Electroretinographic evidence for a photopic system in the rat

Properties of rat cone-mediated electroretinograms during light adaptation

PURPOSE

Our aim was to better understand how to isolate the cone-mediated response in rats. Therefore, we studied the difference of ERGs in the course of light adaptation between 2 and 20 Hz stimulus frequencies.

METHODS

A total of 90 rats divided into 18 different groups were used following overnight dark adaptation. ERGs were recorded against 3 different adapting field luminances (1.15, 1. 50 or 1.75 log cd/m(2)) with a combination of 3 stimulus flash intensities (0.86, 1.30 or 2.03 log cd sec/m( 2)). The responses were obtained at 2 minute intervals for 25 minutes of light adaptation.

RESULTS

The response of the rat cone ERG was large despite the small number of cones. The mean amplitude increased systematically from the dark-adapted value requiring more than 15 minutes to reach an asymptote at 2 Hz stimulation, but only 10 minutes at 20 Hz stimulation. The 2 Hz adaptation curves had biphasic pattern compared to the monophasic 20 Hz curve. This second increase in the amplitude at 2 Hz appeared at around 7-8 minutes as a function of adaptation time. This tendency was most evident when using a low adapting field luminance with a high flash intensity.

CONCLUSIONS

Our results suggest that the rods intrude much more during light adaptation at 2 Hz stimulation in rodents than in humans. Therefore, 20 Hz flicker stimulation can better isolate more the cone-mediated function than 2 Hz stimulation during the course of light adaptation in rats. Furthermore, the functional characteristics of the cone in rats may be different from that in humans.

Regional specialization of the rat retina: catecholamine-containing amacrine cell characterization and distribution

The distribution of catecholaminergic amacrine cells has been investigated in rats by means of immunohistochemical labelling of wholemounted retinas. Two groups of catecholamine-containing cells could be distinguished on the basis of their catecholamine and biosynthetic enzyme content. Both groups could be stained with an anti-tyrosine hydroxylase (TH) antiserum. The first group was composed of large, strongly TH-immunoreactive stellate amacrine cells, located principally in the innermost row of the inner nuclear layer (INL) and sending processes to the outermost sublamina of the inner plexiform layer (IPL). Some were displaced in the IPL or in the ganglion cell layer (GCL). This first group of cells can be regarded as dopaminergic since they were also stained by an anti-dopamine (DA) antiserum. The second group was composed of small, weakly TH-positive cell bodies, located slightly more sclerad within the INL. Their processes were usually not labelled with anti-TH. Identical cells could be better visualized with an anti-phenylethanolamine-N-methyltransferase (PNMT) antiserum. Their processes were observed in the middle sublamina of the IPL. A great number of these cells were displaced in the GCL. They could be regarded as epinephrine cells. Concerning the density and distribution throughout the retina a striking difference was observed between the superior and inferior halves of the retina, whereas a lower difference was observed between the nasal and temporal regions. Almost all the PNMT-immunoreactive cells were located throughout the upper retina, whereas the DA-cells were especially concentrated in the upper temporal quadrant. The distribution of the DA cells parallels that of the ganglion cells whose density is also maximal in the upper temporal retina.

Rod saturation in b-wave of the rat electroretinogram under two different anesthetics

B-wave increment threshold experiments in the rat show that "rod saturation" occurs at different background levels with different anesthetics. Rod saturation builds up over the first 60 sec of light adaptation in pentobarbital anesthetized but not in urethane anesthetized animals. These and other findings suggest that "rod saturation" can occur when the rod photoreceptors themselves are not saturated.

Development of circadian rhythmicity and light responsiveness in the rat suprachiasmatic nucleus: a study using the 2-deoxy[1-14C]glucose method

The suprachiasmatic nucleus (SCN) of the hypothalamus is thought to play a critical role in circadian rhythm generation and entrainment to the light/dark cycle. In adult rats, the SCN shows a circadian rhythm in metabolic activity level as indicated by 2-deoxy[1-14C]glucose uptake. In the present study, the development of this rhythm was investigated. No diurnal difference in uptake was evident in fetal rats 1--2 days before birth. A significant diurnal difference in SCN 2-deoxyglucose uptake was preseno light at night increased the SCN metabolic levels. According to previous studies, on day 1 the SCN is poorly developed and contains few synapses. At this time the retinohypothalamic tract has not yet developed. We found progressive functional maturation of the SCN through day 21, when the rhythm and light responsiveness resembled those of adult rats.

Rods and cones in the mouse retina. I. Structural analysis using light and electron microscopy

Rods and cones of the C57BL/6J mouse retina have been examined by light and electron microscopy to distinguish the structural features of the two photoreceptor types. By light microscopy, cone nuclei are conspicuously different from rod nuclei in 1-2 micrometer plastic sections. Cone nuclei have an irregularly shaped clump of heterochromatin that appears in single sections to be one to three clumps, whereas rod nuclei are more densely stained and have one large, central clump of heterochromatin. Cone nuclei make up approximately 3% of the photoreceptor nuclei in both the central and peripheral retina at all ages examined up to 267 days. Cone nuclei are confined to the outer half of the outer nuclear layer, and more than 50% of the cone nuclei lie adjacent to the outer limiting membrane. By electron microscopy, cones in the mouse retina meet virtually every morphological criterion of mammalian cones. The outer segments are conically shaped. Many, if not all of the outer segment discs are continuous with the outer plasma membrane, whereas almost all of the rod discs are not. Cone outer segments are only about half the length of the rod outer segments, and they are contacted by long, villous pigment epithelial cell processes. The cone inner segment diameter is greater than the outer segment diameter, and the accumulation of mitochondria present at the apical end of the inner segment forms a more conspicuous ellipsoid than in rods. The internal fiber or axon of the cone is larger in diameter than that of the rod, and it terminates in a large synaptic pedicle with multiple ribbon synapses, whereas the rod terminal is a smaller spherule with only a single ribbon synaptic complex.