APOMORPHINE MODIFIES THE VISUAL RESPONSES OF CELLS IN THE RABBITS LATERAL GENICULATE-NUCLEUS

It has been shown that enhancing or reducing dopaminergic activity in the retina modifies the balance between center and surround responses of retinal neurons such as ganglion cells. We investigated how these c hanges are reflected in the dorsal lateral geniculate nucleus (dLGN) b y studying the effects of injections of apomorphine, a mixed D-1 and D -2 agonist of dopamine, on the visual responses of geniculate cells. E xperiments were carried out on anesthetized adult pigmented rabbits. A varnished tungsten microelectrode was used to record single-unit acti vity in the dLGN. The flash electroretinogram was also recorded to mon itor retinal changes and to confirm the success of the injections. Apo morphine was injected intravitreally or intravenously. The results can be summarized as follows. Apomorphine decreased the amplitude of the b-wave of the electroretinogram. For most dLGN cells, apomorphine prod uced a strong reduction in response amplitude evoked by sine-wave grat ing stimuli, presented at various spatial frequencies. Responses to fl ashing spots were also reduced but to a much lesser extent than those to gratings. In addition, the balance between the responses to small a nd large spots changed in favor of large stimuli. Consequently, after injection of apomorphine, the geniculate cells were preferentially act ivated by large-sized flashing stimuli. These data suggest that apomor phine can reduce the spatial contrast sensitivity of cells in the dLGN . This effect could be mediated by the reduction of the strength of la teral inhibition at the retinal level.