Pain and emotion. The body has strong links between the two. This does not mean emotion causes pain. It is much more likely to be the other way around. The body has merciful mechanisms to restrict pain information in emergency survival situations, but chronic pain is not known to participate in such protections, probably because mGluR5 does its own thing. The powerful emotional impact of pain is probably routed via the amygdala, and accounts for the findings of increased brain emotional activity on PET, SPECT, and fMRI in experimental pain studies.
Neugebauer at UTMB is not messing around when he looks at the amygdala in pain. There has been entirely too much nonsense about pain being psychological. It is as physical as any other sensation. The reason for the lapse in logic is that everyone has experienced situations where identical stimulus produced more or less pain under given circumstances. A common example is loud music played at the dental office. Certain behaviorists have extended this idea beyond all reason until pain has been relegated to a nonexistent status, a figment of the imagination. In the face of this has been the development of rats with central pain, nearly identical in every respect to that in humans.
Unless “The Secret of Nimh” was a nonfiction cartoon, and Don Bluth* is actually a neuroscientist, rats don’t really have a psychology behind their pain. Rats are very practical but are not known to score especially high on abstract reasoning, the kind that underlies so much of psychology.
Still, to give the devil his due, it is not only fair but scientific to ask, “What ARE the emotional parts of the brain doing when they light up on brain scans during pain?” Basically, it is a cause and effect debate. Which came first? The chicken or the egg. If bad attitude comes first, then it is quite impossible to explain the consistent predictable pain from capsaicin, which is no respecter of persons or degrees from prestigious universities.
Neugebauer has made research on pain psychology respectable by really looking at the amygdala. This structure, which along with the hippocampus is an important part of the Papez circuit, or circuit of brain centers which handle emotion, IS important in pain.
Anatomical studies have now shown it is the laterocapsular division of the central nucleus of the amygdala which is the “nociceptive amygdala”. Most older studies just say central nucleus, but they mean the pain part of the amygdala. Neugebauer has looked at mGluR1 and MGluR5 receptor subtypes in pain processing under normal conditions AND in pain-related sensitization. Neurons in the central nucleus are abbreviated CeA neurons. See Li and Neugebauer J Neurophysiol. 2004 Jan;91(1):13-24 AND J Neurophysiol. 2003 Feb;89(2):716-27
As a prior article has stated the metabotropic glutamate receptors under discussion are G protein-coupled. Microdialysis was used to inject both agonists (mimic) and antagonists (blocker) of the mGluR receptors during the study of the neurons. DHPG mimics mGluR1 and DHPG mimics mGluR 5. CPCCOEt was used as the mGluR5 antagonist. (MPEP was also used in one of the studies as the antagonist for mGluR 5). The results showed that MGluR1 is designed to work in normal pain, but is capable of changing with time. By comparison, MGluR5 modulates both normal and chronic pain. Remember, we are talking about activity in the AMYGDALA, which would normally mean the creation of an emotional RESPONSE to pain, not the cause of it.
It is getting pretty late in the day for behaviorists to hold on to outdated ideas that all or nearly all pain has its origin in psychological events. What is known is that the emotional centers of the brain react to pain, usually fairly strongly, and that the metabotropic glutamate receptors differ in this respect, with MGluR5 appearing to be at the center of the action for chronic pain. Down the road, blockers of these receptor subtypes may form the basis for pharmacological treatment. Since those with central pain know the devastating emotional impact of pain, there will be great demand for anything which might lead to emotional calming, as well as anything which actually stops pain.
After all, there is pretty good evidence that much of the pain relief derived from today’s medicines is really sedative in nature anyway. Specific drugs to target the amygdala cannot hurt.
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Bluth’s prescience as a filmmaker has been demonstrated by the current attempt to inject human genes into rats. The agreement is that if any of the rats demonstrate human behavior, they will be terminated and the experiments ended. One must ask, “What would be the attributes of human behavior to look for”. Possibly, the desire to experiment on other rats? A willingness to kill out of spite or over religious ideas? Fascination with Britney Spears? Lust for Haagen Das? Pursuit by the IRS for taxes? No, the first evaluations will be attentive preoccupation with television.
