Social touch is an essential component of communication. Little is known about the
underlying pathways and mechanisms. The hypothalamus is a major regulatory center
of
rodent social behavior. It is also likely to be involved in the control of instinctive
behaviors in
humans. It is conceivable that ascending sensory pathways carrying information on
social
touch might project directly to the hypothalamus. Here, we discovered a novel neuronal
pathway from the posterior intralaminar thalamic nucleus (PIL) to the medial preoptic
area
(MPOA) is involved in control of social grooming. First, we determined the effect
of
chemogenetic stimulation of PIL neurons on social interactions between familiar adult
female rats. Activity-dependent tagging of PIL neurons was performed in rats experiencing
physical social contacts. The selective chemogenetic stimulation of the preoptic areaprojecting
PIL neurons was performed using double viral injections and also by CNO
administration directly into the preoptic area. We found that neurons in the PIL and
MPOA
were naturally activated by physical contact between female rats and also by chemogenetic
stimulation of PIL neurons. Chemogenetic activation of these neurons increased social
grooming between familiar rats as did selective activation of the PIL-MPOA pathway.
Neurons projecting from the PIL to the MPOA express the neuropeptide parathyroid
hormone 2 (PTH2) and central infusion of its receptor antagonist diminished social
grooming.
We showed its increased expression in the PIL in response to social interaction. Finally,
we
showed similarity in the anatomical organization of the PIL-MPOA circuit in the rat
and
human brain. We propose that the discovered PIL-MPOA neuronal pathway facilitates
physical contacts in both rodents and human. Therefore, the pathway as well as the
PTH2
neuropeptide and its receptor should be investigated in the future in disorders where
deficits
in direct social interactions are found, such as autism spectrum disorder.
