Circadian control of migratory restlessness and the effects of exogenous melatonin in the brambling, Fringilla montifringilla

Circadian pacemakers control both “daytime” activity and nocturnal restlessness of migratory birds, and the daily rhythm of melatonin release from the pineal has been suggested to be involved in the control of migratory activity. To study the phase relations between the two activity components during entrainment and when free running, locomotor activity of bramblings (Fringilla montifringilla) was recorded continuously under a 12:12 “cool light” to “warm light” cycle (CL:WL, ca. 5000 K and ca. 2500 K, respectively) or blue light to red light cycle (BL:RL, maxima at 440 and 650 nm, respectively) at different irradiance ratios. Migratory activity was expressed primarily during the WL or RL phase of the light cycles. Under free-running conditions, the circadian periods τ correlated with the phase relations between day and night (migratory) activity components during preceding entrainment. Bramblings with migratory activity had significantly longer τ at constant light intensity than the same individuals without migratory activity. Birds with migratory activity reentrained faster after a 6h phase shift of the CL:WL cycle than birds without migratory activity. When exogenous melatonin was given in the drinking water (200 μg/mL 1% ethanol or 0.86 mM) to bramblings exposed to 12:12 CL:WL cycles with constant irradiance, the amounts of activity, which were initially higher during the WL phase of the light cycle, were suppressed to similar low levels during both light phases. The systematic changes in the amounts of activity during melatonin treatment were not correlated with consistent changes in entrainment status. The data support the hypothesis that changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by either allowing or inhibiting nocturnal activity. (Chronobiology International, 17(4), 471-488, 2000)     

Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night

Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only “twilight” periods defined by the sun’s elevation below the horizon at midday; we term this “midday twilight.” Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light environment is relevant to behavioral and visual processes. We reveal from acoustic field observations that the zooplankton community is undergoing diel vertical migration (DVM) behavior. Furthermore, using electroretinogram (ERG) recording under constant darkness, we show that the main migratory species, Arctic krill (Thysanoessa inermis) show endogenous increases in visual sensitivity during the subjective night. This change in sensitivity is comparable to that under exogenous dim light acclimations, although differences in speed of vision suggest separate mechanisms. We conclude that the extremely weak midday twilight experienced by krill at high latitudes during the darkest parts of the year has physiological and ecological relevance.

This study shows that ambient light cycles set an internal rhythm that controls visual sensitivity of Arctic krill during the Polar Night – the darkest part of the year, when the sun remains below the horizon all day. This demonstrates that biologically relevant photoperiods can be achieved during this time of "midday twilight".     

Distribution of Exopolymeric Substances in the Littoral Sediments of an Oligotrophic Lake

Bacteria and algae release exopolymeric substances (EPS) that perform a wide range of important functions in aquatic and terrestrial systems. In this study we measured EPS in sediments at nine littoral sites around a shallow oligotrophic basin, and tested whether the concentration and composition of EPS was related to sediment characteristics. The concentrations of both loosely bound (colloidal) and tightly bound (capsular) EPS carbohydrates ranged up to ~800 µg glucose equiv. cm^–2 and were well within the range of concentrations reported from marine intertidal flats, where EPS play an important role in stabilizing sediments, affecting nutrient exchanges between sediments and the water column, feeding benthic invertebrates, and sequestering and increasing the transfer of contaminants to food webs. Proteins were an important component of the EPS in these littoral sediments, with protein:carbohydrate ratios of ~0.4. In summer, the concentrations of most EPS fractions were positively related (P < 0.05) to the porewater and organic matter content of the sediments. Capsular EPS concentrations were lower in the fall, with a simultaneous increase in colloidal proteins but not in colloidal carbohydrates. This suggests that the carbohydrates in this colloidal EPS may be more labile than the proteins. Our results suggest that exopolymeric substances could be an important, but neglected, component of littoral sediments in lakes.     

Serotonin N-acetyltransferase (NAT) activity in hen retina and pineal gland: pharmacological induction at noon and antagonism of the light-evoked suppression at night

effective pharmacological procedures are described which markedly increase activity of serotonin N-acetyltransferase (NAT), the key regulatory enzyme in melatonin biosynthesis, during the daytime (in light) and counteract suppressive effects of light on NAI activity at night in the hen retina and pineal gland. Of the tested compounds, and their combinations, the most effective were: “aminophylline + spiroperidol + alpha-methyl-p-tyrosine” for the retina, and “aminophylline + yohimbine (+ alpha-methyl-p-tyrosine)” for the pineal gland. The results give strong support to the concept that the dopaminergic (C₂-receptor) and noradrenergic (alpha₂-adrenergic receptor) mechanisms control NAT activity, and melatonin synthesis, in the hen retina and pineal gland, respectively.     

Chelicerae as male grasping organs in scorpions: sexual dimorphism and associated behaviour

Specialised structures that enable males to grasp females during sexual interactions are highly susceptible to selection and thus diverge relatively rapidly over evolutionary time. These structures are often used to test hypotheses regarding sexual selection such as sexually antagonistic co-evolution and sexual selection by female choice. In the present study, we determine whether there is a relationship between a novel record of scorpion sexual dimorphism, the sexual dimorphism of chelicerae (CSD), and the presence of the mating behaviour termed “cheliceral grip” (CG). The presence of both traits in the order Scorpiones is also reviewed from a phylogenetic perspective. The results confirm a strong relationship between CSD and the presence of CG. The morphological and behavioural patterns associated with “CSD–CG” are opposed to the predictions postulated by the hypothesis of sexually antagonistic co-evolution. However, if the female shows resistance after the deposition of the spermatophore, the possibility that the male exerts pressure as a “cryptic form” of coercion to prevent the interruption of mating cannot be ruled out completely. Female choice by “mechanical fit” could be another explanation for some aspects of the CG's contact zone. The possibility that the “CG–CSD” complex has evolved under natural selection in order to ensure sperm transfer is also considered.     

Food supply to the seafloor in the Pacific Ocean after the Cretaceous/Paleogene boundary event

Deep-sea benthic foraminifera show important but transient assemblage changes at the Cretaceous/Paleogene (K/Pg) boundary, when many biota suffered severe extinction. We quantitatively analyzed benthic foraminiferal assemblages from lower bathyal–upper abyssal (1500–2000 m) northwest Pacific ODP Site 1210 (Shatsky Rise) and compared the results with published data on assemblages at lower bathyal (~ 1500 m) Pacific DSDP Site 465 (Hess Rise) to gain insight in paleoecological and paleoenvironmental changes at that time.At both sites, diversity and heterogeneity rapidly decreased across the K/Pg boundary, then recovered. Species assemblages at both sites show a similar pattern of turnover from the uppermost Maastrichtian into the lowermost Danian: 1) The relative abundance of buliminids (indicative of a generally high food supply) increases towards the uppermost Cretaceous, and peaks rapidly just above the K/Pg boundary, coeval with a peak in benthic foraminiferal accumulation rate (BFAR), a proxy for food supply. 2) A peak in relative abundance of Stensioeina beccariiformis, a cosmopolitan form generally more common at the middle than at the lower bathyal sites, occurs just above the buliminid peak. 3) The relative abundance of Nuttallides truempyi, a more oligotrophic form, decreases at the boundary, then increases above the peak in Stensioeina beccariiformis. The food supply to the deep sea in the Pacific Ocean thus apparently increased rather than decreased in the earliest Danian. The low benthic diversity during a time of high food supply indicates a stressed environment. This stress might have been caused by reorganization of the planktic ecosystem: primary producer niches vacated by the mass extinction of calcifying nannoplankton may have been rapidly (<10 kyr) filled by other, possibly opportunistic, primary producers, leading to delivery of another type of food, and/or irregular food delivery through a succession of opportunistic blooms.The deep-sea benthic foraminiferal data thus are in strong disagreement with the widely accepted hypothesis that the global deep-sea floor became severely food-depleted following the K/Pg extinction due to the mass extinction of primary producers (“Strangelove Ocean Model”) or to the collapse of the biotic pump (“Living Ocean Model”).     

PRODUCTION OF MUCILAGE BY THE ADRIATIC EPIPELIC DIATOM CYLINDROTHECA CLOSTERIUM (BACILLARIOPHYCEAE) UNDER NUTRIENT LIMITATION

The carbon partitioning of the epipelic diatom Cylindrotheca closterium (Ehrenberg) Reiman and Lewin isolated from the Adriatic Sea was studied in the laboratory under varying scenarios of nutrient limitation. Total number of cells, photosynthesis measured at 695 μmol photons·m ^{− 2}·s ^{− 1} irradiance (P_{695- μ mol}), chlorophyll ( a + c ) content, respiration, extracellular polymeric substances (EPS), total particulate carbohydrate (TPC), and dissolved carbohydrate were evaluated under nitrogen and phosphorus deficiencies in culture. The highest total number of cells was found in the control, whereas the nitrogen-limited treatment showed the lowest value. During the transition phase of growth, photosynthesis in the nitrogen-limited treatment was 3-fold lower than in the phosphorus-limited treatment and 4-fold lower than in the control. Differences in respiration rates and chlorophyll ( a + c ) content were even more marked. Dissolved carbohydrate remained the same in all the treatments, whereas during the transition and stationary phase, EPS presented the highest values under phosphorus limitation and the lowest in the control treatment. The production of EPS was closely linked to the periods of carbon assimilation (transition phase) in the nutrient depleted treatments, especially in the phosphorus-limited treatment. These results point out the relevance of the nutrient imbalance (nitrogen or phosphorus) in the production of EPS by the benthic or resuspended diatoms and suggest that these diatoms play an important role in nutrient-unbalanced systems like sediments or marine snow.     

Caridina nilotica in Lake Victoria: abundance, biomass, and diel vertical migration

Caridina nilotica (Decapoda: Atyidae) in offshore waters of Lake Victoria were investigated with both day and night sampling over a period of two years. Offshore populations are mainly planktonic rather than benthic, and the animals exhibit diel vertical migrations into near-surface waters at night. These changes in diel abundance as well as the size-frequency distribution of the migrating shrimp suggest that the migratory behavior is in response to visual planktivory, because only the very smallest individuals (2–4 mm) remain in surface waters during the day. During October 1992, abundances were estimated both by vertical net sampling and by underwater video transect methods. Concordance was established between abundances estimated by the two methods. Only about 9% (night) to 14% (day) of the Caridina population appeared to be epibenthic. We suggest that the behavior of the animal is consistent with the hypothesis that it is not a strict detritivore as previously reported; rather it may engage in facultative planktivory, especially at night.     

Optimized isolation procedure for obtaining strongly actinide binding exopolymeric substances (EPS) from two bacteria (Sagittula stellata and Pseudomonas fluorescens Biovar II)

Different chemical extractants (NaCl, EDTA, HCl and NaOH) and physical methods (ultrasonication and heating) were examined by their efficacies of extracting “attached” exopolymeric substances (EPS) secreted by marine bacterium Sagittula stellata (SS) and terrestrial bacterium Pseudomonas fluorescens Biovar II (PF). Extraction by 0.5 N HCl for 3 h was best for SS while extraction by 0.05 N NaCl for 3–5 h was regarded as optimal for PF. Improvements in EPS purification included a pre-diafiltration step to remove the broth material and reduce the solution volume, thus the usage of ethanol, and time. The EPS harvested at the optimal time and purified by the improved method were enriched in polysaccharides, with smaller amounts of proteins, thus having amphiphilic properties. Isoelectric focusing of ²³⁴Th or ²⁴⁰Pu labeled EPS showed both actinides were strongly bound to macromolecules with low pI, similar to reported marine or soil colloidal natural organic matter (NOM).     

Changes in the composition of extracellular polymeric substances in activated sludge during anaerobic storage

Changes in the chemical composition of organic compounds in total activated sludge, activated sludge extracellular polymeric substances (EPS), and sludge bulk water during anaerobic storage (12 days) were studied. The background for the study was that anaerobic storage of activated sludge, which often takes place at wastewater treatment plants before dewatering, causes a deterioration of the dewaterability. The reasons are not known at present, but may be related to changes in exopolymer composition of the flocs. The results showed that a fast decrease in total sludge protein and carbohydrate took place within 3 days of anaerobic storage as a result of degradation processes, which accounted for approximately 20% of the organic fraction. The amount of uronic acids and humic compounds remained almost constant in the sludge. The EPS were extracted from the floc matrix using a cationexchange resin. In the EPS matrix a similar initial (2–3 days) degradation of proteins and carbohydrate took place, whereas the content of DNA and uronic acids showed minor changes. The extractability of humic compounds increased during the first 3 days of storage. No changes in extractability of the carbohydrate were observed. A fraction of the EPS protein was found to be difficult to extract but was observed to be degraded during the anaerobic storage. The EPS composition was further characterized by high-performance size-exclusion chromatography analysis obtained by on-line UV detection and post-column detection of proteins, carbohydrates, humic compounds and DNA. Four fractions of polysaccharides were found, of which only one was responsible for the decrease in the carbohydrate content observed with storage time. The fraction was presumably of low molecular mass. Humic compounds and volatile fatty acids (acetate and propionate) were released to the bulk water from the flocs during the storage. A possible mechanism to explain the reduced dewaterability developed during anaerobic storage, partly because of the observed changes in EPS, is discussed.     

Selective activation of neuromuscular compartments within the human trapezius muscle

Task-dependent differences in relative activity between “functional” subdivisions within human muscles are well documented. Contrary, independent voluntary control of anatomical subdivisions, termed neuromuscular compartments is not observed in human muscles. Therefore, the main aim of this study was to investigate whether subdivisions within the human trapezius can be independently activated by voluntary command using biofeedback guidance. Bipolar electromyographical electrodes were situated on four subdivisions of the trapezius muscle. The threshold for “active” and “rest” for each subdivision was set to >12% and <1.5% of the maximal electromyographical amplitude recorded during a maximal voluntary contraction. After 1 h with biofeedback from each of the four trapezius subdivisions, 11 of 15 subjects learned selective activation of at least one of the four anatomical subdivisions of the trapezius muscle. All subjects managed to voluntarily activate the lower subdivisions independently from the upper subdivisions. Half of the subjects succeeded to voluntarily activate both upper subdivisions independently from the two lower subdivisions. These findings show that anatomical subdivisions of the human trapezius muscle can be independently activated by voluntary command, indicating neuromuscular compartmentalization of the trapezius muscle. The independent activation of the upper and lower subdivisions of the trapezius is in accordance with the selective innervation by the fine cranial and main branch of the accessory nerve to the upper and lower subdivisions. These findings provide new insight into motor control characteristics, learning possibilities, and function of the clinically relevant human trapezius muscle.     

DAILY FLUCTUATIONS OF EXOPOLYMERS IN CULTURES OF THE BENTHIC DIATOMS CYLINDROTHECA CLOSTERIUM AND NITZSCHIA SP. (BACILLARIOPHYCEAE)1

Dynamics in the production of extracellular polymeric substances (EPS) were investigated for the benthic diatoms Cylindrotheca closterium (Ehrenberg) and Nitzschia sp. The effect of growth phase and light:dark conditions were examined using axenic cultures. Two EPS fractions were distinguished. Soluble EPS was recovered from the culture supernatant and represented polysaccharides that were only loosely associated with the cells. Bound EPS was extracted from the cells using warm (30° C) water and was more closely associated with the diatom aggregates. Concentrations of EPS exceeded internal concentrations of sugar throughout growth, indicating that EPS production is important in these organisms. Soluble and bound EPS revealed distinct differences in daily dynamics during the course of growth. Soluble EPS was produced continuously once cultures entered the stationary phase. During the stationary phase, chl a‐normalized EPS production rates equaled 6.4 and 3.4 d ^{? 1} for C. closterium and Nitzschia sp., respectively. In contrast, production of bound EPS occurred only in the light and was highest during the exponential phase. Up to 90% of the attached EPS that was produced in the light was degraded during the subsequent dark period. The monosaccharide distribution of EPS was constant during the course of the experiment. The soluble EPS consisted of high amounts of galactose and glucuronic acid, relative to rhamnose, glucose, xylose/mannose, and galacturonic acid. In contrast, glucose was the dominant monosaccharide present in the bound EPS. These differences suggest that the production of the two distinct EPS fractions is under different metabolic controls and probably serves different cellular functions.     

Production of yam microtubers using a temporary immersion system

Yam clones ‚Pacala Duclos’ and ‚Belep’ in temporary immersion system culture showed favourable results on shoot growth stage and in the development of microtubers in comparison with solid culture media. Cultures in temporary immersion systems in both clones obtained a higher microtuber number per plant, with greater fresh weight and diameter in comparison with solid culture media. Besides, 45 and 47% of microtubers greater than 3.0 gFW for ‚Belep’ and ‚Pacala Duclos’ clones respectively, were obtained. Those tubers may be planted without acclimatization and may be stored for a prolonged period of time.     

Optical Properties and Light Climate in Lake Verevi

The optical properties and light climate during the ice-free period in the highly stratified Lake Verevi (Estonia) have been studied together with other lakes in same region since 1994. The upper water layer above the thermocline belongs to class “moderate” by optical classification of Estonian lakes but can turn “turbid” (concentration of chlorophyll a up to 73 mg m⁻³ and total suspended matter up to 13.2 g m⁻³) during late summer blooms. In the blue part of the spectrum, light is mainly attenuated by dissolved organic matter and in red part notably scattering but also absorption by phytoplanktonic pigments effect the spectral distribution of underwater light. Consequently, the underwater light is of greenish-yellow color (550–650 nm). Rapid change in optical properties occurs with an increase of all optically active substances close to thermocline (2.5–6 m). Optical measurements are often hampered beneath this layer so that modeling of the depth distribution of the diffuse attenuation coefficient is an useful compliment to field measurements. K_d,PAR ranges from 0.8 to 2.9 m⁻¹ in the surface layer, and model results suggest that it may be up to 5.8 m⁻¹ in the optically dense layer. This forms a barrier for light penetration into the hypolimnion.     

Benthic colonization and succession on temperate sublittoral rocky cliffs

Patterns of benthic colonization and succession were investigated on a temperate rocky cliff (Aegean Sea, Eastern Mediterranean). Cement and ceramic panels deployed on the rocky substratum at 25-30 m depth were sampled every 3 months over a 2-year period yielding 28 floral and 156 animal species. Diversity, cover and abundance had low values at short immersion periods and increased at the long ones. The vagile fauna responded to the increasing habitat complexity offered by the development of algal turfs and sessile species, and the community structure remained highly dynamic. Unlike the duration of immersion, the type of substratum didn't affect species colonization except for decapods that showed a preference for cement panels. Three to four stages of succession were recorded over immersion periods, according to vagile and sessile biota, respectively, during which species richness and abundance increased. The structure of the developed communities on both artificial materials differed from the natural algal-dominated benthic community of the same area, suggesting that the recovery of rocky shore communities on temperate cliffs is time consuming.     

Changes in hydrogen peroxide homeostasis and cytokinin levels contribute to the regulation of shade-induced senescence in wheat leaves

In a previous work we demonstrated that the suppression of blue light in shaded leaves of wheat increases their senescence rate and the development of oxidative stress symptoms. In order to better understand the interaction between the oxidative metabolism and light spectral quality in the regulation of leaf senescence, we studied the evolution of H₂O₂ concentration, protein oxidation, proteolytic activity and cytokinin content in excised leaves, either illuminated (control, “C”) or shaded under blue (“B”, high blue light transmission) or green (“G”, very low blue light transmission) light filters. H₂O₂ concentration significantly increased during the first 9 h after treatment initiation, an effect that was consistently higher in treatments B and C. Leaves from these treatments showed lower chlorophyll and protein degradation rates, lower concentration of oxidized proteins, and maintained higher levels of the cytokinin isopentenyl-adenosine than those from treatment G. When moderate H₂O₂ concentrations were supplied during 6–9 h after the onset of the shade treatments, senescence rate in treatment G was delayed, while the opposite effect was observed in the presence of the H₂O₂ scavengers catalase and, to a lesser extent, dimethylthiourea. These effects were accompanied by an increment or a decrement, respectively, of catalase activity, suggesting that the early changes in H₂O₂ homeostasis in leaves from treatments B and C may contribute to the prevention rather than to the induction of further oxidative damage. Altogether our results show that the suppression of blue light transmission in shaded leaves act as a stress signal that increases their sensitivity to oxidative stress and accelerates cell death.     

A high standard metabolic rate constrains juvenile growth

The allocation of energy to various components of an individual's energy budget is often viewed as a competitive process. As such, a tradeoff may exist between production (growth) and maintenance metabolism. One view of a potential tradeoff, termed “the principle of allocation”, suggests that individuals with lower maintenance metabolic expenditures may have higher growth rates. To determine whether such a tradeoff exists, I analyzed the relationship between growth rate and maintenance metabolism of 225 juvenile snapping turtles housed in the laboratory. I measured growth from hatching to 6 months of age, and then measured oxygen consumption and calculated standard metabolic rate. Mean growth rate was 0.19 g d⁻ and mean standard metabolic rate (SMR) was 1.41 kJ d⁻. Maintenance metabolism and growth were negatively correlated after both were adjusted for body mass. The results support the “principle of allocation” theory: individuals with higher standard metabolic rates tended to have low growth rates.     

CIRCADIAN CONTROL OF MIGRATORY RESTLESSNESS AND THE EFFECTS OF EXOGENOUS MELATONIN IN THE BRAMBLING,FRINGILLA MONTIFRINGILLA

Circadian pacemakers control both “daytime” activity and nocturnal restlessness of migratory birds, and the daily rhythm of melatonin release from the pineal has been suggested to be involved in the control of migratory activity. To study the phase relations between the two activity components during entrainment and when free running, locomotor activity of bramblings (Fringilla montifringilla) was recorded continuously under a 12:12 “cool light” to “warm light” cycle (CL:WL, ca. 5000 K and ca. 2500 K, respectively) or blue light to red light cycle (BL:RL, maxima at 440 and 650 nm, respectively) at different irradiance ratios. Migratory activity was expressed primarily during the WL or RL phase of the light cycles. Under free-running conditions, the circadian periods τ correlated with the phase relations between day and night (migratory) activity components during preceding entrainment. Bramblings with migratory activity had significantly longer τ at constant light intensity than the same individuals without migratory activity. Birds with migratory activity reentrained faster after a 6h phase shift of the CL:WL cycle than birds without migratory activity. When exogenous melatonin was given in the drinking water (200 μg/mL 1% ethanol or 0.86 mM) to bramblings exposed to 12:12 CL:WL cycles with constant irradiance, the amounts of activity, which were initially higher during the WL phase of the light cycle, were suppressed to similar low levels during both light phases. The systematic changes in the amounts of activity during melatonin treatment were not correlated with consistent changes in entrainment status. The data support the hypothesis that changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by either allowing or inhibiting nocturnal activity. (Chronobiology International, 17(4), 471–488, 2000)     

Diel activity and thermoregulatory behavior of a fully aquatic frog: Xenopus laevis

Ten adult Xenopus laevis were tested individually for 48-hr periods, following an initial 24-hr introductory period, in electronic shuttleboxes which allowed them to control water temperatures without operant conditioning. Locomotor activity was recorded via photocell-monitored light beams. The frogs were nocturnal, being nearly twice as active at night as during the day. The mean preferred temperature was 22.4°C, with no significant difference between night (22.5°C) and day (22.3°C), although the modal preferendum shifted from 24°C by day to 22°C at night, with a corresponding change in skewness. The range of voluntarily occupied temperatures was 14–32°C by day and 14–29°C at night. The median thermal preferendum was 22°C both day and night.     

Chronic ethanol intake alters circadian period-responses to brief light pulses in rats

Although chronic alcohol intake is associated with widespread disruptions of sleep-wake cycles and other daily biological rhythms in both human alcoholics and experimental animals, the extent to which the chronobiological effects of alcohol are mediated by effects on the underlying circadian pacemaker remains unknown. Nevertheless, recent studies indicate that both adult and perinatal ethanol treatments may alter the free-running period and photic responsiveness of the circadian pacemaker. The present experiment was designed to further characterize the effects of chronic ethanol intake on the response of the rat circadian pacemaker to brief light pulses. Ethanol-treated and control animals were exposed to 15-min light pulses during either early or late subjective night on the first day of constant darkness following entrainment to a 12:12 light-dark cycle. Relative to pulses delivered during early subjective night and to “no-pulse” conditions, light pulses delivered during late subjective night resulted in period-shortening after-effects under constant darkness, but only in control animals, not in ethanol-treated animals. These results indicate that chronic ethanol intake reduces the responsiveness of the circadian pacemaker to acute photic stimulation, and suggest that the chronobiological disruptions seen in human alcoholics are due in part to alterations in circadian pacemaker function.