2000, Number 1
Salud Mental 2000; 23 (1)
Sotres-Bayon F, Pellicer F
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ABSTRACTPain is a recognizable event, with an aversive emotional content a defined localizable topology and duration. The International Association for the Study of Pain has defined it as an unpleasant sensorial and emotional experience associated to a real or potential tissue injury, which includes a series of behaviors related to pain, visible or audible, that can be modified by learning.
A classification for its temporality has been suggested: acute or phasic pain and chronic or tonic pain. Acute pain is conducted by the lateral spinothalamic system, it is brief and its intensity decays rapidly. Chronic pain is persistent and is conducted by the medial spinothalamic system.
Chronic pain animal models have been developed with which it is intended to clear out the operating way of a complex phenomenon that requires more rational therapeutic alternatives. There are three basic models: irritative central focus, deafferentation and hiperestimulation. Our group has developed an hiperestimulation paradigm by means of an intraplantar carrageenan (CAR) injection, which triggers a selfinjury behavior (SIB) associated to the unpleasant and persistent painful input. This abnormal behavior is exhibited in human syndromes such as: Lesh Nyhan, Gilles de la Tourette, congenital analgesia, and psychiatric disorders: obsessive-compulsive, depression and schizophrenia. In animals, this behavior is associated to disestesic, spontaneous pains, hiperalgesia and allodynia, that, as well as chronic pain, is very variable between individuals and involve genetic, cognitive and affective factors not yet well identified.
The limbic system has been related to affective and cognitive processes. One of the brain areas linked to the limbic system is the anterior cingulate cortex (ACC). Ablution of this cortex (cingulotomy) diminishes the unpleasant painful sensation in some patients that suffer chronic pain.
On the other hand, one of the structures close related to the limbic system, and more specifically to the ACC, is the ventral tegmental area (VTA). Several works postulate the dopaminergic activity of this nucleus over the ACC. It receives neural projections from the VTA, mainly dopaminergic, by means of the medial forebrain bundle. It has been reported that in the rat, the electrical stimulation of the VTA increases the behavioral response threshold to nociceptive stimulation. Moreover, there is electrophysiological evidence that nociceptive neuronal activity in the ACC is susceptible of inhibition by means of VTA electrical stimulation. In neurectomized rats it has been observed that self-stimulation of the VTA decreases the autotomy behavior; on the contrary, the lesioning of this nucleus induces an increase in the above mentioned behavior.
In this review we emphasize the importance of the dopaminergic mesolimbic system in the complexity of the affective experience of pain.