8.14, 8.15 - Neurochemistry and drugs at synapses

Chemical signals and drug targets

A drug can change communication between neurones by acting at a particular step in chemical transmission.

At a typical chemical synapse, an action potential opens voltage-gated calcium channels in the presynaptic terminal. Calcium entry triggers vesicles to fuse with its membrane and release transmitter by exocytosis. Transmitter diffuses across the cleft and binds to matching receptors on the postsynaptic membrane. Receptor activation changes the postsynaptic response, directly through ion channels or indirectly through signalling inside the cell. The effect depends on the receptor and circuit: more transmitter does not always mean more postsynaptic action potentials.

The signal must also end. Acetylcholine is broken down in the cleft by acetylcholinesterase. For dopamine and serotonin, important removal mechanisms include transport back into the presynaptic neurone, called reuptake. Recovered molecules may be repackaged into vesicles or broken down by enzymes. A reuptake transporter moves transmitter across a membrane; a receptor detects it. These proteins have different jobs.

An agonist activates a receptor, whereas an antagonist binds without activating it and can prevent activation by the transmitter. Drugs can also act away from receptors: they may change transmitter synthesis, storage, release, reuptake or breakdown. These possibilities give researchers distinct drug targets, the molecules or processes through which a drug acts.

This table predicts an immediate mechanism. Whether a change helps an illness depends on which neurones and receptors are affected, how circuits adapt, and the drug’s unwanted effects.

Dopamine loss in Parkinson’s disease

Parkinson’s disease illustrates how the loss of a particular population of neurones changes chemical signalling in connected brain regions.

Dopamine-producing neurones in the substantia nigra normally send signals to the striatum, part of the basal ganglia involved in controlling movement. Dopamine modulates the activity of these circuits, helping appropriate movements to be selected and carried out smoothly. It is not a substance that contracts muscle directly.

In Parkinson’s disease, many of these neurones degenerate and die. Fewer functional terminals supply less dopamine to the striatum. The resulting change in signalling disrupts movement-control circuits, contributing to slow movement, muscle rigidity and resting tremor. Parkinson’s also involves other changes, so dopamine loss does not explain every symptom. NINDS describes this neuronal loss and its movement consequences.

There are two different problems here: a reduced chemical signal and loss of the cells that produce it. A treatment that compensates for the chemical shortage may improve circuit function while leaving the underlying cell loss unresolved. Likewise, an improved movement score is evidence about movement; it is not, by itself, a measurement of how many neurones survive.

L-Dopa as a dopamine precursor

L-Dopa can compensate for reduced dopamine signalling because it supplies a starting material for making dopamine inside the brain.

L-Dopa, also called levodopa, is a dopamine precursor. Dopamine itself does not readily cross the blood–brain barrier, the selectively permeable barrier between blood and brain tissue. L-Dopa can cross using a transport system. In the brain, an enzyme converts it to dopamine. The resulting dopamine can be stored and released by functioning neurones, increasing the dopamine available to act on receptors in movement-control circuits. L-Dopa is therefore neither dopamine itself nor simply a molecule that occupies a dopamine receptor. NINDS explains the barrier and conversion mechanism.

The reasoning is a sequence: provide a precursor that reaches the brain → synthesise more dopamine → increase available dopaminergic signalling → improve some movement symptoms. A molecule that is converted before it reaches the brain may act elsewhere and supply less useful precursor to the intended site. Drug development therefore considers delivery as well as the target reaction.

L-Dopa does not replace dead neurones. A benefit during treatment must be distinguished from evidence that a drug slows degeneration. In the LEAP trial, people with early Parkinson’s were randomly assigned to start levodopa earlier or later; the trial did not find a disease-modifying effect over its 80-week period. That result concerns the tested conditions and follow-up, rather than proving what every future treatment will do. The original trial reports the comparison and conclusion.

Serotonin, depression and evidence

Serotonin signalling is involved in brain circuits that influence mood and other functions, but its relationship with depression is more complex than a single chemical shortage.

The historical serotonin hypothesis of depression proposed that reduced serotonin activity contributes to depression. It helped direct research towards serotonin receptors, synthesis and removal. Some research has found altered serotonergic function in selected groups, but this does not establish that everyone with depression has low serotonin or that low serotonin is its sole cause. Genetic, biological, environmental and psychological factors can interact in depression. NIMH gives this multifactorial account.

For example, a 2023 PET study compared 17 antidepressant-free people experiencing a major depressive episode with 20 people without depression. Its authors interpreted the brain-imaging response to a pharmacological challenge as evidence of reduced serotonin release capacity in the depressed group. This was a small selected comparison using an indirect imaging measure, not a test showing that a serotonin deficit preceded and caused depression in every individual. Other researchers disputed the strength of its inference. The finding supports continued investigation, with replication and careful attention to the measurement. The original study describes its design and findings.

Selective serotonin reuptake inhibitors (SSRIs) act on the serotonin transporter, SERT, in the presynaptic membrane. Inhibiting reuptake slows serotonin clearance and can increase its extracellular availability. SSRIs do not supply serotonin and do not need to mimic it at the postsynaptic receptor. Immediate transporter inhibition and the eventual change in symptoms are different observations; changes in receptors and circuits over time may also matter. Human SERT research distinguishes inhibitor binding from transported substrates.