Final answer:
Substance P (SP) is an endogenous pain modulator that acts as a neurotransmitter and neuromodulator in the central and peripheral nervous system. It is the preferred ligand for the neurokinin NK1 receptor in pain processing pathways. Endomorphin-2 and endomorphin-1 interact with the SP1 binding site, but not the µ-opioid receptor.
Step-by-step explanation:
Substance P (SP) is an endogenous pain modulator that belongs to the tachykinin family. It acts as a neurotransmitter and neuromodulator in both the central and peripheral nervous system. SP is the preferred endogenous ligand for the neurokinin NK1 receptor, which is expressed in pain processing pathways in the spinal cord.
Endomorphin-2 (EM-2) and endomorphin-1 (EM-1) are opioid receptor agonists that interact with the binding site of SP1, a metabolite of SP. EM-2 has a high affinity for the SP1 binding site, while EM-1 has a lower affinity. However, no specific binding or activation of the µ-opioid receptor was observed, indicating a specific target protein for SP1.