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 when it was administered during the early hours of the morning (01:20–02:20 h). The one case in which early morning administration provoked an ‘excitation’ was in a cat without a regular rhythm. Units tested were in Rexed's laminae I, III, IV and V. These results suggest that an endogenous opioid factor is released into the circulation in amounts which vary on a diurnal cycle and that this factor crosses the blood-brain barrier at the spinal level to exert an inhibitory effect on transmission specifically in nociceptive pathways at the spinal level.