Protection of mature oligodendrocytes by inhibitors of caspases and calpains

Calcium influx and calpain activation mediate preclinical retinal neurodegeneration in autoimmune optic neuritis

Optic neuritis is a common manifestation of multiple sclerosis, an inflammatory demyelinating disease of the CNS. Recently, the neurodegenerative component of multiple sclerosis has come under focus particularly because permanent disability in patients correlates well with neurodegeneration; and observations in both humans and multiple sclerosis animal models highlight neurodegeneration of retinal ganglion cells as an early event. After myelin oligodendrocyte glycoprotein immunization of Brown Norway rats, significant retinal ganglion cell loss precedes the onset of pathologically defined autoimmune optic neuritis. To study the role calcium and calpain activation may play in mediating early degeneration, manganese-enhanced magnetic resonance imaging was used to monitor preclinical calcium elevations in the retina and optic nerve of myelin oligodendrocyte glycoprotein-immunized Brown Norway rats. Calcium elevation correlated with an increase in calpain activation during the induction phase of optic neuritis, as revealed by increased calpain-specific cleavage of spectrin. The relevance of early calpain activation to neurodegeneration during disease induction was addressed by performing treatment studies with the calpain inhibitor calpeptin. Treatment not only reduced calpain activity but also protected retinal ganglion cells from preclinical degeneration. These data indicate that elevation of retinal calcium levels and calpain activation are early events in autoimmune optic neuritis, providing a potential therapeutic target for neuroprotection.

Proteolipid protein dimerization at cysteine 108: Implications for protein structure

Proteolipid protein (PLP) and its alternatively spliced isoform DM20 comprise ∼50% of central nervous system (CNS) myelin protein. The two proteins are identical in sequence except for the presence of a 35 amino sequence within the intracellular loop of PLP that is absent in DM20. In this work, we compared the expression of PLP/DM20 in transfected cells, oligodendrocytes and brain. In all 3 tissues, PLP exists as both a monomer and a disulfide-linked dimer; in contrast, DM20 is found mainly as a monomer. PLP dimers were increased by both chemical crosslinking and incubation with hydrogen peroxide, and were mediated by a cysteine at amino acid 108, located within the proximal intracellular loop of both PLP and DM20. The PLP-specific sequence thus influences the accessibility of this cysteine to chemical modification, perhaps as a result of altering protein structure. Consistent with these findings, several mutant PLPs known to cause Pelizaeus-Merzbacher disease form predominantly disulfide-linked, high molecular weight aggregates in transfected COS7 cells that are arrested in the ER and are associated with increased expression of CHOP, a part of the cellular response to unfolded proteins. In contrast, the same mutations in DM20 accumulate fewer high molecular weight disulfide-linked species that are expressed at the cell surface, and are not associated with increased CHOP. Taken together, these data suggest that mutant PLP multimerization, mediated in part by way of cysteine 108, may be part of the pathogenesis of Pelizaeus-Merzbacher disease.

Cannabidiol induces intracellular calcium elevation and cytotoxicity in oligodendrocytes

Heavy marijuana use has been linked to white matter histological alterations. However, the impact of cannabis constituents on oligodendroglial pathophysiology remains poorly understood. Here, we investigated the in vitro effects of cannabidiol, the main nonpsychoactive marijuana component, on oligodendrocytes. Exposure to cannabidiol induced an intracellular Ca(2+) rise in optic nerve oligodendrocytes that was not primarily mediated by entry from the extracellular space, nor by interactions with ryanodine or IP(3) receptors. Application of the mitochondrial protonophore carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP; 1 μM) completely prevented subsequent cannabidiol-induced Ca(2+) responses. Conversely, the increase in cytosolic Ca(2+) levels elicited by FCCP was reduced after previous exposure to cannabidiol, further suggesting that the mitochondria acts as the source of cannabidiol-evoked Ca(2+) rise in oligodendrocytes. n addition, brief exposure to cannabidiol (100 nM-10 μM) led to a concentration-dependent decrease of oligodendroglial viability that was not prevented by antagonists of CB(1), CB(2), vanilloid, A(2A) or PPARγ receptors, but was instead reduced in the absence of extracellular Ca(2+). The oligodendrotoxic effect of cannabidiol was partially blocked by inhibitors of caspase-3, -8 and -9, PARP-1 and calpains, suggesting the activation of caspase-dependent and -independent death pathways. Cannabidiol also elicited a concentration-dependent alteration of mitochondrial membrane potential, and an increase in reactive oxygen species (ROS) that was reduced in the absence of extracellular Ca(2+). Finally, cannabidiol-induced cytotoxicity was partially prevented by the ROS scavenger trolox. Together, these results suggest that cannabidiol causes intracellular Ca(2+) dysregulation which can lead to oligodendrocytes demise.

A transcription factor map as revealed by a genome-wide gene expression analysis of whole-blood mRNA transcriptome in multiple sclerosis

Background

Several lines of evidence suggest that transcription factors are involved in the pathogenesis of Multiple Sclerosis (MS) but complete mapping of the whole network has been elusive. One of the reasons is that there are several clinical subtypes of MS and transcription factors that may be involved in one subtype may not be in others. We investigate the possibility that this network could be mapped using microarray technologies and contemporary bioinformatics methods on a dataset derived from whole blood in 99 untreated MS patients (36 Relapse Remitting MS, 43 Primary Progressive MS, and 20 Secondary Progressive MS) and 45 age-matched healthy controls.

Methodology/Principal Findings

We have used two different analytical methodologies: a non-standard differential expression analysis and a differential co-expression analysis, which have converged on a significant number of regulatory motifs that are statistically overrepresented in genes that are either differentially expressed (or differentially co-expressed) in cases and controls (e.g., V$KROX_Q6, p-value <3.31E-6; V$CREBP1_Q2, p-value <9.93E-6, V$YY1_02, p-value <1.65E-5).

Conclusions/Significance

Our analysis uncovered a network of transcription factors that potentially dysregulate several genes in MS or one or more of its disease subtypes. The most significant transcription factor motifs were for the Early Growth Response EGR/KROX family, ATF2, YY1 (Yin and Yang 1), E2F-1/DP-1 and E2F-4/DP-2 heterodimers, SOX5, and CREB and ATF families. These transcription factors are involved in early T-lymphocyte specification and commitment as well as in oligodendrocyte dedifferentiation and development, both pathways that have significant biological plausibility in MS causation.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Leukemia inhibitory factor induces an antiapoptotic response in oligodendrocytes through Akt-phosphorylation and up-regulation of 14-3-3

Leukemia inhibitory factor (LIF) promotes the survival of oligodendrocytes (OLG) both in vitro and in an animal model of multiple sclerosis. Here, we show that LIF protects mature rat OLG cultures selectively against the combined insult of the proinflammatory cytokines interferon-gamma and tumor necrosis factor-alpha, but it does not protect against oxidative stress nor against staurosporine induced apoptosis. We further demonstrate that LIF activates the janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) and the phosphatidylinositol 3 kinase/Akt pathway in mature OLG. We show that LIF protection is independent of suppressors of cytokine signaling and Bcl-2 mRNA expression levels. To gain further insight into the protective mechanism, a quantitative proteomic approach (DIGE) was applied to identify differentially expressed proteins in LIF-treated OLG. Our results indicate that LIF induces a shift in the cellular machinery toward a prosurvival execution program, illustrated by an enhanced expression of isoforms of the antiapoptotic molecule 14-3-3. These data provide further insight into the mechanisms of LIF-mediated protection of mature OLGs.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.

Caspase-3 activity is reduced after spinal cord injury in mice lacking dynorphin: differential effects on glia and neurons

Dynorphins are endogenous opioid peptide products of the prodynorphin gene. An extensive literature suggests that dynorphins have deleterious effects on CNS injury outcome. We thus examined whether a deficiency of dynorphin would protect against tissue damage after spinal cord injury (SCI), and if individual cell types would be specifically affected. Wild-type and prodynorphin(-/-) mice received a moderate contusion injury at 10th thoracic vertebrae (T10). Caspase-3 activity at the injury site was significantly decreased in tissue homogenates from prodynorphin(-/-) mice after 4 h. We examined frozen sections at 4 h post-injury by immunostaining for active caspase-3. At 3-4 mm rostral or caudal to the injury, >90% of all neurons, astrocytes and oligodendrocytes expressed active caspase-3 in both wild-type and knockout mice. At 6-7 mm, there were fewer caspase-3(+) oligodendrocytes and astrocytes than at 3-4 mm. Importantly, caspase-3 activation was significantly lower in prodynorphin(-/-) oligodendrocytes and astrocytes, as compared with wild-type mice. In contrast, while caspase-3 expression in neurons also declined with further distance from the injury, there was no effect of genotype. Radioimmunoassay showed that dynorphin A(1-17) was regionally increased in wild-type injured versus sham-injured tissues, although levels of the prodynorphin processing product Arg(6)-Leu-enkephalin were unchanged. Our results indicate that dynorphin peptides affect the extent of post-injury caspase-3 activation, and that glia are especially sensitive to these effects. By promoting caspase-3 activation, dynorphin peptides likely increase the probability of glial apoptosis after SCI. While normally beneficial, our findings suggest that prodynorphin or its peptide products become maladaptive following SCI and contribute to secondary injury.

Cell suicide and caspases

Programmed cell death or apoptosis is a well regulated physiological form of cellular autodestruction. It plays an essential role in embryonic development, homeostasis, remodeling, surveillance, and host defense mechanisms. Conversely dysregulation of apoptosis, resulting in either too less or excessive cell death is implicated in pathogenesis of stroke, myocardial infarction, neurodegenerative diseases, cancer and autoimmmune disorders. Apoptosis is coordinated by a family of cysteine proteinases called caspases, which dismantle the cell by targeting panoply of proteins. The mammalian caspase family contains 14 members, a subset participates in cellular demise and the remaining are involved in the processing of pro-inflammatory cytokines. We have tried to develop a simplified picture of basic apoptotic mechanisms on the basis of recent insights into the area.

Cyclic GMP-dependent pathways protect differentiated oligodendrocytes from multiple types of injury

The cyclic GMP analog 8-bromo-cyclic GMP (8-Br-cGMP) protects differentiated murine oligodendrocytes (OLs) from caspase-mediated death initiated by staurosporine, thapsigargin or kainate. Caspase-independent death caused by high levels of NO is also partially prevented by 8-Br-cGMP. Inhibitors of protein kinase G (cGMP-dependent protein kinase, cGK) reversed protection, supporting involvement of cGK. Since NO stimulates soluble guanylate cyclase, increasing cGMP, we treated OLs with low levels of NO and observed partial protection against thapsigargin, staurosporine and kainate. Two inhibitors of mitochondrial pore transition (MPT), cyclosporin A and bongkrekic acid, were poorly protective, indicating that cGMP is not acting primarily by blocking MPT. 8Br-cGMP was more effective than 8Br-cAMP in protecting against staurosporine or release of intracellular Ca(++) by thapsigargin. The cAMP analog exhibited little or no protection against kainate or high levels of NO. Thus cGK signaling is more effective than protein kinase A or phosphodiesterase 3 signaling in preventing OL death.

Aspartoacylase is a regulated nuclear‐cytoplasmic enzyme

Mutations in the gene for aspartoacylase (ASPA), which catalyzes deacetylation of N-acetyl-L-aspartate in the central nervous system (CNS), result in Canavan Disease, a fatal dysmyelinating disease. Consistent with its role in supplying acetate for myelin lipid synthesis, ASPA is thought to be cytoplasmic. Here we describe the occurrence of ASPA within nuclei of rat brain and kidney, and in cultured rodent oligodendrocytes. Immunohistochemistry showed cytoplasmic and nuclear ASPA staining, the specificity of which was demonstrated by its absence from tissues of the Tremor rat, an ASPA-null mutant. Subcellular fractionation analysis revealed low enzyme activity against NAA in nuclear fractions from normal rats. Whereas two recent reports have indicated that ASPA exists as a dimer, size-exclusion chromatography of subcellular fractions showed ASPA is an active monomer in both subcellular fractions. Western blotting detected ASPA as a single 38 kD band. Because ASPA is small enough to passively diffuse into the nucleus, we constructed, expressed, and detected in COS-7 cells a green fluorescent protein-human ASPA (GFP-hASPA) fusion protein larger than the permissible size for the nuclear pore complex. GFP-hASPA was enzymatically active and showed mixed nuclear-cytoplasmic distribution. We conclude that ASPA is a regulated nuclear-cytoplasmic protein that may have distinct functional roles in the two cellular compartments.

Calpeptin provides functional neuroprotection to rat retinal ganglion cells following Ca2+ influx

Apoptosis of retinal ganglion cells (RGCs) impairs vision in glaucoma patients. RGCs are also degenerated in multiple sclerosis (MS), resulting in loss of visual perception in MS patients. We examined the involvement of calpain and caspase cascades in apoptosis of the rat retinal ganglion cell line RGC-5 following 24 h of exposure to 250 nM ionomycin (IMN) or 300 units/ml interferon-gamma (IFN-gamma) and then evaluated functional neuroprotection with 2 microM calpeptin (CP, a calpain-specific inhibitor). Morphological and biochemical features of apoptosis were detected in RGC-5 cells following exposure to IMN or IFN-gamma. Fura-2 assay determined significant increases in intracellular free [Ca2+] following exposure to IMN or IFN-gamma. Pretreatment with CP for 1 h prevented Ca2+ influx, proteolytic activities, and apoptosis in RGC-5 cells. Western blot analyses showed an increase in activities of calpain and caspase-12, upregulation of Bax:Bcl-2 ratio, release of cytochrome c from mitochondria, and increase in caspase-9 and caspase-3 activities during apoptosis. Increased caspase-3 activity was also confirmed by a colorimetric assay. Activation of caspase-8 and cleavage of Bid to tBid in RGC-5 cells following exposure to IFN-gamma indicated co-operation between extrinsic and intrinsic pathways of apoptosis. Patch-clamp recordings showed that pretreatment with CP attenuated apoptosis and maintained normal whole-cell membrane potential, indicating functional neuroprotection. Taken together, our results demonstrated that Ca2+ overload could be responsible for activation of calpain and caspase cascades leading to apoptotic death of RGC-5 cells and CP provided functional neuroprotection.

Protein co-expression with axonal injury in multiple sclerosis plaques

Damage to axons in acute multiple sclerosis (MS) lesions is now well established but the mechanisms of this damage remain obscure. Here we have applied a panel of antibodies that identify cell populations and proteins contained in them with a view to detecting those cells and proteins that are localised particularly closely to damaged axons in acute, sub-acute and border-active MS plaques. Results are expressed semi-quantitatively and graphs produced that show that many of the markers show enhanced expression at sites of axon damage. However, the sharpest increase in expression in relation to axon damage was seen for Calpain I (micro-calpain), inducible nitric oxide synthase and MMP-2, suggesting that these proteins may form part of a group of proteins responsible for the initiation of myelin and/or axon damage seen in MS lesions.

A sublethal dose of TNFalpha potentiates kainate-induced excitotoxicity in optic nerve oligodendrocytes

Glutamate receptor-induced cell death, known as excitotoxicity in both neurons and oligodendrocytes, has been implicated as a common pathway of cell death in numerous central nervous system (CNS) diseases and trauma. Research in both neuronal and oligodendrocyte excitotoxicity has examined glutamate's receptor-mediated effects on CNS cells, and explored strategies to protect cells exposed to the elevated glutamate levels that occur in CNS trauma and disease. Proinflammatory cytokines are also elevated in the injured CNS, and have also been implicated in CNS cell death. Recently, several laboratories have examined cytokines' effects on neuronal and glial excitotoxicity. Here, we review literature concerning the dynamic susceptibility of both neurons and oligodendrocytes to excitotoxicity, and present new data from our laboratory showing that the susceptibility of oligodendrocytes to excitotoxicity is acutely potentiated by the proinflammatory cytokine TNFalpha.

The tumor suppressor DAL-1/4.1B and protein methylation cooperate in inducing apoptosis in MCF-7 breast cancer cells

Background

DAL-1 (Differentially Expressed in Adenocarcinoma of the Lung)/4.1B is a member of the protein 4.1 superfamily that has been shown to suppress growth in lung, breast and brain tumor cells. In the case of the caspase-3 deficient MCF-7 breast cancer cells, this growth suppression has been shown to be partially mediated by the induction of apoptosis. However the exact mechanism of action of DAL-1/4.1B is unknown. Recently, protein arginine N-methyltransferase 3 (PRMT3) was identified as a DAL-1/4.1B interacting protein. Protein arginine methyltransferases (PRMTs) posttranslationally methylate the arginine residues of proteins, a modification which has been implicated in the regulation of multiple cellular processes including nuclear-cytoplasmic transport, signal transduction, and transcription.

Results

To investigate the role of protein methylation in cell death induced by DAL-1/4.1B, DAL-1/4.1B-inducible MCF-7 cells were examined for apoptosis and caspase activation in the absence and presence of the protein methylation inhibitor adenosine dialdehyde (AdOX). Flow cytometry analysis revealed that apoptosis was primarily associated with the activation of caspase 8, and inhibition of this activation blocked the ability of DAL-1/4.1B to induce cell death.

Conclusion

These results suggest that protein methylation cooperates with DAL-1/4.1B-associated caspase 8-specific activation to induce apoptosis in breast cancer cells.

Inhibitors of calpain activation (PD150606 and E-64) and renal ischemia-reperfusion injury

Calpain activation has been implicated in the development of ischemia-reperfusion (I-R) injury. Here we investigate the effects of two inhibitors of calpain activity, PD150606 and E-64, on the renal dysfunction and injury caused by I-R of rat kidneys in vivo. Male Wistar rats were administered PD150606 or E-64 (3mg/kg i.p.) or vehicle (10%, v/v, DMSO) 30min prior to I-R. Rats were subjected to bilateral renal ischemia (45min) followed by reperfusion (6h). Serum and urinary biochemical indicators of renal dysfunction and injury were measured; serum creatinine (for glomerular dysfunction), fractional excretion of Na(+) (FE(Na), for tubular dysfunction) and urinary N-acetyl-beta-d-glucosaminidase (NAG, for tubular injury). Additionally, kidney tissues were used for histological analysis of renal injury, immunohistochemical analysis of intercellular adhesion molecule-1 (ICAM-1) expression and nitrotyrosine formation. Renal myeloperoxidase (MPO) activity (for polymorphonuclear leukocyte infiltration) and malondialdehyde (MDA) levels (for tissue lipid peroxidation) were determined. Both PD150606 and E-64 significantly reduced the increases in serum creatinine, FE(Na) and NAG caused by renal I-R, indicating attenuation of renal dysfunction and injury and reduced histological evidence of renal damage caused by I-R. Both PD150606 and E-64 markedly reduced the evidence of oxidative stress (ICAM-1 expression, MPO activity, MDA levels) and nitrosative stress (nitrotyrosine formation) in rat kidneys subjected to I-R. These findings provide the first evidence that calpain inhibitors can reduce the renal dysfunction and injury caused by I-R of the kidney and may be useful in enhancing the tolerance of the kidney against renal injury associated with aortovascular surgery or renal transplantation.

Expression of P0 glycoprotein in CNS glia: Effects of overexpression in N20.1 cells

To examine effects of expression of the PNS myelin P0 glycoprotein in glial cells of CNS lineage, we transfected murine N20.1 glial cells with a rat P0 cDNA. A stably transfected cell line expressing high levels of P0 message showed P0 immunostaining, along with changes in morphology. Polymerase chain reaction (PCR) identified the predicted rat P0 sequence in the transfected N20.1 cells and further revealed low levels of mouse P0 message in the nontransfected cells and in primary mouse astrocytes. This is the first evidence of endogenous expression of message for P0 glycoprotein in CNS glia. Quantitative RT-PCR confirmed the expression of rat P0 mRNA in the transfected N20.1 cells, at levels about 400 times greater than murine P0 in nontransfected cells. A 27-kD band was detected in the transfected cells by Western blot with P0 antibody, but not in mock-transfected or nontransfected N20.1 cells. Immunocytochemistry following permeabilization showed intracellular vesicular localization of P0 in the cytoplasm and perinuclear rings in transfected cells, with a similar pattern but much lower levels in nontransfected cells. Faint surface staining for P0 protein without permeabilization was seen only on the transfected cells. A few transfected cells with membrane sheets stained more intensely for surface P0. Quantitative RT-PCR was used to determine if P0 overexpression altered expression of other myelin-related genes compared with glial fibrillary acidic protein (GFAP); the ratios of myelin basic protein (MBP)/GFAP and proteolipid protein (PLP)/GFAP were increased 2- to 3-fold in the P0-transfected cells. We conclude that P0 overexpression alters N20.1 gene expression and cell morphology, and shifts the cells from astroglial to oligodendroglial phenotype.

Regulation of sphingomyelinases in cells of the oligodendrocyte lineage

Controversy exists regarding the nature of the "executioner" sphingomyelinase (SMase) in cells and its subcellular localization. A new fluorescence-based assay with the substrate 6-hexadecanoylamino-4-methylumbelliferyl-phosphorylcholine allowed rapid and reliable microassays of neutral (N) and acid (A) SMase activity in cell extracts from primary cultures of neonatal rat oligodendrocytes (OPC) and a human oligodendroglioma cell line (HOG). Total SMase activity was much higher in OPC than in HOG cells. Both staurosporine and tumor necrosis factor-alpha (TNF-alpha) induced apoptosis and activated NSMase in a multiphasic manner in both OPC and HOG cells. The increase in caspase 8 activity preceded the 1 hr peak of NSMase activation, which was followed by caspase 3 activation. In contrast, ASMase activity, which constituted >90% of the total SMase activity, was unresponsive to proapoptotic drugs. Neither reducing ASMase levels by 50% by pretreatment with desipramine nor inhibiting sphingolipid synthesis by 50% with fumonisin B1 had any effect on cell death. Isolation of sphingolipid-rich plasma membrane microdomains (rafts) from the cells by sucrose density gradient ultracentrifugation revealed an enrichment of sphingomyelin, ceramide, and caspase 8. Proapoptotic drugs such as staurosporine promoted the translocation of NSMase to the raft fraction. In contrast, ASMase, other lysosomal hydrolases, and caspase 3 remained absent from rafts even after staurosporine treatment. The staurosporine-induced concomitant increase of ceramide in the raft fraction and caspase 3 in the cytosol could be mimicked by the addition of exogenous bacterial SMase. We conclude that caspase 8 activates NSMase in rafts in oligodendrocytes and that the downstream apoptotic signal is via caspase 3.

Thapsigargin suppresses cochlear potentials and DPOAEs and is toxic to hair cells

Thapsigargin, a drug that inhibits sarco-endoplasmic reticulum Ca(2+) ATPases (SERCAs), was infused into the perilymph compartment of the guinea pig cochlea in increasing concentrations (0.1-10 microM) while sound evoked cochlear potentials were monitored. Thapsigargin significantly suppressed the compound action potential of the auditory nerve, cochlear microphonics, and increased N(1) latency at low (56 dB SPL) and high intensity (92 dB SPL) levels of sound, suppressed low intensity sound evoked summating potential (SP) and greatly increased the magnitude of the high intensity sound evoked SP. At 10 microM, the drug suppressed the cubic distortion product otoacoustic emissions (2f(1)-f(2)=8 kHz, f(2)=12 kHz) evoked by both high and low intensity primaries (45, 60, 70 dB SPL). Thapsigargin (10 microM; 30 min) increased the endocochlear potential slightly (5 mV). In chronic animals, thapsigargin (10 microM; 60 min) destroyed many outer hair cells and some inner hair cells, especially in the basal turns. These effects are consistent with the hypothesis that the inhibition of the SERCAs affects the function of the cochlear amplifier and outer hair cells to a greater degree than it affects other functions of the cochlea.