Neuropharmacological profile of a novel and selective ligand of the sigma site: SR 31742A

Behavioral Effects of a Potential Novel TAAR1 Antagonist

The trace amine associated receptor 1 (TAAR1) is a G-protein coupled receptor expressed in the monoaminergic regions of the brain, and represents a potential novel therapeutic target for the treatment of neurological disorders. While selective agonists for TAAR1 have been successfully identified, only one high affinity TAAR1 antagonist has been described thus far. We previously identified four potential low potency TAAR1 antagonists through an in silico screen on a TAAR1 homology model. One of the identified antagonists (compound 22) was predicted to have favorable physicochemical properties, which would allow the drug to cross the blood brain barrier. In vivo studies were therefore carried out and showed that compound 22 potentiates amphetamine- and cocaine-mediated locomotor activity. Furthermore, electrophysiology experiments demonstrated that compound 22 increased firing of dopamine neurons similar to EPPTB, the only known TAAR1 antagonist. In order to assess whether the effects of compound 22 were mediated through TAAR1, experiments were carried out on TAAR1-KO mice. The results showed that compound 22 is able to enhance amphetamine- and cocaine-mediated locomotor activity, even in TAAR1-KO mice, suggesting that the in vivo effects of this compound are not mediated by TAAR1. In collaboration with Psychoactive Drug Screening Program, we attempted to determine the targets for compound 22. Psychoactive Drug Screening Program (PDSP) results suggested several potential targets for compound 22 including, the dopamine, norepinephrine and serotonin transporters; as well as sigma 1 and 2 receptors. Our follow-up studies using heterologous cell systems showed that the dopamine transporter is not a target of compound 22. Therefore, the biological target of compound 22 mediating its psychoactive effects still remains unknown.

Pharmacology and therapeutic potential of sigma(1) receptor ligands

Sigma (σ) receptors, initially described as a subtype of opioid receptors, are now considered unique receptors. Pharmacological studies have distinguished two types of σ receptors, termed σ₁ and σ₂. Of these two subtypes, the σ₁ receptor has been cloned in humans and rodents, and its amino acid sequence shows no homology with other mammalian proteins. Several psychoactive drugs show high to moderate affinity for σ₁ receptors, including the antipsychotic haloperidol, the antidepressant drugs fluvoxamine and sertraline, and the psychostimulants cocaine and methamphetamine; in addition, the anticonvulsant drug phenytoin allosterically modulates σ₁ receptors. Certain neurosteroids are known to interact with σ₁ receptors, and have been proposed to be their endogenous ligands. These receptors are located in the plasma membrane and in subcellular membranes, particularly in the endoplasmic reticulum, where they play a modulatory role in intracellular Ca²⁺ signaling. Sigma₁ receptors also play a modulatory role in the activity of some ion channels and in several neurotransmitter systems, mainly in glutamatergic neurotransmission. In accordance with their widespread modulatory role, σ₁ receptor ligands have been proposed to be useful in several therapeutic fields such as amnesic and cognitive deficits, depression and anxiety, schizophrenia, analgesia, and against some effects of drugs of abuse (such as cocaine and methamphetamine). In this review we provide an overview of the present knowledge of σ₁ receptors, focussing on σ₁ ligand neuropharmacology and the role of σ₁ receptors in behavioral animal studies, which have contributed greatly to the potential therapeutic applications of σ₁ ligands.

Dopamine-1 receptor agonist, but not cocaine, modulates sigma(1) gene expression in SVG cells

It has been hypothesized that sigma(1) receptors (sigma(1)Rs) are involved in the effects of cocaine abuse. Many in vitro and in vivo studies have already indicated an influence of sigma(1)R ligands on dopaminergic transmission; however, the direct effect on the brain is poorly understood. Herein we describe the effects of cocaine and the selective dopamine-1 receptor (D(1)R) agonist, (+)-SKF38393, on gene expression of the sigma(1)R in a human fetal astrocyte cell line (SVG cells). This study provides the first evidence for the expression of sigma(1)RmRNAin these cells. Our results show that treatment of SVG cells with various cocaine concentrations for several time durations showed no significant alterations in sigma(1)R gene expression, as detected by real-time quantitative RT-PCR, whereas treating cells for 24 h with (+)-SKF38393 caused a significant down-regulation in sigma(1) transcripts. This (+)-SKF38393-induced effect was blocked by the D(1)R selective antagonist (+)-SCH23390. These results suggest that the effect of cocaine on sigma(1) gene expression in the brain might be indirect and mediated through D(1)R.

Neurochemical, electrophysiological and pharmacological profiles of the selective inhibitor of the glycine transporter-1 SSR504734, a potential new type of antipsychotic

Noncompetitive N-methyl-D-aspartate (NMDA) blockers induce schizophrenic-like symptoms in humans, presumably by impairing glutamatergic transmission. Therefore, a compound potentiating this neurotransmission, by increasing extracellular levels of glycine (a requisite co-agonist of glutamate), could possess antipsychotic activity. Blocking the glycine transporter-1 (GlyT1) should, by increasing extracellular glycine levels, potentiate glutamatergic neurotransmission. SSR504734, a selective and reversible inhibitor of human, rat, and mouse GlyT1 (IC50=18, 15, and 38 nM, respectively), blocked reversibly the ex vivo uptake of glycine (mouse cortical homogenates: ID50: 5 mg/kg i.p.), rapidly and for a long duration. In vivo, it increased (minimal efficacious dose (MED): 3 mg/kg i.p.) extracellular levels of glycine in the rat prefrontal cortex (PFC). This resulted in an enhanced glutamatergic neurotransmission, as SSR504734 potentiated NMDA-mediated excitatory postsynaptic currents (EPSCs) in rat hippocampal slices (minimal efficacious concentration (MEC): 0.5 microM) and intrastriatal glycine-induced rotations in mice (MED: 1 mg/kg i.p.). It normalized activity in rat models of hippocampal and PFC hypofunctioning (through activation of presynaptic CB1 receptors): it reversed the decrease in electrically evoked [3H]acetylcholine release in hippocampal slices (MEC: 10 nM) and the reduction of PFC neurons firing (MED: 0.3 mg/kg i.v.). SSR504734 prevented ketamine-induced metabolic activation in mice limbic areas and reversed MK-801-induced hyperactivity and increase in EEG spectral energy in mice and rats, respectively (MED: 10-30 mg/kg i.p.). In schizophrenia models, it normalized a spontaneous prepulse inhibition deficit in DBA/2 mice (MED: 15 mg/kg i.p.), and reversed hypersensitivity to locomotor effects of d-amphetamine and selective attention deficits (MED: 1-3 mg/kg i.p.) in adult rats treated neonatally with phencyclidine. Finally, it increased extracellular dopamine in rat PFC (MED: 10 mg/kg i.p.). The compound showed additional activity in depression/anxiety models, such as the chronic mild stress in mice (10 mg/kg i.p.), ultrasonic distress calls in rat pups separated from their mother (MED: 1 mg/kg s.c.), and the increased latency of paradoxical sleep in rats (MED: 30 mg/kg i.p.). In conclusion, SSR504734 is a potent and selective GlyT1 inhibitor, exhibiting activity in schizophrenia, anxiety and depression models. By targeting one of the primary causes of schizophrenia (hypoglutamatergy), it is expected to be efficacious not only against positive but also negative symptoms, cognitive deficits, and comorbid depression/anxiety states.

Involvement of the sigma 1 receptor in the modulation of dopaminergic transmission by amantadine

Pharmacological effects of amantadine on dopaminergic transmission are proposed to result from an uncompetitive antagonism at glutamate N-methyl-D-aspartate (NMDA) receptors. However, our previous studies examining amantadine-mediated dopamine receptor regulation in the rat striatum revealed a discrepancy from a direct interference with glutamate transmission. Preliminary in vitro binding data from the literature suggested the interaction of amantadine with the sigma1 receptor. Therefore, we have now further characterized the pharmacological properties of amantadine and memantine at this receptor and investigated its involvement in the modulation of striatal dopaminergic transmission. Our binding studies using [3H]-(+)SKF-10,047 indicated that amantadine and memantine behave as ligands of the sigma(1) receptor in rat forebrain homogenates (Ki values of 7.44 +/- 0.82 and 2.60 +/- 0.62 microm, respectively). In NG108-15 neuroblastoma cells, both drugs (amantadine (100 microm) and memantine (10 microm)) potentiated the bradykinin-induced mobilization of intracellular Ca2+, mimicking the effect of the sigma1 receptor agonist PRE-084 (1 microm). Finally, we previously showed that in striatal membranes from amantadine-treated rats, the functional coupling of dopamine receptors with G-proteins was enhanced. Similarly, PRE-084 dose-dependently increased the [35S]GTPgammaS binding induced by dopamine (Emax 28 and 26% of basal, 0.3 and 1 mg/kg PRE-084, respectively). By contrast, BD1047, which is without effect on its own, antagonized the effects of amantadine and PRE-084. Together, these data demonstrate that aminoadamantanes behave as sigma1 receptor agonists, and confirm an involvement of this receptor in modulating dopamine receptors exerted by therapeutically relevant concentrations of amantadine.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

MS-377, a novel selective sigma(1) receptor ligand, reverses phencyclidine-induced release of dopamine and serotonin in rat brain

A novel selective sigma(1) receptor ligand, (R)-(+)-1-(4-chlorophenyl)-3-[4-(2-methoxyethyl)piperazin-1-yl]methyl-2-pyrrolidinone L-tartrate (MS-377), inhibits phencyclidine (1-(1-phenylcyclohexyl)piperidine; PCP)-induced behaviors in animal models. In this study, we measured extracellular dopamine and serotonin levels in the rat brain after treatment with MS-377 alone, using in vivo microdialysis. We also examined the effects of MS-377 on extracellular dopamine and serotonin levels in the rat medial prefrontal cortex after treatment with PCP. MS-377 itself had no significant effects on dopamine release in the striatum (10 mg/kg, p.o.) nor on dopamine or serotonin release in the medial prefrontal cortex (1 and 10 mg/kg, p.o.). PCP (3 mg/kg, i.p.) markedly increased dopamine and serotonin release in the medial prefrontal cortex. MS-377 (1 mg/kg, p.o.), when administered 60 min prior to PCP, significantly attenuated this effect of PCP. These results suggest that the inhibitory effects of MS-377 on PCP-induced behaviors are partly mediated by inhibition of the increase in dopamine and serotonin release in the rat medial prefrontal cortex caused by PCP.

Subchronic treatment with either clozapine, olanzapine or haloperidol produces a hyposensitive response of the rat cortical cells to N-methyl-D-aspartate

Using the technique of intracellular recording in in vitro brain slice preparations, we examined the effects produced by repeated administration of the antipsychotic drugs clozapine, olanzapine and haloperidol, on N-methyl-D-aspartic acid-induced responses in pyramidal cells of the rat medial prefrontal cortex. Rats were anesthetized and decapitated 24h after the conclusion of daily intraperitoneal injection with either clozapine (25mg/kg), olanzapine (1, 5 or 10mg/kg) or haloperidol (0.5mg/kg) for 21 days, and the slices from medial prefrontal cortex were used for electrophysiological recordings. The concentration-response curves for N-methyl-D-aspartic acid to activate cortical cells shifted markedly to the right in rats which received the subchronic antipsychotic drug treatment, compared with those obtained from rats which received repeated injections of vehicle (1ml/kg/day, i.p. for 21 days). In addition, repeated exposure to antipsychotic drugs caused a significant reduction in the ability of these antipsychotic drugs to augment the N-methyl-D-aspartic acid-induced inward current in pyramidal cells of the rat medial prefrontal cortex. Repeated administration of haloperidol, but not clozapine or olanzapine, significantly hyperpolarized the resting membrane potential and increased membrane resistance in pyramidal cells of the medial prefrontal cortex. Moreover, subchronic treatment with haloperidol, but not clozapine or olanzapine, depressed (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-induced responses. The desensitized response of medial prefrontal cortex cells to N-methyl-D-aspartic acid could be the result of a compensatory response to the facilitating action of antipsychotic drugs on N-methyl-D-aspartic acid receptor-mediated transmission. The inhibitory action of haloperidol on (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid responses may also contribute to the rightward shift of the N-methyl-D-aspartic acid concentration-response curve.Thus, the present study suggests that the atypical antipsychotic drugs, clozapine and olanzapine, as well as the typical antipsychotic drug haloperidol strongly modulate glutamatergic transmission after prolonged treatment. This might be an important factor in the mechanisms by which these drugs alleviate symptoms in schizophrenic patients.

The pharmacology of latent inhibition as an animal model of schizophrenia

The nature of the primary symptoms of schizophrenia and our lack of knowledge of its underlying cause both contribute to the difficulty of generating convincing animal models of schizophrenia. A more recent approach to investigating the biological basis of schizophrenia has been to use information processing models of the disease to link psychotic phenomena to their neural basis. Schizophrenics are impaired in a number of experimental cognitive tasks that support this approach, including sensory gating tasks and models of selective attention such as latent inhibition (LI). LI refers to a process in which noncontingent presentation of a stimulus attenuates its ability to enter into subsequent associations, and it has received much attention because it is widely considered to relate to the cognitive abnormalities that characterise acute schizophrenia. Several claims have been made for LI having face and construct validity for schizophrenia. In this review of the pharmacological studies carried out with LI we examine its claim to predictive validity and the role of methodological considerations in drug effects. The data reviewed demonstrate that facilitation of low levels of LI is strongly related to demonstrated antipsychotic activity in man and all major antipsychotic drugs, both typical and atypical, have been shown to potentiate LI using a variety of protocols. Very few compounds without antipsychotic activity are active in this model. In contrast, disruption of LI occurs with a wide range of drugs and the relationship with psychotomimetic potential is less clear. Although reversal of disrupted LI has also been used as a model for antipsychotic acticity, mostly using amphetamine-induced disruption, insufficient studies have been carried out to evaluate its claim to predictive validity. However, like facilitation, it is sensitive to both typical and atypical antipsychotic agents. The data we have reviewed here demonstrate that facilitation of LI and, perhaps to a lesser extent, reversal of disrupted LI fulfil the criteria for predictive validity.

Pharmacological and molecular targets in the search for novel antipsychotics

The recent enthusiasm among clinicians for the so-called 'atypical antipsychotics' has both improved treatment for schizophrenic patients and provided a welcome stimulus for basic research on antipsychotic mechanisms. Even the newer drugs have shortcomings, and research is underway aimed at identifying novel agents with greater efficacy and safety. Much of this effort is directed towards compounds which, in addition to blocking dopamine receptors, also act on other neurotransmitter receptors such as 5-HT2, 5-HT1A and alpha2-adrenergic receptors. However, there is also a large amount of scientific activity seeking to discover and develop selective dopamine receptor subtype antagonists (including compounds which specifically block D3 or D4 receptors) or drugs that specifically target the dopamine autoreceptor. Finally, a number of drug development programmes are searching for non-dopaminergic antipsychotics. Drugs that do not have affinity for dopamine receptors but act through neurotensin, sigma or cannabinoid CB1 receptors or glutamatergic mechanisms are currently being evaluated. If any of these agents prove to have clinical efficacy this may lead to a third generation of antipsychotics. It is likely, however, that the mechanisms of action of such drugs will nevertheless imply the intimate involvement of dopaminergic pathways.

Influence of 3-PPP, a sigma receptor ligand, on the anticonvulsive action of conventional antiepileptic drugs

(+)-3-(3-Hydroxyphenyl)-N-(1-propyl)-piperidine (3-PPP; a sigma receptor ligand), administered at 30 mg kg-1, 30 min before the test, significantly decreased the electroconvulsive threshold in mice, being ineffective in lower doses. 3-PPP (20 mg kg-1) diminished the protective activity of diphenylhydantoin, phenobarbital and valproate, but not that of carbamazepine against maximal electroshock. The effect of 3-PPP upon the electroconvulsive threshold and the 3-PPP-induced inhibition of the protective action of antiepileptics was reversed by haloperidol (0.5 mg kg-1). Moreover, 3-PPP did not alter the total and free plasma levels of antiepileptic drugs, so a pharmacokinetic interaction is not probable. The combined treatment of 3-PPP with antiepileptic drugs, providing a 50% protection against maximal electroshock, did not affect motor performance in mice, although resulted in significant long-term memory deficits. Our data indicate that sigma receptor-mediated events may play some role in seizure processes in the central nervous system and can modulate the protective activity of some conventional antiepileptic drugs.

Acute and chronic administration of the selective sigma1 receptor agonist SA4503 significantly alters the activity of midbrain dopamine neurons in rats: An in vivo electrophysiological study

In this study, we examined the effect of the acute and repeated administration of the selective sigma (sigma)1 receptor agonist 1-(3, 4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) on the number and firing pattern of spontaneously active dopamine (DA) neurons in substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single unit recording. The intravenous administration of SA4503 (0.01-1.28 mg/kg) did not significantly alter the firing rate or pattern of spontaneously active DA neurons in either the SNC or VTA. A single injection of either 0.1 or 0.3 mg/kg i.p. of SA4503 did not alter the number of spontaneously active SNC and VTA DA neurons. In contrast, a single injection of 1 mg/kg i.p. of SA4503 produced a significant decrease and increase in the number of spontaneously active SNC and VTA DA neurons, respectively. Overall, the firing pattern parameters of spontaneously active SNC DA neurons were altered more significantly than those of spontaneously active VTA DA neurons following the acute administration of SA4503. The repeated administration (one injection per day for 21 days) of 0.3 and 1 mg/kg i.p. of SA4503 produced a significant increase in the number of spontaneously active VTA DA neurons. In addition, the repeated administration of SA4503 produced a greater alteration of the firing pattern of spontaneously active VTA compared to SNC DA neurons. Our results suggest that the administration of SA4503 significantly alters the activity of spontaneously active midbrain DA neurons, particularly those in the VTA following repeated administration.

Electrophysiological effects of E-5842, a sigma1 receptor ligand and potential atypical antipsychotic, on A9 and A10 dopamine neurons

Extracellular single unit recording techniques were used to study the effects of the novel potential atypical antipsychotic E-5842, (4-(4-fluorophenyl)-1,2,3,6-tetrahydro-1-[4-(1,2,4-triazol-1-il)bu tyl]pyridine citrate), a preferential sigma1 receptor ligand, on the activity of dopamine cells in substantia nigra pars compacta (A9) and ventral tegmental area (A10) in anesthetized rats. Acute i.v. administration of E-5842 (up to 3.2 mg kg(-1)) did not change the spontaneous activity of the dopamine neurons, which still responded to the inhibitory effect of a subsequent administration of high dose of apomorphine. Acute administration of E-5842 (20 mg kg(-1), i.p.) did not change the number of spontaneously active A9 or A10 dopamine cells. Chronic administration of E-5842 (20 mg kg(-1) day(-1) x 21 days, s.c.) decreased the number of spontaneously active A10 but not A9, dopamine neurons. This effect was reversed by the administration of apomorphine, thus, indicating a possible depolarization inactivation phenomenon. Our results suggest an influence of E-5842 on dopaminergic neurotransmission, although the exact mechanism remains unknown. The effect of E-5842 on A10 is similar, in some ways, to the effects observed with several atypical antipsychotics and suggest the atypicality of the compound and that E-5842 may exert its antipsychotic effects without causing significant extrapyramidal side effects.

Design, synthesis, structure-activity relationships, and biological characterization of novel arylalkoxyphenylalkylamine sigma ligands as potential antipsychotic drugs

sigma Receptor antagonists may be effective antipsychotic drugs that do not induce motor side effects caused by ingestion of classical drugs such as haloperidol. We obtained evidence that 1-(2-dipropylaminoethyl)-4-methoxy-6H-dibenzo[b,d]pyran hydrochloride 2a had selective affinity for sigma receptor over dopamine D2 receptor. This compound was designed to eliminate two bonds of apomorphine 1 to produce structural flexibility for the nitrogen atom and to bridge two benzene rings with a -CH2O- bond to maintain the planar structure. In light of the evidence, N, N-dipropyl-2-(4-methoxy-3-benzyloxylphenyl)ethylamine hydrochloride 10b was designed. Since compound 10b had eliminated a biphenyl bond of 6H-dibenzo[b,d]pyran derivative 2a, it might be more released from the rigid structure of apomorphine 1 than compound 2a. The chemical modification of compound 10b led to the discovery that N, N-dipropyl-2- [4-methoxy-3-(2-phenylethoxyl)phenyl]ethylamine hydrochloride 10g (NE- 100), the best compound among arylalkoxyphenylalkylamine derivatives 3, had a high and selective affinity for sigma receptor and had a potent activity in an animal model when the drug was given orally. We report here the design, synthesis, structure-activity relationships, and biological characterization of novel arylalkoxyphenylalkylamine derivatives 3.

Involvement of sigma receptors in the modulation of the glutamatergic/NMDA neurotransmission in the dopaminergic systems

Extracellular single-unit recordings and iontophoresis were used to examine the effects of different selective sigma receptor ligands on dopaminergic and glutamatergic N-methyl-D-aspartate (NMDA) neurotransmissions both in origin (A10 and A9 areas) and terminal (nucleus accumbens and caudate nucleus) regions of the rat mesolimbic and nigrostriatal dopaminergic systems. The selective sigma1 receptor ligands 2-[4-(4-methoxy-benzyl)piperazin-1-yl-methyl]4-oxo[4H]-benzo-th iazolin-2-one (S-21377), systemically administered (1.2 mg/kg, i.v., cumulative dose), and 2[(4-benzyl piperazin-1-yl) mothyl] naphthalene, dichiorydrate (S-21378), iontophoretically applied, slightly increased the spontaneous firing rate and potentiated the NMDA-induced neuronal activation of dopaminergic neurons in the A9 and A10 regions. (+)N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-butyl-2-N (JO-1784), another selective sigma1 receptor ligand produced no or little effect in these areas. The systemic administration of the selective sigma2 receptor ligand 1,4-bis-spiro[isobenzofuran-1(3H), 4'-piperidin-1'yl]butane (Lu 29-252) (2 mg/kg, i.v., cumulative dose) did not modify the firing activity of A9 and A10 dopaminergic neurons, but significantly potentiated the NMDA-induced increase in firing activity of A10 dopaminergic neurons. None of the sigma receptor ligands tested had any effects on the dopamine-induced suppression of firing. In the nucleus accumbens, the systemic administration of (JO-1784), (40 microg/kg, i.v.), (+)-pentazocine (30 microg/kg, i.v.), another selective sigma1 receptor ligand, and of the non selective sigma1 receptor ligand di-tolyl-guanidine (DTG) (20 microg/i.v.) produced a significant increase of NMDA-induced neuronal activation. Microiontophoretic applications of JO-1784 also potentiated the NMDA response. They also increased significantly the suppressant effect of dopamine on NMDA and kainate-induced activations of accumbens neurons. In the caudate nucleus, (+)-pentazocine, but not JO-1784, potentiated slightly the neuronal response to NMDA. None of the sigma receptor ligands tested did modify significantly the responses of caudate and accumbens neurons to kainate. These findings suggest that at least two subtypes of sigma1 receptors may affect differentially the glutamate NMDA neurotransmission in the terminal and origin regions of the mesolimbic and nigrostriatal dopaminergic systems. These results also demonstrate the existence of a functional interaction between sigma2 and NMDA receptors in the A10 region.

Biphasic modulatory action of the selective sigma receptor ligand SR 31742A on N-methyl-D-aspartate-induced neuronal responses in the frontal cortex

The technique of intracellular recording was used to assess the effect of SR 31742A, a selective sigma receptor ligand, on N-methyl-d-aspartate (NMDA) and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated responses in pyramidal cells of the rat medial prefrontal cortex in vitro brain slice preparations. Bath application of SR 31742A produced a biphasic effect on NMDA responses: SR 31742A facilitated and inhibited NMDA-induced inward current at low (0.01, 0.05 and 0.1 microM) and higher (0.5, 1 and 10 microM) concentrations, respectively. The potentiating effect reached the peak (366%) at 0.1 microM, with an estimated EC50 value of 23 nM. Correspondingly, SR 31742A also produced a similar biphasic modulatory action on excitatory postsynaptic potentials or currents (EPSPs/EPSCs) evoked by electrical stimulation of the forceps minor. In contrast, SR 31742A produced a modest potentiation of AMPA responses at the concentrations from 0.01 to 1 microM. The potentiating action of SR 31742A on NMDA-receptor mediated neurotransmission may account for, at least partially, its antipsychotic and cognitive-enhancing potential, whereas the inhibitory action on NMDA responses at higher concentrations may be related to the purported neuroprotective action of sigma receptor ligands.

Neuroprotective effect of eliprodil: attenuation of a conditioned freezing deficit induced by traumatic injury of the right parietal cortex in the rat

We have previously demonstrated that a lateral fluid percussion-induced traumatic lesion of the right parietal cortex can lead to a deficit in a conditioned freezing response and that this deficit can be attenuated by both pre- and postlesion administration of the NMDA receptor antagonist dizocilpine. In the present study, we investigated the effects of eliprodil, a noncompetitive NMDA receptor antagonist acting at the polyamine modulatory site, which also acts as a Ca2+ channel blocker, on the trauma-induced conditioned freezing deficit. Eliprodil produced a 50% reduction in this deficit when administered as three 1 mg/kg injections i.v. at 15 min, 6 h, and 24 h following the lesion. Approximately the same degree of protection was afforded when 2 x 1.5 mg/kg were administered 6 and 24 h and equally at 12 and 24 h after surgery (56% and 59%, respectively). A single treatment (3 mg/kg) at 24 h was ineffective against the deficit. The protection afforded with treatment at 6 and 24 h after lesion was dose dependent, with a minimal active dose of 2 x 0.75 mg/kg. These data complement those previously published on the ability of eliprodil to reduce lesion volume following traumatic brain injury and show, in addition, that the neuroprotective effect has functional consequences.

Involvement of cortical neurotensin in the regulation of rat meso-cortico-limbic dopamine neurons: evidence from changes in the number of spontaneously active A10 cells after neurotensin receptor blockade

In order to further assess the role of endogenous neurotensin on midbrain dopaminergic neuronal function, the effects of the selective neurotensin receptor antagonists SR 48692 and SR 48527 were investigated on the number of spontaneously active A9 and A10 dopaminergic neurons in rats. Single intraperitoneal administration of SR 48692 (0.1-3 mg/kg) dose-dependently increased the number of active A10, but not A9 cells. SR 48527 (1 mg/kg) had a similar profile, but not SR 49711, its low affinity R-enantiomer, indicating that the effects observed were mediated through neurotensin receptor blockade. Five-week treatment with SR 48692 (3 mg/kg/day) produced a significant decrease of the number of active A10, but not A9 cells, which was reversed by apomorphine, suggesting that these cells were under depolarization block. Single co-administration of inactive doses of SR 48692 (0.1 mg/kg) and haloperidol (0.0625 mg/kg) significantly increased the number of active A10 cells. Conversely, co-administered active doses of SR 48692 or SR 48527 and haloperidol (1 and 0.25 mg/kg, respectively) induced an apomorphine-sensitive decrease of the number of A10 active cells. Finally, SR 48692 (10 mg/kg) modified neither accumbal nor cortical basal DA release. Local micro-injection of SR 48692 (10[-11]-10[-9] M), but not that of SR 49711 (10[-9] M), into the prefrontal cortex, increased the number of active A10 cells in a concentration-dependent manner. These results suggest that neurotensin receptor blockade counteracts a tonic inhibitory regulation by endogenous neurotensin of mesolimbic dopaminergic function and indicate that the prefrontal cortex is critically involved in this regulation.

Kappa opioid receptor agonists inhibit sigma-1 (sigma 1) receptor binding in guinea-pig brain, liver and spleen: autoradiographical evidence

The present study examined whether the kappa-opioid agonists U50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N[-2-(1-pyrrolidinyl)- cyclohexyl]-benzeacetamide methane sulphonate), bremazocine, spiradoline and ICI 197067 bind to sigma sites in guinea-pig tissues using in vitro, semi-quantitative receptor autoradiography and receptor binding, and compared the binding profile so obtained with those for several selective sigma ligands. Guinea-pigs were killed and their brians, livers and spleens were removed, tissue sections cut and processed for sigma binding site autoradiography using (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-[3H]-3-PPP), or tissue was wiped and determined by liquid scintillation. Serial slide-mounted sections were incubated with 9-10 concentrations (1 nM-10 microM) of kappa opioids and their potency to inhibit (+)-[3H]-3-PPP binding compared with that of the sigma ligands haloperidol, DTG (1,3 di(o)-tolylguanidine), (+)-3-PPP, (+) and (-)pentazocine, SR 31742A and rimcazole (n = 3, duplicate determinations). Binding of (+)-[3H]-3-PPP to untreated, matched serial tissue sections was used as control. Kd values were estimated in brain, liver and spleen using quantitative, saturation binding analysis, IC50 values were determined from the binding data obtained by slide wiping experiments for each drug, and Ki values were calculated using the Cheng-Prussoff equation. All four kappa opioids inhibited (+)-[3H]-3-PPP binding to sigma 1-receptors with order of potency: brain: U50,488H = spiradoline > bremazocine > ICI 197067; liver: spiradoline > U50,488H > ICI 197067 > bremazocine; spleen: U50,488H > spiradoline > ICI 197067 > bremazocine. By comparison, the sigma ligands inhibited (+)-[3H]-3-PPP binding to matched, serial slide-mounted brain tissue sections (similar results in liver and spleen) with order of potency: SR 31742A > haloperidol > (+)pentazocine > (+)-3-PPP > DTG > (-)pentazocine > rimcazole. (+)-[3H]-3-PPP autoradiography confirmed these binding data. It is concluded that the kappa opioids tested moderately inhibit (+)-[3H]-3-PPP binding to sigma 1-receptors in guinea-pig brain, liver and spleen tissue with Ki values comparable to some selective sigma ligands and therefore are not opioid selective.

sigma Receptor antagonists block the development of sensitization to cocaine

The effects of putative sigma receptor antagonists, BMY-14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine), rimcazole and SR-31742A (cis-3-(hexahydroazepin-1-yl)1-(3-chloro-4- cyclohexylphenyl)propene-1), on the development of behavioral sensitization induced by repeated administration of cocaine were investigated. Acute intraperitoneal injection of 15 mg/kg cocaine in rats induced moderate hyperactivity which mainly consisted of sniffing and rearing. These acute effects of cocaine were hardly affected by co-administration of the sigma receptor antagonists, except that BMY-14802 enhanced, but not significantly cocaine-induced locomotion. While repeated cocaine administration induced a progressive increase in stereotyped behaviors and resulted in sensitization, every sigma receptor antagonists tested attenuated the development of sensitization to cocaine. These prophylactic effects of sigma receptor antagonists against cocaine-induced sensitization were confirmed by the challenge test with cocaine alone after an abstinence. These results were consistent with results of our previous study which revealed that BMY-14802 blocked the sensitization to methamphetamine, another psychostimulant. Therefore, sigma receptors play a crucial role in the development of the psychostimulant-induced sensitization phenomenon, which is a pharmacological model of schizophrenia.

Inotropic action of sigma receptor ligands in isolated cardiac myocytes from adult rats

High affinity binding sites for sigma receptor ligands were found in membranes of cardiac myocytes from adult rats. The sigma receptor ligand (+)-3-hydroxyphenyl-N-(1-propyl)piperidine ((+)-3-PPP) binds with a Kd of 17.9 +/- 4.0 nM and a Bmax of 275 +/- 32.1 fmol/mg protein. Competition experiments of (+)-pentazocine with [3H]1,3-di-O-tolylguanidine ([3H]DTG) binding yielded a Ki of 6.1 +/- 1.3 nM. The majority of the sites (> 80%) were of the sigma 1 subtype. Exposure of isolated cardiomyocytes from adult rats to (+)-3-PPP (10 nM-1.0 microM) caused a marked concentration-dependent increase in the amplitude of systolic cell contraction, reaching 149% of control level, with an apparent ED50 value of 4.5 nM. The increase in the contraction amplitude was markedly inhibited by pretreatment with verapamil or thapsigargin. An increase in the amplitude of [Ca2+]i transients, similar to that in the amplitude of cell contraction, was observed in indo-1-loaded cardiomyocytes exposed to 0.1 microM (+)-3-PPP. Exposure to 10 nM of haloperidol or (+)-pentazocine induced an increase in the amplitude of contraction, reaching 188% and 138% (respectively) of control level. A lower concentration of haloperidol or (+)-pentazocine (1 nM) did not induce an increase in the contraction amplitude but rather reduced the amplitude to 70-80% of control.

Effects of the sigma receptor ligand SR 31742A on neurotensin biosynthesis in rat basal ganglia

The effects of SR 31742A, a specific sigma ligand, were investigated on neurotensin (NT) biosynthesis in the basal ganglia of the rat. Both single and repeated treatments with either SR 31742A (20 mg/kg i.p.) or haloperidol (1 mg/kg i.p.) increased the concentration of NT-like immunoreactivity (NT-li) in the nucleus accumbens. In contrast to haloperidol, the administration of SR 31742A failed to increase the concentration of NT-li in the caudate-putamen. We have further investigated drug-induced variations in NT biosynthesis by studying NT/neuromedin N (NT/NN) mRNA levels in the nucleus accumbens and the ventral tegmental area of the rat following SR 31742A administration. The NT/NN mRNA levels in the ventral tegmental area were increased by a maximum of fifteen fold (7 h at 20 mg/kg i.p.). A lower increase in NT/NN mRNA levels was elicited in the nucleus accumbens. These results suggest that the increase in NT-li observed after SR 31742A treatment, like that produced by haloperidol, may result from an increase of NT biosynthesis. Furthermore, the effects of SR 31742A on NT metabolism are similar to those of atypical antipsychotics, since they appear to be selective for the limbic system.

Pharmacological evidence for the involvement of sigma sites in DTG-induced contralateral circling in rats

The central distribution of sigma sites labelled by di-o-tolylguanidine (DTG), a compound which has specific affinity for sigma sites, and its ability to produce postural movements, are consistent with the hypothesis that sigma sites may play a functional role in the regulation of movement. The aim of the present study was to evaluate the specificity of the circling behaviour induced by unilateral intranigral injection of DTG in rats. As previously described, DTG produced dose-dependent unilateral rotations (2.5-20 nmol/rat). A similar dose-dependent circling behaviour was observed with DMTG and (+) NANM (3-40 nmol/rat), compounds which bind to both sigma and PCP sites, and with haloperidol (3-20 nmol/rat) whereas raclopride and D,L-sulpiride did not elicit any circling (10 nmol/rat). DTG-induced circling after intranigral injection (10 nmol/rat) was decreased in a dose-dependent manner by rimcazole (20-40 mg/kg, i.p.), a selective ligand for sigma sites, and by BMY 14802 (3, 10, 30 mg/kg, i.p.), ifenprodil and eliprodil (1, 3, 10 mg/kg, i.p.), non-selective sigma ligands. In contrast, naloxone (1 mg/kg, s.c.) and CGS 19755 (1, 3, 10 mg/kg, i.p.) did not change the DTG-induced circling. Eliprodil failed to inhibit circling produced by compounds devoid of any affinity for sigma sites such as APV, dizocilpine or muscimol, indicating the specificity of the inhibition observed with eliprodil on the DTG-induced circling.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of SR 48692, a selective non-peptide neurotensin receptor antagonist, on two dopamine-dependent behavioural responses in mice and rats

One major mechanism underlying the central action of neurotensin is an interaction with the function of dopamine (DA)-containing neurons. In addition, direct or indirect DA agonists have been reported to promote neurotensin release. We have found that SR 48692, a non-peptide neurotensin receptor antagonist (0.04-0.64 mg/kg orally), antagonizes (50-65%) yawning induced by apomorphine (0.07 mg/kg SC) or bromocriptine (2 mg/kg IP) in rats, and turning behaviour induced by intrastriatal injection of apomorphine (0.25 micrograms), (+) SKF 38393 (0.1 micrograms), bromocriptine (0.01 ng) or (+) amphetamine (10 micrograms) in mice. Other apomorphine-induced effects in mice and rats such as climbing, hypothermia, hypo- and hyper-locomotion, penile erections and stereotypies were not significantly modified by SR 48692. Taken together, these data suggest that neurotensin may play a permissive role in the expression of some but not all behavioural responses to DA receptor stimulation.

Allosteric modulation of peripheral sigma binding sites by a new selective ligand: SR 31747

The interactions of a new compound SR 31747 with sigma sites were examined in rat spleen membranes and in human peripheral blood leukocytes (PBL). Nanomolar concentrations of SR 31747 selectively inhibited in a non-competitive manner the binding of the prototypic sigma ligands [3H](+)-pentazocine, [3H](+)-3PPP and [3H]DTG on rat spleen membranes. Characterization of SR 31747 binding sites using [3H]SR 31747 as a ligand showed that this compound binds reversibly, with high affinity to one class of sites on rat spleen membranes (Kd 0.66 nM, Bmax 5646 fmol/mg protein). The pharmacological profile of [3H]SR 31747 binding sites was consistent with the presence of specific sites distinct from classical sigma 1 and sigma 2 receptor subtypes strongly suggesting an allosteric modulation of sigma sites by SR 31747. Similarly, [3H]SR 31747 binding sites were demonstrated on human PBL and also on purified subpopulations of human mononuclear cells (granulocytes, NK cells, T4, T8 and B lymphocytes). Administered to mice by i.p. or oral route 30 min before sacrifice, SR 31747 strongly inhibited the binding of [3H](+)-3PPP to mice spleen membranes with ED50 values of 0.18 and 1.43 mg/kg, respectively. Taken together these results could suggest a potential immunological activity of SR 31747 either directly or through allosteric modulation of peripheral sigma sites.

The sigma receptor ligand SR 31742A increases neurotensin in the nucleus accumbens but not in the caudate-putamen of the rat

The effects of SR 31742A, a specific sigma site ligand, were investigated on regional neurotensin concentrations in rat brain. Both acute and chronic (21-day) treatment with either SR 31742A (20 mg/kg i.p.) or haloperidol (1 mg/kg i.p.) increased the neurotensin-like immunoreactivity in the nucleus accumbens. In contrast to haloperidol, the administration of SR 31742A failed to increase the concentration of neurotensin-like immunoreactivity in the caudate-putamen. Thus, the effects of SR 31742A appear to be selective for the limbic system.

NE-100, a novel sigma receptor ligand: in vivo tests

It has been suggested that sigma receptor antagonists may be useful as antipsychotic drugs. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) is a novel compound with high affinity for the sigma receptor (IC50 = 4.16 nM), but low affinity (IC50 > 10,000 nM) for D1, D2, 5-HT1A, 5-HT2 and phencyclidine (PCP) receptors. The head-weaving behavior induced by either (+)SKF10047 or PCP was dose-dependently antagonized by NE-100 with oral ED50 at 0.27 and 0.12 mg/kg, respectively. NE-100 did not affect dopamine agonists-induced stereotyped behavior and/or hyperactivity. NE-100 failed to induce catalepsy in rats. These findings indicate that NE-100 may have antipsychotic activity without the liability of motor side effects typical of neuroleptics.