The catecholamine content of hypothalamic nerve cells after acute exposure to cold and thyroxine administration.

1. In female rats, the intensity of the catecholamine fluorescence was determined in nerve cells of the arcuate and periventricular hypothalamic nuclei by means of a microfluorimetrical technique which is based on the fluorescence method of Falck & Hillarp. Frequency distributions were established by grouping the nerve cells according to their fluorescence intensity which was referred to that of a noradrenaline standard.2. When rats were exposed to a cold environment (4 degrees C) on dioestrous day 1, the frequency distribution of relative fluorescence intensity was shifted towards classes of higher fluorescence intensity within 10 min. The mean relative fluorescence intensity and the symmetry of the distribution did not change further during the next 20 min.3. The change in the frequency distribution was almost completely prevented by L-thyroxine (50 mug/kg) administered 3 hr before exposure to cold.4. The direction of the cold-induced change in the frequency distribution is interpreted as reflecting an increase in neuronal activity. Since it was observed in nerve cells which belong to catecholamine-containing tubero-infundibular neurones, it might be related to the release of thyrotropin. Differences in the frequency distributions of the anterior and posterior tuberal regions are discussed with regard to possible functional implications.