John Hughes

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 α2δ 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 μg/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.

Gabapentin and pregabalin, but not morphine and amitriptyline, block both static and dynamic components of mechanical allodynia induced by streptozocin in the rat

A single injection of streptozocin (50 mg/kg, i.p.) led to the development of static and dynamic allodynia in the rat. The two responses were detected, respectively, by application of pressure using von Frey hairs or lightly stroking the hind paw with a cotton bud. Static allodynia was present in the majority of the animals within 10 days following streptozocin. In contrast, dynamic allodynia took almost twice as long to develop and was only present in approximately 60% of rats. Morphine (1-3 mg/kg, s.c.) and amitriptyline (0.25-2.0 mg/kg, p.o.) dose-dependently blocked static allodynia. However, neither of the compounds was effective against dynamic allodynia. In contrast, gabapentin (10-100 mg/kg, p.o.) and the related compound pregabalin (3-30 mg/kg, p.o.) dose-dependently blocked both types of allodynia. However, the corresponding R-enantiomer (10-100 mg/kg, p.o.) of pregabalin, was found to be inactive. The intrathecal administration of gabapentin dose-dependently (1-100 microg/animal) blocked both static and dynamic allodynia. In contrast, administration of similar doses of gabapentin into the hind paw failed to block these responses. It is suggested that in this model of neuropathic pain dynamic allodynia is mediated by A beta-fibres and the static type involves small diameter nociceptive fibres. These data suggest that gabapentin and pregabalin possess a superior antiallodynic profile than morphine and amitriptyline, and may represent a novel class of therapeutic agents for the treatment of neuropathic pain.

Detection of static and dynamic components of mechanical allodynia in rat models of neuropathic pain: are they signalled by distinct primary sensory neurones?

In the present study, chronic constrictive injury (CCI model) of the sciatic nerve or tight ligation of L5 and L6 spinal nerves (Chung model) produced both dynamic and static components of mechanical allodynia in rats. The two responses were detected, respectively, by lightly stroking the hind paw with cotton wool or application of pressure using von Frey hairs. Animals with spinal nerve ligation developed both types of responses at a faster rate compared to animals with the CCI. Morphine (1-3 mg/kg, s.c.) dose-dependently blocked static but not dynamic allodynia. In contrast, pregabalin (previously S-isobutylgaba and CI-1008) dose-dependently (3-30 mg/kg, p.o.) blocked both types of allodynia. In CCI animals, two administrations of capsaicin (100 microg/50 microl) into the plantar surface of the ipsilateral paw at 1-h intervals blocked the maintenance of thermal hyperalgesia without affecting either static or dynamic allodynia. The similar administration of a further two doses of capsaicin into the same animals blocked the maintenance of static allodynia without affecting the dynamic response. These data indicate that thermal hyperalgesia, static and dynamic allodynia are respectively signalled by C-, Adelta- and Abeta/capsaicin insensitive Adelta- primary sensory neurones. It is suggested that pregabalin possesses a superior antiallodynic profile than morphine and may represent a novel class of therapeutic agents for the treatment of neuropathic pain.

Gabapentin inhibits the substance P-facilitated K+-evoked release of [3H]glutamate from rat caudal trigeminal nucleus slices

&NA; The effect of gabapentin on the release of the spinal sensory neurotransmitter glutamate has been investigated in an in vitro model using a perfused thin slice preparation from the rat brainstem containing the spinal trigeminal caudal subnucleus (Sp5C) and pre‐incubated with [3H]glutamate. Addition of excess K+ to the perfusing solution increased the content of tritium in the perfusate. The prior addition of substance P increased this index of glutamate release in a concentration‐dependent manner, with the mean maximum of around 50% increase obtained at 1–3 &mgr;M. The action of substance P to increase the evoked release of glutamate was blocked by the antagonist CP‐99994, suggesting a specific involvement of the NK1 receptor in mediating the facilitatory effect. On its own, gabapentin at up to 100 &mgr;M did not modify the baseline level of K+‐evoked release of glutamate; however, gabapentin caused a concentration‐dependent decrease of the facilitatory effect of substance P (EC50=6.49 &mgr;M). The R‐(−)‐ and S‐(+)‐isomers of 3‐isobutylgaba were then tested against the increase in K+‐evoked release of glutamate by substance P. S‐(+)‐3‐isobutylgaba (pregabalin) at 30 &mgr;M acted like gabapentin to reduce the substance P‐mediated increase of release almost to the baseline level of K+‐evoked release, while in contrast the R‐(−)‐isomer at this concentration produced no reduction, and rather a trend towards a further enhancement of the potentiating effect of substance P. In conclusion, we have found and characterized an effect of gabapentin that is of possible mechanistic relevance to the anti‐hyperalgesic/allodynic actions of this compound.

Further evidence for the role of the α2δ subunit of voltage dependent calcium channels in models of neuropathic pain

Current analgesic therapy is dominated by NSAIDs and opiates, however these agents have limited efficacy in the treatment of neuropathic pain. The novel anticonvulsant agent gabapentin (Neurontin) has been shown to be an effective treatment for neuropathic pain in the clinic. Recent studies have demonstrated that gabapentin selectively interacts with the α2δ subunit of voltage dependent calcium channels (VDCCs) which may be important in its mechanism of action. Previous studies have identified a gabapentin analogue, 3‐methyl gabapentin, that stereoselectively interacts with the α2δ subunit of VDCCs. Thus, whilst (1S,3R) 3‐methyl gabapentin binds to the α2δ protein with high affinity (IC50=42 nM), the corresponding (1R,3R) isomer is 300 times weaker ( Bryans et al., 1998 : J. Med. Chem., 41, 1838–1845). The present study examines the activity of diastereoisomers of 3‐methyl gabapentin in two rat models of neuropathic pain to assess the importance of an interaction with the α2δ subunit of VDCCs. (1S,3R) 3‐methyl‐gabapentin dose‐dependently (10–100 mg kg−1, p.o.) blocked the maintenance of static allodynia in the rat streptozocin and Chung models of neuropathic pain with MEDs of 30 mg kg−1. This isomer also dose‐dependently blocked the maintenance of dynamic allodynia in both models with respective MEDs of 30 and 100 mg kg−1. In contrast, (1R,3R) 3‐methyl gabapentin (100 mg kg−1, p.o.) failed to block either static or dynamic allodynia in the streptozocin model. It is concluded that these data further support the hypothesis that the α2δ subunit of VDCCs plays an important role in the maintenance of mechanical hypersensitivity in models of neuropathic pain.

Relaxations of the isolated portal vein of the rabbit induced by nicotine and electrical stimulation

1 A pharmacological analysis of the inhibitory innervation of the isolated portal vein of the rabbit has been made. 2 In untreated preparations, transmural stimulation elicited a long‐lasting relaxation at low frequencies (0·2–1 Hz); at higher frequencies a contraction followed by a prolonged after‐relaxation occurred. Tetrodotoxin abolished the contractions but a higher dose was required to abolish the relaxations. Veratrine lowered the threshold of stimulation for producing relaxations in the untreated vein. The relaxations were unaffected by hyoscine or hexamethonium. They were reduced or altered by antagonists of α‐adrenoceptors for catecholamines and by adrenergic neurone blockade. They were sometimes slightly reduced by antagonists of β‐adrenoceptors. 3 In the presence of antagonists of α‐adrenoceptors, electrical stimulation elicited relaxations which increased with frequency of stimulation and became maximal at 20–30 Hz. These relaxations were partially reduced by antagonists of β‐adrenoceptors, or by adrenergic neurone block; the antagonisms were more pronounced at the higher frequencies of stimulation. Noradrenaline also caused relaxations which were abolished by β‐adrenoceptor blocking drugs. Cocaine increased the sensitivity to noradrenaline by 7–8 fold after α‐adrenoceptor blockade but had little or no effect on the relaxations induced by electrical stimulation at high frequencies. 4 In the presence of antagonists of α‐ and β‐adrenoceptors, or adrenergic neurone blocking agents, or in veins taken from rabbits pretreated with reserpine, electrical stimulation elicited rapid relaxations which were greatest at 20–30 Hz. These relaxations were increased by veratrine and abolished by tetrodotoxin or by storing the vein for 9 days at 4° C. They were unaffected by antagonists of acetylcholine, or by dipyridamole. 5 Prostaglandins E1, E2 and F2α inhibited contractions elicited by electrical stimulation and noradrenaline, but in higher doses caused contractions themselves. 6 Nicotine (10−6–10−5 g/ml) relaxed the portal vein; higher concentrations elicited mixed inhibitory and excitatory effects. All these effects were abolished by tetrodotoxin, cocaine, hexamethonium or storage. The contractor effects were abolished by drugs or procedures that blocked adrenergic mechanisms. 7 The relaxations produced by nicotine in untreated preparations and in veins from rabbits pretreated with reserpine were mediated mainly by a non‐adrenergic non‐cholinergic nervous mechanism. Relaxations induced by nicotine in the presence of antagonists of α‐adrenoceptors were only partially antagonized by antagonists of β‐adrenoceptors. 8 It was concluded that all the effects of nicotine and transmural stimulation were mediated by nerves. Part of the inhibitory effects was mediated by non‐adrenergic, non‐cholinergic nerves.

The tachykinin NK1 receptor antagonist PD 154075 blocks cisplatin-induced delayed emesis in the ferret

The activity of a selective tachykinin NK1 receptor antagonist, PD 154075 ([(2-benzofuran)-CH2OCO]-(R)-alpha-MeTrp-(S)-NHCH(CH3) Ph), was examined in radioligand binding studies, in a [Sar9,Met(O2)11]substance P-induced foot-tapping model in the gerbil, and in cisplatin-induced acute and delayed emesis in the ferret. In radioligand binding studies, PD 154075 showed nanomolar affinity for the human, guinea-pig, gerbil, dog and ferret NK1 receptors with an approximate 300 times lower affinity for the rodent NK1 receptor. Using NK2,NK3 receptors and a range of other receptor ligands, PD 154075 was shown to exhibit a high degree of selectivity and specificity for the human type NK1 receptor. Following subcutaneous administration PD 154075 dose dependently (1-100 mg/kg) antagonised the centrally mediated [Sar9,Met(O2)11] substance P-induced foot tapping in the gerbil with a minimum effective dose (MED) of 10 mg/kg. The ability of PD 154075 to readily penetrate into the brain following oral administration was confirmed by its extraction and high performance liquid chromatography assay from the rat brain. PD 154075 was shown to achieve a relatively fast and sustained brain concentration (brain/plasma ratios ranged from 0.27 to 0.41 during the time period of 0.25-12 h). Further pharmacokinetic studies revealed that the absolute oral bioavailability of PD 154075 in the rat was (mean +/- S.D.) 49 +/- 15%. PD 154075 (1-30 mg/kg, i.p.) dose dependently antagonised the acute vomiting and retching in the ferret measured for 4 h following administration of cisplatin (10 mg/kg, i.p.) with a MED of 3 mg/kg. The administration of a lower dose of cisplatin (5 mg/kg, i.p.) in the ferret induces both an acute (day 1) and delayed (days 2 and 3) phase of emesis. The i.p. administration of PD 154075, 10 mg/kg three times a day for 3 days, almost completely blocked both the acute and delayed emetic responses. In the same study, the 5-HT3 receptor antagonist ondansetron (1 mg/kg, i.p., t.i.d.) was also very effective against the acute emetic response observed during the first 4 h following cisplatin, but it was only weakly active against the delayed response. In conclusion, PD 154075 is a selective and specific high affinity NK1 receptor antagonist with good oral bioavailability which is effective against both acute and delayed emesis induced by cisplatin in the ferret.

Enadoline, a selective κ-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.

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.

Activation of c-fos mRNA in the brain by the κ-opioid receptor agonist enadoline and the NMDA receptor antagonist dizocilpine

Using in situ hybridization, we have shown that the kappa-opioid receptor agonist enadoline (CI-977) and the non-competitive NMDA receptor antagonist dizocilpine (MK-901) induced the immediate early gene c-fos in dorsal medial thalamic nuclei. Dizocilpine, and not enadoline, also induced c-fos in the posterior cingulate cortex and retrosplenial cortex. Enadolines stimulation of c-fos mRNA was dose and time dependent and completely inhibited by pretreatment with naltrexone. Morphine did not stimulate c-fos in the thalamus. It is suggested that the stimulation of c-fos with enadoline represents a specific kappa-opioid receptor effect.