Loss of NR1 Subunit of NMDARs in Primary Sensory Neurons Leads to Hyperexcitability and Pain Hypersensitivity: Involvement of Ca²⁺-Activated Small Conductance Potassium Channels

Abstract

It is well established that activation of NMDARs plays an essential role in spinal cord synaptic plasticity (i.e., central sensitization) and pain hypersensitivity after tissue injury. Despite prominent expression of NMDARs in DRG primary sensory neurons, the unique role of peripheral NMDARs in regulating intrinsic neuronal excitability and pain sensitivity is not well understood, in part due to the lack of selective molecular tools. To address this problem, we used Advillin-Cre driver to delete the NR1 subunit of NMDARs selectively in DRG neurons. In NR1 conditional knock-out (NR1-cKO) mice, NR1 expression is absent in DRG neurons but remains normal in spinal cord neurons; NMDA-induced currents are also eliminated in DRG neurons of these mice. Surprisingly, NR1-cKO mice displayed mechanical and thermal hypersensitivity compared with wild-type littermates. NR1-deficient DRG neurons show increased excitability, as indicated by increased frequency of action potentials, and enhanced excitatory synaptic transmission in spinal cord slices, as indicated by increased frequency of miniature EPSCs. This hyperexcitability can be reproduced by the NMDAR antagonist APV and by Ca 2ϩ -activated slow conductance K ϩ (SK) channel blocker apamin. Furthermore, NR1-positive DRG neurons coexpress SK1/SK2 and apamin-sensitive afterhyperpolarization currents are elevated by NMDA and suppressed by APV in these neurons. Our findings reveal the hitherto unsuspected role of NMDARs in controlling the intrinsic excitability of primary sensory neurons possibly via Ca 2ϩ -activated SK channels. Our results also call attention to potential opposing effects of NMDAR antagonists as a treatment for pain and other neurological disorders.

Optogenetic activation of mechanically insensitive afferents in mouse colorectum reveals chemosensitivity

“…Advillin-Cre mice were used to drive the expression of ChR2 in DRG neurons because, compared with other Creexpressing mouse lines, Advillin induces expression in a greater proportion of DRG neurons and apparently does not bias expression in only a subset(s) of DRG neurons (17,28). We confirmed by immunohistochemistry abundant expression of ChR2 in most L6 DRG neurons as well as at terminal endings in the colorectum, consistent with previous findings (41).…”

Identifying local and descending inputs for primary sensory neurons

“…Basbaum, Scherrer, and their colleagues previously showed that DOR, which is the high-affinity receptor for enkephalin, is mainly expressed in nonpeptidergic and in myelinated sensory fibers, and DOR agonists reduce acute mechanical but not heat pain (73). They further from their soma when activated, a process referred to as intraganglion neural transmission (56,57). Basbaum and colleagues have observed intraganglion transfer of WGA between DRG neurons (58).…”

Estrogens stimulate serotonin neurons to inhibit binge-like eating in mice

“…The slices were bathed in oxygenated aCSF (52) (32°C-34°C) at a flow rate of approximately 2 ml/min. Patch pipettes with resistances of 3 to 5 MΩ were filled with solution containing 126 mM K gluconate, 10 mM NaCl, 10 mM EGTA, 1 mM MgCl 2 , 2 mM Na-ATP, and 0.1 mM Mg-GTP (adjusted to pH 7.3 with KOH) (34). Voltage clamp was used to record SK-like currents.…”