Intracellular calcium responses encode action potential firing in spinal cord lamina I neurons

• Published in: The Journal of neuroscience 2020-06, Vol.40 (23), p.4439-4456
• Main Authors: Harding, Erika K, Boivin, Bruno, Salter, Michael W
• Format: Article
• Language: English
• Subjects: Action potential; Back propagation; Calcium; Calcium (intracellular); Calcium channels; Calcium channels (voltage-gated); Calcium imaging; Calcium sequestration; Calcium signalling; Chronic pain; Compartments; Cytosol; Dendrites; Dendritic spines; Electrophysiology; Intracellular; Intracellular signalling; Ions; Neurons; Pain; Plastic properties; Plasticity; Ryanodine; Spinal cord; Spinal plasticity
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.0206-20.2020

Maladaptive plasticity of neurons in lamina I of the spinal cord is a lynchpin for the development of chronic pain, and is critically dependent upon intracellular calcium signaling. However, the relationship between neuronal activity and intracellular calcium in these neurons is unknown. Here we combined two-photon calcium imaging with whole-cell electrophysiology to determine how action potential firing drives calcium responses within subcellular compartments of male rat spinal cord lamina I neurons. We found that single action potentials generated at the soma increase calcium concentration in the somatic cytosol and nucleus, and these calcium responses invade dendrites and dendritic spines by active backpropagation. Calcium responses in each compartment were dependent upon voltage-gated calcium channels, and somatic and nuclear calcium responses were amplified by release of calcium from ryanodine-sensitive intracellular stores. Grouping single action potential-evoked calcium responses by neuron type demonstrated their presence in all defined types, as well as a high degree of similarity in calcium responses between neuron types. With bursts of action potentials, we found that calcium responses have the capacity to encode action potential frequency and number in all compartments, with action potential number being preferentially encoded. Together, these findings indicate that intracellular calcium serves as a readout of neuronal activity within lamina I neurons, providing a unifying mechanism through which activity may regulate plasticity, including that seen in chronic pain. Despite their critical role in both acute pain sensation and chronic pain, little is known of the fundamental physiology of spinal cord lamina I neurons. This is especially the case with respect to calcium dynamics within these neurons, which could regulate maladaptive plasticity observed in chronic pain. By combining two-photon calcium imaging and patch-clamp electrophysiological recordings from lamina I neurons, we found that action potential firing induces calcium responses within the somatic cytosol, nucleus, dendrites, and dendritic spines of lamina I neurons. Our findings demonstrate the presence of actively backpropagating action potentials, shifting our understanding of how these neurons process information, such that calcium provides a mechanism for lamina I neurons to track their own activity.