The Endocannabinoid/Cannabinoid Receptor 2 System Protects Against Cisplatin-Induced Hearing Loss

Abstract

Previous studies have demonstrated the presence of cannabinoid 2 receptor (CB2R) in the rat cochlea which was induced by cisplatin. In an organ of Corti-derived cell culture model, it was also shown that an agonist of the CB2R protected these cells against cisplatin-induced apoptosis. In the current study, we determined the distribution of CB2R in the mouse and rat cochleae and examined whether these receptors provide protection against cisplatin-induced hearing loss. In a knock-in mouse model expressing the CB2R tagged with green fluorescent protein, we show distribution of CB2R in the organ of Corti, stria vascularis, spiral ligament and spiral ganglion cells. A similar distribution of CB2R was observed in the rat cochlea using a polyclonal antibody against CB2R. Trans-tympanic administration of (2-methyl-1-propyl-1H-indol-3-yl)-1-naphthalenylmethanone (JWH015), a selective agonist of the CB2R, protected against cisplatin-induced hearing loss which was reversed by blockade of this receptor with 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630), an antagonist of CB2R. JWH015 also reduced the loss of outer hair cells (OHCs) in the organ of Corti, loss of inner hair cell (IHC) ribbon synapses and loss of Na+/K+-ATPase immunoreactivity in the stria vascularis. Administration of AM630 alone produced significant hearing loss (measured by auditory brainstem responses) which was not associated with loss of OHCs, but led to reductions in the levels of IHC ribbon synapses and strial Na+/K+-ATPase immunoreactivity. Furthermore, knock-down of CB2R by trans-tympanic administration of siRNA sensitized the cochlea to cisplatin-induced hearing loss at the low and middle frequencies. Hearing loss induced by cisplatin and AM630 in the rat was associated with increased expression of genes for oxidative stress and inflammatory proteins in the rat cochlea. In vitro studies indicate that JWH015 did not alter cisplatin-induced killing of cancer cells suggesting this agent could be safely used during cisplatin chemotherapy. These data unmask a protective role of the cochlear endocannabinoid/CB2R system which appears tonically active under normal conditions to preserve normal hearing. However, an exogenous agonist is needed to boost the activity of endocannabinoid/CB2R system for protection against a more traumatic cochlear insult, as observed with cisplatin administration.

Cnr2 is important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

“…The underlying mode of action of Cnr2 in any of those cells/tissue remains to be determined. However, both groups demonstrated that Cnr2 expression was up-regulated after Cisplatin exposure [11,12]. This was strengthening an earlier claim for an anti-apoptotic role for Cnr2 in cisplatin-treated cultured cells of an auditory cells line [76].…”

“…Strong immunolabelling of Cnr2 was found in the stria vascularis (SV), and maybe more surprisingly in all inner HCs (IHCs), as well as in the afferent neurites and cell bodies of spiral neurons [11]. All these localizations were confirmed independently, and additional cochlear expression was found in the spiral ligament (SL), the outer HCs (OHCs) and a subset of support cells (SCs), namely the inner and outer pillar cells (IPCs and OPCs respectively) [12]. Furthermore, this group showed that Cnr2 was co-localizing with markers of fibroblasts in the SL, basal cells in the SV, and ribbon synapse (Ribeye B or CtBP2) in the IHCs [12].…”

“…First, it prevented apoptosis in OHCs. Second, it reduced loss of ribbon synapses in IHCs, and third, it maintained Na + / and K + -ATPase activity in SV and SL [12]. All three aspects are strong contributors to optimal hearing.…”

“…In sensory organs, Cnr2 expression was described in several cell types like photoreceptors of the adult retina in various mammals [9] as well as in fish [10]. More recently, Cnr2 expression was found in several cell types of the rodent inner ear like sensory hair cells (HCs) of the Organ of Corti [11,12]. However, little is known about the developmental expression pattern or the role of cnr2 in those epithelia.…”

Mechanisms of Cisplatin-Induced Ototoxicity and Prevention

“…This effect was found to be mediated in part by inhibition of STAT1, thereby preventing cell death in the cochlea. 43 An extract from green tea epigallocatechin-3gallate (EGCG) was shown to protect against cisplatin ototoxicity in the rat and tumor-bearing mouse without interference with the tumor killing efficacy of cisplatin. 15 EGCG protected against cisplatin-induced hair cell damage, ABR threshold shifts, and prevented a decrease in the strial Na/K-ATPase activity.…”