5.1.2. Paw-withdrawal test

This test is completely comparable to the test of D'Amour and Smith [59] but have the benefit that it does not involve the pre-eminent organ of thermoregulation in rats and mice, i.e. the tail [60]. In this test, a paw is exposed to radiant heat that had previously been swollen by a subcutaneous injection of carrageenan. By exposure to ultraviolet rays, inflammation can also be produced. Heat applied to a freely moving animal is an advantage of these types of tests [61].

## 5.1.3. Hot-plate model

procedures (neurolysis, thermo lesion,) and neurosurgery. Advance medicine provides many

bark

Flacurtiaceae Leaves and

Scientific name/common name Family Parts used Medicinal used References

Myrtus communis Myrtaceae Leaves Narcotic analgesic [50] Tribulus terrestris Zygophyllaceae Aerial Narcotic analgesic [51] Sinapis arvensis Solanaceae Aerial Narcotic analgesic [51]

fruit

Peganum harmala Zygophyllaceae Whole plant Narcotic analgesic [51] Hibiscus rosa sinensis Malvaceae leaves Analgesic [52] Stylosanthes fruticosa Papilionaceae Whole plant Analgesic [53] Polyalthia longifolia Annonaceae Leaves Analgesic [53] Ficus glomerata Moraceae Bark and leaves Toothache, analgesic [53] Baugainvillea spectabilis Nyctaginaceae Leaves Analgesic [53] Toona ciliata Meliaceae heart wood Analgesic [53] Sida acuta Malvaceae whole plant Analgesic [53] Chococca brachiata Rubiaceae Root Anti-inflammatory, analgesics [54] Bauhinia racemosa Caesalpiniaceae Stem bark Analgesic [54]

Narcotic analgesic [51]

Anti-inflammatory, analgesics [54]

Leaves Anti-inflammatory, analgesics [54]

Due to obvious adverse effect of synthetic drugs, herbal medicinal plants are focusing to develop newer analgesic agents with fewer side effects. Some plants having analgesic activity

For the activation of cutaneous receptors, heat is a suitable stimulus. Nociceptive stimulation origin can be far apart from its target, for example radiant heat from a lamp in a direct touch

5. Experimental models for screening of analgesic substances

The animal models employed for screening of analgesic agents include

different methods for the management of pain.

scaber

Table 5. Some plant sources under study to develop new analgesics.

Withania somnifera Solanaceae Leaves and

328 Pain Relief - From Analgesics to Alternative Therapies

5.1. Pain-state models using thermal stimuli

4. Plant sources of analgesics

Elephantopus scaber Elephantopus

are given in Table 5.

Casearia sylvestris Swartz. (wild

coffee)

In this test, a mouse or rat is presented into an open-ended cylindrical space with a floor composed of metallic plate that is heated by boiling liquid or a thermoderm [62]. Two behavioural components are produced by heating the plate at constant temperature that is calculated in terms of their reaction times, namely jumping and paw licking. In terms of analgesic chemicals, the paw licking behaviour is influenced only by opioids. On the contrary, by using less powerful analgesics, for example, paracetamol or acetylsalicylic acid, the jumping reaction time can be increased, especially when the temperature of the plate is 50�C or less or if the temperature is changing incrementally and in linear fashion, e.g. from 43 to 52�C at 2.5�C/min [63]. The behaviour is more complex in the rat and relatively stereotyped in the mouse like it sniffs, licks its forepaws, licks its hind paws, straightens up, and stamps its feet, starts and stops washing itself, among other things. These behaviours have been labelled 'chaotic defensive movements' [64].

#### 5.1.4. Pain-state models using cold stimuli

For stimulation and measurement of pain in mice, a new animal model has been developed and designed. This laboratory model (M-model) basically consists of four parts (i) perspex-box, (ii) M-Zone, (iii) ice-tray and (iv) ice floor. At the start, the mouse is exposed to different parts of the M-model mainly M-Zone for about 60 s, so that the mouse is sensitive of the existence of M-Zone prior to the initiation of the experiment. From the top/ceiling of the perspex box, the animal is inserted. The ice tray containing of ice block is slide onto the floor of the perspex box. When the animal is not able to bear the cold surface of ice floor, it escapes to M-Zone. The time taken by the animal to run away into the M-Zone (Flight-Zone) when placed on the ice-floor is called endurance time. This time is recorded with the help of a stopwatch. In general, mice take about 4–6 s to escape into the M-Zone to evade ice floor. Separate groups of animals are pretreated with narcotics such as butorphanol (partial opioid agonist, 2 mg/kg, s. c), tramadol (opioid agonist, 5 mg/kg, s. c), pentazocine (10 mg/kg, s. c) and non-narcotic analgesics such as ketoprofen (non-selective COX inhibitor, 5 mg/kg, p. o), diclofenac (non-selective COX inhibitor, 15 mg/kg, i. p) and meloxicam (preferential COX-2 inhibitor, 5 mg/kg, s. c) to determine their effect on endurance time. This time is recorded at 0, 15, 30, 45, 60, 120 and 180 min after administration of the standard drugs [65].
