Reward Circuitry Plasticity in Pain Perception and Modulation

The basal ganglia are a chain of subcortical nuclei that facilitate action selection. Two striatal projection systems--so-called direct and indirect pathways--form the functional backbone of the basal ganglia circuit. Twenty years ago, investigators proposed that the striatum's ability to use dopamine (DA) rise and fall to control action selection was due to the segregation of D(1) and D(2) DA receptors in direct- and indirect-pathway spiny projection neurons. Although this hypothesis sparked a debate, the evidence that has accumulated since then clearly supports this model. Recent advances in the means of marking neural circuits with optical or molecular reporters have revealed a clear-cut dichotomy between these two cell types at the molecular, anatomical, and physiological levels. The contrast provided by these studies has provided new insights into how the striatum responds to fluctuations in DA signaling and how diseases that alter this signaling change striatal function.

Acute pain and emotion are processed in two forebrain networks, and the cingulate cortex is involved in both. Although Brodmann's cingulate gyrus had two divisions and was not based on any functional criteria, functional imaging studies still use this model. However, recent cytoarchitectural studies of the cingulate gyrus support a four-region model, with subregions, that is based on connections and qualitatively unique functions. Although the activity evoked by pain and emotion has been widely reported, some view them as emergent products of the brain rather than of small aggregates of neurons. Here, we assess pain and emotion in each cingulate subregion, and assess whether pain is co-localized with negative affect. Amazingly, these activation patterns do not simply overlap.

The literature on psychological factors in neck and back pain was systematically searched and reviewed. To summarize current knowledge concerning the role of psychological variables in the etiology and development of neck and back pain. Recent conceptions of spinal pain, especially chronic back pain, have highlighted the role of psychological factors. Numerous studies subsequently have examined the effects of various psychological factors in neck and back pain. There is a need to review this material to ascertain what conclusions may be drawn. Medical and psychological databases and cross-referencing were used to locate 913 potentially relevant articles. A table of 37 studies was constructed, consisting only of studies with prospective designs to ensure quality. Each study was reviewed for the population studied, the psychological predictor variables, and the outcome. The available literature indicated a clear link between psychological variables and neck and back pain. The prospective studies indicated that psychological variables were related to the onset of pain, and to acute, subacute, and chronic pain. Stress, distress, or anxiety as well as mood and emotions, cognitive functioning, and pain behavior all were found to be significant factors. Personality factors produced mixed results. Although the level of evidence was low, abuse also was found to be a potentially significant factor. Psychological factors play a significant role not only in chronic pain, but also in the etiology of acute pain, particularly in the transition to chronic problems. Specific types of psychological variables emerge and may be important in distinct developmental time frames, also implying that assessment and intervention need to reflect these variables. Still, psychological factors account for only a portion of the variance, thereby highlighting the multidimensional view. Because the methodologic quality of the studies varied considerably, future research should focus on improving quality and addressing new questions such as the mechanism, the developmental time factor, and the relevance that these risk factors have for intervention.