How is the affinity of psychoactive substances measured in scientific research?

Affinity is measured by the strength of the bond between a substance and a specific receptor. In scientific practice, the difference in affinity between individual psychoactive substances can be as much as 10,000-fold.

Why are mice used in psychiatric drug research?

Mice are used because mammalian brains are structurally and functionally very similar to human brains, containing nearly identical receptors. This allows scientists to observe effects like the reversal of induced depression through SSRI antidepressants.

What determines if a substance is psychoactive?

A substance is considered psychoactive if it is sufficiently lipophilic to penetrate the blood-brain barrier and interact with receptors in the central nervous system.

Scientific research of psychoactive substances in practice

Testing psychoactive substances or psychiatric drugs on people is dangerous and is not done in practice. First, it is tested on a laboratory animal and only then on a human.

A chemist will produce some unknown substance that is assumed to be psychoactive. It is not certain what effects this substance will have and what affinity it will have for individual receptors . The difference in affinity between individual psychoactive substances can be as much as 10,000-fold.
https://en.wikipedia.org/wiki/Affinity_%28pharmacology%29

The mammal has a very similar brain to that of a human, there are only minimal differences.
Scientists also conducted experiments in which they artificially induced depression in mice by exposing them to severe stressful situations over a long period. In severe depression, we know from psychiatric medicine that a person cannot even move, walk, or get up from a place. Scientists observed something similar in mice when they observed severe depression in them, in the form that they could not even move from the spot. Later, the scientists administered SSRI antidepressants to the mice, and the mice returned to normal – they started moving and functioning like healthy ones – meaning the animals were cured of depression . It is interesting that animals can also suffer from depression and can also be treated with antidepressants.
With this scientific experiment, I want to say that both human and animal brains are very similar and contain exactly the same receptors as humans (with only very small differences, e.g., an animal may have additional receptor subtypes compared to a human, while others may be missing).

So, in scientific practice, psychoactive substances can be experimented with on mice. Under the appropriate instruments, it is possible to see which receptors the substance acts on, with what affinity, and whether it acts as an agonist or antagonist. And just as with the animal, this will also apply to humans – there are no significant differences.

The scientific team must include a chemist who creates new chemical compounds, a pharmacologist, and a neurologist or psychiatrist, or a veterinarian. Alternatively, the doctor can be replaced by a veterinarian.

Animals are treated as gently as possible. One animal can be used to test several substances.
The death of an animal is necessary only when determining the toxicity of a given substance. This means that the dose at which the substance is lethal is expressed in units of g/kg or mg/kg . Thus, it is calculated based on the ratio of the animal's weight to the weight of the active substance.
(Even the usual average recreational dose of a psychoactive substance is expressed in mg/kg, i.e. how many milligrams per kilogram of a person's weight)
https://en.wikipedia.org/wiki/Toxicity

Positron Emission Tomography
I think that radioactive substances, e.g. radioactive serotonin, or I don't know what else, are used as imaging methods for receptor activity. On the devices, it is possible to see neurotransmitter substances in the brain as well as the activity of the receptors themselves. (Today, it is already necessary to drink a radioactive liquid for an examination on a CT scanner. The radiation level is very low and the decay time is very short. It does not endanger the patient's health and is safe)

Which substances are psychoactive?
Psychoactive substances are only those that can penetrate the blood-brain barrier. Only substances that are sufficiently lipophilic, i.e., soluble in fats, can penetrate.