The role of cytoplasmic calcium in photoreceptor light adaptation.

The process of light adaptation in vertebrate rod and cone photoreceptors is believed to involve a diffusible cytoplasmic messenger. Two lines of evidence indicate that photoreceptor light adaptation is mediated by a light-induced fall in cytoplasmic calcium concentration (Ca2+i). First, if changes in calcium concentration are slowed by the incorporation of calcium chelators into the photoreceptor cytoplasm then light adaptation is slowed also. Second, if the normal control of Ca2+i is prevented by simultaneously minimising calcium influx and efflux across the outer segment membrane by means of external solution changes, then all of the manifestations of light adaptation are abolished. Furthermore, recent results show that changes in Ca2+i imposed in the absence of light are sufficient to cause at least some of the manifestations of light adaptation. Together these results indicate that calcium acts as the messenger of light adaptation in the photoreceptors of both lower and higher vertebrates.     

The crustacean eye: dark/light adaptation, polarization sensitivity, flicker fusion frequency, and photoreceptor damage

Compound eyes, nauplius eyes, frontal organs, intracerebral ocelli, and caudal photoreceptors are the main light and darkness detectors in crustaceans, but they need not be present all at once in an individual and in some crustaceans no photoreceptors whatsoever are known. Compound eye designs reflect on their functions and have evolved to allow the eye to operate optimally under a variety of environmental conditions. Dark-light-adaptational changes manifest themselves in pigment granule translocations, cell movements, and optical adjustments which fine-tune an eye's performance to rapid and unpredictable fluctuations in ambient light intensities as well as to the slower and predictable light level changes associated with day and night oscillations. Recycling of photoreceptive membrane and light-induced membrane collapse are superficially similar events that involve the transduction cascade, intracellular calcium, and membrane fatty acid composition, but which differ in aetiology and longterm consequence. Responses to intermittant illumination and linearly polarized light evoke in the eye of many crustaceans characteristic responses that appear to be attuned to each species' special needs. How the visual responses are processed more centrally and to what extent a crustacean makes behavioural use of e-vector discrimination and flickering lights are questions, however, that still have not been satisfactorily answered for the vast majority of all crustacean species. The degree of light-induced photoreceptor damage depends on a large number of variables, but once manifest, it tends to be progressive and irreversible. Concomittant temperature stress aggravates the situation and there is evidence that free radicals and lipid hydroperoxides are involved.     

Multiple phosphorylation of rhodopsin and the in vivo chemistry underlying rod photoreceptor dark adaptation

Dark adaptation requires timely deactivation of phototransduction and efficient regeneration of visual pigment. No previous study has directly compared the kinetics of dark adaptation with rates of the various chemical reactions that influence it. To accomplish this, we developed a novel rapid-quench/mass spectrometry-based method to establish the initial kinetics and site specificity of light-stimulated rhodopsin phosphorylation in mouse retinas. We also measured phosphorylation and dephosphorylation, regeneration of rhodopsin, and reduction of all-trans retinal all under identical in vivo conditions. Dark adaptation was monitored by electroretinography. We found that rhodopsin is multiply phosphorylated and then dephosphorylated in an ordered fashion following exposure to light. Initially during dark adaptation, transduction activity wanes as multiple phosphates accumulate. Thereafter, full recovery of photosensitivity coincides with regeneration and dephosphorylation of rhodopsin.     

High prion and PrPSc levels but delayed onset of disease in scrapie-inoculated mice heterozygous for a disrupted PrP gene.

BACKGROUND: It has been proposed that the prion, the infectious agent of transmissible spongiform encephalopathies, is PrPSc, a post-translationally modified form of the normal host protein PrPC. We showed previously that mice devoid of PrPC (Prn-p0/0) are completely resistant to scrapie. We now report on the unexpected response of heterozygous (Prn-p0/+) mice to scrapie infection. MATERIALS AND METHODS: Prn-p0/+, Prn-p0/0 and Prn-p+/+ mice were obtained from crosses of Prn-p0/+ mice. Mice were inoculated intracerebrally with mouse-adapted scrapie agent and the clinical progression of the disease recorded. Mice were sacrificed at intervals, PrPSc was determined as protease-resistant PrP and the prion titer by the incubation time assay. RESULTS: Prn-p0/+ mice, which have about half the normal level of PrPC in their brains, show enhanced resistance to scrapie, as manifested by a significant delay in onset and progression of clinical disease. However, while in wild type animals an increase in prion titer and PrPSc levels is followed within weeks by scrapie symptoms and death, heterozygous Prn-p0/+ mice remain free of symptoms for many months despite similar levels of scrapie infectivity and PrPSc. CONCLUSIONS: Our findings extend previous reports showing an inverse relationship between PrP expression level and incubation time for scrapie. However, contrary to expectation, overall accumulation of PrPSc and prions to a high level do not necessarily lead to clinical disease. These findings raise the question whether high titers of prion infectivity could also persist for long periods under natural circumstances in the absence of clinical symptoms.     

The role of cyclic nucleotides in the processes of adaptation and alteration of body radiosensitivity

The experimental data on the role of the cyclic nucleotides (cAMP, cGMP) in the mammalian integrating systems and their significance in the processes of adaptation and radiosensitivity change of animals are analysed. The changes in the system of cyclic nucleotides (CN) are reported for a number of morbid conditions at the subunit level. The CN system changes as a body response to the extreme action including ionizing radiation are revealed. A concept on a phase correlation between adrenergic and cholinergic system activity in development of adaptive reaction of a cell and a body on the whole during radiation or the action of some other factors is emphasized. The significance of cAMP/cGMP ratio calculation (coefficient K) is underlined for the analysis and prediction of the morbid state outcome.     

Resting and concanavalin-A stimulated levels of cyclic nucleotides in splenic cells of aging mice with spontaneous cancers.

The resting levels of cyclic 3′, 5′ -adenosine monophosphate (cAMP) and cyclic 3′, 5′ -guanosine monophosphate (cGMP) in splenic lymphoid cells of 25 aged (C57BL/10 × C3H)F₁ hybrid mice with spontaneous tumors, including 5 with hepatoma, 10 with lung tumor, 2 with lymphoma, and 8 with several varieties of tumor, as well as in 18 young and 13 tumor-free aging mice, were measured. The alterations in cyclic nucleotide levels in spleen cells characteristic of normal aging in tumor-free animals may be additionally influenced by the occurrence of spontaneous neoplasia. Furthermore, the levels may vary with different types of late-life tumors. For example, levels of cAMP in resting spleen cells of old mice with hepatomas were not different than in age-matched controls, whereas spleen, cells from old mice with lung tumors showed exceedingly high levels of resting cAMP. Upon $\text{in vitro}$ stimulation by Con-A, the splenic lymphoid cells from mice bearing spontaneous late-life lung and liver tumors displayed different kinetic patterns of percent changes in cAMP, cGMP and cAMP/cGMP ratios when compared to either young or age-matched tumor-free controls. Thus, both resting and Con-A stimulated levels of cAMP and cGMP and their ratios in splenic lymphoid cells may be affected by spontaneous cancer elsewhere in the body, including cancer of non-lymphoid type and origin. These findings plus the known functional decline in immune response capacity and the increase in spontaneous tumor incidence with age may suggest the existence of a complex relationship among cyclic nucleotide levels, immunity, aging, and cancer.     

The effects of nitroglycerin on cyclic nucleotides in the coronary artery in vivo

We examined the effects of nitroglycerin (NGL) on cyclic AMP (c-AMP) and cyclic GMP (c-GMP) in the coronary artery at 15 sec, 30 sec, 60 sec, and 3 min after the injection of NGL (0.02 mg/kg i.v.) in vivo. The relaxant effect of NGL was significantly correlated to an increase in the c-GMP concentration of the coronary artery. The c-AMP concentrations were not significantly changed at any time during the time response studies. We observed purely in vivo that there was a close correlation between an increase in c-GMP concentration after treatment with NGL and relaxation of the canine coronary artery. This study suggests that intracellular c-GMP may be involved with the biologic events leading to smooth muscle relaxation.     

Effects of light and dark upon photoreceptor synapses in the retina of Xenopus laevis

Summary The adrenergic innervation of the juxtaglomerular complex was studied in kidneys from mice, rats, guinea-pigs, rabbits, cats, dogs, pigs, monkeys, and humans using fluorescence histochemistry of neuronal nor-adrenaline and autoradiography of ³H-noradrenaline. The localization of the nerves was established by phase contrast optics or by perfusing the vascular system with India ink. Adrenergic nerve terminals, exhibiting a formaldehyde-induced fluorescence and having the ability to take up and accumulate ³H-noradrenaline, were easily identified when they enclosed the glomerular afferent arteriole. They continued in between and close to the macula densa and lacis cells to supply the glomerular efferent arteriole. The nerves could be seen to accompany this arteriole for a considerable distance until they branched off to the vasa recta in the juxtamedullary region and to adjacent cortical veins. This innervation pattern was found to be a constant feature except in kidneys from guinea-pigs and cats, in which post-glomerular adrenergic nerves were not found in some of the superficial glomerular units. The fluorescence in all adrenergic fibres supplying the juxtaglomerular complex disappeared after removal of the aortico-renal ganglion, showing that they belong to a common system of renal sympathetic nerves.This work is dedicated to Professor Wolfgang Bargmann in honour of his seventieth birthday, January 26, 1976     

A model for the mechanism of light and dark adaptation of vertebrate cones

A model is proposed for the mechanism of light and dark adaptation of vertebrate cones, especially for the one of operating curves shifting during light and dark adaptation, on the basis of physiological results. The mechanism is modeled in terms of bleaching levels and background effects through horizontal cell feedback loops. Furthermore, the spectral sensitivity of vertebrate cones is examined with the model. Simulations of the model are made and the results of the simulations extremely coincide with experimental results.     

Post-embryonic photoreceptor development and dark/light adaptation in the spittle bug Philaenus spumarius (L.) (Homoptera, Cercopidae)

The aims of this paper have been to describe (1) the general structure of the compound eye of the spittle bug Philaenus spumarius, (2) the eye's post-embryonic development, (3) photomechanical changes upon dark/light adaptation in the eye, and (4) how leaving the semi-aquatic foam bubble and turning into an adult affects the organization of the eye. Spittle bugs, irrespective of size or sex, possess apposition type compound eyes. The eye's major components (i.e. facet, cornea, cone and rhabdom) grow rather isometrically from the smallest nymph to the adult. Photomechanical changes can occur during both nymphal and adult phases and manifest themselves through pigment granules and mitochondria migrating to and away from the rhabdom, and rhabdom diameters varying with time of day and ambient light level. When a nymph transforms into an adult, its compound eyes’ dorsoventral axes widen, facet diameters increase, facet shapes turn from circular to pentagonal and hexagonal, the cornea thickens and the rhabdoms become thinner. The agile adults, free from the foam that surrounds the nymphs, can be expected to need their vision more than the nymphs, and the changes in eye structure do, indeed, indicate that the adults have superior visual acuity. A thicker cornea in the adults reduces water loss and protects the compound eye from mechanical and light-induced damage: protection given to the nymphs by their foam bubbles.     

Detection of light intensity in the frog retina: evidence from dark and light adaptation

The frog retina was stimulated with light flashes homogeneous in space but not time. The time heterogeneity of stimulation was created by abrupt change of a referent stimulus for a stimulus with different luminance. Such changes form a time pattern, as well as sharp borders of luminance between the neighbor areas of the visual field form a spatial pattern. The electroretinogram recorded in response to presentation of a triad of stimuli: the onset of a short flash of homogeneous light after long dark (or light) adaptation of a retina, brief sequence of the referent and test light flashes varied in luminance, and the offset, with returning to the initial level of adaptation. It was shown that responses of the retina under conditions of time heterogeneity of stimulation could be divided in two types as well as under conditions of spatial heterogeneity. Such a dual change in amplitude confirms our earlier hypothesis on the existence of two mechanisms of luminance coding in the frog retina. The first mechanism encodes power characteristics of light, it forms the information on the absolute level of the environmental luminance. Its activity is connected basically with receptors and cells of the external plexiform layer of the frog's retina. It is responsible for the b-wave of the electroretinogram. The other mechanism associated with RERG is based on a vector code of stimuli. This mechanism forms the information on spatial and time differentiation of the light flow in the visual field and is connected basically with cells of the internal plexiform layer. The results suggest that the frog retina has the individual mechanism for time pattern detection, distinguishing it from the homogeneous light flow in a similar way as in case of spatial light pattern detection. It is possible that the first mechanism is responsible for the detection of any new stimulus in general, irrespective of its specificity, whereas the second mechanism serves for the measurement of suprathreshold differences between stimuli.     

Non-genomic effects of progestins--inhibition of cell growth and increased intracellular levels of cyclic nucleotides

The anti-proliferative effect of progestins was studied in human transformed cell lines from the uterine cervix (C-4I, C33A and Me-180). Progestins caused a concentration-dependent inhibition of proliferation. The maximum tested concentration (2.6-3.2 microM) inhibited C-4I cell growth by the following order of potency: progesterone (56%) > medroxyprogesterone (38%) > megestrol acetate (25%). The sensitivity, expressed as I(25) (the concentration that caused 25% inhibition of growth), showed the same order: progesterone (7.7 nM) > medroxyprogesterone (78 nM) > megestrol acetate (570 nM). The intracellular levels of cGMP and cAMP were elevated and the cellular export of these cyclic nucleotides was inhibited by a similar order of potency. The C-4I cell line was devoid of progesterone-, estrogen-, androgen- and glucocorticoid-receptors. In addition, the antiprogestins mifepristone, onapristone and ZK-112993 did not block the anti-proliferative effect of progesterone. On the other hand, antiprogestins (2.3 nM) appeared to have some progesterone-like ("mimetic") activity with inhibition of C-4I cell growth; mifepristone (11%), onapristone (12%) and ZK-112993 (16%). The observed effects of progestins and antiprogestins on C-4I cells were also presented in C33A cells (16% androgen receptor positive) and Me-180 cells (22% progesterone receptor positive, 9% androgen receptor positive and 17% glucocorticoid receptor positive). This study suggests that a non-genomic mechanism contributes to the anti-proliferative effect of progestins.     

Sodium specificity of the effect of cyclic nucleotides on the conductance of cytoplasmic membranes of the photoreceptor

The effect of cyclic nucleotides (3',5'-cAMP and 3',5'-cGMP) on the membrane of the retina rod outer segment under intracellular dialysis conditions was shown to possess sodium specificity, its value in the medium with normal Na+ content being 4-20 times above that in sodium free solution. The results obtained permit a conclusion that the effect observed is due to the increased conductance of the cytoplasmic membrane of the outer segment and support the assumption concerning the mediator role of cyclic nucleotides in the photoreceptor act.