As Ivan Pavlov famously demonstrated with his science hounds, brains are extremely malleable and easily reprogrammed. This can be a good thing, as it helps us learn new behaviors, although when these form into bad habits things can get a bit problematic. To learn more about what drives this process, scientists have developed a new technique that allows them to watch the brain becoming hooked in real time.

Presenting their work yesterday at a meeting of the American Chemical Society, the researchers revealed how they injected detectors called cell-based neurotransmitter fluorescent engineered reporters – or CNiFERs for short – into the brains of mice. These are special receptors that light up when they detect the presence of certain neurotransmitters, thereby giving scientists a glimpse of exactly which chemicals are being transmitted in the brain at a given point in time.

This represents a major step forward for neuroscientists, as while measuring electrical brain activity is relatively simple, observing neurotransmitter activity is considerably more difficult – particularly given the fact that some of these substances, such as dopamine and noradrenaline, are virtually identical and therefore indistinguishable.

Rehashing Pavlov’s iconic experiment, the team trained mice to associate the sound of a bell ringing with a sugary treat. At first, the mice didn’t respond at all to the bell, and experienced a surge of dopamine – the neurotransmitter associated with pleasure – only after receiving the treat.

^{CNiFERs light up when they detect dopamine. Slesinger & Kleinfeld labs}

However, as they began to associate the two events, their brains started to release dopamine immediately after hearing the bell, even before they had their sugary reward. This response became increasingly immediate as the association become stronger.

Noradrenaline, meanwhile, is associated with alertness, and while scientists have long suspected that it plays a role in the formation of habits and addictions, its exact involvement in this process remains unknown.

Crucially, because CNiFERs can distinguish between dopamine and noradrenaline, the researchers were able to accurately observe and separate the activity of these two neurotransmitters. Interestingly, they discovered that the timing of noradrenaline release did not appear to be affected as the mice learned to associate the bell with the treat.

Knowing this could help scientists pinpoint the precise role that different neurotransmitters play in the formation of habits and addictions, and could therefore enable them to design new treatments that target particular compounds in the brain.