Hey, fellow users. The past couple of days I have been writing up some information I am interested in concerning the role of glutamate in the effects of hallucinogens and the possible effects of mGluR2 antagonists as psychoactives. As always this has engendered a lot of research and me coming across a lot of things I didn't know before... Basically what I have below is a very rough draft of a summary of what I've come across and I was wondering if anyone could help me address some of the inadequacies and inaccuracies in my knowledge and explanation.
Things in red are points I've specifically noticed but ANY advice or additions would be greatly appreciated! Thanks in advance... and for the myriad of knowledge I've gained from bluelight so far.
COMPLEXITIES: WHAT MAKES HALLUCINOGENIC ACTION?
(below are the main sources of information I've referred to, which I intend on properly referencing later on)
http://www.sciencedirect.com/science/article/pii/S0896627307000281
http://www.nature.com/npp/journal/v28/n1/full/1300013a.html
http://www.bluelight.ru/vb/threads/425678-Hallucinogens-A-Common-Mechanism-of-Action
http://www.nature.com/npp/journal/v23/n5/full/1395544a.html
http://onlinelibrary.wiley.com/doi/10.1046/j.1471-4159.2000.0750889.x/full
http://en.wikipedia.org/wiki/5-HT2A_receptor#Distribution
http://www.iuphar-db.org/DATABASE/ObjectDisplayForward?objectId=290#Tissues
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2743172/
Distinguishing the Singularity of Psychedelics:
From the figure below, the distinguishing feature of the hallucinogens is egr-2 expression - whilst both LSD and lisuride induce c-fos expression, lisuride has no effect on egr-2. Increasing the concentration of LSD also increases the magnitude of the egr-2 response, whilst the c-fos response increases for an increased concentration of LSD and lisuride.
Cortical Fos is a proto-oncogene which is altered by phosphorylation. It is often used as a measurement as it is generally expressed after action potentials in a neuron, and is a consequence of glutamate receptor activation. Egr-2, or “early growth response 2”, is a transcriptional factor and is involved in the formation of myelin [More info needed]. Hallucinogenic drugs such as DOI have been shown to increase glutamate concentration in the prefrontal cortex; and the consequent increase in c-fos expression can be reversed by blocking glutamate receptors on the post-synaptic membrane. Relevant to our glutamate hypothesis is the fact Group II glutamate receptor agonists and positive allosteric modulators reduce the effect of DOI, as well as 5-HT2A antagonists (but not 5-HT2C). Use of an AMPA antagonist also blocked the c-fos response on administration of DOI. [Expand]
Whilst the induction of egr-2 is clearly a distinct feature of the hallucinogens, it is only a measurement of a difference in the regulation of pathways responsible for the unique behavioural effects of these compounds. This information from an earlier article by the same team asserts that psychedelic 2AR agonists have a different effect on G-protein signalling, shown in the below figure taken from the Neuroskeptic article:
Both the serotonergic and glutaminergic receptors are G-protein coupled. It is known that the 5-HT2A receptors (or 2AR) signal by Gαq, stimulating phospholipase C. Conversely, mGluR2 acts via Gαi, which inhibits adenylyl cyclase and affects calcium and potassium ion channels. Inhibiting PLC-β eliminates the gene response of both the psychoactive LSD and its inactive yet structurally very similar lisuride - which suggests that while this response is necessary, it is not a unique feature of hallucinogenic action. However, more interestingly, pre-treatment of cortical cultures with Peritussis toxin (PTX) greatly reduced the response of LSD but not that of lisuride. Gi/o proteins are PTX-sensitive, which suggests that the co-activation of both Gi/o and Gq/11 is specific to hallucinogenic action as a signalling pathway not initiated by non-hallucinogenic 2AR ligands. Since Gi/o has been suggested to activate Src by Gβγ subunits; this pathway was also investigated - and similarly the inhibition of the kinase Src by PP2 reduced the LSD response as to be indistinguishable from that of lisuride. Crucially inhibition of PI3 kinases also activated by Gβγ subunits had no effect on the response, indicating that the Src pathway [Expand] is unique to hallucinogenic action due to regulation of the Gi/o proteins. Since egr-2 is Gi/o dependent mGluR2 seems to be important for hallucinogen-specific signalling.
A Glutamatergic Mechanism:
Activation of mGluR2/3 suppresses the increase in EPSP/Cs [Expand on EPSPs and how they are affected by hallucinogens?] due to 5-HT2A receptors and administration of an antagonist increases both the frequency and amplitude of EPSCs due to serotonin. This suggests a regulatory role - that group II metabotropic glutamate receptors act as autoreceptors on terminals, so that 5-HT2A receptor activation induces glutamate release. In layer V pyramidal cells this potentially occurs via a presynaptic mechanism. However, since 2AR are predominately postsynaptically located a retrograde messenger may also be involved to produce this effect. This asynchronous [Explain?] glutamate release could take place by increasing residual Ca2+ in excitatory nerve terminals, activating synaptotagmin III - a calcium-binding protein that “may be involved in exocytosis of secretory vesicles through Ca2+ and phospholipid binding to the C2 domain or may serve as Ca2+ sensors in the process of vesicular trafficking and exocytosis”. Since Gαq acts via PLC pathway, the resultant IP3 would cause the release of Ca2+ from intracellular stores. However, an observed dependence on extracellular Ca2+ requires entry either by voltage-gated channels, or potentially reverse Na+/Ca2+ exchange (NCXr) due to increased intracellular Na+. [Consider alternative mechanisms?]
The similarities between the effects of many psychoactives may be explained by this glutamatergic pathway activated by drugs of different classes - and the differences explained by the different ways in which this pathway is induced, as well as the effects on serotonergic and other receptors. For example, application of phencyclidine (PCP) did not increase EPSCs in layer V pyramidal cells of the prefrontal cortex - i.e. it does not act in the same way that the 2AR agonist hallucinogens do to increase glutamate levels, suggesting a distinct mechanism by which NMDA antagonists produce this effect that could account for their differing subjective effects. A blockade of NMDA receptors is known to increase glutamate in the medial prefrontal cortex and result in an increase in the activation of AMPA receptors. [Examine other pathways, cycles etc.?]
The distribution of receptors in the brain is undoubtedly a key aspect of their effects. 5-HT2A is expressed “near most of the serotoninergic terminal rich areas, including neocortex (mainly prefrontal, parietal, and somatosensory cortex) and the olfactory tubercle. Especially high concentrations of this receptor on the apical dendrites of pyramidal cells in layer V of the cortex may modulate cognitive processes, by enhancing glutamate release followed by a complex range of interactions with the 5-HT1A, GABAA, adenosine A1, AMPA, mGluR2/3, mGlu5, and OX2 receptors.” Specifically, mGluR2 receptors are “localized in layer II and III of the frontal, visual and sensorimotor cortex, several limbic areas including the entorhinal cortex, hippocampus, and amygdala. Also within the basal ganglia including caudate, putamen and globus pallidus. Multiple regions of the thalamus, and the granular cell layer of the cerebellum.”
Obviously co-localisation of the two is key for the role of the 2AR-mGluR2 complex. Most 2AR-positive cells in the layer V mouse somatosensory cortex were also mGluR2-positive, as well as in cortical pyramidal neurons - where the primary effect of hallucinogens seems to take effect. The difference between HCs and NHCs seems to be that “only HCs stabilize a receptor conformation that recruits the signaling pathways responsible for their neurobehavioral effects.” This is supported by the fact that both categories of ligand activate overlapping genes - i.e. the common induction of c-fos, and is also consistent with the existence of multiple agonist states. The “standard model” posits that the efficacy of a ligand is based upon its ability to alter the position of equilibrium between two conformational states of the receptor; the active and the inactive. Therefore agonists have a higher affinity for the active (R*), inverse agonists higher for the inactive (R); and neutral antagonists have an equal affinity for both states, and so no effect on basal activity. This is known as the ternary complex model. However, evidence such as hallucinogenic responses suggests that receptors can adopt multiple active conformations (R*n), each of which preferentially activate different signalling pathways and have differences in potency and intrinsic activity.
[Not sure where to go from here and there are undoubtedly tonnes of things I've missed out!]
Things in red are points I've specifically noticed but ANY advice or additions would be greatly appreciated! Thanks in advance... and for the myriad of knowledge I've gained from bluelight so far.
COMPLEXITIES: WHAT MAKES HALLUCINOGENIC ACTION?
(below are the main sources of information I've referred to, which I intend on properly referencing later on)
http://www.sciencedirect.com/science/article/pii/S0896627307000281
http://www.nature.com/npp/journal/v28/n1/full/1300013a.html
http://www.bluelight.ru/vb/threads/425678-Hallucinogens-A-Common-Mechanism-of-Action
http://www.nature.com/npp/journal/v23/n5/full/1395544a.html
http://onlinelibrary.wiley.com/doi/10.1046/j.1471-4159.2000.0750889.x/full
http://en.wikipedia.org/wiki/5-HT2A_receptor#Distribution
http://www.iuphar-db.org/DATABASE/ObjectDisplayForward?objectId=290#Tissues
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2743172/
Distinguishing the Singularity of Psychedelics:
From the figure below, the distinguishing feature of the hallucinogens is egr-2 expression - whilst both LSD and lisuride induce c-fos expression, lisuride has no effect on egr-2. Increasing the concentration of LSD also increases the magnitude of the egr-2 response, whilst the c-fos response increases for an increased concentration of LSD and lisuride.
Cortical Fos is a proto-oncogene which is altered by phosphorylation. It is often used as a measurement as it is generally expressed after action potentials in a neuron, and is a consequence of glutamate receptor activation. Egr-2, or “early growth response 2”, is a transcriptional factor and is involved in the formation of myelin [More info needed]. Hallucinogenic drugs such as DOI have been shown to increase glutamate concentration in the prefrontal cortex; and the consequent increase in c-fos expression can be reversed by blocking glutamate receptors on the post-synaptic membrane. Relevant to our glutamate hypothesis is the fact Group II glutamate receptor agonists and positive allosteric modulators reduce the effect of DOI, as well as 5-HT2A antagonists (but not 5-HT2C). Use of an AMPA antagonist also blocked the c-fos response on administration of DOI. [Expand]
Whilst the induction of egr-2 is clearly a distinct feature of the hallucinogens, it is only a measurement of a difference in the regulation of pathways responsible for the unique behavioural effects of these compounds. This information from an earlier article by the same team asserts that psychedelic 2AR agonists have a different effect on G-protein signalling, shown in the below figure taken from the Neuroskeptic article:
Both the serotonergic and glutaminergic receptors are G-protein coupled. It is known that the 5-HT2A receptors (or 2AR) signal by Gαq, stimulating phospholipase C. Conversely, mGluR2 acts via Gαi, which inhibits adenylyl cyclase and affects calcium and potassium ion channels. Inhibiting PLC-β eliminates the gene response of both the psychoactive LSD and its inactive yet structurally very similar lisuride - which suggests that while this response is necessary, it is not a unique feature of hallucinogenic action. However, more interestingly, pre-treatment of cortical cultures with Peritussis toxin (PTX) greatly reduced the response of LSD but not that of lisuride. Gi/o proteins are PTX-sensitive, which suggests that the co-activation of both Gi/o and Gq/11 is specific to hallucinogenic action as a signalling pathway not initiated by non-hallucinogenic 2AR ligands. Since Gi/o has been suggested to activate Src by Gβγ subunits; this pathway was also investigated - and similarly the inhibition of the kinase Src by PP2 reduced the LSD response as to be indistinguishable from that of lisuride. Crucially inhibition of PI3 kinases also activated by Gβγ subunits had no effect on the response, indicating that the Src pathway [Expand] is unique to hallucinogenic action due to regulation of the Gi/o proteins. Since egr-2 is Gi/o dependent mGluR2 seems to be important for hallucinogen-specific signalling.
A Glutamatergic Mechanism:
Activation of mGluR2/3 suppresses the increase in EPSP/Cs [Expand on EPSPs and how they are affected by hallucinogens?] due to 5-HT2A receptors and administration of an antagonist increases both the frequency and amplitude of EPSCs due to serotonin. This suggests a regulatory role - that group II metabotropic glutamate receptors act as autoreceptors on terminals, so that 5-HT2A receptor activation induces glutamate release. In layer V pyramidal cells this potentially occurs via a presynaptic mechanism. However, since 2AR are predominately postsynaptically located a retrograde messenger may also be involved to produce this effect. This asynchronous [Explain?] glutamate release could take place by increasing residual Ca2+ in excitatory nerve terminals, activating synaptotagmin III - a calcium-binding protein that “may be involved in exocytosis of secretory vesicles through Ca2+ and phospholipid binding to the C2 domain or may serve as Ca2+ sensors in the process of vesicular trafficking and exocytosis”. Since Gαq acts via PLC pathway, the resultant IP3 would cause the release of Ca2+ from intracellular stores. However, an observed dependence on extracellular Ca2+ requires entry either by voltage-gated channels, or potentially reverse Na+/Ca2+ exchange (NCXr) due to increased intracellular Na+. [Consider alternative mechanisms?]
The similarities between the effects of many psychoactives may be explained by this glutamatergic pathway activated by drugs of different classes - and the differences explained by the different ways in which this pathway is induced, as well as the effects on serotonergic and other receptors. For example, application of phencyclidine (PCP) did not increase EPSCs in layer V pyramidal cells of the prefrontal cortex - i.e. it does not act in the same way that the 2AR agonist hallucinogens do to increase glutamate levels, suggesting a distinct mechanism by which NMDA antagonists produce this effect that could account for their differing subjective effects. A blockade of NMDA receptors is known to increase glutamate in the medial prefrontal cortex and result in an increase in the activation of AMPA receptors. [Examine other pathways, cycles etc.?]
The distribution of receptors in the brain is undoubtedly a key aspect of their effects. 5-HT2A is expressed “near most of the serotoninergic terminal rich areas, including neocortex (mainly prefrontal, parietal, and somatosensory cortex) and the olfactory tubercle. Especially high concentrations of this receptor on the apical dendrites of pyramidal cells in layer V of the cortex may modulate cognitive processes, by enhancing glutamate release followed by a complex range of interactions with the 5-HT1A, GABAA, adenosine A1, AMPA, mGluR2/3, mGlu5, and OX2 receptors.” Specifically, mGluR2 receptors are “localized in layer II and III of the frontal, visual and sensorimotor cortex, several limbic areas including the entorhinal cortex, hippocampus, and amygdala. Also within the basal ganglia including caudate, putamen and globus pallidus. Multiple regions of the thalamus, and the granular cell layer of the cerebellum.”
Obviously co-localisation of the two is key for the role of the 2AR-mGluR2 complex. Most 2AR-positive cells in the layer V mouse somatosensory cortex were also mGluR2-positive, as well as in cortical pyramidal neurons - where the primary effect of hallucinogens seems to take effect. The difference between HCs and NHCs seems to be that “only HCs stabilize a receptor conformation that recruits the signaling pathways responsible for their neurobehavioral effects.” This is supported by the fact that both categories of ligand activate overlapping genes - i.e. the common induction of c-fos, and is also consistent with the existence of multiple agonist states. The “standard model” posits that the efficacy of a ligand is based upon its ability to alter the position of equilibrium between two conformational states of the receptor; the active and the inactive. Therefore agonists have a higher affinity for the active (R*), inverse agonists higher for the inactive (R); and neutral antagonists have an equal affinity for both states, and so no effect on basal activity. This is known as the ternary complex model. However, evidence such as hallucinogenic responses suggests that receptors can adopt multiple active conformations (R*n), each of which preferentially activate different signalling pathways and have differences in potency and intrinsic activity.
[Not sure where to go from here and there are undoubtedly tonnes of things I've missed out!]
