The preoptic nucleus: the probable location of the circadian pacemaker of the hagfish, Eptatretus burgeri.

By recording the locomotor activity rhythm of hagfish, Eptatretus burgeri, in which surgical lesions of the preoptic nucleus (PON) were made, we searched for the location of the circadian pacemaker in the hypothalamus. The characteristic rhythms were lost in animals lacking the anterior part of the hypothalamus, which includes the PON. Cuts in front 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 high-frequency lesion generator also caused loss of rhythms. These findings indicate that the pacemaker may be located in the PON, which has retinofugal connections.