While neurotransmitter identity was once considered singular and immutable for mature neurons, it is now appreciated that one neuron can release multiple neuroactive substances (cotransmission) whose identities can even change over time. To explore the mechanisms that tune the suite of transmitters a neuron releases, we developed transcriptional and translational reporters for cholinergic, glutamatergic, and GABAergic signaling in Drosophila. We show that many glutamatergic and GABAergic cells also transcribe cholinergic genes, but fail to accumulate cholinergic effector proteins. Suppression of cholinergic signaling involves posttranscriptional regulation of cholinergic transcripts by the microRNA miR-190; chronic loss of miR-190 function allows expression of cholinergic machinery, reducing and fragmenting sleep. Using a "translation-trap" strategy, we show that neurons in these populations have episodes of transient translation of cholinergic proteins, demonstrating that suppression of cotransmission is actively modulated. Posttranscriptional restriction of fast transmitter cotransmission provides a mechanism allowing reversible tuning of neuronal output.