| In cats, information from the retina through the lateral geniculate nucleus (LGN) to the visual cortex flows along three physiologically and morphologically distinct neuronal streams---W, X and Y. The LGN is a thalamic nucleus that modulates and then relays retinal information to the visual cortex. Geniculate interneurons and perigeniculate (PGN) neurons constitute two sources of the inhibitory input to the dLGN. Both of these cells use the inhibitory neurotransmitter gamma-aminobutyric acid (GABA; LeVay and Ferster, 1979; Weber and Kalil, 1983; Madarasz et al., 1985) and have been shown to provide extensive GABAergic innervation of the dLGN thalamocortical cells through their axonal and dendritic terminals (Hamos et al., 1985; Montero and Singer, 1985; Montero, 1986; 1987; 1991; Cucchiaro et al., 1991).;It is well established that interneurons and PGN cells affect transfer of the visual information in the X visual pathway. However, organization of inhibitory circuitry of the Y visual pathway is still unclear.;To address this issue we examined organization of synaptic connections of Y thalamocortical cells, interneurons, and PGN neurons in the A and magnocellular C laminae of the LGN and PGN by using anatomical techniques.;We found that in the A and magnocellular C laminae of the LGN interneurons receive synaptic input from the Y retinogeniculate axons (≈40%) and that in the magnocellular C lamina geniculate interneurons contact Y thalamocortical cells (≈99%). This indicates that interneurons are present in the Y visual pathway and therefore affect transfer of the visual information in this pathway.;We also found that distal dendrites of the Y thalamocortical cells located in the PGN receive most of their input from the non-GABAergic corticothalamic (≈67%) and cholinergic axons (≈23%), while input from GABAergic PGN neurons was found to be the sparsest (≈10%). Thus GABAergic PGN neurons do not provide significant input to and therefore do not have significant inhibitory influence on the circuitry of the Y visual pathway. |
