Final answer:
The μ-opioid receptor (MOR) is responsible for respiratory depression, physical dependence, supraspinal analgesia, and muscle rigidity caused by opioids, with naloxone being an antagonist used to reverse these effects in overdose situations.
Step-by-step explanation:
The opioid receptor responsible for respiratory depression, physical dependence, supraspinal analgesia, and muscle rigidity is the μ-opioid receptor (MOR). This receptor is one of several types of opioid receptors in the body and is the primary site of action for most opioids, including morphine, heroin, methadone, and fentanyl. When opioids bind to μ-opioid receptors, they can produce analgesic effects, which are helpful for pain management.
However, the activation of these receptors also leads to adverse effects such as respiratory depression, which is a leading cause of overdose deaths. Physical dependence can develop from prolonged opioid receptor activation, prompting withdrawal symptoms upon cessation. Furthermore, agents like naloxone are used to counteract these effects in cases of overdose by blocking the opioid receptors.