Reducing auditory nerve excitability by acute antagonism of Ca2+-permeable AMPA receptors

Hearing depends on glutamatergic synaptic transmission mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs are tetramers, where inclusion of the GluA2 subunit reduces overall channel conductance and C

Distribution of Dexamethasone and Preservation of Inner Ear Function following Intratympanic Delivery of a Gel-Based Formulation

Intratympanic (IT) delivery of drugs to the ear is increasingly used for both clinical and research purposes. One limitation of IT delivery is that drugs are rapidly lost from the middle ear by a number of processes, so that prolonged delivery of drug is technically difficult. In the present study, the delivery characteristics of a poloxamer hydrogel formulation containing dexamethasone (dex) were evaluated. The gel is liquid at room temperature, allowing IT injection, but transitions to a gel at body temperature, providing a prolonged residence time in the middle ear. A 50-μl volume of control or dex-containing gel (dex-gel) was injected through the tympanic membrane of guinea pigs. Cochlear function was assessed with cochlear action potential and acoustic emission thresholds measured immediately, 6 or 24 h after IT gel injection. After 6- or 24-hour treatment with dex-gel, perilymph drug gradients along the cochlea were assessed by taking samples sequentially from the apex, and endolymph was sampled from the basal turn. Control gel injections caused small changes in sound field calibrations and functional measures for low-frequency stimuli, consistent with an induced conductive loss. Within 24 h, responses returned to normal. Twenty-four hours after dex-gel injection, low-frequency changes remained as the dex-gel was retained better in the middle ear, but there was no indication of high-frequency loss. While perilymph sample data showed that dex gradients were substantially lower than after single injections of dex solution, quantitative analysis of this result suggests that some dex may have entered the perilymph through the thin bone in the apical region of the cochlea. Endolymph levels of dex remained lower than those in the perilymph. This study confirms that a poloxamer hydrogel-based dex formulation provides an effective method for a prolonged delivery, providing a more uniform distribution of drug in the inner ear

Perilymph pharmacokinetics of marker applied through a cochlear implant in guinea pigs.

Patients undergoing cochlear implantation could benefit from a simultaneous application of drugs into the ear, helping preserve residual low-frequency hearing and afferent nerve fiber populations. One way to apply drugs is to incorporate a cannula into the implant, through which drug solution is driven. For such an approach, perilymph concentrations achieved and the distribution in the ear over time have not previously been documented. We used FITC-labeled dextran as a marker, delivering it into perilymph of guinea pigs at 10 or 100 nL/min though a cannula incorporated into a cochlear implant with the outlet in the mid basal turn. After injections of varying duration (2 hours, 1 day or 7 days) perilymph was collected from the cochlear apex using a sequential sampling technique, allowing dextran levels and gradients along scala tympani to be quantified. Data were interpreted quantitatively using computer simulations of the experiments. For injections of 2 hours duration, dextran levels were critically influenced by the presence or absence of fluid leakage at the cochleostomy site. When the cochleostomy was fluid-tight, substantially higher perilymph levels were achieved at the injection site, with concentration declining along scala tympani towards the apex. Contrary to expectations, large dextran gradients along scala tympani persisted after 24 hours of sustained injection and were still present in some animals after 7 days injection. Functional changes associated with implantation and dextran delivery, and the histological state of the implant and cannula were also documented. The persistent longitudinal gradients of dextan along the ear were not readily explained by computer simulations of the experiments based on prior pharmacokinetic data. One explanation is that inner ear pharmacokinetics are altered in the period after cochlear implantation, possibly by a permeabilization of the blood-labyrinth barrier as part of the immune response to the implant

Measured dextran concentrations in perilymph samples.

<p>Dextran concentrations were measured for sequential fluid samples collected from the apex of implanted animals in which the cochleostomy was leaking during injection (A) or in which there was no detectable leak from the cochleostomy during the 2 hr injection period (B). Injections were performed at 100 nL/min either with a WPI Ultrapump (solid circles) or with iPRECIO SMP-200 pumps (solid triangles). Group average curves are shown in black. With leakage at the cochleostomy, the peak of the average marker concentration occurred in the 3^rd sample and averaged 271 μM (SD 103, n = 7) while with no leakage at the cochleostomy the peak of the average marker concentration occurred in the 4^th sample and averaged 529 μM (SD 138, n = 4). The predicted curve (from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183374#pone.0183374.g003" target="_blank">Fig 3C</a>; 2 hr) is shown for comparison (gray diamonds). Measured concentrations were typically lower than predicted.</p

The implant-cannula-pump system ready for implantation.

<p>The pump reservoir was filled with 10 mM FITC-dextran solution in a bicarbonate-buffered artificial perilymph. The pump was coupled to the polyimide cannula of the implant with a 10 mm length of 21G stainless tubing, fixed to the polyimide tubing with cyanoacrylate. The stainless coupler was sutured to the implant to limit strain on the polyimide tubing.</p

Perilymph measurements at 3 time points compared.

<p>(A) Group mean curves for the three injection durations of 2hr, 24 hr or 7 days. The peak is higher for 24 hr injection than for 2 hr, but is lower after 7 days. Sample 1 concentration was lowest at 2 hrs and increased progressively with time. (B) Calculated ratio between sample 3 and sample 1 for individual animals in the study. The fitted regression line suggests that uniform distribution of dextran along ST (3:1 ratio of 1) would typically take about 11 days to achieve. Also shown on the curve is the change of 3:1 ratio with time initially predicted by simulations of the experiment.</p

Measurements compared to simulations of the experiments.

<p>Open symbols: Calculated sample curves for 2 hr (green) and 24 hr (purple) injections using the same pharmacokinetic parameters. In these simulations we implemented a base-apex gradient of elimination (800 min half time at the base, transitioning to 30 min half time at the apex) and a base-apex gradient of volume flow (0.06 μL entering at the base, transitioning to zero at the apex). This scenario permitted the gradients along ST shown by samples 1–3 to be approximated after 2hr and 24 hr injections. Solid symbols: Measured group average curves. In the case of 24 hr injections the measured curve is also shown excluding one animal that had an abnormal decline in the later samples, which largely accounted for the difference between the measured and calculated curves in samples 6–10. The group mean for 7 day injections is also shown, but the calculated curve for 7 days of injection was identical to that at 24 hours (purple, open symbols) so it has been omitted.</p

Functional changes caused by implantation.

<p>Mean CAP threshold curves (A) and acoustic emission threshold curves (B) for 5 experimental groups with group numbers given below for CAP and emissions respectively. Unimplanted: (Green: 16, 7); Immediately after implantation (Brown: 9, 1); 1 day after implantation with injection (Purple, 5, 5); 7 days after implantation where injection failed (Blue, 4, 4); 7 days after implantation where injection was successful (Dark Blue, 3, 3). The implant location is shown on a frequency/distance plot for the guinea pig (C), adapted to show the relationship with distance along scala tympani (blue curve). The frequency range for significant CAP sensitivity loss corresponds to the middle region of the implant.</p