We demonstrate the existence of a reversible entropic thermal effect in nematic liquid crystals, when one increases (or let decay) the orientational ordering with an external electric field. The system, close to the isotropic nematic transition, is quantitatively described by a Landau-Khalatnikov model. We measure the orientational order parameter S dynamics by an optical method. We do observe heating and cooling using S as a transient thermometer. When cooling, metastable states can be created. One goes from a fast adiabatic regime towards a very slow diffusion limited regime, creating a thermal bottleneck.