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Max-Planck-Institut für physiologische und klinische Forschung, W. G. Kerckhoff-Institut, D-6350 Bad Nauheim, Federal Republic of Germany

1. Five Pekin ducks were chronically implanted with a device allowing thermal stimulation of the hypothalamus and simultaneous recording of hypothalamic single unit activity on the conscious animals in repeated experiments. In addition, the core temperature of the animals could be lowered by means of a thermode tube which was placed in the colon and was perfused by a cold solution. Hypothalamic temperature was measured in the centre of the hypothalamic thermode array; core temperature was measured in the axillary pit.

2. Each unit was tested in two periods of hypothalamic ramp cooling, one was performed at normal core temperature, and the other at a lowered core temperature during sustained intestinal cooling.

3. Among forty-six neurones exhibiting a local Q₁₀ > 2 of their discharge rate, intestinal cooling was found to activate 26% (fall feed-back units), to inhibit 44% (rise feed-back units), and not to affect 30% (non-reactive units). The local Q₁₀ values of the fall feed-back units were, on average, significantly higher than those of the rise feed-back units.

4. By multiple linear regression analysis the thermal coefficients (impulses/sec. °C) relating unit discharge to hypothalamic (local) and to core (remote) temperature changes were evaluated. The fall feed-back units exhibited average local temperature coefficients of 0·79±0·11 and remote coefficients of -2·75±0·56 (means±S.E. of mean); the corresponding coefficients of the rise feed-back units were determined as 0·44±0·08 and +2·33±0·41.

5. The results of this study support the hypothesis that the activity of hypothalamic neurones conveying extrahypothalamic cold signals is depressed more by hypothalamic cooling than that of the neurons conveying extrahypothalamic warm signals. This would explain the paradoxical effects of hypothalamic cooling on thermoregulatory effector activity in birds.

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