The Complex Relationship Between Dopamine and Acetylcholine

Dopamine is famous as the brain’s reward and motivation chemical, while acetylcholine is better known for memory and for making muscles contract. Deep in the brain, though, the two work as partners, each turning the other up or down second by second.¹ That’s why some problems people blame on “too little dopamine” partly involve the pair falling out of step, and why some treatments target acetylcholine instead. Scientists are also rewriting the textbook idea that the two simply sit at opposite ends of a seesaw.²

Where the Two Chemicals Meet

The main meeting place is the striatum, a cluster of tissue deep in each half of the brain that helps choose movements and learn habits. Dopamine arrives there from cells in the midbrain, whose long fibers branch through it. The acetylcholine, by contrast, comes mostly from a small population of local cells scattered among those dopamine fibers.³ There are few of them, but each is well connected. Because both chemicals are released in the same small space, each can reach the other’s endings directly.

When you reach for a mug or settle into a familiar routine, the striatum helps pick that action from many possible ones and suppresses the rest. It doesn’t issue the muscle commands itself; it shapes what the motor areas of the brain are allowed to do.

Each chemical has its own job in that space. Dopamine signals help mark what is worth doing and help movements start smoothly. Acetylcholine cells fire steadily in the background and change their pattern around important events.⁴ They also release glutamate, an excitatory messenger, so they send more than one signal.⁵ Some dopamine cells likewise release glutamate alongside dopamine to excite acetylcholine cells.⁶

How Each Chemical Turns the Other Up or Down

Acetylcholine can release dopamine directly. Dopamine nerve endings carry nicotinic receptors, which are docking sites for acetylcholine.⁷ When acetylcholine binds them, the endings release dopamine without any signal from the dopamine cell itself.⁸ This mainly happens when many acetylcholine cells fire together, not from their steady background activity.⁹

Dopamine talks back. Acetylcholine cells carry two kinds of dopamine receptor, D2 and D5.¹⁰ Dopamine acting on D2 receptors helps these cells briefly go quiet after a burst.¹¹ Activating D5 receptors does the opposite and can wipe out that quiet pause.¹² In one experiment, D5 receptors also boosted the cells’ response to calming signals from neighbors.¹³ So dopamine can both damp and excite acetylcholine cells, depending on which receptor it reaches.

An acetylcholine cell normally fires in a slow, steady rhythm. A burst is a brief run of faster firing, often set off by something important such as a cue or a surprise. The pause is a short silence right afterward, and dopamine acting on D2 receptors helps make it, so the signal stands out from the background.

The influence loops back through other receptors too. Acetylcholine also acts on slower docking sites called M4 receptors that help keep dopamine in check.¹⁴ Mice lacking M4 receptors had higher resting dopamine and bigger dopamine surges from stimulant drugs.¹⁵ Because the effect depends on the receptor more than on the chemical, drugs can be aimed at one receptor type without disturbing the whole system, which matters for several medicines discussed below.

Why the Seesaw Picture Is Too Simple

For decades, doctors described movement health as a balance in which dopamine and acetylcholine pull in opposite directions.¹⁶ A related model says acetylcholine boosts dopamine release while dopamine holds acetylcholine back.¹⁷ But acetylcholine cells don’t always depend on dopamine and can react differently to rewarding events.¹⁸

Timing studies make the tidy story harder to keep. Earlier monkey recordings found acetylcholine cells paused just as dopamine cells fired to a surprise reward.¹⁹ But when rats were tracked with identified cells during a decision task, the pauses and the dopamine reward signals did not line up.²⁰ Newer imaging shows the two chemicals rise and fall in opposite rhythms across many behaviors, not only around rewards.²¹ Input from the thalamus, a relay center, also shapes the pair, and it helps explain why the balance idea is being revised.²²

Recording methods are part of the reason. Older work often had to guess which cells it was listening to, while newer tools can light up specific cell types, and they revealed patterns the older picture missed. The balance idea still explains why some drugs work; it is incomplete rather than wrong.

Parkinson’s Disease and the Acetylcholine Surplus

Parkinson’s disease involves the loss of the dopamine cells that feed the striatum. Without dopamine’s restraint, the striatum enters a state of excess acetylcholine activity.²³ In a mouse model of Parkinson’s, a potassium channel that normally keeps acetylcholine cells calm stopped working properly.²⁴ This imbalance is thought to drive the thinking and flexible-learning problems of Parkinson’s too, not just the movement symptoms.²⁵

Medicines that block acetylcholine, called anticholinergics, such as trihexyphenidyl and benztropine, are among the oldest Parkinson’s treatments. They are also used for stiffness and tremor caused by antipsychotic drugs that block dopamine.²⁶ The reasoning is that blocking acetylcholine M1 receptors restores the balance upset when dopamine signaling is too weak.²⁷

They tend to help tremor and stiffness more than slowness of movement, so they suit some people’s symptom patterns better than others. Doctors tend to consider them more for younger patients, because older adults are more vulnerable to the side effects: dry mouth, constipation, blurred vision and confusion. They are an add-on that sits alongside levodopa and other treatments, not a replacement.

If you’re helping a parent or partner with Parkinson’s, it’s reasonable to ask whether memory or confusion risk outweighs the benefit for an older person. Mention any new confusion that appears after starting such a medicine. And don’t stop Parkinson’s medicines abruptly without guidance.

How Levodopa Side Effects Involve Acetylcholine

Levodopa (L-DOPA), the main Parkinson’s drug, is turned into dopamine in the brain. After years of use, many people develop dyskinesia, meaning uncontrolled writhing or jerking movements.²⁸ In mice given repeated levodopa, acetylcholine cells fire faster and respond more strongly to dopamine.²⁹ D5 receptors on these cells help drive this overactivity.³⁰ So an overexcited acetylcholine system, not dopamine alone, helps produce the extra movements.³¹

Calming that system reduced the movements in animals. Lowering acetylcholine activity or removing these cells reduced the uncontrolled extra movements.³² An acetylcholine-blocking drug reduced them in parkinsonian mice,³³ and blocking D5 receptors restored the cells’ normal quiet pause in dyskinetic mice.³⁴

These findings point to possible future treatments, but they are not yet standard human care. Managing the extra movements today mostly means adjusting levodopa timing and dose with a neurologist.

Dyskinesia is often fluid and writhing, while tremor is rhythmic shaking. Dyskinesia often peaks when medicine levels are highest, so a timed diary of doses and movements gives the doctor something concrete to work with.

A New Kind of Schizophrenia Medicine

Standard antipsychotics work by blocking dopamine D2 receptors. That approach has limits and side effects, including movement problems.³⁵ Dopamine in the striatum helps movements start smoothly, so blocking its receptors there can produce stiffness, tremor or restlessness.

A different route goes through acetylcholine’s slower docking sites. The M1 and M4 types help regulate dopamine and glutamate signaling.³⁶ Xanomeline, which activates these receptors, damps dopamine release in animal tests.³⁷ Its anti-dopamine effect appears to work mainly through M4 receptors.³⁸ Paired with trospium, which blocks acetylcholine outside the brain to reduce stomach side effects, it became the first non-dopamine-blocking schizophrenia medicine approved in the US, in 2024. This approach may address a wider range of symptoms than dopamine blockers.³⁹

For someone on schizophrenia treatment, the newer drug works differently, so side effects differ too. Common effects include nausea and constipation. It may suit people troubled by movement side effects, and any switch between medicines should be planned with a psychiatrist.

Nicotine Hijacks the Partnership

Nicotine binds the same nicotinic receptors that acetylcholine uses.⁴⁰ The cell responds as if its own messenger had arrived. But acetylcholine is cleared within moments, while nicotine lingers, so the receptors are stimulated far longer. Nicotine activates dopamine cells in a midbrain reward area, sending dopamine surges into reward circuits.⁴¹ That activation can last from several minutes up to an hour,⁴² which is why the effect outlasts the cigarette itself. This lasting boost is thought to be central to nicotine dependence.⁴³

Quitting is hard because the brain adapts to frequent dopamine boosts, and cravings reflect that adaptation. Varenicline partially activates the same receptors to ease cravings, and nicotine replacement delivers steadier, lower doses.

Vaping and smoking both deliver nicotine to the same receptors. Fast delivery strengthens the habit loop, and switching products doesn’t remove the dependence.

Learning, Habits and Attention

Acetylcholine cells act as gatekeepers for dopamine’s learning effects. Dopamine helps weaken some striatal connections during learning only by lowering acetylcholine activity through D2 receptors on acetylcholine cells.⁴⁴ That makes these cells key go-betweens in dopamine-driven learning.⁴⁵ A dopamine peak must also coincide with an acetylcholine dip for one type of striatal cell to register a positive signal.⁴⁶ Requiring a dopamine peak and an acetylcholine dip together makes the signal more selective than dopamine alone.

The partnership extends beyond the striatum. In the front of the brain, both acetylcholine and dopamine receptors tune working memory, the ability to hold information in mind.⁴⁷ Acetylcholine release rises when tasks demand more attention.⁴⁸

Because acetylcholine matters for memory, medicines that block it can worsen memory in older adults. Some allergy and bladder pills have this effect too, so a pharmacist can review the total anticholinergic load across everything a person takes.