If the brain is constantly trying to reach a stasis, how is any medication supposed to help chemical imbalance disorders like ADHD and Depression?

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We know the brain is always trying to find a stasis which is why we build tolerance to caffeine, alcohol, etc. If that is the case, how would it be possible for any external medication to come in and fix your brain chemistry without creating a tolerance. Are people with imbalance disorders doomed to always ride a rollercoaster of tolerance building and detoxing. For example, wouldn’t any attempt to increase dopamine levels in people with ADHD just cause the brain to eventually bring the new levels back down to the disordered levels?

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The brain’s mechanisms for reaching homeostasis are imperfect.

To make an analogy, consider a thermostat. It shuts off the heater when the temp exceeds the limit.

That means your house is always going to be at that limit right? No. Because imagine if your medication is another big heater that heats the house. The heater shuts off, but that doesn’t mean the temp stays within the limit. Your meds carry it further.

Now another analogy. This one involves more complex “equations” for homeostasis. The thermostats equation is like A – B.

If A (target temp) is greater than B (actual temp) it turns the heater on. That’s basically “If A – B > 0”

But your brain might be balancing multiple factors. For example imagine it’s considering “how much dopamine there is compared to the set point, crossed with how much effort it takes to make more dopamine”.

So let’s say the brain’s production of dopamine has a nonlinear cost curve. The first few molecules are easy and they get more expensive after that.

That will lead to a curve with a peak somewhere. That peak will be “how much dopamine you have”.

Adding medication alters some parameters so that there’s still a curve with a peak, but the peak is in a new place now.

Similar to how pricing works in a market. This concept in math is an “equilibrium”.

And most of the homeostatic systems in the body are evolved to respond to inputs in a way that they settle at some equilibrium, but no part of the system actually has that specific equilibrium point “programmed in”. Instead of the body’s rule being like “Ensure there is X amount of dopamine present”, the rule is more like “When you detect dopamine at 20% of these receptors, shut down dopamine production”

Here an analogy is a bathtub filling with water with the drain open. Outflow increases with depth, meaning there is a particular equilibrium depth for the tub. But if you add another source of water flowing in, it still settles at an equilibrium it’s just a new one.

That’s a classic example in differential equations, which is a great class to take to
understand how feedback systems like this work.

It’s all related to therapeutic levels of the drug.

The concept is to day-after-day reach the levels that provide benefit while not reaching toxic levels (high) or ineffective levels (low).

Not easy or straightforward…

A lot of people have provided adequate responses. I just wanted to add once you find the “magic dose” it rarely works long term. The brain adapts and over time there is a need to change dosages or medications.