Clinical implications
Schizophrenia
Following the discovery that some antipsychotic drugs bind to sigma receptors, the suggestion arose that sigma receptors may be involved in schizophrenia and in the mode of action of antipsychotic drugs. Support for this hypothesis arose from the observation that the density of sigma receptors was dramatically reduced in several brain regions of post-mortem brains from schizophrenic patients. Such changes appeared to be restricted to the sigma receptor and did not involve the NMDA receptor or the PCP receptor. Whether such findings implicate alterations in sigma receptor function in schizophrenia is uncertain as it is possible that the changes in the density of these receptors is a function of the duration of treatment with neuroleptics. Support for the possible involvement of sigma receptors in schizophrenia, and in the action of antipsychotic drugs comes from the observation that haloperidol had a high affinity for these receptors in rat brain. Furthermore rimcazole, a putative neuroleptic, was found to have a high affinity for sigma receptors with little action on dopamine receptors. Several other sigma-selective ligands were also developed as possible neuroleptics. Unfortunately, despite convincing pre-clinical data showing that many of the sigma-selective ligands were active in animal models predictive of antipsychotic activity, none proved to have efficacy in clinical trials. It would therefore seem that the sigma ligands so far developed are unlikely to become the novel neuroleptics of the future.
Movement disorders
The most common symptomatic dystonias result from the administration of neuroleptics and occur as acute dystonic reactions or as tardive dyskinesia. The dystonias are disorders that involve sustained, involuntary muscle contractions and abnormal posture which interferes with normal motor function. Dystonias can be focal, as in the case of torticollis in which the neck involuntarily rotates, or they may be progressive and generalized as in torsion dystonia in which the body slowly becomes contorted. Torsian dystonia is familial and recent studies have identified a defective gene which may be responsible. Acute dystonic reactions occurring following the administration of potent neuroleptics are reported primarily in young men and usually develop shortly after the start of therapy. By contrast, tardive dystonia occurs following chronic neuroleptic treatments; as with tardive dyskinesia, symptoms often begin after the abrupt withdrawal of the neuroleptic. Although less severe than acute dystonic reactions, tardive dystonia is frequently permanent and difficult to treat. Until recently, the cause of dystonia has been assumed to involve a dysfunction of the basal ganglia. However, it is now known that most patients with lesions of the basal ganglia show no evidence of dystonia while those patients with dystonia exhibit little biochemical or anatomical change in basal ganglia function. More recently, there is clinical evidence that dystonia is associated with lesions of the brainstem and the cerebellum. The cerebellum is closely linked to the red nucleus which contains a high density of sigma receptors but few dopamine, serotonin or glutamate receptors. The brainstem region is also implicated in the hereditary mutant mouse model of dystonia in which the symptoms are known to be associated with both brainstem and cerebellar lesions. The presence of sigma receptors in anatomical structures that control movement and posture provides indirect evidence for the link between sigma receptors and dystonia. Further support for the involvement of these receptors is provided by the effects induced by the direct administration of sigma ligands into the red nucleus of rats; the degree of dystonia produced is directly proportional to the affinity of the drug for the sigma receptors. Additional experimental support for the involvement of sigma receptors in idiopathic dystonias comes from studies on a strain of rats which can develop a lethal dystonia but which are free of any identifiable anatomical lesions. It would appear that the density of sigma receptors is dramatically reduced compared to their non-affected litter-mates.
Regarding neuroleptic-induced dystonias, it is well known that typical neuroleptics cause catalepsy in rats and movement disorders in man. By contrast, the atypical neuroleptics clozapine and sulpiride have a low propensity to cause movement disorders in man even though they have established antipsychotic effects. These atypical neuroleptics, unlike many of the typical neuroleptics, have a low affinity for sigma receptors which lends support to the hypothesis that the dystonias produced by typical neuroleptics are related to their affinity for sigma receptors in the brainstem–cerebellar region.
Neurodegenerative disorders
So far all the evidence implicating the neuroprotective action of sigma ligands has been based on animal models of stroke or neurodegeneration. Several sigma ligands such as igmesine (JO 1784), NPC26377, ifenprodil and eliprodil have been shown to protect gerbils against ischaemic insult resulting from the bilateral occlusion of the carotid arteries; this is a popular experimental model of stroke. Similarly, ifenprodil and eliprodil, which have high affinity for sigma receptors in rat brain, are effective in protecting the mouse against focal cerebral ischaemia when administered after the induction of ischaemia. It would appear that the neuroprotective action is due to modulation of the polyamine site on the NMDA-glutamate receptor. However, as sigma ligands such as DTG, 3-PPP and BM4 14802 (which lack affinity for the NMDA glutamate receptor) have no neuroprotective action in the mouse model of focal cerebral ischaemia, it is uncertain whether highly selective sigma ligands would be effective in focal ischae mia in man. In other experimental studies, the potent sigma ligand igmesine has been shown to potentiate the potassium-evoked release of acetylcholine from rat hippocampal slices in vitro, an effect which is blocked by haloperidol. This suggests that igmesine may act as a sigma-1 agonist and may facilitate memory formation. Further evidence for this possibility is provided by the anti-amnestic action of igmesine in scopolamine-treated rats. These experimental studies suggest that sigma ligands, particularly sigma-1 agonists, may have therapeutic potential in the treatment of stroke and possibly in facilitating memory formation in the aged brain. Only doubleblind clinical trials of drugs such as igmesine, which appear to be relatively devoid of peripheral organ toxicity, will determine whether the various animal models of memory deficit and neurodegeneration are really predictive of potential therapeutic activity.
Anxiety and depression
There is experimental evidence to show that representative drugs for most classes of antidepressants have a modest affinity for sigma-1 receptors in vitro. Some antidepressants, such as sertraline and the monoamine oxidase- A inhibitor clorgyline, are moderately potent ligands for their receptor site. However, more recent studies have indicated that the most important final common pathway for the action of antidepressants involves the modulation of the NMDA-glutamate receptor possibly via the sigma receptor. It therefore seems uncertain that potent and selective sigma ligands will form the basis of a new group of antidepressants. However, there is more convincing experimental evidence to suggest that sigma ligands could have anxiolytic or anti-stress activity. Thus igmesine and DTG have been shown to block environmentally induced stress or corticotrophin-releasing factor induced colonic activity in the rat. Recently there has been renewed interest in the clinical development of igmesine as an antidepressant. Other experimental studies have shown that selective sigma ligands such as Lu 28-178 are potent anxiolytics in rodent models of anxiety.
The future of sigma receptor ligands
Besides the obvious need to develop highly potent and selective drugs for the sigma-1 and sigma-2 receptor sites, knowledge of the precise structures of the sigma receptors is required in order to establish firmly their identity. The presence of sigma receptors in the brain, in the gastrointestinal tract and endocrine and immune systems suggests that there must be endogenous factors that act as agonists and antagonists for these receptors. To date the nature of these endogenous factors is unknown but there is experimental evidence to implicate some neuropeptides (such as neuropeptides- Y and PYY) and steroids such as progesterone and deoxycorticosterone as putative ligands. In addition to the need for more detailed experimental studies to characterize the cellular mechanism of action of the different types of sigma receptors it is also essential to broaden the clinical profile of these drugs. So far, attention has been almost exclusively directed at the action of relatively non-selective sigma ligands in the treatment of psychotic disorders. The experimental findings that sigma compounds may have putative neuroprotective and anxiolytic/anti-stress effects will hopefully encourage the further development of the highly selective sigma compounds for their therapeutic application.
Schizophrenia
Following the discovery that some antipsychotic drugs bind to sigma receptors, the suggestion arose that sigma receptors may be involved in schizophrenia and in the mode of action of antipsychotic drugs. Support for this hypothesis arose from the observation that the density of sigma receptors was dramatically reduced in several brain regions of post-mortem brains from schizophrenic patients. Such changes appeared to be restricted to the sigma receptor and did not involve the NMDA receptor or the PCP receptor. Whether such findings implicate alterations in sigma receptor function in schizophrenia is uncertain as it is possible that the changes in the density of these receptors is a function of the duration of treatment with neuroleptics. Support for the possible involvement of sigma receptors in schizophrenia, and in the action of antipsychotic drugs comes from the observation that haloperidol had a high affinity for these receptors in rat brain. Furthermore rimcazole, a putative neuroleptic, was found to have a high affinity for sigma receptors with little action on dopamine receptors. Several other sigma-selective ligands were also developed as possible neuroleptics. Unfortunately, despite convincing pre-clinical data showing that many of the sigma-selective ligands were active in animal models predictive of antipsychotic activity, none proved to have efficacy in clinical trials. It would therefore seem that the sigma ligands so far developed are unlikely to become the novel neuroleptics of the future.
Movement disorders
The most common symptomatic dystonias result from the administration of neuroleptics and occur as acute dystonic reactions or as tardive dyskinesia. The dystonias are disorders that involve sustained, involuntary muscle contractions and abnormal posture which interferes with normal motor function. Dystonias can be focal, as in the case of torticollis in which the neck involuntarily rotates, or they may be progressive and generalized as in torsion dystonia in which the body slowly becomes contorted. Torsian dystonia is familial and recent studies have identified a defective gene which may be responsible. Acute dystonic reactions occurring following the administration of potent neuroleptics are reported primarily in young men and usually develop shortly after the start of therapy. By contrast, tardive dystonia occurs following chronic neuroleptic treatments; as with tardive dyskinesia, symptoms often begin after the abrupt withdrawal of the neuroleptic. Although less severe than acute dystonic reactions, tardive dystonia is frequently permanent and difficult to treat. Until recently, the cause of dystonia has been assumed to involve a dysfunction of the basal ganglia. However, it is now known that most patients with lesions of the basal ganglia show no evidence of dystonia while those patients with dystonia exhibit little biochemical or anatomical change in basal ganglia function. More recently, there is clinical evidence that dystonia is associated with lesions of the brainstem and the cerebellum. The cerebellum is closely linked to the red nucleus which contains a high density of sigma receptors but few dopamine, serotonin or glutamate receptors. The brainstem region is also implicated in the hereditary mutant mouse model of dystonia in which the symptoms are known to be associated with both brainstem and cerebellar lesions. The presence of sigma receptors in anatomical structures that control movement and posture provides indirect evidence for the link between sigma receptors and dystonia. Further support for the involvement of these receptors is provided by the effects induced by the direct administration of sigma ligands into the red nucleus of rats; the degree of dystonia produced is directly proportional to the affinity of the drug for the sigma receptors. Additional experimental support for the involvement of sigma receptors in idiopathic dystonias comes from studies on a strain of rats which can develop a lethal dystonia but which are free of any identifiable anatomical lesions. It would appear that the density of sigma receptors is dramatically reduced compared to their non-affected litter-mates.
Regarding neuroleptic-induced dystonias, it is well known that typical neuroleptics cause catalepsy in rats and movement disorders in man. By contrast, the atypical neuroleptics clozapine and sulpiride have a low propensity to cause movement disorders in man even though they have established antipsychotic effects. These atypical neuroleptics, unlike many of the typical neuroleptics, have a low affinity for sigma receptors which lends support to the hypothesis that the dystonias produced by typical neuroleptics are related to their affinity for sigma receptors in the brainstem–cerebellar region.
Neurodegenerative disorders
So far all the evidence implicating the neuroprotective action of sigma ligands has been based on animal models of stroke or neurodegeneration. Several sigma ligands such as igmesine (JO 1784), NPC26377, ifenprodil and eliprodil have been shown to protect gerbils against ischaemic insult resulting from the bilateral occlusion of the carotid arteries; this is a popular experimental model of stroke. Similarly, ifenprodil and eliprodil, which have high affinity for sigma receptors in rat brain, are effective in protecting the mouse against focal cerebral ischaemia when administered after the induction of ischaemia. It would appear that the neuroprotective action is due to modulation of the polyamine site on the NMDA-glutamate receptor. However, as sigma ligands such as DTG, 3-PPP and BM4 14802 (which lack affinity for the NMDA glutamate receptor) have no neuroprotective action in the mouse model of focal cerebral ischaemia, it is uncertain whether highly selective sigma ligands would be effective in focal ischae mia in man. In other experimental studies, the potent sigma ligand igmesine has been shown to potentiate the potassium-evoked release of acetylcholine from rat hippocampal slices in vitro, an effect which is blocked by haloperidol. This suggests that igmesine may act as a sigma-1 agonist and may facilitate memory formation. Further evidence for this possibility is provided by the anti-amnestic action of igmesine in scopolamine-treated rats. These experimental studies suggest that sigma ligands, particularly sigma-1 agonists, may have therapeutic potential in the treatment of stroke and possibly in facilitating memory formation in the aged brain. Only doubleblind clinical trials of drugs such as igmesine, which appear to be relatively devoid of peripheral organ toxicity, will determine whether the various animal models of memory deficit and neurodegeneration are really predictive of potential therapeutic activity.
Anxiety and depression
There is experimental evidence to show that representative drugs for most classes of antidepressants have a modest affinity for sigma-1 receptors in vitro. Some antidepressants, such as sertraline and the monoamine oxidase- A inhibitor clorgyline, are moderately potent ligands for their receptor site. However, more recent studies have indicated that the most important final common pathway for the action of antidepressants involves the modulation of the NMDA-glutamate receptor possibly via the sigma receptor. It therefore seems uncertain that potent and selective sigma ligands will form the basis of a new group of antidepressants. However, there is more convincing experimental evidence to suggest that sigma ligands could have anxiolytic or anti-stress activity. Thus igmesine and DTG have been shown to block environmentally induced stress or corticotrophin-releasing factor induced colonic activity in the rat. Recently there has been renewed interest in the clinical development of igmesine as an antidepressant. Other experimental studies have shown that selective sigma ligands such as Lu 28-178 are potent anxiolytics in rodent models of anxiety.
The future of sigma receptor ligands
Besides the obvious need to develop highly potent and selective drugs for the sigma-1 and sigma-2 receptor sites, knowledge of the precise structures of the sigma receptors is required in order to establish firmly their identity. The presence of sigma receptors in the brain, in the gastrointestinal tract and endocrine and immune systems suggests that there must be endogenous factors that act as agonists and antagonists for these receptors. To date the nature of these endogenous factors is unknown but there is experimental evidence to implicate some neuropeptides (such as neuropeptides- Y and PYY) and steroids such as progesterone and deoxycorticosterone as putative ligands. In addition to the need for more detailed experimental studies to characterize the cellular mechanism of action of the different types of sigma receptors it is also essential to broaden the clinical profile of these drugs. So far, attention has been almost exclusively directed at the action of relatively non-selective sigma ligands in the treatment of psychotic disorders. The experimental findings that sigma compounds may have putative neuroprotective and anxiolytic/anti-stress effects will hopefully encourage the further development of the highly selective sigma compounds for their therapeutic application.
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