Atrial natriuretic peptide-like immunoreactive cells in the guinea pig inner ear

Atrial Natriuretic Peptide Promotes Neurite Outgrowth and Survival of Cochlear Spiral Ganglion Neurons in vitro Through NPR-A/cGMP/PKG Signaling

Sensorineural hearing loss (SNHL) is a dominant public health issue affecting millions of people around the globe, which is correlated with the irreversible deterioration of the hair cells and spiral ganglion neurons (SGNs) within the cochlea. Strategies using bioactive molecules that regulate neurite regeneration and neuronal survival to reestablish connections between auditory epithelium or implanted electrodes and SGN neurites would become attractive therapeutic candidates for SNHL. As an intracellular second messenger, cyclic guanosine-3’,5’-monophosphate (cGMP) can be synthesized through activation of particulate guanylate cyclase-coupled natriuretic peptide receptors (NPRs) by natriuretic peptides, which in turn modulates multiple aspects of neuronal functions including neuronal development and neuronal survival. As a cardiac-derived hormone, atrial natriuretic peptide (ANP), and its specific receptors (NPR-A and NPR-C) are broadly expressed in the nervous system where they might be involved in the maintenance of diverse neural functions. Despite former literatures and our reports indicating the existence of ANP and its receptors within the inner ear, particularly in the spiral ganglion, their potential regulatory mechanisms underlying functional properties of auditory neurons are still incompletely understood. Our recently published investigation revealed that ANP could promote the neurite outgrowth of SGNs by activating NPR-A/cGMP/PKG cascade in a dose-dependent manner. In the present research, the influence of ANP and its receptor-mediated downstream signaling pathways on neurite outgrowth, neurite attraction, and neuronal survival of SGNs in vitro was evaluated by employing cultures of organotypic explant and dissociated neuron from postnatal rats. Our data indicated that ANP could support and attract neurite outgrowth of SGNs and possess a high capacity to improve neuronal survival of SGNs against glutamate-induced excitotoxicity by triggering the NPR-A/cGMP/PKG pathway. The neuroregenerative and neuroprotective effects of ANP/NPRA/cGMP/PKG-dependent signaling on SGNs would represent an attractive therapeutic candidate for hearing impairment.

Atrial Natriuretic Peptide Improves Neurite Outgrowth from Spiral Ganglion Neurons In Vitro through a cGMP-Dependent Manner

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the spiral ganglion (SG), while their degeneration or loss would cause sensorineural hearing loss. As a cardiac-derived hormone, atrial natriuretic peptide (ANP) plays a critical role in cardiovascular homeostasis through binding to its functional receptors (NPR-A and NPR-C). ANP and its receptors are widely expressed in the mammalian nervous system where they could be implicated in the regulation of multiple neural functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors in the inner ear, their presence within the cochlear SG and their regulatory roles during auditory neurotransmission and development remain largely unknown. Based on our previous findings, we investigated the expression patterns of ANP and its receptors in the cochlear SG and dissociated SGNs and determined the influence of ANP on neurite outgrowth in vitro by using organotypic SG explants and dissociated SGN cultures from postnatal rats. We have demonstrated that ANP and its receptors are expressed in neurons within the cochlear SG of postnatal rat, while ANP may promote neurite outgrowth of SGNs via the NPR-A/cGMP/PKG pathway in a dose-dependent manner. These results indicate that ANP would play a role in normal neuritogenesis of SGN during cochlear development and represents a potential therapeutic candidate to enhance regeneration and regrowth of SGN neurites.

Guanylyl Cyclase A/cGMP Signaling Slows Hidden, Age- and Acoustic Trauma-Induced Hearing Loss

In the inner ear, cyclic guanosine monophosphate (cGMP) signaling has been described as facilitating otoprotection, which was previously observed through elevated cGMP levels achieved by phosphodiesterase 5 inhibition. However, to date, the upstream guanylyl cyclase (GC) subtype eliciting cGMP production is unknown. Here, we show that mice with a genetic disruption of the gene encoding the cGMP generator GC-A, the receptor for atrial and B-type natriuretic peptides, display a greater vulnerability of hair cells to hidden hearing loss and noise- and age-dependent hearing loss. This vulnerability was associated with GC-A expression in spiral ganglia and outer hair cells (OHCs) but not in inner hair cells (IHCs). GC-A knockout mice exhibited elevated hearing thresholds, most pronounced for the detection of high-frequency tones. Deficits in OHC input–output functions in high-frequency regions were already present in young GC-A-deficient mice, with no signs of an accelerated progression of age-related hearing loss or higher vulnerability to acoustic trauma. OHCs in these frequency regions in young GC-A knockout mice exhibited diminished levels of KCNQ4 expression, which is the dominant K⁺ channel in OHCs, and decreased activation of poly (ADP-ribose) polymerase-1, an enzyme involved in DNA repair. Further, GC-A knockout mice had IHC synapse impairments and reduced amplitudes of auditory brainstem responses that progressed with age and with acoustic trauma, in contrast to OHCs, when compared to GC-A wild-type littermates. We conclude that GC-A/cGMP-dependent signaling pathways have otoprotective functions and GC-A gene disruption differentially contributes to hair-cell damage in a healthy, aged, or injured system. Thus, augmentation of natriuretic peptide GC-A signaling likely has potential to overcome hidden and noise-induced hearing loss, as well as presbycusis.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

Expression patterns of atrial natriuretic peptide and its receptors within the cochlear spiral ganglion of the postnatal rat

The spiral ganglion, which is primarily composed of spiral ganglion neurons and satellite glial cells, transmits auditory information from sensory hair cells to the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac, neuronal and glial functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors (NPR-A and NPR-C) in the inner ear, their presence within the cochlear spiral ganglion and their regulatory roles during auditory neurotransmission and development is not known. Here we investigated the expression patterns and levels of ANP and its receptors within the cochlear spiral ganglion of the postnatal rat using immunofluorescence and immunoelectron microscopy techniques, reverse transcription-polymerase chain reaction and Western blot analysis. We have demonstrated that ANP and its receptors colocalize in both subtypes of spiral ganglion neurons and in perineuronal satellite glial cells. Furthermore, we have analyzed differential expression levels associated with both mRNA and protein of ANP and its receptors within the rat spiral ganglion during postnatal development. Collectively, our research provides direct evidence for the presence and synthesis of ANP and its receptors in both neuronal and non-neuronal cells within the cochlear spiral ganglion, suggesting possible roles for ANP in modulating neuronal and glial functions, as well as neuron-satellite glial cell communication, within the spiral ganglion during auditory neurotransmission and development.

Expression and localization of atrial natriuretic peptide and its receptors in rat spiral ganglion neurons

Spiral ganglion neurons (SGNs) are the primary auditory neurons in the inner ear, conveying auditory information between sensory hair cells and the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac and neuronal functions. While the localization of ANP and its receptors (NPR-A and NPR-C) in the inner ear has been widely studied, there is only limited information regarding their localization in cochlear SGNs and their regulatory roles during primary auditory neurotransmission. Here we have investigated the presence of ANP and its receptors in the cochlear spiral ganglion of the postnatal rat using immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. ANP and its receptors are expressed in the cochlear SGNs at both the mRNA and protein level, and co-localize in the cochlear SGNs as shown by immunofluorescence. Our research provides a direct evidence for the presence and synthesis of ANP as well as its receptors in the cochlear SGNs, suggesting a possible role for ANP in modulating the neuronal functions of SGNs via its receptors.

Is there a role for atrial peptides in the labyrinthine “disease”?

The important role of atrial natriuretic peptides (ANP) in regulating blood pressure and changing vascular permeability has been widely studied and assessed during the last ten years. Considering the characteristics of this mechanism, which is responsible for a hypotensive and hypovolemic effect, and the possible role of hypotension associated with a default of autoregulatory sympathetic reactions in inner ear unexplained disorders, it seems reasonable to hypothesize a possible involvement of ANP system in the genesis of such disorders. As a matter of fact, the presence of specific receptors for ANP in the inner ear has been widely reported in studies concerning both rat and human inner ear, although their precise role in the labyrinthine homeostasis has not been satisfactory explained until now. Some aspects concerning vascular and fluid regulation of the inner ear under different conditions still remain not totally clear, and consequently a detailed explanation to the possible mechanism causing inner ear disorders of functional origin is lacking; from this point of view, an investigation on the serum level of ANP in subjects with labyrinthine affections of uncertain origin could be of some utility in contributing to assess the role of this system in the inner ear fluid regulation and in the inner ear perfusion and to investigate on the possible influence of an abnormal ANP release in some kind of inner ear damage.

Modification of Atrial Natriuretic Peptide Receptor Expression in the Rat Inner Ear

HYPOTHESIS

The purpose of this animal study was to confirm the presence of all three atrial natriuretic peptide (ANP) receptor subtypes in the rat inner ear and compare the expression of each receptor after inner ear injection of ANP, phosphate-buffered saline, or a solution containing ANP incubated with anti-ANP antibody (to block upregulation).

BACKGROUND

Receptors for ANP and related compounds have been localized in the inner ear of animals and humans. A previous study at this institution demonstrated the ability to up-regulate the expression of the three ANP receptors (ANP-A, ANP-B, ANP-C) in response to round window injection of ANP in the rat inner ear.

METHODS

After surgical exposure, the round window of female Lewis rats was injected with various concentrations of ANP, ANP plus anti-ANP antibody, or control. Animals were killed 24 hours after injection, inner ear tissues were harvested and homogenized, and RNA was isolated for reverse-transcription polymerase chain reaction.

RESULTS

Electrophoresis showed the presence of all three receptor subtypes with exposure to phosphate-buffered saline. Expression was significantly higher 24 hours after injection with the two concentrations of ANP. This increase was partially blocked with increasing relative concentrations of anti-ANP antibody.

CONCLUSIONS

These findings confirm the presence and responsiveness of ANP receptors in the rat inner ear. The ability to block up-regulation with the antibody provides a potential new research tool for manipulating the function of this hormone system in experimental models and, ultimately, in understanding the mechanisms of fluid homeostasis in the inner ear.

Ante mortem ruptures of the endolymphatic membranes in children: real finding or artifact?

We undertook this study to determine whether or not ante mortem ruptures of the endolymphatic membranes occurred naturally in the infantile labyrinth, what were the most frequent locations and to assess the statistical correlation of an earlier finding of bulging of Reissner's membrane in pediatric temporal bones. We examined 128 temporal bones from 80 children between the ages of newborn to 12 years with an average age of 15.4 months. All temporal bones were prepared by the celloidin technique and studied by light microscopy. The endolymphatic membranes of the cochlear duct, saccule, utricle and semicircular ducts were examined for ruptures. A rupture of the saccule and utricle was considered to be an ante mortem event if its edges were rolled and bound with fine adhesions and could be identified in adjacent sections. Ante mortem ruptures were seen in 11.1% of the specimens and occurred in 2.4% in Reissner's membrane, in 6.3% in the saccular membrane and in 2.4% in the utricular membrane. In these specimens, the histopathological changes included atrophy of the stria vascularis and collapse of the organ of Corti, which provided further evidence of an ante mortem event. Infants with extralabyrinthine congenital anomalies or children who were treated with chemotherapy were more likely to develop ruptures of the endolymphatic membranes if they also received aminoglycosides.

Atrial natriuretic peptide receptor upregulation in the rat inner ear

The purpose of this study was to further examine whether fluid homeostasis in the endolymphatic system could be regulated by a locally effective paracrine system involving atrial natriuretic peptides (ANPs) and their receptors. We assessed the biologic activity of the 3 ANP receptors (ANP-A, ANP-B, ANP-C) in the rat inner ear by measuring receptor upregulation after inner ear administration of ANPs. After appropriate anesthesia, female Lewis rats were injected with ANP via the round window. The animals were sacrificed 24 hours later, and RNA was isolated for reverse transcription-polymerase chain reaction (RT-PCR). Electrophoresis of RT-PCR products showed the presence of all 3 ANP receptor genes in both injected and control animals. Gene expression was significantly higher 24 hours after injection. These findings demonstrate that ANP receptors in the inner ear can be upregulated after injection of ANPs.

The natriuretic peptides: universal volume controllers

Since the discovery of the natriuretic effect of atrial natriuretic peptide (ANP), a family of other natriuretic peptides similar to ANP were isolated, including atriopeptin, vessel dilator, long-acting natriuretic peptide, urodilatin, and brain natriuretic peptide (BNP) to name a few. ANP was noted to possess natriuretic and diuretic properties that controlled increases in intravascular volume. ANP was also found to be elevated in conditions of increased intraocular pressure and biliary obstruction. BNP was found to be elevated in conditions of increased intracranial pressure, pointing towards its role in controlling cerebrospinal fluid volume. While at the cellular level, ANP controlled individual cell size. This makes the natriuretic peptides not only controllers of intravascular volume, but also modulators of a myriad of cavity volumes down to the control of individual cell volume.

C-type natriuretic peptide-like immunoreactivity in the rat inner ear

C-type natriuretic peptide (CNP) is a member of the atrial natriuretic peptide family (ANP family). The family also includes ANP and brain natriuretic peptide (BNP). These peptides regulate the homeostasis of body fluid and blood pressure as a neuropeptide in the central nervous system as well as a cardiac hormone in the periphery. We have recently reported the expression of CNP mRNA in the inner ear. To assess the possible physiological role of CNP in the inner ear, we investigated the localization of CNP peptide in the rat inner ear by immunohistochemistry at the light and electron microscopic level. CNP-like immunoreactivity was widely distributed in the secretory and the neuronal portion of the inner ear, i.e. the spiral ligament, the dark cell region of the utriculus, the epithelium of the endolymphatic sac, the spiral ganglion cells and the vestibular ganglion cells. The results suggest that CNP may play a role in the homeostasis of the perilymph and endolymph and may also influence nerve activities in the inner ear.

Cyclic GMP and outer hair cell electromotility

The aim of this study is to examine the effect of phosphorylation pathways on the electrically evoked fast motile response of isolated outer hair cells (OHCs). Transcellular electrical stimulation was applied in the microchamber to guinea pig OHCs and motility was measured before and after drug application. Forskolin (adenylate cyclase activator), phorbol 12-myristate 13-acetate (PMA, protein kinase C activator) and dibutyryl 3',5'-cyclic guanosine monophosphate (cGMP agonist) were studied. As controls, L15 medium and dimethyl-sulfoxide (DMSO) were used. In each group, 12 cells were measured. Forskolin and PMA were dissolved in 0.1% DMSO to render them membrane permeable. DMSO by itself caused a statistically significant electromotility magnitude decrease. Forskolin and PMA could not reverse the motility decrease due to DMSO, the effects seen in their presence were the same as observed with DMSO alone. Thus, neither 3',5'-cyclic AMP-dependent protein kinase nor calcium/phospholipid-dependent protein kinase appear to have modulatory effects on electromotility. Dibutyryl cGMP (DBcGMP), in concentrations of 200 microM, elicited a significant electromotility magnitude increase. The DBcGMP effect could be inhibited by co-application of 200 microM DBcGMP and 100 microM 8-Rp-pCPT-cGMPS (8-4-chlorophenylthio-guanosine 3',5'-cyclic monophosphothioate, Rp isomer, a cGMP antagonist). Our results suggest that OHC electromotility is modulated by a cGMP-dependent pathway.

Expression of membrane-bound and cytosolic guanylyl cyclases in the rat inner ear

Membrane-bound guanylyl cyclases (GCs) are peptide hormone receptors whereas the cytosolic isoforms are receptors for nitric oxide. In the inner ear, the membrane-bound GCs may be involved in the regulation of fluid homeostasis and the cytosolic forms possibly play a role in signal processing and regulation of local blood flow. In this comprehensive study, we examined, qualitatively and quantitatively, the transcription pattern of all known GC isoforms in the inner ear from rat by RT-PCR. The tissues used were endolymphatic sac, stria vascularis, organ of Corti, organ of Corti outer hair cells, cochlear nerve, Reissner's membrane, vestibular dark cells, and vestibular sensory cells. We show that multiple particulate (GC-A, GC-B, GC-D, GC-E, GC-F and GC-G) and several subunits of the heterodimeric cytosolic GCs (alpha1, alpha2, beta1 and beta2) are expressed, albeit at highly different levels. GC-C was not found. GC-A and the soluble subunits alpha1 and beta1 were transcribed ubiquitously. GC-B was present in all tissues except stria vascularis, which contained GC-A and traces of GC-E and GC-G. GC-B was by far the predominant membrane-bound isoform in the organ of Corti (86%), Reissner's membrane (75%) and the vestibulum (80%). Surprisingly, GC-E, a retinal isoform, was detected in significant amounts in the cochlear nerve (8%) and in the organ of Corti (4%). Although the cytosolic GC is a heterodimer composed of an alpha and a beta subunit, the mRNA transcription of these subunits was not stoichiometric. Particularly in the vestibulum, the transcription of the beta1 subunits was at least four-fold higher than of the alpha1 subunit. The data are compatible with earlier suggestions that membrane receptor GCs may be involved in the control of inner ear electrolyte and fluid composition whereas NO-stimulated GC isoforms mainly participate in the regulation of blood flow and supporting cell physiology.

RT-PCR analysis of mRNA expression of natriuretic peptide family and their receptors in rat inner ear

To assess the possible physiological role of the atrial natriuretic peptide (ANP) family, we investigated the expression of mRNA of ANP, brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and their receptors in rat inner ear using the reverse transcription-polymerase chain reaction method. ANP and CNP message bands were detected in the inner ear, but the BNP message band was not. Amplification products of the expected sizes of ANP-A, ANP-B and ANP-C receptors were detected in the inner ear. These results suggest that natriuretic peptide family may influence the function of the inner ear through the ANP-A, ANP-B, and ANP-C receptors.

Localization of soluble guanylate cyclase activity in the guinea pig cochlea suggests involvement in regulation of blood flow and supporting cell physiology

Although the nitric oxide/cGMP pathway has many important roles in biology, studies of this system in the mammalian cochlea have focused on the first enzyme in the pathway, nitric oxide synthase (NOS). However, characterization of the NO receptor, soluble guanylate cyclase (sGC), is crucial to determine the cells targeted by NO and to develop rational hypotheses of the function of this pathway in auditory processing. In this study we characterized guinea pig cochlear sGC by determining its enzymatic activity and cellular localization. In cytosolic fractions of auditory nerve, lateral wall tissues, and cochlear neuroepithelium, addition of NO donors resulted in three- to 15-fold increases in cGMP formation. NO-stimulated sGC activity was not detected in particulate fractions. We also localized cochlear sGC activity through immunocytochemical detection of NO-stimulated cGMP. sGC activity was detected in Hensen's and Deiters' cells of the organ of Corti, as well as in vascular pericytes surrounding small capillaries in the lateral wall tissues and sensory neuroepithelium. sGC activity was not observed in sensory cells. Using NADPH-diaphorase histochemistry, NOS was localized to pillar cells and nerve fibers underlying hair cells. These results indicate that the NO/cGMP pathway may influence diverse elements of the auditory system, including cochlear blood flow and supporting cell physiology.

Effect of glycerol on electrochemical composition of endolymph and perilymph in the rat

Glycerol (2 g/kg body weight), or 0.15 M NaCl for control animals, was administered to rats by i.v. injection. The dose was chosen in order to obtain an osmolarity increase in plasma of about 15 mosm/l 1 h after the glycerol administration, an increase which is similar to that observed in the human glycerol dehydration test. Endolymph and perilymph were sampled from the basal turn of the cochlea; cerebrospinal fluid (CSF) was sampled from cisterna magna. Plasma osmolarity, endocochlear potential, Na and K concentrations in endolymph, perilymph and CSF were determined 1 and 2 h after the glycerol injection. Compared with control animals, glycerol induced an increase in Na and K concentration in perilymph and endolymph, respectively, 1 and 2 h after the glycerol injection. No modification of the endocochlear potential was observed. These results are compatible with an increase in inner ear fluids osmolarity induced by glycerol.

Ultrastructural localization of G-protein GS in the lateral wall of the guinea pig cochlear duct

Immunocytochemical localization of a GTP-binding protein, Gs, in the various cells of the lateral wall of guinea pig cochlear duct was investigated using a post-embedding immunogold method with antibody raised against a synthetic decapeptide (RMHLRQYELL) encoding the C-terminus of the alpha-subunit of Gs. In the stria vascularis, labeling was observed on the basolateral membrane infoldings of marginal cells, on the juxtaposed membrane of intermediate cells, and on the cell membrane of basal cell. In contrast, no significant labeling was observed on the luminal membrane of marginal cells. Immunoreactivity also was detected on the cell membranes of various other cells. These include spiral prominence epithelial cells, fibrocytes of spiral ligament, external sulcus cells, and epithelial and mesothelial cells of Reissner's membrane. Adenylylcyclase has been functionally implicated in some of the cell types with membranes labeled in this study. The significance of these findings is briefly discussed.

Atrial natriuretic peptide participates in the regulation of vestibular blood flow

Atrial natriuretic peptide (ANP) is a cardiac hormone which exerts natriuretic, diuretic and vasorelaxant effects. Among the many organs and vascular beds populated with ANP receptors (Genest and Cantin, 1988) are the vestibular and auditory organs of the inner ear (Lamprecht and Meyer zum Gottesberge, 1988). The purpose of the current study was to assess the potential influence of ANP on vestibular blood flow in the guinea pig. The inner ear was exposed with a posterior-lateral approach medially through the mastoid cortex. The laser Doppler probe was placed adjacent to the ampulla of the posterior semicircular canal. Baseline measurements of mean blood pressure (BP), heart rate (HR) and vestibular blood flow were established. ANP dissolved in physiologic saline was infused intravenously at concentrations of 15, 150 or 300 ng/kg/min at 10 microliters/min for 30 min. Measurements were recorded during the infusion and for a recovery period of 65 min. The control group was treated equivalently and infused with 0.15 M NaCl. Baseline BP and HR for all animals were 40.1 +/- 6.67 and 190 +/- 15.7, respectively. BP, HR and vestibular blood flow remained stable during the baseline, control and recovery conditions of saline infused subjects. Infusion of ANP (15 ng/kg/min) induced a mild elevation of BP followed by a small decrease in pressure during the post-infusion period. Vestibular blood flow showed a decrease to approximately 20% below baseline during infusion and stabilized at this level during the recovery period. Infusion of higher concentrations of ANP (150 and 300 ng/kg/min) induced a similar pattern of BP change in a dose-dependent manner. Vestibular blood flow, however, evidenced significant elevations during the post-infusion periods for both concentrations. These increases (22% and 26%, for 150 and 300 ng/kg/min, respectively) were significantly different from vestibular blood flow changes in the saline and low dose groups. The HR remained stable for baseline, infusion and recovery periods for each of the ANP infused subjects. This investigation demonstrates the systemic and local effects of ANP suggest a possible role for ANP in local regulation of vestibular blood flow.

The volume protective natriuretic peptide system in the inner ear. Comparison between vestibular and cochlear compartments

It has been suggested that the family of natriuretic peptides (NP) has a protective role in volume overloading. Specific binding sites for atrial natriuretic peptide (ANP) and the kidney analog urodilatin (URO) were identified and quantified with computerized autoradiography and biochemical assay in the cryosection in the cochlear and vestibular (utricle/ampulla) tissue. Immunohistochemistry was used to identify and localize NP-like immunoreactive cells. Different levels of specific receptors between and within the inner ear compartments were detected. The presence of specific receptors for NP, as well as unequal distribution of NP-immunoreactivity between the compartments (in certain parts of the cochlea and the endolymphatic sac), may indicate a local autocrine and/or paracrine action of these peptide systems (presumably as a result of the integration of the different peptide effects), independent of their action via the more conventional systemic route, in addition to differences in response of the inner ear compartments to the load. The present results on specific binding of ANP and URO in the inner ear tissue may suggest physiological homology between the inner ear and the kidney. Moreover, a similar role of NP in these organs is suggested.

Binding sites of atrial natriuretic peptide (ANP) in the mammalian cochlea and stimulation of cyclic GMP synthesis

The distribution of binding sites for atrial natriuretic peptide (ANP) has been examined in frozen sections of the guinea pig inner ear by means of autoradiography. The highest density was found in the stria vascularis of all cochlear turns. In membrane preparations of stria vascularis in vitro, the production of the second messenger cGMP was strongly stimulated by synthetic ANP in a dose dependent manner. Adenylate cyclase was neither stimulated nor inhibited by ANP, thus suggesting, that the binding sites coincide with an ANP receptor, which is coupled to guanylate cyclase but not negatively coupled to an adenylate cyclase molecule. The production of cyclic GMP could not be reduced by GDP-beta S, a strong inhibitor of the Gs protein. We conclude the existence of an ANP receptor-guanylate cyclase signal transfer system, similar to the beta 2 receptor-adenylate cyclase system in the inner ear, without coupling to a G protein. ANP might play a role in sodium and water regulation of the endolymph and might antagonize the action of vasopressin.