The anti-inflammatory selective melanocortin receptor subtype 4 agonist, RO27-3225, fails to prevent acoustic trauma-induced tinnitus in rats

Pharmacologic treatments in preclinical tinnitus models with special focus on Ginkgo biloba leaf extract EGb 761®

Tinnitus is defined as the perception of sound in the absence of external acoustic stimuli. Frequent comorbidities or associated factors are depression, anxiety, concentration problems, insomnia, resignation, helplessness, headache, bruxism, or social isolation, just to name a few. Although many therapeutic approaches have already been tested with varying success, there still is no cure available for tinnitus. The search for an effective treatment has been hampered by the fact that the mechanisms of tinnitus development are still not fully understood, although several models are available and discussed in this review. Our review will give a brief overview about preclinical models, presenting the heterogeneity of tinnitus sub-types depending on the different inner ear and brain structures involved in tinnitus etiology and pathogenesis. Based on these models we introduce the different target structures and transmitter systems implicated in tinnitus development and provide an extensive overview on preclinical drug-based therapeutic approaches that have been explored in various animal models. As the special extract from Ginkgo biloba leaves EGb 761® has been the most widely tested drug in both non-clinical tinnitus models as well as in clinical trials, a special focus will be given to EGb 761®. The efficacy of terpene lactones, flavone glycosides and proanthocyanidines with their distinct contribution to the overall efficacy profile of the multi-constituent drug EGb 761® will be discussed.

Stimulation of Melanocortin Receptor-4 (MC4R) Prevents Mitochondrial Damage Induced by Binge Ethanol Protocol in Adolescent Rat Hippocampus

Binge drinking is a common pattern of adolescent alcohol consumption characterized by a high alcohol intake within a short period of time; which may seriously affect brain function, triggering in some cases an addictive behavior. Current evidence indicates that alcohol addictive conduct is related to the impairment of the Melanocortin System (MCS). This system participates in the regulation of food intake and promotes anti-inflammatory response in the brain. However, the cellular mechanisms involved in the protective effects induced by MCS against binge-alcohol intoxication are still unknown. Here, we studied the effects of MCS activation on mitochondrial and oxidative damage induced by a binge-like protocol in the hippocampus of adolescent rats. We used a pharmacological activator of MC4R (RO27-3225) and evaluated its effects against oxidative injury, mitochondrial failure, and bioenergetics impairment induced by binge ethanol protocol in the hippocampus of adolescent's rats. Our results indicate that MC4R agonist reduces hippocampal oxidative damage promoting antioxidant (Nrf-2) and mitochondrial biogenesis (PGC1-alpha) pathways in animals subjected to the binge-like protocol. Additionally, MC4R activation prevented mitochondrial potential loss and increased mitochondrial mass that were significantly reduced by binge ethanol protocol. Finally, RO27-3225 treatment increased ATP production and mitochondrial respiratory complex expression in adolescent rats exposed to ethanol. Altogether, these findings show that activation of the MCS pathway through MC4R prevents these negative effects of binge ethanol protocol, suggesting a possible role of the MCS in the reduction of the neurotoxic effects induced by alcohol intoxication in adolescents.

Effects of RO27-3225 on neurogenesis, PDGFRβ+ cells and neuroinflammation after cerebral infarction

Cerebral infarction causes severe social and economic burdens to patients due to its high morbidity and mortality rates, and the available treatments are limited. RO27-3225 is a highly selective melanocortin receptor 4 agonist that alleviates damage in many nervous system diseases, such as cerebral hemorrhage, traumatic brain injury and chronic neurodegenerative diseases. However, the effect of RO27-3225 on cerebral infarction remains unclear. In this study, we used a mouse model of transient middle cerebral artery occlusion (tMCAO) and administered RO27-3225 or saline to the mice through intraperitoneal injection. RO27-3225 increased the number of Nestin⁺/BrdU⁺ cells and doublecortin (DCX)⁺/BrdU⁺ cells in the subventricular zone (SVZ) and the number of DCX⁺/BrdU⁺ cells in the peri-infarct area on day 7 after tMCAO. Furthermore, RO27-3225 decreased the number of activated microglia (Iba1⁺ cells with a specific morphology) and the expression levels of Iba1, TNFα, IL6, and iNOS proteins and increased the number of PDGFRβ⁺ cells in the peri-infarct region on day 3 after tMCAO. Finally, RO27-3225-treated mice exhibited significantly decreased infarct volumes, brain water contents, and neurological deficits after cerebral infarction. Thus, RO27-3225 can improve the outcomes following cerebral infarction, partially by regulating neurogenesis in the SVZ, PDGFRβ⁺ cell survival and neuroinflammation in the peri-infarct zone. Our research reveals that RO27-3225 is a potential new treatment for cerebral infarction.

Pharmacological Evaluation of Drugs in Animal Models of Tinnitus

Despite the pressing need for effective drug treatments for tinnitus, currently, there is no single drug that is approved by the FDA for this purpose. Instead, a wide range of unproven over-the-counter tinnitus remedies are available on the market with little or no benefit for tinnitus but with potential harm and adverse effects. Animal models of tinnitus have played a critical role in exploring the pathophysiology of tinnitus, identifying therapeutic targets and evaluating novel and existing drugs for tinnitus treatment. This review summarises and compares the studies on pharmacological evaluation of tinnitus treatment in different animal models based on the pharmacological properties of the drug and provides insights into future directions for tinnitus drug discovery.

A Conditioned Behavioral Paradigm for Assessing Onset and Lasting Tinnitus in Rats

Numerous behavioral paradigms have been developed to assess tinnitus-like behavior in animals. Nevertheless, they are often limited by prolonged training requirements, as well as an inability to simultaneously assess onset and lasting tinnitus behavior, tinnitus pitch or duration, or tinnitus presence without grouping data from multiple animals or testing sessions. To enhance behavioral testing of tinnitus, we developed a conditioned licking suppression paradigm to determine the pitch(s) of both onset and lasting tinnitus-like behavior within individual animals. Rats learned to lick water during broadband or narrowband noises, and to suppress licking to avoid footshocks during silence. After noise exposure, rats significantly increased licking during silent trials, suggesting onset tinnitus-like behavior. Lasting tinnitus-behavior, however, was exhibited in about half of noise-exposed rats through 7 weeks post-exposure tested. Licking activity during narrowband sound trials remained unchanged following noise exposure, while ABR hearing thresholds fully recovered and were comparable between tinnitus⁽⁺⁾ and tinnitus^(-) rats. To assess another tinnitus inducer, rats were injected with sodium salicylate. They demonstrated high pitch tinnitus-like behavior, but later recovered by 5 days post-injection. Further control studies showed that 1): sham noise-exposed rats tested with footshock did not exhibit tinnitus-like behavior, and 2): noise-exposed or sham rats tested without footshocks showed no fundamental changes in behavior compared to those tested with shocks. Together, these results demonstrate that this paradigm can efficiently test the development of noise- and salicylate-induced tinnitus behavior. The ability to assess tinnitus individually, over time, and without averaging data enables us to realistically address tinnitus in a clinically relevant way. Thus, we believe that this optimized behavioral paradigm will facilitate investigations into the mechanisms of tinnitus and development of effective treatments.

Cell proliferation in the cochlear nucleus following acoustic trauma in rat

Our previous studies have suggested that surgical lesions of the rat cochlea induce cell proliferation in the cochlear nucleus (CN) that may be related to neurogenesis. The aim of the present study was to further investigate the nature of cell proliferation in the CN, following acoustic trauma that has previously been shown to induce tinnitus in rats. Rats were subjected either to a unilateral acoustic trauma (16-kHz pure tone, 115dB for 1h under anesthesia) or a sham procedure. Bromodeoxyuridine (BrdU) immunohistochemistry was used to measure cell proliferation and newborn cell survival; an antibody to interleukin-6 was used to investigate inflammatory responses; and double immunolabeling for BrdU and Ki-67, BrdU and CD-11b, and BrdU and doublecortin (DCX), was used to investigate the origin of the proliferating cells. There was a time-dependent increase in the number of BrdU(+ve) cells in the CN following acoustic trauma; however, the number of BrdU(+ve) cells that survived was comparable to that of control animals at 4 weeks post-trauma. Cell proliferation was unlikely to be due to proliferating inflammatory cells as a result of a trauma-induced inflammatory response as the IL-6 expression level was comparable between sham and exposed groups. Immunolabeling revealed the BrdU(+ve) cells to co-express Ki-67 and DCX, but not CD-11b. However, there was no difference in DCX expression between sham and exposed animals. The results suggest that DCX-expressing cells in the CN may proliferate in response to acoustic trauma; however, the proportion of cells proliferating and the survival rate of the newborn cells may not support functional neurogenesis in the CN.