  We study the evolution of scalar curvature perturbations in a brane-world
inflation model in a 5D Anti-de Sitter spacetime. The inflaton perturbations
are confined to a 4D brane but they are coupled to the 5D bulk metric
perturbations. We numerically solve full coupled equations for the inflaton
perturbations and the 5D metric perturbations using Hawkins-Lidsey inflationary
model. At an initial time, we assume that the bulk is unperturbed. We find that
the inflaton perturbations at high energies are strongly coupled to the bulk
metric perturbations even on subhorizon scales, leading to the suppression of
the amplitude of the comoving curvature perturbations at a horizon crossing.
This indicates that the linear perturbations of the inflaton field does not
obey the usual 4D Klein-Gordon equation due to the coupling to 5D gravitational
field on small scales and it is required to quantise the coupled brane-bulk
system in a consistent way in order to calculate the spectrum of the scalar
perturbations in a brane-world inflation.
