Subpopulation of dorsal horn neurons displays enhanced N-methyl-D-aspartate receptor function after chronic morphine exposure.
BACKGROUND
Morphine tolerance may be attributed to enhancement of glutamatergic neurotransmission, in particular to increased function of the N-methyl-D-aspartate (NMDA) receptor. The cellular mechanisms responsible for these changes remain poorly defined. The authors identified and characterized a specific subpopulation of dorsal horn neurons, displaying NMDA receptor plasticity in response to chronic morphine administration.


METHODS
The authors undertook current clamped and voltage clamped recordings of NMDA receptor-mediated responses from cultured rat dorsal horn neurons that were untreated or treated for 7 days with 1 or 100 microm morphine.


RESULTS
Smaller (capacitance < or = 22 pF), tonic firing neurons showed a significantly enhanced NMDA receptor-mediated peak current after prolonged morphine treatment, whereas larger and phasic firing neurons showed no enhancement. With high-concentration but not low-concentration morphine treatment, Mg2+ blockade of NMDA receptors at resting membrane potentials was reduced. Furthermore, the chronic opioid-induced increase in NMDA current was attenuated by pretreatment with either a mu-opioid receptor inhibitor (naloxone) or an NMDA receptor inhibitor (2-amino-5-phosphonovalerate) (low-concentration > high-concentration morphine).


CONCLUSIONS
In an electrophysiologically defined subpopulation of dorsal horn neurons, enhanced NMDA receptor function after chronic morphine exposure was shown to be mechanistically dependent on morphine concentration and sensitive to both NMDA and mu-opioid receptor antagonism. Therefore, these changes observed in this population of sensory spinal neurons can be used to study the development and prevention of opioid tolerance described in multiple laboratory and clinical reports.
Zhao
Min
M
Brain and Behaviour Group, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada.
Joo
Daisy Therese
DT
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D000818:Animals; D008274:Magnesium / Q000494:pharmacology; D009020:Morphine / Q000494:pharmacology*; D016202:N-Methylaspartate / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D020671:Posterior Horn Cells / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects* / Q000502:physiology
D016194:Receptors, N-Methyl-D-Aspartate; D009270:Naloxone; D016202:N-Methylaspartate; D015763:2-Amino-5-phosphonovalerate; D009020:Morphine; D008274:Magnesium