Which of the following is NOT considered a neurotransmitter involved in mood regulation?

Insulin is not classified as a neurotransmitter involved in mood regulation. Instead, it is a hormone produced by the pancreas that regulates blood sugar levels in the body. While insulin plays crucial roles in metabolism and energy regulation, its functions are distinct from neurotransmitters, which are chemical messengers that transmit signals across synapses in the nervous system.

In contrast, dopamine, serotonin, and GABA are all well-recognized neurotransmitters that significantly influence mood and emotional state. Dopamine is often associated with pleasure and reward processes, serotonin is commonly linked to mood stabilization and the feeling of well-being, and GABA serves as the primary inhibitory neurotransmitter, which helps to reduce anxiety and promote relaxation. Each of these neurotransmitters interacts with various receptors in the brain to modulate emotional responses and mental health, unlike insulin, which operates within a different biochemical context.