Presynaptic MAST Kinase Controls Bidirectional Post-SynapticResponses to Convey Stimulus Valence in C. elegans

Presynaptic plasticity is known to modulate the strength of synaptic transmission. However, it remains unknown whether regulation in presynaptic neurons alters the directionality −positive or negative− of postsynaptic responses. We report here that the C. elegans homologs of MAST kinase, Stomatin and Diacylglycerol kinase act in a thermosensory neuron to elicit in its postsynaptic neuron an excitatory or inhibitory response that correlates with the valence of thermal stimuli. By monitoring neural activity of the valence-coding interneuron in freely behaving animals, we show that the alteration between excitatory and inhibitory responses of the interneuron is mediated by controlling the balance of two opposing signals released from the presynaptic neuron. These alternative transmissions further generate opposing behavioral outputs necessary for the navigation on thermal gradients. Our findings reveal the previously unrecognized capability of presynaptic regulation to evoke bidirectional postsynaptic responses and suggest a molecular mechanism of determining stimulus valence.