Apomorphine modifies the visual responses of cells in the rabbit's 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 changes are reflected in the dorsal lateral geniculate nucleus (dLGN) by studying the effects of injections of apomorphine, a mixed D1 and D2 agonist of dopamine, on the visual responses of geniculate cells. Experiments were carried out on anesthetized adult pigmented rabbits. A varnished tungsten microelectrode was used to record single-unit activity in the dLGN. The flash electroretinogram was also recorded to monitor retinal changes and to confirm the success of the injections. Apomorphine 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 produced a strong reduction in response amplitude evoked by sine-wave grating stimuli, presented at various spatial frequencies. Responses to flashing spots were also reduced but to a much lesser extent than those to gratings. In addition, the balance between the responses to small and large spots changed in favor of large stimuli. Consequently, after injection of apomorphine, the geniculate cells were preferentially activated by large-sized flashing stimuli. These data suggest that apomorphine can reduce the spatial contrast sensitivity of cells in the dLGN. This effect could be mediated by the reduction of the strength of lateral inhibition at the retinal level.