5.2. Pain-state models using mechanical stimuli

#### 5.2.1. Strain gauges

In this test, an increasing amount of pressure is applied to a punctiform area on the hind paw or, far less frequently, on the tail. The tail or paw is wedged between a plane surface and a blunt point mounted on top of a system of cog wheels with a cursor that can be moved in the direction of length of a graduated beam [66]. When the pressure increases, following step wise reactions occurs, i.e. the reflex removal of the paw or a complex movement of the animal to free its captured limb and at last a vocal response is noticed. Randall and Selitto with the aim of enhancing the sensitivity of the test offer comparison of thresholds seen with an inflamed paw and with a healthy paw [67].

#### 5.2.2. von-Frey filaments

The key method for the study of pain in animal models is the assessment of mechanosensitivity. This is frequently executed with the use of von-Frey filaments in an up-down testing model. This is the most commonly used method for measuring pain in animals described by Vivancos [68] for mechanosensitivity testing in rodents. Though, in this method, animals are getting a changeable amount of stimuli which may direct the animals in distinctive groups getting diverse testing experiences that affect their subsequent responses. In order to standardize the measurement of mechano-sensitivity, a simplified up-down method (SUDO) for reckoning paw withdrawal threshold (PWT) with von-Frey filaments has been developed that uses a constant number of five stimuli per test [69].

#### 5.3. Pain-state models using electrical stimuli

#### 5.3.1. Electrical stimulation of the tail

Progressively escalating strength of electrical stimuli can be applied in range (lasting for some milliseconds) through subcutaneous electrodes positioned in the tail of the mouse or the rat. One can see the following: when such slowly increasing intensities of electrical stimuli are applied from invariable voltages 40–50 V, i.e., the impulse movement of the tail, vocalization occurs at the time of stimulation, and then, utterance continuing ahead for the period of stimulation. Due to the electrical current, the animal may be died. Morphine or morphine-like drugs are useful in this model [70].
