Developmental changes in environmentally induced analgesia.

Previous studies have shown that exposure to inescapable, front-paw shock produces an opioid-mediated analgesia. Additionally, research has revealed that the majority of opioid receptors in the central nervous system are formed between birth and adulthood. The purpose of the present experiment was to examine the relationship between the development of the opioid receptors and the function of the endogenous opioid pain-inhibitory system activated by shock. Ten-day-old, 28-day-old and 5-7-month-old rats were exposed to 90 s of front-paw shock (1.6 mA). Results revealed that 10-day-old rats displayed lower levels of shock-induced analgesia than 28-day-old and 5-7-month-old rats. This age-related difference in shock-induced analgesia confirms a parallel in the development of opioid receptors and the function of an endogenous pain-inhibitory system. In addition, injection of naloxone produced an increased analgesia in the 10-day-old rats. In the 28-day-old rats naloxone completely blocked the shock-induced analgesia while in the 5-7-month-old rats naloxone only partially attenuated analgesia. This age-related difference (28-day-old vs 5-7-month-old) in the effectiveness of naloxone in blocking shock-induced analgesia suggests the involvement of a non-opioid analgesia system in the 5-7-month-old rats that is not present in the 28-day-old rats. This last difference led to the speculation that the non-opioid analgesia system develops more slowly than the opioid system.