Figure 2. Sound stress increases PGE₂ hyperalgesia in masseter muscle.

Administration of PGE₂ (1 μg in 20 μl) produced a marked decrease in mechanical nociceptive threshold in the masseter muscle 1 h post-injection that returns to near baseline in control (non-stressed) rats (n=8), when tested 4 h post-injection. A similar PGE₂ hyperalgesia was present in rats 1 d after the last session of chronic unpredictable sound stress (n=6). In contrast, 14 d after sound stress (n=8), PGE₂ hyperalgesia was still near maximal 4 h post-injection (P<0.001). Inset figure shows that exposure to sound stress had little effect on nociceptive threshold (6.8% increase).