Minocycline Inhibits Hyperpolarization-activated Currents In Rat Spinal Substantia Gelatinosa Neurons

Objective:Chronic pain is a clinical problem with high incidence which underlying mechanisms remain elusive. It is an important public health issue that seriously impacts on patients’ health, and is hard to manage. Minocycline, a semisynthetic tetracycline derivative, is able to provide distinct analgesic effect. However, the underlying cellular mechanisms have not been fully clarified yet. Here, by using whole-cell patch-clamp in spinal cord slices, we investigated the effects of minocycline on hyperpolarization-activated current(Ih) in rat substantia gelatinosa(SG) neurons. Methods:Male Spraueu-Dawley rats(3-5 weeks old) were deeply anesthetized with urethane intraperitoneally. After transcardial perfusion and extraction of the lumbosacral spinal cords(L1-S3), the pia-arachnoid membrane as well as all dorsal and ventral roots were removed with fine forceps. Spinal cords were then cut into 300 μm thick transverse slices. Whole-cell patch clamp technique was applied to record Ih before and after bath application of various concentration of minocycline(1-300 μM) in SG neurons. Results:1. Ih currents were observed in nearly 50% of recorded neurons, which could be blocked by ZD7288 and Cs Cl;2. SG neurons show Ih in different proportions with respect to cell morphological classification;3. Minocycline could inhibit Ih rapidly and reversibly in SG neurons;4. Minocycline inhibited Ih in a concentration-dependent manner with an IC50 of 34 μM.5. Minocycline shifted the activation curve of Ih towards the hyperpolarizing direction and thus slowed down its activation.6. The application of minocycline did not significantly change the reverse potential of Ih. Conclusions:Minocycline could not only decrease the amplitude and current density of Ih, but also change the current activation kinetics through acting on hyperpolarizationactivated cyclic nucleotide-gated cation channels(HCN) in SG neurons. These data indicated that the inhibition effect of minocycline on Ih could contribute to its analgesic effect.