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)plugin-autotooltip__plain plugin-autotooltip_bigLithium (treatment for bipolar disorder)

Lithium, normally in the form of lithium carbonate (Li2CO3) is widely recognised as the 'gold standard' prescribed drug for treating bipolar disorder, depression and mania.

In widespread use since the early 1970s, lithium carbonate was the first, mentioned above, also works by an as-yet-unknown mechanism.