Gamma-aminobutyric acid and glutamic acid levels in the auditory pathway of rats with chronic tinnitus: A direct determination using high resolution point-resolved proton magnetic resonance spectroscopy (1H-MRS)

Damage to the auditory system following high-level sound exposure reduces afferent input. Homeostatic mechanisms appear to compensate for the loss. Overcompensation may produce the sensation of sound without an objective physical correlate, i.e., tinnitus. Several potential compensatory neural processes have been identified, such as increased spontaneous activity. The cellular mechanisms enabling such compensatory processes may involve down-regulation of inhibitory neurotransmission mediated by γ-amino butyric acid (GABA), and/or up-regulation of excitatory neurotransmission, mediated by glutamic acid (Glu). Because central processing systems are integrated and well regulated, compensatory changes in one system may produce reactive changes in others. Some or all may be relevant to tinnitus. To examine the roles of GABA and Glu in tinnitus, high-resolution point resolved proton magnetic-resonance spectroscopy (1H-MRS) was used to quantify their levels in the dorsal cochlear nucleus (DCN), inferior colliculus (IC), medial geniculate body( (MGB), and primary auditory cortex (A1) of rats. Chronic tinnitus was produced by a single high-level unilateral exposure to noise, and was measured using a psychophysical procedure sensitive to tinnitus. Decreased GABA levels were evident only in the MGB, with the greatest decrease, relative to unexposed controls, obtained in the contralateral MGB. Small GABA increases may have been present bilaterally in A1 and in the contralateral DCN. Although Glu levels showed considerable variation, Glu was moderately and bilaterally elevated both in the DCN and in A1. In the MGB Glu was increased ipsilaterally but decreased contralaterally. These bidirectional and region-specific alterations in GABA and Glu may reflect large-scale changes in inhibitory and excitatory equilibrium accompanying chronic tinnitus. The present results also suggest that targeting both neurotransmitter systems may be optimal in developing more effective therapeutics

Local NMDA receptor blockade attenuates chronic tinnitus and associated brain activity in an animal model.

Chronic tinnitus has no broadly effective treatment. Identification of specific markers for tinnitus should facilitate the development of effective therapeutics. Recently it was shown that glutamatergic blockade in the cerebellar paraflocculus, using an antagonist cocktail was successful in reducing chronic tinnitus. The present experiment examined the effect of selective N-methyl d-aspartate (NMDA) receptor blockade on tinnitus and associated spontaneous brain activity in a rat model. The NMDA antagonist, D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.5 mM), was continuously infused for 2 weeks directly to the ipsilateral paraflocculus of rats with tinnitus induced months prior by unilateral noise exposure. Treated rats were compared to untreated normal controls without tinnitus, and to untreated positive controls with tinnitus. D-AP5 significantly decreased tinnitus within three days of beginning treatment, and continued to significantly reduce tinnitus throughout the course of treatment and for 23 days thereafter, at which time testing was halted. At the conclusion of psychophysical testing, neural activity was assessed using manganese enhanced magnetic resonance imaging (MEMRI). In agreement with previous research, untreated animals with chronic tinnitus showed significantly elevated bilateral activity in their paraflocculus and brainstem cochlear nuclei, but not in mid or forebrain structures. In contrast, D-AP5-treated-tinnitus animals showed significantly less bilateral parafloccular and dorsal cochlear nucleus activity, as well as significantly less contralateral ventral cochlear nucleus activity. It was concluded that NMDA-mediated glutamatergic transmission in the paraflocculus appears to be a necessary component of chronic noise-induced tinnitus in a rat model. Additionally, it was confirmed that in this model, elevated spontaneous activity in the cerebellar paraflocculus and auditory brainstem is associated with tinnitus

RatAtlas 2.0_Locked.pdf

A digital rat brain atlas, in PDF format, was derived from high-resolution <i>in situ</i> MRI images. The images depict transverse, sagittal, and horizontal views in contiguous 0.2 mm slices, with a planar resolution of 50 µm. Each slice was indexed to the skull surface as well as the skull landmarks Bregma and Lambda. Using a common interactive file format enables users to customize the images for their application, including image reproduction with or without background or labels. The objective was to provide a supplementary guide for stereotaxic neurosurgery that permits more accurate visualization of targets and determination of target coordinates in Long Evans rats

COAL AND CHAR STUDIES BY ADVANCED EMR TECHNIQUES

Advanced electronic magnetic resonance (EMR) as well as nuclear magnetic resonance (NMR) methods have been used to examine properties of coals, chars, and molecular species related to constituents of coal. During the span of this grant, progress was made on construction and applications to coals and chars of two high frequency EMR systems particularly appropriate for such studies--48 GHz and 95 GHz electron magnetic resonance spectrometer, on new low-frequency dynamic nuclear polarization (DNP) experiments to examine the interaction between water and the surfaces of suspended char particulates in slurries, and on a variety of proton nuclear magnetic resonance (NMR) techniques to measure characteristics of the water directly in contact with the surfaces and pore spaces of carbonaceous particulates

Representative MEMRI scans of an exposed (tinnitus) untreated rat (left column), an unexposed (no-tinnitus) untreated rat (center column), and an exposed treated rat (right column).

<p>Panel rows are indexed with respect to the auditory nerve (dark arrows in row 0). The row labels indicate distance in mm rostral (positive values) or caudal (negative values) to the auditory nerve. The dorsal cochlear nucleus (dark arrows) appears in row −0.6 and the paraflocculus (white arrows) in row −0.9. The scale bar (top right panel) is 2 mm, R and L indicate the right and left hemispheres, respectively.</p

Typical AOI boundaries used in MEMRI quantification.

<p>A. Paraflocculus (PFL) blue, dorsal cochlear nucleus (DCN) green, posterior ventral cochlear nucleus, red. The 2 mm scale bar applies to all panels. B. Anterior cochlear nucleus (AVCN). C. Inferior colliculus (IC). D. Medial geniculate body (MGB).</p

Psychophysical performance of Exposed (square data points) and Unexposed (round data points) groups.

<p>Prior to D-AP5 treatment, exposed animals were divided into two equal subgroups, one of which was randomly assigned to receive D-AP5 (open, square data points). A downward shift with respect to Unexposed controls was indicative of tinnitus. <b>A</b>. Pre-D-AP5 treatment there was no difference between the two exposed groups (error bars in all panels indicate the standard error of the mean) and both were significantly different than the Unexposed group (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 1). <b>B</b>. After 72 hrs of D-AP5 infusion the treated Exposed group significantly diverged from the untreated Exposed group (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 2), indicating decreased tinnitus. <b>C</b>. The D-AP5 treatment effect persisted for 23 days after discontinuation of drug infusion (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 3).</p

Post D-AP5 spontaneous neural activity indicated by manganese enhanced magnetic resonance imaging (MEMRI).

<p>Depicted are normalized data (AOI / adjacent muscle area). Elevation was evident in the ipsilateral paraflocculus (PFL) and dorsal cochlear nucleus (DCN) of Exposed untreated, but not in Exposed D-AP5 treated animals, in comparison to Unexposed controls (top panel). Contralateral elevation (bottom panel) was evident in the PFL, DCN, and the anterior ventral cochlear nuclei (AVCN). Posterior ventral cochlear nucleus (PVCN), inferior colliculus (IC), medial geniculate body (MGB). Error bars indicate mean deviation (*** p<0.0001; * p<0.05); statistical summary in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t002" target="_blank">Table 2</a>.</p