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The effects of histamine and its antagonists on the cochlear microphonic and the compound action potential of the guinea pig
Auris Nasus Larynx 28 (2001) 219– 222 www.elsevier.com/locate/anl
The effects of histamine and its antagonists on the cochlear microphonic and the compound action potential of the guinea pig Ryosei Minoda *, 1, Takafumi Toriya 1, Keisuke Masuyama, Eiji Yumoto Department of Otolaryngology, Head and Neck Surgery, Kumamoto Uni6ersity School of Medicine, 1 -1 -1 A. Honjo, Kumamoto 860 -8556, Japan Received 21 August 2000; received in revised form 9 January 2001; accepted 19 January 2001
Abstract Object: we studied the effects of histamine, the H1 receptor antagonist pyrilamine, and the H2 receptor antagonist cimetidine on the cochlear potential of guinea pigs (cochlear microphonic, CM; compound action potential, CAP). Methods: histamine was applied into the cochlear perilymph at three different dosages (10 mM, 50 mM or 10 mM). Pyrilamine and cimetidine were applied at 50 mM each. Results: histamine increased the CAP at 10 and 50 mM without any significant effects on the CM. The effects of histamine at 50 mM were suppressed by the 50-mM of pyrilamine and cimetidine. At 10 mM of histamine, CAP and CM amplitudes were significantly decreased. Conclusion: in low concentrations, histamine may act as an extracellular signal on inner hair cells (IHCs) or it may stimulate the afferent nerve by binding to their H1 and H2 receptors. A possible explanation for the inhibitory effects of histamine at 10-mM dosage was apparently found in that the effects of the high concentration may be supraphysiological; and furthermore, there is a difference in the mechanism by which histamine exerts its effects mediated by the histamine receptors on the cochlea. © 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: H1 antagonist; H2 antagonist; Cochlear microphonic; Compound action potential
1. Introduction Few studies have addressed the effects of histamine on the cochlea in mammalians [1], while Bledsoe et al. [2] reported that histamine had no effect on the Xenopus lae6is lateral line. In mammals, only Bobbin and Thompson [3] reported that intra-cochlear histamine infusion in the guinea pig at 10 mM produced a slight decrease in the compound action potential (CAP) amplitude; Housley et al. [4] reported that histamine at 0.1 – 100 mM has a facilitatory effect on afferent transmission in the semicircular canal of the frog Rana pipiens, while at concentrations greater than 1 mM, histamine produced a reduction of the afferent firing rate. Studies at lower concentrations are needed to clarify the detailed physiological effects of histamine on * Corresponding author. Tel.: + 81-96-3735255; fax: +81-963735256. E-mail address: [email protected] (R. Minoda). 1 Ryosei Minoda and Takafumi Toriya equally contributed to this work.
the cochlea of the guinea pig. In the present study, we examined the effects of a lower concentration of histamine than that used by Bobbin and Thompson on the cochlear function potential (CM, CAP) in guinea pigs.
2. Materials and methods Hartley guinea pigs with positive Preyer’s reflexes, weighing 300 –400 g, were anesthetized with sodium pentobarbital (30 mg/kg, i.p.). After tracheotomy, the animals were immobilized with suxamethonium chloride (4 mg, i.m.) and maintained under artificial respiration with room air. Rectal temperature was maintained at 389 1°C with a heating pad. ECG was monitored continuously throughout all experiments. Additional anesthetic agents and muscle relaxants were administered as required to assure a constant deep level of anesthesia. The tympanic bulla was exposed using a ventrolateral approach. The cochlea was prepared for perfusion by making two holes in the cochlea basal turn. A hole was
0385-8146/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 3 8 5 - 8 1 4 6 ( 0 1 ) 0 0 0 5 1 - 7
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R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
made in the scala tympani for introduction of the perfusates and another hole was made in the scala vestibuli to allow the effluent to escape. The artificial perilymph (aPL) employed in these experiments had a composition of NaCl 137 mM, KCl 5 mM, CaCl2 2 mM, MgCl2 1 mM, NaH2PO4 1 mM, NaHCO3 12 mM, and glucose 11 mM with a resulting pH of 7.4. This perilymph solution did not affect the cochlear potentials [5]. The following drugs were tested: histamine dihydrochloride (Wako, Osaka, Japan); pyrilamine (Sigma Chemical Co., St Louis, MO, USA); and cimetidine (SK&F, Welwyn Garden City, Hertfordshire, UK). A fresh batch of medication for each case was diluted with new bicarbonate buffered artificial perilymph solution, adjusted to pH 7.4 as required, and applied to the cochlea by perfusing the perilymph compartment from the basal turn of the scala tympani to the basal turn of the scala vestibuli at 2.5 ml/min using a micropipette coupled to a syringe pump. In groups of six animals, the initial perfusion of aPL without medication was followed by successive perfusions with the agent solution. The medication effects were tested using cochlear potentials before any perfusions and immediately (within 1 min) after each perfusion. The CAP and CM were recorded by an electrodiagnostic testing system (Navigator, Bio-logic System Corp., Mundelein, IN, USA). The active electrode was 200-mm stainless steel wire (insulated except for the tip) placed in the basal turn of the scala tympani. An Ag/AgCl reference electrode was placed in the neck muscle. The responses were amplified by a gain of 1000, filtered with a band pass of 0.1 Hz to 10 kHz, averaged over 128 sweeps, and stored on hard disk. Acoustic stimuli were generated by the same system and delivered via an inserted earphone (E.A.R tone, Carbot Safety Co. Auditory Systems Division, Indianapolis, IN, USA). An 8 kHz tone burst of 90 dBSPL having 1 ms rise/fall time, and 10 ms duration was repeated at a rate of 5 times per second. The stimuli were phase-locked, and in all experiments both condensation and rarefaction stimuli were used. CAP was obtained by adding the results of stimuli of the two polarities, while CM was obtained by subtracting the results of the stimuli of the two polarities. Values of CAP and CM amplitude are expressed as mean 9 S.E. Non-parametric statistical analyses were used to determine statistical significance. Differences were considered significant when P B 0.05.
mean9S.E. of CAP amplitude was 1143.59 51.3 (n = 5), mean9S.E. of CM amplitude was 832.1938.1 (n= 5). All medication effects are shown as percent changes of potentials recorded after the various treatments were related to those recorded following the pre-medication artificial perilymph perfusion. Fig. 1 shows the effects of 10 mM, 50 mM and 10 mM dose of histamine on CAP and CM amplitudes. CAP amplitudes were significantly increased at 10 and 50 mM (P B0.05); although, histamine had no significant effect on CM amplitude at these concentrations. At 10 mM, CAP and CM amplitude were significantly decreased (PB0.05). To observe whether the increases in CAP amplitude at 10 and 50 mM histamine were mediated by H1 or H2 receptors, either the H1 receptor antagonist pyrilamine or the H2 receptor antagonist cimetidine was dissolved along with histamine in the aPL. Histamine was used at a concentration of 50 mM, because 50 mM histamine had a greater effect on the CAP amplitude than at 10 mM. Pyrilamine and cimetidine were used as the concentration of 50 mM, as Housley et al. [1] reported that both pyrilamine and cimetidine at the concentrations of
3. Results The initial aPL perfusion had no effect on the cochlear potentials (data not shown). Initial absolute potentials were recorded before the various treatments:
Fig. 1. Effects of histamine on CAP and CM amplitudes. (A) Percentage change in CAP amplitude (n = 6). (B) Percentage change in CM amplitude (n = 6). Data are mean 9S.E. *, PB 0.05 versus values after perfusion with aPL. N.S. is not significant.
R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
Fig. 2. Pyrilamine and cimetidine mediation of the effects of histamine on CAP amplitude. (A) Pyrilamine significantly inhibited the histamine-induced CAP amplitude elevation (n= 6). (B) Cimetidine significantly inhibited the histamine-induced CAP amplitude elevation (n= 6). CAP amplitude expressed as the percent change in CAP amplitude. Data are mean 9S.E. *, PB 0.05 versus values after the application of histamine by itself.
10 mM produced a reduction of the afferent firing rate on guinea pig semicircular canals. The H1 and H2 receptor antagonists were perfused without histamine following artificial perilymph perfusion. No significant changes were observed with either antagonist (data not shown). Pyrilamine and cimetidine (Fig. 2) significantly inhibited the histamine-induced elevation of CAP amplitude (PB 0.05).
4. Discussion In this study, 10 and 50 mM histamine increased CAP amplitude without affecting the CM. This result verifies that histamine involvement is limited to the CAP generation mechanism. According to Bobbin and Thompson [3] the CAP represents the synchronous discharge of many cochlear afferent nerve fibers and is an indirect indicator of afferent nerve fiber activity. An increase in
221
the CAP magnitude, therefore, simply indicates that many fibers are discharging in synchrony. Considering the above points, the increased CAP amplitude with no change in CM could be explained as follows. Outer hair cells (OHCs) suppress the action potential via an efferent medial olivo-cochlear bundle [6]. The possibility is that histamine may suppress OHC or efferent fiber functions; consequently, IHCs may become activated and effectively stimulate afferent fibers. The lack of a shift in CM suggests that the OHCs are largely unaffected by histamine [7]. Histamine though may act on synaptic sites on afferent nerve fibers. As discussed by Bobbin and Thompson [3], external application of the afferent transmitters could induce both spontaneous asynchronous discharge to an acoustic stimulus and a reduction in CAP amplitude. If histamine is an afferent transmitter and acts on postsynaptic sites, then a CAP amplitude decrease should be observed; contrarily, however, in this study, an increase in CAP amplitude was observed: then the possibility that histamine acts as an afferent transmitter can be discounted. Another possibility worth considering seems to be that histamine may affect the afferent nerves directly via efferent fibers synapsing with afferent dendrites beneath the IHCs. A CAP amplitude increase without an effect on the CM dovetails with this explanation. Recently, Yamaguchi and Ohmori [8] reported that the efferent synapse may function as an excitatory synapse on the afferent fibers. Histamine could stimulate efferent fibers beneath the IHCs and consequently the afferent nerves would become activated. Histamine may also act directly on IHCs to stimulate the release of neurotransmitters, which would thereby increase CAP amplitude. When extracellular signals such as hormones or neurotransmitters act on IHCs, secondary messengers such as protein kinase C are activated and neurotransmitter release is thereby, induced. Histamine may, therefore, act as an extracellular signal to stimulate neurotransmitter release from IHCs [9]. The increase in CAP amplitude at 50 mM histamine was suppressed by H1 and H2 receptor antagonists. This finding suggests that histamine may act on the cochlea via both H1 and H2 receptors, which are suspected to exist in the cochlea. Bobbin and Thompson [3] reported that intra-cochlear histamine perfusion at 10 mM in the guinea pig produced an 1898% decrease in CAP amplitude. Our study showed that 10-mM histamine significantly reduced both the CAP amplitudes (12.29 4.2%) and also the CM amplitudes (14.19 4.0%). The CAP amplitude was significantly increased at 10 mM and at 50 mM with no change in CM amplitude. Although Bobbin and Thompson [3] concluded that histamine was not an afferent neurotransmitter because the CAP amplitude
222
R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
reduction was small, the statistically significant CAP amplitude reduction in our study suggests that histamine has some effect on the CAP generation mechanism at high concentrations; the significant CM amplitude reduction in our study at 10 mM suggests that OHCs or the stria vascularis may be involved. The reduction of both CAP and CM amplitudes at 10 mM may also reflect the effects of a supraphysiological dose of histamine. During inflammation, histamine, released by the basophils and mastocytes, can reach local concentrations of 1 mM, and can act on neighboring tissues and cells [10]. The concentration of 10-mM histamine may be higher than the concentration in a purely physiologic condition. CAP and CM amplitude reduction may be caused by hyperosmolarity of the perfusate at 10 mM histamine. In our study, osmolarity of the perfusate at 10 mM histamine is 331 mOsm/l (data not shown), while the osmolarity of a normal condition guinea pig is about 290 mOsm/l. Suckfull et al. [11] reported that increment of perilymphatic osmolarity can lead to a disturbance of OHC. Juhn et al. [12] reported that perilymph osmolality elevation induced by the intravenous administration of epinephrine can induce a decrease in the CAP amplitude; moreover, Wakiaone et al. [13] reported that increments of perilymphatic osmolarity can lead to a decline endocochlear potential. CAP and CM amplitude reduction in our study at 10 mM may be caused by the effect of supraphysiological hyperosmolarity. A further explanation for the opposition between the effects of low concentrations and the effects of high concentrations of histamine may be that the inhibitory effects seen in high concentrations may be caused by some other mechanism: H1, H2, or H3 receptors — while the excitatory effects in low concentrations may be actuated via H1 and H2 receptors. In summary, physiologically, histamine plays a role in the cochlea of the guinea pig. It may act as an extracellular signal on IHCs or it may stimulate the afferent nerve via efferent fibers beneath the IHCs by binding to H1 and H2 receptors in the cochlea. The modulatory actions of histamine in the cochlea are similar to the actions of histamine on equilibrium organs, which have been reported earlier. The inhibitory effects of histamine were found in supraphysiological concentrations, while the excitatory effects of histamine were found in low concentrations. A possible explanation for this opposition may be that the effects of high concentrations may be supraphysiological; also there appears to be a difference in the mechanism by which .
histamine effects are mediated by histamine receptors on the cochlea at high and at low concentrations.
Acknowledgements The authors are grateful for the advice given by Dr Nozomu Mori, Department of Otorhinolaryngology, Kagawa Medical University and Dr Masafumi Yoshida, Department of Otorhinolaryngology, University of Occupational and Environmental Health, Japan.
References [1] Eybalin M. Neurotransmitters and neuromodulators of the mammalian cochlea. Physiol Rev 1993;73:309 – 73. [2] Bledsoe SC Jr, Sinard RJ, Allen SJ. Analysis of histamine as a hair-cell transmitter in the lateral line of Xenopus lae6is. Hearing Res 1989;38:81 – 93. [3] Bobbin RP, Thompson MH. Effects of putative transmitters on afferent cochlear transmission. Ann Otol Rhinol Laryngol 1978;87:185 – 90. [4] Housley GD, Norris CH, Guth PS. Histamine and related substances influence neurotransmission in the semicircular canal. Hearing Res 1988;35:87 – 98. [5] Bobbin RP, Ceasar G. Kynurenic acid and gamma-D-glutamylaminomethylsulfonic acid suppress the compound action potential of the auditory nerve. Hearing Res 1987;25:77 – 81 Sohmer HS. [6] The effect of contralateral olivo-cochlear bundle stimulation on the cochlear potentials evoked by acoustic stimuli of various frequencies and intensities. Acta Otolaryngol Stockh 1965; 60: 59 – 70. [7] Fechter LD, Liu Y. Trimethyltin disrupts N1 sensitivity, but has limited effects on the summating potential and cochlear microphonic. Hearing Res 1994;78:189 – 96. [8] Yamaguchi K, Ohmori H. Suppression of the slow K+ current by cholinergic agonists in cultured chick cochlear ganglion neurones. J Physiol 1993;464:213 – 28. [9] Kitano I, Nario K, Mori N, Matsunaga T. The effect of protein kinase C stimulator and inhibitor on cochlear potentials in the guinea pig. Hearing Res 1995;85:11 – 7. [10] Adams GK, Lichtenstein L. In vitro studies of antigen-induced bronchospasm: effect of antihistamine and SRS-A antagonist on response of sensitized guinea pig and human airways to antigen. J Immunol 1979;122:555 – 62. [11] Suckfull M, Winkler G, Thein E, Raab S, Schorn K, Mees K. Changes in serum osmolarity influence the function of outer hair cells. Acta Otolaryngol 1999;119:316 – 21. [12] Juhn SK, Prado S, Rybak L. Effect of urea on osmolality of perilymph. Arch Otolaryngol 1979;105(9):538 – 41. [13] Wakizono S, Komune S, Uemura T. Susceptibility of the endocochlear potential to pH and osmolarity changes in the perilymph of the cochlea in the guinea pig. Eur Arch Otorhinolaryngol 1990;247:97 – 9.
The effects of histamine and its antagonists on the cochlear microphonic and the compound action potential of the guinea pig Ryosei Minoda *, 1, Takafumi Toriya 1, Keisuke Masuyama, Eiji Yumoto Department of Otolaryngology, Head and Neck Surgery, Kumamoto Uni6ersity School of Medicine, 1 -1 -1 A. Honjo, Kumamoto 860 -8556, Japan Received 21 August 2000; received in revised form 9 January 2001; accepted 19 January 2001
Abstract Object: we studied the effects of histamine, the H1 receptor antagonist pyrilamine, and the H2 receptor antagonist cimetidine on the cochlear potential of guinea pigs (cochlear microphonic, CM; compound action potential, CAP). Methods: histamine was applied into the cochlear perilymph at three different dosages (10 mM, 50 mM or 10 mM). Pyrilamine and cimetidine were applied at 50 mM each. Results: histamine increased the CAP at 10 and 50 mM without any significant effects on the CM. The effects of histamine at 50 mM were suppressed by the 50-mM of pyrilamine and cimetidine. At 10 mM of histamine, CAP and CM amplitudes were significantly decreased. Conclusion: in low concentrations, histamine may act as an extracellular signal on inner hair cells (IHCs) or it may stimulate the afferent nerve by binding to their H1 and H2 receptors. A possible explanation for the inhibitory effects of histamine at 10-mM dosage was apparently found in that the effects of the high concentration may be supraphysiological; and furthermore, there is a difference in the mechanism by which histamine exerts its effects mediated by the histamine receptors on the cochlea. © 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: H1 antagonist; H2 antagonist; Cochlear microphonic; Compound action potential
1. Introduction Few studies have addressed the effects of histamine on the cochlea in mammalians [1], while Bledsoe et al. [2] reported that histamine had no effect on the Xenopus lae6is lateral line. In mammals, only Bobbin and Thompson [3] reported that intra-cochlear histamine infusion in the guinea pig at 10 mM produced a slight decrease in the compound action potential (CAP) amplitude; Housley et al. [4] reported that histamine at 0.1 – 100 mM has a facilitatory effect on afferent transmission in the semicircular canal of the frog Rana pipiens, while at concentrations greater than 1 mM, histamine produced a reduction of the afferent firing rate. Studies at lower concentrations are needed to clarify the detailed physiological effects of histamine on * Corresponding author. Tel.: + 81-96-3735255; fax: +81-963735256. E-mail address: [email protected] (R. Minoda). 1 Ryosei Minoda and Takafumi Toriya equally contributed to this work.
the cochlea of the guinea pig. In the present study, we examined the effects of a lower concentration of histamine than that used by Bobbin and Thompson on the cochlear function potential (CM, CAP) in guinea pigs.
2. Materials and methods Hartley guinea pigs with positive Preyer’s reflexes, weighing 300 –400 g, were anesthetized with sodium pentobarbital (30 mg/kg, i.p.). After tracheotomy, the animals were immobilized with suxamethonium chloride (4 mg, i.m.) and maintained under artificial respiration with room air. Rectal temperature was maintained at 389 1°C with a heating pad. ECG was monitored continuously throughout all experiments. Additional anesthetic agents and muscle relaxants were administered as required to assure a constant deep level of anesthesia. The tympanic bulla was exposed using a ventrolateral approach. The cochlea was prepared for perfusion by making two holes in the cochlea basal turn. A hole was
0385-8146/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 3 8 5 - 8 1 4 6 ( 0 1 ) 0 0 0 5 1 - 7
220
R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
made in the scala tympani for introduction of the perfusates and another hole was made in the scala vestibuli to allow the effluent to escape. The artificial perilymph (aPL) employed in these experiments had a composition of NaCl 137 mM, KCl 5 mM, CaCl2 2 mM, MgCl2 1 mM, NaH2PO4 1 mM, NaHCO3 12 mM, and glucose 11 mM with a resulting pH of 7.4. This perilymph solution did not affect the cochlear potentials [5]. The following drugs were tested: histamine dihydrochloride (Wako, Osaka, Japan); pyrilamine (Sigma Chemical Co., St Louis, MO, USA); and cimetidine (SK&F, Welwyn Garden City, Hertfordshire, UK). A fresh batch of medication for each case was diluted with new bicarbonate buffered artificial perilymph solution, adjusted to pH 7.4 as required, and applied to the cochlea by perfusing the perilymph compartment from the basal turn of the scala tympani to the basal turn of the scala vestibuli at 2.5 ml/min using a micropipette coupled to a syringe pump. In groups of six animals, the initial perfusion of aPL without medication was followed by successive perfusions with the agent solution. The medication effects were tested using cochlear potentials before any perfusions and immediately (within 1 min) after each perfusion. The CAP and CM were recorded by an electrodiagnostic testing system (Navigator, Bio-logic System Corp., Mundelein, IN, USA). The active electrode was 200-mm stainless steel wire (insulated except for the tip) placed in the basal turn of the scala tympani. An Ag/AgCl reference electrode was placed in the neck muscle. The responses were amplified by a gain of 1000, filtered with a band pass of 0.1 Hz to 10 kHz, averaged over 128 sweeps, and stored on hard disk. Acoustic stimuli were generated by the same system and delivered via an inserted earphone (E.A.R tone, Carbot Safety Co. Auditory Systems Division, Indianapolis, IN, USA). An 8 kHz tone burst of 90 dBSPL having 1 ms rise/fall time, and 10 ms duration was repeated at a rate of 5 times per second. The stimuli were phase-locked, and in all experiments both condensation and rarefaction stimuli were used. CAP was obtained by adding the results of stimuli of the two polarities, while CM was obtained by subtracting the results of the stimuli of the two polarities. Values of CAP and CM amplitude are expressed as mean 9 S.E. Non-parametric statistical analyses were used to determine statistical significance. Differences were considered significant when P B 0.05.
mean9S.E. of CAP amplitude was 1143.59 51.3 (n = 5), mean9S.E. of CM amplitude was 832.1938.1 (n= 5). All medication effects are shown as percent changes of potentials recorded after the various treatments were related to those recorded following the pre-medication artificial perilymph perfusion. Fig. 1 shows the effects of 10 mM, 50 mM and 10 mM dose of histamine on CAP and CM amplitudes. CAP amplitudes were significantly increased at 10 and 50 mM (P B0.05); although, histamine had no significant effect on CM amplitude at these concentrations. At 10 mM, CAP and CM amplitude were significantly decreased (PB0.05). To observe whether the increases in CAP amplitude at 10 and 50 mM histamine were mediated by H1 or H2 receptors, either the H1 receptor antagonist pyrilamine or the H2 receptor antagonist cimetidine was dissolved along with histamine in the aPL. Histamine was used at a concentration of 50 mM, because 50 mM histamine had a greater effect on the CAP amplitude than at 10 mM. Pyrilamine and cimetidine were used as the concentration of 50 mM, as Housley et al. [1] reported that both pyrilamine and cimetidine at the concentrations of
3. Results The initial aPL perfusion had no effect on the cochlear potentials (data not shown). Initial absolute potentials were recorded before the various treatments:
Fig. 1. Effects of histamine on CAP and CM amplitudes. (A) Percentage change in CAP amplitude (n = 6). (B) Percentage change in CM amplitude (n = 6). Data are mean 9S.E. *, PB 0.05 versus values after perfusion with aPL. N.S. is not significant.
R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
Fig. 2. Pyrilamine and cimetidine mediation of the effects of histamine on CAP amplitude. (A) Pyrilamine significantly inhibited the histamine-induced CAP amplitude elevation (n= 6). (B) Cimetidine significantly inhibited the histamine-induced CAP amplitude elevation (n= 6). CAP amplitude expressed as the percent change in CAP amplitude. Data are mean 9S.E. *, PB 0.05 versus values after the application of histamine by itself.
10 mM produced a reduction of the afferent firing rate on guinea pig semicircular canals. The H1 and H2 receptor antagonists were perfused without histamine following artificial perilymph perfusion. No significant changes were observed with either antagonist (data not shown). Pyrilamine and cimetidine (Fig. 2) significantly inhibited the histamine-induced elevation of CAP amplitude (PB 0.05).
4. Discussion In this study, 10 and 50 mM histamine increased CAP amplitude without affecting the CM. This result verifies that histamine involvement is limited to the CAP generation mechanism. According to Bobbin and Thompson [3] the CAP represents the synchronous discharge of many cochlear afferent nerve fibers and is an indirect indicator of afferent nerve fiber activity. An increase in
221
the CAP magnitude, therefore, simply indicates that many fibers are discharging in synchrony. Considering the above points, the increased CAP amplitude with no change in CM could be explained as follows. Outer hair cells (OHCs) suppress the action potential via an efferent medial olivo-cochlear bundle [6]. The possibility is that histamine may suppress OHC or efferent fiber functions; consequently, IHCs may become activated and effectively stimulate afferent fibers. The lack of a shift in CM suggests that the OHCs are largely unaffected by histamine [7]. Histamine though may act on synaptic sites on afferent nerve fibers. As discussed by Bobbin and Thompson [3], external application of the afferent transmitters could induce both spontaneous asynchronous discharge to an acoustic stimulus and a reduction in CAP amplitude. If histamine is an afferent transmitter and acts on postsynaptic sites, then a CAP amplitude decrease should be observed; contrarily, however, in this study, an increase in CAP amplitude was observed: then the possibility that histamine acts as an afferent transmitter can be discounted. Another possibility worth considering seems to be that histamine may affect the afferent nerves directly via efferent fibers synapsing with afferent dendrites beneath the IHCs. A CAP amplitude increase without an effect on the CM dovetails with this explanation. Recently, Yamaguchi and Ohmori [8] reported that the efferent synapse may function as an excitatory synapse on the afferent fibers. Histamine could stimulate efferent fibers beneath the IHCs and consequently the afferent nerves would become activated. Histamine may also act directly on IHCs to stimulate the release of neurotransmitters, which would thereby increase CAP amplitude. When extracellular signals such as hormones or neurotransmitters act on IHCs, secondary messengers such as protein kinase C are activated and neurotransmitter release is thereby, induced. Histamine may, therefore, act as an extracellular signal to stimulate neurotransmitter release from IHCs [9]. The increase in CAP amplitude at 50 mM histamine was suppressed by H1 and H2 receptor antagonists. This finding suggests that histamine may act on the cochlea via both H1 and H2 receptors, which are suspected to exist in the cochlea. Bobbin and Thompson [3] reported that intra-cochlear histamine perfusion at 10 mM in the guinea pig produced an 1898% decrease in CAP amplitude. Our study showed that 10-mM histamine significantly reduced both the CAP amplitudes (12.29 4.2%) and also the CM amplitudes (14.19 4.0%). The CAP amplitude was significantly increased at 10 mM and at 50 mM with no change in CM amplitude. Although Bobbin and Thompson [3] concluded that histamine was not an afferent neurotransmitter because the CAP amplitude
222
R. Minoda et al. / Auris Nasus Larynx 28 (2001) 219–222
reduction was small, the statistically significant CAP amplitude reduction in our study suggests that histamine has some effect on the CAP generation mechanism at high concentrations; the significant CM amplitude reduction in our study at 10 mM suggests that OHCs or the stria vascularis may be involved. The reduction of both CAP and CM amplitudes at 10 mM may also reflect the effects of a supraphysiological dose of histamine. During inflammation, histamine, released by the basophils and mastocytes, can reach local concentrations of 1 mM, and can act on neighboring tissues and cells [10]. The concentration of 10-mM histamine may be higher than the concentration in a purely physiologic condition. CAP and CM amplitude reduction may be caused by hyperosmolarity of the perfusate at 10 mM histamine. In our study, osmolarity of the perfusate at 10 mM histamine is 331 mOsm/l (data not shown), while the osmolarity of a normal condition guinea pig is about 290 mOsm/l. Suckfull et al. [11] reported that increment of perilymphatic osmolarity can lead to a disturbance of OHC. Juhn et al. [12] reported that perilymph osmolality elevation induced by the intravenous administration of epinephrine can induce a decrease in the CAP amplitude; moreover, Wakiaone et al. [13] reported that increments of perilymphatic osmolarity can lead to a decline endocochlear potential. CAP and CM amplitude reduction in our study at 10 mM may be caused by the effect of supraphysiological hyperosmolarity. A further explanation for the opposition between the effects of low concentrations and the effects of high concentrations of histamine may be that the inhibitory effects seen in high concentrations may be caused by some other mechanism: H1, H2, or H3 receptors — while the excitatory effects in low concentrations may be actuated via H1 and H2 receptors. In summary, physiologically, histamine plays a role in the cochlea of the guinea pig. It may act as an extracellular signal on IHCs or it may stimulate the afferent nerve via efferent fibers beneath the IHCs by binding to H1 and H2 receptors in the cochlea. The modulatory actions of histamine in the cochlea are similar to the actions of histamine on equilibrium organs, which have been reported earlier. The inhibitory effects of histamine were found in supraphysiological concentrations, while the excitatory effects of histamine were found in low concentrations. A possible explanation for this opposition may be that the effects of high concentrations may be supraphysiological; also there appears to be a difference in the mechanism by which .
histamine effects are mediated by histamine receptors on the cochlea at high and at low concentrations.
Acknowledgements The authors are grateful for the advice given by Dr Nozomu Mori, Department of Otorhinolaryngology, Kagawa Medical University and Dr Masafumi Yoshida, Department of Otorhinolaryngology, University of Occupational and Environmental Health, Japan.
References [1] Eybalin M. Neurotransmitters and neuromodulators of the mammalian cochlea. Physiol Rev 1993;73:309 – 73. [2] Bledsoe SC Jr, Sinard RJ, Allen SJ. Analysis of histamine as a hair-cell transmitter in the lateral line of Xenopus lae6is. Hearing Res 1989;38:81 – 93. [3] Bobbin RP, Thompson MH. Effects of putative transmitters on afferent cochlear transmission. Ann Otol Rhinol Laryngol 1978;87:185 – 90. [4] Housley GD, Norris CH, Guth PS. Histamine and related substances influence neurotransmission in the semicircular canal. Hearing Res 1988;35:87 – 98. [5] Bobbin RP, Ceasar G. Kynurenic acid and gamma-D-glutamylaminomethylsulfonic acid suppress the compound action potential of the auditory nerve. Hearing Res 1987;25:77 – 81 Sohmer HS. [6] The effect of contralateral olivo-cochlear bundle stimulation on the cochlear potentials evoked by acoustic stimuli of various frequencies and intensities. Acta Otolaryngol Stockh 1965; 60: 59 – 70. [7] Fechter LD, Liu Y. Trimethyltin disrupts N1 sensitivity, but has limited effects on the summating potential and cochlear microphonic. Hearing Res 1994;78:189 – 96. [8] Yamaguchi K, Ohmori H. Suppression of the slow K+ current by cholinergic agonists in cultured chick cochlear ganglion neurones. J Physiol 1993;464:213 – 28. [9] Kitano I, Nario K, Mori N, Matsunaga T. The effect of protein kinase C stimulator and inhibitor on cochlear potentials in the guinea pig. Hearing Res 1995;85:11 – 7. [10] Adams GK, Lichtenstein L. In vitro studies of antigen-induced bronchospasm: effect of antihistamine and SRS-A antagonist on response of sensitized guinea pig and human airways to antigen. J Immunol 1979;122:555 – 62. [11] Suckfull M, Winkler G, Thein E, Raab S, Schorn K, Mees K. Changes in serum osmolarity influence the function of outer hair cells. Acta Otolaryngol 1999;119:316 – 21. [12] Juhn SK, Prado S, Rybak L. Effect of urea on osmolality of perilymph. Arch Otolaryngol 1979;105(9):538 – 41. [13] Wakizono S, Komune S, Uemura T. Susceptibility of the endocochlear potential to pH and osmolarity changes in the perilymph of the cochlea in the guinea pig. Eur Arch Otorhinolaryngol 1990;247:97 – 9.