Can you tell me about neuroendocrinology q! how come we can't make an inference of what hormones in the blood is in the brain?

Blood brain barrier: The blood brain barrier are cells that regulate what gets into the CNS. For example, insulin levels fluctuate widely in blood, based on food, but CNS insulin fluctuates much less. Also, many hormones act locally in brain. For example, serotonin, dopamine etc are high at the nerve synapse, but most is then taken up and plasma levels are very low.