Subcutaneous administration of botulinum toxin A reduces formalin-induced pain :

A method for assessing pain and analgesia in rats and cats is described. The procedure involves subcutaneous injection of dilute formalin into the forepaw, after which the animal's responses are rated according to objective behavioral criteria. The formalin test is a statistically valid technique which has two advantages over other pain tests: (1) little or no restraint is necessary, permitting unhindered observation of the complete range of behavioral responses; and (2) the pain stimulus is continuous rather than transient, thus bearing greater resemblance to most clinical pain. The analgesic effects of morphine, meperidine, and stimulation of the periaqueductal grey matter are evaluated using this test.

Peripheral tissue damage or nerve injury often leads to pathological pain processes, such as spontaneous pain, hyperalgesia and allodynia, that persist for years or decades after all possible tissue healing has occurred. Although peripheral neural mechanisms, such as nociceptor sensitization and neuroma formation, contribute to these pathological pain processes, recent evidence indicates that changes in central neural function may also play a significant role. In this review, we examine the clinical and experimental evidence which points to a contribution of central neural plasticity to the development of pathological pain. We also assess the physiological, biochemical, cellular and molecular mechanisms that underlie plasticity induced in the central nervous system (CNS) in response to noxious peripheral stimulation. Finally, we examine theories which have been proposed to explain how injury or noxious stimulation lead to alterations in CNS function which influence subsequent pain experience.