Tracing Touch Signals to the Pleasure Centre

I never cease to be impressed by what neuroscientists are able to discover, and the range of methods and procedures that produce these discoveries. Researchers have recently reported that they have discovered the function of cells found in the skin of female mice. These cells have been known for a long time and they are named after the skin cell receptors they possess.

However, researchers didn’t know the role these cells played. When gently touched, typically by paintbrush stroking, these cells become active. When they were stroked this way, the mice continually returned to the location where they had received the stroking. The prompted researchers to suspect that the mice found this to be pleasurable.

Now this discovery has been extended in important ways. Because simply stroking these hairs was very imprecise, it was important to gain more control over the individual cells to be activated. Researchers used a technique called optogenetics to engineer these cells so they respond to light.

Neurons typically become activated when the energy source, in this case touch, reaches them. Optogenetics involves introducing proteins to the cell’s membrane that respond to stimulation by light, which then activates the cell.  In this way, they can achieve precise control over the cells that are activated. Because the mouse’s back has the most hair, and thus would be most sensitive to touch, they chose these cells to engineer, and then shaved the back of the mice.

Shining light on these cells was revealing. The mice immediately assumed a sexually-receptive posture known as lordosis. The females arches her back to allow the male better access. They also repeatedly returned to the light chamber where they had been stimulated. In contrast, shining light on non-engineered mice had no effect on their posture, nor did they have any location preferences.

Further experiments have now shown that activating these cells with light causes the release of dopamine in the ventral tegmental area (VTA) and the nucleus accumbens (NA), which are key parts of the brain’s pleasure pathway (see Ch 7 in my book).

The really remarkable part of this story, if you are a neuroscientist, is how the researchers then proceeded to trace the individual neurons in this touch pathway that reached the VTA and NA. Just to describe one process used, researchers located the terminals of these touch neurons inside the spinal cord. A single neuron projection, the axon, travels through the arm (paw) into the spinal cord. Researchers were able to modify the terminals of these axons, to precisely control, again with light, when these terminals released neurotransmitter and to record from the neurons on which they terminated. Using similar techniques, they were able to identify each neuron in the pathway reaching the pleasure centres.

They were also able to conclude that these cells were necessary for the lordosis posture to occur. In fact, the researchers concluded that in mice where these touch cells had been destroyed, female mice found male advances to be aversive. This provides support for the conclusion that these touch pathways not only stimulate sexual behaviour, but also that they provide pleasure signals during this process.

What’s so interesting about this is that while humans are very aware of the pleasurable nature of touch for sex as well as a range of other conditions, such as hugging, or massage, there has been some debate over whether the same is true for other animals. This research seems to confirm this idea. Not only that, but it has significantly advanced our knowledge of a) the specific neural pathway through which the touch sensation is transmitted, and b) the utility of the wide range of techniques, including many applications of optogenetics, in making important discoveries about how animals and, by extension, humans work.