Extrasynaptic release of dopamine in a retinal neuron: Activity dependenceand transmitter modulation

Extrasynaptic release of dopamine is well documented, but its relation to t he physiological activity of the neuron is unclear. Here we show that in ab sence of presynaptic active zones, solitary cell bodies of retinal dopamine rgic neurons release by exocytosis packets of similar to 40,000 molecules o f dopamine at irregular intervals and low frequency. The release is trigger ed by the action potentials that the neurons generate in a rhythmic fashion upon removal of all synaptic influences and therefore depends upon the ele ctrical events at the neuronal surface. Furthermore, it is stimulated by ka inate and abolished by GABA and quinpirole, an agonist at the D-2 dopamine receptor. Since the somatic receptors for these ligands are extrasynaptic, we suggest that the composition of the extracellular fluid directly modulat es extrasynaptic release.