Most pet owners have seen their furry friends get soaked before. After a second of dripping, they spray water everywhere with the wet dog shake. From dogs, cats, and rats to lions, tigers, and bears, nearly all hairy mammals will shake themselves dry when their fur gets wet. However, despite wet dog shakes being so common, scientists don’t know what causes this behavior.
Our skin’s sense of touch can be extremely sensitive. Most people can easily distinguish the gentle touch of a loved one, a soft breeze, and the movement of a pest on our skin. We sense touch through our nervous system, using specialized cells called somatosensory neurons to detect changes to our skin. Hairy animals have 12 unique types of somatosensory neurons that sense and respond to touch. One such neuron is the C-LTMR, which only reacts to gentle, slow-moving stimuli. When these neurons sense something, they respond strongly by sending powerful electrical signals through the nervous system. A team of researchers wondered whether these neurons are responsible for the wet dog shake.
The researchers first measured this behavior in mice to investigate the connection between C-LTMR neurons and wet dog shake. They used a dropper to place a sunflower seed oil droplet on the necks of 10 mice and monitored their behavior for 5 minutes. The mice began to wet dog shake within an average of 10 seconds of placing the droplet, often 3 to 4 times every 18 seconds. After the first minute, the mice stopped shaking as often and instead groomed their fur and scratched themselves.
The researchers then wanted to know whether the wet dog shake was an automatic behavior caused by neurons that sensed these droplets. To test this, they used the genetic engineering method CRISPR to remove the gene responsible for making the touch sensor of neurons, Piezo2. CRISPR uses a copy of the Piezo2 gene to find the mouse cell’s own Piezo2 gene. CRISPR then breaks the gene, preventing the mouse cell from making a touch sensor. When the researchers placed oil droplets on the engineered mice’s necks, they did not wet dog shake. The researchers concluded that touch is the primary mechanism that causes automatic wet dog shake behavior.
Many somatosensory neurons use Piezo2 to sense touch, preventing the researchers from identifying the neurons responsible for the wet dog shake. However, several genes are only produced by specific types of neurons. The researchers targeted these genes using genetic engineering to make them sensitive to light, a process called optogenetics. These light-sensitive genes produce proteins that activate when the scientists shine a blue laser on them. The researchers used optogenetics to create light-sensitive C-LTMR neurons and compared them to other light-sensitive somatosensory neurons.
When the researchers shone a light on the skin of the 7 light-sensitive C-LTMR mice, the mice did the wet dog shake 60% of the time. However, mice with other light-sensitive neurons did not wet dog shake in response to the laser. The researchers noticed that the light-sensitive C-LTMR mice responded the most when the laser was on their necks, 80% of the time, instead of their backs or thighs, 30% and 0% of the time, respectively.
The researchers thought this could be a sign that C-LTMR neurons send their signals through other neurons in the spine called spinoparabrachial neurons or SPNs. To test this, the researchers grew SPNs in petri dishes with optogenetic C-LTMR neurons. When they shone the laser on the C-LTMR neurons, they measured an electrical signal in the SPNs with an electrode, showing that these neurons were communicating.
The researchers concluded that the wet dog shake is caused specifically by C-LTMR neurons in the spine. They hope future research builds on their findings to better understand the connections between C-LTMR neurons and the brain. Continuing these studies could impact how scientists understand the wet dog shake in mammals and how hypersensitivity and other touch-based conditions develop in humans.
