CHRONIC PAIN MODELS 
The past decade has seen the proliferation of animal models to study the effects of tissue and 
nerve injury on the development of persistent or chronic pain. In most of these studies, the 
animals are awake and perceive pain. These models attempt to mimic human clinical 
conditions. A major purpose of such studies is to further knowledge that can ultimately be 
applied to the management of acute and chronic pain in humans and animals. There is a 
special need to demonstrate responsibility in the proper treatment of animals that participate 
in these experiments. The animals should be exposed to the minimal pain necessary to carry 
out the experiment. Models of inflammation that may produce more persistent pain include 
the injection of carrageenan or Complete Freund’s adjuvant into the foot pad (Dubner, 1994). 
These models result in persistent pain that mimics the time course of postoperative pain or 
other types of persistent injury. Studies have shown that the impact of the inflamed limb on 
the rat’s behavior is minimal and the rats will use the limb for support if necessary. Recently 
developed models indicate that partial nerve injury in the rat results in signs of hyperalgesia 
and spontaneous pain and mimic neuropathic pain conditions (Dubner, 1994). These 
neuropathic pain models have been adapted to mice recently for studies of transgenic animals. 
All of the inflammation and nerve injury models that attempt to mimic human pain conditions 
produce pain that the animal cannot control. Therefore, it is important that investigators 
assess the level of pain in these animals and provide analgesic agents when it does not 
interfere with the purpose of the experiment. Pain in these studies can be inferred from 
ongoing behavioral variables such as feeding and drinking, sleep-waking cycle, grooming, 
guarding of the limb, and social behavior. Major changes in such behaviors may indicate that 
the animal is in considerable pain and the experiment should be terminated. 
OTHER CONSIDERATIONS 
Although the concept of using minimal levels of intensity of shock, as with any stressor, is an 
important one, research has shown that higher intensities or numbers of shock sometimes 
need to be used in certain types of studies. First, in stress research, the effect of reduced 
movement can be achieved after 40 inescapable shocks; interference with learning begins to 
occur after 80 shocks but is clearer after 120 shocks (Minor et al, 1988). Second, research on 
punishment has shown that using gradually increasing shock intensities results in 
habituation. That is, the level of shock ultimately required to produce the desired suppression 
of responding will likely be higher than if a higher shock level had been used initially. 
Because there is considerable adaptation to shock if it continues for many sessions or if it is 
given in chronic form, shock may have disadvantages for long-term stressor experiments 
unless adaptation per se is under study. Third, the same shock applied to the same body 
region sequentially activates different neural pathways that regulate pain as the number of 
shocks increase. 
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