Effects of chronic salicylate on GABAergic activity in rat inferior colliculus

Abstract

It is well accepted that salicylate ototoxicity results in reversible tinnitus in humans. Salicylate-induced tinnitus may be an example of plasticity of the central auditory system and could potentially serve as a model to further understand mechanisms of tinnitus generation. This study examined levels of glutamic acid decarboxylase (GAD) and the binding characteristics of the GABA_A receptor in auditory brainstem structures of Long–Evans rats chronically treated with salicylate. Western blotting revealed a significant 63% (P<0.008) elevation of GAD levels in the inferior colliculus (IC) of salicylate-treated subjects. This occurred in subjects demonstrating behavioral evidence of tinnitus. Muscimol saturation analysis was indicative of a salicylate-related increase in receptor affinity. Linear regression of [³H]muscimol saturation analysis data revealed a significant (P<0.05) reduction in K_d values in whole IC (−48%), as well as in the central nucleus of IC (CIC, −58%) and combined external and dorsal cortex of IC (E/DCIC, −46%). The number of GABA_A binding sites (B_max) were also significantly (P<0.05) decreased. These changes were observed only in central auditory structures. This suggests that GAD expression and GABA_A receptor binding characteristics may be altered with chronic exposure to sodium salicylate and these changes may represent aberrant plasticity clinically experienced as tinnitus.

Introduction

Tinnitus may be defined as an auditory perception without an external auditory source. It is frequently, but not invariably, associated with a loss of peripheral auditory sensitivity. Tinnitus can occur with normal cochlear function as well as after complete peripheral deafferentation (Eggermont, 1990, Jastreboff, 1990). Furthermore, variable degrees of tinnitus are often associated with identical levels of hearing loss. The psychoacoustical properties of tinnitus (perceived loudness and pitch) are also highly variable between individuals with seemingly identical cochlear function (Meikle, 1995). Although tinnitus is a frequent and often intrusive consequence of cochlear dysfunction, it has been difficult to study because of its subjective nature and variable clinical presentation.

Plasticity of the adult mammalian nervous system has been demonstrated to occur in a number of sensory modalities and central neural structures in response to sensory deprivation (Abbott et al., 1999, Benson et al., 1997, Fuchs and Salazar, 1998, Liang et al., 1996, Potashner et al., 1997, Recanzone et al., 1993, Suneja et al., 1998, Sugita, 1996). Cortical tonotopic organization is altered both in aging and following selective deafferentation (Robertson and Irvine, 1989, Willott, 1984, Willott et al., 1993). Altered receptor characteristics, neurotransmitter expression, and synaptic structural change provide possible mechanisms for central auditory plastic changes aimed at improving auditory detection, frequency discrimination, binaural hearing and topographic representation. While plasticity may be an adaptive response to altered stimulus environments, plasticity may also result in abnormal sensory perception. Tinnitus may be one such pathological manifestation of auditory plasticity. The functional, physiological, and neurochemical changes that result in tinnitus, however, are not presently known. Brummett (1995) has proposed that tinnitus may result from a loss of GABA (γ-aminobutyric acid)-mediated inhibition within the inferior colliculus (IC) and consequently an increase in neural activity from the IC to the auditory cortex. Szczepaniak and Moller (1995) have shown that noise exposure alters auditory processing within the IC through loss of GABA-mediated inhibition, and suggest that this loss of inhibition may underlie the development of tinnitus.

GABA is the major inhibitory neurotransmitter of the central nervous system. GABAergic function is altered after sensory deprivation (Hensch et al., 1998, Melzer and Smith, 1998, Suneja et al., 1998, Vees et al., 1998), and in association with auditory-evoked and drug-induced seizures (Faingold et al., 1994, Houser and Esclapez, 1996, Roberts et al., 1985). The inhibitory neurotransmitters GABA and glycine are critical for high-fidelity coding of acoustic information in the auditory brainstem. Hypersensitivity of the central auditory pathway occurs after partial deafferentation and preliminary findings suggest that the GABA system in the IC may be impaired under such conditions (Abbott et al., 1999, Bledsoe et al., 1995, Suneja et al., 1998, Wang et al., 1996). Thus, loss of GABA-mediated inhibitory control over neural activity in auditory pathways may have pathological behavioral consequences (Bauer et al., 1999, Gerken et al., 1984).

The mechanism underlying development of salicylate-induced tinnitus is unknown. Although a number of studies have examined the effect of salicylate toxicity on cochlear function (Dieler et al., 1991, Shehata et al., 1991), auditory nerve activity nerve (Evans et al., 1981, Evans and Borerwe, 1982, Martin et al., 1993, Schreiner and Snyder, 1987, Stypulkowski, 1990), and central auditory neural activity (Jastreboff and Sasaki, 1986, Manabe et al., 1997, Wallhausser-Franke, 1997, Eggermont and Kenmochi, 1998, Ochi and Eggermont, 1996), there are no studies which specifically address the effect of chronic salicylate exposure on the GABA system.

The goal of the present study was to address the hypothesis that a change in GABA-mediated inhibition may result in tinnitus. The specific aims of the experiment were to: (1) examine the effect of salicylate on the expression of glutamic acid decarboxylase (GAD) and the binding characteristics of the GABA receptor within the IC; and (2) to determine if the GABAergic changes within the IC are associated with the development of tinnitus. Levels of GAD and GABA-receptor binding affinity were assessed in rats chronically exposed to salicylate. Behavioral evidence of salicylate-induced tinnitus was tested and detected in one group of subjects. Detailed methods and results of the behavioral experiments are published elsewhere (Bauer et al., 1999). Briefly, subjects were divided into a control group or a group that chronically received salicylate-treated drinking water. Subjects were trained to respond in an operant task (press a lever) with auditory test conditions. Subjects were trained to refrain from lever-pressing under conditions of silence, and to lever-press when an auditory perception occurred. The perception of tinnitus was detected by comparing discrimination performance of experimental and control groups under different stimulus conditions. A separate group of subjects that was not behaviorally tested was also assessed for salicylate-induced changes in GABA receptor binding activity.