Pharmacological comparison of the sigma recognition site labelled by [3H]haloperidol in human and rat cerebellum

5-substituted derivatives of 6-halogeno-3-((2-(S)-azetidinyl)methoxy)pyridine and 6-halogeno-3-((2-(S)-pyrrolidinyl)methoxy)pyridine with low picomolar affinity for alpha4beta2 nicotinic acetylcholine receptor and wide range of lipophilicity: potential probes for imaging with positron emission tomography

Potential positron emission tomography (PET) ligands with low picomolar affinity at the nicotinic acetylcholine receptor (nAChR) and with lipophilicity (log D) ranging from -1.6 to +1.5 have been synthesized. Most members of the series, which are derivatives of 5-substituted-6-halogeno-A-85380, exhibited a higher binding affinity at alpha4beta2-nAChRs than epibatidine. An analysis, by molecular modeling, revealed an important role of the orientation of the additional heterocyclic ring on the binding affinity of the ligands with nAChRs. The existing nicotinic pharmacophore models do not accommodate this finding. Two compounds of the series, 6-[(18)F]fluoro-5-(pyridin-3-yl)-A-85380 ([(18)F]31) and 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-[(18)F]fluoropyridin-5-yl)pyridine) ([(18)F]35), were radiolabeled with (18)F. Comparison of PET data for [(18)F]31 and 2-[(18)F]FA shows the influence of lipophilicity on the binding potential. Our recent PET studies with [(18)F]35 demonstrated that its binding potential values in Rhesus monkey brain were ca. 2.5 times those of 2-[(18)F]FA. Therefore, [(18)F]35 and several other members of the series, when radiolabeled, will be suitable for quantitative imaging of extrathalamic nAChRs.

Antitussive activity of sigma-1 receptor agonists in the guinea-pig

1. Current antitussive medications have limited efficacy and often contain the opiate-like agent dextromethorphan (DEX). The mechanism whereby DEX inhibits cough is ill defined. DEX displays affinity at both NMDA and sigma receptors, suggesting that the antitussive activity may involve central or peripheral activity at either of these receptors. This study examined and compared the antitussive activity of DEX and various putative sigma receptor agonists in the guinea-pig citric-acid cough model. 2. Intraperitoneal (i.p.) administration of DEX (30 mg kg(-1)) and the sigma-1 agonists SKF-10,047 (1-5 mg kg(-1)), Pre-084 (5 mg kg(-1)), and carbetapentane (1-5 mg kg(-1)) inhibited citric-acid-induced cough in guinea-pigs. Intraperitoneal administration of a sigma-1 antagonist, BD 1047 (1-5 mg kg(-1)), reversed the inhibition of cough elicited by SKF-10,047. In addition, two structurally dissimilar sigma agonists SKF-10,047 (1 mg ml(-1)) and Pre-084 (1 mg ml(-1)) inhibited cough when administered by aerosol. 3. Aerosolized BD 1047 (1 mg ml(-1), 30 min) prevented the antitussive action of SKF-10,047 (5 mg kg(-1)) or DEX (30 mg kg(-1)) given by i.p. administration and, likewise, i.p. administration of BD 1047 (5 mg kg(-1)) prevented the antitussive action of SKF-10,047 given by aerosol (1 mg ml(-1)). 4. These results therefore support the argument that antitussive effects of DEX may be mediated via sigma receptors, since both systemic and aerosol administration of sigma-1 receptor agonists inhibit citric-acid-induced cough in guinea-pigs. While significant systemic exposure is possible with aerosol administration, the very low doses administered (estimated <0.3 mg kg(-1)) suggest that there may be a peripheral component to the antitussive effect.

Correlation between neuroleptic binding to sigma(1) and sigma(2) receptors and acute dystonic reactions

Acute dystonic reactions are motor side effects that occur soon after the initiation of neuroleptic treatment. Although earlier studies indicate that these abnormal movements can be induced in animals and humans via activation of sigma receptors, the relative contribution of the different sigma receptor subtypes is unknown. Since sigma(1) and sigma(2) receptor are differentially represented in motor regions of the brain, the affinities of 17 neuroleptics for these sigma receptor subtypes were determined using competition binding studies. The results revealed that most neuroleptics do not exhibit selectivity for either of the sigma receptor subtypes, as reflected by a significant correlation between the affinities of the neuroleptics for sigma(1) vs. sigma(2) receptors. Moreover, when the sigma binding affinities of the neuroleptics were correlated with the tendency of the drugs to produce acute dystonic reactions in humans, there was a significant correlation for both subtypes. Together with earlier studies in animals, the data suggest that neuroleptic-induced motor side effects can be mediated through both sigma(1) and sigma(2) receptors.

Effect of short-term and long-term treatments with sigma ligands on the N-methyl-D-aspartate response in the CA3 region of the rat dorsal hippocampus

1. Long-term treatments with the sigma ligand haloperidol decrease the density of sigma receptors in mammalian CNS. We have shown that sigma ligands, such as di(2-tolyl)guanidin (DTG), potentiate dose-dependently, with bell-shaped dose-response curves, the neuronal response of pyramidal neurones to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. sigma Ligands producing such a potentiation were denoted 'agonists'. This potentiation was suppressed by low doses of other sigma ligands denoted 'antagonists'. High doses of DTG and JO-1784 did not modify the NMDA response but acted as 'antagonists' by suppressing the potentiation induced by sigma 'agonists'. 2. Following a 21-day treatment with haloperidol as well as with high doses of DTG or JO-1784, after a 48 h washout, the acute administration of sigma 'agonists' failed to induce any potentiation of the NMDA response. Following a 21 day treatment with a low dose of DTG or JO-1784, after a 48 h washout, the neuronal response to microiontophoretic applications of NMDA was markedly increased. A 21 day treatment with low or high doses of (+)-pentazocine, after a 48 h washout, did not produce any change. 3. Following a two day treatment with a high dose of haloperidol, DTG, JO-1784 and (+)-pentazocine, after a 24 h washout, the potentiation of the NMDA response induced by the acute administration of the sigma 'agonists' was unchanged. 4. With the minipumps on board, with DTG and JO-1784, a dose-dependent enhancement of the NMDA response was seen but no effect was observed in the groups of rats treated at the same doses with haloperidol or (+)-pentazocine. 5. The present data suggest that long-term treatments with sigma 'antagonists' induce a desensitization of the th receptors, whereas long-term treatments with th 'agonists' induce a supersensitivity of the th receptors.

Modulation of NMDA and dopaminergic neurotransmissions by sigma ligands: possible implications for the treatment of psychiatric disorders

Sigma (sigma) receptors, improperly classified as belonging to the opiate receptor family when discovered in 1976, were subsequently confused with phencyclidine binding sites for several years. It's only recently, with the emergence of new selective ligands that their functional significance could be meaningfully addressed. Several subtypes of sigma receptors are present in high densities in the limbic structures as well as in motor-related areas of the CNS. Different lines of evidence suggest that a major role for sigma receptors might be to regulate the activity of the glutamatergic system via the modulation one of its subtype of receptor, the NMDA receptor. This modulation of the glutamatergic system could in turn interfere with the dopaminergic neurotransmission with which, however, sigma ligands could also interact directly. The potential involvement of sigma receptors in schizophrenia has been considered ever since their discovery. The initial suggestion to this respect emerged from the observation that several of the earliest sigma ligands induced psychotomimetic symptoms such as delusions, hallucinations and depersonalization. This link was later reinforced with the demonstration that several neuroleptics, such as haloperidol, have a high affinity for sigma receptors, whereas, some new molecules with a high affinity for sigma receptors, but a low affinity for dopaminergic receptors demonstrated a "neuroleptic-like" pharmacological profile. However, the therapeutic efficacy of selective sigma ligands in schizophrenia has not yet been established and it has even been suggested that sigma receptors might be responsible for some side effects of the classical neuroleptics. The possible implication of sigma receptors in affective disorders has also been suggested by reports showing that some antidepressant drugs have a high affinity for sigma receptors and that long-term treatments with anti- depressant drugs, even with those devoid of affinity for sigma receptors, modify their binding characteristics. In conclusion, indirect evidence suggests possible etiological and/or therapeutic roles for sigma receptors in some psychiatric disorders. However, despite several attempts, no clear indications of a therapeutic efficacy of sigma ligands has yet emerged. More selective ligands and fundamental studies on the respective role of the different subtypes of sigma receptors are needed before clear concepts can be formulated. p3

Blockade by sigma site ligands of high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones

1. The effects of a series of structurally-dissimilar sigma site ligands were examined on high voltage-activated Ca2+ channel activity in two preparations of cultured hippocampal pyramidal neurones. 2. In mouse hippocampal neurones under whole-cell voltage-clamp, voltage-activated Ca2+ channel currents carried by barium ions (IBa) were reduced with the rank order (IC50 values in microM): 1S,2R-(-)-cis-N-methyl-N-[2-(3,4-dichlorophenyl)ethyl]- 2-(1-pyrrolidinyl)cyclohexylamine (7.8) > rimcazole (13) > haloperidol (16) > ifenprodil (18) > opipramol (32) > carbetapentane (40) = 1-benzylspiro[1,2,3,4-tetrahydronaphthalene-1,4-piperidine] (42) > caramiphen (47) > dextromethorphan (73). At the highest concentrations tested, the compounds almost abolished IBa in the absence of any other pharmacological agent. 3. The current-voltage characteristics of the whole-cell IBa were unaffected by the test compounds. The drug-induced block was rapid in onset and offset, with the exceptions of carbetapentane and caramiphen where full block was achieved only after two to three voltage-activated currents and was associated with an apparent increase in the rate of inactivation of IBa. 4. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, rises in intracellular free Ca2+ concentration evoked by transient exposure to 50 mM K(+)-containing medium, either in the absence or in the presence of 10 microM nifedipine (to block L-type high voltage-activated Ca2+ channels), were also reversibly attenuated by the sigma ligands. The rank order potencies for the compounds in these experimental paradigms were similar to that observed for blockade of IBa in the electrophysiological studies. 5. These results indicate that, at micromolar concentrations, the compounds tested block multiple subtypes of high voltage-activated Ca2+ channels. These actions, which do not appear to be mediated by high-affinity sigma binding sites, may play a role in some of the functional effects previously described for the compounds.

Biphasic effects of sigma ligands on the neuronal response to N-methyl-D-aspartate

Previous studies from our laboratory have demonstrated that low doses of selective sigma (sigma) ligands potentiate the neuronal response to N-methyl-D-aspartate (NMDA) in the CA3 region of the rat dorsal hippocampus. Sertraline and clorgyline, two antidepressant drugs with a high affinity for receptors, also potentiate, at low doses, the NMDA response; however, when administered at higher sigma doses, the degree of potentiation induced by these two ligands progressively decreases (Bergeron et al. 1993). In the present experiments, the selective sigma ligands DTG, (+)pentazocine, BD-737, JO-1784 and L-687,384 were studied to determine if they would also generate bell-shaped dose-response curves. These ligands were administered intravenously at doses ranging from 1 micrograms/kg to 1 mg/kg or applied by microiontophoresis. They potentiated selectively, with bell-shaped dose-response curves, the NMDA-induced activation of pyramidal neurons in the CA3 region of the rat dorsal hippocampus. The potentiation of the NMDA response following the intravenous administration of a low dose of a sigma ligand persisted for at least 60 min, after which point in time a second injection of the same dose induced the same degree of potentiation. Moreover, a sustained potentiation was obtained during prolonged microiontophoretic applications of a sigma ligand. These two latter series of observations suggest that the lack of effect of the high doses of sigma ligands is not related to a rapid desensitisation of sigma receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of L-687,384, a sigma receptor ligand, on NMDA-induced currents in cultured rat hippocampal pyramidal neurons

The actions of the sigma receptor ligand L-687,384 were studied on N-methyl-D-aspartate (NMDA)-induced currents recorded from outside-out patches obtained from cultured rat hippocampal pyramidal neurons and on NMDA-evoked rises in [Ca2+]i in the same preparation. L-687,384 did not change the magnitudes of unitary NMDA currents or frequency of channel-openings but diminished channel-open probability by decreasing mean open time. This action was consistent with a voltage-dependent open-channel block of the NMDA channel by L-687,384 with a blocking rate constant of 5.9 x 10(6) M-1 s-1 at -80 mV. L-687,384 also reduced NMDA-evoked rises in [Ca2+]i in Fura-2-loaded neurons with an apparent IC50 value of 49 +/- 8 microM. The results demonstrate that L-687,384 acts as an antagonist at the NMDA receptor-channel complex.

Chronic treatment with the monoamine oxidase inhibitors clorgyline and pargyline down-regulates non-adrenoceptor [3H]-idazoxan binding sites in the rat brain

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate non-adrenoceptor idazoxan binding sites (NAIBS) in the rat brain after treatment with various psychotropic drugs. 2. Chronic treatment (14 days) with the monoamine oxidase (MAO) inhibitors clorgyline (0.3-10 mg kg-1, i.p.) and pargyline (10 mg kg-1, i.p.), but not with Ro 41-1049 (1 mg kg-1, i.p.), markedly decreased (30-50%) the density of NAIBS in the cerebral cortex without any apparent change in the affinity of the radioligand. 3. Acute (1 day) and/or chronic treatments (14 days) with other psychotropic drugs such as desipramine (3 mg kg-1, i.p.), cocaine (10 mg kg-1, i.p.), reserpine (0.12 mg kg-1, s.c.), haloperidol (1 mg kg-1, i.p.) and diazepam (10 mg kg-1, i.p.) did not alter the density of NAIBS in the cerebral cortex. 4. In vitro, the propargylamines clorgyline, pargyline and deprenyl displaced the binding of [3H]-idazoxan to NAIBS from two distinct sites, but only clorgyline displayed an apparent very high affinity for a relevant population of NAIBS (KiH = 40 pM; KiL = 10.6 microM). The structurally diverse MAO inhibitors Ro 16-6491 (selective for MAO-B) and Ro 41-1049 (selective for MAO-A), as well as the other psychotropic drugs (desipramine, cocaine, reserpine and haloperidol) displaced the binding of [3H]-idazoxan to NAIBS monophasically and with very low potencies. As expected, the MAO inhibitors clorgyline and Ro 41-1049 displaced the binding of [3H]-Ro 41-1049 to MAO-A monophasically and with high potencies (Ki values: 0.18 nM and 22 nM, respectively). In contrast, idazoxan displayed very low affinity (Ki =40 microM) against the binding of pH]-Ro 41-1049 to MAO-A. These results disprove a direct interaction between [3H]-idazoxan and the enzyme MAO.5. Preincubation of cortical membranes with clorgyline (10-9M or 10-6 M for 30 min) or pargyline(10-6 M or 10-5M for 30 min), reduced by 30-50% and by 17-30%, respectively, the total density of NAIBS without any apparent change in the affinity of the radioligand. Preincubation with 10-6M clorgyline did not alter the affinity of cirazoline for the two populations of NAIBS, but reduced by 60%the binding of [3H]-idazoxan to the high affinity site without affecting the binding of the radioligand to the low affinity site. These results indicate that the two MAO inhibitors irreversibly block the binding of[3H]-idazoxan to NAIBS.6. In vivo, however, various acute treatments with clorgyline (1-20 mg kg-1, i.p.) for different time intervals (6-48 h) did not alter the density of NAIBS. In vivo, only very high doses of clorgyline (40 and 80 mg kg-1, i.p.) induced modest decreases (21-28%) in the density of NAIBS in the cerebral cortex.7. Together the results indicate that the irreversible binding of clorgyline and pargyline to NAIBSfound in vitro does not fully explain the marked decreases in the density of NAIBS found in vivo after the chronic treatments. It is suggested that the down-regulation of NAIBS induced in vivo by clorgyline and pargyline, through a direct or indirect mechanism, may have functional implications.