Which neurotransmitter is associated with the regulation of blood flow and metabolism by the Suprachiasmic Nucleus?

The neurotransmitter associated with the regulation of blood flow and metabolism by the Suprachiasmatic Nucleus (SCN) is norepinephrine. The SCN is known as the body's primary circadian clock, and it plays a critical role in controlling various physiological processes, including those related to sleep-wake cycles and metabolic functions.

Norepinephrine is particularly important in this context as it influences a variety of bodily responses through its action on the sympathetic nervous system, including the regulation of blood flow. It affects vascular tone and distribution of blood flow to various organs based on the body's metabolic needs. Additionally, norepinephrine is involved in the modulation of various hormonal secretions, which further ties into metabolism and energy balance.

In contrast, while dopamine is involved in reward and pleasure, serotonin is primarily associated with mood regulation. GABA, on the other hand, is an inhibitory neurotransmitter that generally functions to calm neuronal activity. These roles, while significant, do not directly correlate to the functions of the SCN in regulating blood flow and metabolism as norepinephrine does. Thus, the clarity with which norepinephrine is linked to the functionalities of the Suprachiasmatic Nucleus makes it the correct choice in this context.