THE PREOPTIC NUCLEUS - THE PROBABLE LOCATION OF THE CIRCADIAN PACEMAKER OF THE HAGFISH, EPTATRETUS-BURGERI

By recording the locomotor activity rhythm of hagfish, Eptatretus burg eri, in which surgical lesions of the preoptic nucleus (PON) were made , we searched for the location of the circadian pacemaker in the hypot halamus. The characteristic rhythms were lost in animals lacking the a nterior part of the hypothalamus, which includes the PON. Cuts in fron t of the PON did not affect the rhythm, but cuts behind the PON caused the animals to loose their rhythms. Destruction of the PON with a hig h-frequency lesion generator also caused loss of rhythms. These findin gs indicate that the pacemaker may be located in the PON, which has re tinofugal connections.