Different mechanisms are involved in the respiratory depression and analgesia induced by neurotensin in rats

The tridecapeptide neurotensin (NT), injected intracerebroventricularly in rats, induced a naloxone-insensitive and dose-dependent analgesia (tail-flick test), the ED50 being 3.22 nmol/animal. Higher doses induced a dose-dependent respiratory depression, mostly accounted for by a reduction of frequency; the ED50 for this action was 144.6 nmol/animal. In contrast to analgesia, the respiratory depression was antagonized by naloxone, suggesting the possibility that NT and opioid systems might interact at the level of respiration-related nuclei.