The retinal dopaminergic system is a global regulator of retinal
function. Apart from the fact that the rates of dopamine synthesis and
release are increased by increasing illumination, the visual image
parameters that influence dopaminergic function are mostly unknown.
Roles for spatial and temporal frequency and image contrast are
suggested by the effects of form-deprivation with a diffusing goggle.
Form-deprivation reduces the rates of dopamine synthesis and release,
and induces myopia, which is prevented by dopamine agonists. Our
purpose here was to identify visual stimulus parameters that activate
dopaminergic amacrine cells and elicit dopamine release. White Leghorn
cockerels 4–7 days old were exposed to 2 h of form-deprivation,
reduced light intensity, or stimuli of varied temporal or spatial
frequency. Activation of dopaminergic neurons, labeled for tyrosine
hydroxylase (TH), was assessed with immunocytochemistry for c-Fos, and
dopamine release was measured by HPLC analysis of dopamine metabolite
accumulation in the vitreous body. Form-deprivation did not reduce TH+
cell activation or vitreal dopamine metabolite accumulation any more
than did neutral-density filters of approximately equal transmittance.
TH+ cell activation and vitreal metabolite accumulation were not
affected significantly by exposure to 2, 5, 10, 15, or 20 Hz
stroboscopic stimulation on a dark background, or by sine-wave gratings
of 0.089, 0.44, 0.89, 1.04, or 3.13 cycles/deg compared to a
uniform gray target of equal mean luminance. These data indicate that
the retinal dopaminergic system does not respond readily to short-term
changes in visual stimulus parameters, other than light intensity,
under the conditions of these experiments.