ATP and Adenosine—Fundamental Cellular Communication Signals

For more than a century, neuroscience has been built around a neuron-centered worldview. The heroes of this story are familiar neurotransmitters: dopamine, serotonin, norepinephrine, acetylcholine, glutamate, and GABA. Textbooks describe neurons communicating across synapses in brain circuits using these specialized chemical messengers. The vast ‘connectome’ network of axons and dendrites was believed to determine mental events. Neurons were viewed as the only true information-processing units.

The three other main brain cells—astrocytes, microglia, and oligodendrocytes—were given the name glial cells, for the Greek word meaning glue. For decades they were viewed as having mostly supportive roles, either structural or metabolic. While this framework of understanding neurons and glial cells produced enormous advances, it also created blind spots.

Two recent developments have totally changed this view.  One is the increasing evidence of complex signaling among glial cells, and their great significance in determining brain function. The second is the emergence of the purinergic signaling system. Hidden beneath the neuron-centric view is another communication system that is older, more universal, and ultimately more fundamental than any specialized neurotransmitter system. This underlying system uses ATP, ADP, AMP, and adenosine—molecules collectively known as purines.

The gradual acceptance of purinergic signaling represents one of the most important conceptual revolutions in modern biology. It is forcing scientists to rethink not only the nervous system, but the nature of cellular communication itself.

The story begins with ATP. Every biology student learns that ATP is the universal energy currency of life. ATP powers muscle contraction, protein synthesis, membrane transport, molecular motors, and countless other cellular activities. Because ATP is viewed primarily as an intracellular energy molecule, scientists assumed its role ended there.

Then came the radical proposal of Geoffrey Burnstock. Beginning in the 1970s, Burnstock argued that ATP was not merely an energy molecule; it was also a neurotransmitter and a signal used by all cells. He suggested that cells actively released ATP into the extracellular environment and that neighboring cells possessed receptors specifically designed to detect it.

His idea was met with skepticism and hostility. Many scientists believed ATP was too important metabolically to serve as a signaling molecule. Others argued that observed effects could be explained by known neurotransmitters. For years, purinergic signaling remained outside mainstream neuroscience.

 While still not widely understood today, ATP has now been proven to be one of the most widespread signaling molecules in biology. ATP does not behave like dopamine, serotonin, acetylcholine, or any of the other well-known neurotransmitters. Instead, ATP signaling has an entirely different level of biological organization. Unlike classical neurotransmitters that are primarily associated with specialized neuronal circuits (and increasingly glial brain cells and immune cells, as well), ATP is released by virtually every cell type in the body, including muscle, bone, skin, fascia, blood vessels, and every organ. All these cells release ATP when undergoing stress, inflammation, injury, or mechanical stimulation.

This purine signaling is, in fact, a universal language of cellular communication. Dopamine neurons communicate primarily within dopamine circuits. Serotonin neurons communicate primarily within serotonin systems. But ATP communication occurs across virtually all biological systems simultaneously.

The purinergic system links nervous system activity, immune responses, metabolic states, tissue repair, vascular regulation, inflammation, development, and cellular stress into a single integrated signaling network. This fundamentally challenges the traditional neuron-centered model of neuroscience. The brain is not just a collection of neurons connected by wires. It is a vast multicellular communication network in which neurons, glia, immune cells, vascular cells, and metabolic systems continuously exchange information.

For example, glial cells use the same neurotransmitters as neurons but also utilize ATP purinergic signaling. Astrocytes release ATP to communicate with neighboring astrocytes and to influence neuronal firing, synaptic strength, blood flow, and metabolic support. Microglia, the immune cells of the brain, use ATP gradients to navigate toward sites of injury. ATP serves as a distress signal that alerts surrounding cells to danger. Oligodendrocytes respond to purinergic signals during myelination and repair.

The complexity of the purinergic signaling system greatly increases when ATP in the extracellular space is broken down into three other molecules that are also part of the signaling. Once released at the neuronal synapse, for example, ATP is rapidly converted into ADP, then AMP, and finally adenosine by specialized enzymes. Each of these molecules is also a signaling molecule with multiple types of receptors. 

Currently, 19 distinct purine receptors have been discovered in this vast signaling network. But these 19 receptors do not reflect the extent of the purine system’s capacities and complexity. The same signals have different effects in different contexts and different tissues, and can convey information about levels of cellular activity, energy consumption, tissue health, inflammation, and environmental conditions.

There are other factors that increase complexity in the purinergic signaling system. Receptors are made of subunits that can combine in different ways to make a wider range of receptors with greater functional diversity. When ATP is released a rapid effect occurs, but then it is broken down into three other molecules that trigger different effects in various slower timeframes. There are also spatial and temporal patterns of ATP release, such as increasing and decreasing intensity, and pulsating at different rates. ATP is released in different patterns when neurons fire intensely, when tissues become metabolically active, when cells are injured, and when inflammation occurs.

Adenosine adds a layer of regulation for the effects of ATP signals. As ATP is degraded, adenosine accumulates. ATP typically signals activity. Adenosine typically signals recovery. ATP stimulates. Adenosine moderates. This produces a remarkable feedback system for the stimulating effects of ATP. Adenosine signals suppress excessive neuronal activity, reduce excitotoxicity, modulate inflammation, protect tissues from overexertion, and promote sleep pressure for the organism.

Purinergic signaling is by far the earliest signaling system in evolution. Serotonin signaling evolved later, and dopamine appeared even later. Complex nervous systems emerged long after ATP already existed. ATP signaling was present in some of the earliest microbe cells on Earth and powered primitive metabolism.

Rather than evolving specifically for nervous systems, the purinergic system was inherited from ancient cellular communication networks that predated multicellular life. The specialized neurotransmitters are later evolutionary refinements built upon a much older purinergic foundation. This perspective explains why purinergic signaling is so universal. The system was never designed exclusively for neurons. It was already functioning as a cellular communication platform long before neurons evolved.

The brain is an integrated ecosystem of cells constantly exchanging neuronal and purinergic information about activity, energy, stress, injury, and environmental conditions. ATP, ADP, AMP, and adenosine form one of the central communication networks that make this integration possible. Dopamine, serotonin, and glutamate are less like master regulators and more like specialized dialects spoken within particular neural circuits. The purines operate at a deeper level. They connect local events to global physiological states as the oldest, most fundamental, and most pervasive form of communication in all of life.

As neuroscience continues to integrate immunology, metabolism, systems biology, and intercellular communication, the importance of purinergic signaling will grow.