The term stimulant or psychostimulant isn’t well defined. Cocaine, amphetamine, methylphenidate, modafinil, armodafinil, caffeine, and nicotine belong to this class of drugs, which are called stimulants for the marked sensorimotor activation that occurs in response to drug administration. Stimulants are characterized by their ability to increase alertness, heighten arousal, and cause behavioral excitement.

Stimulants have a rich history which is beyond the scope of this post. 

Today, psychostimulants are prescribed for the treatment of attention-deficit hyperactivity disorder (ADHD), narcolepsy, chronic fatigue, depression, and cancer-related fatigue to name a few. Stimulants are also drugs of abuse (such as cocaine, illicit methamphetamine, nicotine, and caffeine).

Let’s take a closer look at psychostimulants and how they work. 

Methylphenidate (MPH) acts by inhibiting the dopamine and norepinephrine transporters which increases the concentrations of dopamine and norepinephrine in the synapse. See the figure below.

In individuals with attention and/or concentration problems, there may be a problem with how the brain is processing sensory input. Our brains spend an enormous amount of energy (up to 20-30% of all energy used by your body) processing information below our level of awareness. In fact, only a very small percentage of brain activity contributes to our conscious awareness (roughly 15%). The rest of the activity is all the unconscious processing, integrating, and analyzing of information that ultimately results in complex behavior. Much of the brain’s energy is spent “deciding” which signals are relevant and need to be brought to conscious awareness.

Think of all the activities we do that we aren’t even aware of.

While walking down the street talking with someone, do you actively feel your left big toe? Well, no, not unless you have pain or stub your toe. We aren’t aware of our left big toe because it’s irrelevant to what we are doing. But this doesn’t mean those signals are physiologically absent.

Dopamine and norepinephrine are neurotransmitters in the brain that act like the tuners of a piano. The strings of the piano that create the sounds represent the glutamate and GABA neurons that are the primary excitatory and inhibitory neurotransmitters in the mammalian brain, respectively. Dopamine and norepinephrine are there to tighten the strings so the music sounds good. No one likes a song that’s out of tune. That is, dopamine and norepinephrine are those “tuners” of the brain–they modulate communication between neurons. They help our brain decide what to ignore and what to focus on.

In fact, norepinephrine in the prefrontal cortex (PFC) plays a role in enhancing relevant and important signals so that we focus on relevant and important things. Low-to-moderate concentrations of norepinephrine (NE) mediate these actions by acting preferentially on postsynaptic 𝛼2A-adrenoceptors. However, as the concentration of norepinephrine increases, norepinephrine begins stimulating 𝛼1 and 𝛽-adrenoceptors. Stimulation of 𝛼1-adrenoceptors and 𝛽-adrenoceptors (which occurs in high stress states) impairs our ability to focus.

When the NE concentration is too low, the signal strength (i.e., our ability to focus on things) is low. As the NE concentration increases so does the signal strength (i.e., our ability to focus on things) until it reaches a peak. Any additional increase in NE impairs, rather than enhances, our ability to focus. This explains the inverted U shaped curves depicted below.

Dopamine in the prefrontal cortex (PFC) plays a role in filtering out the irrelevant stimuli. That is, dopamine D1 receptors in the prefrontal cortex reduce the “noise” or irrelevant stimuli so that we can focus on relevant and important things without being distracted.

When DA levels are too low, all incoming signals, whether they are relevant or not, are treated in the same way. Therefore, it becomes difficult to focus on a single task as there are too many distracting stimuli. However, as the concentration of DA increases to moderate levels, it will decrease ‘noise’ by stimulating D1 receptors. This results in decreased firing of neurons to irrelevant inputs in PFC networks.

When DA levels are too high, D1 receptors in the prefrontal cortex are overstimulated and the brain’s ability to filter out the noise declines. Stressful situations and illicit drug use can cause dopamine levels to be too high.

Therefore, medications like amphetamines (Vyvanse, Adderall), methylphenidates (Ritalin, Concerta, Focalin), bupropion (Wellbutrin), and atomoxetine (Strattera) alter norepinephrine and/or dopamine levels to “enhance the signal” while “reducing the noise,” respectively. 

Medications (or illicit drugs) that enhance dopamine too much in certain regions of the brain may cause us to “hyper focus” or “fixate” our attention on unproductive tasks. In addition, the euphoria and motivational reinforcement that results from overstimulation of dopamine receptors in the nucleus accumbens increases the risk for addiction and drug abuse.

In summary, we don’t want too much stimulation of dopamine (D1) receptors because this is associated with euphoria, hyper focus (like scrubbing the floor with a toothbrush), impaired attention, and drug addiction. We don’t want too little stimulation of dopamine (D1) receptors because this is associated with anhedonia, depression, lack of motivation, and apathy.

The same goes for norepinephrine. We don’t want too much norepinephrine because then we will feel symptoms associated with the fight or flight response such as anxiety, hypervigilance, racing heart, sweating, and shortness of breath. We don’t want too little norepinephrine because then we will feel symptoms like fatigue, depression, drowsiness, and weakness.

Therefore, we want our DA and NE to be not too hot and not too cold, but just right (yes, like Goldilocks). This is why controlled doses of stimulants can be very beneficial for some people.