Lamotrigine is used as an anticonvulsant and mood stabilizer. Currently prescribed in cases of epilepsy, Lennox–Gastaut syndrome and bipolar disorder. Its active biochemical pathways are not known.
more info at Wikipedia

“The action mechanisms underlying the mood-stabilizing effects of lamotrigine are unknown at present but recent studies have produced interesting leads. Lamotrigine modulates various ion channels, altering neuronal excitability. The use-dependent inhibition of neuronal firing by lamotrigine is potentially important because it could result in attenuating supranormal neuronal activities that are possibly associated with bipolar disorder. Lamotrigine inhibits the release of glutamate, similarly to lithium, and its possible association with mood-stabilizing or antidepressant effects needs to be further examined.” see The current understanding of lamotrigine as a mood stabilizer. J Clin Psychiatry. 2004 Jun;65(6):791-804.

Note that Lithium (treatment for bipolar disorder), mentioned above, also works by an as yet unknown mechanism.