CEREBROSPINAL-FLUID BETA-ENDORPHIN IN MODELS OF HYPERALGESIA IN THE RAT

Cerebrospinal fluid (CSF) obtained by acute percutaneous puncture of t he cisternal membrane of the halothane anesthetized rat has low but me asurable concentrations of beta-endorphin-like immunoreactivity( beta- EPir: 32.8 +/- 3.0 pmol/l). Chromatographic separation of beta-EPir sh owed that authentic beta-endorphin(1-31) was the main component of bet a-EPir in cisternal CSF. Subcutaneous injection of 5% formalin in the hind paws did not increase beta-EPir in cisternal CSF. Rats with tacti le paw hyperalgesia evoked by unilateral ligation of the L(5/6) nerve soots 2 weeks earlier had beta-EPir concentrations that did not differ from sham operated or unoperated control animals. In contrast, capsai cin injected in the hindpaws increased the mean beta-EPir concentratio n compared to saline injections (P = 0.006) 45 min after emerging from anesthesia following injection. These results show that acute activat ion of C fibers (by capsaicin) will evoke the release of beta-endorphi n into the CSF, suggesting activation of the beta-endorphin terminal s ystems in the brain/midbrain. The failure of formalin injections to re lease beta-EPir to CSF may be due to specificity of the afferent stimu lus evoking beta-EPir release, a lower stimulus intensity, and/or the duration of the stimulus generated by formalin. The normal concentrati ons of beta-EPir found in the hyperalgesic state following nerve injur y suggest that the supraspinal beta-endorphin system does not display tonic changes under such conditions.