Pharmacologic characterization of imidazoline receptor proteins identified by immunologic techniques and other methods

Tolerance and cross-tolerance to the antinociceptive effects of oxycodone and the imidazoline I2 receptor agonist phenyzoline in adult male rats

RATIONALE

Emerging evidence suggests the potential utility of combining opioids with imidazoline I₂ receptor agonists for chronic pain. However, chronic pain management requires prolonged pharmacotherapy, and the consequence of such combination therapy remains unclear.

OBJECTIVE

This study examined the anti-hyperalgesic effect of the opioid oxycodone, the selective I₂ receptor agonist phenyzoline, alone and in combination, during prolonged treatment.

METHODS

Von Frey filament test was used to examine the anti-hyperalgesic effect of drugs in complete Freund's adjuvant (CFA)-induced inflammatory pain or chronic constriction injury (CCI)-induced neuropathic pain in rats. Twice-daily treatment with oxycodone and phenyzoline, alone or in combination, was continued until the development of significant tolerance (oxycodone) or as long as 19 days passed (phenyzoline).

RESULTS

In rats receiving CFA or CCI manipulation, mechanical hyperalgesia was dose-dependently reversed by oxycodone and phenyzoline. Twice-daily treatment with 2 × ED₅₀ dose of oxycodone for 7 days led to significant antinociceptive tolerance to oxycodone but not cross-tolerance to phenyzoline. Similarly, twice-daily treatment with 2 × ED₅₀ dose of phenyzoline for 19 days led to significant antinociceptive tolerance to phenyzoline but not cross-tolerance to oxycodone. Twice-daily treatment with the combined oxycodone and phenyzoline using different ratios (1:3, 1:1 and 3: 1) at the doses that were functionally equivalent to the treatment doses of oxycodone and phenyzoline for 13-19 days generally led to delayed antinociceptive tolerance.

CONCLUSIONS

Combination therapy with oxycodone and I₂ receptor agonists maintains prolonged antinociceptive effectiveness with reduced propensity to develop tolerance.

Phospholipase C-dependent hydrolysis of phosphatidylinositol 4,5-bisphosphate underlies agmatine-induced suppression of N-type Ca2+ channel in rat celiac ganglion neurons

Agmatine suppresses peripheral sympathetic tone by modulating Cav2.2 channels in peripheral sympathetic neurons. However, the detailed cellular signaling mechanism underlying the agmatine-induced Cav2.2 inhibition remains unclear. Therefore, in the present study, we investigated the electrophysiological mechanism for the agmatine-induced inhibition of Cav2.2 current (I_Cav2.2) in rat celiac ganglion (CG) neurons. Consistent with previous reports, agmatine inhibited I_Cav2.2 in a VI manner. The agmatine-induced inhibition of the I_Cav2.2 current was also almost completely hindered by the blockade of the imidazoline I₂ receptor (IR₂), and an IR₂ agonist mimicked the inhibitory effect of agmatine on I_Cav2.2, implying involvement of IR₂. The agmatine-induced I_Cav2.2 inhibition was significantly hampered by the blockade of G protein or phospholipase C (PLC), but not by the pretreatment with pertussis toxin. In addition, diC8-phosphatidylinositol 4,5-bisphosphate (PIP₂) dialysis nearly completely hampered agmatine-induced inhibition, which became irreversible when PIP₂ resynthesis was blocked. These results suggest that in rat peripheral sympathetic neurons, agmatine-induced IR₂ activation suppresses Cav2.2 channel voltage-independently, and that the PLC-dependent PIP₂ hydrolysis is responsible for the agmatine-induced suppression of the Cav2.2 channel.

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.

Interactions between imidazoline I2 receptor ligands and acetaminophen in adult male rats: antinociception and schedule-controlled responding

RATIONALE

Recent evidence suggests that imidazoline I2 receptor ligands are suitable for combination therapy with opioids. Quantitative analysis of I2 receptor ligands combined with non-opioid drugs is necessary for the justification of alternative pain therapies.

OBJECTIVE

This study systematically examined the antihyperalgesic and response rate-suppressing effects of selective I2 receptor ligands (2-BFI and phenyzoline) alone and in combination with acetaminophen.

METHODS

Von Frey and Hargreaves tests were used to examine the antihyperalgesic effects of drugs in complete Freund's adjuvant (CFA)-induced inflammatory pain in rats. Food-reinforced schedule-controlled responding was used to assess the rate-suppressing effects of study drugs. Dose-addition and isobolographic analyses were used to assess drug-drug interactions for all assays.

RESULTS

2-BFI (3.2-17.8 mg/kg, i.p.), phenyzoline (17.8-100 mg/kg, i.p.), and acetaminophen (56-178 mg/kg, i.p.) all dose-dependently produced significant antinociceptive effects. When studied as combinations, 2-BFI and acetaminophen produced infra-additive to additive interactions while phenyzoline and acetaminophen produced additive to supra-additive interactions. The same drug combinations suppressed response rate in a supra-additive manner.

CONCLUSIONS

Quantitative analysis of the antihyperalgesic and response rate-suppressing effects suggests that I2 receptor ligands are not well suited to combination therapy with acetaminophen.

Immunodetection and subcellular distribution of imidazoline receptor proteins with three antibodies in mouse and human brains: Effects of treatments with I1- and I2-imidazoline drugs

Various imidazoline receptor (IR) proteins have been proposed to mediate the effects of selective I1- and I2-IR drugs. However, the association of these IR-binding proteins with classic I1- and I2-radioligand binding sites remains somewhat controversial. In this study, three IR antibodies (anti-NISCH and anti-nischarin for I1-IRs; and anti-IRBP for I1/I2-IRs) were used to immunodetect, characterize and compare IR protein patterns in brain (mouse and human; total homogenate, subcellular fractionation, grey and white matter) and some cell systems (neurones, astrocytes, human platelets). Various immunoreactive IRs (specific molecular weight bands coincidently detected with the different antibodies) were related to I1-IR (167 kDa, 105/115 kDa and 85 kDa proteins) or I2-IR (66 kDa, 45 kDa and 30 kDa proteins) types. The biochemical characterization of cortical 167 kDa protein, localized in the membrane/cytosol but not in the nucleus, indicated that this I1-IR also forms part of higher order nischarin-related complexes. The contents of I1-IR (167 kDa, 105/115 kDa, and 85 kDa) proteins in mouse brain cortex were upregulated by treatment with I1-drugs (moxonidine, efaroxan) but not with I2-drugs (BU-224, LSL 61122). Conversely, the contents of I2-IR (66 kDa, 45 kDa and 30 kDa) proteins in mouse brain cortex were modulated by treatment with I2-drugs (decreases after BU-224 and LSL 61122, and increases after idazoxan) but not with I1-drugs (with the exception of moxonidine). These findings further indicate that brain immunoreactive IR proteins exist in multiple forms that can be grouped in the already known I1- and I2-IR types, which are expressed both in neurones and astrocytes.

Anti-hyperalgesic effects of imidazoline I2 receptor ligands in a rat model of inflammatory pain: interactions with oxycodone

RATIONALE

Emerging preclinical evidence suggests that imidazoline I2 receptor ligands may be effective analgesics. Quantitative analysis of the combined I2 receptor ligands and opioids is needed for the justification of combination therapy.

OBJECTIVE

This study systematically examined the anti-hyperalgesic and response rate-suppressing effects of selective I2 receptor ligands (2-BFI and phenyzoline) alone and in combination with oxycodone in rats.

METHODS

Von Frey filament test was used to examine the anti-hyperalgesic effects of drugs in a rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. Schedule-controlled responding was used to assess the rate-altering effects of study drugs. Duration of actions of individual drugs (2-BFI, phenyzoline, and oxycodone) alone or in combination was studied. Dose-addition analysis was employed to assess the anti-hyperalgesic interactions between drugs.

RESULTS

Oxycodone (0.1-3.2 mg/kg, i.p.), 2-BFI (1-17.8 mg/kg, i.p.), and phenyzoline (17.8-56 mg/kg, i.p.) all dose-dependently produced significant antinociceptive effects. When studied as combinations, 2-BFI and oxycodone produced additive interactions while phenyzoline and oxycodone produced supra-additive interactions under all fixed ratios. The same drug combinations did not alter or significantly reduced the operant responding depending on the ratios of the drug combinations.

CONCLUSIONS

Quantitative analysis of the anti-hyperalgesic effects of I2 receptor ligands strongly supports the therapeutic potential of I2 receptor ligands against inflammatory pain. In addition, the data reveal that phenyzoline is superior to the prototypic I2 receptor ligand 2-BFI for the management of pain and warrants further consideration as a novel analgesic.

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.

Chronic treatment with selective I2-imidazoline receptor ligands decreases the content of pro-apoptotic markers in rat brain

Selective I(2)-imidazoline receptor ligands induce neuroprotection through various molecular mechanisms including blockade of N-methyl-D-aspartate (NMDA) receptors. To investigate new neuroprotective mechanisms associated with I(2)-imidazoline receptors, the effects of selective (2-styryl-2-imidazoline (LSL 61122), 2-(2-benzofuranyl)-2-imidazoline (2-BFI), 2-(4,5-dihydroimidazol-2-yl) quinoline hydrochloride (BU-224)) and non-selective (idazoxan) I(2)-drugs on canonical apoptotic pathways were assessed in rat brain cortex. The acute treatment with LSL 61122 (10 mg/kg) reduced the content of mitochondrial (pro-apoptotic) Bax (-33%) and cytochrome c (-31%), which was prevented by idazoxan, an I(2)-receptor antagonist. The sustained stimulation of I(2)-imidazoline receptors with selective drugs (10 mg/kg, every 12 h for seven days) was associated with down-regulation of key components of the extrinsic (Fas receptor: -20%; Fas associated protein with death domain (FADD) adaptor: -47-54%) and/or intrinsic (Bax: -20-23%; cytochrome c: -22-28%) apoptotic signalling and/or up-regulation of survival anti-apoptotic factors (p-Ser194 FADD/FADD ratio: +1.6-2.5-fold; and/or Bcl-2/Bax ratio: +1.5-fold), which in the long-term could dampen cell death in the brain. Similar chronic treatments with LSL 60101 (the imidazole analogue of 2-BFI) and idazoxan (a mixed I(2)/α(2)-ligand) did not induce significant alterations of pro- or anti-apoptotic proteins. The disclosed anti-apoptotic mechanisms of selective I(2)-imidazoline drugs may work in concert with other molecular mechanisms of neuroprotection (e.g. blockade of NMDA receptors) that are engaged by I(2)-ligands.

Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons

Presynaptic imidazoline receptors (R(i-pre)) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of R(i-pre)-induced inhibition of NA release is unknown. We, therefore, investigated the effect of R(i-pre) activation on voltage-dependent Ca(2+) channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 μM), an R(i-pre) agonist as well as an α-adrenoceptor (R(α)) agonist, decreased Ca(2+) currents (I(Ca)) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 μM), which blocks the α(2)-adrenoceptor (R(α2)), cirazoline still inhibited I(Ca) by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 μM), which blocks R(i-pre) as well as R(α2). In addition, pretreatment with LY320135 (10 μM), another R(i-pre) antagonist, in combination with low-dose rauwolscine (3 μM), also blocked the R(α2)-resistant effect of cirazoline. Addition of guanosine-5-O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml(1)) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 μM) suppressed M current in SCG neurons cultured overnight. Finally, omega-conotoxin (omega-CgTx) GVIA (1 μM) obstructed cirazoline-induced current inhibition, and cirazoline (30 μM) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of omega-CgTx. Taken together, our results suggest that activation of R(i-pre) in SCG neurons reduced N-type I(Ca) in a pertussis toxin- and voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Identification of an imidazoline binding protein: creatine kinase and an imidazoline-2 binding site

Drugs that bind to imidazoline binding proteins have major physiological actions. To date, three subtypes of such proteins, I(1), I(2) and I(3), have been proposed, although characterisations of these binding proteins are lacking. I(2) binding sites are found throughout the brain, particularly dense in the arcuate nucleus of the hypothalamus. Selective I(2) ligands demonstrate antidepressant-like activity and the identity of the proteins that respond to such ligands remained unknown until now. Here we report the isolation of a approximately 45 kDa imidazoline binding protein from rabbit and rat brain using a high affinity ligand for the I(2) subtype, 2-BFI, to generate an affinity column. Following protein sequencing of the isolated approximately 45 kDa imidazoline binding protein, we identified it to be brain creatine kinase (B-CK). B-CK shows high binding capacity to selective I(2) ligands; [(3)H]-2-BFI (5 nM) specifically bound to B-CK (2330+/-815 fmol mg protein(-1)). We predicted an I(2) binding pocket near the active site of B-CK using molecular modelling. Furthermore, B-CK activity was inhibited by a selective I(2) irreversible ligand, where 20 microM BU99006 reduced the enzyme activity by 16%, confirming the interaction between B-CK and the I(2) ligand. In summary, we have identified B-CK to be the approximately 45 kDa imidazoline binding protein and we have demonstrated the existence of an I(2) binding site within this enzyme. The importance of B-CK in regulating neuronal activity and neurotransmitter release may well explain the various actions of I(2) ligands in brain and the alterations in densities of I(2) binding sites in psychiatric disorders.

Involvement of I2-imidazoline binding sites in positive and negative morphine analgesia modulatory effects

Some studies, suggesting the involvement of I(2)-imidazoline binding sites (I(2)-IBS) in morphine analgesia modulation, prompted us to examine on mice antinociceptive assays the effect produced by 1 (phenyzoline), that in view of its high I(2)-IBS affinity and high I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors might be considered the first interesting I(2)-IBS ligand. The study was also applied to its ortho phenyl derivative 2 (diphenyzoline), designed and prepared in order to produce a possible modification of the biological profile of 1. Diphenyzoline (2) retains a significant I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors. Moreover, by the functional assays 1 and 2 proved inactive at all alpha(2)-adrenoreceptors subtypes up to 10(-3) M. As expected, phenyzoline and diphenyzoline, which are structurally related, highlighted an interesting "positive" or "negative", respectively, morphine analgesia modulatory effect. In fact, 1 (s.c. 10 mg/kg) enhanced morphine analgesia (60% and 40% in mouse tail-flick and mouse hot-plate, respectively), while 2 (s.c. 10 mg/kg) decreased it (-41% and -20%, respectively). The ability to decrease morphine analgesia had never been observed before in I(2)-IBS ligands. These effects were not affected by i.p. treatment of animals with yohimbine (a selective alpha(2)-adrenoreceptor antagonist, 0.625 mg/kg) or efaroxan (an I(1)-IBS/alpha(2)-adrenoreceptor antagonist, 1.0 mg/kg). In contrast, they were completely reversed by i.p. treatment of animals with idazoxan (an I(2)-IBS/alpha(2)-adrenoreceptor antagonist, 2 mg/kg). Moreover, compound 2, in mouse tail-flick test, was able to potentiate by 23% the naloxone-induced decrease of morphine analgesia. Therefore, the results of this study indicate the crucial involvement of I(2)-IBS in the morphine analgesia modulatory effects of 1 and 2.

Autoradiographical distribution of imidazoline binding sites in monoamine oxidase A deficient mice

This study has used receptor autoradiography to characterize imidazoline binding sites (I-BS) in monoamine oxidase (MAO) A knockout and wild-type mice. A comparison between MAO-A and MAO-B, binding of the endogenous beta-carboline [(3)H]harmane, and I-BS, has been made using sections from brain and kidney. The loss of binding to MAO-A in the knockout animals was confirmed using the selective radioligand [(3)H]Ro41-1049, with labelling reduced to background levels. The binding of [(3)H]Ro19-6327 to MAO-B was unaffected, indicating no change in this isoform in response to the loss of MAO-A. A reduction in binding to the I(2)-BS, as labelled by both [(3)H]idazoxan and [(3)H]2-BFI (2-(2-benzofuranyl)-2-imidazoline), was seen in the MAO-A knockout animals in both brain and kidney sections, whereas binding to the I(1)-BS in kidney sections remained unchanged. The loss of I(2) binding was found to be regionally dependent and was positively correlated with the relative expression of MAO-A in specific regions in the wild-type animals. Using the MAO-A knockout mice it was also possible to demonstrate a non-MAO-A population of binding sites labelled by the putative I-BS endogenous ligand, harmane.

Characterisation of imidazoline I2 binding sites in pig brain

The imidazoline I2 binding sites in the central nervous system have previously been described in several different species including rat, mouse, rabbit and frog. The present study has investigated the imidazoline I2 binding site, and its relationship to the monoamine oxidase isoforms, in pig whole brain and compared the results obtained with data from other species. Results from saturation binding studies revealed that the imidazoline I2-selective ligand, [3H]2BFI (2-(2-benzofuranyl)-2-imidazoline) labelled a single saturable population of sites with a KD=6.6 nM and Bmax=771.7 fmol/mg protein. The pharmacological characterisation of the sites was similar to that previously reported with a rank order of potency for the imidazoline I2 ligands of 2BFI>BU224>Idazoxan>BU226. Displacement by the imidazoline I1 ligands was low affinity and the monoamine oxidase inhibitors displaced with micromolar affinity. The majority of compounds displaced the binding in a monophasic manner, however, displacement by the putative endogenous ligand, harmane was biphasic. The relative populations of the two monoamine oxidase isoforms revealed a 10 fold greater expression of monoamine oxidase B relative to monoamine oxidase A. These data confirm the presence of imidazoline I2 binding sites in pig brain and show that their pharmacology is characteristic of that seen in other species. The proportion of monoamine oxidase A and B expressed in the pig brain is similar to that seen in the human brain therefore, given the association between imidazoline I2 binding sites and monoamine oxidase, the pig may provide a more useful model for human imidazoline I2 binding sites than other species such as the rat.

Autoradiographic localisation of [3H]2-BFI imidazoline I2 binding sites in mouse brain

Imidazoline I2 binding sites are heterogeneous in nature and have been observed in the brain of a number of species. Development of specific imidazoline I2 radioligands, such as [3H]2-BFI and [3H]BU224, that have a high affinity for the imidazoline I2 binding site, has enabled the central distribution of these sites to be mapped. Extensive studies have been conducted on the rat brain with a number of radioligands. However, to date a comprehensive analysis of imidazoline I2 ligand binding in mouse brain has not been completed. In the present work we describe levels of [3H]2-BFI specific binding found throughout the mouse brain. [3H]2-BFI (2 nM) showed discrete regional distribution which was readily displaced by saturating concentrations of the specific imidazoline I2 ligand BU224. The highest levels of [3H]2-BFI specific binding were found in the dorsal raphe, paraventricular thalamus and nucleus accumbens. Moderate levels were found throughout the lining of the aqueduct, lateral ventricle, lateral 4th ventricle, 4th ventricle, 3rd ventricle, but not the dorsal 3rd ventricle. Based on the loss of [3H]idazoxan binding in brain homogenates from monoamine oxidase-A and B (MAO-A and MAO-B) deficient mice it has been suggested that imidazoline I2 binding sites are predominantly on MAO. Consistent with this hypothesis the regional distribution of [3H]2-BFI shows some overlap with that previously reported for MAO. However, in the rat imidazoline I2 binding sites have been shown to be heterogeneous in nature and it is likely [3H]2-BFI is binding to multiple imidazoline I2 binding sites within mouse brain.

Imidazoline binding sites and their ligands: an overview of the different chemical structures

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

Differential expression of alpha2-adrenoceptor vs. imidazoline binding sites in postmortem orbitofrontal cortex and amygdala of depressed subjects

Clonidine is a well established antihypertensive agent that is also used effectively to treat a variety of psychiatric disorders. Clonidine is a prototypic imidazoline compound that acts as an alpha(2)-adrenergic agonist but possesses nearly equivalent affinity for non-adrenergic imidazoline binding sites (I-sites). Receptor autoradiography of [(3)H]-clonidine binding presented herein compares densities of alpha(2)-adrenoceptors and I-sites (under a noradrenergic-mask) in Brodmann's area 47 of the left orbitofrontal cortex (OFC) and in six amygdaloid nuclei of subjects with major depression (n=12) vs. controls with no psychiatric history (n=11). Postmortem diagnoses were made from psychiatric interviews with next-of-kin. [(3)H]-Clonidine binding to alpha(2)-adrenoceptors in each of six OFC layers was lower, although not reaching statistical significance in any one layer by multivariate analysis, in depressives vs. control subjects. Binding to I-sites was conversely higher in depressives compared to control OFC layers, but did not reach statistical significance alone. However, the ratios of alpha(2)-adrenoceptor : I-sites in all six layers of OFC of depressed subjects were nearly half that of control subjects (P<0.008). In amygdalas from a different group of depressed patients there were no changes in alpha(2)-adrenoceptors or I-sites, or their ratios, compared with controls. The results support previous western blot data indicating a cortex-selective shift away from alpha(2)AR towards I-site preponderance in depressed patients.

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.

Imidazoline(2) (I(2)) binding site- and alpha(2)-adrenoceptor-mediated modulation of central noradrenergic and HPA axis function in control rats and chronically stressed rats with adjuvant-induced arthritis

The aim of this study was to investigate imidazoline(2) (I(2)) binding site- and alpha(2)-adrenoceptor-mediated control of central noradrenergic and HPA axis activity in control rats and chronically stressed rats with adjuvant-induced arthritis (AA). Basal levels of extracellular nonadrenaline (NA) in the region of the hypothalamic paraventricular nucleus (PVN) of AA rats were significantly greater than controls. Both the I(2) binding site selective ligand BU224 (10 mg kg(-1) i.p.) and the alpha(2)-adrenoceptor antagonist RX821002 (2.5 mg kg(-1) i.p.) significantly elevated extracellular levels of NA in the PVN region and plasma corticosterone (CORT) in a rapid and transient manner in both control and AA rats. The noradrenergic response of AA rats to BU224 was significantly enhanced compared with drug treated controls. There was a significant correlation between extracellular NA in the PVN region and plasma CORT following BU224 and RX821002. In conclusion, central noradrenergic and HPA axis activity in control and chronically stressed AA rats appear to be under the control of both I(2) binding sites and alpha(2)-adrenoceptors. Increased basal levels of extracellular NA in the PVN region of AA rats suggests increased noradrenergic activity in these animals which is modulated to a greater extent by I(2) binding sites than by alpha(2)-adrenoceptors.

Region-dependent effects of acute and chronic tranylcypromine in vivo on [3H]2-BFI binding to brain imidazoline I(2) sites

An imidazoline I(2) site has been localised to monoamine oxidase. However, in vitro studies of the effect of monoamine oxidase inhibitors on imidazoline I(2)-site radioligand binding have produced conflicting findings. Using the technique of autoradiography, we examined the effect of in vivo administration of the irreversible monoamine oxidase inhibitor tranylcypromine on binding of the imidazoline I(2) site-specific ligand [3H]2-(-2-benzofuranyl)-2-imidazoline ([3H]2-BFI) in four rat brain nuclei which are known to possess a high density of imidazoline I(2) sites, together with cerebral cortex and cerebellum which show weaker binding. A single acute pre-treatment with tranylcypromine significantly increased imidazoline I(2) site-specific binding in four regions: arcuate nucleus, interpeduncular nucleus, pineal gland and area postrema, but effects in cortical areas and cerebellum were not significant. The extent of the increase was proportional to the control binding in each region. In contrast, five daily treatments with the same dose of tranylcypromine significantly reduced [3H]2-BFI binding in these same areas. The potential role of monoamine oxidase isoforms in these changes is discussed.

Selective imidazoline I2 ligands do not show antidepressant-like activity in the forced swim test in mice

Clonidine is an adrenergic agonist with high affinity for alpha2 adrenoceptors that also has affinity for imidazoline receptors. Clonidine has previously been shown to reduce immobility in the forced swim test (FST) in mice. In the present study, this effect was blocked by idazoxan (0.06 mg/kg s.c.) and by yohimbine (1.0 mg/kg s.c.) suggesting that clonidine's effects in this test are mediated via its action at alpha2 sites. Imidazoline I2 site ligands have been shown to inhibit monoamine oxidase and thus may also have antidepressant activity. Three compounds with selective affinity for I2 receptors (BU224, BU239, BDF 8082) were also tested in the FST. These compounds showed no activity either alone or in combination with a subthreshold dose of imipramine in the FST. These results suggest that I2 receptor ligands do not show antidepressant-like activity in the FST in mice. Furthermore the activity of the mixed alpha2/I1 agonist clonidine is most likely to be due to its action at alpha2 sites.

Imidazoline receptor proteins are decreased in the hippocampus of individuals with major depression

BACKGROUND

A downregulation of I(2)-imidazoline binding sites has been reported in frontal cortices of depressed suicide victims, according to I(2)-radioligand binding and confirmed by Western blotting. We now report Western blots of imidazoline receptor proteins in hippocampi of subjects with and without depression at the time of death.

METHODS

Postmortem diagnoses were obtained from 17 cases of Axis I major depressive disorder and 17 cases without Axis I psychopathology. No psychotropic compounds were found in body fluids. Hippocampi were removed, sectioned, and assessed histologically. Throughout the analysis, each major depressive disorder sample was paired with a sample from a psychiatrically healthy subject based on equivalent life spans and postmortem delays. The antiserum was identical to that used in previous studies that reported a downregulation of cortical 29/30-kd imidazoline receptor-binding proteins in depression.

RESULTS

A triad of imidazoline receptor-binding protein bands (40-50 kd) was detected in the human hippocampus. Subjects with major depressive disorder had significantly less intensity in each imidazoline receptor-binding proteins band compared with control subjects (p =. 01 for overall bands).

CONCLUSIONS

The present results can be aligned with previous reports of downregulation of I(2)-radioligand binding sites in both cortices and platelets of depressed patients.

Imidazoline I(2)-receptors and spinal reflexes in the decerebrated rabbit

Idazoxan potentiates spinal reflexes in the decerebrated rabbit, an effect that has been attributed to antagonism of tonic noradrenergic inhibition. It is now known that this drug has a higher affinity for I(2)-imidazoline receptors than alpha(2)-receptors. The roles of I(2)-receptors in modulating transmission in spinal reflex pathways have been investigated using the selective ligands RX 821029 and RS-45041-190, and, as I(2)-receptors are closely associated with monoamine oxidase (MAO), the MAO inhibitors pargyline and clorgyline. In decerebrated rabbits with an intact spinal cord, intrathecal doses of 5-365 microg (cumulative) of the I(2)-ligands augmented, to 150-180% of pre-drug levels, the reflex responses of medial gastrocnemius motoneurones to electrical stimulation of the sural nerve, and significantly increased mean arterial blood pressure by approximately 10 mmHg over pre-drug values. Neither MAO inhibitor had significant effects on reflexes, but the highest dose of the MAO-A selective agent clorgyline (365 microg cumulative) caused a significant rise in blood pressure of 6 mmHg. Neither the I(2)-ligands nor the MAO inhibitors prevented the further enhancement of reflexes or blood pressure by subsequent administration of the selective alpha(2)-adrenoceptor antagonist RX 821002. In decerebrated, spinalized rabbits, intrathecal RS-45041-190 (60 microg, single dose) increased spinal reflex responses to 109% of pre-drug values, a significantly smaller effect than that seen in non-spinal preparations, and had no effect on blood pressure. These data show that imidazoline I(2)-receptors can influence somatic and autonomic motor outflows. These effects should be taken into account when interpreting the spinal effects of imidazoline-based adrenoceptor-active drugs.

Imidazoline receptor antisera-selected (IRAS) cDNA: cloning and characterization

The imidazoline-1 receptor (IR1) is considered a novel target for drug discovery. Toward cloning an IR1, a truncated cDNA clone was isolated from a human hippocampal lambda gt11 cDNA expression library by relying on the selectivity of two antisera directed against candidate IR proteins. Amplification reactions were performed to extend the 5' and 3' ends of this cDNA, followed by end-to-end PCR and conventional cloning. The resultant 5131-basepair molecule, designated imidazoline receptor-antisera-selected (IRAS) cDNA, was shown to encode a 1504-amino acid protein (IRAS-1). No relation exists between the amino acid sequence of IRAS-1 and proteins known to bind imidazolines (e.g., it is not an alpha2-adrenoceptor or monoamine oxidase subtype). However, certain sequences within IRAS-1 are consistent with signaling motifs found in cytokine receptors, as previously suggested for an IR1. An acidic region in IRAS-1 having an amino acid sequence nearly identical to that of ryanodine receptors led to the demonstration that ruthenium red, a dye that binds the acidic region in ryanodine receptors, also stained IRAS-1 as a 167-kD band on SDS gels and inhibited radioligand binding of native I1 sites in untransfected PC-12 cells (a source of authentic I1 binding sites). Two epitope-selective antisera were also generated against IRAS-1, and both reacted with the same 167-kD band on Western blots. In a host-cell-specific manner, transfection of IRAS cDNA into Chinese hamster ovary cells led to high-affinity I1 binding sites by criteria of nanomolar affinity for moxonidine and rilmenidine. Thus, IRAS-1 is the first protein discovered with characteristics of an IR1.