Capsaicin fails to produce disturbances of autonomic heat and cold defence in an avian species (Anas platyrhynchos)

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Abstract
Capsaicin was intravenously administered to adult domestic ducks of 1.8–2.6 kg body weight, with a cumulative dose of 1.0 g/kg body weight given in 4–6 single infusions at intervals of 2–3 days. There were no acute, nociceptive or hypothermic effects, as typically seen in mammals. Before and after capsaicin treatment respiratory evaporative heat loss (REHL, w·kg−1), breathing frequency (BF, min−1) and metabolic heat production (M, w·kg−1) were determined in a warm environment (35–38°C) as a function of core temperature, measured in the esophagus (T es), which was altered by graded heat extraction with a colonic thermode. Capsaicin treatment reduced the rate at which REHL increased with increasing BF, however, this was compensated by a steeper increase of BF with risingT es so that the relationship betweenT es and REHL remained unchanged. TheT es threshold for activation of M was increased by 0.3°C and the slope reduced by 27% after capsaicin, but identical maximum M values were attained before and after capsaicin at identical degrees of hypothermia. Skin temperature measurements revealed no influence of capsaicin on the thresholdT es values for skin vasoconstriction. It is concluded that capsaicin fails to exert effects in birds on those afferents and central neurons which are involved in thermo- and nociception, in contrast to mammals in which these perceptive functions become severely impaired.