A mechanism for the analgesic effect of neurotensin as revealed by behavioral and electrophysiological techniques.

Behavioral and electrophysiological techniques were used to examine the effects of local injections of neurotensin (NT) into the periaqueductal gray (PAG). The results of the behavioral experiments showed that injection of NT into the PAG produced dose-dependent analgesia that lasted for as long as 60 min and was not blocked by naloxone. However, electrolytic lesions of the nucleus raphe magnus (NRM) and its surrounding area, abolished the analgesic effect of NT. Electrophysiological experiments indicated that micro-pressure application of NT onto neurons in the PAG had a predominantly excitatory effect. Furthermore, it was shown that injection of NT into the PAG produced excitation of the NRM neurons. It is concluded that NT produces its analgesic effect by excitation of PAG neurons which leads to activation of the pain inhibitory system that originates from the NRM and its surrounding areas in the medulla.