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

Abstract

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 2 h 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 2 h 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.

Introduction

Subjective tinnitus affects 15-17% of the population and approximately 2% experience severe or disabling tinnitus (Axelsson and Ringdahl, 1989, Cooper, 1994). The phantom sound of tinnitus often begins with the onset of hearing loss induced by acoustic overstimulation (Atherley et al., 1968, Axelsson and Hamernik, 1987, Loeb and Smith, 1967) or ototoxic drugs (Day et al., 1989, Gerharz et al., 1995, Kopelman et al., 1988); however, not every hearing impaired subject develops tinnitus. Since the biological mechanisms that give rise to tinnitus are still poorly understood, acoustic overstimulation and ototoxic drugs are typically used to try to induce tinnitus in animal models in order to study its neural correlates. Because there are individual differences in susceptibility to noise or drug induced tinnitus, it is important to employ behavioral techniques that permit tinnitus to be assessed in individual animals treated with a particular tinnitus inducing agent in order to determine if tinnitus is present or absent at a particular time (Guitton et al., 2003, Heffner and Harrington, 2002, Kaltenbach et al., 2002, Lobarinas et al., 2004, Rachel et al., 2002).

Because high doses of salicylate induce tinnitus in humans (Brien, 1993, Day et al., 1989, McFadden et al., 1984, Myers and Bernstein, 1965) in a predictable, dose dependent manner, many animal studies have used sodium salicylate to investigate the behavioral manifestation of tinnitus (Guitton et al., 2003, Jastreboff et al., 1988, Lobarinas et al., in press). The minimum dose needed to induce tinnitus in rats is on the order of 150 mg/kg (Jastreboff and Brennan, 1994, Lobarinas et al., 2004). However, the minimum effective dose for inducing tinnitus varies from day-to-day, animal-to-animal and the number of days of treatment (Guitton et al., 2003, Lobarinas et al., in press). We are unaware of any behavioral reports of salicylate-induced tinnitus in other animal models; however, such information would be valuable given that salicylate is often used to investigate the neural mechanisms of tinnitus in other species (Basta and Ernst, 2004, Chen and Jastreboff, 1995, Evans and Borerwe, 1982, Manabe et al., 1998, Muller et al., 2003, Ochi and Eggermont, 1996).

Although much is known about the perceptual characteristics of tinnitus (Tyler, 1985); the neural mechanisms that give rise to salicylate-induced tinnitus are not fully understood. Some neurophysiological studies indicate that high doses of salicylate increase spontaneous activity in the auditory nerve (Evans et al., 1981). Others, however, have found the opposite, spontaneous rates are reduced in neurons tuned to low frequencies and are unchanged in fibers tuned to high frequencies after salicylate; maximum sound evoked activity was unchanged (Muller et al., 2003). Recordings from the inferior colliculus and auditory cortex (AC) after salicylate have produced variable results; in some cases, spontaneous rate increased, in others it decreased or showed no significant change (Basta and Ernst, 2004, Chen and Jastreboff, 1995, Eggermont and Kenmochi, 1998, Jastreboff and Sasaki, 1986, Ma et al., 2006, Ochi and Eggermont, 1996). The variability in the results could be due to species differences, type of anesthetic, salicylate dose, or the particular subdivision or cell type studied. The one feature that all of the neurophysiological studies do have in common is that measurements were obtained from anesthetized animals. Since anesthetics have a profound effect on spontaneous and sound evoked activity at multiple levels along the auditory pathway (Astl et al., 1996, Evans and Nelson, 1973, Tennigkeit et al., 1997, Verbny et al., 2005, Zurita et al., 1994), the changes in neural activity seen in anesthetized animals may not accurately reflect the neural correlates of tinnitus in conscious animals that can perceive their tinnitus. The purpose of the present study was to investigate the neural correlates of salicylate-induced tinnitus in conscious animals capable of perceiving the phantom sound of tinnitus. To accomplish this, we implanted a chronic, 16 channel microwire electrode array into the AC of the rat and used it to monitor the local field potentials and spike discharges of neurons in the AC of awake-rats before and after treatment with a high dose of sodium salicylate. We used two behavioral paradigms, schedule induced polydipsia avoidance conditioning (SIPAC) and a new paradigm, gap pre-pulse inhibition of acoustic startle (GPIAS), to document the presence of salicylate induced tinnitus, its pitch and the recovery from salicylate-induced tinnitus.