Morphine, but not atropine, blocks nociceptor-driven activity in rat dorsal hippocampal neurones

Neurones in the CA1 region of the dorsal hippocampus of urethane-anaesthetized rats were recorded extracellularly with microelectrodes, and tested for their response to non-noxious and noxious peripheral stimuli. None of the cells responded to non-noxious stimuli but 91 of 216 cells were excited by noxious stimuli. Morphine (5.0 mg/kg i.v.) increased the background firing rate in 7 of 10 cells tested and blocked the response to noxious heating of the tail in all cases. Naloxone (0.5 mg/kg i.v.) reversed these effects of morphine. Atropine (0.1 plus 0.2 mg/kg i.v.), in another group of animals, produced only transient changes of an inconsistent nature in 7 cells tested.