A role for spinal, but not supraspinal, alpha(2) adrenergic receptors in the actions of improgan, a powerful, non-opioid analgesic

Characterization of the Antinociceptive Mechanisms of Khat Extract (Catha edulis) in Mice

This study investigated the antinociceptive mechanisms of khat extract (100, 200, and 400 mg/kg, i.p.) in four pain models: two thermic (hot plate, tail-flick) and two chemical (acetic acid, formalin) models. Male mice were pretreated intraperitoneally (i.p.) with the opioid receptor blocker naloxone (5 mg/kg), the cholinergic antagonist atropine (2 mg/kg), the selective α₁ blocker prazosin (1 mg/kg), the dopamine D₂ antagonist haloperidol (1.5 mg/kg), or the GABA_A receptor antagonist, bicuculline (1 mg/kg) 15 minutes prior to i.p. injection of khat extract (400 mg/kg). Khat extract reduced the nociceptive response of mice in the four pain tests. Naloxone significantly inhibited the antinociceptive effect of khat extract in the hot plate, tail-flick, and the first phase of formalin tests. Bicuculline significantly antagonized the antinociceptive effect of khat extract on the hot plate and tail-flick tests. Haloperidol significantly reversed the antinociceptive effect of khat extract on the tail-flick test and the first phase of formalin test. These results provide strong evidence that the antinociceptive activity of khat extract is mediated via opioidergic, GABAergic, and dopaminergic pathways. The mechanism of the antinociceptive action of khat may be linked to the different types of pain generated in animal models.

Efficacy of improgan, a non-opioid analgesic, in neuropathic pain

Improgan, a non-opioid analgesic, is known to act in the rodent brain stem to produce highly effective antinociception in several acute pain tests. However, improgan has not been studied in any models of chronic pain. To assess the efficacy of improgan in an animal model of neuropathic pain, the effects of this drug were studied on mechanical allodynia following unilateral spinal nerve ligation (SNL) in rats. Intracerebroventricular (icv) improgan (40-80 μg) produced complete, reversible, dose-dependent attenuation of hind paw mechanical allodynia for up to 1h after administration, with no noticeable behavioral or motor side effects. Intracerebral (ic) microinjections of improgan (5-30 μg) into the rostral ventromedial medulla (RVM) also reversed the allodynia, showing this brain area to be an important site for improgan's action. The recently-demonstrated suppression of RVM ON-cell activity by improgan may account for the presently-observed anti-allodynic activity. The present findings suggest that brain-penetrating, improgan-like drugs developed for human use could be effective medications for the treatment of neuropathic pain.

Non-opioid antinociception produced by brain stem injections of improgan: significance of local, but not cross-regional, cannabinoid mechanisms

Improgan, a cimetidine derivative which lacks activity at known histamine, opioid or cannabinoid receptors, acts by an unknown mechanism in the periaqueductal gray (PAG) and raphe magnus (RM) to stimulate descending, analgesic circuits. These circuits may utilize cannabinoid mechanisms. To characterize further the nature of these circuits, the effects of intracerebral (i.c.) microinjections of rimonabant (a CB(1) receptor inverse agonist) were studied on antinociceptive responses following i.c. microinjections of improgan and the cannabinoid agonist WIN 55,212 (WIN) in rats. Separate intra-RM injections of improgan (30 microg) and WIN (8 microg) produced near-maximal antinociception on both the hot plate (HP) and tail flick (TF) nociceptive tests. Pretreatment with intra-RM rimonabant (20 microg) antagonized the antinociception produced by both intra-RM improgan and intra-RM WIN, but had no effects when given alone. Similar studies with improgan demonstrated rimonabant-sensitive sites within the dorsal and ventrolateral PAG. However, intra-RM pretreatment with rimonabant had no effect on antinociceptive responses following intra-PAG improgan. These studies show that improgan activates pain-relieving mechanisms in the PAG and the RM, both of which may utilize local cannabinoid mechanisms.

Possible involvement of supraspinal opioid and GABA receptors in CDP-choline-induced antinociception in acute pain models in rats

Cytidine-5'-diphosphate choline (CDP-choline; citicoline) is an essential endogenous compound normally produced by the organism and is a source of cytidine and choline. Our recent studies on acute pain models demonstrate that intracerebroventricularly administered CDP-choline produces antinociception via supraspinal alpha-7 nicotinic acetylcholine receptors-mediated mechanism in rats. However, it remains to be elucidated which other supraspinal mechanisms are involved in the antinociceptive effect of CDP-choline. In this study, we investigated the role of the supraspinal opioidergic, GABAergic, alpha-adrenergic and serotonergic receptors in CDP-choline-induced antinociception. The antinociceptive effect of CDP-choline was evoked by the intracerebroventricular (i.c.v.) administration. Two different pain models were utilized: thermal paw withdrawal test and mechanical paw pressure test. The i.c.v. administration of CDP-choline (0.5, 1.0 and 2.0 micromol) produced dose-dependent antinociception. Non-specific opioid receptor antagonist naloxone (10 microg; i.c.v.) and GABA(B) receptor antagonist CGP-35348 (20 microg; i.c.v.) pretreatments inhibited the antinociceptive effects of CDP-choline (1.0 micromol; i.c.v.). In contrast, the alpha-1 adrenergic receptor antagonist prazosin (20 microg; i.c.v.), alpha-2 adrenergic receptor antagonist yohimbine (30 microg; i.c.v.) and non-specific serotonin receptor antagonist methysergide (20 microg; i.c.v.) pretreatments had no effect on CDP-choline-induced antinociception in the thermal paw withdrawal test and in the mechanical paw pressure test. Therefore, it can be postulated that i.c.v. administered CDP-choline exerts antinociceptive effect mediated by supraspinal opioid and GABA(B) receptors in acute pain models. Furthermore, supraspinal alpha-adrenergic and serotonergic receptors do not appear to be involved in the antinociceptive effect of CDP-choline.

CC12, a high-affinity ligand for [3H]cimetidine binding, is an improgan antagonist

Improgan, a chemical congener of cimetidine, is a highly effective non-opioid analgesic when injected into the CNS. Despite extensive characterization, neither the improgan receptor, nor a pharmacological antagonist of improgan has been previously described. Presently, the specific binding of [(3)H]cimetidine (3HCIM) in brain fractions was used to discover 4(5)-((4-iodobenzyl)thiomethyl)-1H-imidazole, which behaved in vivo as the first improgan antagonist. The synthesis and pharmacological properties of this drug (named CC12) are described herein. In rats, CC12 (50-500nmol, i.c.v.) produced dose-dependent inhibition of improgan (200-400nmol) antinociception on the tail flick and hot plate tests. When given alone to rats, CC12 had no effects on nociceptive latencies, or on other observable behavioral or motor functions. Maximal inhibitory effects of CC12 (500nmol) were fully surmounted with a large i.c.v. dose of improgan (800nmol), demonstrating competitive antagonism. In mice, CC12 (200-400nmol, i.c.v.) behaved as a partial agonist, producing incomplete improgan antagonism, but also limited antinociception when given alone. Radioligand binding, receptor autoradiography, and electrophysiology experiments showed that CC12's antagonist properties are not explained by activity at 25 sites relevant to analgesia, including known receptors for cannabinoids, opioids or histamine. The use of CC12 as an improgan antagonist will facilitate the characterization of improgan analgesia. Furthermore, because CC12 was also found presently to inhibit opioid and cannabinoid antinociception, it is suggested that this drug modifies a biochemical mechanism shared by several classes of analgesics. Elucidation of this mechanism will enhance understanding of the biochemistry of pain relief.

Cannabinoid-improgan cross-tolerance: Improgan is a cannabinomimetic analgesic lacking affinity at the cannabinoid CB1 receptor

Improgan is a non-opioid analgesic which does not act at known histamine or cannabinoid receptors. Because improgan antinociception is blocked by low doses of a cannabinoid CB1 antagonist, the present experiments determined if development of cannabinoid tolerance in mice would alter improgan antinociception. Twice-daily injections of Delta9-tetrahydrocannabinol (THC, 10 mg/kg, s.c.) for 3.5 days induced 47-54% and 42-56% reductions in cannabinoid (WIN 55,212-2, 20 microg, i.c.v.) and improgan (30 microg, i.c.v.) antinociception, respectively, as compared with responses from vehicle-treated groups. Because improgan lacks cannabinoid-like side effects in rats, and does not act directly on cannabinoid CB1 receptors, the finding that development of cannabinoid tolerance reduces improgan antinociception suggests that this drug may release endocannabinoids, or activate novel cannabinoid sites. Either possibility offers the potential for developing new types of analgesics.

Antinociceptive activity of chemical congeners of improgan: optimization of side chain length leads to the discovery of a new, potent, non-opioid analgesic

Improgan is a chemical congener of the H2 antagonist cimetidine which shows the profile of a highly effective analgesic when administered directly into the CNS. Although the improgan receptor is unknown, improgan activates analgesic pathways which are independent of opioids, but may utilize cannabinoid mechanisms. To discover selective, potent, improgan-like drugs, seven compounds chemically related to improgan were synthesized and tested for antinociceptive activity in rats after intracerebroventricular (icv) administration. Among a series of improgan congeners in which the alkyl chain length of improgan ((-CH2)3-) was varied, five compounds showed full agonist antinociceptive activity with potencies greater than that of improgan. VUF5420 (containing (-CH2)4-, EC50 = 86.1 nmol) produced maximal antinociceptive activity after doses which showed no motor impairment or other obvious toxicity, and was 2.3-fold more potent than improgan (EC50 = 199.5 nmol). As found previously with improgan, VUF5420-induced antinociception was unaffected by administration of the opioid antagonist naltrexone, but was inhibited by the CB1 antagonist SR141716A, suggesting a non-opioid, cannabinoid-related analgesic action. However, VUF5420 showed very low affinity (Kd approximately 10 microM) on CB1-receptor activation of 35S-GTPgammaS binding, indicating that this drug does not directly interact with the CB1 receptor in vivo. The present results show that VUF5420 is a high potency, improgan-like, non-opioid analgesic which may indirectly activate cannabinoid pain-relieving mechanisms.

The effects of alpha2-adrenoceptor agents on anti-hyperalgesic effects of carbamazepine and oxcarbazepine in a rat model of inflammatory pain

In this study, the effects of yohimbine (alpha2-adrenoceptor antagonist) and clonidine (alpha2-adrenoceptor agonist) on anti-hyperalgesia induced by carbamazepine and oxcarbazepine in a rat model of inflammatory pain were investigated. Carbamazepine (10-40 mg/kg; i.p.) and oxcarbazepine (40-160 mg/kg; i.p.) caused a significant dose-dependent reduction of the paw inflammatory hyperalgesia induced by concanavalin A (Con A, intraplantarly) in a paw pressure test in rats. Yohimbine (1-3 mg/kg; i.p.) significantly depressed the anti-hyperalgesic effects of carbamazepine and oxcarbazepine, in a dose- and time-dependent manner. Both drug mixtures (carbamazepine-clonidine and oxcarbazepine-clonidine) administered in fixed-dose fractions of the ED50 (1/2, 1/4 and 1/8) caused significant and dose-dependent reduction of the hyperalgesia induced by Con A. Isobolographic analysis revealed a significant synergistic (supra-additive) anti-hyperalgesic effect of both combinations tested. These results indicate that anti-hyperalgesic effects of carbamazepine and oxcarbazepine are, at least partially, mediated by activation of adrenergic alpha2-receptors. In addition, synergistic interaction for anti-hyperalgesia between carbamazepine and clonidine, as well as oxcarbazepine and clonidine in a model of inflammatory hyperalgesia, was demonstrated.

Antinociceptive, brain-penetrating derivatives related to improgan, a non-opioid analgesic

The antinociceptive profile of selected histamine H(2) and histamine H(3) receptor antagonists led to the discovery of improgan, a non-brain-penetrating analgesic agent which does not act on known histamine receptors. Because no chemical congener of improgan has yet been discovered which has both antinociceptive and brain-penetrating properties, the present study investigated the antinociceptive effects of a series of chemical compounds related to zolantidine, a brain-penetrating histamine H(2) receptor antagonist. The drugs studied presently contain the piperidinomethylphenoxy (PMPO) moiety, hypothesized to introduce brain-penetrating characteristics. Following intracerebroventricular (i.c.v.) dosing in rats, six of eight drugs produced dose- and time-related antinociception on both the tail flick and hot plate tests over a nearly eight-fold range of potencies. Ataxia and other motor side effects were observed after high doses of these drugs, but two of the compounds (SKF94674 and loxtidine) produced maximal antinociception at doses which were completely devoid of these motor effects. Consistent with the hypothesis that PMPO-containing drugs are brain-penetrating analgesics, SKF94674 and another derivative (JB-9322) showed dose-dependent antinociceptive activity 15 to 30 min after systemic dosing in mice, but these effects were accompanied by seizures and death beginning 45 min after dosing. Other drugs showed a similar pattern of antinociceptive and toxic effects. In addition, loxtidine produced seizures without antinociception, whereas zolantidine produced neither effect after systemic dosing in mice. Although several of the drugs tested have histamine H(2) receptor antagonist activity, neither the antinociception nor the toxicity was correlated with histamine H(2) receptor activity. The present results are the first to demonstrate the existence of brain-penetrating antinociceptive agents chemically related to zolantidine and improgan, but further studies are needed to understand the mechanisms of both the pain relief and toxicity produced by these agents.

Absence of 5-HT3 and cholinergic mechanisms in improgan antinociception

Improgan, an analgesic derived from histamine antagonists, acts in the brain stem to activate descending non-opioid, pain-relieving circuits, but the mechanism of action of this drug remains elusive. Because improgan has a moderate affinity for 5-HT(3) receptors, and, since cholinergic and serotonergic drugs can modulate descending analgesic circuits, roles for 5-HT(3), nicotinic and muscarinic receptors in improgan antinociception were presently investigated in rats. Improgan (80 microg, icv) induced nearly maximal inhibition of hot plate and tail flick nociceptive responses, and these actions we unaffected by antagonists of muscarinic (atropine, 5.9 mg/kg, i.p.) and nicotinic (mecamylamine, 2 mg/kg, i.p.) receptors. Control experiments verified that these antagonist treatments were maximally effective against muscarinic and nicotinic antinociception in both tests. In addition, improgan antinociception was unaffected by icv pretreatment with a 5-HT(3) antagonist (ondansetron, 20 microg). When given alone, icv treatment with neither this antagonist nor a 5-HT(3) agonist (m-chlorophenylbiguanide, 1000 nmol, icv) modified thermal nociceptive latencies. These results show no role for supraspinal cholinergic and 5-HT(3) receptors in improgan antinociception. The findings help to narrow the search for the relevant mediators of the action of this novel analgesic agent.

Activation of brain stem nuclei by improgan, a non-opioid analgesic

Improgan is a compound developed from histamine antagonists which shows the pre-clinical profile of a highly effective, non-opioid analgesic when administered into the rodent CNS. Pharmacological studies suggest that improgan activates descending pain-relieving circuits, but the brain and spinal sites of action of this drug have not been previously studied. Presently, the effects of intracerebral and intrathecal microinjections of improgan were evaluated on thermal nociceptive responses in rats. Improgan produced large, dose- and time-related reductions in nociceptive responses following administration into the ventrolateral periaqueductal gray (PAG), the dorsal PAG, and the rostral ventromedial medulla (RVM). The drug had no measurable effects after injections into the caudate nucleus, basolateral amygdala, hippocampus, ventromedial hypothalamus, superior colliculi, ventrolateral medulla, or the spinal subarachnoid space. Inactivation of the RVM by muscimol microinjections completely attenuated antincociceptive responses produced by intraventricular improgan. These findings, taken with earlier results, show that, like opioids and cannabinoids, improgan acts in the PAG and RVM to activate descending analgesic systems. Unlike these other analgesics, improgan does not act in the spinal cord or in CNS areas outside of the brain stem.

Improgan antinociception does not require neuronal histamine or histamine receptors

Improgan, a chemical congener of the H(2) antagonist cimetidine, induces antinociception following intracerebroventricular (i.c.v.) administration in rodents, but the mechanism of action of this compound remains unknown. Because the chemical structure of improgan closely resembles those of histamine and certain histamine blockers, and because neuronal histamine is known to participate in pain-relieving responses, the antinociceptive actions of improgan were evaluated in mice containing null mutations in the genes for three histamine receptors (H(1), H(2), and H(3)) and also in the gene for histidine decarboxylase (the histamine biosynthetic enzyme). Similar to earlier findings in Swiss-Webster mice, improgan induced maximal, reversible, dose-related reductions in thermal nociceptive responses in ICR mice, but neither pre-improgan (baseline) nor post-improgan nociceptive latencies were changed in any of the mutant mice as compared with wild-type controls. Improgan also had weak inhibitory activity in vitro (pK(i)=4.7-4.9) on specific binding to three recently-discovered, recombinant isoforms of the rat H(3) receptor (H(3A), H(3B), and H(3C)). The present findings strongly support the hypothesis that neuronal histamine and its receptors fail to play a role in improgan-induced antinociception.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).