Neuroscientists have various ways of activating specific neurons in animal brains to determine the cells' roles in neural circuits. A twist on these techniques that its inventors say is less labour-intensive and invasive involves capsaicin — the compound that makes chilli peppers hot — and its receptor.
Richard Palmiter at the University of Washington in Seattle and his colleagues genetically modified mice so that they expressed a cell-surface receptor for capsaicin only in brain neurons that release the neurotransmitter dopamine, and not in the neurons of the peripheral nervous system that are responsible for the painful response to capsaicin.
When the researchers fed or injected the mice with capsaicin, they found that the compound activated brain dopamine neurons only and stimulated behaviour consistent with increased dopamine release. Similar results were seen in mice with the capsaicin receptor present only on cells that release another neurotransmitter, serotonin.
The researchers could reverse and repeat the effects of capsaicin, as the compound activated neurons for no more than 10 minutes after each administration and did not injure the mice.