Thermal stimulation of hypothalamic neurones in unanaesthetized rabbits.

1. A technique has been devised for recording unit activity in the anterior hypothalamus of conscious rabbits during the controlled displacement of local temperature by 1-2 degrees C. The region at 1 and 2 mm from the mid line was explored.2. All the units studied showed spontaneous activity before thermal stimulation with a mean rate of 9 impulses/sec (range 1/16 sec to 65/sec).3. Twenty-seven (10%) of the recorded neurones showed a change in firing rate which could be related to the temperature changes. Twenty-one of the cells were ;warm-sensitive' and were excited when temperature was raised or inhibited when it was lowered. The other six units were ;cold-sensitive' and showed the opposite type of response.4. Apart from this directional grouping, it was possible to classify the responses into four categories: A, five cells whose firing rate was always proportional to local temperature over a range from 2 degrees C below to 2 degrees C above body temperature; B, six cells whose average level of firing changed during the period of observation, but whose sensitivity to temperature was not affected; C, eight cells which showed a threshold and were only affected by temperature above or below a certain level; D, four cells whose changes in frequency either led or lagged behind the temperature changes.5. The positions of these sensitive units in the hypothalamus did not show any apparent pattern, except that 75% of them were found 1 mm lateral to the mid line; the remaining 25% were 2 mm lateral.