“…Blocking selectins, ICAM-1, integrins ␣ 4 and  2 , or chemokines CXCL1 and CXCL2/3 in vivo substantially decreased opioid-producing immune cells in inflamed tissue or injured nerves (Machelska et al, 1998(Machelska et al, , 2004Brack et al, 2004c;Labuz et al, 2009). In addition, the recruitment of opioid-containing cells is dependent on neurokinin-1 receptors (which bind substance P) (Rittner et al, 2007b), on sensory and sympathetic neurons (Kager et al, 2010), and may be regulated by adhesion to neurons (Hua et al, 2006). Central mechanisms also play a role because intrathecally administered analgesic doses of morphine led to a decreased number of END-positive leukocytes in inflamed rat paws (Schmitt et al, 2003).…”