Retinal and cortical afferents to the dorsal lateral geniculate nucleus of the turtle, Emys orbicularis: A combined axonal tracing, glutamate, and GABA immunocytochemical electron microscopic study

The dorsal lateral geniculate nucleus (GLd) of the turtle Emys orbicularis has been analyzed with axonal tracing methods and immunocytochemical techniques for glutamate (GLU) and γ‐aminobutyric acid (GABA), in combination with a quantitative study of the morphologic characteristics, distribution, and synaptology of the retinofugal and corticofugal terminals. Ultrastructural observations show that the vast majority of retinal terminals (Rtr) have clear, rounded synaptic vesicles and account for 16% of all profiles containing synaptic vesicles (PCSV). Their synaptic index (0.5) is low, and they make three times more contacts with the dendrites of projection cells than with those of interneurons. A low proportion of retinal terminals of a second category contain pleomorphic synaptic vesicles and are highly GABA immunoreactive. Axon terminals, unlabeled after intraocular injection of the tracer (SR), smaller in size and with more rounded clear synaptic vesicles, longer synaptic differentiations, and higher synaptic index than Rtr terminals, account for 19.7% of all PCSV and make asymmetric synaptic contacts with large dendrites of projection cells and less with the dendrites of interneurons. Some SR have been unambiguously identified as corticofugal terminals (Cg), either after cortical injection of the tracer (16%) or cortical lesion (37%). Retinal and Cg/SR terminals are spatially segregated within the GLd. Both are highly GLU immunoreactive, with the highest density of labeling over synaptic vesicles, suggesting that these terminals may use GLU as neurotransmitter. The level of GLU immunoreactivity of GABA‐positive profiles is half that of Rtr and Cg/SR terminals and is greatest over mitochondria, possibly reflecting the ‘metabolic’ pool of GLU that serves as a precursor in the formation of GABA. J. Comp. Neurol. 391:470–490, 1998. © 1998 Wiley‐Liss, Inc.