Biol. Bull. 207: 153. (October 2004)
© 2004 Marine Biological Laboratory
Circadian Clock: Where Is It Located in the Limulus Brain?
T. Saito1,
K. Mori2 and
R. Barlow1
1 SUNY Upstate Medical University, Syracuse, New York
2 Tsukuba University, Tsukuba, Japan
At night a circadian clock in the brain of the horseshoe crab, Limulus polyphemus, transmits efferent optic nerve activity to the ventral, median, and lateral eyes. The efferent input increases the sensitivity of the lateral eyes, enabling the animals to find mates under dim nocturnal levels of illumination. We investigated the source of the clock-generated efferent optic nerve activity. A previous study reported that cell bodies of the efferent nerve fibers are located in the cheliceral ganglia, bilateral clusters of neurons at the lateral, posterior edges of the protocerebrum (Calman and Battelle, Vis. Neurosci. 6:481–495, 1991). The cells were identified by retrograde transport of Neurobiotin in optic nerve fibers. We report here that retrograde transport of cobalt plus nickel in the optic nerve fibers reveals neuronal cell bodies in the central body of the protocerebrum. The large U-shaped central body is posterior and medial to the primary visual centers, the lamina and medulla. Situated near the center of the protocerebrum, the bilateral clusters of neurons of the central body are linked by a connecting commissure. Retrograde transport reveals a small number of neurons (
15) consistent with electrophysiological recordings (R. Barlow, J. Neurosci. 3:856–870, 1983). We attempted to locate the source of clock-generated efferent activity by selective lesions. Lesioning the circumesophageal connectives, including the cheliceral ganglion, did not block the efferent activity, nor did lesions of the protocerebrum posterior to the central body and of the dorsal portion (10%) of the central body. Efferent activity was abolished only by a deep lesion of the central body, that is, an ablation of the entire central body. We conclude that neurons in or below the central body are the sources of clock-generated efferent optic nerve activity. Attempts to record clock-like efferent activity from these neurons have not succeeded.
Support by NSF, NEI, NIMH, RPB and Lions of Central New York.
