Acetylcholine — The First Messenger

Every neurotransmitter in this series was discovered after acetylcholine, because acetylcholine was the one that proved chemical transmission existed at all. In the early twentieth century, the question was genuinely open: did neurons communicate by electrical continuity or by releasing a substance? The answer arrived in 1921, when the pharmacologist Otto Loewi woke in the night with an experiment in mind, scribbled it down, could not read his own handwriting in the morning, and — in the most fortunate coincidence in the history of neuroscience — had the same dream again the following night. This time he went straight to the laboratory. He stimulated the vagus nerve of an isolated frog heart until it slowed, then transferred the fluid bathing that heart onto a second, unstimulated heart. The second heart slowed too. Something chemical had crossed over. Loewi called it Vagusstoff; it turned out to be acetylcholine.

Its most visible job is at the neuromuscular junction. Every voluntary movement you make — turning a page, walking uphill, speaking — happens because motor neurons release acetylcholine onto muscle fibers, where it binds nicotinic receptors and triggers contraction. This is one of the few places in the body where a single neurotransmitter has a single, unambiguous, all-or-nothing job. It is also why the neuromuscular junction is such a vulnerable target: paralytic toxins, autoimmune diseases like myasthenia gravis, and surgical muscle relaxants all converge on this one synapse.

Inside the brain, acetylcholine plays a subtler role. Rather than carrying point-to-point messages, clusters of cholinergic neurons in the basal forebrain send diffuse projections across the entire cortex, functioning less like a telephone line and more like a stage light. When acetylcholine levels rise, cortical circuits become more sensitive to incoming sensory information and less dominated by their own internal chatter. This is roughly what attention feels like from the inside. A parallel pathway running into the hippocampus supports the encoding of new memories, and another set of cholinergic neurons in the brainstem helps generate REM sleep. Alertness, learning, and dreaming are all, in different ways, cholinergic states.

The clinical importance of this became clear through Alzheimer's disease. Among the earliest and most consistent findings in Alzheimer's brains is degeneration of exactly those basal forebrain cholinergic neurons, and the severity of that loss tracks with cognitive decline. This observation produced the cholinergic hypothesis and, eventually, the first approved drugs for the disease — cholinesterase inhibitors, which block the enzyme that breaks acetylcholine down and thereby leave more of it in the synapse. These drugs treat symptoms rather than the underlying disease, and their benefits are modest, but they remain a clear demonstration of how much of ordinary cognition rests on this one system.

That same enzyme is where acetylcholine intersects with environmental science, and the intersection is not a gentle one. Acetylcholinesterase is one of the fastest enzymes known, and it has to be: without rapid clearance, a synapse that fires once would keep firing. Organophosphate and carbamate insecticides work by disabling it. Acetylcholine accumulates, receptors are overwhelmed, and in acute poisoning the result is a cholinergic crisis — convulsions, respiratory failure, death. The chemistry is the same one behind nerve agents, which is not a coincidence of history but a direct lineage. At the far lower doses encountered in agricultural communities, the concern shifts to children: epidemiological cohorts following prenatal exposure to organophosphates such as chlorpyrifos have reported associations with reduced IQ and attention problems, findings that shaped regulatory decisions in several countries even as debate over exposure thresholds continues.

A second class of pesticides attacks the same system from the opposite direction. Neonicotinoids do not block the enzyme; they mimic acetylcholine itself, binding insect nicotinic receptors far more tightly than they bind vertebrate ones. That selectivity is the selling point. But bees are insects too, and sublethal doses do not need to kill a forager to matter — impaired navigation, disrupted foraging, and weakened colony growth have all been documented in field and laboratory studies, contributing to restrictions on several neonicotinoids in the European Union. It is a striking illustration of a theme this series keeps returning to: the molecules of behavior are deeply conserved across the tree of life, and a chemical designed to exploit that conservation in one species will rarely stop there. The first neurotransmitter ever discovered has become one of the clearest examples of neuroscience and environmental policy being the same conversation.