Glucocorticoid regulation of lactate release from spinal astrocytes contributes to the induction of spinal LTP of C-fiber-evoked field potentials and the development of mechanical allodynia

High-frequency stimulation (HFS) of the sciatic nerve leads to long-term potentiation (LTP) at C-fiber synapse and long-lasting pain hypersensitivity. The underlying mechanisms, however, are still unclear. In the present study, we investigated the involvement of astrocytes derived l-lactate in the spinal dorsal horn subsequent to glucocorticoid (GC) secretion into the plasma in this process using Sprague-Dawley rats and Aldh1L1-CreERT2 mice of either sex. We found that HFS increased l-lactate and monocarboxylate transporters 1/2 (MCT1/2) in the spinal dorsal horn. Inhibition of glycogenolysis or blocking lactate transport prevented the induction of spinal LTP following HFS. Furthermore, Chemogenetical inhibition of dorsal horn astrocytes, which were activated by HFS, prevented spinal LTP, alleviated the mechanical allodynia and the decreased the level l-lactate and GFAP expression in the dorsal horn following HFS. In contrast, Chemogenetics activation of dorsal horn astrocytes in naïve rats induced spinal LTP as well as mechanical allodynia, and increased GFAP expression and l-lactate. Application of l-lactate directly to the spinal cord of naïve rats induced spinal LTP, mechanical allodynia, and increased spinal expression of p-ERK. Importantly, HFS increased GC in the plasma and glucocorticoid receptor (GR) expression in spinal astrocytes, adrenalectomy or knocking down of GR in astrocytes by using Cre-Loxp system blocked the mechanical allodynia, prevented the spinal LTP and the enhancement of lactate after HFS. These results show that lactate released from spinal astrocytes following glucocorticoid release into the plasma enhance synaptic transmission at the C-fiber synapse and underlie pain chronicity.