Hallucinogens induce a NMDA and highly diffusion dependent glutamate release.

[BilZ0r]

Neuropsychopharmacology. 2005 Nov 2;
Hallucinogen-Induced UP States in the Brain Slice of Rat Prefrontal Cortex: Role of Glutamate Spillover and NR2B-NMDA Receptors.

Lambe EK, Aghajanian GK.

Department of Psychiatry, Yale School of Medicine, New Haven, CT, USA.

Psychedelic hallucinogens (eg LSD or DOI) induce disturbances of mood, perception, and cognition through stimulation of serotonin 5-HT(2A) receptors. While these drugs are not proconvulsant, they have been shown by microdialysis to increase extracellular glutamate in the prefrontal cortex. Electrophysiological studies in the rat prefrontal slice have shown that both LSD and DOI enhance a prolonged, late wave of glutamate release onto layer V pyramidal neurons after an electrical stimulus. Here, we hypothesize that the network activity underlying this UP state involves glutamate spillover from excitatory synapses. To test this hypothesis, we raised the viscosity of the extracellular solution by adding the inert macromolecule dextran ( approximately 1 mM) that is known to retard glutamate overflow into the extrasynaptic space. Dextran suppressed the UP state or late excitatory postsynaptic current (EPSC), but neither the fast EPSC, the traditional polysynaptic EPSC, nor a synaptic form of 5-HT(2A)-mediated transmission (serotonin-induced spontaneous EPSCs). Consistent with the previous work showing that extrasynaptic glutamate transmission in adult depends on NR2B-containing NMDA receptors, we found that NR2B-selective antagonists, ifenprodil and Ro25-6981, also suppressed the late EPSCs. The effect of psychedelic hallucinogens on UP states could be partially mimicked by inhibiting glutamate uptake but only after blocking inhibitory group II metabotropic glutamate receptors. This difference suggests that hallucinogens increase glutamate spillover in a phasic manner unlike glutamate uptake inhibitors. Increases in glutamate spillover have been suggested to recruit synapses not directly in the pathway activated by the electrical stimulus. Such recruitment could account for certain cognitive, affective, and sensory perturbations generated by psychedelic hallucinogens.Neuropsychopharmacology advance online publication, 2 November 2005; doi:10.1038/sj.npp.1300944.

An interesting one, more of Aghajanains asynchronous transmitter release. He concludes from the data that DOI and LSD stimulate glutamate spill-over from the synapse, and somehow this is dependent on NR2B-containing NMDA recepors. I'm not sure if I buy it; because the only way I can see of potenting glutamate spill over is to either a) increase glutamate release or b) inhibit the glutamate transporters. It's not a) because the synchronous glutamate release doesn't get any bigger and I can't see it being b) because they can't mimic this effect with glutamate transporter inhibitors.

The NR2B story is interesting, because the NR2B is both a presynaptic NMDA receptor and an extrasynaptic receptor. The NMDA receptor isn't mediating the asynchronous event directly, because it's previously been shown to be blocked by AMPA... so somehow the NR2B receptor is what is causing the spill over...

I propose that extasynaptic presynaptic NR2B-subunit containing NMDA receptors are activated subsequent to DOI, these allow Ca2+ entery (during presynaptic depolarization) which then cause the release of glutamate from non-synaptic sites, which diffuses to the postsyanptic sites... this explains the time lag, and why it seem to be mediated by a whole different set of vesicular proteins.

 

[Jamshyd]

Excuse me if my proposal is too simplistic, but 5-HT antagonists have a general excitatory effect on the CNS. Perhaps the release of glutamate is not induced in a cause-and-effect manner, but rather through a "mirroring" of the anxious (to use the word loosely) psychological state created by 5-HT antagonism?

Perhaps using something like benzodiazepines while duplicating this experiment would help determine if the spillover is indeed caused as a direct response?

 

[BilZ0r]

Excuse me if my proposal is too simplistic, but 5-HT antagonists have a general excitatory effect on the CNS

I think that is a vast over simplification... I mean, I don't even know what you mean....

. Perhaps the release of glutamate is not induced in a cause-and-effect manner, but rather through a "mirroring" of the anxious (to use the word loosely) psychological state created by 5-HT antagonism?

Hallucinogens are 5-HT2A agonists, not antagonists... These experiments were conducted in slices of brain tissue about 300µm thick, not awake animals... there was no emotion.

 

[Jamshyd]

I guess I wasn't thorough in my reading the article. I assumed the experiments were done on live animals and then their brain tissue was examined.

Now that we're talking about the effects of psychoactive (ie. you need to be alive to feel/think) on dead animals, I would guess that this whole experiment is not too enlightening.

As for my "vast oversimplification", I admit I am not as familiar with the 5-HT system as other systems (and even then, I am still not too familiar), but my idea was based on the fact that LSD (or DOI) is what is traditionally used in experiments concerning Serotonin antagonism, serotonin mainly being an inhibitory transmitter.

Psychedelics, as far as I know, are at best mixed agonists/antagonists at different subtypes.

Feel free to correct me if I'm wrong on any of these accounts.

In any case, I apologize if my post came out as blundering ignorance. I still do hold the thought that testing pharmacology that deals with consciousness on dead body tissue is big stabbing in the dark.

 

[BilZ0r]

Although most hallucinogens are partial 5-HT2A agonists, it is because of their agonist effects are hallucinogenic.. some hallucinogens like 5-MeO-DMT and and DOB are very close full agonists at certain 2nd messanger pathways. And I wouldn't hold onto the idea that "Serotonin is an inhibitory transmitter". Different serotonin receptors can have either generally excitatory or generally inhibitory effects depending on where in the brain the are expressed. Thing about serotonin syndrome; it causes depression of conciousness, but hyper-reflexia and seizures.

Now that we're talking about the effects of psychoactive (ie. you need to be alive to feel/think) on dead animals, I would guess that this whole experiment is not too enlightening.

If you take that stance, then we don't know that hallucinogens are 5-HT2A agonists, we don't know what causes epilepsy, I mean, we really know nothing about the brain at all.

One just needs to sythesis the things that can only be learned in in vitro situations with things that can be only be found in in vivo situations. Unfortunately hallucinogenic experiences are very subjective, without a known physiological cause, so investigating them requires a novel approach. For example, there's the possibility that at least some of the increased glutamate release caused in the cortex is mediated by the 5-HT2A receptor mediated release of arachidonic acid, via the action of phospholipase A2 (PLA2). If that is true, and glutamate release mediates hallucinogens, the PLA2 inhibitors should block DOI-induced hallucinations. That hypothesis could only have been legitamately generated from in vitro data, however the last part can only be legitamitly tested in humans...

 

[Jamshyd]

Thanks for clarifying the part about Serotonin. I admit I had been over-generalizing.