Pain intensity and duration can be enhanced by prior challenge: initial evidence suggestive of a role of microglial priming.
Activation of spinal microglia and consequent release of proinflammatory mediators facilitate pain. Under certain conditions, responses of activated microglia can become enhanced. Enhanced microglial production of proinflammatory products may result from priming (sensitization), similar to macrophage priming. We hypothesized that if spinal microglia were primed by an initial inflammatory challenge, subsequent challenges may create enhanced pain. Here, we used a "two-hit" paradigm using 2 successive challenges, which affect overlapping populations of spinal microglia, presented 2 weeks apart. Mechanical allodynia and/or activation of spinal glia were assessed. Initially, laparotomy preceded systemic lipopolysaccharide (LPS). Prior laparotomy caused prolonged microglial (not astrocyte) activation plus enhanced LPS-induced allodynia. In this "two-hit" paradigm, minocycline, a microglial activation inhibitor, significantly reduced later exaggerated pain induced by prior surgery when minocycline was administered intrathecally for 5 days starting either at the time of surgery or 5 days before LPS administration. To test generality of the priming effect, subcutaneous formalin preceded intrathecal HIV-1 gp120, which activates spinal microglia and causes robust allodynia. Prior formalin enhanced intrathecal gp120-induced allodynia, suggesting that microglial priming is not limited to laparotomy and again supporting a spinal site of action. Therefore, spinal microglial priming may increase vulnerability to pain enhancement.


CONCLUSIONS
Spinal microglia may become "primed" (sensitized) following their activation by disparate forms of peripheral trauma/inflammation. As a result, such primed microglia may overrespond to subsequent challenges, thereby enhancing pain intensity and duration.
Hains
Leah E
LE
Department of Psychology and Neuroscience, and the Center for Neuroscience, University of Colorado at Boulder, Boulder, Colorado 80309-0345, USA.
Loram
Lisa C
LC
Weiseler
Julie L
JL
Frank
Matthew G
MG
Bloss
Erik B
EB
Sholar
Paige
P
Taylor
Frederick R
FR
Harrison
Jacqueline A
JA
Martin
Thomas J
TJ
Eisenach
James C
JC
Maier
Steven F
SF
Watkins
Linda R
LR
D000818:Animals; D039481:CD11b Antigen / Q000378:metabolism; D004195:Disease Models, Animal; D019317:Evidence-Based Medicine; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D015699:HIV Envelope Protein gp120 / Q000008:administration & dosage; D006930:Hyperalgesia / Q000175:diagnosis / Q000473:pathology; D007278:Injections, Spinal; D007813:Laparotomy / Q000009:adverse effects; D008297:Male; D017628:Microglia / Q000378:metabolism / Q000473:pathology* / Q000821:virology; D010146:Pain / Q000378:metabolism* / Q000473:pathology* / Q000821:virology; D010147:Pain Measurement / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000378:metabolism / Q000473:pathology / Q000821:virology; D013997:Time Factors
D039481:CD11b Antigen; D005904:Glial Fibrillary Acidic Protein; D015699:HIV Envelope Protein gp120; C514793:gp120 protein, Human immunodeficiency virus 1