RESEARCH RODS IN THE ANTELOPE
NOTE GROUND
SQUIRREL’
STEVEXK. FISHER.GERALD H. JACOBS,DOS W. AYDERWN and MARJ-IN S. SIL~ER.~X Departments of Biological Sciences and Psychology. University of California. Santa Barbara. CA 93106, U.S.A.
(Received
13 Janttary
1976; in rrtfsedform
The ground-dwelling sciurids have conventionally been assumed to constitute one of the few groups of animals whose member species have all-cone retinas. However, over the past few months evidence has been presented to show that for at least some of the species in this group the claim for a pure-cone retina cannot be substantiated. Thus, using a variety of anatomical criteria, four species of ground-dwelling sciurids (prairie dogs, Mexican ground squirrels, 1j-line ground squirrels, and California ground squirrels) have all been shown to have a small number of retinal rods in addition to a large population of cones (West and Dowling, 1975; Jacobs. Fisher, Anderson and Silverman. 1976: Anderson and Fisher, 1976). Although the number of rods in these retinas is small. less than 10% of the total number of photoreceptors. it has also been demonstrated that rodgenerated signals can be clearly seen in the electroretinograms (ERGS) recorded from these animals (Green and Dowling. 1975; Jacobs et al., 1976). In addition to the four species referred to above, there is another ground-dwelling sciurid which has been the frequent subject of vision experiments. This animal, the antelope ground squirrel (Ammospermophilrls [eucurus), has been used in behavioral (Crescitelli and Pollack, 1965; t966; 1972). anatomical (Johnston and Gardner. 1959). and eIectrophys~ological (Crescitelli. 1961; 1962) studies of vision. Ail of these studies indicate a belief that the antelope ground squirrel has an all-cone retina; indeed. there are no results in any of these studies inconsistent with this supposition. To determine if the antelope ground squirrel in fact has a pure-cone retina, or whether like the other species recently studied has a small popuIation of rods, we have examined both the anatomy of the photore~ptors in this species and the nature of the ERG recorded from the intact eye. RESULTS
AYD
DtSCLSSlON
In order to anatomically determine the types of photoreceptors present one antelope ground squirrel was prepared for both autoradiographv and electron microscopy. The squirrel (200 g) was injected intraperitonealiy with 30mCi of (4,S-3H)-L-ieucine tsp. ’ This work was supported by research grants (EY-00105. EY-00888, and EY-03602) from the National Eye Institute. The authors thank Dr. J. Lee Kavanau (CCL.\) for providing the squirrels used in this study, 875
I February
1976)
activity 51 Ciimw; 389 Ci/mg; Amersham Searle) dissolved in 2.0 ml of distilled water. After 72 hr the animal was given a lethal dose of sodium pentobarbito1 and fixed by intracardiac perfusion of a glutaraldehyde-picric acid fixative as described in detail elsewhere (Jacobs et al., 1976; Anderson and Fisher. 1976). The retina was postfixed in 2”; buffered osmium tetroxide dehydrated in ethanol, and embedded in Araldite. For autoradiography, 0.5 .um sections were placed on pre-cleaned glass slides and dipped into a 1: 1 aqueous solution of Kodak Nuclear Track Emulsion (NTB-2) maintained at AO’C:Autoradiograms were developed for 2 min in Dektol (17’C) diluted 2: I with water and placed in acid fixer for 5 min. After washing in distilled water the autoradiograms were stained with a 1: 1 mixture of I:‘, methylene blue and 1% azure II. Light micrographs were taken on 35 mm Kodak high contrast copy film. Sections for histological examination by light and electron microscopy were taken from the same retina as was used for autoradiography. Sections for Ii&t microscopy were stained with the methylene blueazure II mixture or a saturated aqueous solution of p-phenylenediamine while thin sections for electron microscopy were placed on either bar or mesh grids. stained with 1% aqueous uranyl acetate (20 min) and lead citrate (10 min). and examined in a Siemens 101 electron microscope. Using structural criteria. two photoreceptor types were easily identified in the electron micrographs One type can be clearly classified as identical to rods described in other species of ground squirrels on the basis of outer segment ultrastructure (Jacobs et al.. 1976; Anderson and Fisher, 1976). the presence of “pale” or “lucent” cytoplasm (West and Dowling. 1975), and shape and organization of the synaptic terminal (West and Dowling. 1975; Jacobs cr al.. 1976) (see Figs. l-4). Using the pale cytoplasma as a criterion for recognizing rods in light microscopy. counts of photoreceptors were made in 1 kernsections taken from the central portion of the eye. Out of 975 receptors 56 (5.7%) could be classified as rods, a value close to that reported for other gound-dwelling sciurids (West and Dowling, 1975; Jacobs t’r nl.. 1976). The outer and inner segments of both rods and cones occupy a single row across the retina. Both types of outer segments are cylindrical and abut the pigment epithelium at the same level (Fig. 1). Rod outer segments are slightly longer and thinner than those of the cones (averaging 8 x 1.5 pm compared
to h.> ‘* 2 rim r:spectivel?~. R~lct o~t;‘r ssgments m the ~11slop2 ground sqtIirrsI ari: somposzd of ;I stack of indspsndznt discs surrounded h! a plasma msmbran2 (Fig. 3) VIhzrsas th2 conr outer ssgments hav2 a continuouslq foldsd msmbranc forming the lower one-third of thsir length (Fig. +--characteristics shartd aith othsr ground squirrel rods and cones (Jacobs rif ctl.. 1976: Andrrson and Fisher. 1976). The photoreceptor tsrminals in this sp2cies ar2 also bcr); simibr to those drscribed in othrr ground squirrels (West and Donling. 1975: Jacobs “r &. 1976) inasmuch as the con2 t2rminals hav2 a typical cxpandsd pedicls shaps n-hi12 ths rods terminate with a blunt rounded shape on ths border of th2 out2r plexiform laqcr (Figs. 1 and 2). As has also been prsxiouslk dsscribzd for other ground squirrel rod terminals ()E’est and Dowling. 1975: Jacobs t’r rd.. lY76i thos2 in this speci2s contain ~2~214 synaptic ribbons (Fig. 2) and organization of post-synaptic proctxses at th2 ribbon synapse often appears as a triadic ~~rrang2ment whm examined in single thin ssctions. In the autoradiograms of the antelope ground squirxl retina two distinct labeling pattrrns wert: seen in the photorrcsptor outzr segments (Fig. 5). Thes2 conform to the labzling patterns consistsntly d2scrib2d for a baristy of v2rtebrate rods and conss (Young. 1967: 1971;~. b: Anderson and Fisher. 1975. 1976). Aftsr 72 hr of labeling Lvith ‘H-lsucine a small population of outsr segments show the accumulation of lab&d protein into a distinct band which is displaced about 3.1 yrn from thz outer ssgment has2 (Fig. 5). C2lIs which show this patttxn of labeling also ha/v2 th2 “pals” c! toplasm characteristic of th2 receptors identified as rods by electron microscopy (Fig. 51. The majorit) of outer segments do not show the accumulation of radioacti\t: protein into discrete bands but rathsr show a diffuse labelin! over th2 entir2 outer segmsnt (Fig. 5). a picture t!plcal of vertebrat2 conss. ERG evidence for rod operation was sought in the sam2 manner as in our previous experiments (Jacobs t’t (II.. 1976). In order to maaimize a possible rod signal. ERGS were rscorded from thoroughly darkadapt2d animals using large field. low-intsnsity stimuli coupled with rssponse avsraging. Specifically. ERGS wsr2 recorded from an2sthetized antslope ground squirrsls bvith insulated stainless steel electrodes lvhich were insertsd through th2 cornea into the post2rior chamber. An indifferent electrode ~vas sewn into th2 skin above the 242. Thz ERG signals were recorded differentially through an amplifizr having a
bandF:lss at‘ IJ.z-IO@.) HZ. Signals t’rom rile ,\nlpiii: dark adapted for :I pcrioc! of at Icast 45 min prior to data collzction. .Aft2r that. dim monochromatic t2st flashes (100 msec duration) \\2rt‘ presented to the 2~2 once ever) 10 s2c. Rrsponses to a minimum of 10 such ilashzs w2r2 averaged for sach ~~~~~elength-intensity combin~~tion. Spectral locations from 40 to 6% nm. in st2ps of 20 nm. were’ csplored in uns>stsmatic ord2r. All ERG mensursments wsre based on the base-to-peak amplitud2 ot‘ the b-\\a\s. Sp2ctral sensitivit) functions were obtained irom the ERG of th2 dark-ndaptsd antelope ground squirrel bq drtzrmining for 2ach test wav212ngth‘the stimulus snrrrgy rsquirsd to produce a criterion signal having an amplituds of IO ,uV. Ths results of this procedurr: ar2 shown in Fig. 6. Ths data shown in the top part of that figure (circles and triangies) ar2 th2 ssnsitivit) vaiuss for IWO antelop ground squirrels (one male. one female). Those sensitivity values are plottsd from an 2qual quanta1 base and they arr: correctsd for the preretinal absorbance oi light bb the lens in this spsciss using measurements reported b! Crcscit2lli and Pollack (1966). The data th2r2fore rspr2s2nt the spectral sensitivity of th2 dark-adapr2d retina of this sp2cies. The agreem2nt hctivcen th2 t\vo animals was remarkably high-thr: mean diffsrencr in sensitivitl; for all test wavelengths \vas less than 0.1 log units. The solid line in th2 top portion of Fig. 6 represents the sensitivity curve: for a nomogram photopigment (Dartnail. 1953) having a pzak at 33.3 nm. The correspondents betusen ths data and the nomogram is vsry good and it is thus apparent that the spectral sensitivity of the ey2 of th2 dark-adaptsd antslope ground squirrel is that eupect2d of a retina containing a typical mammalian rhodopdn. ,\lthough our primar! interest +vas in rods and th2
Fig. I. A low power electron mxropraph of rhe photoreceptor layer rn the anrciope ground squirrel retina. Receptors identified as rods (RI have paler cytoplasm and slightly longer and thinner outer segments than those identified as cones tC). This field of view was selected to demonstr~~fe the pr:sence of rods in the retina of this species: it is, however, atypical in that rods predominate. 4330 x Fig. 2. ,\n electron micrograph of the rod synaptic terminal sho\ving the general shaps of these terminals as well as their pale qtoplasm. the presence of synaptic wsicles. and five sqndptic ribbons. t6.ooox.
Fig. 3. An electron micrograph of the base of a rod outer segment. The stack of discs comprising the outer segment is surrounded b) the plasma membrane of the cdl. li’.joo x Fig. 4. .An electron microgaph of the base of a cone outer segment. The IOWC~ one-third af the cons outer segment is composed of invaginations (arrous) of the plasma membrane. 30.0O~I1 X. Fig. 5. A light micropraph of an autoradiogram of the photoreceptors after 73 hr labeling \%ith ‘Hkucinr. There is accumulation of labeled protein into a distinct band in the rod (R) outer segment (arrow., shils the cons outer segment shosn {bracket) is evitnly labeled over its entire length. 300 y
Figs. 1-5. ~titiny
pccgr 876
877
Research Note
these ground squirrel species may show some individual variation either in the number of rods present or in the effectiveness of the rod contribution to retinal signals. In summary. in both structure and function the retina of the antelope ground squirrel appears similar to the retinas of other ground squirrels recently examined in the sense that it contains an operational rod system in addition to a much more obvious cone system. Whether or not any of the species of grounddwelling sciurids have an all-cone retina remains an experimental question but that possibility appears increasingly unlikely. REFERESCES
D. H. and Fisher S. K. (1975) Disc shedding in rodlike and conelike photoreceptors of tree squirrels. Scitprxr 187. 953-955. Anderson D. H. and Fisher S. K. (1976) The photoreceptors of diurnal squirrels: outer segment structure. disc shedding, and protein renewal. J. L’ltrasrruct. Rrs. (in press). Crescirclli F. (1961) The electrorstinogram of the antelope ground squirrel. b.ision Res. 1. 139-153. Crescitelli F. (1962) Some characteristics of on- and offresponses to Hashes of colored light in ground squirrel visual system. J. camp. Srftrol. 25. 141-151. Crescitelli F. and Pollack J. D. (1965) Color vision in the antelope ground squirrel. Science 150. 131&1318. Crescitclli F. and Pollack J. D. (1966) Investigations into color vision of the ground squirrel. In Aspects of CONpurutirr Ophthnlmo~oy~ (Edited by Graham-Jones 0.). Pergamon Press. Oxford. Crescitrlli F. and Pollack J. D. 11972) Dichromacy in the antelope ground squirrel. i’ision Rex 12. 15j3-iSY6. Dartnall H. J. A. (1953) The interoretation of soectral sensitivitv curves. Br. med. h11. 9.‘24-30. ’ Green D. G. and Dowling J. E. (1975) Electrophysiological evidence for rod-like receptors in the gray squirrel. ground squirrel and prairie dog retinas. J. camp. .~rurol. 159. 161472. Jacobs G. H.. Fisher S. K.. Anderson D. H. and Silverman M. S. (1976) Scotopic and photopic vision in the California ground squirrel: physiological and anatomical evidence. J. camp. Neural. 163, 209-227. Johnston J. P. and Gardner E. (1959) Central connections of the optic nerve in mammals with pure-cone retinae. Anat. Rec. 134. 205-216. West R. W. and Dowling J. E. (1975) Anatomical evidence for cone and rod-like receptors in the gray squirrel. ground squirrel. and prairie dog. J. camp. .Vrwol. 159. 439160. Young R. W. (1967) The renewal of photoreceptor cell outer segments. J. Cdl Biol. 33. 61-72. Young R. W. (197la) An hypothesis to account for a basic distinction between rods and cones. vision Rex. 11. l-j. Young R. W. (1971b) The renewal of rod and cone ourer segments in the rhesus monke:. J. Cell Biol. 19. 303-318. Anderson
Fig. 6. Spectral sensitivity of the ERG recorded from the antelope ground squirrel. The data plotted at the top are sensitivity values for two dark-adapted animals (criterion amplitude of 10 pV). The solid line is that for a nomogram photopigment having a 5OOnm peak. The data shown at the bottom were obtained from a light-adapted antelope ground squirrel (criterion amplitude of 3OpV). All sensitivity values are based on responses to stimuli having equal numbers of quanta at each test wavelength. These values are also corrected for pre-retinal absorbance in this species. eye, we also measured spectral sensitivity for conditions of light adaptation for purposes of comparison. The values shown in the lower half of Fig. 6 represent the spectral sensitivity (criterion amplitude of 30 pV) of the ERG of one antelope ground squirrel whose eye was continuously exposed to an achromatic adaptation light having a luminance of 1 logcd/m’. As noted previously (Crescitelli and Pollack. 1972). this function peaks at about 525 nm and. thus, the Pukinje shift in this species is accompanied by a shift in peak sensitivity of approx 25 nm. Although the data shown in Fig. 6 clearly suggest the operation of rods in the retina of the antelope ground squirrel, it is worth pointing out that in at least one preparation we examined it proved impossible to find unambiguous evidence for a dark-adapted spectral sensitivity function having a 500 nm peak. This is noteworthy because it parallels our experience in much more extensive measurements made on the California ground squirrel (Jacobs er a[.. 1976) in which it also proved impossible to find clear evidence for a function having a 500 nm peak in each of the individuals tested. Insofar as these ERG data are adequate indices. this suggests the possibility that dark-adapted
NOTE GROUND
SQUIRREL’
STEVEXK. FISHER.GERALD H. JACOBS,DOS W. AYDERWN and MARJ-IN S. SIL~ER.~X Departments of Biological Sciences and Psychology. University of California. Santa Barbara. CA 93106, U.S.A.
(Received
13 Janttary
1976; in rrtfsedform
The ground-dwelling sciurids have conventionally been assumed to constitute one of the few groups of animals whose member species have all-cone retinas. However, over the past few months evidence has been presented to show that for at least some of the species in this group the claim for a pure-cone retina cannot be substantiated. Thus, using a variety of anatomical criteria, four species of ground-dwelling sciurids (prairie dogs, Mexican ground squirrels, 1j-line ground squirrels, and California ground squirrels) have all been shown to have a small number of retinal rods in addition to a large population of cones (West and Dowling, 1975; Jacobs. Fisher, Anderson and Silverman. 1976: Anderson and Fisher, 1976). Although the number of rods in these retinas is small. less than 10% of the total number of photoreceptors. it has also been demonstrated that rodgenerated signals can be clearly seen in the electroretinograms (ERGS) recorded from these animals (Green and Dowling. 1975; Jacobs et al., 1976). In addition to the four species referred to above, there is another ground-dwelling sciurid which has been the frequent subject of vision experiments. This animal, the antelope ground squirrel (Ammospermophilrls [eucurus), has been used in behavioral (Crescitelli and Pollack, 1965; t966; 1972). anatomical (Johnston and Gardner. 1959). and eIectrophys~ological (Crescitelli. 1961; 1962) studies of vision. Ail of these studies indicate a belief that the antelope ground squirrel has an all-cone retina; indeed. there are no results in any of these studies inconsistent with this supposition. To determine if the antelope ground squirrel in fact has a pure-cone retina, or whether like the other species recently studied has a small popuIation of rods, we have examined both the anatomy of the photore~ptors in this species and the nature of the ERG recorded from the intact eye. RESULTS
AYD
DtSCLSSlON
In order to anatomically determine the types of photoreceptors present one antelope ground squirrel was prepared for both autoradiographv and electron microscopy. The squirrel (200 g) was injected intraperitonealiy with 30mCi of (4,S-3H)-L-ieucine tsp. ’ This work was supported by research grants (EY-00105. EY-00888, and EY-03602) from the National Eye Institute. The authors thank Dr. J. Lee Kavanau (CCL.\) for providing the squirrels used in this study, 875
I February
1976)
activity 51 Ciimw; 389 Ci/mg; Amersham Searle) dissolved in 2.0 ml of distilled water. After 72 hr the animal was given a lethal dose of sodium pentobarbito1 and fixed by intracardiac perfusion of a glutaraldehyde-picric acid fixative as described in detail elsewhere (Jacobs et al., 1976; Anderson and Fisher. 1976). The retina was postfixed in 2”; buffered osmium tetroxide dehydrated in ethanol, and embedded in Araldite. For autoradiography, 0.5 .um sections were placed on pre-cleaned glass slides and dipped into a 1: 1 aqueous solution of Kodak Nuclear Track Emulsion (NTB-2) maintained at AO’C:Autoradiograms were developed for 2 min in Dektol (17’C) diluted 2: I with water and placed in acid fixer for 5 min. After washing in distilled water the autoradiograms were stained with a 1: 1 mixture of I:‘, methylene blue and 1% azure II. Light micrographs were taken on 35 mm Kodak high contrast copy film. Sections for histological examination by light and electron microscopy were taken from the same retina as was used for autoradiography. Sections for Ii&t microscopy were stained with the methylene blueazure II mixture or a saturated aqueous solution of p-phenylenediamine while thin sections for electron microscopy were placed on either bar or mesh grids. stained with 1% aqueous uranyl acetate (20 min) and lead citrate (10 min). and examined in a Siemens 101 electron microscope. Using structural criteria. two photoreceptor types were easily identified in the electron micrographs One type can be clearly classified as identical to rods described in other species of ground squirrels on the basis of outer segment ultrastructure (Jacobs et al.. 1976; Anderson and Fisher, 1976). the presence of “pale” or “lucent” cytoplasm (West and Dowling. 1975), and shape and organization of the synaptic terminal (West and Dowling. 1975; Jacobs cr al.. 1976) (see Figs. l-4). Using the pale cytoplasma as a criterion for recognizing rods in light microscopy. counts of photoreceptors were made in 1 kernsections taken from the central portion of the eye. Out of 975 receptors 56 (5.7%) could be classified as rods, a value close to that reported for other gound-dwelling sciurids (West and Dowling, 1975; Jacobs t’r nl.. 1976). The outer and inner segments of both rods and cones occupy a single row across the retina. Both types of outer segments are cylindrical and abut the pigment epithelium at the same level (Fig. 1). Rod outer segments are slightly longer and thinner than those of the cones (averaging 8 x 1.5 pm compared
to h.> ‘* 2 rim r:spectivel?~. R~lct o~t;‘r ssgments m the ~11slop2 ground sqtIirrsI ari: somposzd of ;I stack of indspsndznt discs surrounded h! a plasma msmbran2 (Fig. 3) VIhzrsas th2 conr outer ssgments hav2 a continuouslq foldsd msmbranc forming the lower one-third of thsir length (Fig. +--characteristics shartd aith othsr ground squirrel rods and cones (Jacobs rif ctl.. 1976: Andrrson and Fisher. 1976). The photoreceptor tsrminals in this sp2cies ar2 also bcr); simibr to those drscribed in othrr ground squirrels (West and Donling. 1975: Jacobs “r &. 1976) inasmuch as the con2 t2rminals hav2 a typical cxpandsd pedicls shaps n-hi12 ths rods terminate with a blunt rounded shape on ths border of th2 out2r plexiform laqcr (Figs. 1 and 2). As has also been prsxiouslk dsscribzd for other ground squirrel rod terminals ()E’est and Dowling. 1975: Jacobs t’r rd.. lY76i thos2 in this speci2s contain ~2~214 synaptic ribbons (Fig. 2) and organization of post-synaptic proctxses at th2 ribbon synapse often appears as a triadic ~~rrang2ment whm examined in single thin ssctions. In the autoradiograms of the antelope ground squirxl retina two distinct labeling pattrrns wert: seen in the photorrcsptor outzr segments (Fig. 5). Thes2 conform to the labzling patterns consistsntly d2scrib2d for a baristy of v2rtebrate rods and conss (Young. 1967: 1971;~. b: Anderson and Fisher. 1975. 1976). Aftsr 72 hr of labeling Lvith ‘H-lsucine a small population of outsr segments show the accumulation of lab&d protein into a distinct band which is displaced about 3.1 yrn from thz outer ssgment has2 (Fig. 5). C2lIs which show this patttxn of labeling also ha/v2 th2 “pals” c! toplasm characteristic of th2 receptors identified as rods by electron microscopy (Fig. 51. The majorit) of outer segments do not show the accumulation of radioacti\t: protein into discrete bands but rathsr show a diffuse labelin! over th2 entir2 outer segmsnt (Fig. 5). a picture t!plcal of vertebrat2 conss. ERG evidence for rod operation was sought in the sam2 manner as in our previous experiments (Jacobs t’t (II.. 1976). In order to maaimize a possible rod signal. ERGS were rscorded from thoroughly darkadapt2d animals using large field. low-intsnsity stimuli coupled with rssponse avsraging. Specifically. ERGS wsr2 recorded from an2sthetized antslope ground squirrsls bvith insulated stainless steel electrodes lvhich were insertsd through th2 cornea into the post2rior chamber. An indifferent electrode ~vas sewn into th2 skin above the 242. Thz ERG signals were recorded differentially through an amplifizr having a
bandF:lss at‘ IJ.z-IO@.) HZ. Signals t’rom rile ,\nlpiii: dark adapted for :I pcrioc! of at Icast 45 min prior to data collzction. .Aft2r that. dim monochromatic t2st flashes (100 msec duration) \\2rt‘ presented to the 2~2 once ever) 10 s2c. Rrsponses to a minimum of 10 such ilashzs w2r2 averaged for sach ~~~~~elength-intensity combin~~tion. Spectral locations from 40 to 6% nm. in st2ps of 20 nm. were’ csplored in uns>stsmatic ord2r. All ERG mensursments wsre based on the base-to-peak amplitud2 ot‘ the b-\\a\s. Sp2ctral sensitivit) functions were obtained irom the ERG of th2 dark-ndaptsd antelope ground squirrel bq drtzrmining for 2ach test wav212ngth‘the stimulus snrrrgy rsquirsd to produce a criterion signal having an amplituds of IO ,uV. Ths results of this procedurr: ar2 shown in Fig. 6. Ths data shown in the top part of that figure (circles and triangies) ar2 th2 ssnsitivit) vaiuss for IWO antelop ground squirrels (one male. one female). Those sensitivity values are plottsd from an 2qual quanta1 base and they arr: correctsd for the preretinal absorbance oi light bb the lens in this spsciss using measurements reported b! Crcscit2lli and Pollack (1966). The data th2r2fore rspr2s2nt the spectral sensitivity of th2 dark-adapr2d retina of this sp2cies. The agreem2nt hctivcen th2 t\vo animals was remarkably high-thr: mean diffsrencr in sensitivitl; for all test wavelengths \vas less than 0.1 log units. The solid line in th2 top portion of Fig. 6 represents the sensitivity curve: for a nomogram photopigment (Dartnail. 1953) having a pzak at 33.3 nm. The correspondents betusen ths data and the nomogram is vsry good and it is thus apparent that the spectral sensitivity of the ey2 of th2 dark-adaptsd antslope ground squirrel is that eupect2d of a retina containing a typical mammalian rhodopdn. ,\lthough our primar! interest +vas in rods and th2
Fig. I. A low power electron mxropraph of rhe photoreceptor layer rn the anrciope ground squirrel retina. Receptors identified as rods (RI have paler cytoplasm and slightly longer and thinner outer segments than those identified as cones tC). This field of view was selected to demonstr~~fe the pr:sence of rods in the retina of this species: it is, however, atypical in that rods predominate. 4330 x Fig. 2. ,\n electron micrograph of the rod synaptic terminal sho\ving the general shaps of these terminals as well as their pale qtoplasm. the presence of synaptic wsicles. and five sqndptic ribbons. t6.ooox.
Fig. 3. An electron micrograph of the base of a rod outer segment. The stack of discs comprising the outer segment is surrounded b) the plasma membrane of the cdl. li’.joo x Fig. 4. .An electron microgaph of the base of a cone outer segment. The IOWC~ one-third af the cons outer segment is composed of invaginations (arrous) of the plasma membrane. 30.0O~I1 X. Fig. 5. A light micropraph of an autoradiogram of the photoreceptors after 73 hr labeling \%ith ‘Hkucinr. There is accumulation of labeled protein into a distinct band in the rod (R) outer segment (arrow., shils the cons outer segment shosn {bracket) is evitnly labeled over its entire length. 300 y
Figs. 1-5. ~titiny
pccgr 876
877
Research Note
these ground squirrel species may show some individual variation either in the number of rods present or in the effectiveness of the rod contribution to retinal signals. In summary. in both structure and function the retina of the antelope ground squirrel appears similar to the retinas of other ground squirrels recently examined in the sense that it contains an operational rod system in addition to a much more obvious cone system. Whether or not any of the species of grounddwelling sciurids have an all-cone retina remains an experimental question but that possibility appears increasingly unlikely. REFERESCES
D. H. and Fisher S. K. (1975) Disc shedding in rodlike and conelike photoreceptors of tree squirrels. Scitprxr 187. 953-955. Anderson D. H. and Fisher S. K. (1976) The photoreceptors of diurnal squirrels: outer segment structure. disc shedding, and protein renewal. J. L’ltrasrruct. Rrs. (in press). Crescirclli F. (1961) The electrorstinogram of the antelope ground squirrel. b.ision Res. 1. 139-153. Crescitelli F. (1962) Some characteristics of on- and offresponses to Hashes of colored light in ground squirrel visual system. J. camp. Srftrol. 25. 141-151. Crescitelli F. and Pollack J. D. (1965) Color vision in the antelope ground squirrel. Science 150. 131&1318. Crescitclli F. and Pollack J. D. (1966) Investigations into color vision of the ground squirrel. In Aspects of CONpurutirr Ophthnlmo~oy~ (Edited by Graham-Jones 0.). Pergamon Press. Oxford. Crescitrlli F. and Pollack J. D. 11972) Dichromacy in the antelope ground squirrel. i’ision Rex 12. 15j3-iSY6. Dartnall H. J. A. (1953) The interoretation of soectral sensitivitv curves. Br. med. h11. 9.‘24-30. ’ Green D. G. and Dowling J. E. (1975) Electrophysiological evidence for rod-like receptors in the gray squirrel. ground squirrel and prairie dog retinas. J. camp. .~rurol. 159. 161472. Jacobs G. H.. Fisher S. K.. Anderson D. H. and Silverman M. S. (1976) Scotopic and photopic vision in the California ground squirrel: physiological and anatomical evidence. J. camp. Neural. 163, 209-227. Johnston J. P. and Gardner E. (1959) Central connections of the optic nerve in mammals with pure-cone retinae. Anat. Rec. 134. 205-216. West R. W. and Dowling J. E. (1975) Anatomical evidence for cone and rod-like receptors in the gray squirrel. ground squirrel. and prairie dog. J. camp. .Vrwol. 159. 439160. Young R. W. (1967) The renewal of photoreceptor cell outer segments. J. Cdl Biol. 33. 61-72. Young R. W. (197la) An hypothesis to account for a basic distinction between rods and cones. vision Rex. 11. l-j. Young R. W. (1971b) The renewal of rod and cone ourer segments in the rhesus monke:. J. Cell Biol. 19. 303-318. Anderson
Fig. 6. Spectral sensitivity of the ERG recorded from the antelope ground squirrel. The data plotted at the top are sensitivity values for two dark-adapted animals (criterion amplitude of 10 pV). The solid line is that for a nomogram photopigment having a 5OOnm peak. The data shown at the bottom were obtained from a light-adapted antelope ground squirrel (criterion amplitude of 3OpV). All sensitivity values are based on responses to stimuli having equal numbers of quanta at each test wavelength. These values are also corrected for pre-retinal absorbance in this species. eye, we also measured spectral sensitivity for conditions of light adaptation for purposes of comparison. The values shown in the lower half of Fig. 6 represent the spectral sensitivity (criterion amplitude of 30 pV) of the ERG of one antelope ground squirrel whose eye was continuously exposed to an achromatic adaptation light having a luminance of 1 logcd/m’. As noted previously (Crescitelli and Pollack. 1972). this function peaks at about 525 nm and. thus, the Pukinje shift in this species is accompanied by a shift in peak sensitivity of approx 25 nm. Although the data shown in Fig. 6 clearly suggest the operation of rods in the retina of the antelope ground squirrel, it is worth pointing out that in at least one preparation we examined it proved impossible to find unambiguous evidence for a dark-adapted spectral sensitivity function having a 500 nm peak. This is noteworthy because it parallels our experience in much more extensive measurements made on the California ground squirrel (Jacobs er a[.. 1976) in which it also proved impossible to find clear evidence for a function having a 500 nm peak in each of the individuals tested. Insofar as these ERG data are adequate indices. this suggests the possibility that dark-adapted
