nlme function examples for dose-respone

Hi,

I want to fit nonlinear dose-response curves, as "fun(X,a,b,c)", for
each of our 5 trail locations. Our data basis is something like

location plot year dose response

For each location there are 4 plots as repetitions (over 3 years). So
the interactions "location*year" and "location*plot" should be random
effects.

There are some examples in "Mixed-Effects Models in S and S-PLUS"
(Pinheiro and Bates), but I do not see how they can help me for my
model. Of course I can start with something like

mod <- nlme(response ~ fun(dose,a,b,c)
            , fixed = list(a ~ 1, b ~ 1, c ~ 1)
            , random = list(a ~ 1, b ~ 1, c ~ 1)
            , groups = ~location
            , data=dat
            , start= ... )

But that is not what I want. How do you describe that you want one fit
for each of the five locations and that "location*year" and
"location*plot" or something similar are random effects?

Do you have some other examples that fit better to this problem setting?
I welcome any tips.

thx
Christof