The peptidic antidepressant spadin interacts with prefrontal 5-HT4 and mGluR2 receptors in the control of serotonergic function

This study investigates the mechanism of action of spadin, a putative fast-acting peptidic antidepressant (AD) and a functional blocker of the K⁺ TREK-1 channel, in relation with the medial prefrontal cortex (mPFC)–dorsal raphé (DRN) serotonergic (5-HT) neurons connectivity. Spadin increased 5-HT neuron firing rate by 113 %, an augmentation abolished after electrolytic lesion of the mPFC. Among the few receptor subtypes known to modulate TREK-1, the stimulation of 5-HT₄ receptors and the blockade of mGluR_2/3 ones both activated 5-HT impulse flow, effects also suppressed by mPFC lesion. The combination of spadin with the 5-HT₄ agonist RS 67333 paradoxically reduced 5-HT firing, an effect reversed by acutely administering the 5-HT_1A agonist flesinoxan. It also had a robust synergetic effect on the expression of Zif268 within the DRN. Together, these results strongly suggest that 5-HT neurons underwent a state of depolarization block, and that the mechanisms underlying the influences exerted by spadin and RS 67333 are additive and independent from each other. In contrast, the mGluR_2/3 antagonist LY 341495 occluded the effect of spadin, showing that it likely depends on mPFC TREK-1 channels coupled to mGluR_2/3 receptors. These in vivo electrophysiological data were confirmed by in vitro Ca²⁺ cell imaging performed in cultured cortical neurons. Altogether, our results indicate that spadin, as a natural compound, constitutes a very good candidate to explore the “glutamatergic path” of fast-acting AD research. In addition, they provide the first evidence of 5-HT depolarization block, showing that the combination of 5-HT activators for strategies of AD augmentation should be performed with extreme caution.