Theta-Burst Stimulation of Primary Afferents Drives Long-Term Potentiation in the Spinal Cord and Persistent Pain via α2δ-1-Bound NMDA Receptors

Abstract

Long-term potentiation (LTP) and long-term depression (LTD) in the spinal dorsal horn reflect activity-dependent synaptic plasticity and central sensitization in chronic pain. Tetanic high-frequency stimulation is commonly used to induce LTP in the spinal cord. However, primary afferent nerves often display low-frequency, rhythmic bursting discharges in painful conditions. Here, we determined how theta-burst stimulation (TBS) of primary afferents impacts spinal cord synaptic plasticity and nociception in male and female mice. We found that TBS induced more LTP, whereas tetanic stimulation induced more LTD, in mouse spinal lamina II neurons. TBS triggered LTP, but not LTD, in 50% of excitatory neurons expressing vesicular glutamate transporter-2 (VGluT2). By contrast, TBS induced LTD and LTP in 12–16% of vesicular GABA transporter (VGAT)-expressing inhibitory neurons. Nerve injury significantly increased the prevalence of TBS-induced LTP in VGluT2-expressing, but not VGAT-expressing, lamina II neurons. Blocking NMDARs, inhibiting α2δ-1 with gabapentin, or α2δ-1 knockout abolished TBS-induced LTP in lamina II neurons. Also, disrupting the α2δ-1–NMDAR interaction with α2δ-1Tat peptide prevented TBS-induced LTP in VGluT2-expressing neurons. Furthermore, TBS of the sciatic nerve induced long-lasting allodynia and hyperalgesia in wild-type, but not α2δ-1 knockout, mice. TBS significantly increased the α2δ-1–NMDAR interaction and synaptic trafficking in the spinal cord. In addition, treatment with NMDAR antagonists, gabapentin, or α2δ-1Tat peptide reversed TBS-induced pain hypersensitivity. Therefore, TBS-induced primary afferent input causes a neuropathic pain-like phenotype and LTP predominantly in excitatory dorsal horn neurons via α2δ-1-dependent NMDAR activation. α2δ-1-bound NMDARs may be targeted for reducing chronic pain development at the onset of tissue/nerve injury.

SIGNIFICANCE STATEMENT Spinal dorsal horn synaptic plasticity is a hallmark of chronic pain. Although sensory nerves display rhythmic bursting discharges at theta frequencies during painful conditions, the significance of this naturally occurring firing activity in the induction of spinal synaptic plasticity is largely unknown. In this study, we found that theta-burst stimulation (TBS) of sensory nerves induced LTP mainly in excitatory dorsal horn neurons and that the prevalence of TBS-induced LTP was potentiated by nerve injury. This TBS-driven synaptic plasticity required α2δ-1 and its interaction with NMDARs. Furthermore, TBS of sensory nerves induced persistent pain, which was maintained by α2δ-1-bound NMDARs. Thus, TBS-induced LTP at primary afferent–dorsal horn neuron synapses is an appropriate cellular model for studying mechanisms of chronic pain.