Compromising tyrosine hydroxylase function establishes a delusion-like temporal profile of reinforcement by dopamine neurons in Drosophila

Abstract

For a proper representation of the causal structure of the world, one must consider both evidence for and evidence against causality. To take punishment as an example, the causality of a stimulus is reasonable if the stimulus precedes punishment, whereas causality can be ruled out if the punishment occurred first. This is reflected in the associative principle of timing-dependent valence reversal: aversive memories are formed when a stimulus occurs before the punishment, whereas memories of appetitive valence are observed when a stimulus is presented upon its relieving termination. We map the temporal profile of punishment induced by optogenetic activation of the PPL1-01 neuron in the fly Drosophila melanogaster, and find that impairment of tyrosine hydroxylase function, either acutely by pharmacological methods or by cell-specific RNAi, i) enhances learning with a time gap between stimulus and PPL1-01 punishment (trace conditioning), ii) impairs learning when the stimulus immediately precedes PPL1-01 punishment (delay conditioning), and iii) prevents learning about a stimulus presented after PPL1-01 punishment has ceased (relief conditioning). This implies a delusion-like state in which causality is attributed to cues that do not merit it (better trace conditioning), whereas both credible evidence for and credible evidence against causality is not properly appreciated (worse delay and relief conditioning). Under conditions of low dopamine, we furthermore observe a compensatory role for serotonin that is pronounced in trace conditioning, weaker in delay conditioning, and absent in relief conditioning. We discuss a disturbed dopamine-serotonin balance as an endophenotype for the positive and cognitive symptoms in schizophrenia.