Melatonin receptor activation increases glutamatergic synaptic transmission in the rat medial lateral habenula

Melatonin (MLT) is secreted from the pineal gland and mediates its physiological effects through activation of two G protein-coupled receptors, MT₁ and MT₂. These receptors are expressed in several brain areas, including the habenular complex, a pair of nuclei that relay information from forebrain to midbrain and modulate a plethora of behaviors, including sleep, mood, and pain. However, so far, the precise mechanisms by which MLT control the function of habenula neurons remain unknown. Using whole cell recordings from male rat brain slices, we examined the effects of MLT on the excitability of medial lateral habenula (MLHb) neurons. We found that MLT had no significant effects on the intrinsic excitability of MLHb neurons, but profoundly increased the amplitude of glutamate-mediated evoked excitatory post-synaptic currents (EPSC). The increase in strength of glutamate synapses onto MLHb neurons was mediated by an increase in glutamate release. The MLT-induced increase in glutamatergic synaptic transmission was blocked by the competitive MT₁/MT₂ receptor antagonist luzindole (LUZ). These results unravel a potential cellular mechanism by which MLT receptor activation enhances the excitability of MLHb neurons. The MLT-mediated control of glutamatergic inputs to the MLHb may play a key role in the modulation of various behaviors controlled by the habenular complex.