The influence of hypothalamic temperature on the activity of warm-excited neurones, which responded to skin warming with an increased firing rate, in the ventro-basal (VB) complex of the thalamus of rats was examined electrophysiologically. 2. The warm-excited neurones were classified into three types: hypothalamus-cold neurones in which the firing rate increased with hypothalamic cooling, hypothalamus-warm neurones in which the firing rate increased with hypothalamic warming and hypothalamus-insensitive neurones in which the firing rate was not affected by hypothalamic temperature. The majority of hypothalamus-cold and hypothalamus-warm neurones increased their firing rate at hypothalamic temperature below and above 38 degrees C, respectively. 3. The threshold temperature at which hypothalamus-warm neurones responded to skin warming was lowered by hypothalamic warming. However, hypothalamus-cold neurones responded to lower skin temperatures during hypothalamic cooling. 4. These results show that the neuronal activity of the VB complex in the thalamus, responding to skin warming, is affected by hypothalamic temperature. Thus thermal information from the peripheral thermoreceptors is modulated by hypothalamic temperature at the level of the relay nuclei of the thalamus.
