Mu receptors are the target of opioids. Opioids have various side effects, many of which are due to Mu receptors found in all below EXCEPT.

A. Peripheral neurons: Mu opioid receptors are present on peripheral sensory neurons, contributing to analgesia by reducing nociceptive signaling. Activation here can also modulate inflammatory pain responses. B. Brain and spinal cord: Central nervous system mu receptors mediate analgesia, euphoria, respiratory depression, sedation, and dependence. Most of the clinically significant opioid effects occur through these central receptors. C. Gastrointestinal tract: Mu receptors in the GI tract reduce gastrointestinal motility, leading to common side effects such as constipation, delayed gastric emptying, and nausea. This is a well-known peripheral effect of opioids. D. Renal arteries: Mu receptors are not significantly expressed in renal arterial tissue. Opioid-related side effects do not arise from direct action on the renal vasculature, making this the exception among the listed sites.