Itch processing in the brain

Abstract

Itch is a sensation defined as the urge to scratch. The central mechanisms of itch are being increasingly studied. These studies are usually based on experimental itch induction methods, which can be classified into the following categories: histamine-induced, induction by other non-histamine chemicals (e.g. cowhage), physically induced (e.g. electrical) and mentally induced (e.g. audio-visual). Because pain has been more extensively studied, some extrapolations to itch can be proposed and verified by experiments. Recent studies suggest that the itch-processing network in the brain could be disrupted in certain diseases. This disruption could be related to the implication of new regions or the exclusion of already engaged brain regions from itch-processing network in the brain.

Pruritus as a Distinctive Feature of Type 2 Inflammation

“…Specific receptors located on these fibers can be activated by a large number of endogenous itch-inducing agents released by keratinocytes, immune cells, or neighboring neuronal afferent and exogenous agents, such as proteases released by Staphylococcus aureus . These sensory neurons have cell bodies in the dorsal root ganglion (DRG) and project primary afferents to the skin, and they send projections to the dorsal horn of the spinal cord, where they synapse with second- or third-order neurons that come together to form part of the spinothalamic tract, which then ascends up to the thalamus and proceed to the somatosensory and the anterior cingulate cortex [ 8 ]. Second- and third-order neurons in the spinal cord might have either excitatory or inhibitory functions [ 9 ].…”

Itch Matrixes

“…Knowledge of itch processing in the brain is growing thanks to brain imaging ( 2 , 68 ). A better understanding of interactions between itch matrixes would allow a better understanding of pruritus in different cutaneous or extra-cutaneous etiologies ( 69 ).…”

“…In a recent meta-analysis from our group based on a systematic review we have identified brain regions reported to be responsible for central mechanisms of itch processing ( 1 ). We also have discussed the central mechanisms of itch proceeding in the brain more in depth in a review paper ( 2 ). The research papers that have studied central mechanism of itch are presented in Table 1 while their results are presented in Table 2 .…”

Associations Between Interindividual Differences, Expectations and Placebo and Nocebo Effects in Itch

“…Moreover, interaction between cortex and periaqueductal gray (PAG) was enhanced in nocebo responding in itch ( van de Sand et al, 2018 ). Activation of the PAG in particular has been implicated in descending pain control and reward function ( Becerra et al, 2001 ), but is central to itch processing as well ( Najafi et al, 2021 ). Interestingly, PAG deactivation deriving from reward system activation following scratching has been found to relieve itch, which may suggest distinct mechanisms for itch compared to pain relief ( Papoiu et al, 2013 ).…”