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At the end of class on Thursday we discussed the notion that synaptic activity varies with time. The amount of neurotransmitter released and the number of receptors for that chemical all may change with repeated use (or disuse).  I was intrigued by this because of its parallel to drug use and withdrawal. For example, when caffeine is consumed and present in the brain, it binds to the receptor for the neurotransmitter adenosine, which serves to regulate blood pressure among other things. An increase in the amount of caffeine one consumes (the amount of this chemical that is consistently present in the brain) is then related to an increase in the amount of adenosine receptors present. This is known as tolerance. If a person were to stop consuming caffeine for a prolonged period of time, the receptors would initially still be present and a larger amount of adenosine than usual would bind to these receptors. Thus, caffeine withdrawal effects (headache, nausea) are in fact caused by the binding of adenosine to these receptors in the absence of caffeine.

I wonder whether other chemicals, such as the endorphins released after long, uninterrupted workouts, have the same effect as caffeine in creating more receptors in the brain. Endorphins resemble opiates in their ability to block pain and cause one to feel well and happy. Should it be expected than an individual who exercises everyday has a greater number of receptors for endorphins than a person who rarely exercises? Are they happier? What would happen to that individual if they stopped exercising suddenly for a prolonged period of time? Can these individuals experience withdrawals from strenuous exercise (pain, depression)? Would some other chemicals begin to bind to the numerous receptors causing other symptoms? Or, perhaps the amount of endorphins produced is small and the effects are fleeting (limited to a short period of time after exercise). Perhaps a lack of exercise would cause no observable detriment in one's behavior.