Tac1-Expressing Neurons in the Periaqueductal Gray Facilitate the Itch-Scratching Cycle via Descending Regulation

Uncontrollable itch-scratching cycles lead to serious skin damage in patients with chronic itch. However, the neural mechanism promoting the itch-scratching cycle remains elusive. Here, we report that tachykinin 1 (Tact)-expressing glutamatergic neurons in the lateral and ventrolateral periaqueductal gray (l/vlPAG) facilitate the itch-scratching cycle. We found that l/vlPAG neurons exhibited scratching-behavior-related neural activity and that itch-evoked scratching behavior was impaired after suppressing the activity of l/vlPAG neurons. Furthermore, we showed that the activity of Tac1-expressing glutamatergic neurons in the l/vlPAG was elevated during itch-induced scratching behavior and that ablating or suppressing the activity of these neurons decreased itch-induced scratching behavior. Importantly, activation of Tac1-expressing neurons induced robust spontaneous scratching and grooming behaviors. The scratching behavior evoked by Tac1-expressing neuron activation was suppressed by ablation of spinal neurons expressing gastrin-releasing peptide receptor (GRPR), the key relay neurons for itch. These results suggest that Tac1-expressing neurons in the l/vlPAG promote itch-scratching cycles.