Impaired Lactate Release in Dorsal CA1 Astrocytes Contributed to Nociceptive Sensitization and Comorbid Memory Deficits in Rodents.

BACKGROUND: Memory deficits are a common comorbid disorder in patients suffering from neuropathic pain. The mechanisms underlying the comorbidities remain elusive. The hypothesis of this study was that impaired lactate release from dysfunctional astrocytes in dorsal hippocampal CA1 contributed to memory deficits. METHODS: A spared nerve injury model was established to induce both pain and memory deficits in rats and mice of both sexes. von Frey tests, novel object recognition, and conditioned place preference tests were applied to evaluate the behaviors. Whole-cell recording, fiber photometry, Western blotting, and immunohistochemistry combined with intracranial injections were used to explore the underlying mechanisms. RESULTS: Animals with spared sciatic nerve injury that had displayed nociception sensitization or memory deficit comorbidities demonstrated a reduction in the intrinsic excitability of pyramidal neurons, accompanied by reduced Ca2+ activation in astrocytes (ΔF/F, sham: 6 ± 2%; comorbidity: 2 ± 0.4%) and a decrease in the expression of glial fibrillary acidic protein and lactate levels in the dorsal CA1. Exogenous lactate supply or increasing endogenous lactate release by chemogenetic activation of astrocytes alleviated this comorbidity by enhancing the cell excitability (129 ± 4 vs. 88 ± 10 for 3.5 mM lactate) and potentiating N-methyl-d-aspartate receptor-mediated excitatory postsynaptic potentials of pyramidal neurons. In contrast, inhibition of lactate synthesis, blocking lactate transporters, or chemogenetic inhibition of astrocytes resulted in comorbidity-like behaviors in naive animals. Notably, ß2-adrenergic receptors in astrocytes but not neurons were downregulated in dorsal CA1 after spared nerve injury. Microinjection of a ß2 receptor agonist into dorsal CA1 or activation of the noradrenergic projections onto the hippocampus from the locus coeruleus alleviated the comorbidity, possibly by increasing lactate release. CONCLUSIONS: Impaired lactate release from dysfunctional astrocytes, which could be rescued by activation of the locus coeruleus, led to nociception and memory deficits after peripheral nerve injury.

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.