Final answer:
Buprenorphine is the partial mu agonist that can treat pain in lower doses and acts as an antagonist at higher doses.
Step-by-step explanation:
The substance that is a partial mu agonist which treats pain at lower doses and antagonises the receptor at high doses is Buprenorphine. Buprenorphine has a unique pharmacological profile that allows it to act as an analgesic at lower doses due to its partial agonist activity at the mu-opioid receptor. However, at higher doses, it can block the effects of other full agonists by acting as an antagonist. This is in contrast to other substances like Naloxone, which is a narcotic antagonist and not an agonist at any dose, and Fentanyl, which is a full opioid agonist with no antagonist properties. Methadone is a full opioid agonist, which is used in maintenance therapy for opioid addiction and also as an analgesic, but does not have antagonist properties at high doses.