DISTRIBUTION OF SYNAPSES IN THE LATERAL GENICULATE-NUCLEUS OF THE CAT- DIFFERENCES BETWEEN LAMINAE-A AND LAMINAE-A1 AND BETWEEN RELAY CELLS AND INTERNEURONS

Laminae A and A1 of the lateral geniculate nucleus in the cat are gene rally considered to be a structurally and functionally matched pair of inputs from two eyes, although there are subtle light microscopic and physiological differences. The present study aims to display ultrastr uctural differences between these two laminae based on electron micros copic observances on the connectivity patterns of their afferents onto two main cell types: relay cells, and interneurons present in this nu cleus. In a design of population measurement from randomized sample ar eas in laminae A and A1 from six brains, all synaptic contacts made by three terminal types of the geniculate nucleus were identified, and a number of relative distribution properties were analyzed. When the A- laminae were considered as a homogeneous structure, the distribution o f the three terminal types on geniculate cells was similar to previous ly reported results, confirming the validity of the sampling strategie s used; RLP (retinal) terminals provided one-fifth of all synapses, wh ereas RD (from cortex and brainstem) and F (inhibitory) types constitu ted one-half and one-third, respectively. The relay cells alone receiv ed a similar composition of afferents. However, interneurons alone rec eived approximately equal amounts of synapses from the three sources. Similar analyses comparing the distributions in lamina A and A1 reveal ed that RD and F terminals, but not RLP terminals, innervate these two laminae differently; more RD and fewer F terminals were found in lami na A1. This difference was also present in the distribution of termina ls on relay cells alone, but not on interneurons. These results sugges t that (1) retinal terminals form a significantly larger fraction of t he input to interneurons than to relay cells; correspondingly, cortex and brainstem provide a smaller fraction of all inputs to interneurons than to relay cells; and (2) laminae A and A1 are not strictly equiva lent projection sites of the two retinae. The results are discussed in relation to the Y-cell subpopulation in lamina A1 that is involved in corticotectal, as well as corticogeniculate circuits, as opposed to Y -cells of lamina A that are involved in only the latter. (C) 1998 Wile y-Liss, Inc.