A GLIAL MECHANISM OF ANTIDEPRESSANT ACTION: FLUOXETINE POTENTIATES ATP-DRIVEN CAMP SIGNALLING IN ASTROCYTES

Antidepressant use has doubled in the last two decades, yet a third of patients don't respond to treatment. Selective serotonin reuptake inhibitors (SSRIs) like fluoxetine (FLX) are the most commonly prescribed antidepressants. Their therapeutic efficacy was initially proposed to be mediated by their ability to increase brain serotonin (5-HT) by inhibiting its reuptake. However, evidence of antidepressant effects is independent of monoamine changes, suggesting that FLX targets other pathways.
Astrocytes, once seen as merely supporting cells, are now widely accepted to play crucial roles in emotional regulation. While cAMP and Ca²⁺ signalling are vital for astrocyte function, their specific roles in emotional regulation or antidepressant mechanisms remain largely unclear.
We aimed to elucidate the effects of antidepressant FLX on intracellular signalling in astrocytes.
Live cell imaging of primary rat astrocytes showed FLX (5 minutes; 10µM) increases cAMP in adenylate cyclase-dependent manner by 27% without affecting Ca²⁺signalling. Antagonising the 5-HT 2B (5-HT_2B; LY266097) and adenosine 2B receptors (A_2B; PSB603) revealed that FLX requires both receptors to increase cAMP. Using a luminescence assay, we demonstrated that FLX engages 5-HT_2BR to potentiate ATP release from astrocytes. Depletion of microglia in primary astrocyte cultures (PLX5622, 10 µM, 7 days), which normally comprise ~3% of the culture and have been shown to convert ATP to adenosine, abolished the ability of fluoxetine to potentiate cAMP signalling.
In summary, FLX enhances ATP release via 5-HT_2BR, which microglia convert to ADO. ADO activates A_2BR, thus raising intracellular cAMP levels.