (PCP), thienylcyclohexylpiperidine (TCP), (+)-N-allylnormetazocine [+)-NANM, (+)-SKF 10,047) and ketamine also blocked NMDLA-induced seizures with a rank order of potency of MK-801 greater than PCP greater than TCP = (+)-NANM greater than ketamine. The competitive NMDA receptor antagonist, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked NMDLA-induced seizures with an ED50 of 4.5 mg/kg, 22- and 560-fold more potently than the competitive antagonists, 2-DL-amino-7-phosphonoheptanoic acid (2-APH) and 2-DL-amino-5-phosphonovaleric acid (2-APV), respectively. MK-801 was the most potent of the non-competitive antagonists to induce a motor syndrome including head weaving, body rolling, increased locomotion and ataxia, characteristic of the behavioural response to PCP in the mouse. The syndrome was also present following injection of the competitive NMDA receptor antagonists, although they were generally less potent (probably a reflection of poor brain penetration) and less efficacious than the non-competitive antagonists. For all compounds except CPP, the anticonvulsant ED50 dose was close to the minimum effective dose to induce motor stimulation: CPP was 5- to 10-fold more potent as an anticonvulsant.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for receptors of N-methyl-D-aspartic acid in the discriminative stimulus properties of phencyclidine.

Ketamine and (+)-N-allylnormetazocine ((+)-NANM) were found to generalize in a rat operant drug discrimination paradigm to the interoceptive stimulus induced by phencyclidine (PCP). Intraperitoneal administration of the non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, MK-801, and intracerebroventricular injection of the competitive antagonist, 2-DL-amino-7-phosphonoheptanoic acid (2-APH), also resulted in a dose-dependent generalisation to the PCP discriminative stimulus. The results suggest that NMDA receptor antagonism may play an important role in the mediation of the discriminative stimulus properties of PCP. The low potency of MK-801 and 2-APH to displace [3H](+)-NANM binding in vitro argues against an involvement of the haloperidol-sensitive sigma recognition site in the behaviour.