  By using new and previous measurements of the $ab$-plane conductivity
$\sigma_1^{ab} (\omega,T)$ of La$_{2-x}$Sr$_x$CuO$_{4}$ (LSCO) it is shown that
the spectral weight $W = \int_0^\Omega {\sigma_1^{ab} (\omega,T) d\omega}$
obeys the same law $W = W_0 - B(\Omega) T^2$ which holds for a conventional
metal like gold, for $\Omega$'s below the plasma frequency. However
$B(\Omega)$, which measures the "thermal response" of the charge system, in
LSCO exhibits a peculiar behavior which points towards correlation effects. In
terms of hopping models, $B(\Omega)$ is directly related to an energy scale
$t_T$, smaller by one order of magnitude than the full bandwidth $t_0 \sim
W_0$.
