NEURONAL ACTIVITY PATTERNS IN THE RAT VISUAL SYSTEM: THE ROLE OF DORSAL LATERAL GENICULATE NUCLEUS IN RETINO-CORTICAL TRANSMISSION

Aims: to characterize and analyze the responses of rat dorsal Lateral Geniculate Nucleus (dLGN) to different kinds of retinal inputs and outline its role in retino-cortical transmission. Methods: various light stimulation approaches were used together with pharmacological manipulations. A selective blocker of the bipolar ON cells (LAP4) and a potassium channel blocker (4-AP) were employed produce highly clean (LAP4, pure OFF output) and noisy (4-AP) responses to visual stimuli. The effect on geniculo-cortical transmission was studied via Visual Evoked Potential recordings, while the effect on the activation pattern of the dLGN was evaluated via immunostainings for the protein c-fos. Results: the rat dLGN showed a differential activation of excitatory and inhibitory neurons in the two experimental conditions, thus playing a great role in regulating retino-cortical transmission of different visual inputs. Indeed, it appeared to act as an effective filter for the noisy stimuli (4-AP) activating a strong inhibitory circuitry and effectively suppressing the information flow to the visual cortex. We also analyzed the spatial distribution of cells expressing c-fos in dLGN obtaining clear evidence for a non homogeneous arrangement of active cells. Conclusions: these data confirm the important role of dLGN in visual processing, with either suppression or enhancement of sensory transmission depending on the characteristics of the input signals, presumably according to the amount of information conveyed. The inhomogeneous distribution of active cells could be related either to a peculiar distribution of synapses and synaptic weights or to a strong shaping of active regions due to inhibitory circuits