Glutamate and dopamine modulate synaptic plasticity in horizontal cell dendrites of fish retina.

Horizontal cell dendrites protruding into the cone pedicles in fish retina exhibit a light-dependent plasticity. In a light-adapted retina they form numerous spinules having membrane densities at their tips. These spinules disappear during dark adaptation. Experiments with light- or dark-adapted retinas which were incubated in glutamate or its agonists and antagonists, respectively, revealed that this putative cone transmitter is able to reduce the expression of spinules in a light-adapted retina. Dopamine, on the other hand, induces the formation of spinules in a dark-adapted retina and haloperidol reduces the expression in a light-adapted retina. These data suggest a control of spinules plasticity through two retinal neurotransmitter systems.