Fig. 4. Effect of liposome injection on opioid-containing leukocytes and on cold-water swim stress (CWS)–induced antinociception: Rats (n = 7/group) were injected according to the schedule described in  figure 3. Clodronate-containing liposomes induced a significant reduction of 3E7⁺opioid-containing leukocytes at both time points (  A , *  P < 0.05,  t test;  black bar = phosphate-buffered saline [PBS];  white bar =clodronate-containing liposomes;  dashed bar = empty liposomes). A separate set of rats (n = 6/group) was injected as described above. Paw pressure threshold (PPT) increases after CWS were significantly lower in rats injected with clodronate-containing liposomes (  B , *  P < 0.05,  t test; MPE = maximum possible effect). All data are presented as mean ± SEM.