del2 function using convn and imfilter. Help

I am performing a diffusion regularization using the del2 function. My exercise consists to achieve the same result with convn and imfilter functions.

The code lines I need to change are:

uxuy = zeros(N,N,2);
...
grad_param = - 2*diff.*grad - lambda*4*[reshape(del2(uxuy(:,:,1)),[N*N,1])   reshape(del2(uxuy(:,:,2)),[N*N,1])];

Since I am working on a discrete domain, I've tried the following approach ( http://en.wikipedia.org/wiki/Discrete_Laplace_operator )

% Image Processing . 2D-filter
h_laplacian = [0 1 0; 1 -4 1 ; 0 1 0];
a1 = imfilter(uxuy(:,:,1), h_laplacian, 'replicate');
a2 = imfilter(uxuy(:,:,2), h_laplacian, 'replicate');
grad_param = - 2*diff.*grad - lambda*4*[reshape(a1,[N*N,1])   reshape(a2,[N*N,1])];

, but under the same conditions the result is not the same ( http://imgur.com/a/xwARZ )

So far I am not using the convn function. What am I doing wrong? Can anyone give me some hints?

Cheers.