Diurnal variation in the effect of angiotensin II on nociception.

The aim of this study was to investigate the effect of angiotensin II (Ang II) on nociception at particular time points of a 24-h cycle using different pain stimuli. A parallel investigation of phasic and tonic pain tests revealed different diurnal patterns of pain responses. Phasic pain test (mechanical paw pressure) in rats was characterized with shortest latencies during the dark phase, when the average of motor activity is greatest. Ang II (0.1 microg/animal) increased the latency of pain responses to mechanical and thermal stimulations mainly during the active dark phase. With regard to tonic pain, regardless of a weak circadian fluctuation of the number of pain responses (writhes) in mice, there was a tendency to attenuate the diurnal pattern of nociception. In contrast to the effect of Ang II on the phasic pain, it exerted an antinociceptive effect in the writhing test during the light phase. In summary, Ang II exerted an antinociceptive effect at the time points that have naturally high pain sensitivity.