To date, 50 neuronal growth factors have been identified, of which BDNF and NGF are of particular importance. The specific roles of BDNF and NGF for some neurons are well known; for example, BDNF promotes the function and survival of cortical neurons, and NGF is known to activate the function of basal forebrain cholinergic neurons and prevents cell death. NGF plays an important role in Alzheimer disease (Tuszynski et al. 2015). The present data suggest that BDNF is the top1 key protein in SG for the three processes PoS, AcouStim, and Tin; NGF has a comparable significance for Tin. Together with neurotrophin-3 (NT3), and neurotrophin-4 (NT4), BDNF and NGF belong to the group of important neurotrophins. Neurotrophins are soluble factors that play an important role in the survival and differentiation of neurons, in axon and dendrite growth, in synaptogenesis, and in synaptic transmission. The function of neurons requires communication between the cell body, axon, dendrites, and synaptic nerve terminals, some of which are spatially distant and require intracellular transport processes to reach the appropriate site (Scott-Solomon and Kuruvilla 2018). Neurotrophins are formed as precursors, the mature form is obtained by proteolytic cleavage. Release can be regulated constitutively and by activation (Fig. 5).

BDNF is involved in all elementary regulatory processes, such as survival and differentiation of neurons, axonal growth, dendrite growth, migration, path finding, transcription, translation, transport, secretion, maturation, differentiation, growth, regeneration or migration, synaptic transmission, and plasticity (Green et al. 2012). An important feature for the multiple influences of BDNF on synaptic transmission and plasticity could be the structure of the BDNF gene, which has eight exons and four different promoters (Reichardt 2006). The expression of each exon can be regulated by its own promoter. Exon IV seems to have a special role; its expression is controlled by calcium-response elements (Singer et al. 2008). Recently, regulation at the microRNA level via BDNF-AS has become increasingly recognized as being important for Tin (Appendix 3; Yuksel et al. 2023). Also has the loss of activity-driven BDNF promotor activation following deafferentation discussed to contribute to central hyperexcitability during tinnitus, through the specific role of BDNF to stabilize inhibitory GABA-ergic circuits (Knipper et al. 2022; Tan et al. 2007).

NGF (Nerve growth factor, also known as NGFB; Beta-NGF) is a member of the NGF-beta family and binds the NTRK1 receptor with high affinity. An important biological function of NGF in the inner ear is to ensure the survival of SGNs, including their dendrites (Dai et al. 2004; Nicoletti et al. 2022).

Neurotrophins exert their function as regulatory proteins for survival and for synaptic transmission by binding and activation of NTRK and NGFR receptors. The TRK family of receptors consists of three members: NTRK1, NTRK2, and NTRK3 (TRKA, TRKB, TRKC). Activation of the receptors by neurotrophins promotes neuronal survival, neuronal differentiation, axon and neurite growth, and synaptic transmission and plasticity. Binding of BDNF alters the structure of the receptor (dimerization, mutual phosphorylation); the receptor thus binds a number of adaptor and signaling proteins and activates, among others, protein kinase B (AKT), mitogen-activated protein kinase (MAPK), and other transcriptional processes via the transcription factor CREB (Nicoletti et al. 2022). The affinity for binding of the different neurotrophins to the kinase receptors differs: BDNF binds and activates mainly NTRK2, NGF-NTRK1, and NT3-NTRK3. NT3 also binds NTRK2 and NTRK1 with weaker affinity (Fig. 6; Scott-Solomon and Kuruvilla 2018; Regua et al. 2019). The expression of neurotrophins and of the receptors influences to a considerable extent the biological effects.

Neurotrophins and their receptors (according to Green et al. 2012)

NTRK2 (Neurotrophic Receptor Tyrosine Kinase 2; also known as BDNF/NT3 Growth Factors Receptor, TRKB-Tropomyosin-Related Kinase B) is a cellular component of the postsynaptic membrane and acts together with BDNF in the modulation of synaptic transmission. The BDNF-NTRK2 axis is also involved in the formation of synaptic spines (Zagrebelsky et al. 2020). These are spiny extensions of synapses that allow more precise and spatial signal transmission. They are also part of excitatory synapses in the auditory system and play a major role in synaptic plasticity. The changes in dendritic morphology and their excitability may be associated with the development of tinnitus (Yang et al. 2012). NTRK1 is not a specific component of the synapse and of trans-synaptic signaling. Basically, NTRK1 and NTRK2 activate the same metabolic pathways: PI3K, MAPK, and PLCy (Fig. 6; Regua et al. 2019). Upon dimeric NGF ligand binding, it undergoes homodimerization, autophosphorylation, and activation (Regua et al. 2019). Extracellular ligand NGF binds NTRK1 and NGFR receptors and activates cellular signaling cascades to regulate neuronal proliferation, differentiation, and survival. SGN express NTRK1, NTRK2, and NTRK3 and are protected in vitro by BDNF and NT3 in their function and survival in the face of damaging influences. It is known that after deafferentation of SGN, they die as a result of the loss of hair cells; electrical stimulation promotes survival of SGN after hair cell loss (Hansen et al. 2001).

NGFR (nerve growth factor receptor; also known as P75NTR, p75NTR, TNFRSF16 protein tumor necrosis factor receptor superfamily member 16) is a low-affinity receptor for NGF, BDNF, NTF3, and NTF4 (Fig. 5). It is part of the presynaptic membrane and is involved in the modulation of synaptic transmission. In development, NGFR plays an important role in cell differentiation and cell survival and death. While all neurotrophins bind and activate the NGFR receptor only with low affinity, all pro-neurotrophins bind it with high affinity (Green et al. 2012). NGFR is expressed in glial cells as well as neurons; the special glia cell known as the spiral ganglion Schwann cell (SGSCs), which myelinate SGN, represent a source of neurotrophic factors. SGSCs express sortilin, a NGFR co-receptor for pro-neurotrophins (Blondeau et al. 2018; Chen et al. 2005). SGSC cell density appears to be subject to tight regulation of proliferation and cell death upon hair cell injury (Provenzano et al. 2011). In general, NGFR has atrophic effects in the CNS, promoting apoptosis, inhibiting neurite growth, and decreasing synaptic activity.

Candidate key proteins of the PoS process are BDNF-GDNF and OTOF-CACNA1D. BDNF and GDNF are extremely important as modulators of synaptic transmission in mature cells of the CNS (Ferrini et al. 2021). Together, BDNF and GDNF are very effective in their actions on survival after ototoxic drugs or noise (Altschuler et al. 1999). GDNF (glial cell line-derived neurotrophic factor) is an important survival and growth factor for SGN (Green et al. 2012). The GDNF receptor is a complex consisting of a member of the GDNF receptor family (GFRa, GDNF family receptor alpha) and the receptor protein tyrosine kinase RET (Fig. 4). RET (proto-oncogene tyrosine protein kinase receptor), a high-score interaction protein of GDNF, is a receptor for neurotrophic factors of the GDNF family. Binding of RET and a member of the GFR family activates fundamental signals for neuronal developmental processes, such as cell proliferation, neuronal navigation, cell migration, cell differentiation, and cell death (Hamada and Lasek 2020). Disruption of c-Ret phosphorylation during development can lead to congenital hearing loss, with degeneration of the SGN in mice (Ohgami et al. 2010).

OTOF (Otoferlin) is a protein that plays a key role in calcium-regulated synaptic transmission from cochlear synapses. Otoferlin simultaneously binds multiple copies of the calcium voltage-gated channel subunit alpha1 D (CACNA1D, also known as Cav1.3 neuronal L-type channel) of the calcium ion channel of neurons and arms of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex) protein complex. Both are required in spatial proximity to allow rapid transmission of the sound signal (Wei et al. 2020). Sound transmission at ribbon synapses between inner hair cells and type1 spiral ganglion neurons are strongly dependent on Ca2+-mediated neurotransmitter release mediated by otoferlin and other factors (Takago et al. 2018). CACNA1D is a subunit of calcium channels that regulates the influx of calcium into neural excitable cells and regulates processes, such as neurotransmitter release or gene expression. The alpha1 subunit largely determines the properties of the channel that is important for the translation of sound-induced depolarization into neurotransmitter release. Mutation of the CACNA1D is related to congenital deafness (Baig et al. 2011). Efficient neurotransmitter release requires localization of the calcium channel in the presynaptic terminal near the synaptic vesicle and the SNAP25 release complex (Simms and Zamponi 2014). SNAP25 (synaptosomal-associated protein 25), a high-score interaction protein of CACNA1D, is involved in the molecular regulation of neurotransmitter release, in particular in vesicle docking and membrane fusion (Roux et al. 2006). SNAP25 is a component of the SNARE complex that is closely associated with Otoferlin. Signal transduction in spiral ganglia is under the influence of BDNF and NT3 (Flores-Otero et al. 2007). In the PoS network, SNAP25 and BDNF appear to have a bridging function between the top1 and top2 key proteins (Bolat et al. 2022).

In summary, the proteins BDNF/GDNF/RET and OTOF/CACNA1D/SNAP25 appear to be important in the process of sound perception under control conditions. These proteins represent the biological GO processes of synaptic vesicle priming and synaptic vesicle exocytosis (DAVID). In the PoS process, the protein pair BDNF-GDNF is important for the survival of the cells, and the protein pair OTOF-CACNA1D is important for the fine regulation of synaptic transmission. The network (Fig. 4) suggests that the regulation of survival processes (top1 protein pair) and signaling processes (top2 protein pair) occurs through the interaction of SNAP25 and BDNF.

Candidate key proteins of the AcouStim process are BDNF-NTRK2 und TH-CALB1. In comparison to PoS, AcouStim is an activated status of the auditory system. Under these conditions BDNF and its high-affinity receptor NTRK2 become the important modulator of synaptic transmission. The top2 HSIP pair is TH (tyrosine hydroxylase) and CALB1 (Calbindin 1). TH catalyzes the conversion of l-tyrosine to l-dihydroxyphenylalanine (l-Dopa) and that is the rate-limiting step in the biosynthesis of the catecholamines dopamine, noradrenaline, and adrenaline. Dopamine (DA) receptor expression has been demonstrated in the spiral ganglion (Inoue et al. 2006); all five DA receptors were identified on the soma of SGN (Reijntjes and Pyott 2016). The expression of tyrosine hydroxylase was detected in type II afferent neurons of mice (Vyas et al. 2017; Wu et al. 2018). DA is an important neurotransmitter of the lateral efferent dopaminergic system that regulates the excitability of the afferent dendrites of the SGN at the base of the IHC. Wu et al. (2020) demonstrated that DA is also involved in efferent neurons of the lateral olivocochlear region that modulate auditory nerve activity. Inhibition or modulation of BDNF-NTRK2 activity occurs by efferent neurons of the lateral olivocochlear (LOC) system via regulation of TH expression. DA seems to regulate the neurotransmitter release from hair cells and thus the firing rate of SG neurons and to modulate excitatory and inhibitory synaptic transmission pre and postsynaptically (Jacob and Nienborg 2018). A role in protecting the inner ear from excitotoxicity has been widely attributed to dopamine (Lamas et al. 2017).

CALB1 (Calbindin1; also known as CALB; D-28 K) belongs to the group of calcium-binding proteins, such as calmodulin and troponin C. It is involved in the regulation of pre- and postsynaptic cytosolic calcium levels, and it acts as a buffer for calcium in the pre- and postsynaptic cytosol. Studies of CALB1 in the auditory system are not yet available. CALB1 protein regulates DA release and DA transport in the ventral striatum by regulating Ca++ influx (Brimblecombe et al. 2019). CALB1 protein is also thought to regulate the activation of glutamate receptors by Ca++ influx. Calb1-knockout experiments suggest that CALB1 modifies not only DA release, but also the temporo-spatial coupling of Ca++ influx and DA release. CALB1 is involved in the regulation of long-term synaptic potentiation. Raynard et al. (2022) showed that overexpression of CALB1 is protective against increasing levels of Ca++ in aging cells. CALB1 interacts closely with PVALB (Parvalbumin), which belongs to the group of Ca++-binding proteins that includes calbindin (CALB1) and calretinin (CALB2). Parvalbumin is a small cytosolic protein that binds Ca++. Its physiological function is to date insufficiently studied, but it is assumed to be a calcium buffer. Thus, it is able to regulate Ca++-dependent metabolic and electrical processes, particularly of gamma-aminobutyric acid (GABA) interneurons (Permyakov and Uversky 2022).

Calcium-binding proteins have been detected in various inhibitory interneurons. The physiological function of these proteins is to regulate Ca++ activity, Ca++ transport, and the activity of various enzymes. There is evidence that neurons with high concentrations of these Ca++ buffers are more resistant to degenerative processes (Idrizbegovic et al. 2006; Schwaller 2020; Permyakov and Uversky 2022). Parvalbumin-positive interneurons belong to the class of GABA-ergic inhibitory neurons of the CNS that have a high axonal conduction velocity. In the cortex, parvalbumin-positive neurons play a major role in plasticity (Rupert and Shea 2022). The expression of PVALB varies depending on the activity of the PVALB-producing neurons. Loss of parvalbumin-positive interneuron activity is involved in the development of tinnitus (Knipper et al. 2020).

In summary, the response of the SG cells to acoustic stimulation is the activation of the BDNF-NTRK2 axis. The activity of the BDNF-NTRK2 system is regulated by the DA-Calcium system. An exaggerated response with cell damage is apparently prevented by regulation via the calcium system. PVALB seems to be of central importance, as it has close interactions with all members of this network (Fig. 4).

The key proteins of tinnitus differ from those assigned to PoS and AcouStim not only in the proteins themselves but also by the observation that the HDPs BDNF and NGF show HSIs with NGFR and NTRK1 at approximately the same level of interaction. BDNF and NGF have similar high-degree values (Table 3). In addition, the key proteins interact with a variety of other proteins, of which we have selected here those with the highest CS (Fig. 4). Thus, the key proteins BDNF-NTRK1 and NGF-NGFR interact so closely that it is not possible to distinguish which protein pair has particular dominance. Probably there will be a competition of the receptors for the neurotrophins, thus the concentrations and activities of the receptors or neurotrophins will determine the dominant effect.

Both BDNF and NGF play important roles in neurogenesis, differentiation, survival, and synaptic plasticity (Huang and Reichardt 2001). BDNF and NGF can have similar effects on neurons, but at different locations in the cell, depending on the localization of the receptors. However, NGF and BDNF can also have different effects, e.g., NGF can specifically promote survival of cholinergic neurons (Nordvall et al. 2022). This is significant, in that cholinergic input to the hippocampus after noise exposure is important for tinnitus induction (Zhang et al. 2019).

The mechanism of interactions between the four proteins in SG are not well known in detail. It is quite possible that the interaction of BDNF and NTRK1 reflects a connection between BDNF and NGF (1–14), a peptide of the N terminus of NGF (Naletova et al. 2019). Manohar et al. (2016) demonstrated that noise exposure upregulates the mRNA expression of Ntrk1 in the cochlear nucleus as a result of inflammation, neuropathic pain, microglial activation, and the migration of nerve fibers. The interactions of BDNF and NTRK1 might be related to the metabolic effects of NTRK1 on redox status, autophagy, glucose homeostasis, energy, and cholesterol metabolism (Avcı 2021; Colardo et al. 2021; Boecking et al. 2022).

The receptors of BDNF, NTRK2, and NGFR are expressed in SG. The interaction of BDNF and NGFR likely occurs via the action of preBDNF and may lead to apoptosis or inhibition of neurite growth. In primary dissociated SGN, proBDNF has neither toxic nor protective effects. Simultaneous administration of BDNF and proBDNF seems rather to be beneficial for the positive effect of BDNF (Schulze et al. 2022). It is known that SGN degenerate (loss of peripheral and central SGN synapses) without hair cells dying, e.g., in various forms of age-related hearing loss (neural ARHL). Survival of SGN depends strongly on the presence of neurotrophic factors, such as BDNF, NT3, or the glial cell factor GDNF (Shityakov et al. 2021). Several in vitro and in vivo studies show that neurotrophins promote SGN survival and synapse formation (Schulze et al. 2022). It can be assumed that further genetic and environmental factors are responsible for tinnitus (Bao and Ohlemiller 2010).

During embryonic development, NGF and its receptors are significantly expressed in humans. After birth, expression decreases significantly and remains low in adulthood. NGF is involved in the protection of degenerating neurons and in the targeted growth of nerve fibers. Low blood NGF levels seem to be associated with sensorineural hearing loss and tinnitus (Salvinelli et al. 2015). In mouse models, NGF therapy was successful in preserving SGN structure and function (Shityakov et al. 2021). NGF is produced by various cells in the body; it reaches a blood level that may well exert physiological effects.

In the CNS, NGF is present only in the form as proNGF. NGF and the precursor form proNGF appear to be critical for neuronal cell survival and cognitive function. Decreasing NTRK1 expression reduces cell survival; proNGF loses neurotrophic function in the absence of NTRK1, and it promotes apoptosis. Maintaining the balance between NTRK1 and NGFR at axon terminals is critical for the pro-survival or pro-apoptosis effects of proNGF. ProNGF appears to act neurotrophically in the presence of NTRK1, but apoptotically in the absence of Ntrk1 (Kropf and Fahnestock 2021).

NGF is known to promote the survival of basal forebrain cholinergic nuclei neurons in vitro and in vivo. NGF is produced in the cortex and hippocampus and is retrogradely transported to cholinergic neurons of the basal forebrain. These neurons play a major role in cognition and attention and in Alzheimer’s disease (Shityakov et al. 2021). It acts through two cell surface receptors, NTRK1 and NGFR; this involves the formation of a trimeric complex and the activation of signaling pathways that promote cell survival. Binding of NGF to the receptors can occur at axon terminals. Transport into the cell body involves retrograde transport (Canu et al. 2017). The problem with NGF therapy is that NGF is a large and polar protein and cannot cross the blood–brain barrier when administered peripherally. NGF has severe negative effects on some neurons, e.g., loss of pain perception (Tuszynski et al. 2015). Because NGF is detectable in physiologically effective amounts in the blood and NTRK1 receptors are expressed in the spiral ganglion, therapeutic trials have been undertaken for sensorineural hearing loss and tinnitus (Zhou et al. 2009; Salvinelli et al. 2015). Disturbances in the secretion of these neurotrophins lead to irreversible degeneration of the SGN. In contrast to mature NGF, the interaction of the precursor form (proNGF) and sortilin promotes neuronal apoptosis (Tauris et al. 2011; Vaegter et al. 2011; Carlo 2013).

The four key proteins (BDNF, NTRK1, NGF, NGFR) show high-score interactions to NTF3, to several growth factors (FGF2, EGF, GDNF, NTF3, VEGFA), and to regulatory proteins (PTPN11; Fig. 4). Neurotrophin 3 (NTF3, NT3) belongs to the neurotrophin family, with NTRK3 as the high-affinity receptor (Fig. 5). NTF3 binds and activates NTRK1 and NTRK2 with weaker affinity (Green et al. 2012). In the developmental phase, NTF3 is a critical factor for the survival of auditory neurons and for the formation of ribbon synapses on inner hair cells (Wan et al. 2014). It is known for its regulation of the formation of IHC-SGN synapses in the neonatal period (Cassinotti et al. 2022). NTF3 also plays a role in the recovery of synaptic connections after noise trauma (Wan et al.