  We consider the D2-brane probe action in the gravity background dual to N
coincident Dp-branes by treating the separation between the D2- and Dp-branes
as a nondynamical parameter for $p=2,4,6$. The gauge coupling, the core size of
a non-BPS instanton and the mass gap of the compact U(1) gauge theory in the
D2-brane are determined as a function of the separation in the type IIA gravity
region. The results are interpreted in terms of the 2+1D U(1) gauge theory
coupled with the matter fields which are also strongly coupled with the p+1D
SU(N) gauge field. It is shown that strong coupling of the matter fields to the
SU(N) gauge field can drastically modify their screening of the U(1) gauge
field. The non-perturbative dependence of the U(1) gauge coupling on the energy
scale is obtained.
