2. Chronic Pain and Depression are Biologically Linked
The reward circuitry of the brain has been long studied for its role in reinforcing behaviors, drug abuse, and most relevant to our current topic, mood disorders. It is controlled by the mesolimbic system. Its primary role is to produce and transmit the neurochemical dopamine from the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Dopamine, also known as the reward chemical, is released when you eat good food, use recreational drugs, score during a game, solve a puzzle, you name it. Its release gives you a sense of reward any time you accomplish a goal or do something highly desirable. Dysregulation of dopamine in the mesolimbic circuit has been linked to mood disorders like depression and anxiety. Chronic pain can also dysregulate the mesolimbic system, particularly in the VTA-NAc dopaminergic pathway.2 Its dysregulation can attenuate dopamine transmission, changing the reinforcing properties of reward-type stimuli to the individual. The brains of mice with chronic pain release less dopamine after they are given drugs like cocaine and morphine (drugs known to stimulate dopamine release).2 On top of that, they also exhibit less reward-related behaviors such as spending more time in chambers where the drugs were present.2 Individuals with depression also express less reward-related behaviors. These effects in mice solidify the link between chronic pain and depression.
The mesolimbic dopaminergic pathways are also responsible for the salience circuitry in the brain.3 Salience is the importance our brain gives to a certain stimulus or physiological state. In the context of pain, salience can magnify pain as well as minimize it. It can be attributed to why we can experience pain from emotional trauma in the absence of physical damage and how we can attain robust analgesic (pain-relieving) effects from placebo (inactive) treatments. We already know that chronic pain can dysregulate dopamine transmission in the mesolimbic system. The salience value for a given sensory input can also become abnormal due to alterations in the brain structures involved in this system.5 Essentially, in the same way chronic pain can affect our mood it can also affect how we evaluate our pain.
Click page 3 to read about how certain weather conditions induce more pain