Discriminative stimulus produced by the imidazoline I2 site ligand, 2 -BFI

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I1, I2, and I3) have been proposed and the understanding of each has seen differing progress over the decades. I1 receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I1/α 2-adrenoceptor selectivity. Newer I1 receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α 2-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I2 receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I2 receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I2 receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I2 receptor-based first-in-class nonopioid analgesic. The understanding of I3 receptors is relatively limited. Existing data suggest that I3 receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I1 and I2) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

The Discriminative Stimulus Properties of Drugs Used to Treat Depression and Anxiety

Drug discrimination is a powerful tool for evaluating the stimulus effects of psychoactive drugs and for linking these effects to pharmacological mechanisms. This chapter reviews the primary findings from drug discrimination studies of antidepressant and anxiolytic drugs, including novel pharmacological mechanisms. The stimulus properties revealed from these animal studies largely correspond to the receptor affinities of antidepressant and anxiolytic drugs, indicating that subjective effects may correspond to either therapeutic or side effects of these medications. We discuss drug discrimination findings concerning adjunctive medications and novel pharmacologic strategies in antidepressant and anxiolytic research. Future directions for drug discrimination work include an urgent need to explore the subjective effects of medications in animal models, to better understand shifts in stimulus sensitivity during prolonged treatments, and to further characterize stimulus effects in female subjects. We conclude that drug discrimination is an informative preclinical procedure that reveals the interoceptive effects of pharmacological mechanisms as they relate to behaviors that are not captured in other preclinical models.

Mechanisms of imidazoline I2 receptor agonist-induced antinociception in rats: involvement of monoaminergic neurotransmission

BACKGROUND AND PURPOSE

Although the antinociceptive efficacies of imidazoline I₂ receptor agonists have been established, the exact post-receptor mechanisms remain unknown. This study tested the hypothesis that monoaminergic transmission is critical for I₂ receptor agonist-induced antinociception.

EXPERIMENTAL APPROACH

von Frey filaments were used to assess antinociceptive effects of two I₂ receptor agonists, 2-BFI and CR4056 on chronic constriction injury (CCI)-induced neuropathic pain or complete Freund's adjuvant (CFA)-induced inflammatory pain in rats. Rectal temperature was measured to assess hypothermic effects of 2-BFI. A two-lever drug discrimination paradigm in which rats were trained to discriminate 5.6 mg·kg^-1 2-BFI (i.p.) from its vehicle was used to examine the discriminative stimulus effects of 2-BFI. In each experiment, pharmacological mechanisms were investigated by combining 2-BFI or CR4056 with various pharmacological manipulations of the monoaminergic system including selective reuptake inhibition, monoamine depletion and monoamine receptor antagonism.

KEY RESULTS

In the CCI model, selective reuptake inhibitors of 5-HT (fluoxetine) or noradrenaline (desipramine), but not dopamine (GBR12909), enhanced 2-BFI-induced antinociception. Selective depletion of 5-HT or noradrenaline almost abolished 2-BFI-induced antinociception. 5-HT_1A , 5-HT_2A and α₁ -adrenoceptor antagonists, but not other monoaminergic antagonists, attenuated 2-BFI and CR4056-induced antinociception in CCI and/or CFA models. However, none of these monoamine receptor antagonists significantly altered 2-BFI-induced hypothermia or discriminative stimulus effects.

CONCLUSIONS AND IMPLICATIONS

Antinociception induced by I₂ receptor agonists was mediated by serotonergic and noradrenergic mechanisms with 5-HT_1A , 5-HT_2A and α₁ -adrenoceptor being particularly important. In contrast, the hypothermic and discriminative stimulus effects of I₂ receptor agonists were mediated by distinct, independent mechanisms.

Role of intracellular Ca2+ signaling in the antinociceptive and discriminative stimulus effects of the imidazoline I2 receptor agonist 2-BFI in rats

RATIONALE

Recent research has established the imidazoline I₂ receptor as a promising target for the development of novel analgesics. However, despite an increasing understanding of imidazoline I₂ receptor-mediated behavioral effects, little is known about post-I₂-receptor signaling mechanisms.

OBJECTIVE

This study examined the effects of several inhibitors of Ca²⁺ signaling mechanisms on two behavioral effects of the prototypical imidazoline I₂ receptor ligand 2-(2-benzofuranyl)-2-imidazoline (2-BFI).

METHODS

The von Frey filament test was used to examine the antinociceptive effects of 2-BFI in complete Freund's adjuvant (CFA)-induced inflammatory pain in rats. A two-lever drug discrimination paradigm in which rats were trained to discriminate 5.6 mg/kg (intraperitoneally) 2-BFI from its vehicle was used to examine the discriminative stimulus effects of 2-BFI.

RESULTS

The L-type Ca²⁺ channel blockers verapamil and nimodipine, the calmodulin antagonist W-7, and the internal Ca²⁺ release inhibitor ryanodine all attenuated the antinociceptive effects of 2-BFI. Oxycodone- and acetaminophen-induced antinociception was unaffected by pretreatment with the Ca²⁺ channel blockers. Rats learned to reliably discriminate 5.6 mg/kg 2-BFI from saline. The I₂ receptor agonists BU224, RS45041, tracizoline, and CR4056 all fully substituted for 5.6 mg/kg 2-BFI while idazoxan, S22687, 2,5-dimethoxy-4-methylamphetamine (DOM), and phenyzoline produced partial or no substitution. Verapamil, nimodipine, and W-7 did not alter the discriminative stimulus effects of 2-BFI.

CONCLUSION

These results indicate that the antinociceptive effects of 2-BFI involve intracellular Ca²⁺ elevation and/or downstream Ca²⁺/calmodulin signaling, whereas the discriminative stimulus effects of 2-BFI are mediated by a distinct, independent mechanism.

Imidazoline I2 receptors: An update

Since first introduced more than two decades ago, the research in imidazoline I₂ receptors has been steadily increasing. This review provides an update on the current status of I₂ receptor pharmacology. Imidazoline I₂ receptors or I₂ binding sites refer to several (at least four) different proteins that bind to [³H]-idazoxan and [³H]-2-BFI with high affinity. The molecular identities of the proteins remain elusive. One of the proteins (45kD) seems to be consistent with the identity of brain creatine kinase. The biological functions of I₂ receptors have been primarily unveiled by the studies of selective I₂ receptor ligands. Accumulating evidence suggests that I₂ receptor ligands are effective analgesics for persistent and chronic painful conditions such as inflammatory, neuropathic and postoperative pain. One selective I₂ receptor ligand, CR4056, has been advanced to phase II clinical trial with the therapeutic indication of chronic inflammatory pain (osteoarthritis). The expansion to the treatment of other chronic pain conditions should be expected if CR4056 could eventually be approved as a new drug. I₂ receptor ligands also demonstrate robust discriminative stimulus activity and induce a characteristic discriminative cue in animals. Biochemical and preclinical in vivo investigations also suggest that I₂ receptor ligands have neuroprotective activity and modulate body temperature. The emerging discrepancies of a range of purported selective I₂ receptor ligands suggest different pharmacological effects mediated by discrete I₂ receptor components which likely attribute to the I₂ receptor-related proteins. It is proposed that the I₂ receptors represent an emerging drug target for the treatment of neurological disorders such as pain and stroke, and deserve more research attention to translate preclinical findings to pharmacotherapies.

Antinociceptive effects of imidazoline I2 receptor agonists in the formalin test in rats

The imidazoline I2 receptor is an emerging drug target for analgesics. This study extended previous studies by examining the antinociceptive effects of three I2 receptor agonists (2-BFI, BU224, and CR4056) in the formalin test. The receptor mechanisms and anatomical mediation of I2 receptor agonist-induced antinociception were also examined. Formalin-induced flinching responses (2%, 50 μl) were quantified after treatment with I2 receptor agonists alone or in combination with the I2 receptor antagonist idazoxan. Anatomical mediation was studied by locally administering 2-BFI into the plantar surface or into the right lateral ventricle through cannulae (intracerebroventricular). The locomotor activity was also examined after central (intracerebroventricular) administration of 2-BFI. 2-BFI (1-10 mg/kg, intraperitoneal) and BU224 (1-10 mg/kg, intraperitoneal) attenuated the spontaneous flinching response observed during 10 min (phase 1) and 20-60 min (phase 2) following formalin treatment, whereas CR4056 (1-32 mg/kg, intraperitoneal) decreased only phase 2 flinching response. The I2 receptor antagonist idazoxan attenuated the antinociceptive effects of 2-BFI and BU224 during phase 1, but not phase 2. Peripheral administration of 2-BFI (1-10 mg/kg, intraplantar) to the hind paw of rats had no antinociceptive effect. In contrast, centrally delivered 2-BFI (10-100 µg, intracerebroventricular) dose-dependently attenuated phase 1 and phase 2 flinching at doses that did not reduce the locomotor activity. Together, these data revealed the differential antinociceptive effects of I2 receptor agonists and the differential antagonism profiles by idazoxan, suggesting the involvement of different I2 receptor subtypes in reducing different phases of formalin-induced pain-like behaviors. In addition, the results also suggest the central mediation of I2 receptor agonist-induced antinociceptive actions.

Discriminative stimulus effects of the imidazoline I2 receptor ligands BU224 and phenyzoline in rats

Although imidazoline I2 receptor ligands have been used as discriminative stimuli, the role of efficacy of I2 receptor ligands as a critical determinant in drug discrimination has not been explored. This study characterized the discriminative stimulus effects of selective imidazoline I2 receptor ligands BU224 (a low-efficacy I2 receptor ligand) and phenyzoline (a higher efficacy I2 receptor ligand) in rats. Two groups of male Sprague-Dawley rats were trained to discriminate 5.6mg/kg BU224 or 32mg/kg phenyzoline (i.p.) from their vehicle in a two-lever food-reinforced drug discrimination procedure, respectively. All rats acquired the discriminations after an average of 18 (BU224) and 56 (phenyzoline) training sessions, respectively. BU224 and phenyzoline completely substituted for one another symmetrically. Several I2 receptor ligands (tracizoline, CR4056, RS45041, and idazoxan) all occasioned>80% drug-associated lever responding in both discriminations. The I2 receptor ligand 2-BFI and a monoamine oxidase inhibitor harmane occasioned>80% drug-associated lever responding in rats discriminating BU224. Other drugs that occasioned partial or less substitution to BU224 cue included clonidine, methamphetamine, ketamine, morphine, methadone and agmatine. Clonidine, methamphetamine and morphine also only produced partial substitution to phenyzoline cue. Naltrexone, dopamine D2 receptor antagonist haloperidol and serotonin (5-HT)2A receptor antagonist MDL100907 failed to alter the discriminative stimulus effects of BU224 or phenyzoline. Combined, these results are the first to demonstrate that BU224 and phenyzoline can serve as discriminative stimuli and that the low-efficacy I2 receptor ligand BU224 shares similar discriminative stimulus effects with higher-efficacy I2 receptor ligands such as phenyzoline and 2-BFI.

Behavioral effects of the imidazoline I(2) receptor ligand BU99006 in rats

The imidazoline I2 receptor ligand BU99006 binds to and attenuates effects mediated by I2 receptors in vitro, although its effects in vivo have not been studied previously. This study examined the effects of BU99006 in two behavioral assays in rats: hypothermia and 2-BFI discrimination. BU99006 (3.2-15 mg/kg, intraperitoneally) produced a dose-dependent hypothermic effect (rectal temperature), which was antagonized by the I2 receptor antagonist idazoxan. BU99006 (3.2 or 10 mg/kg administered 10 min or 2 h before the session, respectively) did not significantly alter hypothermia produced by the I2 receptor agonist 2-BFI (10 mg/kg). In rats discriminating 5.6 mg/kg 2-BFI, BU99006 (1.78-17.8 mg/kg, intraperitoneally) produced 40 and 82% responding on the 2-BFI-associated lever when it was administered immediately or 2 h before the test sessions, respectively. BU99006 enhanced the discriminative stimulus and rate-suppressing effects of 2-BFI. Collectively, these data suggest that BU99006 is an imidazoline I2 receptor agonist with no evidence of I2 receptor antagonism in rats.

Discriminative stimulus effects of the novel imidazoline I₂ receptor ligand CR4056 in rats

This study examined whether a novel imidazoline I₂ receptor ligand CR4056 could serve as a discriminative stimulus and whether it shares similar discriminative stimulus effects with other reported I₂ receptor ligands. Eight male Sprague-Dawley rats were trained to discriminate 10.0 mg/kg CR4056 (i.p.) from vehicle in a two-lever food-reinforced drug discrimination procedure. Once rats acquired the discrimination, substitution and combination studies were conducted to elucidate the underlying receptor mechanisms. All rats acquired CR4056 discrimination after an average of 26 training sessions. Several I₂ receptor ligands (phenyzoline, tracizoline, RS45041, and idazoxan, 3.2–75 mg/kg, i.p.) all occasioned > 80% CR4056-associated lever responding. Other drugs that occasioned partial or no CR4056-associated lever responding included methamphetamine, ketamine, the endogenous imidazoline ligand agmatine, the monoamine oxidase (MAO) inhibitor harmane, the α₂-adrenoceptor agonist clonidine, the μ-opioid receptor agonists morphine and methadone, and the selective I₂ receptor ligands BU224 and 2-BFI. The α₁ adrenoceptor antagonist WB4101, α₂ adrenoceptor antagonist yohimbine and μ-opioid receptor antagonist naltrexone failed to alter the stimulus effects of CR4056. Together, these results show that CR4056 can serve as a discriminative stimulus in rats, which demonstrates high pharmacological specificity and appears to be mediated by imidazoline I₂ receptors.

Locomotor effects of imidazoline I2-site-specific ligands and monoamine oxidase inhibitors in rats with a unilateral 6-hydroxydopamine lesion of the nigrostriatal pathway

The present study examined the ability of the selective imidazoline I(2)-site ligands 2-(-2-benzofuranyl)-2-imidazoline (2-BFI) and 2-[4,5-dihydroimidaz-2-yl]-quinoline (BU224) and selected monoamine oxidase (MAO) inhibitors to evoke locomotor activity in rats bearing a lesion of the nigrostriatal pathway. Male Sprague-Dawley rats were injected with 12.5 microg 6-hydroxydopamine (6-OHDA) into the right median forebrain bundle to induce a unilateral lesion of the nigrostriatal tract. After 6 weeks, test drugs were administered either alone or in combination with L-DOPA (l-3,4-dihydroxyphenylamine) and the circling behaviour of animals was monitored as an index of anti-Parkinsonian activity. Intraperitoneal (i.p.) administration of the irreversible MAO-B inhibitor deprenyl (20 mg kg(-1)) or the imidazoline I(2)-site ligands BU224 (14 mg kg(-1)) and 2-BFI (7 and 14 mg kg(-1)) produced significant increases in ipsiversive rotations compared to vehicle controls totaling, at the highest respective doses tested, 521 +/-120, 131 +/- 37 and 92.5 +/- 16.3 net contraversive rotations in 30 (deprenyl) or 60 (BU224 and 2-BFI) min. In contrast, the reversible MAO-A inhibitor moclobemide (2.5-10 mg kg(-1)) and the reversible MAO-B inhibitor lazabemide (2.5-10 mg kg(-1)) failed to instigate significant rotational behaviour compared to vehicle. Coadministration of lazabemide (10 mg kg(-1)), moclobemide (10 mg kg(-1)) or 2-BFI (14 mg kg(-1)) with L-DOPA (20 mg kg(-1)) significantly increased either the duration or total number of contraversive rotations emitted over the testing period in comparison to L-DOPA alone. These data suggest that I(2)-specific ligands have dual effects in the 6-OHDA-lesioned rat model of Parkinson's disease; a first effect associated with an increase in activity in the intact hemisphere, probably via an increase in striatal dopamine content, and a secondary action which, through the previously documented inhibition of MAO-A and/or MAO-B, increases the availability of dopamine produced by L-DOPA.

Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats

RATIONALE

Because of their potential therapeutic effects, N-methyl-D-aspartate (NMDA) receptor antagonists have been investigated for clinical use. Unfortunately, many channel-blocking antagonists have been associated with the production of side effects, including motor impairment and phencyclidine (PCP)-like subjective effects.

OBJECTIVE

This study investigated the relationship between NMDA receptor channel blockade and production of PCP-like side effects by evaluating a variety of NMDA channel blockers with different binding characteristics for the production of PCP-like discriminative stimulus effects.

METHODS

The NMDA channel blockers were tested in rats trained to discriminate 2 mg/kg PCP, i.p., from saline using a standard two-lever drug discrimination procedure with responding under a fixed ratio (FR) 32 schedule of food reinforcement.

RESULTS

The high-affinity channel blockers PD 138289, PD 137889 and FR 115427, produced full, dose-dependent substitution for PCP. Of the moderate-affinity channel blockers, MRZ 2/579 fully substituted for PCP while 1-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline, 8-(2-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline and alaproclate produced partial substitution. Drugs with the lowest affinity for the channel site and/or higher affinity for non-NMDA CNS sites, antazoline, idazoxan, 1-phenyl-1,2,3,4-tetrahydroisoquinoline, alpha-benzyl- N-methylphenethylamine and orphenadrine, failed to substitute for PCP.

CONCLUSIONS

The results demonstrate that the cellular actions of the individual channel-blocking NMDA antagonists, in particular affinity for the channel site and NMDA receptor specificity, are important determinants of their discriminative stimulus effects. While higher affinity channel blockers show a correlation between affinity and PCP-like discriminative stimulus effects, behavioral disruption through action at non-NMDA receptors probably prevents achieving sufficient concentrations of the lower affinity compounds at NMDA receptors to produce PCP-like discriminative stimulus effects.

Potential serotonergic and noradrenergic involvement in the discriminative stimulus effects of the selective imidazoline I2-site ligand 2-BFI

The functional significance of imidazoline I2 binding sites is unknown but microdialysis studies have indicated that the administration of I2-site ligands leads to an increase in extracellular levels of monoamines. The specific I2-site ligand 2-(-2-benzofuranyl)-2-imidazoline (2-BFI) generates a cue in drug discrimination, thereby indicating functional consequences of I2-site ligand binding. In the present work, we explored the ability of selective noradrenergic and serotonergic ligands to substitute for 2-BFI. Hooded Lister rats were trained in two-lever operant chambers with condensed milk reward to distinguish 2-BFI (7 mg/kg) from saline vehicle, by pressing the correct lever to a predetermined success criterion. Training sessions were then interspersed with sessions in which animals were administered test substances and the proportion of lever presses on the 2-BFI-associated lever (substitution) recorded. Several agents exhibited significant partial substitution for 2-BFI: The monoamine-releasing agents D-amphetamine and fenfluramine dose-dependently substituted for 2-BFI, while norepinephrine (desipramine, reboxetine) and serotonin (clomipramine, citalopram) reuptake inhibitors substituted at one or more doses. Further investigation using specific receptor agonists and antagonists indicated a possible role for activation of alpha1-adrenoceptors but failed to support involvement of alpha2-adenoceptor, beta-adrenoceptor or 5-HT1A receptor activation. These results support the concept that the 2-BFI cue may contain both noradrenergic and serotonergic components.

Characterization of the discriminable stimulus produced by 2-BFI: effects of imidazoline I(2)-site ligands, MAOIs, beta-carbolines, agmatine and ibogaine

1. The molecular nature and functions of the I(2) subtype of imidazoline binding sites are unknown but evidence suggests an association with monoamine oxidase (MAO). Rats can distinguish the selective imidazoline I(2)-site ligand 2-BFI from vehicle in drug discrimination, indicating functional consequences of occupation of these sites. We have used drug discrimination to investigate the nature of the discriminable stimulus, especially in relation to MAO inhibition. 2. Following training to distinguish 2-BFI 7 mg kg(-1) i.p. from saline vehicle in two-lever operant-chambers, male Hooded Lister rats underwent sessions where test substances were given instead and the proportion of lever presses on the 2-BFI-associated lever (substitution) recorded. 3. 2-BFI; its cogeners BU216, BU224, BU226 and LSL60101; the reversible MAO-A inhibitors moclobemide and RO41-1049; the beta-carbolines harmane, norharmane and harmaline which also reversibly inhibit MAO-A, and the anti-addictive substance ibogaine exhibited potent, dose-dependent substitution for 2-BFI. 4. Agmatine, and LSL60125 substituted at one dose only. The reversible MAO-B inhibitors lazabemide and RO16-1649; the sigma(2)-site ligand SKF10,047 and the I(2A)-site ligand, amiloride, failed to substitute. The irreversible inhibitor of MAO, deprenyl, substituted for 2-BFI while clorgyline did not. 5. These results suggest imidazoline I(2) site ligands produce a common discriminable stimulus that appears associated with reversible inhibition of MAO-A rather than MAO-B, possibly through increases in extracellular concentration of one or more monoamines. Ibogaine exhibits a commonality in its subjective effects with those of I(2)-site ligands.