  Using the relativistic Vlasov-Uehling-Uhlenbeck (RVUU) equation based on mean
fields from the nonlinear relativistic models, we study the effect of medium
modification of pion production threshold on the total pion yield and the
$\pi^-/\pi^+$ ratio in Au+Au collisions. We find that the in-medium threshold
effect enhances both the total pion yield and the $\pi^-/\pi^+$ ratio, compared
to those without this effect. Furthermore, including the medium modification of
the pion production threshold in asymmetric nuclear matter leads to a larger
$\pi^-/\pi^+$ ratio for the $NL\rho\delta$ model with a stiffer symmetry energy
than the $NL\rho$ model with a softer symmetry energy, opposite to that found
without the in-medium threshold effect. Experimental data from the FOPI
Collaboration are reproduced after including a density-dependent cross section
for $\Delta$ baryon production from nucleon-nucleon collisions, which
suppresses the total pion yield but hardly changes the $\pi^-/\pi^+$ ratio. The
large errors in the experimentally measured $\pi^-/\pi^+$ ratio prevent,
however, the distinction between the predictions from the $NL\rho$ and
$NL\rho\delta$ models.
