This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Substance P, actions are all except \nSelect and explain. \nA. Vasoconstriction, B. Pain transmission, C. Axon reflex, D. Peristalsis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Substance P saw at\n\nPrimary pain nerve endings in the dorsal horn of the spinal cord\nHypothalamus\nNigrostriatal pathway of basal ganglia\n\nIt is probably involved in pain transmission, axon reflex and peristalsis.', 'role': 'assistant'}]