Ординатура / Офтальмология / Английские материалы / Eye, Retina, and Visual System of the Mouse_Chalupa, Williams_2008

Aqueous humor inflow

Aqueous humor in the anterior eye is a clear fluid that contains the nutrients needed by the lens and the cornea, tissues that need to be avascular to serve their respective optical functions. The site of aqueous humor formation is the ciliary body, where aqueous humor is secreted into the posterior chamber by an active, energy-dependent secretory process involving the ciliary epithelium. The structural details of the ciliary epithelium that enable its secretory function have been extensively studied in humans and traditional laboratory animal models of eye research, such as monkeys and rabbits, and have been reviewed elsewhere (Tamm and Lütjen-Drecoll, 1996). In contrast, the existing information on the structure of the ciliary epithelium in the mouse eye is limited. The available data, however, strongly indicate no major structural or functional differences between mouse ciliary epithelium and that of other mammalian species. Accordingly, the ciliary epithelium in the mouse eye consists of two layers (figure 9.1). The outer layer is formed by the pigmented ciliary epithelium (PE), which is continuous with the retinal pigmented epithelium in the back of the eye and derives from the outer layer of the optic cup during embryonic eye development (Pei and Rhodin, 1970). The inner layer is formed by the nonpigmented ciliary epithelium (NPE), which is continuous with the retina and shares its origin from the inner layer of the optic cup. The mouse ciliary epithelium expresses critical requirements for aqueous humor secretion such as tight junctions between NPE cells as the site of the blood-aqueous barrier (Calera et al., 2006), and Na+/Ka+-ATPase for secretory activity (Wetzel and Sweadner, 2001). As in other mammalian species (Tamm and Lütjen-Drecoll, 1996), both epithelial layers are extensively coupled by gap junctions (Calera et al., 2006) and work synergistically to achieve aqueous humor secretion by facilitating movement of NaCl and water between NPE and PE (Civan and Macknight, 2004). Accordingly, in mutant mice that express no gap junctions between NPE and PE, secretion of aqueous humor is critically impaired (Calera et al., 2006). The total volume of aqueous humor in the mouse eye has been reported at 5.9 ± 0.5 μl, and the rate of aqueous humor formation as measured in the NIH Swiss

White mouse has been reported at 0.18 ± 0.05 μl/min (Aihara et al., 2003a).

The ciliary processes covered by NPE and PE (pars plicata region) show a more irregular arrangement than that of other species, as they are not strictly parallel but rather overlap and intersect with each other (Napier and Kidson, 2005). As in other mammalian species, the fibers of the zonular apparatus are mainly attached to the NPE in the valleys between the ciliary processes and in the pars plana region, behind the ciliary processes and immediately adjacent to the sensory retina (figures 9.1 and 9.2).

The ciliary muscle in the mouse eye is localized in the posterior parts of the ciliary body, underneath the pars plana region of the ciliary epithelium (see figure 9.1). It is considerably smaller than in humans and higher primates (Tamm, 2002). In meridional or sagittal sections through the anterior eye, mouse ciliary muscle usually consists of only four to six smooth muscle cells, indicating there is no significant accommodative power in the mouse eye.

Aqueous humor outflow

After it is secreted into the posterior chamber, aqueous humor passes through the pupil into the anterior chamber. It leaves the eye via two outflow pathways, the conventional outflow pathway, through the trabecular meshwork, and the unconventional or uveoscleral outflow pathway, through the ciliary muscle, and the supraciliary and suprachoroidal spaces. The entrance to both outflow pathways is localized in the iridocorneal or chamber angle of the anterior chamber. The architecture of the outflow pathways in the mouse eye shows several distinct structural differences from that in humans or primates. The most obvious difference relates to the position of the ciliary body. In humans and primates, the ciliary body is localized posterior to the trabecular meshwork outflow pathways. In contrast, in the mouse eye, the root of the ciliary body completely covers the trabecular meshwork outflow pathways (see figure 9.1A and B). The ciliary body is attached to the cornea by pectinate ligaments that originate near the junction between iris and ciliary body and attach to the periphery of the cornea anterior to the trabecular meshwork. Aqueous humor passes through the pectinate

Figure 9.1 Light micrographs of Schlemm’s canal (SC) and trabecular meshwork (white arrows) in the chamber angle of a Balb/c mouse (A) and a C57/Bl6 mouse (B) at 6 weeks of age. Internal to the trabecular meshwork is the root of the ciliary body, which is connected to the periphery of the cornea by strands of the pectinate ligament (PL, black arrow). Posterior to the pectinate ligament are uveal connective tissue strands with large spaces in between (Fontana’s spaces [FS]), which are connected to the anterior chamber (AC) and filled with aqueous humor. At the apex of the ciliary body, ciliary processes (CP) are formed. Behind the ciliary processes, in the pars plana region, zonular fibers (ZF) take their origin from the nonpigmented ciliary epithelium. In the C57/Bl6 background, the chamber angle is somewhat narrower than in the Balb/c background, and Fontana’s spaces are smaller. CM, ciliary muscle; NPE, nonpigmented ciliary epithelium; PE, pigmented ciliary epithelium.

ligaments into a meshwork of uveal connective tissue strands that are covered by flat cells. Between the uveal strands are large empty spaces, termed “Fontana’s spaces” (Duke-Elder, 1958; Franz, 1934; Rohen, 1964, 1982; Tripathi, 1977). The number of uveal strands and Fontana’s spaces, as well as the width of the iridocorneal angle, differs between different mouse strains. Balb/c mice have usually a wide-open angle with numerous uveal strands (see figure 9.1A), whereas the angle is somewhat narrower in mice with a C57/Bl6 background (see figure 9.1B). A detailed analysis of the structural

Figure 9.2 Frontal section through Schlemm’s canal (SC) in a Balb/c mouse at 6 weeks of age. Schlemm’s canal (white arrows) is clearly circumferentially oriented and covered along its entire length by the trabecular meshwork (black arrows). Collector channels (CC) in the sclera (S) are in places in contact with Schlemm’s canal (white arrowhead). White stars mark connective tissue septa in the lumen of Schlemm’s canal. Internal to the trabecular meshwork is the root of the ciliary body, consisting of a meshwork of uveal connective tissue strands. Large open spaces (Fontana’s spaces [FS]) that are open to the anterior chamber and are filled with aqueous humor are localized between the strands. At the apex of the ciliary body, ciliary processes (CP) are found at regular intervals. In the valley between the processes, zonular fibers (ZF) take their origin from the nonpigmented ciliary epithelium (double arrows).

differences of the outflow pathways in different mouse strains is beyond the scope of this discussion. It should be noted, however, that in eyes fixed by immersion only, the iridocorneal angle of the mouse eye is usually considerably collapsed. To get an impression of the architecture of the outflow pathways under the normal condition of continuous flow of aqueous humor, eyes need to be fixed by perfusion via the heart, or, even better, by perfusion with fixative via the anterior chamber.

After entering the spaces between the uveal strands of the iridocorneal angle, aqueous humor either passes into Schlemm’s canal via the trabecular meshwork or enters the ciliary muscle and the supraciliary spaces of the uveoscleral outflow pathways. It is reasonable to assume that, as in other mammalian species ( Johnson, 2006; Johnson and Erickson, 2000), the mouse trabecular meshwork outflow pathways are the site that provides resistance to aqueous humor outflow. In response to this resistance, intraocular pressure (IOP) is generated, until the pressure is high enough to drive aqueous humor out of the eye, against the resistance. IOP varies considerably between individual mouse strains. In strains with low IOP, such as Balb/c, IOP is 10 mm Hg, whereas in strains with higher pressure, such as CBA/J, IOP has been measured at 18 mm Hg (Savinova et al., 2001). So far, no structural or molecular differences between the outflow pathways of strains with low or high IOP have been identified. IOP in the mouse eye shows a 24-hour pattern, with low levels at daytime and higher levels during nighttime (Aihara et al., 2003c).

Similar to the situation in humans and primates and very unlike to that in rabbits, pigs and cows, Schlemm’s canal in the mouse eye is a circumferentially oriented vessel (see figure 9.2). Schlemm’s canal is in contact with collector channels in the sclera (see figure 9.2), which will carry aqueous humor to the episcleral vessel system outside the eye (Aihara et al., 2003b). In places, there are connective tissue septa present in the lumen of Schlemm’s canal that are in contact with the connective tissue of the adjacent sclera (see figure 9.2).

The trabecular meshwork

As in humans and primates, the mouse trabecular meshwork is divided into an inner part, which consists of connective tissue strands or lamellae that are covered by flat trabecular cells, and an outer juxtacanalicular connective tissue (JCT; figure 9.3) (Tamm et al., 1999). The inner part usually does not form more than one or two lamellae, which are fixed posteriorly at the sclera behind Schlemm’s canal and anteriorly at the periphery of the cornea. Accordingly, the inner part of the mouse trabecular meshwork resembles the corneoscleral meshwork in the human eye. The outer JCT does not form organized lamellae but resembles a loose connective tissue that is 1–2 μm wide in the mouse eye. In the JCT, there are trabecular cells that are in contact with the endothelial layer of Schlemm’s canal and with the cells covering the strands of the outer corneoscleral meshwork. The intercellular spaces of the JCT are filled with sparse extracellular matrix components, mostly in the form of fine fibrillar material (Tamm et al., 1999). In the posterior spaces of the JCT, electron-dense fiber bundles are present that resemble the elastic-like fibers, which are a characteristic component of

Figure 9.3 Electron micrograph of the trabecular meshwork in a C57/Bl6 mouse at 6 weeks of age. The meshwork consists of an inner part with two connective tissue lamellae (arrows) covered by flat trabecular meshwork cells, and the outer juxtacanalicular connective tissue ( JCT) next to the endothelium of the inner wall of Schlemm’s canal (SC).

the JCT in the human eye (Lütjen-Drecoll and Rohen, 2001). In humans, the elastic-like fibers in the JCT are covered by a sheath of largely amorphous material that increases with age. In patients with advanced primary openangle glaucoma (POAG), this increase is even more pronounced, and the sheath material forms the “sheath-derived plaque material,” a hallmark of the JCT in eyes with POAG. Elastic-like fibers in the mouse trabecular meshwork appear not to be surrounded by a sheath material comparable to that in humans. The fetal development of the mouse trabecular meshwork has been reviewed recently (Cvekl and Tamm, 2004; Smith et al., 2001) and is not discussed here.

The cells of Schlemm’s canal inner wall endothelium in the mouse eye are connected by tight junctions and adher- ens-type junctions, very similar to the situation that is found in the human eye (Ethier, 2002; Johnson, 2006; Johnson and Erickson, 2000). When eyes are fixed by perfusion, numerous outpouchings of the inner wall into the lumen of Schlemm’s canal are found. The outpouchings, which form in response to aqueous humor flow, are characteristic of the inner wall endothelium in the mammalian eye and have been called giant vacuoles (Ethier, 2002; Johnson, 2006; Tripathi, 1977; Tripathi and Tripathi, 1972). A second characteristic feature of the inner wall endothelium is the presence of large intraor intercellular pores that open in response to aqueous humor flow and are often associated with giant vacuoles. Although pores have not been systematically analyzed in the mouse inner wall endothelium, they can be readily observed (figure 9.4D), indicating there is no substantial difference between the mouse inner wall endothelium and that of other mammalian species.

In summary, the structural details of the trabecular meshwork outflow pathways are remarkably similar between the

mouse eye and that of higher primates and humans, and the mouse appears to be an excellent animal model for molecular and experimental studies on the mechanisms that control aqueous humor outflow resistance.

Uveoscleral outflow pathways

In contrast to trabecular meshwork outflow, uveoscleral outflow is anatomically less well defined and understood. The pathway was first described by Anders Bill, who observed that large tracers, as markers of bulk flow, exited the anterior chamber through the ciliary body into the supraciliary space and out through the sclera into the extraocular tissues (Bill, 1975). Uveoscleral outflow is largely pressure independent and does not contribute substantially to the formation of IOP (Johnson and Erickson, 2000). Fluid in this pathway ultimately drains into the lymphatic system. Comparable findings have been reported for the mouse eye after injection of fluorescent 70-kDa dextran into the anterior chamber of NIH Swiss mice (Lindsey and Weinreb, 2002). After injection, tracer was observed first in the ciliary muscle and

Figure 9.4 Ultrastructural details of the juxtacanalicular region in the trabecular meshwork of a C57/Bl6 mouse. The intercellular spaces in the JCT are filled with fine fibrillar material (arrows in A), and by electron-dense elastic-like fibers (arrows in B). The endothelial cells (En) of Schlemm’s canal (SC) inner wall endothelium are

connected with each other by tight junctions (white arrows in C) and adherens-type junctions (black arrows in C). When eyes are fixed by perfusion, numerous outpouchings (“giant vacuoles” [GV]) of the inner wall endothelium are present. Pores in the inner wall endothelium are often found at the apex of giant vacuoles (arrow in D).

anterior choroid, and later in the equatorial choroid and within the stroma of the adjacent equatorial sclera. According to data of Aihara and co-workers (2003a), almost 80% of the aqueous humor leaves the mouse eye via the uveoscleral pathways. Assuming these data are correct, uveoscleral outflow in the mouse eye would be considerably higher as in other mammalian species such as humans, monkeys, and rabbits (Bill, 1962, 1971, 1975; Bill and Phillips, 1971). In mutant mice with larger abnormalities of the chamber angle, such as heterozygous Pax6 −/+ mice that do not form a differentiated trabecular meshwork and in which Schlemm’s canal is absent (Baulmann et al., 2002), aqueous humor outflow probably uses exclusively the uveoscleral outflow pathways.

acknowledgments Work was supported by a grant from the Deutsche Forschungsgemeinschaft (TA 115/15-1). We thank Margit Schimmel for expert technical assistance.

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Functions of cones

Cone photoreceptors subserve day vision; their most critical function is to enable the retina to generate signals in the presence of bright light, escaping the saturation that rods undergo at modest intensity (Rodieck, 1998). In addition to this primary function, cones in retinas expressing multiple cone opsins provide the signals necessary for discriminating objects on the basis of the spectral content of the light they reflect. In humans, cones are the dominant photoreceptor type in the macula, the portion of the retina corresponding to the central 5° of visual space, which projects massively to approximately 10 cm2 (37%) of primary visual cortex (V1). Because of the distinct and critical roles that cones play in normal human vision, cone disease and death, such as occur in age-related macular degeneration, are devastating.

To investigate the molecular mechanisms that allow cones to perform their unique functions and the molecular mechanisms of cone-specific disease, there is a need for mammalian models that allow (1) manipulation of genes expressed in cones, (2) molecular and biochemical characterization of the protein products of such genes, and (3) electrophysiological analysis of cones and their neural circuits. The mouse has become the mammal of choice for the investigation of organ function and the molecular mechanisms of disease. There are a number of reasons for this choice, including the genomic proximity of mice to humans, the array of molecular tools for making targeted gene manipulations in mice, the vast knowledge base of molecular, cellular, and behavioral experimentation involving mice, the relatively short generation time, and the economics of mouse husbandry. Despite these compelling reasons, investigation of the functional consequences of molecularly manipulated conespecific genes in mice has been an elusive goal. Our laboratory has been investigating mouse cones for a number of years, initially with electroretinographic (ERG) methods (Lyubarsky et al., 1999, 2000, 2001) and more recently with single-cell recordings (Nikonov et al., 2005, 2006). This

chapter summarizes our progress and provides a prospectus. We begin with a summary of the basic features of mouse cones.

The anatomy of mouse cones

Numerosity and Retinal Disposition The adult C57BL/6 mouse eye is approximately a sphere of diameter 3.3 mm, and the retina a hemispheric surface of area of 18 mm2 and average thickness of 200 μm; the photoreceptor layer itself is about 100 μm thick (reviewed in Lyubarsky and Pugh, 2004). An excellent inventory of the major cell types of the C57BL/6 mouse retina has been provided, with rods shown to outnumber all other cell types by more than 20:1: at an average density of 4.4 × 105 mm−2, the adult C57BL/6 retina comprises 6.4 × 106 rod photoreceptors ( Jeon et al., 1998). Cones constitute about 3% of the total photoreceptors in the C57BL/6 retina (Carter-Dawson and LaVail, 1979; Jeon et al., 1998), numbering approximately 200,000. Overall, the cone density in the mouse retina appears uniform (there being no fovea or retinal streak), and corresponds to about one cone per retinal pigment epithelium (RPE) cell. It might be thought that the relative paucity of cones implies that the mouse retina has minimal cone signaling capacity. However, several lines of evidence point to the contrary conclusion, among them the large number of cone bipolars ( Jeon et al., 1998), whose depolarizing (“ON”) subclass produces a large ERG b-wave (Lyubarsky et al., 1999).

Mouse cones have a disposition in the photoreceptor layer distinct from that of rods in at least three ways. First, the cell bodies of cones are invariably located at the outermost of the approximately 11–12 rows of nuclei in the outer nuclear layer (ONL), just vitread to the outer limiting membrane (OLM), whereas rod nuclei are distributed throughout the OLM. Second, cone outer segments (OSs) are shorter than those of rods, typically have their base (where disc synthesis occurs) closer to the OLM than rods do, and terminate on average about 10 μm short of the tips of the rod OSs, which

are in contact with the apical surface of RPE cells (CarterDawson and LaVail, 1979). Some of these distinctive features of mouse cones are illustrated in figure 10.1.

Ultrastructure of Mouse Cones The classic investigation of the ultrastructure of mouse cones is that of CarterDawson and LaVail (1979). These investigators found that on average, C57BL/6 mouse cone OSs are shorter than rod OSs (15 μm vs. 25 μm) and somewhat narrower (1.2 μm vs. 1.4 μm), and the cone disc repeat frequency is slightly lower (38 μm−1) than that of rods (41 μm−1). The intradiscal spaces of mouse cone OSs are not all “patent” (open to the extracellular space), as in the classic picture derived from nonmammalian vertebrates such as fishes and amphibia; however, the base of the mouse cone OS typically has 15 or more patent discs, while in the rod typically only five to seven are patent. Moreover, in the more distal OS, patent discs are occasionally seen in mouse cones, but never in rods. Similar ultrastructural details differentiating cones from rods are generally common among mammals (Arikawa et al., 1992).

Retinal Connectivity of Mouse Cones Before focusing further on mouse cones, it is useful to mention briefly some general issues in neuroscience that will be facilitated by their investigation. One such issue is synaptic transmission. The cone pedicle is arguably one of the most complex synaptic specializations in the CNS, comprising as it does 20– 50 presynaptic ribbons, each flanked by numerous vesicles

Figure 10.1 Disposition and numerosity of mouse cones. The image superimposes confocal DIC and fluorescence image of a frozen section of the photoreceptor and RPE cell layer of a mouse retina. The mouse from whose eye the section was made expressed EGFP under the human M/L cone promoter (Fei and Hughes, 2001). See color plate 2.

in preparation for synaptic release (Haverkamp et al., 2000; Wässle, 2004). The synaptic transmitter, glutamate, is released maximally in the dark-adapted state, when cones are most depolarized, and the release is suppressed when the cones hyperpolarize to light. Thus, the pedicle is specialized for massive packaging and delivery of synaptic vesicles, together with recycling of the membrane and transmitter. The full complexity of the cone pedicle, however, is manifest in the variety of postsynaptic contacts.

Mouse cone pedicles, like those of other mammals, make synaptic contact with nine morphologically distinct bipolar cells, which to some extent can also be distinguished by molecular markers (Haverkamp et al., 2003; Ghosh et al., 2004), as well as with several classes of horizontal cells. The nine bipolar types are divided into two principal classes, depolarizing bipolar cells (DBCs) and hyperpolarizing bipolar cells (HBCs), and each class likely appears further divided into “sustained” and “transient” subclasses (DeVries, 2000). DBCs make sign-inverting connections at ribbon synapses, while HBCs make sign-conserving synapses at flat junctions. The sign inversion of the cone → DBC synapse is effected by a GPCR cascade initiated by a metabotropic glutamate receptor (mGluR6) that activates a G protein, Go (Dhingra et al., 2000), leading through yet undetermined signaling components to the opening of cation channels near the invaginating dendritic tips of the bipolars. The signconserving cone → HBC synapses are made via ionotropic (AMPA) receptors.

The significance of the cone synapses and cone bipolars, in the context of the work discussed here describing the physiology of mouse cones, is that the mouse now offers a valuable preparation for investigating the role of many specific molecules in these complex synapses. Thus, for example, synaptic vesicle protein 2 (SVP2) has three isoforms (Sv2a, b, c) expressed in the mouse retina: Sv2a is expressed only in cone synaptic terminals, Sv2b in the ribbon synapses of cones, rods, and bipolar cells, and Sv3c in starburst amacrine terminals (Wang et al., 2003). Genetic manipulations of Sv2s in mouse will be the most likely path to resolving the distinct roles of the different Sv2 isoforms. But assessing the functional consequences of such manipulations requires the investigator to have a clear understanding of the distinctive functional features of the mouse cone system, which we now describe.

Mouse Cone S- and M-opsins; Dorsoventral Gradient of M-Opsin Expression; Coexpression of S and M Cone

Opsins The mouse retina, like that of most mammals, expresses, in addition to the rod pigment rhodopsin, two cone opsins, a mid-wavelength-sensitive opsin (M-opsin) and a short-wavelength-sensitive opsin (S-opsin) (Szel et al., 1992, 1993). (In many primates, including human, three distinct cone opsins are expressed, but two of the three are so closely

homologous—ca. 98% in man [Nathans et al., 1986]—that only two distinct classes of cone opsin are considered to be present.) The two mouse cone opsins have λmax values of approximately 510 nm (M-opsin) and approximately 360 nm (S-opsin), respectively (Jacobs et al., 1991; Sun et al., 1997; Yokoyama et al., 1998). Mouse S-opsin is a member of the SWS1 subfamily of opsins, to which human S-opsin also belongs, while mouse M-opsin belongs to the LW1 subfamily, whose members include the human L and M cone pigments (Yokoyama, 1996; Yokoyama and Yokoyama, 2000; Ebrey and Koutalos, 2001). In mice, M-opsin is expressed in a dorsoventral gradient, with a much higher level of M-opsin in cones of the most dorsal retina, and S-opsin is more purely expressed in the ventral retina (Szel et al., 1992, 1993; Applebury et al., 2000). Most cones of the mouse coexpress both S- and M-opsin, and this coexpression itself exhibits a dorsoventral gradient (Applebury et al., 2000). As discussed in the next section, in individual cones that coexpress them, both S- and M-opsins drive phototransduction.

total population (rod DBCs constituting the remaining twothirds), then the maximum cone DBC postsynaptic current can be estimated to be 500–1,000 pA (Lyubarsky et al., 1999), using the rod a-wave and circulating current as a gauge (figure 10.2).

The b-wave action spectrum comprises two peaks, at approximately 360 nm and 510 nm, corresponding to the S and M cone opsins, respectively (see figure 10.2), with the S-opsin component fourfold more sensitive than the M-opsin component (Lyubarsky et al., 1999). The conedriven b-wave is characterized by oscillatory potentials (Wachtmeister, 1998; Dong et al., 2004), which are negligible or minimal in purely scotopic b-waves. Reliable differences between oscillatory potentials associated with dim-flash responses driven by S-opsin and M-opsin suggest subtle differences in the light-evoked currents of cone DBCs and/or of subsequent retinal neurons with radial extent driven by these bipolar cells (see Lyubarsky et al., 1999, their figures 4 and 9).

Electroretinographic studies of mouse cone function

A full-field ( ganzfeld ) flash delivered to the eye evokes a lightdependent, massed potential, the ERG, which arises from light-driven changes in cellular currents flowing radially in the extracellular spaces of the retina. The initial, cornealnegative potential is known as the a-wave and the subsequent corneal-positive component is known as the b-wave. The a-wave originates primarily from suppression of the photoreceptor “dark” or circulating current (Hagins et al., 1970), while the b-wave has its origin primarily in activation of the light-driven, postsynaptic currents of depolarizing (ON) bipolar cells (DBCs), coupled to the photoreceptors by a metabotropic (mGluR6) receptor cascade (Robson and Frishman, 1995; reviewed in Pugh et al., 1998; Robson and Frishman, 1998).

The Mouse Cone-Driven (Photopic) b-Wave The flashactivated, rodent cone-driven b-wave was first characterized in albino rats by Green (1973). Peachey subsequently characterized aspects of the cone-driven b-wave in mice (Peachey et al., 1993). The ultraviolet (UV) sensitivity of mouse cones was discovered by Jacobs and colleagues, using the flicker ERG (Jacobs et al., 1991). Lyubarsky et al. (1999) extensively investigated the cone-driven b-wave of the C57BL/6 mouse: about one-third of the saturated amplitude (ca. 650 μV) of the dark-adapted mouse b-wave was found to arise from cone-driven cells, so that suppression of the rod a-wave and b-wave with steady background light or strong flashes completely isolates an approximately 180 μV cone component. If this component can be attributed to the complete activation of the postsynaptic current of DBCs, and if cone DBCs constitute approximately one-third of the

Figure 10.2 Spectral sensitivity of the mouse cone-driven ERG b-wave. Each point represents the sensitivity of the mouse conedriven b-wave to brief (ca. 1 ms) flashes of light, expressed in photon units (mean ± SEM, n = 3 or more mice). The data have been normalized at 510 nm. The smooth curves are photopigment templates (Lamb, 1995), with λmax values of 360 nm and 510 nm, respectively, corresponding to the λmax values of the two mouse cone photopigments (see the text), but in the case of the M cone opsin, extended in the near UV, according to Lyubarsky et al. (1999). Note that the sensitivity at the secondary maximum at 510 nm is approximately one-fourth that of the primary maximum at 360 nm. The sensitivity of the secondary maximum at 510 nm varies with age, strain, and light-rearing conditions, varying from one-half to one-sixth that of the primary maximum (Lyubarsky et al., 2002). Other investigations have found systematic variation in the number and responsivity of cones with age and strain in mice (Gresh et al., 2003).