Regulation of Nociceptive Plasticity Threshold and DARPP-32 Phosphorylation in Spinal Dorsal Horn Neurons by Convergent Dopamine and Glutamate Inputs.

Itsaso Buesa,Jon Jatsu Azkue,Zigor Aira

doi:10.1371/journal.pone.0162416

Abstract

Dopamine can influence NMDA receptor function and regulate glutamate-triggered long-term changes in synaptic strength in several regions of the CNS. In spinal cord, regulation of the threshold of synaptic plasticity may determine the proneness to undergo sensitization and hyperresponsiveness to noxious input. In the current study, we increased endogenous dopamine levels in the dorsal horn by using re-uptake inhibitor GBR 12935. During the so-induced hyperdopaminergic transmission, conditioning low-frequency (1 Hz) stimulation (LFS) to the sciatic nerve induced long-term potentiation (LTP) of C-fiber-evoked potentials in dorsal horn neurons. The magnitude of LTP was attenuated by blockade of either dopamine D1-like receptors (D1LRs) by with SCH 23390 or NMDA receptor subunit NR2B with antagonist Ro25-6981. Conditioning LFS during GBR 12935 administration increased phosphorylation of dopamine- and cAMP-regulated phosphoprotein of Mr 32kDa (DARPP-32) at threonine 34 residue in synaptosomal (P3) fraction of dorsal horn homogenates, as assessed by Western blot analysis, which was partially prevented by NR2B blockade prior to conditioning stimulation. Conditioning LFS also was followed by higher co-localization of phosphorylated form of NR2B at tyrosine 1472 and pDARPP-32Thr34- with postsynaptic marker PSD-95 in transverse L5 dorsal horn sections. Such increase could be significantly attenuated by D1LR blockade with SCH 23390. The current results support that coincidental endogenous recruitment of D1LRs and NR2B in dorsal horn synapses plays a role in regulating afferent-induced nociceptive plasticity. Parallel increases in DARPP-32 phosphorylation upon LTP induction suggests a role for this phosphoprotein as intracellular detector of convergent D1L- and NMDA receptor activation.

Highlights

Synaptic plasticity at glutamatergic synapses, including long-term potentiation (LTP) of nociceptive neurotransmission, is held as a critical cellular generator of spinal sensitization and persistent pain states [1,2,3,4]
We investigated how synaptic activation of D1-like receptors (D1LRs) and NMDA receptor subunit 2B can contribute to regulating the threshold of afferent-induced synaptic plasticity in dorsal horn neurons
Concurrent activation of D1LRs and NR2B decreases the threshold of afferent-induced synaptic plasticity in dorsal horn neurons

Summary

Introduction

Synaptic plasticity at glutamatergic synapses, including long-term potentiation (LTP) of nociceptive neurotransmission, is held as a critical cellular generator of spinal sensitization and persistent pain states [1,2,3,4]. Recent work has revealed that sustained dopaminergic, D1LR-mediated input to the spinal dorsal horn can shift the NMDA receptor to an enhanced activation state and lower the threshold of synaptic potentiation of C-fiber-evoked excitation and [15,16]. These data imply that increased dopaminergic neurotransmission at the dorsal horn, which has been shown to occur in states of sustained pain [17], holds the potential to profoundly alter nociception by increasing proneness to synaptic plasticity. We assessed how recruitment of D1LRs and NMDA receptors influence DARPP-32 phosphorylation at residue Thr and phosphorylation of NMDA receptor subunit 2B at Tyr1472 at synaptic compartment in dorsal horn neurons during hyperdopaminergic transmission

Materials and Methods

Results

Discussion