The SSRI citalopram affects fetal thalamic axon responsiveness to netrin-1 in vitro independently of SERT antagonism

Serotonin (5-hydroxytryptamine, 5-HT) signaling is thought to modulate nervous system development. Genetic and pharmacological studies support the idea that altered 5-HT signaling during development can have enduring consequences on brain function and behavior. Recently, we discovered that 5-HT can modulate thalamic axon guidance in vitro and in vivo. Embryonic thalamic axons transiently express the 5-HT transporter (SERT; Slc6a4) and accumulate 5-HT, suggesting that the SERT activity of these axons may regulate 5-HT-modulated guidance cues. We tested whether pharmacologically blocking SERT using selective 5-HT reuptake inhibitors (SSRIs) would impact the action of 5-HT on thalamic axon responses to netrin-1 in vitro. Surprisingly, we observed that two high-affinity SSRIs, racemic citalopram ((RS)-CIT) and paroxetine, affect the outgrowth of embryonic thalamic axons, but differ with respect to their dependence on SERT blockade. Using a recently developed 'citalopram insensitive' transgenic mouse line and in vitro pharmacology, we show that the effect of (RS)-CIT effect is SERT independent, but rather arises from R-CIT activation of the orphan sigma-1 receptor(σ1, Oprs1). Our results reveal a novel σ1 activity in modulating axon guidance and a 5-HT independent action of a widely prescribed SSRI. By extension, (RS)-CIT and possibly other structurally similar SSRIs may have other off-target actions that can impact neural development and contribute to therapeutic efficacy or side effects.