[Comparative psychological aspects of itching and pain]

Vitexin inhibits pain and itch behavior via modulating TRPV4 activity in mice

Itching and pain are distinct unpleasant sensations. The transient receptor potential cation channel subfamily V member 4 (TRPV4) pathway is regarded as a shared pathway that mediates pain and itching. Vitexin (Mujingsu, MJS), a C-glycosylflavonoid, is an effective analgesic. This study aimed to explore the antinociceptive and anti-pruritic effects of MJS and whether its effects are mediated via the TRPV4 pathway. Mice were treated with MJS (7.5 mg/kg) 0.5 h prior to the initiation of the pain or itch modeling process. The results showed that MJS suppressed pain-like behavior in hot plate, thermal infiltration, glacial acetic acid twisting, and formalin tests. Administration of MJS decreased the pruritus response induced by histamine, C48/80, chloroquine and BAM8-22 within 30 min. MJS reduced scratching bouts and lessened the wiping reaction of mice under TRPV4 activation by GSK101 (10 µg/5 μl). MJS inhibited scratching behavior in acetone-ether-water (AEW)-treated mice within 60 min. An H1 receptor antagonist-chlorpheniramine (CLP, 400 mg/kg)-and a TRPV4 antagonist-HC067047 (250 ng/kg), exhibited similar effects to those of MJS. Moreover, MJS ameliorated dry skin itch-associated cutaneous barrier disruption in mice. MJS did not inhibit the expression of TRPV4 in the dorsal root ganglion neurons at L2-L3 in AEW mice. These results indicate that the analgesic and anti-pruritic effects of MJS in acute and chronic pain and itching, as well as itching caused by TRPV4 activation, could be attributed to the TRPV4 pathway modulation.

[Placebo response - Mechanisms and clinical applications]

Research on placebo responses has made major progress in recent years. Placebo responses are psychobiological events, which are created by the entire therapeutic context. They can appear at any time, not only in experimental and clinical settings. Several studies on analgesia-related placebo research showed that patients have higher placebo responses in comparison to healthy participants, which may also last longer. Expectations play a key role in placebo analgesia. They can be induced via three central psychological mechanisms: 1) expectation induced via instructions, 2) expectation induced via classical conditioning and 3) expectation induced via social learning. These mechanisms are controlled by neurobiological structures and modulate pain perception resulting in pain relief by positive expectations and increased pain by negative expectations, the so-called nocebo effect. There is an ongoing discussion that these psychological mechanisms may also play a central role in inducing and maintaining itch-reducing placebo responses. The current state of research suggests that placebo responses could be used in clinical contexts and should not be viewed as being in competition with medications but as an additive increase in efficacy of a pharmacological substance through specifically induced placebo responses. This targeted use is also possible within ethical guidelines. Important prerequisites are that the research results can be transferred from healthy participants to patients and that the placebo responses are reproducible.