There is intense interest in developing methods to regulate proliferation and differentiation of stem cells into neuronal fates for the purposes of regenerative medicine. One way to do this is through in vivo pharmacological engineering using small molecules. However, a key challenge is identification of relevant signaling pathways and therein druggable targets to manipulate stem cell behavior efficiently in vivo. Here, we use the planarian flatworm as a simple chemical-genetic screening model for nervous system regeneration to show that the isoquinoline drug praziquantel (PZQ) acts as a small molecule neurogenic to produce two-headed animals with integrated CNSs following regeneration. Characterization of the entire family of planarian voltage-operated Ca 2+ channel subunits (Ca vα), followed by in vivo RNAi of specific Ca v subunits, revealed that PZQ subverted regeneration by activation of a specific voltage-gated Ca 2+ channel isoform (Cav1A). PZQ-evoked Ca2entry via Cav1A served to inhibit neuronally derived Hedgehog signals, as evidenced by data showing that RNAi of Ca v1A prevented PZQ-evoked bipolarity, Ca 2+ entry, and decreases in wnt1 and wnt11-5 levels. Surprisingly, the action of PZQ was opposed by Ca 2+ influx through a closely related neuronal Cav isoform (Ca v1B), establishing a novel interplay between specific Cav1 channel isoforms, Ca 2+ entry, and neuronal Hedgehog signaling. These data map PZQ efficacy to specific neuronal Ca v complexes in vivo and underscore that both activators (Ca v1A) and inhibitors (Ca v1B) of Ca 2+ influx can act as small molecule neurogenics in vivo on account of the unique coupling of Ca 2+ channels to neuronally derived polarity cues.