Dopaminergic modulation of visual responses in toads. II. Influences of apomorphine on retinal ganglion cells and tectal cells.

The behavioral studies of Part I have shown in common toads that after systemic administration of the dopamine agonist apomorphine the prey-directed orienting turning movements are suppressed while prey snapping is facilitated. Part II focuses on retinal and tectal single cell responses to moving objects. (1) After systemic administration of apomorphine, the discharge rates of retinal class R2 and R3 ganglion cell fibres--recorded from the retino-tectal projection--speeded up in response to visual objects traversing their excitatory receptive fields. This enhancing effect was independent of the recording site in the retino-tectal map. (2) The diameters of the excitatory receptive fields of R2 and R3 neurons doubled their sizes. Probably, apomorphine enhances the center-dominated excitatory responses at the expense of the strength of the inhibitory surround. (3) The apomorphine-induced effects were fully developed 20-35 min after drug administration. (4) At the same time the discharge rates of T5.1 and T5.2 tectal neurons were reduced under apomorphine. The effect was independent of the recording site in the retino-tectal map. The diameters of the excitatory receptive fields of these tectal neurons were not influenced. (5) To changes in configurational stimulus features, the basic pattern of discrimination was maintained. (6) It is suggested that tectal output to the turn-generating motor network--mediated by T5.1 and T5.2 neurons--is modulated by a pretecto-tectal pathway which involves dopaminergic pretectal cells. (7) The enhanced snapping can be interpreted in terms of a modulation of reticular/hypoglossal structures by dopaminergic preoptic/hypothalamic/solitary systems.