Molecular recognition is a fundamental principle in biological systems. The olfactory detection of both food and predators via ecological relevant odorant cues are abilities of eminent evolutionary significance for many species. Pyrazines are such volatile cues, some of which act as both human‐centered key food odorants (KFOs) and semiochemicals. A pyrazine‐selective odorant receptor has been elusive. Here we screened 2,3,5‐trimethylpyrazine, a KFO and semiochemical, and 2,5‐dihydro‐2,4,5‐trimethylthiazoline, an innate fear‐associated non‐KFO, against 616 human odorant receptor variants, in a cell‐based luminescence assay. OR5K1 emerged as sole responding receptor. Tested against a comprehensive collection of 178 KFOs, we newly identified 18 pyrazines and (2R/2S)‐4‐methoxy‐2,5‐dimethylfuran‐3(2H)‐one as agonists. Notably, OR5K1 orthologs in mouse and domesticated species displayed a human‐like, potency‐ranked activation pattern of pyrazines, suggesting a domestication‐led co‐evolution of OR5K1 and its orthologs. In summary, OR5K1 is a specialized olfactory receptor across mammals for the detection of pyrazine‐based key food odors and semiochemicals.

The degradation of aromatic hydrocarbons (benzene, toluene, ethylbenzene, p-xylene, o-xylene, 1,3,5-trimethylbenzene, and naphthalene) under anaerobic conditions was studied in column microcosms developed with aquifer material from the vicinity of a gasoline spill at Seal Beach, California. In one column, which did not receive any electron acceptors other than those naturally present (sulfate and carbon dioxide), more than 60% of the toluene was oxidized to COz. The other six added substrates were not degraded when the toluene supply was constant. In another column, the addition of toluene was discontinued after 126 d, which coincided with the onset of p-xylene degradation. Addition of sulfate, the only electron acceptor that was detected, stimulated the transformation of toluene in batch experiments.

The learning of stimulus-outcome associations allows for predictions about the environment. Ventral striatum and dopaminergic midbrain neurons form a larger network for generating reward prediction signals from sensory cues. Yet, the network plasticity mechanisms to generate predictive signals in these distributed circuits have not been entirely clarified. Also, direct evidence of the underlying interregional assembly formation and information transfer is still missing. Here we show that phasic dopamine is sufficient to reinforce the distinctness of stimulus representations in the ventral striatum even in the absence of reward. Upon such reinforcement, striatal stimulus encoding gives rise to interregional assemblies that drive dopaminergic neurons during stimulus-outcome learning. These assemblies dynamically encode the predicted reward value of conditioned stimuli. Together, our data reveal that ventral striatal and midbrain reward networks form a reinforcing loop to generate reward prediction coding.