Final answer:
Gun handling has been found to increase dopamine in research participants' saliva. Dopamine is a neurotransmitter that plays a key role in the brain's reward and pleasure systems and is also significant in addiction studies.
Step-by-step explanation:
Handling and describing a gun has been found to increase dopamine in research participants' saliva. Dopamine is a neurotransmitter involved in the reward and pleasure centers of the brain, which can increase when engaging in activities that are perceived as exciting or rewarding. Nicotine, for example, has been shown to increase dopamine levels upon binding to specific receptors on neurons. This aspect of dopamine is significant in studies related to brain function and addiction.
Additional research into hormones like cortisol and endorphins indicates that cortisol is involved in stress responses, and endorphins are related to pain inhibition and can be triggered by rigorous aerobic exercise, creating a 'runner's high'. While insulin is mentioned as a hormone produced by the pancreas and associated with blood sugar regulation, it is not typically influenced by gun handling. Thus, the correct answer to the question is 'e. dopamine'.