Figure 3.

Nonlinear computation of receptor activation by dopamine diffusion. (a) Sphere of influence of a single dopaminergic terminal over ${\text{D}}_1$- and ${\text{D}}_2$-type receptors. Time over which the ${\text{EC}}_{50}$ of each receptor type is exceeded is plotted as a function of distance from the terminal. ${\text{D}}_1$- type receptors are insensitive to quantal release at 7 μm radial distance beyond the release site, whereas ${\text{D}}_2$-type receptors can be influenced by a single quantal release through a radial distance of 17 μm. The duration during which the ${\text{EC}}_{50}$ of each receptor type is exceeded decreases monotonically with distance. Broken lines represent spheres of influence of linear uptake kinetics with the nominal $r_{\text{max}}$ and $K_{\text{m}}$ values listed in Table 1. (b) Effect of dopamine reuptake blocker, cocaine, on sphere of influence of ${\text{D}}_1$-type receptors, plotted as receptor activation duration as a function of distance from release site. (c) Same competitive inhibition effect on ${\text{D}}_2$-type receptor activation. Cocaine increases the sphere of influence on ${\text{D}}_2$-type receptors dramatically.