Diurnal variation in excitation of dorsal horn units by naloxone in the spinal cat suggests a circulating opioid factor

Naloxone was administered in intravenous doses of 0.1–0.6 mg/kg in the spinal cat while recording extracellular spike activity from single, nociceptive dorsal horn units in lumbar segments 5–7 in the spinal transected cat. It was found that while naloxone increased on-going activity and excitatory responses to noxious radiant heat stimuli when it was administered during the day and early evening (12:30–20:00h) there was no observable effect …