RETINOPETAL PROJECTIONS IN LAMPREYS

In the lamprey Lampetra fluviatilis the centrifugal visual system is w ell developed. Centrifugal fibers arise from cells of two tegmentomese ncephalic nuclei reticular mesencephalic area and nucleus M5 of Schobe r - as demonstrated by axonal tracing techniques and antidromic neuron al recordings after optic nerve stimulation. Amacrine and ganglion cel ls in the retina are targets of centrifugal innervation, as shown by e lectrophysiology and axonal tracing. Following postembedding immunogol d labeling, 40% of centrifugal axon terminals in the retina express im munoreactivity 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 peroxidas e (HRP) labeling and double immunogold staining were used to investiga te the central organization of the retinal feed-back loop. Some termin als of optic nerve fibers from the retina made direct synaptic contact s on dendrites of centrifugal cells of the reticular mesencephalic are a and nucleus M5. Terminals of non-retinal origin, immunoreactive for GABA and glutamate, also made synapses on centrifugal neurons. Monosyn aptic feed-back loops may be complemented by interneurons as well as b y excitatory and inhibitory inputs of nonvisual origin.