The empirical evidence implicating naturally occurring substances which occur within the mammalian brain and which appear to produce their psychotropic effects by activating specific receptors within the brain. Such substances are termed endocoids and they include the enkephalins and endorphins, which activate specific opioid receptors, the anandamide related compounds, which activate cannabinoid receptors, the endopsychosins and related compounds that activate sigma receptors and natural agonists and antagonists that show an affinity for the benzodiazepine receptors. These different types of endocoids will be discussed in terms of their possible physiological effects.
Endogenous cannabinoids and cannabinoid receptors
The Chinese emperor Shen Nung is believed to have produced the first written account of the medicinal properties of cannabis over 2000 years ago and various formulations of herbal cannabis have been used over the centuries to treat seizures, neuralgia, dysmenorrhoea, insomnia and even gonorrhoea. The hemp plant, Cannabis sativa, from which cannabis and many of the related compounds are obtained, has a long history in medicine. Thus over the centuries the cannabinoids have been used for the treatment of pain, asthma, dysentery, as sedatives, for the suppression of nausea and vomiting and as anticonvulsants. Although the clinical uses of the cannabinoids declined in the 20th century there has been a renewed interest in these natural compounds in recent years for the control of spasticity associated with multiple sclerosis and in the treatment of chronic pain. Such renewed interest coincided with greater attention being paid by the medical profession and society at large to herbal remedies. Understanding the mec hanism of action of the cannabinoids has been advanced by the identification and cloning of specific cannabinoid receptors in the mammalian brain and spleen and the identification of endogenous substances which bind to these receptors. Thus the cannabinoid receptors in the brain are primarily of the CB1 type. These receptors are widely distributed in areas concerned with motor activity (basal ganglia and cerebellum), memory and cognition (cerebral cortex and hippocampus), emotion (amygdala and hippocampus), sensory perception (thalamus) and with endocrine function (hypothalamus and pons). The distribution of radio-labelled tetrahydrocannabinol, the main active ingredient of Cannabis sativa, is similar to the distribution of the CB1 receptors and there is good evidence that the cannabinoids exact their action through these receptors. In addition to the CB1 receptors, CB2 receptors have been identified on macrophages in the spleen where they probably mediate the immunological effects of the cannabinoids. CB1 receptors have also been detected in peripheral tissues.
The discovery of cannabinoid receptors has raised the possibility that therapeutic agents could be developed that may combine the therapeutic uses of the cannabinoids with lack of abuse and drug dependency. The first endogenous substances to be shown to have a high affinity for the cannabinoid receptors were the anandamides, named after the Sanskrit word for ‘‘bliss’’=ananda. Structurally the endogenous ligands for the cannabinoid receptors are unlike those of plant origin. The The system comprising the cannabinoid receptors and endogenous anandamide-related compounds is referred to as the anandamide system. However, it must be borne in mind that endogenous ligands for cannabinoid receptors may exist with properties that differ from those of the anandamide series of compounds. Endogenous parent compound is a derivative of the endogenous fatty acid arachidonic acid, arachidonyl ethanolamide. More recently, two other endogenous unsaturated fatty acid ethanolamides with a high affinity for cannabinoid receptors have been identified in brain tissue. These are homogamma-linolenylethanolamide and docotetraenylethanolamide. While there is convincing evidence that endogenous compounds exist in the mammalian brain that have properties which resemble those of tetrahydrocannabinol, the most potent cannabinoid from a plant source, the question arises regarding the need to postulate the existence of specific receptors for these natural ligands. After all, although opioid peptides have been isolated from brain extracts, the search for other receptor ligands, including those which bind to the benzodiazepine and sigma receptors, has not been nearly as successful. Nevertheless, due to the special nature of receptors which are coupled to G proteins, it is highly probable that there are natural ligands for all such receptors. This is because G proteins are single molecules that do not contain allosteric binding sites, unlike the benzodiazepine–GABA receptor where the benzodiazepine binding site is an allosteric regulatory site for GABA.
For all G protein-coupled receptors, every receptor has an endogenous ligand associated with its binding site. Thus it is reasonable to conclude that the binding sites for the anandamide system in the mammalian brain are true receptor sites through which the physiological changes initiated by the cannabinoids are expressed. Despite the recent advances in molecular biology, the mechanisms of action and the physiological functions of the anandamide system remain obscure. It would appear that the cannabinoid receptors and the anandamides reside within the neurons. Thus unlike the classical neurotransmitters noradrenaline and serotonin, the anandamides are not released into the synaptic cleft and are not involved in interneuronal communication. Instead the anandamides modulate the excitability and inhibitory responsiveness of neurons by acting on cannabinoid heteroceptors located on inhibitory and excitatory terminals. In this way, the cannabinoid receptors reduce the activity of these neurons by decreasing the i nflux of calcium through the calcium channels and increasing the efflux of potassium ions through the potassium channels located on the neuronal membrane. In some regions such as the cerebellum, there is a convergence of the G protein-linked receptors such as the GABA-B, adenosine A1, cannabinoid and kappa opioid receptors that inhibit the activity of adenylate cyclase thereby leading to a reduction in the release of glutamate. Thus it seems possible that the anandamide system modulates the activity of the major neurotransmitter systems including the opioid, prostenoid and glucocorticoid systems.
Sites of action of the cannabinoids
CB1 receptors are present in a high density in the hippocampus and cerebral cortex and the effects of cannabinoids on cognition and memory are undoubtedly related to their activation of the receptors in this brain region. These regions also mediate the effects of the cannabinoids on perception of time, sound, colour and taste. With regard to the motor effects, and effects on posture, of the cannabinoids it would appear that this is related to their agonist action on CB1 receptors located in the basal ganglia and cerebellum. Other central actions of the anandamide system include the hypothalamus (effect on body temperature), the spinal cord (antinociception) and the brain stem (suppression of nausea and vomiting). The discovery that cells of the immune system contain both cannabinoid binding sites and cannabinoid receptor mRNA suggests that the immunosuppressive actions of the naturally occurring cannabinoids are receptor mediated. There is now evidence that cannabinoid receptors occur on spleen cells in rodents and man and in human thymus cells and monocytes, but the receptor density is lower than that occurring in the brain. The B-lymphocytes have been shown to contain the highest quantity of cannabinoid receptor mRNA. The specific binding of cannabinoids to the small intestine and testis has also been reported to occur in different mammalian species. As the peripheral cannabinoid receptor appears to be of the CB2 type which appears to be absent from the brain, there have been attempts to develop selective agonists which would lack psychotropic properties but which would be of therapeutic value as immunosuppressants and in the control of such autoimmune diseases as rheumatoid arthritis. Conversely, CB2 receptor antagonists may act as drugs to enhance immune function. To date, no compounds have reached clinical application despite showing promising pharmacological profiles in the preclinical stages of their development. There is hope that a new approach in which analogues of the anandamides are developed will be more fruitful.
Physiological processes in that endogenous cannabinoids may be act as mediators
The possible physiological importance of the endogenous cannabinoids has largely been based on an extrapolation from the pharmacological properties of the THC-like compounds that are known for their psychotropic effects. Such drugs may differ in action from the endogenous cannabinoids because of their broad range of activity that follows the activation of both the CB1 and CB2 receptors, but also their ability to inhibit membrane bound enzymes and to cause a disruption of the normal function of the phospholipid compounds of neuronal and other membranes. Thus it would be anticipated that endogenous cannabinoids would show more selective actions both in the brain and periphery.
Tolerance is known to develop rapidly to many of the effects of the psychotropic cannabinoids but little is known regarding the mechanisms responsible for the development of tolerance to these drugs. One possibility to account for the development of tolerance is that compensatory decreases in the sensitivity or density of cannabinoid receptors occurs following the prolonged stimulation of these receptors, perhaps by inducing changes in the genetic expression of the receptor protein. This could occur as a result of a decrease in the signal transduction mechanism or in the affinity of the receptor sites for the cannabinoids. There are several in vitro and in vivo experimental studies in support of such mechanisms, but it is presently unproven whether such mechanisms apply to the components of the anandamide system.
Endozepines as endogenous anxiolytic and anxiogenic agents
It has been postulated that, at the cellular level, the symptoms of anxiety can arise because:
1. There is inadequate activity of an endogenous anxiolytic ligand.
2. There is excessive activity of an endogenous inverse agonist at the benzodiazepine receptor site.
3. There is a dysfunctional GABA-A receptor causing a shift in the GABAA complex towards inverse agonist activity.
It is uncertain which of these three possibilities apply to patients with anxiety disorders. There is evidence that the binding of the benzodiazepine receptor antagonist, flumazenil, is lower than normal in patients with panic disorder and that it increases the panic attack frequency in these patients but not in normal subjects. This has been interpreted as a slight shift in the benzodiazepine receptor towards the inverse agonist state.
Three types of endozapines have been isolated. It is known that the betacarbolines can be synthesized in the mammalian brain and that, in vitro, they act as inverse agonists at benzodiazepine receptor sites. Theoretically such compounds could induce anxiety. However, none of these compounds has been isolated in vivo and the original detection of a beta-carboline in the urine of anxious patients was later found to be an artifact, possibly caused by bacterial contamination. A diazepam binding inhibitor has been isolated from mammalian brain and found to be a mixture of two peptides (an octodecaneuropeptide and a trikontatetra neuropeptide) which stimulates neurosteroid synthesis by acting on peripheral benzodiazepine receptors. There are two main neurosteroids present in the mammalian brain which are antagonists of GABA-A receptors, namely dehydroepiandrosterone and its sulphate form (DHEA and DHEAS). These neurosteroids are also synthesized in the adrenal glands. These neurosteroids are known to have multiple effects of brain function by affecting mood, cognition and sleep; they also enhance neuronal plasticity and are neuroprotective. The third group of compounds are the naturally occurring benzodiazepines. Desmethyldiazepam has been isolated from human brains which were stored frozen in the 1930s, at least two decades before the benzodiazepines were developed. While there is no evidence that the benzodiazepine structure can be synthesized enzymatically in the mammalian brain, several other compounds of this type have since been isolated from cattle brain and from human breast milk. One possibility is that gastrointestinal flora can partially synthesize the benzodiazepine molecule and it is also known that plants such as wheat and potatoes are a potential source of diazepam, desmethyldiazepam and lormetazepam. If it is eventually shown that the local brain concentration of these benzodiazepines is sufficiently high to activate the benzodiazepine receptors then the possibility arises that anxiety disorders could result from a lack of these endozepines.
Several species of plant also contain compounds that have been shown to act as agonists on benzodiazepine receptors. These include: Valeriana officinalis which contains hydroxypinoresinol, Matricaria recutita which contains 5,7,4’-trihydroxyflavone, Passiflora coeruleus which contains chrysin and Karmelitter Geist which contains amentoflavin. Hypericum perforatum (St John’s Wort) also contains unknown compounds which have affinity for these receptors. Extracts of these drugs are commonly recommended by herbalists for the treatment of insomnia and anxiety.
Endogenous sleep factors
Early in the 20th century, Pierin in Paris infused the CSF of sleep-deprived dogs into normal dogs and showed that the CSF contained a sleep-inducing (somnogenic) factor. This was thought to be a muramyl peptide but later suggested to be the result of bacterial contamination as these peptides cannot be synthesized by the mammalian brain. Pro-inflammatory cytokines can also induce sleep, the effect depending on the concentration of the cytokine and the time of day. The effect on the sleep profile (increased non-REM and decreased REM sleep) appears to depend on the increased synthesis of prostaglandin D2 and nitric oxide which then alter the circadian rhythm. It is also known that some pro-inflammatory cytokines can affect the reuptake of 5-HT which plays an important role in regulating the sleep–wake profile. The endogenous fatty acid, oleamide, can cause sedation and induce sleep by activating cannabinoid receptors but also by potentiating the action of benzodiazepines on their receptor sites. Whether such action is of physiological relevance is presently unknown.
Function and therapeutic effects of sigma receptors
The sigma opiate receptor were originally proposed by the American neuropharmacologist William R. Martin as the site that mediates the psychotomimetic and stimulatory effects of cyclazocine, pentazocine, Nallyl normetazocine (SKF 10047) and related opiates in humans and dogs. However, there is now considerable evidence to suggest that these effects are not mediated by opioid receptors. Many of the opiates that have psychotomimetic properties also bind with a high affinity to phencyclidine (PCP) receptor sites situated in the channel of the N-methyl-D-aspartate (NMDA) receptor. It now appears from electrophysiological, biochemical, anatomical and molecular studies that there are two distinct sites that bind opioid analgesics that have an affinity for sigma receptors. One site is on the PCP receptor situated in the NMDA receptor. The other sigma site is defined as non-opioid, non-dopaminergic and shows a high affinity for haloperidol and N-allyl normetazocine. Using a highly selective ligand for sigma receptors such as ditolyguanidine (DTG), it has now been possible to separate sigma receptors into two major types. Sigma-1 receptors are the main neuronal type and exhibit a high affinity for centrally acting antitussive and anticonvulsant drugs. The other site has a low affinity for most sigma ligands except DTG and haloperidol. This site is found in the red nucleus and cerebellum (as well as many other brain regions) where it may mediate the motor (dystonic) effects of different types of sigma ligand. Biochemically the sigma-1 and sigma-2 receptors may also be distinguished by the nature of the second messenger to which they are attached. Thus the sigma-1 receptors appear to be linked to guanylyl nucleotide binding proteins (G proteins) whereas the sigma-2 sites are not and may bring about their physiological effects by modulating K+ channels.
Sigma receptors and psychosis
Some 20 years ago, Martin and coworkers proposed that the psychotomimetic effects of pentazocine and related opiate analgesics was due to their effect on sigma receptors. It is now known that the sigma receptors are quite distinct from PCP, opioid, serotonin and dopamine receptors. However, many psychotropic drugs that bind to dopamine, serotonin and PCP receptors also have a high affinity for sigma receptors. For example, haloperidol and the novel benzamide neuroleptic remoxipride bind with high affinity for both D2 and sigma receptors. Nevertheless, there are many potent neuroleptics that have a negligible affinity for sigma receptors and conversely, many sigma ligands that do not apparently have any neuroleptic activity, but it remains a possibility that there could be an involvement of sigma receptors in the pathology of schizophrenia. Thus receptor autoradiographic studies of post-mortem schizophrenic brain have demonstrated a significant reduction of sigma binding sites in the frontal cortex, amygdala and hippocampus without any significant change in the density of PCP binding sites. Therefore, the evidence linking a malfunctional sigma receptor system to schizophrenia, or the use of selective sigma receptor ligands as putative neuroleptics, is inconclusive.
Sigma receptors and the immune and endocrine systems
Experimental evidence suggests that sigma receptors play an important role in regulating and integrating both immune and endocrine functions. In experimental studies, it has been shown that the selective sigma ligand N-allyl-normetazocine stimulates the hypothalamic–pituitary–adrenal axis but suppresses luteinizing hormone and prolactin secretion. A high density of sigma receptors has been identified on human leucocytes and in the rat spleen, testis, ovary and adrenal gland. In human leucocytes it has also been shown that sigma receptors are involved in the second signalling mechanisms that are essential for cellular activation. In addition, sigma receptors have been identified on human and rat T and B cells. There is experimental evidence to show that the suppression of T cell replication, and enhanced activity of monocyte phagocytosis, that occurs in some rodent models of depression, can be effectively reversed by the chronic administration of selective sigma ligands such as igmesine. This suggests that such compounds may be of benefit in correcting the diverse immune and possibly endocrine defects that characterize depression.
Endogenous cannabinoids and cannabinoid receptors
The Chinese emperor Shen Nung is believed to have produced the first written account of the medicinal properties of cannabis over 2000 years ago and various formulations of herbal cannabis have been used over the centuries to treat seizures, neuralgia, dysmenorrhoea, insomnia and even gonorrhoea. The hemp plant, Cannabis sativa, from which cannabis and many of the related compounds are obtained, has a long history in medicine. Thus over the centuries the cannabinoids have been used for the treatment of pain, asthma, dysentery, as sedatives, for the suppression of nausea and vomiting and as anticonvulsants. Although the clinical uses of the cannabinoids declined in the 20th century there has been a renewed interest in these natural compounds in recent years for the control of spasticity associated with multiple sclerosis and in the treatment of chronic pain. Such renewed interest coincided with greater attention being paid by the medical profession and society at large to herbal remedies. Understanding the mec hanism of action of the cannabinoids has been advanced by the identification and cloning of specific cannabinoid receptors in the mammalian brain and spleen and the identification of endogenous substances which bind to these receptors. Thus the cannabinoid receptors in the brain are primarily of the CB1 type. These receptors are widely distributed in areas concerned with motor activity (basal ganglia and cerebellum), memory and cognition (cerebral cortex and hippocampus), emotion (amygdala and hippocampus), sensory perception (thalamus) and with endocrine function (hypothalamus and pons). The distribution of radio-labelled tetrahydrocannabinol, the main active ingredient of Cannabis sativa, is similar to the distribution of the CB1 receptors and there is good evidence that the cannabinoids exact their action through these receptors. In addition to the CB1 receptors, CB2 receptors have been identified on macrophages in the spleen where they probably mediate the immunological effects of the cannabinoids. CB1 receptors have also been detected in peripheral tissues.
The discovery of cannabinoid receptors has raised the possibility that therapeutic agents could be developed that may combine the therapeutic uses of the cannabinoids with lack of abuse and drug dependency. The first endogenous substances to be shown to have a high affinity for the cannabinoid receptors were the anandamides, named after the Sanskrit word for ‘‘bliss’’=ananda. Structurally the endogenous ligands for the cannabinoid receptors are unlike those of plant origin. The The system comprising the cannabinoid receptors and endogenous anandamide-related compounds is referred to as the anandamide system. However, it must be borne in mind that endogenous ligands for cannabinoid receptors may exist with properties that differ from those of the anandamide series of compounds. Endogenous parent compound is a derivative of the endogenous fatty acid arachidonic acid, arachidonyl ethanolamide. More recently, two other endogenous unsaturated fatty acid ethanolamides with a high affinity for cannabinoid receptors have been identified in brain tissue. These are homogamma-linolenylethanolamide and docotetraenylethanolamide. While there is convincing evidence that endogenous compounds exist in the mammalian brain that have properties which resemble those of tetrahydrocannabinol, the most potent cannabinoid from a plant source, the question arises regarding the need to postulate the existence of specific receptors for these natural ligands. After all, although opioid peptides have been isolated from brain extracts, the search for other receptor ligands, including those which bind to the benzodiazepine and sigma receptors, has not been nearly as successful. Nevertheless, due to the special nature of receptors which are coupled to G proteins, it is highly probable that there are natural ligands for all such receptors. This is because G proteins are single molecules that do not contain allosteric binding sites, unlike the benzodiazepine–GABA receptor where the benzodiazepine binding site is an allosteric regulatory site for GABA.
For all G protein-coupled receptors, every receptor has an endogenous ligand associated with its binding site. Thus it is reasonable to conclude that the binding sites for the anandamide system in the mammalian brain are true receptor sites through which the physiological changes initiated by the cannabinoids are expressed. Despite the recent advances in molecular biology, the mechanisms of action and the physiological functions of the anandamide system remain obscure. It would appear that the cannabinoid receptors and the anandamides reside within the neurons. Thus unlike the classical neurotransmitters noradrenaline and serotonin, the anandamides are not released into the synaptic cleft and are not involved in interneuronal communication. Instead the anandamides modulate the excitability and inhibitory responsiveness of neurons by acting on cannabinoid heteroceptors located on inhibitory and excitatory terminals. In this way, the cannabinoid receptors reduce the activity of these neurons by decreasing the i nflux of calcium through the calcium channels and increasing the efflux of potassium ions through the potassium channels located on the neuronal membrane. In some regions such as the cerebellum, there is a convergence of the G protein-linked receptors such as the GABA-B, adenosine A1, cannabinoid and kappa opioid receptors that inhibit the activity of adenylate cyclase thereby leading to a reduction in the release of glutamate. Thus it seems possible that the anandamide system modulates the activity of the major neurotransmitter systems including the opioid, prostenoid and glucocorticoid systems.
Sites of action of the cannabinoids
CB1 receptors are present in a high density in the hippocampus and cerebral cortex and the effects of cannabinoids on cognition and memory are undoubtedly related to their activation of the receptors in this brain region. These regions also mediate the effects of the cannabinoids on perception of time, sound, colour and taste. With regard to the motor effects, and effects on posture, of the cannabinoids it would appear that this is related to their agonist action on CB1 receptors located in the basal ganglia and cerebellum. Other central actions of the anandamide system include the hypothalamus (effect on body temperature), the spinal cord (antinociception) and the brain stem (suppression of nausea and vomiting). The discovery that cells of the immune system contain both cannabinoid binding sites and cannabinoid receptor mRNA suggests that the immunosuppressive actions of the naturally occurring cannabinoids are receptor mediated. There is now evidence that cannabinoid receptors occur on spleen cells in rodents and man and in human thymus cells and monocytes, but the receptor density is lower than that occurring in the brain. The B-lymphocytes have been shown to contain the highest quantity of cannabinoid receptor mRNA. The specific binding of cannabinoids to the small intestine and testis has also been reported to occur in different mammalian species. As the peripheral cannabinoid receptor appears to be of the CB2 type which appears to be absent from the brain, there have been attempts to develop selective agonists which would lack psychotropic properties but which would be of therapeutic value as immunosuppressants and in the control of such autoimmune diseases as rheumatoid arthritis. Conversely, CB2 receptor antagonists may act as drugs to enhance immune function. To date, no compounds have reached clinical application despite showing promising pharmacological profiles in the preclinical stages of their development. There is hope that a new approach in which analogues of the anandamides are developed will be more fruitful.
Physiological processes in that endogenous cannabinoids may be act as mediators
The possible physiological importance of the endogenous cannabinoids has largely been based on an extrapolation from the pharmacological properties of the THC-like compounds that are known for their psychotropic effects. Such drugs may differ in action from the endogenous cannabinoids because of their broad range of activity that follows the activation of both the CB1 and CB2 receptors, but also their ability to inhibit membrane bound enzymes and to cause a disruption of the normal function of the phospholipid compounds of neuronal and other membranes. Thus it would be anticipated that endogenous cannabinoids would show more selective actions both in the brain and periphery.
Tolerance is known to develop rapidly to many of the effects of the psychotropic cannabinoids but little is known regarding the mechanisms responsible for the development of tolerance to these drugs. One possibility to account for the development of tolerance is that compensatory decreases in the sensitivity or density of cannabinoid receptors occurs following the prolonged stimulation of these receptors, perhaps by inducing changes in the genetic expression of the receptor protein. This could occur as a result of a decrease in the signal transduction mechanism or in the affinity of the receptor sites for the cannabinoids. There are several in vitro and in vivo experimental studies in support of such mechanisms, but it is presently unproven whether such mechanisms apply to the components of the anandamide system.
Endozepines as endogenous anxiolytic and anxiogenic agents
It has been postulated that, at the cellular level, the symptoms of anxiety can arise because:
1. There is inadequate activity of an endogenous anxiolytic ligand.
2. There is excessive activity of an endogenous inverse agonist at the benzodiazepine receptor site.
3. There is a dysfunctional GABA-A receptor causing a shift in the GABAA complex towards inverse agonist activity.
It is uncertain which of these three possibilities apply to patients with anxiety disorders. There is evidence that the binding of the benzodiazepine receptor antagonist, flumazenil, is lower than normal in patients with panic disorder and that it increases the panic attack frequency in these patients but not in normal subjects. This has been interpreted as a slight shift in the benzodiazepine receptor towards the inverse agonist state.
Three types of endozapines have been isolated. It is known that the betacarbolines can be synthesized in the mammalian brain and that, in vitro, they act as inverse agonists at benzodiazepine receptor sites. Theoretically such compounds could induce anxiety. However, none of these compounds has been isolated in vivo and the original detection of a beta-carboline in the urine of anxious patients was later found to be an artifact, possibly caused by bacterial contamination. A diazepam binding inhibitor has been isolated from mammalian brain and found to be a mixture of two peptides (an octodecaneuropeptide and a trikontatetra neuropeptide) which stimulates neurosteroid synthesis by acting on peripheral benzodiazepine receptors. There are two main neurosteroids present in the mammalian brain which are antagonists of GABA-A receptors, namely dehydroepiandrosterone and its sulphate form (DHEA and DHEAS). These neurosteroids are also synthesized in the adrenal glands. These neurosteroids are known to have multiple effects of brain function by affecting mood, cognition and sleep; they also enhance neuronal plasticity and are neuroprotective. The third group of compounds are the naturally occurring benzodiazepines. Desmethyldiazepam has been isolated from human brains which were stored frozen in the 1930s, at least two decades before the benzodiazepines were developed. While there is no evidence that the benzodiazepine structure can be synthesized enzymatically in the mammalian brain, several other compounds of this type have since been isolated from cattle brain and from human breast milk. One possibility is that gastrointestinal flora can partially synthesize the benzodiazepine molecule and it is also known that plants such as wheat and potatoes are a potential source of diazepam, desmethyldiazepam and lormetazepam. If it is eventually shown that the local brain concentration of these benzodiazepines is sufficiently high to activate the benzodiazepine receptors then the possibility arises that anxiety disorders could result from a lack of these endozepines.
Several species of plant also contain compounds that have been shown to act as agonists on benzodiazepine receptors. These include: Valeriana officinalis which contains hydroxypinoresinol, Matricaria recutita which contains 5,7,4’-trihydroxyflavone, Passiflora coeruleus which contains chrysin and Karmelitter Geist which contains amentoflavin. Hypericum perforatum (St John’s Wort) also contains unknown compounds which have affinity for these receptors. Extracts of these drugs are commonly recommended by herbalists for the treatment of insomnia and anxiety.
Endogenous sleep factors
Early in the 20th century, Pierin in Paris infused the CSF of sleep-deprived dogs into normal dogs and showed that the CSF contained a sleep-inducing (somnogenic) factor. This was thought to be a muramyl peptide but later suggested to be the result of bacterial contamination as these peptides cannot be synthesized by the mammalian brain. Pro-inflammatory cytokines can also induce sleep, the effect depending on the concentration of the cytokine and the time of day. The effect on the sleep profile (increased non-REM and decreased REM sleep) appears to depend on the increased synthesis of prostaglandin D2 and nitric oxide which then alter the circadian rhythm. It is also known that some pro-inflammatory cytokines can affect the reuptake of 5-HT which plays an important role in regulating the sleep–wake profile. The endogenous fatty acid, oleamide, can cause sedation and induce sleep by activating cannabinoid receptors but also by potentiating the action of benzodiazepines on their receptor sites. Whether such action is of physiological relevance is presently unknown.
Function and therapeutic effects of sigma receptors
The sigma opiate receptor were originally proposed by the American neuropharmacologist William R. Martin as the site that mediates the psychotomimetic and stimulatory effects of cyclazocine, pentazocine, Nallyl normetazocine (SKF 10047) and related opiates in humans and dogs. However, there is now considerable evidence to suggest that these effects are not mediated by opioid receptors. Many of the opiates that have psychotomimetic properties also bind with a high affinity to phencyclidine (PCP) receptor sites situated in the channel of the N-methyl-D-aspartate (NMDA) receptor. It now appears from electrophysiological, biochemical, anatomical and molecular studies that there are two distinct sites that bind opioid analgesics that have an affinity for sigma receptors. One site is on the PCP receptor situated in the NMDA receptor. The other sigma site is defined as non-opioid, non-dopaminergic and shows a high affinity for haloperidol and N-allyl normetazocine. Using a highly selective ligand for sigma receptors such as ditolyguanidine (DTG), it has now been possible to separate sigma receptors into two major types. Sigma-1 receptors are the main neuronal type and exhibit a high affinity for centrally acting antitussive and anticonvulsant drugs. The other site has a low affinity for most sigma ligands except DTG and haloperidol. This site is found in the red nucleus and cerebellum (as well as many other brain regions) where it may mediate the motor (dystonic) effects of different types of sigma ligand. Biochemically the sigma-1 and sigma-2 receptors may also be distinguished by the nature of the second messenger to which they are attached. Thus the sigma-1 receptors appear to be linked to guanylyl nucleotide binding proteins (G proteins) whereas the sigma-2 sites are not and may bring about their physiological effects by modulating K+ channels.
Sigma receptors and psychosis
Some 20 years ago, Martin and coworkers proposed that the psychotomimetic effects of pentazocine and related opiate analgesics was due to their effect on sigma receptors. It is now known that the sigma receptors are quite distinct from PCP, opioid, serotonin and dopamine receptors. However, many psychotropic drugs that bind to dopamine, serotonin and PCP receptors also have a high affinity for sigma receptors. For example, haloperidol and the novel benzamide neuroleptic remoxipride bind with high affinity for both D2 and sigma receptors. Nevertheless, there are many potent neuroleptics that have a negligible affinity for sigma receptors and conversely, many sigma ligands that do not apparently have any neuroleptic activity, but it remains a possibility that there could be an involvement of sigma receptors in the pathology of schizophrenia. Thus receptor autoradiographic studies of post-mortem schizophrenic brain have demonstrated a significant reduction of sigma binding sites in the frontal cortex, amygdala and hippocampus without any significant change in the density of PCP binding sites. Therefore, the evidence linking a malfunctional sigma receptor system to schizophrenia, or the use of selective sigma receptor ligands as putative neuroleptics, is inconclusive.
Sigma receptors and the immune and endocrine systems
Experimental evidence suggests that sigma receptors play an important role in regulating and integrating both immune and endocrine functions. In experimental studies, it has been shown that the selective sigma ligand N-allyl-normetazocine stimulates the hypothalamic–pituitary–adrenal axis but suppresses luteinizing hormone and prolactin secretion. A high density of sigma receptors has been identified on human leucocytes and in the rat spleen, testis, ovary and adrenal gland. In human leucocytes it has also been shown that sigma receptors are involved in the second signalling mechanisms that are essential for cellular activation. In addition, sigma receptors have been identified on human and rat T and B cells. There is experimental evidence to show that the suppression of T cell replication, and enhanced activity of monocyte phagocytosis, that occurs in some rodent models of depression, can be effectively reversed by the chronic administration of selective sigma ligands such as igmesine. This suggests that such compounds may be of benefit in correcting the diverse immune and possibly endocrine defects that characterize depression.
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