Chronic morphine treatment impaired hippocampal long-term potentiation and spatial memory via accumulation of extracellular adenosine acting on adenosine A1 receptors.
Chronic exposure to opiates impairs hippocampal long-term potentiation (LTP) and spatial memory, but the underlying mechanisms remain to be elucidated. Given the well known effects of adenosine, an important neuromodulator, on hippocampal neuronal excitability and synaptic plasticity, we investigated the potential effect of changes in adenosine concentrations on chronic morphine treatment-induced impairment of hippocampal CA1 LTP and spatial memory. We found that chronic treatment in mice with either increasing doses (20-100 mg/kg) of morphine for 7 d or equal daily dose (20 mg/kg) of morphine for 12 d led to a significant increase of hippocampal extracellular adenosine concentrations. Importantly, we found that accumulated adenosine contributed to the inhibition of the hippocampal CA1 LTP and impairment of spatial memory retrieval measured in the Morris water maze. Adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine significantly reversed chronic morphine-induced impairment of hippocampal CA1 LTP and spatial memory. Likewise, adenosine deaminase, which converts adenosine into the inactive metabolite inosine, restored impaired hippocampal CA1 LTP. We further found that adenosine accumulation was attributable to the alteration of adenosine uptake but not adenosine metabolisms. Bidirectional nucleoside transporters (ENT2) appeared to play a key role in the reduction of adenosine uptake. Changes in PKC-alpha/beta activity were correlated with the attenuation of the ENT2 function in the short-term (2 h) but not in the long-term (7 d) period after the termination of morphine treatment. This study reveals a potential mechanism by which chronic exposure to morphine leads to impairment of both hippocampal LTP and spatial memory.
Lu
Gang
G
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institute For Biological Sciences, Chinese Academy of Sciences, Shanghai 201203, China. jgliu@mail.shcnc.ac.cn
Zhou
Qi-Xin
QX
Kang
Shuo
S
Li
Qing-Lin
QL
Zhao
Liang-Cai
LC
Chen
Jia-Dong
JD
Sun
Jian-Feng
JF
Cao
Jun
J
Wang
Yu-Jun
YJ
Chen
Jie
J
Chen
Xiao-Yan
XY
Zhong
Da-Fang
DF
Chi
Zhi-Qiang
ZQ
Xu
Lin
L
Liu
Jing-Gen
JG
D000241:Adenosine / Q000378:metabolism* / Q000633:toxicity; D058907:Adenosine A1 Receptor Agonists; D000818:Animals; D004334:Drug Administration Schedule; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D017774:Long-Term Potentiation / Q000187:drug effects / Q000502:physiology*; D008297:Male; D018782:Maze Learning / Q000187:drug effects / Q000502:physiology*; D008568:Memory / Q000187:drug effects / Q000502:physiology; D051379:Mice; D009020:Morphine / Q000008:administration & dosage* / Q000633:toxicity; D043682:Receptor, Adenosine A1 / Q000378:metabolism*; D013037:Spatial Behavior / Q000187:drug effects / Q000502:physiology
D058907:Adenosine A1 Receptor Agonists; D043682:Receptor, Adenosine A1; D009020:Morphine; D000241:Adenosine