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Hypothalamic thermo‐responsive neurones in the new‐born rat.

1. Single unit activities were recorded from the neurones in the preoptic area and anterior hypothalamus of developing new‐born rats (aged 1‐24 days old) during thermal stimulation of the brain. During the first 2 weeks of life, about 80% of these neurones had low spontaneous firing rates between 0.1 and 5 impulses/sec at 38 degrees C hypothalamic temperature (Thyp). 2. Out of 640 units studied, 118 units increased the firing rate upon elevation of Thyp (warm‐units) and fourteen showed the opposite type of response to temperature changes (cold‐units). Warm‐units were found in the rats of all the age span studied and cold‐units were recorded in the rats more than 8 days old. 3. Thermal coefficients of warm‐units and cold‐units varied between +0.11 and +2.47 and between ‐0.10 and ‐0.49 impulses/sec, degrees C, respectively. Number of warm‐units with higher rates of firing and greater thermal coefficients, comparable to those of warm‐units in the adult, gradually increased with growth. The thermal responsiveness of warm‐units, when expressed by Q10, are already high even in the immediate neonatal period. Their Q10 values were in the range between 2 and 38.5 (mean 6.4). 4. Units responding to extrahypothalamic temperatures were only found in the rats more than 14 days old. 5. All the six warm‐units tested increased the firing rates following subcutaneous injections of capsaicin, while the majority of thermo‐unresponsive units were not affected by this drug. 6. It is suggested that thermo‐responsive neurones in the preoptic area and anterior hypothalamus in the new‐born rat have attained some degree of electrophysiological maturity, despite their slowly firing characteristics.