Coluracetam’s Pharmacological Mechanisms: A Perspective Piece

Reference:

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IUPAC: N-(2,3-dimethyl-5,6,7,8- tetrahydrofuro[2,3-b] quinolin-4-yl)-2- (2-oxopyrrolidin-1-yl)acetamide

Molecular Formula: C 19 H 23 N 3 O 3

Aliases: BCI-540; MKC-231

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Background:

Coluracetam, initially synthesized in Japan, was first introduced to the scientific research community in 1993 as a choline uptake enhancer that selectively affected memory-related mechanisms23. Coluracetam’s mechanisms of action and specific behavioral effects were investigated by the Mitsubishi Tanabe Pharma Corporation under the guise of its suitability for the treatment of Alzheimer’s disease 2–4. Coluracetam’s license was repurposed in 2009 when Brain Cells, Inc. began a 90 patient phase II clinical trial to examine effects on depression and anxiety24. Seeing as its purported rehabilitative effects are postulated to extend to an enhancement of baseline operations, Coluracetam has also gained traction within the online nootropic community 15,22.

Observable Behavioral Effects in Rodents:

In rats with a laboratory induced memory deficit by way of injection of ethylcholine mustard aziridinium ion (AF64A—a selective neurotoxin that elicits degeneration of cholinergic neurons 4 – which massively innervate the hippocampus 5 ,6), the oral administration of Coluracetam (.3ml, 1mg, and 3mg per 10kg body weight once daily for 12 days preceding the test) ameliorated the memory deficit in a delayed non-match to sample task—a classic measure of hippocampal dependent recognition memory 13, compared to control mice injected with saline 9. Results using similar procedures show similar amelioration of memory deficits up to 24hrs after administration in regards to performance on Morris water maze and radial-arm maze tasks 1 – a classic behavioral procedure for studying spatial learning 12. No significant side effects are observed in the rodents at these effective doses 11.

Observable Behavioral Effects in Humans

Taken orally three times daily for six weeks in dosages of 80mg, Coluracetam significantly lowered the severity of self-reported depression in patients with co-morbid major depressive disorder and general anxiety disorder who were previously deemed unresponsive to an average of two antidepressants [12.2 points lower on Hamilton Rating Scale for Depression compared to 5.5 points in placebo group; N=101; p< .0008]20. This effect was selective to individuals with co-morbidity; there was no difference between the overall treatment group and placebo.

Observable Nuerochemical Effects in Rodents:

In scenarios where AF64A selectively decreased hippocampal ACh content, Coluracetam significantly reversed the depletion induced by AF64A at doses of .3mg/kg and 1mg/kg 9. Additionally, 3mg/kg of Coluracetam increased ACh concentration in perfusate in hippocampal slices by 263% compared to the AF64A deficit 2. An extension of this finding showed that the increase in ACh induced by a single administration of Coluracetam revealed no Coluracetam in the brain 3 hours after dosing 2.

Researchers have also observed other neurochemical correlates of Coluracetam ingestion. For example, decreased High affinity choline uptake (HACU) concentration and high Potassium stimulation-induced Acetylcholine (ACh) release (but not basal ACh release) in hippocampal synaptosomes after AF64A administration are reversed to near-pre-deficit levels following the administration of Coluracetam. However, this increase in HACU does not expand to non-AF64A administered rats 2. Furthermore, Coluracetam significantly reduced the decline in repeated depolarization-induced release of ACh in AF64A-treated rats and increased the extracellular ACh basal concentration in the hippocampus of AF64A treated rats.

Interestingly, C-labeled Coluracetam is not detected by radioactivity measurements in the brain 24 hours following a single-day, 7-day, or 14-day administration of 1mg/kg or 3mg/kg 1. However, memory-deficit-reversal benefits are still witnessed during this time. Contrastingly, there is an increase in high-affinity choline uptake (HACU) that follows the behavioral benefits, even in the absence of Coluracetam. Specifically, AF64A administration decreased HACU in the hippocampal synaptosomes to 40-60% levels compared to control rats; Coluracetam started improving this decreased HACU for up to 3 hours after a single-dose administration of 1mg/kg and 3mg/kg. After 8-days of repeated 1mg/kg administration of Coluracetam, HACU in the hippocampal synaptosomes was increased, compared to the AF64A deficit, for up to 24 hours after the last dose. This effected lasted for 48hrs when the dosage of Coluracetam was 3mg/kg during an 8-day administration1. Furthermore, Coluracetam significantly increased high affinity choline uptake (HACU) when it was incubated with the hippocampal synaptosomes of AF64A treated rats, but not of normal rats 11

Mechanisms of Action:

The observation that dosage scales with behavioral effects (e.g induced reversal of AF64A induced working memory deficits), suggests a probable neurochemical correlate for Coluracetam. However, the absence of Coluracetam during the persistence of behavioral enhancements suggests that Coluracetam sets into motion a cascade of longer-lasting neurochemical effects beyond simply playing an agonistic role.

Seeing as the majority of the research that examined the neurochemical effects of Coluracetam was done in the context of rodents altered by way of AF64A to serve as models for Alzheimer’s Disease (AD), it is useful to recognize the mechanisms that make up the altered system. A hallmark of AD is the depletion of cholinergic-related processes that are intimately tied to hippocampal circuitry. The degree of deficit in cholinergic markers (HACU, choline acetyltransferase(ChAT) activity, ACh synethsis, ACh release) has been shown to be most closely correlated with the severity of cognitive imparment in senile dementia and Alzheimer’s disease21, 7 ,8. AF64A can selectively degenerate cholinergic-related processes and, in turn, operationally produce AD-like symptoms. More specifically, AF64A has been shown to inhibit HACU, reduce choline acetyltransferase activity, lower the release and content of ACh in the hippocampus of mice 10,11, and decrease binding of choline transporters 16. Furthermore, in hippocampal slices of AF64A-treated rats, depolarization-induced ACh release is decreased with repetition of stimulations 14. Thus, for all intents and purposes, AF64A-treated animals are considered to model AD in concerns to its largest correlate: diminished ACh related activity.

Coluracetam has been shown to facilitate ACh synthesis in vitro and increase ACh concentration in in-vivo microdyalisis in AF64A-treated rats 2. However, as with any neurotransmitter modulation, this observed increase in ACh can be accomplished in a vast array of ways. For example, Coluracetam could, itself, serve as a cofactor for ACh—this is ruled out, however, due to its absence despite presence of behavioral benefits and increased ACh; it could serve as a reuptake inhibitor—however, Coluracetam did not affect AChE activity 14, which would normally hydrolyze acetylcholine; or it could serve as an acceleration factor for the enzymatic processes who’s downstream results yield a higher presence of ACh. This latter route is most likely the mechanism of action by which Coluracetam operates.

HACU is the system in which choline, an ACh substrate, is up taken from the synapse and utilized for the manufacturing of ACh by way of ChAT. HACU at the presynaptic cholinergic terminal is considered to be a rate-limiting step in the ACh synthesis process because the system’s overall efficiency (velocity) is subject to a variety of environmental factors including, but not limited to, temperature and Choline availability. The speed with which HACU transports choline has correlated with the activity of cholinergic neurons 17. It has been observed that the maximal velocity (V max ) of HACU is decreased in the presence of AF64A-treated rats. This V max is restored in the presence of Coluracetam. However, there was no significant change in the Michaelis-Menten constant (K m ). Seeing as K m is the rate of enzymatic reactions in accordance with the concentration of the substrate, it appears as though Coluracetam is particularly increasing the efficiency of HACU, not just increasing the presence of the substrates necessary for the enzymatic process.

CHT1, a high-affinity choline transporter, has been associated with the up-regulation of HACU 18. CHT1 has been shown to increase its presence in the synaptic membrane during times of low HACU by its release from the cytoplasmic compartment 16. Specifically, CHT1 has been shown to bind to vesicles containing ACh in the presynaptic neuron move into the synaptic membrane when the vesicle is exocytosed 18. CHT1 is increased in synaptic membranes following in-vivo administration of Coluracetam 3. Researchers posit that the increase in choline transporters like CHT1 on the plasma membrane which leads to the rapid availability of choline for ChAT and overall velocity of the HACU system is based on the Coluracetam’s modulation of the vesicular trafficking system of CHT1. Since Coluracetam has been shown to have a direct affinity for CHT1 3, the two’s interaction may increase the ability for CHT1 to be released from the cytoplasm and onto the plasma membrane, where it can carry out its reuptake role. Another theory is that Coluracetam may interfere with the internalization step of CHT1 from the surface of the synaptic membrane to cystolic pool, thus allowing CHT1 to continually aid in choline transportation. Further evidence of this is shown by Coluracetam’s interaction with cystolic anchor proteins that negatively regulate membrane surface expression of CHT1, which increases in AD 19.

Summary

Given that Coluracetam alters ACh availability in the synapse, but only after stimulation induced ACh efflux, it follows that a system responsible for the recycling of that released ACh is the primary mechanism of action by which Coluracetam acts. HACU is that recycling system. For, after ACh is broken down into Choline by AChE in the synapse, the choline needs to reach the cytoplasmic ChAT in order to be synthesized into ACh again. HACU is the “rate-limiting-factor” of ACh production as measured by the system’s overall ability to make choline available for ACh synthesis. Increased CHT1 is responsible for the enhancement of this HACU system by way of accelerating choline transportation back into the presynaptic neuron and, subsequently, increasing the rapidness of available ACh (since ChAT velocity is affected by choline availability). Coluracetam is thought to aid either CHT1’s vesicular-bound release into the synapse, CHT1’s reuptake prevention, or CHT1s’s anchoring stability on the plasma membrane. Coluracetam’s increase of CHT1’s functional availability allows for CHT1 to more adequately perform its task whose downstream result is an increase in density and probability of synaptic ACh concentration. Since cholinergic neuron projection is most prominent in the hippocampus, it is no surprise that Coluracetam ameloriates the working memory deficits imposed by AF64A treatment. As for why, in humans, administration of Coluracetam alleviates anxiety and depression when the two are co-morbid, a clear correlate is unclear. The benefit could be related to the role of a healthy hippocampus in modulating the Default Mode Network25, a network known to be involved in internal trains of thought26.

Author Comment

Why are all the results only seen in the presence of a deficit (AF64-A administration)?; In the highlighted studies used to create this perspective piece, Coluracetam never shows any benefit in control rats. Perhaps…since AF64A decreases ACh at synaptic terminals and ACh concentration is a factor in HACU rate, then HACU rate has more potential for improvement during diminished ACh concentration. Since most enzymatic processes operate at near-optimal efficiency at baseline, it would be hard-pressed to see an effect from transporters like CHT1 on HACU. Perhaps this is why transporters like CHT1 can be released as spare “backups” from the cytoplasm to the plasma membrane only after a toxin-induced damage.

References

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