Inhibition of rubral neurons by a specific ligand for sigma receptors

Animal models of focal dystonia

Animal models indicate that the abnormal movements of focal dystonia result from disordered sensorimotor integration. Sensorimotor integration involves a comparison of sensory information resulting from a movement with the sensory information expected from the movement. Unanticipated sensory signals identified by sensorimotor processing serve as signals to modify the ongoing movement or the planning for subsequent movements. Normally, this process is an effective mechanism to modify neural commands for ongoing movement or for movement planning. Animal models of the focal dystonias spasmodic torticollis, writer's cramp, and benign essential blepharospasm reveal different dysfunctions of sensorimotor integration through which dystonia can arise. Animal models of spasmodic torticollis demonstrate that modifications in a variety of regions are capable of creating abnormal head postures. These data indicate that disruption of neural signals in one structure may mutate the activity pattern of other elements of the neural circuits for movement. The animal model of writer's cramp demonstrates the importance of abnormal sensory processing in generating dystonic movements. Animal models of blepharospasm illustrate how disrupting motor adaptation can produce dystonia. Together, these models show mechanisms by which disruptions in sensorimotor integration can create dystonic movements.

The interaction between neuroactive steroids and the sigma1 receptor function: behavioral consequences and therapeutic opportunities

Steroids, synthesized in peripheral glands or centrally in the brain--the latter being named neurosteroids--exert an important role as modulators of the neuronal activity by interacting with different receptors or ion channels. In addition to the modulation of GABA(A), NMDA or cholinergic receptors, neuroactive steroids interact with an atypical intracellular receptor, the sigma(1) protein. This receptor has been cloned in several species, and highly selective synthetic ligands are available. At the cellular level, sigma1 agonists modulate intracellular calcium mobilization and extracellular calcium influx, NMDA-mediated responses, acetylcholine release, and alter monoaminergic systems. At the behavioral level, the sigma1 receptor is involved in learning and memory processes, the response to stress, depression, neuroprotection and pharmacodependence. Pregnenolone, dehydroepiandrosterone, and their sulfate esters behave as sigma1 agonists, while progesterone is a potent antagonist. This review will detail the physiopathological consequences of these interactions, focusing on recent results on memory and depression. The therapeutical interest of selective sigma1 receptor agonists in alleviating aging-related cognitive deficits will be discussed.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.

Relationship between modulation of the cerebellorubrospinal system in the in vitro turtle brain and changes in motor behavior in rats: effects of novel sigma ligands

Saturation and competition binding studies showed that the turtle brain contains sigma sites labeled by both [3H]di-o-tolylguanidine (DTG) and [3H](+)-pentazocine. There was a significant correlation between the IC50 values of sigma ligands for [3H]DTG sites in the turtle vs. rat brain, suggesting that the sites are comparable in the two species. In contrast, [3H](+)-pentazocine, which primarily labels sigma1 sites in the rodent brain, labels a heterogeneity of sites in the turtle brain. In extracellular recordings from the in vitro turtle brainstem, some sigma ligands enhanced the burst responses of red nucleus (RN) neurons (DTG, haloperidol, BD1031, BD1052, BD1069) while other sigma ligands decreased the burst responses (BD1047, BD1063). Control compounds (turtle Ringer vehicle control, opiate antagonist naloxone, atypical neuroleptic sulpiride) had no significant effects on the RN burst responses recorded from the in vitro turtle brain. The ED50s of the ligands for altering the burst responses in RN neurons from the turtle brain were correlated with their IC50s for turtle brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine; this pattern is identical to that previously reported in rats, where there is a correlation between the potencies of sigma ligands for producing dystonic postures after microinjection into the rat RN and their binding to rat brain sites labeled with [3H]DTG, but not [3H](+)-pentazocine. When the novel sigma ligands were microinjected into the rat RN, dystonic postures were produced by ligands that increased the burst duration of RN neurons in the turtle brain. Novel sigma ligands that reduced the burst responses in the in vitro turtle brain have previously been reported to have no effects on their own when microinjected into the rat RN, but to block the dystonic postures produced by other sigma ligands. Taken together, the data suggest that the opposite effects of the novel ligands in the turtle electrophysiological studies represent the actions of agonists vs. antagonists, and that the directionality of the effects has predictive value for the expected motor effects of the drugs.

The functional sensitisation of sigma receptors following chronic selective serotonin reuptake inhibitor treatment

The purpose of the present study was to investigate the potential impairment of normal motor function following chronic selective serotonin reuptake inhibitor treatment that may result from sensitisation of sigma receptors. Rats were chronically treated with either sertraline, citalopram, paroxetine or fluvoxamine and a selective sigma receptor ligand, di-o-tolylguanidine (DTG), for 28 days. All animals then received an acute intra-rubral injection of either DTG or saline. Following the direct injection of DTG into the red nucleus, rats chronically treated with DTG exhibit a maximal behavioural response characterised as a pronounced dystonia. Animals chronically treated with sertraline and citalopram elicited a response similar to that of control animals following the acute DTG challenge, whereas chronic treatment with paroxetine and fluvoxamine significantly decreased and increased the dystonic response, respectively. Facial spasticity and vacuous chewing movements were associated with, and reflected the extent of, the DTG-induced dystonia. Changes in regional biogenic amine concentrations were also determined. The concentrations of serotonin and noradrenaline were determined in the brain stem and cerebellum following the intra-rubral injection of either saline or DTG in animals that had been chronically treated with a selective serotonin reuptake inhibitor or DTG. There was a significant increase in serotonin concentration in the brain stem as a result of chronic DTG and fluvoxamine treatments. The increase in serotonin correlated with the reported potentiation of dystonia in animals that received 28 days treatment with these drugs. The potentiation of dystonia following chronic DTG and fluvoxamine treatments suggests that these drugs sensitise the sigma2 receptors, an effect that does not appear to be shared by citalopram, sertraline or paroxetine.

The electrical activity is impaired in the red nucleus of dt(sz) mutant hamsters with paroxysmal dystonia: an EEG power spectrum analysis of depth electrode recordings

The genetically dystonic (dt(sz)) hamster is an animal model of paroxysmal dystonia that displays attacks of sustained abnormal movements and postures in response to mild stress. Dysfunctions within the basal ganglia may be critically involved in the pathophysiology of dystonia in mutant hamsters. Furthermore, previous observations from autoradiographic studies pointed to an altered neural activity in the red nucleus (RN). In the present study, computerized EEG spectral analysis of depth electrode recordings from the RN was performed before and after dystonic attacks in freely moving dt(sz) hamsters and compared to age-matched non-dystonic controls. No epileptic activity was seen in any of the recordings, substantiating previous notions that paroxysmal dystonia in these mutants has no epileptogenic basis. The predominant EEG changes in RN of dystonic hamsters were a decrease in total power over the range of 1.25-42.00 Hz, a decrease in maximum power and a shift of frequency at maximum power to lower frequencies. With regard to selected frequency bands, there was a decrease in the alpha, beta and gamma band. Although the observed changes of neural activity in the RN are probably based on a primary dysfunction in related structures, the present data demonstrate its importance in the expression of dystonic movements.

Neuroleptic binding to sigma receptors: possible involvement in neuroleptic-induced acute dystonia

Several antipsychotic drugs, belonging to various chemical classes, were compared for their affinity for the sigma, dopamine-D2, and muscarinic receptors. Many neuroleptic drugs were found to bind with high affinity to sigma 2 receptors, and the binding affinity was clearly different from that observed for dopamine-D2 receptors. The dopaminergic and muscarinic theories for the physiopathology of acute dystonia are not completely satisfactory. Since the sigma receptors were reported to play a role in the control of movement, the high affinity of some neuroleptics for these sites suggests their possible involvement in some side effects, such as drug-induced dystonia. There was a correlation between the clinical incidence of neuroleptic-induced acute dystonia and binding affinity of drugs for the sigma receptor, except for some drugs, with a lower incidence, displaying significant affinity for the cholinergic muscarinic receptor. Therefore, we conclude that the affinity for the sigma receptor might be involved in neuroleptic-induced acute dystonia, but this might be partially corrected by the intrinsic anticholinergic properties of the drug.

Behavioural effects of selective serotonin reuptake inhibitors following direct micro injection into the left red nucleus of the rat

The behavioural effects of selective serotonin reuptake inhibitors (paroxetine, sertraline, citalopram, fluvoxamine, fluoxetine) and reference compounds (N,N'-di(o-tolyl)guanidine, haloperidol, 3-(3-hydroxyphenyl)-N-(l-propyl)piperidine and chlorpromazine) were studied for their ability to produce dystonia and torticollis following direct micro injection into the left red nucleus of the rat, an area of the brain containing a high density of sigma2 receptors but relatively devoid of biogenic amine receptors. Each animal was monitored for abnormalities in posture and movement for a period of 30 min and then sacrificed 40 min following drug administation. Only fluvoxamine (100 nmol) and fluoxetine (100 nmol) elicited acute dystonic behaviour (1-5 min). The onset of dystonia was accompanied by facial spasticity, vacuous chewing movements and grooming behaviour which reflected the extent of dystonia. The dystonic behaviour following the direct intrarubal injection of fluvoxamine and fluoxetine suggest the possible activation of sigma2 receptors while citalopram, sertraline and paroxetine were without effect. The results of this study support the role of sigma2 receptors in the regulation and control of movement and coordination and provides preliminary evidence to suggest the in vivo activity of sigma receptors by fluoxetine and fluvoxamine.

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

Effects of sigma ligands on rat cerebellar Purkinje neuron firing: an iontophoretic study

The electrophysiological responses of rat cerebellar Purkinje neurons to selective sigma ligands applied iontophoretically was examined in urethane anesthetized male Sprague-Dawley rats. 1,3-Di-o-tolylguanidine (DTG), dextrallorphan (DEX), (+)-pentazocine((+)-PENT), (+)-3-(3-Hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP), and the novel diamine BD1008, were ejected from multibarrel pipettes onto individual Purkinje cells. In some neurons, cell firing was inhibited following ejections of all compounds. These inhibitory effects were dose dependent and occurred without changes in spike amplitude or duration, thus ruling out local anesthetic effects as a mechanism. (+)-3-PPP and DEX increased firing rate in 27% and 14% (n = 15, n = 14, respectively) of cells studied. The results of this study indicate that sigma ligands significantly alter the spontaneous firing of Purkinje neurons, consistent with previous work suggesting motor effects of sigma ligands via the rubro-cerebellar circuitry.

Sigma binding parameters in developing rats predict behavioral efficacy of a sigma ligand

The relationship between sigma binding and the behavioral efficacy of a selective sigma ligand was examined in rats of varying ages (30, 45, 60, 75, 90, and 150 days old). Scatchard analyses of the binding of the sigma radioligand [3H]1,3-di-o-tolylguanidine ([3H]DTG) to brain membranes revealed significant age-related differences in binding to both crude synaptosomal and microsomal fractions. The functional significance of these developmental changes in sigma ligand binding was studied by determining the postural effects of rubral microinjections of DTG in age-matched littermates of rats used in the binding studies. The degree of dystonia produced by a single dose of DTG was significantly correlated with the amount of [3H]DTG bound to rat brain synaptosomal membranes at low but not at high concentrations. No significant correlation between binding to the microsomal fraction and drug efficacy was observed. These experimental results were in good agreement with predicted amounts bound as estimated from a Scatchard analysis of the data. The results suggest that sigma binding sites found in brain synaptosomal membranes are functional receptors involved in the control of movement and posture.

Iontophoretic effects of sigma ligands on rubral neurons in the rat

Iontophoretic application of the sigma ligands, 1,3-di-o-tolylguanidine (DTG), dextrallorphan, and (+)-pentazocine reliably inhibited the firing rate of rubral neurons. Dextrallorphan inhibited 87% of the neurons tested, DTG inhibited 76%, and (+)-pentazocine inhibited 50%. These inhibitions were current dependent and occurred without significant changes in spike amplitude or duration, suggesting that local anesthetic effects were not involved. In contrast to the other sigma ligands, iontophoretic application of (+)-3-PPP in the rat red nucleus resulted in very few inhibitions and tended to elicit weak excitations instead. Only 14% of rubral neurons were inhibited by (+)-3PPP, while 36% were excited. Although unusual, (+)-3-PPP has atypical effects when compared to other sigma ligands in numerous functional assays for sigma receptor activity. (+)-3-PPP, therefore, appears to have complex effects and may act through nonsigma mechanisms or through a different type of sigma binding site than the other compounds. The inhibition of firing rate produced by the more typical sigma ligands may contribute to the postural changes produced by microinjection of sigma ligands into the rat red nucleus.

Effects of 1,3-di-o-tolylguanidine (DTG), a sigma ligand, on local cerebral glucose utilization in rat brain

The 2-deoxy-D-[1-14C]glucose ([14C]DG) method was used to examine the effects of the relatively selective sigma ligand 1,3-di-o-tolylguanidine (DTG) on cerebral metabolism in freely moving rats. Each animal received an i.p. injection of DTG (0.2, 1, or 5 mg/kg) or normal saline 20 min prior to the infusion of [14C]DG. DTG induced dose-dependent changes in local cerebral glucose utilization (LCGU) in several motor and limbic structures. Most structures showed increases in LCGU, with a maximum effect at 1 mg/kg. The most profound increases in LCGU were observed in brain regions that are rich in sigma receptors. These included cerebellar and related nuclei (interpositus, lateral and medial cerebellar n., vestibular n., olivary n.), ambiguus n., superior colliculus (superior layers), hippocampus (CA2, CA3, DG), n. basalis of Meynert interpeduncular n., and the substantia nigra pars compacta and pars reticulata. No significant decreases in glucose utilization were observed at any dose. Although the areas affected by DTG are similar to those previously reported for other sigma ligands, future studies employing a range of doses for additional selective sigma ligands must be carried out in order to confirm whether these changes in LCGU were sigma-mediated.

The sigma ligand 1,3-di-o-tolylguanidine depresses amino acid-induced excitation non-selectively in rat brain

The sigma ligand 1,3-di-o-tolylguanidine (DTG) has been applied by microiontophoresis to neurones in the rat hippocampal slice and to neurones in the neocortex and hippocampus of rats anaesthetised with urethane. DTG depressed the excitatory responses of cells to both N-methyl-D-aspartate (NMDA) and quisqualate on a majority of the units tested, in no case causing an enhancement. Haloperidol had no consistent effect of its own and did not prevent the depressant effects of DTG. It is concluded that in the preparations used, DTG did not selectively modify neuronal sensitivity to NMDA.

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

The radioligand binding characteristics of [3H]haloperidol (in the presence of spiperone, 25 nmolL-1) were investigated in rat and human cerebellar membranes. In both rat and human cerebellar membrane preparations saturation studies with [3H]haloperidol (non-specific binding defined by pentazocine, 10 mumolsL-1) demonstrated high affinity saturable specific binding to a homogenous population of binding sites (rat, Bmax 6693 +/- 1242 fmol mg-1 protein, pKD 8.33 +/- 0.08; human, Bmax 2550 +/- 437 fmol mg-1 protein, pKD 8.59 +/- 0.11; mean +/- SEM, n = 3-6). Competition studies employing a wide range of structurally diverse competing compounds displayed that the [3H]haloperidol binding site was pharmacologically similar in both preparations and comparable to sigma recognition sites previously identified in various tissues originating from different species. In addition, with reference to the potential subtypes of sigma recognition sites, the labelling of these sites by low nanomolar concentrations of [3H]haloperidol provides evidence that they belong to the sigma-1 recognition site subtype. The present findings suggest that the pharmacology of the rat and human cerebellar sigma recognition site are directly comparable and provides further supporting evidence towards the use of [3H]haloperidol radioligand binding studies in the rat to detect sigma receptor ligands with potential therapeutic activity.

Electrophysiological effects of phencyclidine and the sigma agonist ditolylguanidine in the cerebellum of the rat

The electrophysiological actions of phencyclidine (PCP) and the sigma agonist 1,3-di(2tolyl)guanidine (DTG) were examined in the cerebellum of urethane-anesthetized rats. The object of the study was to determine if PCP and sigma agonists shared a common mechanism of action. The cerebellar Purkinje neuron was chosen because it has sigma receptors but not N-methyl-D-aspartate receptors, where PCP has additional effects. Both DTG and PCP decreased the spontaneous discharge rate of cerebellar Purkinje neurons after parenteral administration. When the drugs were applied locally to single Purkinje neurons, using pressure ejection through multibarrel micropipettes, both compounds decreased the spontaneous activity of the neurons with equal potency. Previous studies have shown that the actions of PCP in the cerebellum are dependent upon an interaction with noradrenergic terminals from the nucleus locus coeruleus. A similar finding was made in this study for DTG. Elimination of the noradrenergic input by lesion with the neurotoxin, 6-hydroxydopamine, diminished equally the effects of PCP and DTG. Treatment of the animals with haloperidol had similar effects. It is concluded that PCP and the sigma agonist DTG both act as indirect noradrenergic agonists in the cerebellum.

Inhibition of rubral neurons by noxious and non-noxious pressure

The role of the red nucleus (RN) in nociception was investigated in this study. Extracellular recordings from spontaneously active RN neurons were conducted in the rat while noxious pressure was delivered to the hindpaws or tail. Cells in the RN were predominantly inhibited by the stimuli. The units were most responsive when noxious pressure was applied to the contralateral hindpaw. Furthermore, more cells in the magnocellular division of the RN responded to the stimuli than cells in the parvocellular division. Delivery of a graded pressure stimulus to the contralateral hindpaw revealed 4 cell types in the RN: non-responsive cells; cells only responsive during the early, non-noxious portion of the stimulus; cells only responsive during the later, noxious portion of the stimulus; and cells that showed an initial response during the non-noxious part of the stimulus and a second, later response during the noxious portion of the stimulus. To further examine the putative role of the RN in nociception, oxotremorine, gamma-aminobutyric acid (GABA), serotonin, glutamate, and morphine were unilaterally microinjected into the RN and the responses of the animals in the tail flick test were assessed. Only morphine produced a significant antinociception in the animals following intrarubral microinjection. However, it is unclear whether this alteration was mediated through the RN because an antinociception of equal magnitude could be elicited from the reticular formation surrounding the RN and lesions of the RN did not alter the antinociception produced by systemic administration of morphine. Although other explanations cannot be ruled out, it appears that the RN may be involved in coordinating the motor response to pain rather than modulating sensory transmission.

Computer-assisted modeling of multiple dextromethorphan and sigma binding sites in guinea pig brain

Computer-assisted, simultaneous analysis of self- and cross-displacement experiments demonstrated the existence of several binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP), and 1,3-di-o-tolyl guanidine (DTG). Dextromethorphan binds with high affinity to two sites (R1 Kd 50-83 and R2 Kd 8-19 nM) and with low affinity to two additional sites (R3 and R4). (+)-3-PPP binds to one high-affinity (R1 Kd 24-36 nM), to one intermediate-affinity (R3 Kd 210-320 nM), and to two (R2 and R4) low-affinity sites. DTG binds with almost identical high affinity to two different sites (R1 Kd 22-24 and R3 Kd 13-16 nM). These results confirm that dextromethorphan, (+)-3-PPP, and DTG bind to the common DM1/sigma 1 site (R1). The binding of DTG to two different sites with identical affinities precludes the use of this compound as a specific marker for sigma receptors. Besides, haloperidol displaces labeled ligands from both high-affinity DTG sites (R1 and R3) with high affinity. Thus, haloperidol sensitivity should not be used as the single criterion to identify a putative receptor. The resolution of these novel sites also may provide new insights into the multiple effects of antipsychotic drugs. In addition, this investigation has important implications regarding the methods that must be applied to characterize multiple binding sites and their relations with putative receptors.

Interaction of sigma-compounds with receptor-stimulated phosphoinositide metabolism in the rat brain

Sigma-receptors are nonopioid, nondopaminergic receptors that bind with high affinity several antipsychotic drugs and appear to be involved in regulation of posture and movement. The second messenger system coupled to these receptors is still unknown. Recently, an inhibitory effect of various sigma-compounds on carbachol-stimulated phosphoinositide metabolism has been reported. We have investigated the effect of six sigma-compounds on carbachol- and norepinephrine-stimulated 3H-inositol phosphate accumulation in rat cerebral cortex slices. All compounds tested had a dose-dependent inhibitory effect on both systems, although their order of potency differed between neurotransmitters. Pentazocine and 1,3-di-o-tolylguanidine were the most potent inhibitors of carbachol-stimulated phosphoinositide turnover (IC50 = 31.5 and 45.7 microM, respectively), while haloperidol showed the greatest potency on the norepinephrine-coupled system (IC50 = 3.5 microM). In the presence of IC50 concentrations of these inhibitors, the dose-response curves for the agonists were shifted to the right and the EC50 values were significantly increased. Sigma-compounds also inhibited the binding of [3H]quinuclidinyl benzilate to muscarinic receptors and of [3H]prazosin to alpha 1-adrenoceptors in cortical membranes. In the presence of IC50 concentration (11 microM) of 1,3-di-o-tolylguanidine, no differences were found in the maximal number of muscarinic binding sites, whereas the dissociation constant increased approximately fivefold, indicating a decrease of the radioligand's affinity for the receptor. These results indicate that sigma-compounds, at micromolar concentrations, inhibit muscarinic and alpha 1-adrenergic receptor-coupled phosphoinositide metabolism, probably through an interaction with the neurotransmitter recognition sites.

A sigma-like binding site in rat pheochromocytoma (PC12) cells: decreased affinity for (+)-benzomorphans and lower molecular weight suggest a different sigma receptor form from that of guinea pig brain

Two highly selective radiolabeled probes for sigma receptors were found to bind with high affinity and capacity to membranes from undifferentiated PC12 cells. [3H]1,3-di-o-tolylguanidine [( 3H]DTG) bound with Kd = 23.7 +/- 4.6 nM and Bmax = 2025 +/- 660 fmol/mg protein. The Kd and Bmax for [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([3H](+)-3-PPP) were 86.3 +/- 21.6 nM and 1539 +/- 242 fmol/mg protein, respectively. These binding parameters were comparable to those observed in guinea pig brain, although the Kd for [3H](+)-3-PPP was 3-fold higher in the PC12 membranes. Both the PC12 and guinea pig brain sites exhibited high affinity for haloperidol, moderate affinity for phencyclidine (PCP), and negligible affinity for MK-801, apomorphine, and (-)-sulpiride. These data suggest a relationship of the PC12 site to sigma receptors. However, all (+)-opiates [+)-benzomorphans and (+)-morphinans) tested bound with markedly lower affinity to the PC12 site compared to guinea pig brain. These include (+)-N-allylnormetazocine [+)-SKF 10,047), (+)-pentazocine, and dextrallorphan. In fact, PC12 sites exhibited preference for (-)-benzomorphans, the reverse stereoselectivity of guinea pig brain sites. Binding of [3H]N-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) could not be detected, demonstrating absence of PCP receptors on this cell line. Differentiation of cells by treatment with nerve growth factor had no effect on sigma binding parameters. Membranes from guinea pig brain and PC12 cells were photoaffinity-labeled using [3H]azido-di-o-tolylguanidine. In guinea pig brain, a polypeptide of 25 kDa was specifically labeled. However, label was incorporated into polypeptides of 18 kDa and 21 kDa in membranes from PC12 cells. In view of the otherwise similar binding characteristics, the marked differences in affinity for (+)-benzomorphans and molecular weight suggest that PC12 cells contain a molecular form of sigma receptor distinct from that predominant in guinea pig brain. This raises the possibility of multiple sigma receptor types.

Metabolites of haloperidol display preferential activity at sigma receptors compared to dopamine D-2 receptors

Haloperidol bound with equal affinity to sigma and dopamine D-2 receptors (KI = 2.8 nM). Compared to haloperidol, its carbonyl-reduced metabolite bound to sigma receptors with nearly equal affinity. However, reduced haloperidol bound to dopamine receptors with 85-fold lower affinity compared to haloperidol (KI = 239 nM). The chlorophenyl-hydroxy-piperidine metabolite of haloperidol lacked affinity for dopamine receptors, but bound with moderate affinity to sigma receptors (KI = 326 nM). The carboxylic acid metabolite lacked affinity for either receptor. Like haloperidol, (+)-pentazocine, and 1,3-di-o-tolylguanidine, reduced haloperidol potently inhibited the phosphoinositide response to muscarinic agonists in rat brain synaptoneurosomes, an assay which monitors sigma agonist activity. This metabolite also produced a dystonic alteration of head position in rats when microinjected into the red nucleus. However, unlike observations with haloperidol and other sigma ligands, this effect was associated with pathological changes in the red nucleus. Therefore, it cannot be attributed to sigma receptor interactions with certainty. These findings suggest that administration of haloperidol results initially in effects mediated through both dopamine and sigma receptors, but as metabolism proceeds the sigma actions would be expected to decline at a significantly slower rate than the dopaminergic actions.

Age-related differences in the sensitivity of rats to a selective sigma ligand

Differences in sigma binding parameters and behavioral responses to injections of a selective sigma ligand were found in adult rats of different ages. Middle-aged rats (5-6 months old) had fewer sigma binding sites and sites with lower affinity for [3H]di-o-tolylguanidine than young adult animals (2-3 months old). The older animals also exhibited a decreased behavioral response to the selective sigma ligand, di-o-tolylguanidine (DTG). Unilateral microinjection of DTG into the substantia nigra of rats produced fewer net contralateral turns in middle-aged animals, compared to younger rats. Likewise, the postural changes produced by unilateral microinjection of DTG into the red nucleus were less pronounced in the older animals. These data suggest that changes in the number and affinity of sigma binding sites may affect movement and posture as an organism ages.

An examination of the putative sigma-receptor in the mouse isolated vas deferens

1. The effects of several ligands which interact with the sigma-binding site were studied on the electrically-evoked (0.1 Hz) neurogenic twitch contractions of the mouse isolated vas deferens. 2. (+)-3-(3-Hydroxyphenyl)-N-(1-propyl)piperidine [+)-3-PPP) (10(-8) - 10(-5) M), inhibited the neurogenic twitch contractions. This inhibitory action was unaffected by naloxone (10(-6)M), idazoxan (10(-6)M), cocaine (10(-6)M) or tyramine (10(-4)-3 x 10(-4)M), but was abolished by the dopamine D2-antagonist, sulpiride (10(-6)M). Therefore, in order to study the potentiating actions of sigma ligands, sulpiride (10(-6)M) was used to prevent any inhibitory actions mediated via dopamine D2-receptors. 3. In the presence of sulpiride (10(-6)M), haloperidol (10(-6)-10(-5)M), (+)-3-PPP (10(-6)-3 x 10(-4) M) and (+)-N-allyl-N-normetazocine [+)-SKF 10,047) (10(-5)-10(-4)M) each reversibly potentiated the neurogenic twitch contractions in a concentration-dependent manner. The rank order of potency was haloperidol greater than (+)-3-PPP greater than (+)-SKF 10,047. 4. The stereoisomers of 3-PPP displayed stereoselectivity with (+)-3-PPP being more potent than (-)-3-PPP. 5. At a concentration that did not potentiate the twitch contractions, (3 x 10(-7)M), haloperidol did not antagonize the potentiating action of (+)-3-PPP (3 x 10(-5)M). 6. 1,3-Di-O-tolyguanidine (DTG) (10(-8)-10(-5)M) had no effect on the amplitude of twitch contractions and did not affect the potentiating action of (+)-3-PPP (10(-5)-3 x 10(-5)M). 7. It is concluded that a-ligands potentiate neurogenic twitch contractions of the mouse isolated vas deferens via a site that is different from the central sigma-binding site.

Evidence for a multi-site model of the rat brain sigma receptor

Irradiation of rat brain membranes with light of 254 nm, a treatment which modifies ultra-violet absorbing residues in proteins, decreased binding of both [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([ 3H](+)-3-PPP) and [3H]1,3-di-o-tolylguanidine ([3H]DTG) to sigma receptors. For [3H](+)-3-PPP, this was due to a decreased Bmax. In contrast, irradiation markedly increased binding of [3H](+)-N-allylnormetazocine ([3H](+)-SKF 10,047) due to a decrease in the Kd. Both unlabeled DTG and haloperidol were competitive inhibitors of [3H](+)-3-PPP binding to untreated membranes, causing an increase in the Kd and no change in the Bmax. The benzomorphans, (+)-SKF 10,047 and (+)-pentazocine, were uncompetitive inhibitors, causing a decrease in both the Kd and Bmax for [3H](+)-3-PPP. Finally, the ability of DTG and (+)-3-PPP to inhibit binding of [3H](+)-SKF 10,047 was markedly reduced by ultra-violet irradiation, whereas irradiation had little effect on the potency of unlabeled (+)-SKF 10,047 and (+)-pentazocine. These data suggest that sigma-related (+)-benzomorphans and non-benzomorphans interact either with distinct, allosterically coupled sites on the same sigma receptor macromolecule or with different populations of sigma receptor types.

Motor effects of two sigma ligands mediated by nigrostriatal dopamine neurons

(+)-Pentazocine, a potent sigma ligand that lacks affinity for PCP receptors, produced dose-dependent contralateral circling behavior following microinjections in the substantia nigra of rats. This effect was attenuated by 6-hydroxydopamine (6-OHDA) lesions of ascending dopamine neurons and enhanced by systemic injections of amphetamine, 6-OHDA lesions also attenuated the circling produced by another selective sigma ligand, 1,3-di-o-tolylguanidine (DTG). These findings suggest that sigma receptors are involved in the neural control of movement and the regulation of the ascending dopamine system. Since all typical antipsychotic drugs tested bind to sigma receptors with Ki values less than 1 microM, these findings further suggest that sigma receptors may mediate some of the motor side effects of antipsychotic drug therapy.