This script was used for Fig. 6, in which we tried to show
that many parameters, such as the baseline dopamine concentration,
were not restrictive. Hence the experimental responses could be
reproduced with other values for these parameters.

In this particular example, baseline dopamine was at 337 nM (instead of 50).
The doses of TC-7020 and nicotine had to be increased accordingly.
Because this is still an implementation of the desensitization model,
a baseline cholinergic tone was needed to allow for  the drop in dopamine 
efflux after TC-7020 injection. But too strong a tone would weaken, 
through competitive inhibition or saturation, the nicotine response,
which was caused mainly by the a4b2 receptors.

To solve this trade-off we assumed there to be a higher tone at a7 than at a4b2 
receptors. There are indeed experimental indications that a7 receptors are located
closest to the ACh release sites (for instance Bennett et al. J. Neurosci. 2012).
Technically this was done in the script be having a separate choline tone 
(of 30 microM) to which only the a7 receptors are sensitive.