*6.1.2.1 Formalin test*

Subcutaneous or intra-plantar injection of formalin into the paw of rats or mice. The time the animal spends licking and biting its paw is measured [58].

### *6.1.2.2 Abdominal contraction test, Koster test or Writhing test*

The intraperitoneal administration of serosal irritants causes highly stereotyped behaviour in mice and rats [59]. We notice a decrease in the animal's motor activity, a lack of motor coordination, twisting of the dorsal abdominal muscles, abdominal contractions and the entire body (especially the hind legs), (**Figure 3**). The test goes by several names, including the abdominal torsion test, abdominal contraction response or stretch test, but it is more commonly known as the 'torsion test' [49].

In fact, the torsion test is a chemical method used to induce peripheral pain by injecting irritants, such as phenylquinone or acetic acid into mice. Further, the low frequency of contortions makes it possible to deduce the potency and analgesic activity of the active substance. Manifestations of abdominal torsion in mice were first reported by Siegmund et al. [60], as an arching of the back, extension of the hind limbs and contraction of the abdominal muscles [61]. Likewise, the number of abdominal contractions and body stretches are measured [49, 55, 59, 62, 63]. Hence, these methods have the advantage of being able to demonstrate the effects produced by weak analgesics [64].

#### *6.1.3 Mechanical nociception*

Provide responses to stimuli of excessive pressure or mechanical deformation, as well as to breaks in the skin surface (cuts, incisions). These nociceptors, which are often polymodal, have shown to be sensitive to both mechanical deformations and thermal stimuli [56].

#### *6.1.3.1 Von Frey filaments*

The von Frey test is a test of touch sensitivity using von Frey filaments, which can vary in diameter. It is considered the most important test and is almost the only mechanical test that can be used reliably not only in rats but also in mice. In rodents, they are mostly used on the plantar surface, while the animal is on a grid. The expected response is withdrawal of the paw. Rodents exhibit a withdrawal reflex when their paw is touched unexpectedly, indicating the degree of their sensitivity. Filaments of different calibres are applied [52, 64]. The von Frey test allows the

#### **Figure 3.** *Writhing test: The behaviour is described as the paw stretch and contraction of the abdominal muscles.*
