Differential effects of the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline on brain alpha 2-adrenoceptors and I2-imidazoline sites in vitro and in vivo

Correlation of rat cortical Fas-associated death domain (FADD) protein phosphorylation with the severity of spontaneous morphine abstinence syndrome: role of α(2)-adrenoceptors and extracellular signal-regulated kinases

Fas-associated death domain (FADD) phosphorylation was recently implicated in opiate-induced neuroplasticity. To further explore the role of FADD in the mechanisms of morphine-induced physical dependence, the regulation of cortical p-FADD (and their interactions with α(2)-adrenoceptors and other signalling pathways) was assessed during spontaneous opiate withdrawal (SW) in morphine-dependent rats (10-100 mg/kg for 6 days). The main results indicated that oligomeric p-FADD in the cerebral cortex mirrored the time course of morphine SW (12-96 h), which resulted in a striking correlation between p-FADD and the intensity (behavioural scores) of morphine abstinence (Spearman correlation coefficient: 0.59, n = 39, p < 0.0001). The inactivation of brain α(2)-adrenoceptors (EEDQ at SW 12 h) further enhanced morphine abstinence intensity and cortical p-FADD content at SW 24 h. The disruption of ERK1/2 signalling (SL 327 at SW 4 h and SW 8 h) did not alter morphine abstinence at SW 12 h, but it attenuated the behavioural syndrome at SW 24 h. This inhibition of ERK1/2, however, did not prevent the up-regulation of oligomeric p-FADD at SW 12 h and 24 h. These data indicate that cortical oligomeric p-FADD, mainly through an interaction with inhibitory α(2)-adrenoceptors, plays a functional role in the behavioural expression of morphine abstinence in rats.

Alkylation of rat dopamine transporters and blockade of dopamine uptake by EEDQ

Effects of the alkylating agent EEDQ (N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline) on levels of dopamine transporter (DA(T)) and function were examined in caudate-putamen (CPu) tissue from rat brain. EEDQ produced profound, dose-dependent decreases in DA(T) binding in homogenates (IC(50)=78 microM) and frozen sections (IC(75)=200 microM) that were not reversed by washing. EEDQ also blocked uptake of [(3)H]DA in CPu synaptosomes (IC(50)=17 microM). However, single (10 mg/kg) or repeated administration of EEDQ in vivo (15 mg/kg/day x 3) did not alter DA(T) levels or DA uptake in CPu. Pretreatment of rats with alpha-methyl-p-tyrosine and reserpine to deplete endogenous dopamine also failed to lower DA(T) levels in CPu after injections of EEDQ. EEDQ is an effective alkylating agent for DA(T) in vitro, but not to evaluate metabolic turnover or function of DA(T) in vivo. The results encourage development of selective and in vivo-active DA(T)-alkylating agents.

Effects of alkylating agents on dopamine D(3) receptors in rat brain: selective protection by dopamine

Dopamine D(3) receptors are structurally highly homologous to other D(2)-like dopamine receptors, but differ from them pharmacologically. D(3) receptors are notably resistant to alkylation by 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which readily alkylates D(2) receptors. We compared EEDQ with N-(p-isothiocyanatophenethyl)spiperone (NIPS), a selective D(2)-like receptor alkylating agent, for effects on D(3) and D(2) receptors in rat brain using autoradiographic analysis. Neither agent occluded D(3) receptors in vivo at doses that produced substantial blockade of D(2) receptors, even after catecholamine-depleting pretreatments. In vitro, however, D(3) receptors were readily alkylated by both NIPS (IC(50)=40 nM) and EEDQ (IC(50)=12 microM). These effects on D(3) sites were blocked by nM concentrations of dopamine, whereas microM concentrations were required to protect D(2) receptors from the alkylating agents. The findings are consistent with the view that alkylation of D(3) receptors in vivo is prevented by its high affinity for even minor concentrations of endogenous dopamine.

Selective alkylatation of dopamine D2 and D4 receptors in rat brain by N-(p-isothiocyanatophenethyl)spiperone

Effects of the D2-like receptor alkylating agent NIPS (N-[p-isothiocyanatophenethyl]spiperone) on dopamine receptors in rat brain were characterized by radioreceptor assays and quantitative autoradiography. NIPS alkylated D2 and D4 receptors concentration-dependently in brain sections and transfected cells. NIPS also alkylated both receptors dose-dependently in vivo, with no effect on dopamine D1-like or serotonin 5-HT2 receptors at a dose that occluded 75% of D2 and D4 receptors. Pretreatment with D2-like receptor selective antagonist haloperidol completely blocked the effects of NIPS. The findings demonstrate that NIPS selectively alkylates D2 and D4 receptors, indicating its potential utility for studies of these receptors.

The effects of the peptide-coupling agent, EEDQ, on 5-HT2A receptor binding and function in rat frontal cortex

This ex vivo study in rat frontal cortex determined the influence of 5-HT receptor agonists and antagonists on EEDQ-induced depletion of 5-HT2A binding sites and reduction in their functional coupling to phospholipid hydrolysis. Twenty-four hours after EEDQ (6 mg/kg) administration a marked reduction (66%) of cortical 5-HT2A binding sites with no change in binding affinity was observed. The 5HT2A antagonists ritanserin (1 mg/kg), ketanserin (1 and 5 mg/kg), metergoline (3 mg/kg) or the 5HT2A agonist, DOI (3 and 10 mg/kg) also significantly reduced (by 15-44%) these binding sites 24 h after injection. Thirty minute pretreatment with ritanserin, ketanserin, metergoline or DOI (at the doses above) afforded 49-65% protection against the loss of 5-HT2A binding sites induced by EEDQ (6 mg/kg). DOI (10 mg/kg) pretreatment (-24 h) decreased by 26% the accumulation of [3H]inositol phosphates (IPs) evoked by 5-HT (100 microM), but did not affect that produced by DOI (100 microM). Ketanserin (5 mg/kg, -24 h) decreased 5-HT- and DOI-induced IP formation by 65% and 53%, respectively. The EEDQ (6 mg/kg, -24 h)-evoked reductions (-50%) of 5-HT- and DOI-induced IP formation were not altered by DOI (10 mg/kg) or ketanserin (5 mg/kg) given 30 min before EEDQ. G-protein-stimulated IP accumulation was unaffected by EEDQ (6 mg/kg). Overall, EEDQ reduces 5-HT2A binding sites and function in rat frontal cortex, whereas its effects on binding were attenuated by various 5-HT receptor antagonists and agonists, its effects on function was unaltered by these drugs.

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

Biochemical and pharmacologic evidence supports the heterogeneous nature of imidazoline receptors (IRs). However, only monoamine oxidase (MAO) (55- and 61-kD) isozymes have been identified as imidazoline binding site-containing proteins. Idazoxan-binding proteins of approximately 70- and approximately 45-kD of unknown amino acid sequences have been isolated from chromaffin cells and rat brain, respectively. Other proteins of approximately 27-30 to > 80 kD have been visualized by immunologic and photoaffinity labeling techniques in different tissues and species. The specific antiserum that recognizes the approximately 70-, approximately 45-, and approximately 29-kD IR proteins, but not MAO, was used to quantitate these proteins in the rat brain cortex. Treatments (7 days) with the I2-selective imidazoline drugs idazoxan (10 mg/kg), cirazoline (1 mg/kg), and LSL 60101 ([2-(2-benzofuranyl) imidazole; 10 mg/kg]) induced differential changes in these proteins: levels of the approximately 29-kD IR were increased by idazoxan and LSL 60101 (23%), levels of the approximately 45-kD protein only by cirazoline (44%), and those of the approximately 66-kD protein only by idazoxan (50%). These treatments also increased the densities of [3H]-idazoxan (I2) binding sites (32-42%). Chronic treatment with efaroxan, RX821002, and yohimbine (10 mg/kg), which possess very low affinity for I2-IRs, did not alter either their immunoreactivities or the density of I2 sites. Chronic treatment with MAO inhibitors clorgyline and phenelzine (10 mg/kg) and acute treatment with EEDQ (1.6 mg/kg, 6 h) induced decreases in the levels of these IR proteins (17-47%) and I2 sites (31-57%). Significant correlations were found when the mean percentage changes in immunoreactivity of IR proteins were related to the mean percentage changes in the density of I2 sites after treatment with the foregoing drug (r = 0.92, r = 0.69, and r = 0.75 for the approximately 29-, approximately 45-, and approximately 66-kD proteins, respectively). These results indicate that in the rat cerebral cortex, the I2 sites labeled by [3H]idazoxan are heterogeneous and that the related immunoreactive IR proteins contribute differently to the modulation of I2 sites after drug treatment.

The alkylating agent EEDQ facilitates protease-mediated degradation of the human brain alpha2A-adrenoceptor as revealed by a sequence-specific antibody

An antibody against a sequence from the divergent third intracellular loop of the human alpha2A-adrenoceptor, amino acids 262-276, was produced. The antiserum was tested, along with the preimmune serum, by means of ELISA and dot blot assays which demonstrated that the alpha2A-peptide used for the antibody production was recognized by the immune serum. The antibody also recognized the alpha2A-adrenoceptor protein in the human brain (immunoblot analysis). In cortical membranes a major immunoreactive peptide of approximately 70 kDa (mature glycosylated receptor) was detected and after treatment with N-glycosidase F only a approximately 50 kDa peptide (non-glycosylated receptor) was immunodetected. This antibody was used to demonstrate that a chemical modification of the alpha2A-adrenoceptor induced by the alkylating agent EEDQ facilitates the protease-mediated receptor degradation. Thus in vitro, normal receptor degradation (24-44% at 2-4 h) was enhanced by EEDQ (10(-6) M) (38-71% at 2-4 h) but in the presence of protease inhibitors this effect was almost abolished.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Effects of the alkylating agent EEDQ on regulatory G proteins and recovery of agonist and antagonist alpha2-adrenoceptor binding sites in rat brain

The aim of this study was to assess the effect of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced alpha2-adrenoceptor inactivation on regulatory G proteins and the recovery of agonist and antagonist binding sites. EEDQ induced a rapid increase in the abundance of rat brain cortical Galphai1/2 proteins (30% at 6 h) which reached a maximum at 4 days (45%) and which then slowly returned (7-30 days) to control values. EEDQ did not alter significantly the levels of Galphai3 and Galphao proteins. By using the standard monoexponential model, the analysis of the recovery of alpha2-adrenoceptor density (6 h-30 days) with [3H]UK 14304 (bromoxidine) and [3H]RX 821002 (2-metoxy idazoxan) in the cerebral cortex did not reveal differences in receptor turnover parameters. However, the recovery of [3H]UK 14304 binding fitted best to a new biphasic recovery model, suggesting the existence of two distinct phases of recovery of agonist sites (r1 and r2 = 15.7 and 7.4 fmol mg protein(-1) day(-1); k1 and k2 = 0.51 and 0.25 day(-1); (t1/2)1 and (t1/2)2 = 1.4 and 2.7 days). In contrast, the recovery of [3H]RX 821002 antagonist sites did not fit to the biphasic model (r = 8.1, k = 0.14, t1/2 = 4.9). Because agonist binding requires coupling to G proteins, the present results suggest that the rapid over-expression of Galphai1/2 proteins induced by EEDQ is related to the biphasic recovery of [3H]UK 14304 binding. The possible implication of the faster recovery of alpha2-adrenoceptor function after EEDQ inactivation is discussed.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Pharmacological modulation of immunoreactive imidazoline receptor proteins in rat brain: relationship with non-adrenoceptor [3H]-idazoxan binding sites

1. The densities of various imidazoline receptor proteins (with apparent molecular masses of approximately 29/30-45- and 66-kDa) were quantitated by immunoblotting in the rat cerebral cortex after various drug treatments. The modulation of these imidazoline receptor proteins was then compared with the changes in the density of non-adrenoceptor [3H]-idazoxan binding sites (I2-sites) induced by the same drug treatments. 2. Chronic treatment (7 days) with the I2-selective imidazol(in)e drugs idazoxan (10 mg kg-1), cirazoline (1 mg kg-1) and LSL 60101 (10 mg kg-1) differentially increased the immunoreactivity of imidazoline receptor proteins. The levels of the 29/30-kDa protein were increased by idazoxan and LSL 60101 (23%), the levels of the 45-kDa protein only by cirazoline (44%) and those of the 66-kDa protein only by idazoxan (50%). These drug treatments also increased the density of I2-sites (32-42%). 3. Chronic treatment (7 days) with efaroxan (10 mg kg-1), RX821002 (10 mg kg-1) and yohimbine (10 mg kg-1), which possess very low affinity for I2-imidazoline receptors, did not alter either the immunoreactivity of imidazoline receptor proteins or the density of I2-sites. 4. Chronic treatment (7 days) with the monoamine oxidase (MAO) inhibitors clorgyline (10 mg kg-1) and phenelzine (10 mg kg-1) decreased the immunoreactivity of the 29/30-kDa (17-24%), 45-kDa (19%) and 66-kDa (23-31%) imidazoline receptor proteins. The alkylating agent N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (1.6 mg kg-1, 6 h) also decreased the levels of the three imidazoline receptor proteins (20-47%). These drug treatments consistently decreased the density of I2-sites (31-57%). 5. Significant correlations were found when the mean percentage changes in immunoreactivity of imidazoline receptor proteins were related to the mean percentage changes in the density of I2-sites after the various drug treatments (r = 0.92 for the 29/30-kDa protein, r = 0.69 for the 45-kDa protein and r = 0.75 for the 66-kDa protein). 6. In the rat cerebral cortex the I2-imidazoline receptor labelled by [3H]-idazoxan is heterogeneous in nature and the related imidazoline receptor proteins (29/30-, 45- and 66-kDa) detected by immunoblotting contribute differentially to the modulation of I2-sites after drug treatment.

Alpha 2-autoreceptors and alpha 2-heteroreceptors modulating tyrosine and tryptophan hydroxylase activity in the rat brain in vivo: an investigation into the alpha 2-adrenoceptor subtypes

The subtype determination of auto- and hetero-alpha 2-adrenoceptors modulating the synthesis of noradrenaline (NA) and serotonin (5-HT), respectively, was assessed using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. In the cerebral cortex and hippocampus, Org 3770 (non-selective alpha 2-adrenoceptor antagonist, 0.5-10 mg/kg, i.p.) increased (43%-58%) and clonidine (non-selective alpha 2-adrenoceptor agonist, 1 mg/kg) decreased (37%-49%) the synthesis of dopa. Also the antagonist ARC 239 (alpha 2B/C selective, 5-40 mg/kg) increased the synthesis of dopa in cortex (39%-46%) and hippocampus (17%-85%). In contrast, the antagonist BRL 44408 (alpha 2D selective, 1-10 mg/kg) did not increase the synthesis of dopa in cortex, and increased it modestly in hippocampus only. The agonist guanoxabenz (alpha 2B/C selective, 0.03-3 mg/kg) decreased the synthesis of dopa in both brain regions (20%-65%), whereas the agonist oxymetazoline (alpha 2D selective, 0.1-3 mg/kg) failed to do so. These results indicated that the alpha 2-autoreceptors that modulate the synthesis of dopa/NA are probably associated with the alpha 2B/C-subtypes. In cortex and hippocampus, clonidine decreased (35%-53%) the synthesis of 5-HTP but Org 3770 failed to induce the opposite effect (except the 2 mg/kg dose in cortex). BRL 44408 markedly increased the synthesis of 5-HTP in cortex (113%-148%) but not in hippocampus. Similarly, also ARC239 increased the formation of 5-HTP in cortex (36%-48%) but not in hippocampus, where it was decreased (30%-55%). Oxymetazoline decreased the synthesis of 5-HTP in hippocampus (28%-30%) but failed to do so in cortex. Guanoxabenz in the low dose range (0.03-0.3 mg/kg) did not decrease the synthesis of 5-HTP in any brain region. These results indicated that the alpha 2-heteroreceptors that modulate the synthesis of 5-HTP/5-HT may well be different from the proposed alpha 2B/C-autoreceptors modulating the synthesis of dopa/NA. These alpha 2-heteroreceptors appear to be associated with the alpha 2D-subtype.

Differential sensitivity of [3H]7-OH-DPAT-labeled binding sites in rat brain to inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline

The effects of alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the binding of [3H]7-OH-DPAT, a ligand for the D3 dopamine receptor, were assessed in ventral striatal (n. accumbens and olf. tubercle) membranes of adult, male Sprague-Dawley rats. [3H]Spiperone binding to D2-like receptors in striatal membranes was also assayed as a positive control. In vitro, EEDQ was equipotent in inactivating [3H]7-OH-DPAT- and [3H]spiperone-labeled binding sites. In vitro, [3H]spiperone binding was rapidly eliminated in a dose-dependent manner following EEDQ administration. In contrast, [3H]7-OH-DPAT binding was not significantly altered by any dose of the alkylating agent at any time point examined. Depletion of endogenous catecholamines with alpha-methyltyrosine and reserpine revealed a second, higher affinity binding site for [3H]7-OH-DPAT. Administration of EEDQ in catecholamine-depleted animals reduced [3H]7-OH-DPAT labels two distinct populations of binding binding sites in rat brain membranes, only one of which is susceptible to inactivation by EEDQ. These sites may represent high and low affinity states of the D3 receptor. In addition, this discovery may provide a useful method for examining the function of some D3 receptors in brain independent of other monoaminergic systems.

The effects of phenelzine and other monoamine oxidase inhibitor antidepressants on brain and liver I2 imidazoline-preferring receptors

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate I2 imidazoline-preferring receptors in the rat brain and liver after chronic treatment with various irreversible and reversible monoamine oxidase (MAO) inhibitors. 2. Chronic treatment (7-14 days) with the irreversible MAO inhibitors, phenelzine (1-20 mg kg-1, i.p.), isocarboxazid (10 mg kg-1, i.p.), clorgyline (3 mg kg-1, i.p.) and tranylcypromine (10 mg kg-1, i.p.) markedly decreased (21-71%) the density of I2 imidazoline-preferring receptors in the rat brain and liver. In contrast, chronic treatment (7 days) with the reversible MAO-A inhibitors, moclobemide (1 and 10 mg kg-1, i.p.) or chlordimeform (10 mg kg-1, i.p.) or with the reversible MAO-B inhibitor Ro 16-6491 (1 and 10 mg kg-1, i.p.) did not alter the density of I2 imidazoline-preferring receptors in the rat brain and liver; except for the higher dose of Ro 16-6491 which only decreased the density of these putative receptors in the liver (38%). 3. In vitro, phenelzine, clorgyline, 3-phenylpropargylamine, tranylcypromine and chlordimeform displaced the binding of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors from two distinct binding sites. Phenelzine, 3-phenylpropargylamine and tranylcypromine displayed moderate affinity (KiH = 0.3-6 microM) for brain and liver I2 imidazoline-preferring receptors; whereas chlordimeform displayed high affinity (KiH = 6 nM) for these receptors in the two tissues studied, Clorgyline displayed very high affinity for rat brain (KiH = 40 pM) but not for rat liver I2 imidazoline-preferring receptors (KiH = 169 nM). 4. Preincubation of cortical or liver membranes with phenelzine (10-4 M for 30 min) did not alter the total density of I2 imidazoline-preferring receptors, indicating that this irreversible MAO inhibitor does not irreversibly bind to I2 imidazoline-preferring receptors. In contrast, preincubation with 10-6 Mclorgyline reduced by 40% the Bmax of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors.5. Chronic treatment (7 days) with the inducers of cytochrome P-450 enzymes phenobarbitone (40 or 80 mg kg-1, i.p.), 3-methylcholanthrene (20 mg kg-1, i.p.) or 2-methylimidazole (40 mg kg-1, i.p.) did not alter the binding parameters of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors.The compound SKF 525A, a potent inhibitor of cytochrome P-450 enzymes which forms a tight but reversible complex with the haemoprotein, completely displaced with moderate affinity (KiH = 2-10 microM)the specific binding of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors. Preincubation of total liver homogenates with 3 x 10-4 M phenelzine in the presence of 10-3 M NADH, a treatment that irreversibly inactivates the haeme group of cytochrome P-450, did not reduce the density of liver I2 imidazoline-preferring receptors. These results discounted a possible interaction of [3H]-idazoxan with the haeme group of cytochrome P-450 enzymes.6. Together the results indicate that the down-regulation of I2 imidazoline-preferring receptors is associated with an irreversible inactivation of MAO (at least in the brain) that is not related either to the affinity of the MAO inhibitors for I2 imidazoline-preferring receptors or to an irreversible binding to these putative receptors. These findings indicate a novel effect of irreversible MAO inhibitors in the brain and suggest a new target for these compounds that could be of relevance in the treatment of depression, a disease in which an increased density of brain I2 imidazoline-preferring receptors has been reported.

Covariation of alpha 2-adrenoceptor density and function following irreversible antagonism with EEDQ

1. Administration of the irreversible antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, (EEDQ, 2 mg kg-1, i.p.) to mice reduced binding of [3H]-RX 821002 (2-methoxy-idazoxan) to alpha 2-adrenoceptors in whole mouse brain by 75% 24 h later. The receptor binding returned over time only being reduced by 25% by 16 days post administration; the time taken for binding to return to 50% of control levels was estimated to be 5.25 days. 2. EEDQ administration also resulted in the loss of the sedative effect of the alpha 2-adrenoceptor agonist, medetomidine, measured by the holeboard test of directed exploration and locomotor activity. Agonist-induced sedation returned to control values by 8 days post EEDQ administration. 3. EEDQ administration also resulted in the loss of the hypothermic response to medetomidine (0.1 mg kg-1, i.p.). Medetomidine-induced hypothermia returned to control values by 12 days post EEDQ administration. 4. Pretreatment with the selective alpha 2-adrenoceptor antagonist, RX 821002 (0.1-3.0 mg kg-1, i.p.) 45 min before EEDQ prevented the loss of alpha 2-adrenoceptors as well as the blockade of medetomide-induced sedation and hypothermia by EEDQ. 5. The results of these experiments indicate that there is significant receptor reserve for alpha 2-adrenoceptor-mediated behavioural and physiological responses.

The effects of chronic imidazoline drug treatment on glial fibrillary acidic protein concentrations in rat brain

1. The concentration of the astrocytic marker, glial fibrillary acidic protein (GFAP) was quantitated by immunoblotting (western blotting) in the rat brain after treatment with various imidazoline drugs and other agents. 2. Chronic (7 days) but not acute (1 day) treatment with the imidazoline drugs, cirazoline (1 mg kg-1, i.p.) and idazoxan (10 mg kg-1, i.p.), but not with the structurally related alpha 2-adrenoceptor antagonists, RX821002 (2-methoxy idazoxan) (10 mg kg-1, i.p.) and efaroxan (10 mg kg-1, i.p.), markedly increased (45%) GFAP immunoreactivity in the rat cerebral cortex. Chronic treatment (7 days) with yohimbine (10 mg kg-1, i.p.), a non-imidazoline alpha 2-adrenoceptor antagonist, did not significantly modify GFAP immunoreactivity in the cerebral cortex. 3. Chronic treatment (7 days) with cirazoline and idazoxan did not alter the density of brain monoamine oxidase (MAO)-B sites labelled by [3H]-Ro 19-6327 (lazabemide), another relevant astroglial marker. Moreover, these imidazoline drug treatments did not modify the levels of alpha-tubulin in the cerebral cortex. These negative results reinforced the specificity of the effects of imidazoline drugs on GFAP. 4. Irreversible inactivation of brain alpha 2-adrenoceptors (and other neurotransmitters receptors) after treatment with an optimal dose of the peptide-coupling agent EEDQ (1.6 mg kg-1, i.p., for 6-24 h) did not alter GFAP immunoreactivity in the cerebral cortex. These results further disproved the involvement of these receptors on astroglial cells in the tonic control of GFAP levels.5. The binding of [3H]-idazoxan in the presence of 10-6 M (-)-adrenaline was used to quantitate in parallel 12-imidazoline preferring sites in the rat brain after the same treatments. Chronic treatment (7 days) with cirazoline and idazoxan, but not with RX821002, efaroxan or yohimbine, significantly increased (25%) the density of I2-sites in the cerebral cortex. The up-regulation of I2-sites induced by cirazoline was not observed in the liver, a tissue that also expresses 12-sites but lacks glial cells.6. A strong correlation (r = 0.97) was found when the mean percentage changes in GFAP immuno reactivity were related to the mean percentage changes in 12 imidazoline sites after the various drug treatments.7. Together the results suggest a direct physiological function of glial I2-imidazoline preferring sites in the regulation of GFAP levels.