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
Mechanism of analgesia is bya) Nociceptin stimulationb) Nocistatin stimulationc) Anandamide receptorsd) Nicotinic & cholinergic receptors 
Select and explain. 
A. abc, B. acd, C. abd, D. bcd
D: bcd
Answer: D: bcd 
Explanation: Nocistatin stimuli inhibit nociceptors and produce analgesia.
Anandamide (a cannabinoid) produces analgesic effect by binding to CB1 receptors.
Acetylcholine and nicotinic cholinergic receptors (subunits α4 and β 2) are involved in regulation of pain.