Enhanced LTP of primary afferent neurotransmission in AMPA receptor GluR2-deficient mice

Ca²⁺-permeable-AMPA receptors (AMPARs) are expressed in the superficial dorsal horn (SDH, laminae I/II) of the spinal cord, the area involved in transmission and modulation of sensory information, including nociception. A possible role of Ca²⁺-permeable-AMPARs in synaptic strengthening has been suggested in postnatal DH cultures, but their role in the long-lasting activity-dependent synaptic plasticity of primary afferent neurotransmission in the adult mouse SDH has not been investigated. In the present study the role of Ca²⁺-permeable-AMPARs in the regulation of long-lasting synaptic plasticity, specifically long-term potentiation (LTP) and long-term depression (LTD) in the SDH, was investigated using mice deficient in AMPAR GluR2 subunit. We show here that the GluR2 mutants exhibited no changes in passive membrane properties, but a significant increase in rectification of excitatory postsynaptic currents, the finding suggesting increased expression of Ca²⁺-permeable-AMPARs. In the absence of GluR2, high-frequency stimulation (HFS) of small-diameter primary afferent fibers induced LTP that is enhanced and non-saturating in the SDH at both primary afferent Aδ- and/or C-fibers monosynaptic and polysynaptic pathways, whereas neuronal excitability and paired-pulse depression were normal. The LTP could be induced in the presence of the NMDA receptor antagonist d-AP5, and L-type Ca²⁺ channel blockers, suggesting that Ca²⁺-permeable-AMPARs are sufficient to induce LTP in the SDH neurons of adult mouse spinal cord. In contrast, the induction of HFS-LTD is reduced in the SDH of GluR2 mutants. These results suggest an important role for AMPAR GluR2 subunit in regulating synaptic plasticity with potential relevance for long-lasting hypersensitivity in pathological states.