experience. Only when pain becomes severe (approaching pain tolerance levels) is our behavior dominated by attempts to avoid or escape it. That degree of pain needs to be alleviated. In pain studies, giving animals control over the source of pain is an effective way to let them minimize it. In that situation, escape-avoidance behavior is an appropriate adaptive response. An animal displaying such behavior might be experiencing discomfort, but is not yet in distress (see Table 1-3). However, if the animal is denied control of the stimulus and it approaches or exceeds the limit of tolerance, maladaptive behaviors will appear and the animal should be considered to be in distress. In distressed animals, maladaptive behaviors can include persistent attacks on the perceived source of the pain (e.g., electrified grid floors or other parts of the apparatus), prolonged immobilization or ''freezing," self-mutilation of the area of the body receiving the stimulus, or a state of learned helplessness in which the animal gives up and no longer attempts to escape, avoid, or control the stimulus.

In most experiments involving the study of pain, the animal or the investigator can set a limit on the magnitude and duration of the stimulus. The investigator must determine the intensity and duration of the pain that the animal experiences and minimize it.

Four general approaches are available to minimize pain (Dubner, 1987): use of general anesthesia or neurosurgery, use of local anesthesia and analgesia, training of animals to control the stimulus, and control of the stimulus by the investigator.

When an invasive procedure is performed on an appropriately anesthetized animal, there is little concern about pain as long as the animal remains anesthetized. The anesthetic state can be monitored by assessing pupillary size, palpebral and toepinch response, stability of heart rate and blood pressure, and electroencephalographic activity. In a study in which anesthetic agents would confound the data, surgical lesions of the central nervous system can be made under anesthesia, the animal allowed to recover from anesthesia, and data collected on a functional decerebrate with no possibility of conscious* sensation. An alternative to anesthesia or neurosurgery in some studies, and often a valuable adjunct to anesthesia, is the administration of local anesthetic or analgesic agents to eliminate or reduce pain.

Some studies are concerned with pain mechanisms and the relationship between pain-related behavior and neurobiologic processes and require that animals be exposed to actual or potential tissue damage. In others, pain might be a consequence of techniques and methods whose purposes are unrelated to the nature or magnitude of the pain produced. In most such investigations with conscious animals, the animals can be taught to avoid or escape painful stimuli.

An important distinction should be made between reflexes associated with nociception and the maladaptive behaviors associated with distress when pain

*  

The terms conscious and consciousness are used throughout this text to refer to the awake state, the state in which stimuli can be perceived (e.g., "The conscious animal is aware of its surroundings"). Unconsciousness refers to the state in which stimuli are not perceived (e.g., "The anesthetic should produce a loss of consciousness").



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