Retinopetal projections in lampreys.

In the lamprey Lampetra fluviatilis the centrifugal visual system is well developed. Centrifugal fibers arise from cells of two tegmentomesencephalic nuclei-reticular mesencephalic area and nucleus M5 of Schober-as demonstrated by axonal tracing techniques and antidromic neuronal recordings after optic nerve stimulation. Amacrine and ganglion cells in the retina are targets of centrifugal innervation, as shown by electrophysiology and axonal tracing. Following postembedding immunogold labeling, 40% of centrifugal axon terminals in the retina express immunoreactivity to gamma-aminobutyric acid (GABA). About 65% of neurons in M5 and 15% of those in the reticular mesencephalic area have been observed to be immunoreactive for GABA. Combined horseradish peroxidase (HRP) labeling and double immunogold staining were used to investigate the central organization of the retinal feed-back loop. Some terminals of optic nerve fibers from the retina made direct synaptic contacts on dendrites of centrifugal cells of the reticular mesencephalic area and nucleus M5. Terminals of non-retinal origin, immunoreactive for GABA and glutamate, also made synapses on centrifugal neurons. Monosynaptic feed-back loops may be complemented by interneurons as well as by excitatory and inhibitory inputs of non-visual origin.