Neurosteroid Allopregnanolone Suppresses Median Nerve Injury-induced Mechanical Hypersensitivity and Glial Extracellular Signal-regulated Kinase Activation through γ-Aminobutyric Acid Type A Receptor Modulation in the Rat Cuneate Nucleus.
Mechanisms underlying neuropathic pain relief by the neurosteroid allopregnanolone remain uncertain. We investigated if allopregnanolone attenuates glial extracellular signal-regulated kinase (ERK) activation in the cuneate nucleus (CN) concomitant with neuropathic pain relief in median nerve chronic constriction injury (CCI) model rats.



We examined the time course and cellular localization of phosphorylated ERK (p-ERK) in CN after CCI. We subsequently employed microinjection of a mitogen-activated protein kinase kinase (ERK kinase) inhibitor, PD98059, to clarify the role of ERK phosphorylation in neuropathic pain development. Furthermore, we explored the effects of allopregnanolone (by mouth), intra-CN microinjection of γ-aminobutyric acid type A receptor antagonist (bicuculline) or γ-aminobutyric acid type B receptor antagonist (phaclofen) plus allopregnanolone, and allopregnanolone synthesis inhibitor (medroxyprogesterone; subcutaneous) on ERK activation and CCI-induced behavioral hypersensitivity.



At 7 days post-CCI, p-ERK levels in ipsilateral CN were significantly increased and reached a peak. PD98059 microinjection into the CN 1 day after CCI dose-dependently attenuated injury-induced behavioral hypersensitivity (withdrawal threshold [mean ± SD], 7.4 ± 1.1, 8.7 ± 1.0, and 10.3 ± 0.8 g for 2.0, 2.5, and 3.0 mM PD98059, respectively, at 7 days post-CCI; n = 6 for each dose). Double immunofluorescence showed that p-ERK was localized to both astrocytes and microglia. Allopregnanolone significantly diminished CN p-ERK levels, glial activation, proinflammatory cytokines, and behavioral hypersensitivity after CCI. Bicuculline, but not phaclofen, blocked all effects of allopregnanolone. Medroxyprogesterone treatment reduced endogenous CN allopregnanolone and exacerbated nerve injury-induced neuropathic pain.



Median nerve injury-induced CN glial ERK activation modulated the development of behavioral hypersensitivity. Allopregnanolone attenuated glial ERK activation and neuropathic pain via γ-aminobutyric acid type A receptors. Reduced endogenous CN allopregnanolone after medroxyprogesterone administration rendered rats more susceptible to CCI-induced neuropathy.
From the Department of Internal Medicine and Traumatology, National Taiwan University Hospital, Taipei, Taiwan (C.-T.H.); Department of Anatomy and Cell Biology, School of Medicine, College of Medicine, Taipei Medical University, Taipei, Taiwan (S.-H.C.); Department of Anatomy and Cell Biology, College of Medicine, National Taiwan University, Taipei, Taiwan (J.-H.L.); Department of Anatomy, School of Medicine, China Medical University, Taichung, Taiwan (C.-F.C.); and Graduate Institute of Basic Medicine and School of Medicine, College of Medicine, Fu Jen Catholic University, New Taipei City, Taiwan (W.-H.W., Y.-J.T.).
Chun-Ta
CT
Huang
Seu-Hwa
SH
Chen
June-Horng
JH
Lue
Chi-Fen
CF
Chang
Wen-Hsin
WH
Wen
Yi-Ju
YJ
Tsai
D000777:Anesthetics / Q000494:pharmacology; D000818:Animals; D004195:Disease Models, Animal; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D006967:Hypersensitivity / Q000188:drug therapy* / Q000378:metabolism; D008297:Male; D008475:Median Nerve / Q000293:injuries*; D008526:Medulla Oblongata / Q000187:drug effects* / Q000378:metabolism; D009457:Neuroglia / Q000187:drug effects* / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism; D011280:Pregnanolone / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018079:Receptors, GABA / Q000187:drug effects / Q000378:metabolism*
D000777:Anesthetics; D018377:Neurotransmitter Agents; D018079:Receptors, GABA; D011280:Pregnanolone; D048049:Extracellular Signal-Regulated MAP Kinases