Jouriial of Neurochmistri: 1976. Vol. 26, pp. 609-61 1. Pergamon Press Prmled in G r c a t Britain.
SHORT COMMUNICATION The distribution of taurine in the vertebrate retina (Received 26 June 1975. ,4ccepted 12 Angrrst 1975) THE POSSIBLE roles of taurine in neural tissue have been been partially frozen by subjection to a vacuum for the object of several investigations since the discovery of 15-20 min. This providcs very rapid cooling since gas bubhigh levels of this amino acid in nervous tissue (KOECHLIN, bles arc not formed, and consequently even the largest eyes & KACZMAREK,were rapidly frozen without the formation of large ice crys1955; CURTIS & WArKINS, 1965; DAVISON 1971). In the squid giant axon, taurine exists at substantial tals. Tangential 6 PM sections were cut at - 20°C. and dried levels and also is the likely precursor of isethionic acid. under vacuum at -40°C (LOWKY& PASSONNEAI;. 1972). the organic anion which largely neutralizes the high cation The dried scctions were stored at -20°C under vacLlum content of squid axoplasm (KOECHLIN, 1955). Studies have until used. Samples (100-300ng) were dissected at room also been rcportcd indicating that taurine may be involved temperature. weighed on a fishpole balance. and placed in the regulation of K+ permeability in heart muscle (READ in 2p1 of 10 mM-HC1 in an oil well covercd by mineral & WELTY,1965). Recently, evidcnce has accumulated oil ( L ~ R Y & PASSONNEAU, 1972). The identity of the which indicates that taurine might be a synaptic transmit- layers was determined by staining some sections with eosin ter or modulator of neuronal activity (CURTIS& WATKINS, and thionine. The samples in acid were heated at 80°C & KACZMARLK. 1971). An especially impor- for 30min. If necessary. the samples could be stored in 1960; DAVISON tant role for taurine in the retina is indicated by the finding the oil well rack at 4'C for several days before proceeding. of HAYES et al. (1975) that in the cat the absence of dietary Ion exchange. Isolation of taurine was accomplished ustaurine leads first to a drop in plasma and retinal taurine ing a procedure based on the method of Collins (COLLINS. followed by defects in the clectroretinogram, electron 1974). An aliquot (1-1.5 pl) was removed from the oil well microscopic signs of deterioration in the photoreceptors and pipetted onto a column with an approximate 2mm internal diameter packed with Amberlite CG 120. 200-400 and eventual blindness. The retina is particularly well suited for studying taurine mesh. in the H' form (Mallinckrodt) over Dowex AG 1-X10.200-400 mesh in the C1- form (Bio-Rad Laboratorsince extremely high levels of taurine have been reported & DOLENEK. 1958; ies) to a bed height of 14mm. Equal amounts of the two in retinas from several species (KUBICLK et al.. 1972; COHEN resins were used. Taurine was then washed from the BROTHFRTON. 1962; PASANTES-MORALES column with 200 pl of H,O, collcctcd in 7 x 70 mM tubcs, et ul.. 1973; STARR,1973; MACAIONE e l al., 1974). Additionally, several reports have becn published on the physio- and then dried using a Virtis Centrifugal Bio-Dryer (Virtis logical actions of taurine in the retina. Among thcse are Research Equipment). a series of papers by Pasantes-Morales and coworkers showing that taurine is actively taken up by the isolated Reaction with Juorescamine
et al. (1972) for General. The method of UDFNFRIEND el al., 1972) but depresses the retina (PASANTES-MORALES 'h'wave of the electroretinogram. On the other hand, taur- the reaction of fluorescamine with primary amines was ine is released when the retina is stimulated by light (PA- modified to enable the measurement of pmol of taurine in SANTES-MORALES et a!., 1973, 1974) or electrically (PASAN-discrete samples. Such sensitivity has previously been limited to chromatographic flow systems. Increased sensiTES-MORALES et al., 1974). Anatomically, the retina consists of well defined layers tivity was accomplished by decreasing the reaction volume but as yet there have been no reports on the concentrations to I1 pl, thereby reducing the amount of fluorescamine of taurine within the individual layers, although indirect needed. which at the dcsired sensitivity contributed to the and direct evidence has bcen reported indicating that the blank, The fluorescamine concentration in the acetone was outer or receptor containing portion of the retina is en- increased to 1 mg/ml to permit the use o f a smaller propor& VOADEN, tion of acetone. This reduced variability that might arise riched in taurine (COHENet al., 1973; KENNEDY from partial evaporation of the acetone. 1974). Using suitable methods (LOWRY& PASSONNEAU. Spec$c. The dried taurine was redissolved in 9 pl of 250 1972) it is possible to divide the retina into nine separate layers including synaptic and cell body layers. Dctermining mM-bOrate buffer, pH 9.1. Two 111 of fluorescamine (Pierce the taurine concentrations for these layers may help define Chemical Co.) 1 mg/ml in acetone were,added and thc its function in the retina. In this report such data for taur- tubes capped. Several minutes later 9 p1 were removed and addcd to 1 ml of H 2 0 in a 3 ml fluorometer tube. Fluoresine are reported for retinas of five vertebrate species. ccnce was read on a modified Farrand Model A Fluorometer (LOWRY& PASSONNEAU, 1972). METHODS Identity of thc taurine fraction was confirmed by using Tisszrc pi~/)at~~-atio~i. Eyes were removed from chicks and a macro-column (5 mm dia. and a bed vol of 500 pl) on frogs killed by decapitation. rabbits killed by cervical frac- to which whole retinal extract was pipetted. The taurine ture. a rhesus monkey anesthctizcd with Sernylan and pen- fraction was then analyzed on a Beckman Autoanalyzer tobarbital. and a cat anesthetized with ether and then deca- (&HEN et al.. 1973). Only one ninhydrin positive peak, pitated. All animals had been adapted to room light. After identical with taurine standards, was detected in the taurremoval, thc eyes were fro7en in liquid nitrogen which had ine fraction. Figure 1 shows a standard curve obtained 609
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ine in all species. ranging from 365 mmol/kg (dry wt) in the monkey to 195 mmol/kg in the frog. The lowest concentrations of taurine in the neural retina were found in the inner plexiform and ganglion ccll layer. The pigment epithelium of the two species in which it was assayed (cat and chick). contained levels which were half of those of the respective receptor outer segments but this might be expected to vary with the phagosome content of these cells. since these consist of fragments of outer segments. In the choroid of the monkey. both blood and connective tissue combined. no taurine wds detected (less than 10 mmo1,kg dry wt) (Fig. 2). The optic fibre layer of the monkey was found to contain levels of taurine equal to that at the level of the ganglion cell bodies. Thus. all layers of the retina in the 5 species studied contained substantial amounts of taurine.
pYOLES TAURINE ADDED
FIG. I . Taurine standard curve. Fluorescence developed with fluorexamine after passage through the ion-exchange column (see Methods). Identical values were obtained with taurine alone whether passed through the column or assayed directly. The amino acid mixture consisted of 40 pmol each of glutamate, glycine, glutamine, aspartate, GABA, and hypotaurine. One pg of bovine serum albumin was added to the appropriate standards.
by the method when taurine alone, taurine plus an amino acid mixture (including hypotaurine), or taurine plus bovine serum albumin was placed on the column. All 3 conditions yielded the same curve which was linear over a 10-fold taurine concentration range.
RESULTS All 5 species studied yielded the same pattern of retinal taurine distribution (Fig. 2). The ratio of the highest to the lowest taurine concentration within the neural retina varied from 2.2 in the cat to 5.4 in the chick. The outer nuclear layer contained the highest concentration of taur-
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FIG. 2. The distribution of taurine by retinal level in 5 species. CH, choroid; PE, pigment epithelium; OS, photoreceptor outer segments: IS, photoreceptor inner segments; ON, outer nuclear layer; OP, outer plexiform layer: IN, inner nuclear layer: IP, inner plexiform layer; G, ganglion cell layer; OF, optic fibre layer. The species are: C, cat; R. rabbit; M, monkey; Ck, chick; F, frog. Error bars: k S.E.M., each point represents 5 determinations from a single retina.
DISCUSSION The taurine values reported here from five vertebrate retinas confirm previously reported high levels of retinal & DOLFNEK. 1958; BROTHERTON. 1962: taurine (KLIBICEK PASANTES-MORALES et a/.. 1972; COHIX rt a/.. 1973; MACAIONE c't al., 1974). ln addition. they confirm previous evidence that taurine is more concentrated in the outer retina (COHENet al.. 1973; KENNEDY& VOADEN.1974). but is by no means limited to that portion of the retina. The first question is what is the cellular localization of the high taurine concentration in the outer retina. That a very substantial proportion must be located in the photoreceptors is indicated by the high concentration in the outer and inner segment layers where photoreceptors constitute the main cell volume, with only small contributions to the outer layer from pigment cell processes and to the inner layer from Muller cell processes. Furthermore. loss of the receptors in the rat (BROTHERTON, 1962). mouse (COHENrt al.. 1973) and cat (HAY-ES rt al., 1975) is accompanied by a loss of some 707" of retina taurine. The preceding does not, however. rule out the possibility of the presence of taurine in glial cells of Muller. Indeed. (1973) that radiolabelled taurine the finding of EHINGER was concentrated by Miiller cells after its injection into the vitreous. suggests that these cells can rapidly take up this amino acid. On the other hand. preliminary radioautographic studies by YOUNG(1969 and personal communication) of the retinas of the frog and rat. after systcwic administration, showed initial accumulation of radiolabel over the pigment epithelium and later, over the outer retina. In these experiments taurine metabolites could have been included in the radioautograph since the reported results with rats are for 4 1 0 days after administration. If Miiller cells were only a short-term reservoir for taurine, (1973) finding that the bulk it would not explain EHINGER'S of radiolabelled taurine persists in the rctina for more than 20 h. with no change in the radioautographic distribution over this period. STARR& VOADEN(1972) also noted long survival of radiolabelled taurine in the rctina. The data suggest that taurine concentration within receptor cells is highest in the cell bodies. OHMAN& SHELLY (1968) reported that on treatment of retinas from a variety of species with o-phthalaldehyde an intense orange fluorescence was seen at the level of photoreceptor somata and no other. This reagent reacts with all amino acids including taurine and probably with spermine and free histones. Since the concentration of taurine exceeds that of any other amino acid so far measured in retina. it could be
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responsible for thc fluorescence observed by OHUN & BYZOV A. L. & TRIFONOV J. A. (1968) Vision Res. 8. SHELLY.However, taurine is probably distributed more 817-822. evenly in the cell water of thc photoreceptor cells than CtKVETTO L. & PICCOLINOM. (1974) Science, N.Y. 183. Fig. 2 suggests. (The outer segment layer contains a smaller 41 7-418. proportion of cytoplasm than either the inner segment or CoHEN A. I., MCDANIEL M. & ORRH. T. (1973) Invest. outer nuclear layers.) Ophthal. 12. 686693. The work of HAYESet a/. (1975) demonstrates that taur- COLLINS G. G. S. (1974) Brain Res. 76. 447-459. ine is required for the life of the photoreceptors of the CURTIS D. R. & WATKINS J. C. (1960) J . N ~ o c h e i 6. ~~. cat, and in view of the distribution pattern shown in this 117-1 41. report, possibly important for these cells in all species, Thc CLIRTIS D. R . & WATKINS J. C. (1965) Pharmac. Reu. 17. most obvious function of taurine would bc its contribution 347-39 1. to the cell’s osmotic properties. In cephalopods isethionic DAVISON A. N. & KACZMAREK L. K. (1971) Nature. Lond. acid, a taurine derivative. is clearly the preponderant anion 234. 107-108. 1955). Al- DOWLING of the cytoplasm of the giant axons (KOECHLIN. J. E. & RIFTS H. (1973) Nafwa. Lond. 242. though K L E ~ WetI al. (1974) found iselhionatc synthesis 101- 103. from taurine in the chick rctina, STARR& VOAUEN (1972) EHINGER B. (1973) Brain Res. 60. 512-516. did not observe this in thc rat. As [or taurinc’s possible HAASH. L. & HOSLIL. (1973) Braiw Res. 52. 399-402. role as a neurotransmitter. it has been shown to have an HAYES K. C., CAREY R. E. & SCHMIDT S. Y. (1975) Science. inhibitory action on certain neurons (CURTIS& WATKINS, N . Y . 188. 949-950. 1960: KRNJ~VIC, 1964; HAAS& HOSLI,1973).In the current KENNEDY A. S. & VOADENM. J. (1974) J . Netrrochein. 23. study, thc synaptic layer containing the highest level of 1093-1095. taurine was thc outer plexiform layer. Much evidence KLETHIJ., MALLOKGA P. & MANDLLP. (1974) J. Physiol. points to photoreceptors libcrating in the dark a transmitParis 69. 158A. ter which has a depolari7ing action on horizontal cells (BY- KOECHLINB. A. (1955) J . hiophys. hiochem. Cytol. 1. 51 1-529. zov & TRIFONOV, 1968; DOWLING & RIPPS. 1973; CFRVFTK. (1964) I i 7 t Rev. Neurobiol. 7. 41-98. TO & PICCOLINO, 1974). Less of this transmitter is liberated KRNJEVIC KUBICEK R. & DOLEKIK A. (1958) J . Chi-omat. 1. 266268. in the light. Yet. PASANTES-MOKALES et a[.(1974) found LOWRY0. H. & PASSONNEAU J. V. (1972) A Flexible Systhat light flashes resulted in a liberation of radiolabelled tem of Enzymatic Analysis. Academic Press. New York. taurine. As already noted, this group had reported earlier S.. REUGGEK~ P.. DELUCAF. & Tuccr G. (1974) that taurine inhibits the ‘b’ wave of the ERG. a waveform MACAIONE J . Neurochein. 22, 887-891. elicited by light. Unlcss thc release of taurine occurs after the elicitation of the ‘h’wave, or is spatially separated from OHMANS. & SHELLYW. B. (1968) Natirrc., Lond. 220. 378-379. the site of elicitation, there appcars to be a dilcmma here. H., KETHIJ., URBANP. F. & MANDEL Thus. it will be important to locate the source or sources PASANTES-MORALES P. (1972) Physiol. Chem. Physics. 4. 339-348. of the light-induced release of taurine. Clearly there is as H., KETHIJ., URBANP. F. & MANDEL yet no established role for taurine in the retina despite PASANTES-MORALES P. (1973) Brain Res. 51. 375-378. indications of its importance for retinal function. PASANTES-MORALES H., KETHI.I. URBAN . P. F. & MANDEL P. (19741 Expl Brain Res. 19, 131-141. Acknowlr,dgenzeizts-Supportcd by grants from N.I.N.D.S. (5-T01-NSO-5613) H.T.O.; Nat. Eye Inst. (EY-00258) READ W. 0. & WELTYS . D. (1965) in Electrolytes and E.. ed.). Vol. I, pp. 70-85. Cardiovascular Diseuse (BAJUSZ A.I.C.; and U.S.P.H.S. (NS-08862) and the Am. Cancer Williams & Williams, Baltimore. SOC.(BC-4Q) O.H.L. STARRM. S. & VOADEN M. J. (1972) Vision Res. 12. 1261-1269. Departments of Pharmacology and STARRM. S. (1973) Brain R e x 59. 331-338. Ophthalmology. H. T. ORR UDENFRIEND S., STEINS.. BOHLENP., DAIRMAN W., LEMA. I. COHFN Washingtori University School of Mediciize. GRUBER w . & WEIGELE M. (1972) SciC‘?lcC, N.Y. 178, St. Louis, M O 631 10, U.S.A. 0.H. LOWRY 871-872. YOIJNG R. W. (1969) in The Retina (STRAATSMA B. R.. HALL REFERENCES M. 0.:ALLENR. A. & CRESCITELLI F., eds.), pp. 177-210. University of California Press, Los Angeles. BROTHERTON 5. (1962) t x p l Eye Rex 1, 246252.
SHORT COMMUNICATION The distribution of taurine in the vertebrate retina (Received 26 June 1975. ,4ccepted 12 Angrrst 1975) THE POSSIBLE roles of taurine in neural tissue have been been partially frozen by subjection to a vacuum for the object of several investigations since the discovery of 15-20 min. This providcs very rapid cooling since gas bubhigh levels of this amino acid in nervous tissue (KOECHLIN, bles arc not formed, and consequently even the largest eyes & KACZMAREK,were rapidly frozen without the formation of large ice crys1955; CURTIS & WArKINS, 1965; DAVISON 1971). In the squid giant axon, taurine exists at substantial tals. Tangential 6 PM sections were cut at - 20°C. and dried levels and also is the likely precursor of isethionic acid. under vacuum at -40°C (LOWKY& PASSONNEAI;. 1972). the organic anion which largely neutralizes the high cation The dried scctions were stored at -20°C under vacLlum content of squid axoplasm (KOECHLIN, 1955). Studies have until used. Samples (100-300ng) were dissected at room also been rcportcd indicating that taurine may be involved temperature. weighed on a fishpole balance. and placed in the regulation of K+ permeability in heart muscle (READ in 2p1 of 10 mM-HC1 in an oil well covercd by mineral & WELTY,1965). Recently, evidcnce has accumulated oil ( L ~ R Y & PASSONNEAU, 1972). The identity of the which indicates that taurine might be a synaptic transmit- layers was determined by staining some sections with eosin ter or modulator of neuronal activity (CURTIS& WATKINS, and thionine. The samples in acid were heated at 80°C & KACZMARLK. 1971). An especially impor- for 30min. If necessary. the samples could be stored in 1960; DAVISON tant role for taurine in the retina is indicated by the finding the oil well rack at 4'C for several days before proceeding. of HAYES et al. (1975) that in the cat the absence of dietary Ion exchange. Isolation of taurine was accomplished ustaurine leads first to a drop in plasma and retinal taurine ing a procedure based on the method of Collins (COLLINS. followed by defects in the clectroretinogram, electron 1974). An aliquot (1-1.5 pl) was removed from the oil well microscopic signs of deterioration in the photoreceptors and pipetted onto a column with an approximate 2mm internal diameter packed with Amberlite CG 120. 200-400 and eventual blindness. The retina is particularly well suited for studying taurine mesh. in the H' form (Mallinckrodt) over Dowex AG 1-X10.200-400 mesh in the C1- form (Bio-Rad Laboratorsince extremely high levels of taurine have been reported & DOLENEK. 1958; ies) to a bed height of 14mm. Equal amounts of the two in retinas from several species (KUBICLK et al.. 1972; COHEN resins were used. Taurine was then washed from the BROTHFRTON. 1962; PASANTES-MORALES column with 200 pl of H,O, collcctcd in 7 x 70 mM tubcs, et ul.. 1973; STARR,1973; MACAIONE e l al., 1974). Additionally, several reports have becn published on the physio- and then dried using a Virtis Centrifugal Bio-Dryer (Virtis logical actions of taurine in the retina. Among thcse are Research Equipment). a series of papers by Pasantes-Morales and coworkers showing that taurine is actively taken up by the isolated Reaction with Juorescamine
et al. (1972) for General. The method of UDFNFRIEND el al., 1972) but depresses the retina (PASANTES-MORALES 'h'wave of the electroretinogram. On the other hand, taur- the reaction of fluorescamine with primary amines was ine is released when the retina is stimulated by light (PA- modified to enable the measurement of pmol of taurine in SANTES-MORALES et a!., 1973, 1974) or electrically (PASAN-discrete samples. Such sensitivity has previously been limited to chromatographic flow systems. Increased sensiTES-MORALES et al., 1974). Anatomically, the retina consists of well defined layers tivity was accomplished by decreasing the reaction volume but as yet there have been no reports on the concentrations to I1 pl, thereby reducing the amount of fluorescamine of taurine within the individual layers, although indirect needed. which at the dcsired sensitivity contributed to the and direct evidence has bcen reported indicating that the blank, The fluorescamine concentration in the acetone was outer or receptor containing portion of the retina is en- increased to 1 mg/ml to permit the use o f a smaller propor& VOADEN, tion of acetone. This reduced variability that might arise riched in taurine (COHENet al., 1973; KENNEDY from partial evaporation of the acetone. 1974). Using suitable methods (LOWRY& PASSONNEAU. Spec$c. The dried taurine was redissolved in 9 pl of 250 1972) it is possible to divide the retina into nine separate layers including synaptic and cell body layers. Dctermining mM-bOrate buffer, pH 9.1. Two 111 of fluorescamine (Pierce the taurine concentrations for these layers may help define Chemical Co.) 1 mg/ml in acetone were,added and thc its function in the retina. In this report such data for taur- tubes capped. Several minutes later 9 p1 were removed and addcd to 1 ml of H 2 0 in a 3 ml fluorometer tube. Fluoresine are reported for retinas of five vertebrate species. ccnce was read on a modified Farrand Model A Fluorometer (LOWRY& PASSONNEAU, 1972). METHODS Identity of thc taurine fraction was confirmed by using Tisszrc pi~/)at~~-atio~i. Eyes were removed from chicks and a macro-column (5 mm dia. and a bed vol of 500 pl) on frogs killed by decapitation. rabbits killed by cervical frac- to which whole retinal extract was pipetted. The taurine ture. a rhesus monkey anesthctizcd with Sernylan and pen- fraction was then analyzed on a Beckman Autoanalyzer tobarbital. and a cat anesthetized with ether and then deca- (&HEN et al.. 1973). Only one ninhydrin positive peak, pitated. All animals had been adapted to room light. After identical with taurine standards, was detected in the taurremoval, thc eyes were fro7en in liquid nitrogen which had ine fraction. Figure 1 shows a standard curve obtained 609
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ine in all species. ranging from 365 mmol/kg (dry wt) in the monkey to 195 mmol/kg in the frog. The lowest concentrations of taurine in the neural retina were found in the inner plexiform and ganglion ccll layer. The pigment epithelium of the two species in which it was assayed (cat and chick). contained levels which were half of those of the respective receptor outer segments but this might be expected to vary with the phagosome content of these cells. since these consist of fragments of outer segments. In the choroid of the monkey. both blood and connective tissue combined. no taurine wds detected (less than 10 mmo1,kg dry wt) (Fig. 2). The optic fibre layer of the monkey was found to contain levels of taurine equal to that at the level of the ganglion cell bodies. Thus. all layers of the retina in the 5 species studied contained substantial amounts of taurine.
pYOLES TAURINE ADDED
FIG. I . Taurine standard curve. Fluorescence developed with fluorexamine after passage through the ion-exchange column (see Methods). Identical values were obtained with taurine alone whether passed through the column or assayed directly. The amino acid mixture consisted of 40 pmol each of glutamate, glycine, glutamine, aspartate, GABA, and hypotaurine. One pg of bovine serum albumin was added to the appropriate standards.
by the method when taurine alone, taurine plus an amino acid mixture (including hypotaurine), or taurine plus bovine serum albumin was placed on the column. All 3 conditions yielded the same curve which was linear over a 10-fold taurine concentration range.
RESULTS All 5 species studied yielded the same pattern of retinal taurine distribution (Fig. 2). The ratio of the highest to the lowest taurine concentration within the neural retina varied from 2.2 in the cat to 5.4 in the chick. The outer nuclear layer contained the highest concentration of taur-
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FIG. 2. The distribution of taurine by retinal level in 5 species. CH, choroid; PE, pigment epithelium; OS, photoreceptor outer segments: IS, photoreceptor inner segments; ON, outer nuclear layer; OP, outer plexiform layer: IN, inner nuclear layer: IP, inner plexiform layer; G, ganglion cell layer; OF, optic fibre layer. The species are: C, cat; R. rabbit; M, monkey; Ck, chick; F, frog. Error bars: k S.E.M., each point represents 5 determinations from a single retina.
DISCUSSION The taurine values reported here from five vertebrate retinas confirm previously reported high levels of retinal & DOLFNEK. 1958; BROTHERTON. 1962: taurine (KLIBICEK PASANTES-MORALES et a/.. 1972; COHIX rt a/.. 1973; MACAIONE c't al., 1974). ln addition. they confirm previous evidence that taurine is more concentrated in the outer retina (COHENet al.. 1973; KENNEDY& VOADEN.1974). but is by no means limited to that portion of the retina. The first question is what is the cellular localization of the high taurine concentration in the outer retina. That a very substantial proportion must be located in the photoreceptors is indicated by the high concentration in the outer and inner segment layers where photoreceptors constitute the main cell volume, with only small contributions to the outer layer from pigment cell processes and to the inner layer from Muller cell processes. Furthermore. loss of the receptors in the rat (BROTHERTON, 1962). mouse (COHENrt al.. 1973) and cat (HAY-ES rt al., 1975) is accompanied by a loss of some 707" of retina taurine. The preceding does not, however. rule out the possibility of the presence of taurine in glial cells of Muller. Indeed. (1973) that radiolabelled taurine the finding of EHINGER was concentrated by Miiller cells after its injection into the vitreous. suggests that these cells can rapidly take up this amino acid. On the other hand. preliminary radioautographic studies by YOUNG(1969 and personal communication) of the retinas of the frog and rat. after systcwic administration, showed initial accumulation of radiolabel over the pigment epithelium and later, over the outer retina. In these experiments taurine metabolites could have been included in the radioautograph since the reported results with rats are for 4 1 0 days after administration. If Miiller cells were only a short-term reservoir for taurine, (1973) finding that the bulk it would not explain EHINGER'S of radiolabelled taurine persists in the rctina for more than 20 h. with no change in the radioautographic distribution over this period. STARR& VOADEN(1972) also noted long survival of radiolabelled taurine in the rctina. The data suggest that taurine concentration within receptor cells is highest in the cell bodies. OHMAN& SHELLY (1968) reported that on treatment of retinas from a variety of species with o-phthalaldehyde an intense orange fluorescence was seen at the level of photoreceptor somata and no other. This reagent reacts with all amino acids including taurine and probably with spermine and free histones. Since the concentration of taurine exceeds that of any other amino acid so far measured in retina. it could be
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responsible for thc fluorescence observed by OHUN & BYZOV A. L. & TRIFONOV J. A. (1968) Vision Res. 8. SHELLY.However, taurine is probably distributed more 817-822. evenly in the cell water of thc photoreceptor cells than CtKVETTO L. & PICCOLINOM. (1974) Science, N.Y. 183. Fig. 2 suggests. (The outer segment layer contains a smaller 41 7-418. proportion of cytoplasm than either the inner segment or CoHEN A. I., MCDANIEL M. & ORRH. T. (1973) Invest. outer nuclear layers.) Ophthal. 12. 686693. The work of HAYESet a/. (1975) demonstrates that taur- COLLINS G. G. S. (1974) Brain Res. 76. 447-459. ine is required for the life of the photoreceptors of the CURTIS D. R. & WATKINS J. C. (1960) J . N ~ o c h e i 6. ~~. cat, and in view of the distribution pattern shown in this 117-1 41. report, possibly important for these cells in all species, Thc CLIRTIS D. R . & WATKINS J. C. (1965) Pharmac. Reu. 17. most obvious function of taurine would bc its contribution 347-39 1. to the cell’s osmotic properties. In cephalopods isethionic DAVISON A. N. & KACZMAREK L. K. (1971) Nature. Lond. acid, a taurine derivative. is clearly the preponderant anion 234. 107-108. 1955). Al- DOWLING of the cytoplasm of the giant axons (KOECHLIN. J. E. & RIFTS H. (1973) Nafwa. Lond. 242. though K L E ~ WetI al. (1974) found iselhionatc synthesis 101- 103. from taurine in the chick rctina, STARR& VOAUEN (1972) EHINGER B. (1973) Brain Res. 60. 512-516. did not observe this in thc rat. As [or taurinc’s possible HAASH. L. & HOSLIL. (1973) Braiw Res. 52. 399-402. role as a neurotransmitter. it has been shown to have an HAYES K. C., CAREY R. E. & SCHMIDT S. Y. (1975) Science. inhibitory action on certain neurons (CURTIS& WATKINS, N . Y . 188. 949-950. 1960: KRNJ~VIC, 1964; HAAS& HOSLI,1973).In the current KENNEDY A. S. & VOADENM. J. (1974) J . Netrrochein. 23. study, thc synaptic layer containing the highest level of 1093-1095. taurine was thc outer plexiform layer. Much evidence KLETHIJ., MALLOKGA P. & MANDLLP. (1974) J. Physiol. points to photoreceptors libcrating in the dark a transmitParis 69. 158A. ter which has a depolari7ing action on horizontal cells (BY- KOECHLINB. A. (1955) J . hiophys. hiochem. Cytol. 1. 51 1-529. zov & TRIFONOV, 1968; DOWLING & RIPPS. 1973; CFRVFTK. (1964) I i 7 t Rev. Neurobiol. 7. 41-98. TO & PICCOLINO, 1974). Less of this transmitter is liberated KRNJEVIC KUBICEK R. & DOLEKIK A. (1958) J . Chi-omat. 1. 266268. in the light. Yet. PASANTES-MOKALES et a[.(1974) found LOWRY0. H. & PASSONNEAU J. V. (1972) A Flexible Systhat light flashes resulted in a liberation of radiolabelled tem of Enzymatic Analysis. Academic Press. New York. taurine. As already noted, this group had reported earlier S.. REUGGEK~ P.. DELUCAF. & Tuccr G. (1974) that taurine inhibits the ‘b’ wave of the ERG. a waveform MACAIONE J . Neurochein. 22, 887-891. elicited by light. Unlcss thc release of taurine occurs after the elicitation of the ‘h’wave, or is spatially separated from OHMANS. & SHELLYW. B. (1968) Natirrc., Lond. 220. 378-379. the site of elicitation, there appcars to be a dilcmma here. H., KETHIJ., URBANP. F. & MANDEL Thus. it will be important to locate the source or sources PASANTES-MORALES P. (1972) Physiol. Chem. Physics. 4. 339-348. of the light-induced release of taurine. Clearly there is as H., KETHIJ., URBANP. F. & MANDEL yet no established role for taurine in the retina despite PASANTES-MORALES P. (1973) Brain Res. 51. 375-378. indications of its importance for retinal function. PASANTES-MORALES H., KETHI.I. URBAN . P. F. & MANDEL P. (19741 Expl Brain Res. 19, 131-141. Acknowlr,dgenzeizts-Supportcd by grants from N.I.N.D.S. (5-T01-NSO-5613) H.T.O.; Nat. Eye Inst. (EY-00258) READ W. 0. & WELTYS . D. (1965) in Electrolytes and E.. ed.). Vol. I, pp. 70-85. Cardiovascular Diseuse (BAJUSZ A.I.C.; and U.S.P.H.S. (NS-08862) and the Am. Cancer Williams & Williams, Baltimore. SOC.(BC-4Q) O.H.L. STARRM. S. & VOADEN M. J. (1972) Vision Res. 12. 1261-1269. Departments of Pharmacology and STARRM. S. (1973) Brain R e x 59. 331-338. Ophthalmology. H. T. ORR UDENFRIEND S., STEINS.. BOHLENP., DAIRMAN W., LEMA. I. COHFN Washingtori University School of Mediciize. GRUBER w . & WEIGELE M. (1972) SciC‘?lcC, N.Y. 178, St. Louis, M O 631 10, U.S.A. 0.H. LOWRY 871-872. YOIJNG R. W. (1969) in The Retina (STRAATSMA B. R.. HALL REFERENCES M. 0.:ALLENR. A. & CRESCITELLI F., eds.), pp. 177-210. University of California Press, Los Angeles. BROTHERTON 5. (1962) t x p l Eye Rex 1, 246252.
