A possible pathway connecting the photosensitive pineal eye to the swimming central pattern generator in young Xenopus laevis tadpoles.

The pineal eye of young Xenopus laevis tadpoles mediates a swimming response to dimming. Our aim was to define pathways that allow pineal photoreceptors to influence the swimming central pattern generator (CPG) in the hindbrain and spinal cord. Retrograde filling with horseradish peroxidase (HRP) and carboxyfluorescein showed that: (1) pineal ganglion cells do not project to the hindbrain, and (2) diencephalic/mesencephalic descending (D/MD) neurons, which could be contacted by pineal ganglion cell axons, do project to the hindbrain. Lesion experiments demonstrated that ganglion cell axons form ipsilateral and contralateral connections, either of which is sufficient to mediate a swimming response. Latency measurements suggest that the contralateral pathway is stronger than the ipsilateral one. Multiple unit recordings from the midbrain in the region of the D/MD neurons showed short latency activity in response to dimming or a brief current pulse to pineal axons. This activity could last for many seconds after the stimulus. Pharmacological experiments showed that it depended on synaptic excitation and suggested that the ganglion cell transmitter is glutamate. If pineal ganglion cells excite midbrain D/MD neurons on both sides of the brain, the D/MD neuron projections to the hindbrain could excite the swimming CPG and initiate swimming. Copyright (R) 2000 S. Karger AG, Basel