Edward Lobarinas

Salicylate induced tinnitus: behavioral measures and neural activity in auditory cortex of awake rats.

Neurophysiological studies of salicylate-induced tinnitus have generally been carried out under anesthesia, a condition that abolishes the perception of tinnitus and depresses neural activity. To overcome these limitations, measurement of salicylate induced tinnitus were obtained from rats using schedule induced polydipsia avoidance conditioning (SIPAC) and gap pre-pulse inhibition of acoustic startle (GPIAS). Both behavioral measures indicated that tinnitus was present after treatment with 150 and 250 mg/kg of salicylate; measurements with GPIAS indicated that the pitch of the tinnitus was near 16 kHz. Chronically implanted microwire electrode arrays were used to monitor the local field potentials and spontaneous discharge rate from multiunit clusters in the auditory cortex of awake rats before and after treatment with 150 mg/kg of salicylate. The amplitude of the local field potential elicited with 60 dB SPL tone bursts increased significantly 2h after salicylate treatment particularly at 16-20 kHz; frequencies associated with the tinnitus pitch. Field potential amplitudes had largely recovered 1-2 days post-salicylate when behavioral results showed that tinnitus was absent. The mean spontaneous spike recorded from the same multiunit cluster pre- and post-salicylate decreased from 22 spikes/s before treatment to 14 spikes/s 2h post-salicylate and recovered 1 day post-treatment. These preliminary physiology data suggest that salicylate induced tinnitus is associated with sound evoked hyperactivity in auditory cortex and spontaneous hypoactivity.

Salicylate increases the gain of the central auditory system

High doses of salicylate, the anti-inflammatory component of aspirin, induce transient tinnitus and hearing loss. Systemic injection of 250 mg/kg of salicylate, a dose that reliably induces tinnitus in rats, significantly reduced the sound evoked output of the rat cochlea. Paradoxically, salicylate significantly increased the amplitude of the sound-evoked field potential from the auditory cortex (AC) of conscious rats, but not the inferior colliculus (IC). When rats were anesthetized with isoflurane, which increases GABA-mediated inhibition, the salicylate-induced AC amplitude enhancement was abolished, whereas ketamine, which blocks N-methyl-d-aspartate receptors, further increased the salicylate-induced AC amplitude enhancement. Direct application of salicylate to the cochlea, however, reduced the response amplitude of the cochlea, IC and AC, suggesting the AC amplitude enhancement induced by systemic injection of salicylate does not originate from the cochlea. To identify a behavioral correlate of the salicylate-induced AC enhancement, the acoustic startle response was measured before and after salicylate treatment. Salicylate significantly increased the amplitude of the startle response. Collectively, these results suggest that high doses of salicylate increase the gain of the central auditory system, presumably by down-regulating GABA-mediated inhibition, leading to an exaggerated acoustic startle response. The enhanced startle response may be the behavioral correlate of hyperacusis that often accompanies tinnitus and hearing loss.

A novel behavioral paradigm for assessing tinnitus using schedule-induced polydipsia avoidance conditioning (SIP-AC).

A behavioral technique was developed that allowed the onset and recovery of tinnitus to be measured in individual rats treated with different doses of salicylate. Food-restricted rats were self-trained to lick for water during the time between scheduled delivery of food pellets, i.e., schedule-induced polydipsia (SIP). SIP-induced licking was placed under stimulus control by administering foot shock if licks occurred when sound (one of six stimuli, 40 dB SPL) was present; rats were allowed to lick during quiet. After the number of licks-in-quiet (correct response) exceeded 90% of total licks, rats were treated with saline and four different doses of salicylate (50, 100, 150 and 350 mg/kg, intraperitoneally (i.p.); 2 days). Performance was assessed before, during and after treatment. Licks-in-sound remained extremely low with saline and all four salicylate doses indicating that the sounds were audible under all treatment conditions. Licks-in-quiet remained high during the saline control and 50 mg/kg dose of salicylate, behavior consistent with the absence of tinnitus. However, licks-in-quiet showed a statistically significant decline with the 150 and 350 mg/kg dose, behavior consistent with the presence of tinnitus. Licks-in-quiet gradually recovered to baseline level 2-3 days following high-dose salicylate treatments, behavior consistent with the gradual disappearance of tinnitus. The salicylate dose needed to induce tinnitus and the length of recovery are consistent with previous reports, providing support for the method. The ability to obtain sequential estimates of tinnitus-like behavior in an animal after administering a tinnitus-inducing agent could aid in understanding the underlying neural mechanisms and assessing potential treatments.

Noise trauma impairs neurogenesis in the rat hippocampus

The hippocampus, a major site of neurogenesis in the adult brain, plays an important role in memory. Based on earlier observations where exposure to high-intensity noise not only caused hearing loss but also impaired memory function, it is conceivably that noise exposure may suppress hippocampal neurogenesis. To evaluate this possibility, nine rats were unilaterally exposed for 2 h to a high-intensity, narrow band of noise centered at 12 kHz at 126 dB SPL. The rats were also screened for noise-induced tinnitus, a potential stressor which may suppress neurogenesis. Five rats developed persistent tinnitus-like behavior while the other four rats showed no signs of tinnitus. Age-matched sham controls showed no signs of hearing loss or tinnitus. The inner ear and hippocampus were evaluated for sensory hair cell loss and neurogenesis 10 weeks post-exposure. All noise exposed rats showed severe loss of sensory hair cells in the noise-exposed ear, but essentially no damage in the unexposed ear. Frontal sections from the hippocampus were immunolabeled for doublecortin to identify neuronal precursor cells, or Ki67 to label proliferating cells. Noise-exposed rats showed a significant reduction of neuronal precursors and fewer dividing cells as compared to sham controls. However, we could not detect any difference between rats with behavioral evidence of tinnitus versus rats without tinnitus. These results show for the first time that high intensity noise exposure not only damages the cochlea but also causes a significant and persistent decrease in hippocampal neurogenesis that may contribute to functional deficits in memory.

Insensitivity of the audiogram to carboplatin induced inner hair cell loss in chinchillas.

Noise trauma, aging, and ototoxicity preferentially damage the outer hair cells of the inner ear, leading to increased hearing thresholds and poorer frequency resolution. Whereas outer hair cells make synaptic connections with less than 10% of afferent auditory nerve fibers (type-II), inner hair cells make connections with over 90% of afferents (type-I). Despite these extensive connections, little is known about how selective inner hair cell loss impacts hearing. In chinchillas, moderate to high doses of the anticancer compound carboplatin produce selective inner hair cell and type-I afferent loss with little to no effect on outer hair cells. To determine the effects of carboplatin-induced inner hair cell loss on the most widely used clinical measure of hearing, the audiogram, pure-tone thresholds were determined behaviorally before and after 75 mg/kg carboplatin. Following carboplatin treatment, small effects on audiometric thresholds were observed even with extensive inner hair cell losses that exceed 80%. These results suggest that conventional audiometry is insensitive to inner hair cell loss and that only small populations of inner hair cells appear to be necessary for detecting tonal stimuli in a quiet background.

Salicylate- and quinine-induced tinnitus and effects of memantine

Conclusion: Memantine, an antiglutamatergic drug, has been proposed as a treatment for tinnitus. Objectives: The purpose of this study was to determine if memantine would prevent salicylate-induced tinnitus. Local field potentials were also recorded from auditory cortex to determine what effect salicylate, memantine, and the combination of both drugs would have on evoked potential amplitudes. Materials and methods: Schedule induced polydipsia-avoidance conditioning was used to identify the doses of salicylate or quinine that reliably induced tinnitus in rats. Rats were trained to lick for water during quiet intervals and avoid licking during sound intervals. Results: Rats injected with saline or a low dose of sodium salicylate or quinine failed to develop tinnitus-like behaviors. However, high doses of salicylate (150–300 mg/kg/day) or quinine (100–150 mg/kg/day) greatly reduced licks-in-quiet, behavior consistent with the presence of tinnitus. Licks-in-quiet increased slightly when memantine (1.5 or 3 mg/kg/day) was co-administered with salicylate; however, the effect was not statistically significant or dose-dependent. These results indicate that memantine does not completely suppress salicylate-induced tinnitus. Cortical auditory evoked potential amplitude increased after salicylate treatment; co-administration of memantine failed to block this salicylate-induced increase.

Metabolic imaging of rat brain during pharmacologically-induced tinnitus

Although much is known about the perceptual characteristics of tinnitus, its neural origins remain poorly understood. We investigated the pattern of neural activation in central auditory structures using positron emission tomography (PET) imaging in a rat model of salicylate-induced tinnitus. Awake rats were injected with the metabolic tracer, fluorine-18 fluorodeoxyglucose (FDG), once in a quiet state (baseline) and once during salicylate-induced tinnitus. Tinnitus was verified using a behavioral technique. Brain imaging was performed using a high-resolution microPET scanner. Rats underwent magnetic resonance imaging (MRI) and reconstructed MRI and microPET images were fused to identify brain structures. FDG activity in brain regions of interest were quantified and compared. MicroPET imaging showed that FDG activity in the frontal pole was stable between baseline and tinnitus conditions, suggesting it was metabolically inert during tinnitus. Inferior colliculi (p=0.03) and temporal cortices (p=0.003) showed significantly increased FDG activity during tinnitus relative to baseline; activity in the colliculi and temporal cortices increased by 17%+/-21% and 29%+/-20%, respectively. FDG activity in the thalami also increased during tinnitus, but the increase did not reach statistical significance (p=0.07). Our results show increased metabolic activity consistent with neuronal activation in inferior colliculi and auditory cortices of rats during salicylate-induced tinnitus. These results are the first to show that microPET imaging can be used to identify central auditory structures involved in tinnitus and suggest that microPET imaging might be used to evaluate the therapeutic potential of drugs to treat tinnitus.

Comparison of salicylate- and quinine-induced tinnitus in rats: development, time course, and evaluation of audiologic correlates.

Background: Salicylate and quinine have been shown to reliably induce short-term tinnitus when administered at high doses. The present study compared salicylate and quinine-induced tinnitus in rats using the gap prepulse inhibition of acoustic startle (GPIAS). Methods: Twenty-four rats were divided into 2 groups; the first group (n = 12) was injected with salicylate (300 mg kg−1 d−1), whereas the second (n = 12) was treated with quinine orally at a dose of 200 mg kg−1 d−1. Animals were treated daily for 4 consecutive days. All rats were tested for tinnitus and hearing loss before and 2, 24, 48, 72, and 96 hours after the first drug administration. Tinnitus was assessed using GPIAS; hearing function was measured with distortion product otoacoustic emissions (DPOAEs) and auditory brainstem response. Results: Salicylate treatment induced transient tinnitus with a pitch near 16 kHz starting 2 hours posttreatment, persisting over the 4-day treatment period and disappearing 24 hours later. Animals in the quinine group showed GPIAS changes at a higher pitch (20 kHz); however, changes were more variable among animals, and the mean data were not statistically significant. Hearing function varied across treatments. In the salicylate group, high-level DPOAEs were slightly affected; most changes occurred 2 hours posttreatment. Low-level DPOAEs were affected at all frequencies with a progressive dose-dependent effect. In the quinine group, only high-level DPOAEs were affected, mainly at 16 kHz. Conclusion: The present study highlights the similarities and differences in the frequency and the time course of tinnitus and hypoacusis induced by salicylate and quinine. Transient tinnitus was reliably induced pharmacologically with salicylate, whereas hearing loss remained subclinical with only minor changes in DPOAEs.

Does tinnitus "fill in" the silent gaps?

In the basic sciences, many researchers now use gap pre-pulse inhibition of the acoustic startle reflex (GPIAS) to determine if an animal has tinnitus after exposure to an ototoxic drug or intense noise. Tinnitus is assumed to be present if the silent gap in an ongoing narrow band noise (NBN) fails to suppress the startle reflex response evoked by an intense noise burst. The lack of gap pre-pulse inhibition presumably occurs because tinnitus fills in the silent intervals in the background noise. To test the perceptual aspects of this hypothesis, we asked hearing impaired subjects with tinnitus if they could perceive 50 ms silent intervals presented in a NBN, which was located above, below or at the subjects tinnitus pitch. The same tests were performed on normal hearing subjects without tinnitus. All subjects, with and without tinnitus, could detect the 50 ms gaps. Thus, using the stimulus parameters similar to those employed in animal and human GPIAS studies, we found that the tinnitus percept does not fill in the silent interval in a perceptual gap detection task; however, these finding do not rule out the possibility that tinnitus interferes with pre-attentive filtering of sensory stimuli in the GPIAS sensorimotor gating paradigm.

Altered Neuronal Intrinsic Properties and Reduced Synaptic Transmission of the Rat's Medial Geniculate Body in Salicylate-Induced Tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.