Spontaneous multiple unit activity from the preoptic nucleus and changes in blood pressure in the rainbow trout Salmo gairdneri under acute experimental conditions

Multiple unit activity (MUA) from the preoptic nucleus (NPO) blood pressure and heart rate were recorded in 41 anaesthetized and curarized rainbow trout. Mayer waves were frequently observed in blood pressure recordings (83%). Rhythmic MUA from the NPO, recorded in 46% of the experimented trout, occurred preferentially during Mayer waves. The initiation of the period of MUA occurred during low pressure vasomotor tone and large vasomotor pulse pressure inhibited MUA. Sustained decrease in blood pressure induced proportional rise in MUA. These results suggest that cardiovascular inputs influence the electrical activity of neurosecretory cells within the NPO and so may modulate the neurohypophysial hormones secretion.     

Seasonal encoding by the circadian pacemaker of the SCN

The circadian pacemaker of the suprachiasmatic nucleus (SCN) functions as a seasonal clock through its ability to encode day length [1-6]. To investigate the mechanism by which SCN neurons code for day length, we housed mice under long (LD 16:8) and short (LD 8:16) photoperiods. Electrophysiological recordings of multiunit activity (MUA) in the SCN of freely moving mice revealed broad activity profiles in long days and compressed activity profiles in short days. The patterns remained consistent after release of the mice in constant darkness. Recordings of MUA in acutely prepared hypothalamic slices showed similar differences between the SCN electrical activity patterns in vitro in long and short days. In vitro recordings of neuronal subpopulations revealed that the width of the MUA activity profiles was determined by the distribution of phases of contributing units within the SCN. The subpopulation patterns displayed a significantly broader distribution in long days than in short days. Long-term recordings of single-unit activity revealed short durations of elevated activity in both short and long days (3.48 and 3.85 hr, respectively). The data indicate that coding for day length involves plasticity within SCN neuronal networks in which the phase distribution of oscillating neurons carries information on the photoperiod's duration.     

Response of the hypothalamo-corticotropic system to metyrapone in pigeons

Chronic catheterization and miniature recording device allowed plasma corticosterone (B) and hypothalamic multiunit activity (MUA) to be simultaneously obtained from freely behaving, awake pigeons, before and for 4 hrs after intravenous injection of metyrapone. Injection of vehicle (tartric acid : 100 mg/4 ml/kg) led to MUA and B profiles quite similar to stress-induced responses, i.e., a rapid and sustained rebounding increase in hypothalamic firing rate and, shifted by 5-10 min, in plasma B. These responses were progressively attenuating for 90-120 mn. Metyrapone administration induced first a rapid and short MUA and B increase. Then both parameters drastically decreased near zero for about 2 hrs and were slowly restored to initial values within 3-4 hrs. It is suggested that metyrapone treatment inhibited both peripheral (B synthesis) and central (hypothalamic neurons) levels of the corticotropic axis.     

Brain injury following cardiorespiratory arrest in cats. Effects of alphaxalone-alphadolone

The effects of alphaxalone-alphadolone acetate (27.07 microM/kg-7.68 microM/kg) on neurologic injury following acute cerebral ischemia induced by an 8 min cardiorespiratory arrest (CRA) were investigated in cats through the analysis of neurological deficit scores and brain electrical activity; i.e., electroencephalogram (EEG) from parieto-occipital cortices and EEG and multiunit activity (MUA) from mesencephalic reticular formation (MRF). The CRA resulted from electrically induced cardiac arrest and stopping of mechanical ventilation in paralyzed cats which were successfully resuscitated within the immediate 4 min after the end of CRA. Two groups of cats were studied: I. Untreated, which received saline iv; II. Treated, which received alphaxalone-alphadolone acetate iv, 7-9 min after the end of CRA. Neuromuscular blockade and mechanical ventilation were maintained until 8 h following the CRA; then the cats were allowed to recover spontaneous respiratory activity. EEG phenomena were different in untreated and treated cats during this immediate post-arrest period. The former showed rhythmic bursts of fast (12-20 Hz) EEG activity at 1-2 sec intervals from 15-20 min until 3-4 h after the CRA, abundant spikes and delta-like waves. By contrast, administration of alphaxalone-alphadolone acetate resulted in burst suppression EEG pattern during 1 h. Progressive recovery of background EEG activity occurred afterwards. MUA from MRF disappeared during the CRA, however 6 h later the mean MUA frequency in untreated cats ranged between 32-46% and in treated cats 18-27% of their control mean frequencies during paradoxical sleep (100%). Daily electrographic records were performed in all the cats during quiet attentive behavior at each of the five days following the CRA. Significant differences were found in the frequency distributions of MUA from MRF (1st day, p less than 0.01; 5th day, p less than 0.01) as well as in the cortical EEG waves (1st day, p less than 0.01; 5th day, p less than 0.05) before and after the CRA in the untreated group. A wide dispersion of MUA values, and increased proportions of delta and theta-like waves and spindle bursts, besides a significantly high (p less than 0.001) number of spikes occurred in these EEG records the days following the CRA. The frequency distributions of MUA and EEG did not significantly differ before and after the CRA in the treated group; however, a significantly high (p less than 0.05) number of spikes was found in treated cats following the CRA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Damped oscillation of the lateral hypothalamic multineuronal activity synchronized to daily feeding schedules in rats with suprachiasmatic nucleus lesions.

In male Wistar rats with chronically implanted electrodes, multiple-unit activity (MUA) was recorded from the lateral hypothalamus (LH) and ventromedial hypothalamus (VMH). Blinded rats with bilateral suprachiasmatic nucleus (SCN) lesions showed no circadian rhythm in MUA or motor activity when food was available ad libitum. However, under a restricted-feeding schedule (food was available from 1400 to 1600 hr; water was always available) lasting for 10 days, a gradual increase of MUA of the LH developed, starting 3-4 hr prior to the feeding time. The elevated MUA lasted up to 6-7 hr after feeding and subsequently returned to the baseline level. This circadian rhythm of MUA of the LH persisted up to 4 days under total food deprivation, with quickly decreasing amplitude after termination of the schedule. MUA rhythm in VMH was less obvious than that in LH. Also, general motor activity showed a rhythm comparable to that of MUA, but it was less prominent. The elevated MUA in the LH prior to the feeding time may have been neural substrate of anticipatory activity appearing under the restricted-feeding schedule. These findings may suggest the existence of a quickly damping oscillator mechanism in the brain, presumably in the LH, which can be induced by daily feeding cues in the absence of the SCN.     

Estradiol increases multiunit electrical activity in the A15 area of ewes exposed to inhibitory photoperiods

Seasonal anestrus in ewes results from an increase in response to the negative feedback action of estradiol (E(2)). This increase in the inhibitory effects of E(2) is controlled by photoperiod and appears to be mediated, in part, by dopaminergic neurons in the retrochiasmatic area of the hypothalamus (A15 group). This study was designed to test the hypothesis that E(2) increases multiunit electrical activity (MUA) in the A15 during inhibitory long days. MUA was monitored in the retrochiasmatic area of 14 ovariectomized ewes from 4 h before to 24 h after insertion of an E(2)-containing implant subcutaneously. In six of these ewes, MUA activity was also monitored before and after insertion of blank implants. Three of the 14 ewes were excluded from analysis because E(2) failed to inhibit LH. When MUA was recorded within the A15, E(2) produced a gradual increase in MUA that was sustained for 24 h. Blank implants failed to increase MUA in the A15 area, and E(2) did not alter MUA if recording electrodes were outside the A15. These data demonstrate that E(2) increases MUA in the A15 region of ewes and are consistent with the hypothesis that these neurons mediate E(2) negative feedback during long photoperiods.     

The effects of low-speed swimming following exhaustive exercise on metabolic recovery and swimming performance in brook trout (Salvelinus fontinalis)

Experiments were conducted to determine whether low-speed swimming during recovery from exhaustive exercise improved both metabolic recovery and performance during a swimming challenge. For these experiments, brook trout were allowed to recover from exhaustive exercise for 2 h while swimming at 0, 0.5, 1.0, or 1.5 body length (BL) s(-1) or allowed to recover from exhaustive exercise for 1, 2, or 3 h while swimming at 1.0 BL s(-1). At the appropriate interval, either (i) muscle and blood samples were removed from the fish or (ii) fish were assessed for performance (i.e., fatigue time) during a fixed-interval swimming test. Low-speed swimming during recovery from exhaustive exercise resulted in significantly longer fatigue times compared with fish recovering in still water (i.e., 0 BL s(-1)). However, swimming during recovery did not expedite recovery of muscle lactate or blood variables (e.g., lactate, osmolarity, glucose). These observations suggest that metabolic recovery and subsequent swimming performance may not be directly linked and that other factors play a role in swimming recovery in brook trout.     

Non-invasive recording of heart rate and ventilation rate in rainbow trout during rest and swimming. Fish go wireless!

Resting heart rates and ventilation rates in rainbow trout Oncorhynchus mykiss at 15°C are 31·8±1·8 beat min⁻¹ and 53·1±3·7 breaths min⁻¹, respectively. The non-invasive recording system picked up the bioelectric potentials generated by the fish in the water and was based on an array of six silver-silver chloride electrodes covered with agar-gel, which provided a better signal-to-noise ratio than in previously described systems, and allowed the determination of heart rate and ventilation rates at different swimming speeds up to 21 s⁻¹. In concert with the lower rates, the scope for changes in heart rate and ventilation rate during swimming was also considerably larger than in earlier studies (2·4- and 2·0-fold, respectively). Two main conclusions result from this work: (i) short recovery times under 48 h after anaesthesia and surgery are unlikely to provide truly resting heart rates and ventilation rates in trout at 15°C; (ii) heart rate regulation during exercise is more important than previously thought and might account for a larger proportion of the increase in cardiac output observed in swimming trout.     

Exhaustive exercise does not affect the preferred temperature for recovery in juvenile rainbow trout (Oncorhynchus mykiss)

We tested the hypothesis that juvenile rainbow trout (Oncorhynchus mykiss) would select a temperature colder than their acclimation temperature (16 deg +/-1 deg C) to minimize postexhaustive exercise metabolic demands and enhance oxygen availability. After an initial 3-h exploratory period in a thermal gradient (6 degrees -25 degrees C), fish selected a temperature of approximately 14 degrees C and had a baseline exploratory swimming activity of approximately 60 cm min(-1). Subsequently, experimental (chased) fish were individually removed, exhaustively exercised for 1.5 min, and replaced. Both control (unchased) and experimental fish were allowed to explore the thermal gradient for another 2 h. Immediately after being chased, trout had a metabolic profile that was consistent with being exhausted; levels of plasma and muscle lactate were 4.38+/-0.25 mmol L(-1) and 28.0+/-2.0 mmol kg(-1), respectively, and levels of muscle glycogen, adenosine triphosphate, and phosphocreatine were 3.89+/-0.95, 4.23+/-0.62, and 3.07+/-0.73 mmol kg(-1), respectively. Although exploratory swimming activity of the chased fish was significantly lower (by 81%) as compared with control fish during the first 5 min postchase, differences in the mean, median, and mode values for selected temperatures during the next 2 h were neither large (<1 degrees C) nor significant (P>0.05). Contrary to our initial hypothesis, these findings suggest that juvenile rainbow trout do not select a colder temperature to decrease metabolic rate following exhaustive exercise. Instead, rainbow trout selected a temperature marginally cooler than their acclimation temperature (16 degrees C) regardless of whether they had been previously exhausted.     

Diurnal changes in the haemoglobin level, red blood cell number and mean corpuscular haemoglobin in foals during the first 13 weeks of life and in their lactating mothers

1. In six foals and their mothers (of Standard breed) diurnal changes in the haemoglobin level, red blood cell number and mean corpuscular haemoglobin during the first 13 weeks of foal life were studied. 2. Studies begun when a foal reached 7 days of age and were repeated every two weeks till 13 weeks of foal life. Blood was taken every 6 hr in foals and every 4 hr in mares. 3. No diurnal rhythmicity in parameters studied either in foals or in mares was found. 4. Decrease of haemoglobin level in lactating mares (14.0-12.5 g/100 ml) without changes in red blood cell number were observed. Mean corpuscular haemoglobin changed distinctly from one study day to another. 5. In foals were stated: significantly higher haemoglobin level and erythrocyte number and lower mean corpuscular haemoglobin than in mares.     

Coordination and neuromuscular control of rhythmic behaviors in the blue crab,Callinectes sapidus

The stereotypical courtship display (CD) behavior of the male blue crab, Callinectes sapidus, includes an unusual component: the rhythmic waving of the swimming appendages above the carapace. This behavior occurs in a unique context but it resembles two other rhythmic behaviors performed using the swimming legs: sideways swimming and backward swimming. As a first step to understanding the mechanisms that allow the expression of apparently different rhythmic motor patterns, we have examined these behaviors using slow motion video analysis and electromyography of the basal muscles of the swimming legs in freely behaving crabs. The results show that these behaviors are distinguished by four parameters: the frequency of leg waving, the phase relationship between the legs, the presence of a stationary pause in basal muscle activity combined with rotation of the distal leg during CD, and an extended range of motion of these legs during CD and backward swimming, relative to sideways swimming. EMG analysis revealed that during sideways swimming, the sequence of muscular activity between the two legs was different. In contrast, during CD and backward swimming the sequence of activity for these legs is identical.Abbreviations CD courtship display - EMGs electromyograms - CD AMP courtship display in crabs with amputated fifth legs - CD1 crabs that voluntarily used one leg to perform courtship display waving - CD 1–3 courtship waving in cycles 1–3 - CD MID courtship waving after cycles 1–3 - M-C meral-carpal joint     

Energy metabolism of swimming trout (Salmo gairdneri)

Summary The oxidation of 1-¹⁴C palmitate, 2-¹⁴C glucose, 1-¹⁴C lactate, 1-¹⁴C alanine, 1-¹⁴C leucine and 1-¹⁴C glutamate, injected via a cannula into the dorsal aorta, was determined in trout, either at rest, or during swimming at 80% of the maximum sustained speed. The oxygen consumption and the excretion rates of¹⁴CO₂ as well as CO₂ were measured.While the oxygen consumption of swimming trout was about twice as high as of resting trout, the oxidation rates of the injected tracers increased by up to 9 time. Despite the increased importance of blood borne substrates, the estimated contribution to total CO₂ production is about 6% for the resting and 17% for the active trout. The majority of the oxidisable substrates must therefore be endogenous.The mobilization and oxidation rates of lactate, palmitate and leucine were particularly increased during swimming. During rest, palmitate and leucine oxidation rates are low. While oxidation rates of alanine and glutamate are intermediate, those of glucose were found to be extremely low, both at rest and during swimming. The measured RQ values for resting and swimming trout were 0.91 and 0.96 respectively, indicating that protein is the major fuel, since glucose oxidation seems of minor importance.Abbreviations and symbols SA specific activity - tt transit time - decay (time) constant - flux (in % of injected dose per hour) - U_crit critical velocity of sustained swimming - TCO₂ total CO₂     

Sensitization and habituation of the swimming behavior in ascidian larvae to light

Ascidian larvae of Ciona intestinalis change their photic behavior during the course of development. Newly hatched larvae show no response to a light stimulus at any intensity. At 4 hr after hatching, larvae were induced to start to swimming upon the cessation of illumination, and to stop swimming upon the onset of illumination. At a weaker light intensity (5.0 x 10(-3) J/m (2).s), the larvae showed similar responses to either a single stimulus or repeated stimuli of onset and cessation of light until 10 hr after hatching. At a stronger light intensity (3.2 x 10(-1) J/m(2).s), when the stimulus was repeated, they showed sensitization and habituation of the swimming response. At 3 hr after hatching the larvae failed to show any response to an initial stimulus at any intensity of light, but after several repeated stimuli (sensitization) they showed a swimming response at light intensities above 4.0 x 10(-2) J/m (2).s. At 5 hr and with intensity above 1.0 x 10 (-2) J/m(2).s, the larvae showed photoresponses to the first stimulus, but after several repetitions the larvae failed to stop swimming upon the onset of light (habituation). A repeated series of stimuli at stronger intensities of light caused greater habituation; this habituation was retained for about 1 min. Since the larval central nervous system in Ciona is comprised of only about 100 neurons, learning behavior in ascidian larvae should provide insights for a minimal mechanism of memory in vertebrates.     

Ontogeny of Diel Pattern of Stream-Margin Habitat Use by Emerging Brown Trout, Salmo Trutta, in Experimental Channels: Influence of Food and Predator Presence

Age-0 brown trout, Salmo trutta, inhabit shallow and slow-flowing habitats where they can easily maintain stationary swimming positions. However, recent results have shown that they use deeper and faster habitats during daylight than at night, suggesting the occurrence of a nocturnal movement toward stream-margin habitats. Experiments were conducted to describe precisely when this diel pattern of habitat use appears during ontogeny. In two indoor channels, free-embryo brown trout were deposited under the gravel. When emerging, alevins were free to choose between margin (2 cm deep, 0-2 cm s^-1) or deep habitat (12 cm, 2-4 cm s^-1), or to leave the channel (upstream or downstream). During the week of emergence, upstream and downstream catches, fish habitat use (deep habitat or margin), and fish behavior (resting or swimming) were measured by direct observations and trap counts. Three treatments were performed: (1) fish artificially fed on drifting invertebrates, (2) fish exposed to predators (bullhead, Cottus gobio), and (3) control channels (no food, no predator). In control and food channels, a diel pattern of habitat use was observed 1-2 days after the emergence started. Most fish rested in the margin at night, whereas they moved towards the deep habitat during daylight to hold stationary swimming positions. In the presence of bullhead, most trout were cryptic, and visible fish stood in the margin during both daylight and at night. The importance of predation risk and foraging behavior on the ontogeny of the diel pattern of habitat use is discussed. Results support the direct development without larva from free-embryo via alevin in brown trout.     

The recovery of locomotory activity following exhaustive exercise in juvenile rainbow trout (Oncorhynchus mykiss)

This study investigated the recovery of locomotory activity in exhausted juvenile rainbow trout (Oncorhynchus mykiss, approximately 6-10-cm fork length) in response to two conditions: (1) direct transfer to a range of velocities (0-15 cm s(-1)) in a swim flume (forced swimming) and (2) direct transfer to a pool downstream of a swim channel where a choice of velocities was presented: 2-3 cm s(-1) in the lower half of the pool, a range of velocities from 7 to 40 cm s(-1) in the upper half the pool near the channel entrance, and a velocity of 57 cm s(-1) in a swim channel emptying into the pool (volitional swimming). Exhausted trout showed a pronounced delay in the recovery of normal locomotory activity. With forced swimming, the time required to resume swimming was inversely proportional to water velocity. At 15 cm s(-1), almost all exhausted fish recovered immediately, whereas it took about 1 h for recovery at a current of 5 cm s(-1). In contrast, nonexhausted fish responded to imposed velocity with immediate rheotactic responses (orientation and station holding) at all test velocities. In voluntary swim trials, exhausted trout showed a marked preference for holding station in current in the downstream pool (approximately 11 cm s(-1)) but took, on average, 2 h longer than nonexhausted fish to make transits in the swim channel. Moreover, their ground speed in the swim channel was significantly slower. We conclude that swimming performance is impaired for at least 6 h by exhaustive exercise. Maladaptive behaviors during this time include a preference for current near the surface over cover and a reduced capacity for burst activity, both of which would translate into greater predation risk and reduced ability to forage.     

The Bohr effect of the blood in rainbow trout (Salmo gairdnerii). A comparative study with human blood, using precise oxygen equilibrium curves and the Adair model

1. The Bohr effects of trout blood (which exhibits the Root effect) and of human blood were compared. Precise oxygen equilibria were measured with an automatic recording system, on normal trout red blood cell suspensions at pH 7.6 - 8.6, at 10 and 20 degrees C, and on normal human red blood cell suspensions at pH 6.8 - 8.0, at 37 degrees C. 2. The data were fitted to the Adair's stepwise oxygenation model which describes experimental curves with four constants ki (i = 1-4). 3. Adair's scheme successfully fits the equilibrium data for trout and human blood, in the range of conditions examined. 4. The R-state Bohr effect (d log k4/ d pH), is very large in trout blood, indicating a large pH dependence of the R structure, as opposed to human blood. 5. The T-state Bohr effect (d log k1/ d pH), and the overall Bohr effect (d log Pm/ d pH), are equivalent in trout and human blood. 6. The overall Bohr effect is essentially accounted for by the first and fourth oxygenation steps in trout blood and shows a significant effect of temperature. 7. The data attribute a major role to Hb4 in trout blood isotherms and confirm the importance of the C-termini of Beta chains in Bohr and Root effects.     

Bioenergetics of acclimation to permethrin (NRDC-143) by rainbow trout

Metabolic rates associated with sustained, prolonged and critical swimming speeds were examined in 10 g trout exposed to 5% 96 hr LC50 (0.75 microgram X l-1) and 10% 96 hr LC50 (1.50 micrograms X l-1) at 12 degrees C. Permethrin did not influence the metabolic cost for swimming at sustained and prolonged speeds. Basal metabolic rate increased on initial exposure to permethrin reaching maximum values after 7 days and declined to the control level after 13 days in 5% and after 32 days in 10% 96 hr LC50. Critical swimming speeds were adversely affected in a manner reflective of the effects of permethrin on basal metabolic rate. Elevation in basal metabolic rate in fish exposed to permethrin was a result of increased energy requirements due to physiological stress, detoxication and tissue repair.     

Radio-transmitted electromyogram signals as indicators of swimming speed in lake trout and brown trout

Swimming speed and average electromyogram (EMG) pulse intervals were highly correlated in individual lake trout Salvelinus namaycush ( r ²=0·52–0·89) and brown trout Salmo trutta ( r ²=0·45–0·96). High correlations were found also for pooled data in both lake trout ( r ²=0·90) and brown trout of the Emå stock ( r ²=0·96) and Lærdal stock ( r ²=0·96). The linear relationship between swimming speed and average EMG pulse intervals differed significantly among lake trout and the brown trout stocks. This successful calibration of EMGs to swimming speed opens the possibility of recording swimming speed of free swimming lake trout and brown trout in situ . EMGs can also be calibrated to oxygen consumption to record energy expenditure.     

Modifications of acoustic habituation by interruption of visual input in quipazine treated cats

The influence of interruption of the visual input on acoustic habituation was studied in cats before and following the administration of quipazine, 3 mg/kg iv. The characteristics of acoustic habituation were analyzed through the magnitude and temporal course of multiunit activity (MUA) responses elicited in the mesencephalic reticular formation (MRF) by repetitive acoustic stimuli (70 db, 50 Hz trains of 2 sec duration) in 6 freely moving cats with cortical electrodes over the parietal cortex and bipolar electrodes chronically implanted in MRF and basolateral amygdala (AMN). The cats were submitted to repetitive acoustic stimulation during one 30 min period before, and three 30 min periods after drug administration in the following conditions: a) with unmasked eyes; b) with masked eyes by means of dark contact lenses. Persistent attentive behavior, catatonic attitudes, hypersynchronous (6 Hz, 100-150 microV) EEG activity and significant increase of spontaneous MUA in FRM and AMN were induced by quipazine both in the cats tested with unmasked and with masked eyes. This increase of MUA was higher immediately following drug administration and progressively decreased, although MUA values remained significantly higher than controls 110 min after quipazine administration. Acoustic habituation, evidenced through the progressive decrease of MUA responses of MRF to acoustic stimuli, was observed before quipazine administration when the cats were tested with unmasked and with masked eyes; as well as in cats tested with unmasked eyes following drug administration. However, the MUA responses to acoustic stimuli did not decrease in cats with masked eyes during acoustic stimulation periods 0-30 min and 40-70 min after quipazine administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circadian and circahemidian rhythms in plasma prolactin of the ram: seasonal changes

The existence of a circadian rhythm in plasma prolactin of the ram is controversial. Differences among authors can be related to both data sampling (e.g. interindividual changes, time of day, time of year, sampling interval among others) and statistical analyses. To test this hypothesis six adult "Préalpes du Sud" rams were studied individually during 72 hr in January (8 hr of light-16 hr of darkness), April (13L-11D), June (16L-8D) and September (13L-11D). Blood was sampled (vacutainer) from a jugular vein every hour, centrifuged and plasma samples stored at -20 degrees C until prolactin determinations (radioimmunoassay) were made. Individual time series were analysed according to three complementary methods: display of raw data (chronogram), best fitting cosine functions with different period tau (iterative cosinors) and power spectra. Seasonal changes in the 24 hr mean (peak time in June) were confirmed. A circahemidian rhythm (tau = 12 hr) and a circadian rhythm (tau = 24 hr) were validated, respectively in January and April while time series documented in June and September exhibited no rhythmic organization. It seems, therefore, that animals adjusted their rhythmic patterns of prolactin secretion to the increasing (January, April) rather than decreasing (June, September) photofraction (duration of the light span/24 hr).