Published in the January edition of Nature, a new study performed in mice has suggested that there are specialized sensory receptors for pleasant sensations, just as there are for pain and itch. The Caltech group report neurons selectively activated by stroking, but not by other forms of touch (such as poking or pinching). This is the first paper to identify molecularly defined sensory neurons for pleasant touch and most sensory research in the past has focussed on animal models of painful sensation, due its relative detectability. However, the sensation of pleasant touch is also evolutionarily favoured, because it encourages social bonding activities with conspecifics such as grooming.
In a previous study, the group, led by Sophia Vrontou, identified a small subset of unmyelinated sensory neurons expressing MRGPRB4 (a G Protein Coupled Receptor), which was not detectably active in vitro. To further characterise these neurons, they thus developed an original line of genetically-modified mice, using homologous recombination to target a calcium sensing reporter system to MRGPRB4+ neurons. Recording at the level of the spinal cord, the team found these MRGPRB4+ neurons responded selectively to stroking.
However, to demonstrate that these are pleasurable touch sensitive, the researchers needed to show that their stimulation causes pleasure in the mice (i.e. that stimulation of these neurons elicits the subjective and rewarding sensation of pleasure). A conditioned place preference paradigm was used. The mice were given the choice between two compartments over several trials. In the first, control saline solution was administered but in the second, mice were treated with a selective MRGPRB 4 agonist. The mice showed a significant preference for the second. Vrontou claims this was because the agonist elicits a pleasurable sensation, and is acting as a reinforcer for learning.
