Light-induced changes of sensitivity in Limulus ventral photoreceptors

The responses of Limulus ventral photoreceptors to brief test flashes and to longer adapting lights were measured under voltage clamp conditions. When the cell was dark adapted, there was a range of energy of the test flashes over which the peak amplitude of the responses (light-induced currents) was directly proportional to the flash energy. This was also true when test flashes were superposed on adapting stimuli but the proportionality constant (termed peak currently/photon) was reduced. The peak current/photon was attenuated more by brighter adapting stimuli than by less bright adapting stimuli. The peak current/photon is a measure of the sensitivity of the conductance-increase mechanism underlying the light response of the photo-receptor. The response elicited by an adapting stimulus had a large initial transient which declined to a smaller plateau. The peak current/photon decreased sharply during the declining phase of the transient and was relatively stable during the plateau. This indicates that the onset of light adaptation is delayed with respect to the onset of the response to the adapting stimulus. If the adaptational state just before the onset of each of a series of adapting stimuli was constant, the amplitude of the transient was a nearly linear function of intensity. When the total intensity was rapidly doubled (or halved) during a plateau response, the total current approximately doubled (or halved). We argue that the transition from transient to plateau, light-elicited changes of threshold, and the nonlinear function relating the plateau response to stimulus intensity all reflect changes of the responsiveness of the conductance-increase mechanism.     

Diurnal changes in the chilling sensitivity of seedlings

Seedlings of tomato (Lycopersicon esculentum, Mill.) varied diurnally in their sensitivity to chilling temperatures. If chilled near the end of the dark period when they were most sensitive, the time taken to kill half of the seedlings was approximately 3 days, whereas in samples taken 4 hours after the onset of dark, a period of 6 days of chilling was required. Sensitivity dropped rapidly after the onset of the light period. This rhythm was exogenously controlled by the diurnal changes in light, rather than in the temperature. The susceptibility of predawn seedlings could be reduced by exposure to light, by water stress, or by abscisic acid applied to the leaves. However, the subsequent changes in sensitivity to chilling did not correlate with stomatal aperture. Six other chilling-sensitive species showed similar diurnal changes in their chilling sensitivity.     

Diurnal changes in the fine structure of photoreceptors in an abalone,Nordotis discus

Summary The ultrastructure of the synapses in the brain of the monogenean Gastrocotyle trachuri (Platyhelminthes) is described. The synapses consist of one presynaptic terminal separated by a uniformly wide synaptic cleft, from one or more postsynaptic elements. The presynaptic terminals are characterized by the presence of paramembranous dense projections and associated synaptic vesicles. The postsynaptic elements while possessing membrane densities, are usually devoid of vesicles.The structure of the synapses in the brain of Gastrocotyle is compared to synapses from other platyhelminths.     

Six spectral sensitivity classes in crab visual interneurons

Summary Electrophysiological determination of the spectral sensitivity of units from the retina, optic lobes and brain of the crabParagrapsus gaimardii revealed six major colour types. Single peaks occurred in the UV, blue-green, green and yellow, and further units showed either a broad-band sensitivity or double peaks in the violet and yellow.Many of the curves had broad shoulders and offset peaks, markedly different in shape to curves suggested by Dartnall for rhodopsins with the same peak values, but similar to those previously recorded for other crabs. This finding, allied with limited evidence that a number of the colour types changed one from the other depending on illumination level, suggests that at least some of the colour types may result from a filtering of the light reaching a single retinal type.The UV sensitivity, recorded from brain interneurons, is the highest so far found in crabs.The range of colour types recorded provides a good potential substrate for colour vision. It is therefore of interest that a survey of a total of 105 units provided no evidence for colour opponency.Queen Elizabeth II Fellow in Marine Science, 1978–1979     

Spectral sensitivity of photoreceptors in the compound eye of the locust

Three types of receptor with different _max of 360, 430, and 530 nm were found in the locust retina by extracellular recording. Their spectral sensitivity curves were considerably broader than the absorption curves of the corresponding pigments. Possible coefficients of electrical coupling between different receptor types in ommatidia were calculated on the basis of the spectral sensitivity curves obtained for photoreceptors, assuming that each receptor contains only one light-sensitive pigment. The resulting values resembled coefficients measured in the locust by Shaw and Lillywhite. The way in which spectral sensitivity curves spread in comparison with pigment absorption curves may thus be caused by electrical coupling between cells.Institute of Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 69–76, January–February, 1986.     

A quantitative comparison of the time-course of sensitivity changes produced by calcium injection and light adaptation in Limulus ventral photoreceptors

The time-course of light and dark adaptation was quantitatively compared with the time-course of the onset of and recovery from desensitization produced by intracellular calcium injection in Limulus ventral photoreceptors. The onset of light adaptation tended to be faster (by 60-90 s) than the onset of desensitization produced by intracellular Ca++ injection. The initial portion of the time-course of dark adaptation was faster (about 10-20 s) than the time-course of recovery from desensitization produced by intracellular Ca++ injection. The final portion of recovery from Ca++ injection had the same time-course as a comparable dark adaptation.     

Diurnal beta-endorphin changes in human cerebrospinal fluid

Plasma and cerebrospinal fluid (CSF) beta-endorphin levels were determined by a RIA method in seven hydrocephalic male patients. The samples were simultaneously collected every two hours from 8 AM to 12 midnight and every hour from 1 AM to 7 AM. In both plasma and CSF beta-endorphin levels showed significant time-related variations during the 24 hour period. These results suggest the existence of diurnal CSF beta-endorphin variations analogous to those observed in plasma.     

Diurnal variation of insulin clearance and sensitivity in normal man

Sixteen normal healthy volunteers were randomized into two groups, receiving either low doses insulin infusion clamp study (8mU/M2/min) or high dose (40mU/M2/min) to determine the diurnal insulin clearance and sensitivity. Each subject received the assigned dose of insulin clamp twice; one in the morning (0800-1000) and the other in the evening (1800-2000), each with a precedent 9 hours of fasting, respectively. The results showed that there were diurnal variation of serum insulin clearance in the high dose study (AM:791 +/- 54ml/min/M2, PM:947 +/- 53ml/min/M2, p less than 0.01), and the small dose study (AM:411 +/- 32ml/min/M2, PM:716 +/- 87ml/min/M2, p less than 0.001). Diurnal variation of insulin sensitivity as judged by dividing glucose infusion rate by the ambient serum free insulin level (M/FI ration), was only noted in the low dose insulin infusion clamp study (AM:14.6 +/- 2.4, PM:10.5 +/- 1.1, p less than 0.05). In summary, at low physiological levels of insulin the insulin sensitivity is better in the morning, whereas at both high and low insulin levels the insulin clearance of normal subject is greater in the evening. The mechanism of this diurnal variation of insulin clearance and sensitivity awaits further studies.     

Diurnal changes in proline content of desert plants

Summary The diurnal changes in proline and water contents of Zygophyllum quatarense and Francoeuria crispa were investigated under desert conditions. In both plants, the proline content was low before sunrise amounting to 10.42 moles/g fresh weight in Zygophyllum and 6.3 moles/g fresh weight in Francoeuria. By the progress of the day and the intensification of the evaporative power of the atmosphere, the proline content increased considerably in the two plants. The proline content reached maximal values at 5 p.m., reaching 23.36 moles/g in Zygophyllum and 11.16 moles/g in Francoeuria. The proline content of the artificially watered plants was kept at a low level throughout the day. The conditions and role of proline accumulation were discussed.     

Spectral sensitivity of photoreceptors in the compound eye of stingless tropical bees

The spectral sensitivity of photoreceptors of the tropical bees Melipona quadrifasciata, M. marginata, and Trigona spinipes were measured on-line with a rapid constant response amplitude method. The results indicate that although these bees forage in different habitats they have the same set of photoreceptors with sensitivity maxima in ultra-violet, blue and green.     

Diurnal changes in the rabbit's visual evoked potential

Rabbits that have been exposed to the natural cycles of day and night exhibit marked diurnal changes in the shape of their Visual Evoked Potential in constant environmental conditions. The results of exposure to artificial 24 hr light-dark cycles strongly suggest that it is the regular alternation of daylight and darkness which acts as the synchronizing "Zeitgeber" for the V.E.P. rhythm which exists after exposure to the natural cycles of day and night. It would seem further that the V.E.P. changes reflect a square-wave like rhythm in the sensitivity of the visual system to photic stimuli, in which the sensitivity is much higher at night than in the day-time. The probable importance of the diurnal V.E.P. rhythm for the occurrence of daily fluctuations in behaviour is discussed.