Cellular Mechanisms Underlying Suppression Of Rebound Depolarization In Neurons Of Rat Medial Geniculate Body By Sodium Salicylate

BACKGROUND::Sodium salicylate (NaSal), a major component of Aspirin, can penetrate the blood-brain barrier and directly target the central auditory system, including the medial geniculate body (MGB). NaSal is frequently used as a tinnitus inducer in animal models owing to its distinct advantages of consistency and reversibility. Studies have shown that the NaSal induced the hyperactivity and hypersynchronization in auditory neural network and increased the gain of the central auditory system. The MGB, one of the important part of auditory thalamus, relays the ascending auditory information to cortex and receives strong reciprocal descending projections with the corticofugal projections serving as the gating mechanism. Many researchers believe that the MGB is one of many nuclei in the central auditory pathways involved in generation of tinnitus. Neuronal rebound depolarization (RD) is a voltage response to the offset from pre-hyperpolarization of the membrane potential and often results in burst firing of a neuron when robust enough. The RD, as the prominent characteristic of MGB neurons, has important functional significance in auditory information relaying and processing. Previous study showed that the neuronal RD is drastically depressed by NaSal. The purpose of the present study is to investigate the ion channel basis of the RD and the cellular mechanism under the effect of NaSal on the RD.METHODS:In the present studies, the RD was evoked by pre-hyperpolarizing current by the whole-cell current-clamp recordings in the MGB. The horizontal brain slices with MGB were from Wistar rats at P15-23. The generation and regulation mechanisms of the RD were examined by adding channel blockers or receptor agonist in artificial cerebrospinal fluid. In order to identify that the regulatory role of this factors were depended on the change of resting membrane potential, current injection was used to maintain the resting membrane potential at control level to test the effects of the blockers and NaSal. The change of the leak current was used to represent the change of the resting membrane potential in study of the mechanism of the NaSal-induced hyperpolarization in whole-cell voltage-clamp recording.RESULTS:(1) Most MGB neurons showed a robust RD evoked by pre-hyperpolarizing current. The RD was proportional in strength to the magnitude and duration of pre-hyperpolarization. The latency to the peak of the RD was decreased as the magnitude and duration increased. (2) The RD could be blocked by Ni2+ or Mibefradil, blockers for T-type Ca2+channel. The blocking effects of Ni2+ and Mibefradil indicate that the RD in MGB neurons is mediated by activation of T-type Ca2+ channels. (3) Blockage the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in MGB neurons by ZD7288 could hyperpolarize the resting membrane potential and inhibit the RD. Moreover, the inhibition of RD by ZD7288 was depended on the hyperpolarization of resting membrane potential. (4) Ba2+, a blocker of inwardly rectifying potassium (Kir) channels, could significantly depolarize the resting membrane potential to modulate the properties of the RD. (5) Activation of G-protein gated Kir (GIRK) channels by the GABAb receptor agonist baclofen drastically depressed the RD. The Tertinpin-Q, a GIRK channel blocker, could inhibit the suppression of the RD by baclofen, indicating the modulatory effects of GABAB -GIRK pathway in MGB neurons.(6) NaSal reversibly suppressed the RD of MGB neurons. Meanwhile, NaSal hyperpolarized the resting membrane potential and decreased the input resistance. The long latency of the hyperpolarization and the decrease of input resistance suggest that NaSal activates some kinds of metabotropic channels or receptors. (7) NaSal had no effects on the current mediated by T-type Ca2+ channels, which induced the RD. (8) The suppression of the RD and rebound spikes by NaSal could be recover by drawing the resting membrane potential to the control level, indicating that NaSal depresses the RD through hyperpolarizing the resting membrane potential. (9) NaSal had no effects on the HCN current, which play critical role in regulating the resting membrane potential. (10) The NaSal-induced hyperpolarization of the resting membrane potential could be manifested as an increase in the outward leak current under voltage-clamp mode. CGP55845, a blocker for GABAB receptor, could completely block the NaSal-induced increase in the outward leak current, confirming the GABAB receptor as a pharmacological target of NaSal. (11) NaSal failed to hyperpolarize the resting membrane potential when GIRK channels were blocked by Tertiapin-Q. That is, NaSal hyperpolarizes the resting membrane potential through the GABAB-GIRK pathway. (12) Blockage the GABAB-GIRK pathway by CGP55845 could interrupt the suppression of the RD by NaSal, indicating that this pathway is required for NaSal to suppress the RD in MGB neurons.DISSCUSION:Present studies indicate that the RD is mediated by the deinactivation of the T-type Ca2+ channels. As the time-and voltage-dependent properties of the T-type Ca2+ channels, the RD was proportional in strength to the magnitude and duration of pre-hyperpolarization. Because of the critical role of resting membrane potential in regulating the RD in MGB neurons, the factors including the NaSal, HCN and Kir channels, which could modulate the resting membrane potential, will have regulatory effects on the RD. In fact, the suppression of NaSal on the RD is though activating the GABAB-GIRK pathway to hyperpolarize the resting membrane potential. Present studies not only explain the cellular mechaism of the suppression of RD by NaSal but also imply the wide effects of the NaSal in MGB neurons as its role in resting membrane potential. Our study may provide a new way to think about the NaSal-induced tinnitus.