  We look for a non-Gaussian signal in the WMAP 5-year temperature anisotropy
maps by performing a needlet-based data analysis. We use the foreground-reduced
maps obtained by the WMAP team through the optimal combination of the W, V and
Q channels, and perform realistic non-Gaussian simulations in order to
constrain the non-linear coupling parameter $\fnl$. We apply a third-order
estimator of the needlet coefficients skewness and compute the $\chi^2$
statistics of its distribution. We obtain $-80<\fnl<120$ at 95% confidence
level, which is consistent with a Gaussian distribution and comparable to
previous constraints on the non-linear coupling. We then develop an estimator
of $\fnl$ based on the same simulations and we find consistent constraints on
primordial non-Gaussianity. We finally compute the three point correlation
function in needlet space: the constraints on $\fnl$ improve to $-50<\fnl<110$
at 95% confidence level.
