TEMPERATURE SENSITIVITY OF THE SUPRACHIASMATIC NUCLEUS OF GROUND-SQUIRRELS AND RATS IN-VITRO

Temperature compensation of circadian rhythms in neuronal firing rate was investigated in the suprachiasmatic nucleus (SCN) of ground squirr els and rats in vitro. A reduction in SCN temperature from 37 to 25 de grees C reduced peak firing rates by > 70% in rats but only by approxi mate to 21% in squirrels; trough firing rates were marginally altered in both species. In the rat SCN at 25 degrees C, the peak in neuronal activity decreased progressively on successive days and circadian rhyt hms no longer were present by Day 3. There was a 37% reduction in the number of single units detected and an increase in the temporal variab ility of peak firing rates among individual rat SCN neurons at low tem perature. By contrast, single units were readily detected and circadia n rhythms were robust in squirrels at 37 and 25 degrees C; a Q(10) of 0.927 was associated with a shortening of tau by 2 h and a 5-h phase c hange after only 48 h at low temperature. These results suggest that t emperature can have a substantial impact on circadian organization in a mammalian pacemaker considered to be temperature compensated.