1. Neuroscience

Central Nervous System: Is creatine a CNS neurotransmitter?

A range of experiments suggests that creatine, a molecule known for recycling ATP in muscle and brain tissue, may also function as a neurotransmitter in the central nervous system.
https://doi.org/10.7554/eLife.91824
Share this article
Cite this article
  1. Bhagaban Mallik
  2. C Andrew Frank
(2023)
Central Nervous System: Is creatine a CNS neurotransmitter?
eLife 12:e91824.
https://doi.org/10.7554/eLife.91824
  2. Download BibTeX
  3. Download .RIS
  1. Bhagaban Mallik
  2. C Andrew Frank  Is a corresponding author
  1. Department of Anatomy and Cell Biology, University of Iowa, United States

Communication in the nervous system relies on neurons transmitting signals to target cells. This process is facilitated by various chemicals, including neurotransmitters, neuromodulators and neuropeptides (Langley, 1905; Benfenati and Agnati, 1991; Lovinger, 2008). Identifying a new neurotransmitter is no small feat: such a discovery requires extensive investigation and validation, and it might take decades to integrate the collective contributions of many different research groups (Hökfelt, 2010; Contestabile, 2011). Moreover, neurotransmitters that act in peripheral tissues, like muscle, are easier to identify than those that act on the central nervous system (CNS). It is likely that many CNS neurotransmitters have not yet been identified (Curtis et al., 1959; Carlsson et al., 1962; Curtis and Watkins, 1963; Björklund et al., 1968; Andersson, 2000).

To be classified as a neurotransmitter, a molecule should meet several criteria: it needs to be stored within a synaptic vesicle in a neuron and be released upon neuronal stimulation; it needs to act upon a postsynaptic receptor; and afterwards, it needs to be removed or retrieved from the synaptic cleft (Kandel et al., 2021; Purves, 2018; Radian et al., 1986; Pacholczyk et al., 1991). Curiously, several molecules widely accepted as neurotransmitters only meet some, rather than all, of these criteria (Curtis and Watkins, 1960; Felix and Künzle, 1974). Accurately detecting candidate molecules within synaptic vesicles could represent a significant step towards identifying neurotransmitters with a higher certainty.

Now, in eLife, Yi Rao and colleagues at Peking University and other research centers in Beijing – including Xiling Bian, Jiemin Zhu and Xiaobo Jia as joint first authors – report data that potentially uncovers a new neurotransmitter within the mammalian brain (Bian et al., 2023). The researchers used a combination of mass spectrometry, genetics, biochemistry, immunostaining, electrophysiology, and electron microscopy to support this conclusion.

Starting with purified synaptic vesicles (SV) from mouse brains, Bian et al. detected several well-known neurotransmitters in their samples, as well as creatine. The levels of SV creatine were higher than those of other known neurotransmitters, such as acetylcholine and serotonin, but lower than glutamate and gamma-aminobutyric acid (GABA). It has been known for decades that creatine is involved in recycling ATP in both muscle and brain tissue (Wyss and Kaddurah-Daouk, 2000; Brosnan and Brosnan, 2007; Wallimann et al., 2011). And more recently, researchers have suggested that it might also have additional roles in brain function (Ohtsuki et al., 2002; Braissant et al., 2011).

The next step was to dissect the molecular mechanism of creatine function in neurons. Bian et al. demonstrated that creatine was released from stimulated coronal brain slices. Interestingly, creatine release was reduced in slices from mice lacking either the gene that codes for an enzyme called AGAT (which is necessary for creatine production), or the gene that codes for the SLC6A8 creatine transporter. Importantly, the Bian et al. also observed that creatine has an inhibitory effect on a subset of neurons. They also found that SLC6A8 can move creatine into synaptosomes (isolated synaptic structures that contain a machine that helps release neurotransmitters and large numbers of synaptic vesicles). Collectively, these results are consistent with creatine acting like a neurotransmitter and with AGAT and SLC6A8 supporting that function (Figure 1).

Figure 1
Download asset Open asset
Schematic illustration of the synthesis, transport and release of creatine in neurons.

A subtype of glial cell, known as an astrocyte (orange), likely synthesizes creatine molecules (red dots) through a process involving the amino acids glycine (Gly) and arginine (Arg), the enzymes AGAT and GAMT, and other compounds (GAA and SAM; Roschel et al., 2021). It is likely that the creatine molecules are then transported into a synaptic vesicle located in a presynaptic neuron that expresses a creatine transporter (CreaT) called SLC6A8. SNARE proteins (string-like structures) then mediate the release of the vesicles containing the creatine molecules into the synaptic cleft in a calcium-dependent manner, and the creatine molecules go on to bind to an as-yet unidentified creatine receptor (CreaR) on the postsynaptic neuron. It is possible that creatine molecules are also released directly by the astrocytes into the synaptic cleft between the neurons in a calcium-independent manner. AGAT: L-arginine: glycine amidinotransferase; GAA: guanidinoacetate; GAMT: guanidinoacetate methyltransferase; SAM: S-adenosylmethionine.

Image created with BioRender.com.

The work by Bian et al. goes beyond previous studies, which posited that creatine could have neurotransmitter-like properties (Almeida et al., 2006; Peral et al., 2010). Nevertheless, questions remain for future work. Most notably, Bian et al. did not identify a specific postsynaptic receptor for creatine. The researchers speculate that there might be a metabotropic receptor (or receptors) for creatine (Figure 1). Another mystery is that most of the creatine release after high potassium stimulation occurs when there is no extracellular calcium present. This is not consistent with a neurotransmitter role for that portion of the release. Bian et al. speculate that astrocytes might be responsible because astrocytes contain high levels of an enzyme called GAMT, which is involved in the production of creatine. If this idea were correct, then astrocytic creatine could potentially serve a neuromodulatory role. One final puzzle is that AGAT and SLC6A8 are found in different cells in the brain. So if creatine were a neurotransmitter, it suggests a complex model of creatine being synthesized in one cell type and subsequently then transported to another cell type for release (Figure 1).

In summary, Bian et al. report that creatine is a possible neurotransmitter in the central nervous system and that it meets several textbook criteria for a neurotransmitter (Kandel et al., 2021; Purves, 2018). This is a potentially groundbreaking finding that could have implications for understanding brain function and neurotransmission. It may also open new areas of understanding Creatine Transporter Deficiency, which manifests as a collection of intellectual disabilities, language delays and other neurological disorders that are associated with defective SLC6A8 (Salomons et al., 2003).

References

    1. Benfenati F
    2. Agnati LF
    (1991)
    Communication and computation in the central nervous system
    Functional Neurology 6:202–209.
    1. Carlsson A
    2. Falck B
    3. Hillarp NA
    (1962)
    Cellular localization of brain monoamines
    Acta Physiologica Scandinavica 56:1–28.
  15. Book
    1. Kandel ER
    2. Koester JD
    3. Mack SH
    4. Siegelbaum SA
    (2021)
    Principles of Neural Sciences (6th Edition)
    McGraw-Hill.
    1. Lovinger DM
    (2008)
    Communication networks in the brain: neurons, receptors, neurotransmitters, and alcohol
    Alcohol Research & Health 31:196–214.
  21. Book
    1. Purves D
    (2018)
    Neuroscience
    New York: Oxford University Press.
    1. Radian R
    2. Bendahan A
    3. Kanner BI
    (1986)
    Purification and identification of the functional sodium- and chloride-coupled gamma-aminobutyric acid transport glycoprotein from rat brain
    The Journal of Biological Chemistry 261:15437–15441.

Article and author information

Author details

  1. Bhagaban Mallik

    Bhagaban Mallik is in the Department of Anatomy and Cell Biology, University of Iowa, Iowa City, United States
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-7637-3338

  2. C Andrew Frank

    C Andrew Frank is in the Department of Anatomy and Cell Biology, University of Iowa, Iowa City, United States
    For correspondence
    andy-frank@uiowa.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-9599-421X

Publication history

  1. Version of Record published: October 16, 2023 (version 1)

Copyright

© 2023, Mallik and Frank

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 2,804
    views
  • 293
    downloads
  • 0
    citations

Views, downloads and citations are aggregated across all versions of this paper published by eLife.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Bhagaban Mallik
  2. C Andrew Frank
(2023)
Central Nervous System: Is creatine a CNS neurotransmitter?
eLife 12:e91824.
https://doi.org/10.7554/eLife.91824

Categories and tags

Research organism

    1. Related to

    Suggestion of creatine as a new neurotransmitter by approaches ranging from chemical analysis and biochemistry to electrophysiology

    Xiling Bian, Jiemin Zhu ... Yi Rao
  2. Further reading

Further reading

    1. Neuroscience

    Suggestion of creatine as a new neurotransmitter by approaches ranging from chemical analysis and biochemistry to electrophysiology

    Xiling Bian, Jiemin Zhu ... Yi Rao
    Research Article

    The discovery of a new neurotransmitter, especially one in the central nervous system, is both important and difficult. We have been searching for new neurotransmitters for 12 y. We detected creatine (Cr) in synaptic vesicles (SVs) at a level lower than glutamate and gamma-aminobutyric acid but higher than acetylcholine and 5-hydroxytryptamine. SV Cr was reduced in mice lacking either arginine:glycine amidinotransferase (a Cr synthetase) or SLC6A8, a Cr transporter with mutations among the most common causes of intellectual disability in men. Calcium-dependent release of Cr was detected after stimulation in brain slices. Cr release was reduced in Slc6a8 and Agat mutants. Cr inhibited neocortical pyramidal neurons. SLC6A8 was necessary for Cr uptake into synaptosomes. Cr was found by us to be taken up into SVs in an ATP-dependent manner. Our biochemical, chemical, genetic, and electrophysiological results are consistent with the possibility of Cr as a neurotransmitter, though not yet reaching the level of proof for the now classic transmitters. Our novel approach to discover neurotransmitters is to begin with analysis of contents in SVs before defining their function and physiology.

    1. Neuroscience

    EEG-fMRI in awake rat and whole-brain simulations show decreased brain responsiveness to sensory stimulations during absence seizures

    Petteri Stenroos, Isabelle Guillemain ... Emmanuel L Barbier
    Research Article

    In patients suffering absence epilepsy, recurring seizures can significantly decrease their quality of life and lead to yet untreatable comorbidities. Absence seizures are characterized by spike-and-wave discharges on the electroencephalogram associated with a transient alteration of consciousness. However, it is still unknown how the brain responds to external stimuli during and outside of seizures. This study aimed to investigate responsiveness to visual and somatosensory stimulation in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well-established rat model for absence epilepsy. Animals were imaged under non-curarized awake state using a quiet, zero echo time, functional magnetic resonance imaging (fMRI) sequence. Sensory stimulations were applied during interictal and ictal periods. Whole-brain hemodynamic responses were compared between these two states. Additionally, a mean-field simulation model was used to explain the changes of neural responsiveness to visual stimulation between states. During a seizure, whole-brain responses to both sensory stimulations were suppressed and spatially hindered. In the cortex, hemodynamic responses were negatively polarized during seizures, despite the application of a stimulus. The mean-field simulation revealed restricted propagation of activity due to stimulation and agreed well with fMRI findings. Results suggest that sensory processing is hindered or even suppressed by the occurrence of an absence seizure, potentially contributing to decreased responsiveness during this absence epileptic process.

    1. Neuroscience

    Sex-specific resilience of neocortex to food restriction

    Zahid Padamsey, Danai Katsanevaki ... Nathalie L Rochefort
    Research Article

    Mammals have evolved sex-specific adaptations to reduce energy usage in times of food scarcity. These adaptations are well described for peripheral tissue, though much less is known about how the energy-expensive brain adapts to food restriction, and how such adaptations differ across the sexes. Here, we examined how food restriction impacts energy usage and function in the primary visual cortex (V1) of adult male and female mice. Molecular analysis and RNA sequencing in V1 revealed that in males, but not in females, food restriction significantly modulated canonical, energy-regulating pathways, including pathways associated waith AMP-activated protein kinase, peroxisome proliferator-activated receptor alpha, mammalian target of rapamycin, and oxidative phosphorylation. Moreover, we found that in contrast to males, food restriction in females did not significantly affect V1 ATP usage or visual coding precision (assessed by orientation selectivity). Decreased serum leptin is known to be necessary for triggering energy-saving changes in V1 during food restriction. Consistent with this, we found significantly decreased serum leptin in food-restricted males but no significant change in food-restricted females. Collectively, our findings demonstrate that cortical function and energy usage in female mice are more resilient to food restriction than in males. The neocortex, therefore, contributes to sex-specific, energy-saving adaptations in response to food restriction.