The influence of preoperative concentrations of β-endorphin and Met- enkephalin on the duration of analgesia after transurethral resection of prostate

β-Endorphin (β-EP) and methionine-enkephalin (M-EK) are endogenous peptides that play a role in the modification of pain perception and analgesia threshold. In order to understand more about pathophysiology of pain in association with neuroaxial blocks, we evaluated cerebrospinal fluid (CSF) concentrations of β-EP and M-EK prior to spinal anesthesia (SA) in patients undergoing transurethral resection of prostate (TURP) to determine the correlation between preanesthesia concentrations and the duration of postoperative analgesia and opioid requirements. Twenty-five healthy patients undergoing TURP under SA were enrolled. β-EP and M-EK were measured with a competitive radioimmunoassay. Mean preoperative β-EP and M-EK concentrations were 153 ± 44 and 38 ± 5 pg/mL, respectively. Those with β-EP concentrations >153 pg/mL had significantly longer analgesia (P < 0.01), and lower utilization of morphine in the first postoperative day (P < 0.01). Moreover, patients with milder postoperative pain (visual analog scale score <4/10) had significantly higher β-EP concentrations (P < 0.01). A similar correlation was not found with M-EK values. These data suggest that preoperative CSF β-EP, but not M-EK, concentrations correlate with the duration and quality of postoperative analgesia, as well as opioid requirements after spinal anesthesia.