Morphological, biophysical and synaptic properties of glutamatergic neurons of the mouse spinal dorsal horn

Key points Excitatory and inhibitory interneurons of the spinal dorsal horn are critically involved in normal sensory processing and in the generation of pathological pain, but their physiological properties, especially those of excitatory interneurons, are only incompletely characterised. Here, we identified a vGluT2::eGFP BAC transgenic mouse line in which enhanced green fluorescent protein (eGFP) is specifically expressed in a subset of neurons that are likely to be representative of the whole population of excitatory dorsal horn neurons. We compared the physiological properties of vGluT2::eGFP neurons with those of inhibitory neurons in Gad67::eGFP and GlyT2::eGFP transgenic mice: vGluT2::eGFP neurons required stronger depolarising currents than inhibitory neurons to fire action potentials and fired fewer action potentials during prolonged depolarisations. Both excitatory or inhibitory dorsal horn neurons received synaptic input from capsaicin-sensitive fibres and primary afferent fibre-evoked (polysynaptic) inhibitory input. These findings should contribute to a better mechanistic understanding of normal and pathological sensory processing in the spinal dorsal horn. Interneurons of the spinal dorsal horn are central to somatosensory and nociceptive processing. A mechanistic understanding of their function depends on profound knowledge of their intrinsic properties and their integration into dorsal horn circuits. Here, we have used BAC transgenic mice expressing enhanced green fluorescent protein (eGFP) under the control of the vesicular glutamate transporter (vGluT2) gene (vGluT2::eGFP mice) to perform a detailed electrophysiological and morphological characterisation of excitatory dorsal horn neurons, and to compare their properties to those of GABAergic (Gad67::eGFP tagged) and glycinergic (GlyT2::eGFP tagged) neurons. vGluT2::eGFP was detected in about one-third of all excitatory dorsal horn neurons and, as demonstrated by the co-expression of vGluT2::eGFP with different markers of subtypes of glutamatergic neurons, probably labelled a representative fraction of these neurons. Three types of dendritic tree morphologies (vertical, central, and radial), but no islet cell-type morphology, were identified in vGluT2::eGFP neurons. vGluT2::eGFP neurons had more depolarised action potential thresholds and longer action potential durations than inhibitory neurons, while no significant differences were found for the resting membrane potential, input resistance, cell capacitance and after-hyperpolarisation. Delayed firing and single action potential firing were the single most prevalent firing patterns in vGluT2::eGFP neurons of the superficial and deep dorsal horn, respectively. By contrast, tonic firing prevailed in inhibitory interneurons of the dorsal horn. Capsaicin-induced synaptic inputs were detected in about half of the excitatory and inhibitory neurons, and occurred more frequently in superficial than in deep dorsal horn neurons. Primary afferent-evoked (polysynaptic) inhibitory inputs were found in the majority of glutamatergic and glycinergic neurons, but only in less than half of the GABAergic population. Excitatory dorsal horn neurons thus differ from their inhibitory counterparts in several biophysical properties and possibly also in their integration into the local neuronal circuitry.