span>n <span  title="" class="an ">&ensp;</span>M <span  title="" class="an ">&ensp;</span>and <span  title="" class="an ">&ensp;</span>methionine <span  title="" class="an ">&ensp;</span>enkephalin <span  title="" class="an ">&ensp;</span>micro <span  title="" class="an ">&ensp;</span>M <span  title="" class="an ">&ensp;</span>concentration <span  title="" class="an ">&ensp;</span>dependently <span  title="" class="an ">&ensp;</span>inhibited <span  title="" class="an ">&ensp;</span>the <span  title="" cla