A new kind of cell-based, real-time biosensor may turn out to be a neurochemical bloodhound, sniffing out neurotransmitters, doggedly discriminating among diverse signaling molecules. This bloodhound, however, reports not by baying or howling, but by means of fluorescent excitations. Once it joins the hunt for learning mechanisms and pathogenic processes, this biosensor may help scientists run many elusive neurochemical questions to ground.

The new biosensors are called cell-based neurotransmitter fluorescent engineered reporters, or CNiFERs—pronounced, appropriately enough, “sniffers.” To date, CNiFERs have lacked a very discriminating nose. But now scientists report that they have engineered CNiFERs that can discriminate between dopamine and norepinephrine.

These scientists, led by David Kleinfeld, Ph.D., professor of physics at UC San Diego, described their work in the October 26 online issue of Nature Methods, in an article entitled, “Cell-based reporters reveal in vivo dynamics of dopamine and norepinephrine release in murine cortex.”

“CNiFERs were implanted into the frontal cortex of mice to measure the timing of neurotransmitter release during classical conditioning with the use of two-photon microscopy,” wrote the authors. “The onset of DA release correlated with that of licking and shifted from the time of the reward toward that of the cue upon conditioning. In contrast, concurrent release of NE did not correlate with licking or the cue.”

Neuroscientists have long sought to measure dopamine in the brain during controlled conditions. But the chemistry of dopamine is so close to that of norepinephrine that fast and accurate measurements of dopamine, not confounded by other molecules, have eluded scientists.

By finding a way to measure dopamine and norepinephrine with fine resolution in both location and timing, the scientists were able to confirm a long-suspected learning mechanism. Specifically, they found that release of dopamine, but not norepinephrine, tracked anticipatory learning, an aspect of classical conditioning. That is, dopamine was initially released with a reward (sugary water), but began to be released earlier, with a stimulus (the sounding of a tone), as the mice learned to associate this previously neutral signal with something pleasant. In mice that failed to learn or made only a weak association, the anticipatory release of dopamine was reduced as well.

More interesting, perhaps, is the possibility of leveraging the molecular design of the new CNiFERs to give chase to other elusive problems in neurochemistry.

“Previously, CNiFERs were limited to GPCRs that coupled to Gq proteins,” noted the authors of the Nature Methods article. “Yet redirecting the Gi/o-coupled D2 receptor to the PLC-IP3 pathway should now make it possible to create CNiFERs for other Gi/o-coupled receptors, such as those for somatostatin, serotonin and opioids.”

“This work provides a path for the design of cells that report a large and diverse group of signaling molecules in the brain,” Dr. Kleinfeld concluded.