Effects of forskolin and 1,9-dideoxy-forskolin on cochlear potentials

Evidence of a novel transduction pathway mediating detection of polyamines by the zebrafish olfactory system

To better understand the full extent of the odorant detection capabilities of fish, we investigated the olfactory sensitivity of zebrafish to a monoamine and several polyamines using electrophysiological and activity-dependent labeling techniques. Electro-olfactogram (EOG) recording methods established the relative stimulatory effectiveness of these odorants as: spermine >> spermidine approximately agmatine > glutamine > putrescine >or= cadaverine >or= histamine > artificial freshwater. The detection threshold for the potent polyamines was approximately 1 micromol l(-1). Cross-adaptation experiments suggested that multiple receptors are involved in polyamine detection. Three observations indicated that polyamine signaling may involve a transduction cascade distinct from those used by either amino acids or bile salts. Like bile salts and the adenylate cyclase activator forskolin, but unlike amino acid odorants, polyamines failed to stimulate activity-dependent labeling of olfactory sensory neurons with the cation channel permeant probe agmatine, suggesting a signaling pathway different from that used by amino acid stimuli. Also supporting distinct amino acid and polyamine signaling pathways is the finding that altering phospholipase C activity with the inhibitor U-73122 significantly reduced amino acid-evoked responses, but had little effect on polyamine- (or bile salt-) evoked responses. Altering cyclic nucleotide-mediated signaling by adenylate cyclase activation with forskolin, which significantly reduced responses to bile salts, failed to attenuate polyamine responses, suggesting that polyamines and bile salts do not share a common transduction cascade. Collectively, these findings suggest that polyamines are a new class of olfactory stimuli transduced by a receptor-mediated, second messenger signaling pathway that is distinct from those used by amino acids or bile salts.

Signs of endolymphatic hydrops after perilymphatic perfusion of the guinea pig cochlea with cholera toxin; a pharmacological model of acute endolymphatic hydrops

There are indications that endolymph homeostasis is controlled by intracellular cAMP levels in cells surrounding the scala media. Cholera toxin is a potent stimulator of adenylate cyclase, i.e. it increases cAMP levels. We hypothesized that perilymphatic perfusion of cholera toxin might increase endolymph volume by stimulating adenylate cyclase activity, providing us with a pharmacological model of acute endolymphatic hydrops (EH). Guinea pig cochleas were perfused with artificial perilymph (15 min), with or without cholera toxin (10 microg/ml). The endocochlear potential (EP) was measured during and after perfusion. The summating potential (SP), evoked by 2, 4 and 8 kHz tone bursts, was measured via an apically placed electrode 0, 1, 2, 3 and 4 h after perfusion. Thereafter, the cochleas were fixed to enable measurement of the length of Reissner's membrane, reflecting EH. After perfusion the EP increased significantly over time in the cholera toxin group as compared to the controls. Also, the SP increased gradually at all frequencies in the cholera toxin group. Comparison within animals showed that the increase in SP became significant after 2 h at 4 kHz, after 3 h at 2 kHz and after 4 h at 8 kHz. In the control group the SP did not change significantly. The compound action potential (CAP) amplitude decreased monotonically over time at all frequencies in both the cholera toxin group and the control group, but it decreased faster in the cholera toxin group. Also, the cochlear microphonics amplitude decreased over time at all frequencies in both groups, but the decrease was significant only in the cholera toxin group after 3 h at 2 and 4 kHz. Quantification of the length of Reissner's membrane showed a small but insignificant enlargement in the cholera toxin treated animals compared to controls. These results are in accord with our view that EH is accompanied by an increase in SP and a decrease in CAP. Our results partially confirm previous results of Feldman and Brusilow (Proc. Natl. Acad. Sci. USA (1973) 73, 1761-1764). New aspects in relation to that study are the significantly increased EP and SP. In the classical EH model, based on obstruction of the absorptive function of the endolymphatic sac, increased SPs are accompanied by decreased EPs. In this cholera toxin model of EH, it is unlikely that the endolymphatic sac is involved. Apparently, EH can be based on mechanisms located in the cochlea itself as opposed to mechanisms located in the endolymphatic sac.

Expression pattern of aquaporin water channels in the inner ear of the rat. The molecular basis for a water regulation system in the endolymphatic sac

Mammalian aquaporins constitute a family of so far 10 related water channel proteins which mediate osmotically driven water fluxes across the plasma membrane. Because regulation of the ionic composition and osmolality of inner ear fluids is of great functional significance, we investigated the expression patterns of aquaporins in five defined areas of the rat inner ear by RT-PCR. The tissues used were stria vascularis, endolymphatic sac, Reissner's membrane, vestibulum and organ of Corti. Aquaporin 1 transcripts were detected in all tissues and are probably constitutive. Aquaporin 5 was only expressed in the organ of Corti and in Reissner's membrane. We show that aquaporin 2, so far considered to be specific to the principal cells of the renal collecting duct, is expressed in the endolymphatic sac. Aquaporin 2 expression was not detected in any other inner ear region. The postnatal appearance of aquaporin 2 transcripts in the endolymphatic sac resembled that in the kidney, i.e. it increased postnatally until day 4. The full-length DNA for aquaporin 2 was cloned from cDNA of the endolymphatic sac. It had an irrelevant Ile54Thr mutation because it could be functionally expressed in Xenopus oocytes. Also exclusively in the endolymphatic sac of the inner ear, we detected transcripts for aquaporin isoforms 3 and 4 which are known to be expressed in the renal principal cells. In the kidney, aquaporin 2 regulation involves vasopressin-stimulated, cAMP-dependent phosphorylation of Ser256 of aquaporin 2 which is stored in cytosolic vesicles. These storage vesicles also contain a serpentine calcium/polycation-sensing receptor. Vesicle shuffling to the plasma membrane involves proteins such as vesicle-associated membrane protein VAMP2, syntaxin-4 and the small GTPase Rab3a. Using RT-PCR we were able to demonstrate the expression of all of these components. By analogy the data suggest that in the endolymphatic sac of the inner ear a system for cellular water permeability is in place which may share many similarities with that characterized in the principal cells of the renal collecting duct. These findings may have a number of interesting pharmacological implications which need to be addressed in future studies.

Expression pattern of adenylyl cyclase isoforms in the inner ear of the rat by RT-PCR and immunochemical localization of calcineurin in the organ of Corti

Most studies concerning adenylyl cyclases in the inner ear were carried out before the advent of molecular biology. In a PCR approach using cDNAs of six inner ear tissues (stria vascularis, endolymphatic sac, organ of Corti, vestibulum, cochlear and vestibular nerve) we found tissue specific expression of adenylyl cyclase isoforms. Adenylyl cyclases types 2 and 4 are predominant in the fluid controlling tissues, i.e. in the stria vascularis and endolymphatic sac. In the organ of Corti and vestibulum the Ca2+-modulated isoforms types 1, 6 and 9 were expressed. The regulation of adenylyl cyclase 9, which is the major isoform expressed in the organ of Corti, proceeds via the Ca2+-activated protein phosphatase 2B (calcineurin, PPP3). PCR with specific primers for calcineurin demonstrated its abundant expression in the organ of Corti. Using a monoclonal antibody we localized calcineurin immunochemically to the cochlear nerve, the nerve fibers and the inner hair cells. In the cochlear and vestibular nerves a characteristic neuronal expression pattern of adenylyl cyclase isoforms was observed, i.e. adenylyl cyclases types 2, 3 and 8. The functional consequences of the adenylyl cyclase expression pattern in the inner ear are discussed in conjunction with its unique sensory performance.

cAMP increases K+ secretion via activation of apical IsK/KvLQT1 channels in strial marginal cells

In the cochlea, K+ is secreted by electrodiffusion across the apical membrane of strial marginal cells via the IsK/KvLQT1 ('IsK') channel. This channel complex has been reported to be activated in other systems by adenosine 3',5'-cyclic monophosphate (cAMP). Since several reports had suggested that cAMP is a second messenger in the cochlea, the effect of the cAMP pathway on transepithelial K+ secretion by strial marginal cells of the gerbil was studied. Both the transepithelial current (Isc) and K+ flux (JK) across strial marginal cell epithelium were measured; Isc in a micro-Ussing chamber and JK as the gradient of K+ concentration near the apical membrane. The apical membrane current (IIsK) and conductance (gIsK) of IsK channels were recorded with the on-cell macro-patch and the nystatin-perforated whole-cell patch clamp techniques. It has previously been shown that the apical IsK channel constitutes the primary pathway for K+ secretion. Cytoplasmic cAMP was elevated by applying dibutyryl cyclic-AMP (dbcAMP) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) at 37 degrees C. dbcAMP (1 mM) increased Isc by 51 +/- 4% and IIsK in on-cell and whole-cell recordings increased by 214 +/- 63% and 390 +/- 61% above the control value, respectively. IBMX (1 mM) caused transient increases of Isc by 53 +/- 3% and IIsK in on-cell recordings by 177 +/- 75% above the control value. The leak conductance due to all non-IsK channel sources did not change in the presence of dbcAMP or IBMX. dbcAMP (1 mM at 24 degrees C) increased JK by 53 +/- 16% and Isc by 18 +/- 4%. IBMX (1 mM at 24 degrees C) had no effect, suggesting reduced activity of adenylate cyclase at this temperature. Our results demonstrate that the cAMP pathway is constitutively active in strial marginal cells and that the cAMP pathway stimulates transepithelial K+ secretion by increasing IsK channel current rather than by altering another transport pathway.

Role of endolymphatic anion transport in forskolin-induced Cl- activity increase of scala media

To determine the role of anion transport in the forskolin-induced Cl- increase of scala media (SM), effects of forskolin on the EP (endocochlear potential) and Cl- activity (ACl) in SM were examined with double-barrelled Cl(-)-selective microelectrodes. The experiments were carried out on guinea pig cochleae, using a few anion transport inhibitors: IAA-94 for a Cl- channel blocker, bumetanide (BU) for an Na+/K+/2Cl- cotransport blocker, and SITS and DIDS for Cl-/HCO3- exchange blockers. The application of forskolin (200 microM) into scala vestibuli (SV) caused a 20 mEq increase of endolymphatic ACl and a 15 mV elevation of EP, and IAA-94 with forskolin completely abolished these responses. Although each application of BU, SITS or DIDS did not completely suppress EP elevation, the concurrent application of these inhibitors completely suppressed EP with endolymphatic ACl increase. The results indicate the involvement of Cl- channels, Na+/K+/2Cl- cotransport and Cl-/HCO3- exchange in forskolin-induced increase of ACl and EP. The role of adenylate cyclase activation and Cl- transport in endolymph homeostasis was discussed.

Identification of A1 adenosine receptors in rat cochlea coupled to inhibition of adenylyl cyclase

A1 adenosine receptors (A1ARs) are found in a number of tissues in the body where their physiological roles have been identified. In the cochlea, neither the existence of these receptors nor a physiological role of adenosine has been described previously. Membranes prepared from rat cochlea demonstrated high affinity and saturable binding to N6-2-(4-amino-3-[125I]iodophenyl)ethyladenosine ([125I]APNEA), an A1AR agonist, with maximum binding capacity and dissociation constant values being 40.5 +/- 0.5 fmol/mg protein and 1.28 +/- 0.03 nM, respectively. Adenosine analogues competed for [125I]APNEA binding sites with a rank order of potency characteristic of these sites being the A1AR. The [125I]APNEA binding was significantly reduced by pertussis toxin, indicating coupling of these receptors with the Gi and/or Go proteins in cochlear membranes. Photoaffinity labeling of the receptor protein with the A1AR agonist N6-2-(4-azido-3[125I]iodophenyl)ethyladenosine showed specific labeling of a 36-kDa receptor protein. Activation of the A1AR with R-phenylisopropyladenosine (R-PIA) led to inhibition of forskolin-stimulated adenylyl cyclase activity. Amplification of reverse-transcribed RNA derived from cochlear tissue by polymerase chain reaction (using primers for the bovine A1AR) yielded a 770-bp product that hybridized to an A1AR cDNA probe on Southern blots. These data indicate the presence of an inhibitory receptor in the peripheral auditory system, which may play an important role in modulating auditory functions.

Failure of forskolin to elevate the endocochlear potential in kanamycin-poisoned animals

The effect of forskolin (FSK) on the endocochlear potential (EP), K+ activity (AK), Na+ activity (ANa) and Cl- activity (ACl) in scala media (SM) was compared between normal and kanamycin (KM)-poisoned guinea pigs by means of double-barrelled ion-selective microelectrodes. The perfusion of the scala vestibuli (SV) with FSK (200 microM) produced EP elevation in normal animals whereas FSK failed to do it in KM-poisoned animals. FSK increased ACl of SM with no significant change in AK and ANa of SM in both groups of animals. Histological examination of KM-poisoned animals showed damaged outer and inner hair cells with an intact appearance of the stria vascularis. The mechanism underlying the failure of FSK to elevate the EP in KM-poisoned animals is discussed.

The effect of nimodipine on cochlear potentials and Na+/K(+)-ATPase activity in normal and hydropic cochleas of the albino guinea pig

In experimental endolymphatic hydrops (EEH) a decrease in the endocochlear potential (EP) has been reported and is thought to be due to decreased activity of the enzyme Na+/K(+)-ATPase in the stria vascularis. By stimulating Na+/K(+)-ATPase, the EP, and thereby cochlear function as a whole, might be restored. On the other hand, stimulation of stria vascularis Na+/K(+)-ATPase might result in excessive production of endolymph and thus produce or augment hydrops. In this study we have investigated the effect of intraperitoneally applied nimodipine on cochlear potentials and on Na+/K(+)-ATPase activity in the stria vascularis, both in normal cochleas (control) and in cochleas with EEH. Nimodipine is an L-type Ca(2+)-channel blocking agent with Na+/K(+)-ATPase stimulating properties at concentrations as low as 1.5 nM. The compound action potential (CAP), evoked by 2,4 and 8 kHz tone bursts was found to be depressed in the EEH ears with and without nimodipine treatment, and in the nimodipine treated control ears. Statistical analysis (ANOVA) showed that the effects of EEH and nimodipine on the CAP were additive. The negative summating potential (SP), measured extracochlearly at the apex, in response to 4 and 8 kHz tone bursts was significantly enhanced in the EEH ears. Nimodipine treatment did not affect the SP, neither in the control, nor in the EEH ears. Cytochemically, Na+/K(+)-ATPase activity appeared to be decreased in the oedematous stria vascularis of hydropic cochleas. No effect of nimodipine on Na+/K(+)-ATPase activity could be established ultracytochemically, neither in the controls nor in the EEH ears. In the lower turns of some of the nimodipine treated control cochleas a mild hydrops was seen during light-microscopic evaluation. Although it was not possible to prove a stimulatory effect of nimodipine on the enzyme Na+/K(+)-ATPase cytochemically, the finding of mild endolymphatic hydrops in nimodipine treated control ears suggests (a history of) increased endolymph production. This hydrops might be responsible for the depression of the CAP in the nimodipine treated ears.

Antidiuretic hormone restores the endolymphatic longitudinal K+ gradient in the Brattleboro rat cochlea

In the cochlea, endolymph is hyperosmotic to plasma and perilymph. To test the hypothesis that antidiuretic hormone is involved in the modulation of endolymph secretion, the electrochemical composition of cochlear fluids, endolymph and perilymph, was studied in three groups of anaesthetized rats: control Long Evans rats, homozygous Brattleboro rats that are genetically deprived of antidiuretic hormone, and Brattleboro rats that were treated with antidiuretic hormone (dDAVP, 0.5 microgram/100 g body weight/24 h during 8 days). Endolymph was sampled from the scala media at each turn of the cochlea and perilymph from the scala vestibuli. In Long Evans rats, the endocochlear potential, the endolymphatic K+ and Cl- concentrations decreased from base to apex of the cochlea as previously reported in guinea pigs and Sprague Dawley rats. In Brattleboro rats, the endocochlear potential and the Cl- concentration gradients were still present, whereas the K+ concentration gradient were still present, whereas the K+ concentration gradient was absent. This K+ gradient was restored by the administration of dDAVP, which increased the K+ concentration at the base of the cochlea. This work indicates that the K+ secretion in endolymph, and thus the osmolality, may be locally modulated by the antidiuretic hormone, probably via V2 receptors.

Involvement of Cl- transport in forskolin-induced elevation of endocochlear potential

To determine the possible involvement of Cl- transport in the forskolin-induced endocochlear potential (EP) elevation, the effect of forskolin on the EP was examined in Cl(-)-free artificial perilymph (aPL) as well as in the presence of Cl- channel blockers. The perfusion of scala vestibuli (SV) with forskolin (200 microM) dissolved in Cl(-)-free aPL failed to produce an EP elevation, while SV perfusion of forskolin dissolved in normal aPL elevated the EP. The application of DPC and IAA-94 (blockers of cAMP-activated Cl- channel) into SV completely suppressed the forskolin-induced EP elevation, while niflumic acid (a Ca(2+)-activated Cl- channel blocker) failed to do so. IAA-94 applied into scala tympani (ST) did not suppress this EP elevation. The results suggest that adenylate cyclase may modulate the EP by changing Cl- transport between SV and scala media (SM) across Reissner's membrane.

Nonselective cation and Cl channels in luminal membrane of the marginal cell

Single-channel currents of the luminal membrane of marginal cells dissected from the guinea pig cochlea were investigated using the patch-clamp technique. Nonselective cation channels having a linear conductance of 27 pS were activated by depolarization, cytoplasmic Ca2+, and cytoplasmic acidification. Cytoplasmic ATP inactivated the channel. A mixture of 3-isobutyl-1-methylxanthine and forskolin activated a small-conductance Cl channel in the cell-attached mode. On excision in the inside-out mode, the Cl channel was inactivated, but it was reactivated by a cytoplasmic catalytic subunit of protein kinase A with ATP. This Cl channel had a linear conductance of 12 pS, and its activity was little affected by voltage. The sequence of permeation by anions was Br- > Cl > I-. The Cl channel blocker diphenylamine-2-carboxylic acid (3 mM) completely blocked the channel, but 5-nitro-2-(3-phenylpropylamino)-benzoic acid (50 microM) blocked it only partially. The above-mentioned characteristics are similar to those of the well-demonstrated Cl- channel, cystic fibrosis transmembrane regulator.

Effects of adenylate cyclase activation on electrical resistance of scala media

Electrical resistance of scala media at the second turn of the guinea pig cochlea was measured while cochlear adenylate cyclase was activated by perilymphatic perfusion with 2 x 10(-4) M forskolin. The electrical resistance decreased from the pre-forskolin value (6.36 +/- 0.27 k omega to 5.22 +/- 0.33 k omega, n = 5), while the endocochlear potential (EP) increased from the pre-forskolin value (70.2 +/- 2.1 mV to 81.2 +/- 2.8 mV, n = 5). The time course of the electrical resistance change was more rapid than that of the EP change but almost identical to the time course of the change in the Cl- activity of the endolymph. The results suggest that the forskolin-induced decrease in the electrical resistance of the perilymph-endolymph barrier (presumably Reissner's membrane), and the consequent increase in Cl- flux into the endolymph may trigger EP elevation.

Adenylate cyclase modulation of endocochlear potential during suppression of strial Na(+)-K+ ATPase

Forskolin, an adenylate cyclase activator, produces a reversible elevation of the endocochlear potential (EP) (Doi et al., 1990a). To determine whether strial Na(+)-K+ ATPase activity is essential for the forskolin-dependent EP elevation, we examined, by means of K(+)-selective microelectrodes, the effects of forskolin on the EP and the endolymphatic K+ activity ([K+]) while strial Na(+)-K+ ATPase was suppressed by ouabain. Perilymphatic perfusion with ouabain (10(-3) M) decreased the EP from 78.5 +/- 2.4 mV to -27.6 +/- 2.4 mV (N = 8) at 37.9 +/- 3.7 min after the start of perfusion and decreased the [K+] from 138.7 +/- 5.4 mM to 103.7 +/- 3.7 mM (N = 3). Successive perfusion with forskolin (2 x 10(-4) M) with ouabain (10(-3) M) increased the EP by 15.1 +/- 1.5 mV (N = 8) but did not influence the [K+] decrease from 101 +/- 3.6 mM to 95 +/- 1.3 mM (N = 3). Forskolin (2 x 10(-4) M) with ouabain (10(-3) M) without a preceding ouabain perfusion decreased the EP from 76.2 +/- 2.3 mV to -12.9 +/- 1.8 mV (N = 6) at 65.3 +/- 2.1 min after the start of perfusion. These results indicate that adenylate cyclase can modulate the EP in the absence of strial Na(+)-K+ ATPase activity and that adenylate cyclase activation can attenuate the EP drop induced by strial Na(+)-K+ ATPase suppression.

Adenylate cyclase modulation of ion permeability in the guinea pig cochlea: a possible mechanism for the formation of endolymphatic hydrops

The pathophysiological mechanisms leading to endolymphatic hydrops in Meniere's disease are unknown. Changes in ionic permeability of the cellular membranes between the endolymph and the perilymph, which alter the composition and osmolarity of the inner ear fluid, may be a major factor in the etiology of endolymphatic hydrops. To determine the possible involvement of adenylate cyclase in the formation of endolymphatic hydrops, we measured the endolymphatic K+, Na+, Cl- activities (AK, ANa, ACl) and the endocochlear potential (EP) by means of ion-selective microelectrodes while inner ear adenylate cyclase was activated by perilymphatic perfusion with forskolin. We observed a large ACl increase accompanied by an EP increase during forskolin (2 x 10(-4) M) perfusion and a delayed AK decrease after perfusion. No measurable ANa change was observed. These results suggest that adenylate cyclase may regulate Cl- permeability of the endolymph-perilymph barrier and that adenylate cyclase plays a critical role in acute endolymphatic hydrops in Meniere's disease by altering the osmolarity of the endolymph.