Connection re-established: neurotransmission between the medial prefrontal cortex and serotonergic neurons offers perspectives for fast antidepressant action

Rapid Anxiolytic Effects of RS67333, a Serotonin Type 4 Receptor Agonist, and Diazepam, a Benzodiazepine, Are Mediated by Projections From the Prefrontal Cortex to the Dorsal Raphe Nucleus

BACKGROUND

Activation of serotonin (5-HT) type 4 receptors (5-HT₄Rs) has been shown to have anxiolytic effects in a variety of animal models. Characterizing the circuits responsible for these effects should offer insights into new approaches to treat anxiety.

METHODS

We evaluated whether acute 5-HT₄R activation in glutamatergic axon terminals arising from the medial prefrontal cortex (mPFC) to the dorsal raphe nucleus (DRN) induced fast anxiolytic effects. Anxiolytic effects of an acute systemic administration (1.5 mg/kg, intraperitoneally) or intra-mPFC infusion with the 5-HT₄R agonist, RS67333 (0.5 μg/side), were examined in mice. To provide evidence that anxiolytic effects of RS67333 recruited an mPFC-DRN neural circuit, in vivo recordings of firing rate of DRN 5-HT neurons, cerebral 5-HT depletion, and optogenetic activation and silencing were performed.

RESULTS

Acute systemic administration and intra-mPFC infusion of RS67333 produced fast anxiolytic effects and increased DRN 5-HT cell firing. Serotonin depletion prevented anxiolytic effects induced by mPFC infusion of RS67333. Surprisingly the anxiolytic effects of mPFC infusion diazepam (1.5 μg/side) were also blocked by 5-HT depletion. Optogenetically activating mPFC terminals targeting the DRN reduced anxiety, whereas silencing this circuit blocked RS67333 and diazepam mPFC infusion-induced anxiolytic effects. Finally, anxiolytic effects induced by an acute systemic RS67333 or diazepam administration were partially blocked after optogenetically inhibiting cortical glutamatergic terminals in the DRN.

CONCLUSIONS

Our findings suggest that activating 5-HT₄R acutely in the mPFC or targeting mPFC pyramidal cell terminals in the DRN might constitute a strategy to produce a fast anxiolytic response.

CK2 regulates 5-HT4 receptor signaling and modulates depressive-like behavior

The serotonergic neurotransmitter system has been widely implicated in the pathophysiology of mood-related disorders such as anxiety and major depressive disorder (MDD). The onset of therapeutic efficacy of traditional antidepressants is delayed by several weeks. The 5-HT₄ receptor has emerged as a new therapeutic target since agonists of this receptor induce rapid antidepressant-like responses in rodents. Here we show that the 5-HT₄ receptor is regulated by CK2, at transcriptional and post-transcriptional levels. We present evidence, in two different CK2α knockout mouse lines, that this regulation is region-specific, with the 5-HT₄ receptor upregulated in prefrontal cortex (PFC) but not striatum or hippocampus where CK2α is also ablated. 5-HT₄ receptor signaling is enhanced in vitro, as evidenced by enhanced cAMP production or receptor plasma membrane localization in the presence of CK2 inhibitor or shRNA targeting CK2α. In vivo, 5-HT₄ receptor signaling is also upregulated since ERK activation is elevated and sensitive to the inverse agonist, GR113808 in the PFC of CK2α KO mice. Behaviorally, KO mice as well as mice with AAV-mediated deletion of CK2α in the PFC show a robust 'anti-depressed-like' phenotype and display an enhanced response to antidepressant treatment when tested in paradigms for mood and anxiety. Importantly, it is sufficient to overexpress the 5-HT₄ receptor in the mPFC to generate mice with a similar 'anti-depressed-like' phenotype. Our findings identify the mPFC as the region that mediates the effect of enhanced 5-HT₄ receptor activity and CK2 as modulator of 5-HT4 receptor levels in this brain region that regulates mood-related phenotypes.