An important finding in the present study is that the plasma levels of β-endorphin the endogenous ligand for the MOP receptor were significantly lower in the chronic pain patients compared to control subjects. This observation agrees with previous reported findings showing that plasma levels of β-endorphin correlate inversely with pain levels in patients with various pain syndromes [38-42], i.e. the plasma concentration of this opioid peptide are lower in patients with poorly controlled pain but increase with pain relief [42]. Interestingly, in the present study opioid responders had higher plasma concentration of β-endorphin than non-responders.
