Neurons expressing calretinin in the developing feline dorsal lateral geniculate nucleus.

The dorsal lateral geniculate nucleus (LGNd) is a visual thalamic structure primarily responsible for transferring visual information from the retina to the visual cortex. It consists of several layers: A, A1, C magnocellular (Cm), and C parvocellular (Cp) in carnivores; and magnocellular, parvocellular, and koniocellular in primates. Two main populations of geniculate neurons include relay neurons (which consist of magnocellular, parvocellular, and koniocellular types in primates and their corresponding Y, X, and W types in carnivores) and interneurons. In primates, the Ca2+-binding protein calretinin (CR) labels the koniocellular neuron population. However, a similar marker for the corresponding W neurons in carnivores has not yet been looked at. To investigate whether CR could also serve as a marker for W neurons, we examined CR-immunoreactivity (ir) in the LGNd of cats aged 0-70 weeks. The main findings are as follows: (i) The density of CR-ir neurons is higher in A-layers; whereas C-layers are characterized by a higher density of immunoreactive fibers. (ii) In cats aged 0-4 weeks, the largest CR-ir cells are located in the Cp layers; these cells primarily have multipolar soma sized 90-285 μm2. In cats older than 5 weeks, the large CR-ir cells disappear. (iii) To investigate the inhibitory nature of CR-ir neurons, GAD67 labeling was applied. A partial overlap of the two markers was observed, with the fewest double-labeled cells found in the Cp layers. Based on the second finding, we hypothesize that the large CR-ir neurons may represent a transient population of W relay cells in cats.