Comparison of 15N- and 13C-determined parameters of mobility in melittin

Backbone and tryptophan side-chain mobilities in the 26-residue, cytolytic peptide melittin (MLT) were investigated by ¹⁵N and ¹³C NMR. Specifically, inverse-detected ¹⁵N T₁ and steady-state NOE measurements were made at 30 and 51 MHz on MLT at 22 °C enriched with ¹⁵N at six amide positions and in the Trp¹⁹ side chain. Both the disordered MLT monomer (1.2 mM peptide at pH 3.6 in neat water) and -helical MLT tetramer (4.0 mM peptide at pH 5.2 in 150 mM phosphate buffer) were examined. The relaxation data were analyzed in terms of the Lipari and Szabo model-free formalism with three parameters: _m, the correlation time for the overall rotation; S², a site-specific order parameter which is a measure of the amplitude of the internal motion; and _e, a local, effective correlation time of the internal motion. A comparison was made of motional parameters from the ¹⁵N measurements and from ¹³C measurements on MLT, the latter having been made here and previously [Kemple et al. (1997) Biochemistry, 36, 1678–1688]. _m and _e values were consistent from data on the two nuclei. In the MLT monomer, S² values for the backbone N-H and C-H vectors in the same residue were similar in value but in the tetramer the N-H order parameters were about 0.2 units larger than the C-H order parameters. The Trp side-chain N-H and C-H order parameters, and _e values were generally similar in both the monomer and tetramer. Implications of these results regarding the dynamics of MLT are examined.     

Sodium channel opening as a precursor to inactivation

The time constant of the process producing the delay in Na inactivation development as determined by the two pulse method (_delay) was extracted and compared to that of the slowest Na activation process ₃ for the I _Na during the conditioning pulse of that same determination. _delay and two pulse inactivation _c values were computer generated using a nonlinear least squares algorithm. _h and single pulse inactivation _h values were independently generated for each determination also with the aid of the computer using the same non-linear least squares algorithm. In one determination at 2 mV, _c was 4.68 and _delay 0.494 ms while _h was 4.70 and ₃ 0.491 ms for a _c/_h of 0.996 and a _delay/₃ of 1.006. Mean _delay/₃ from five determinations in four axons, both Cs and K perfused, and spanning a potential range of-27 to 2mV was 1.068. The precursor process to inactivation is channel opening. Some fraction of channels presumably inactivate via another route where prior channel opening is not required.     

Determination of internal dynamics of deoxyriboses in the DNA hexamer d(CGTACG)2 by 1H NMR

The conformations and internal dynamics of the deoxyriboses of d(CGTACG)₂ have been determined by NMR measurements at 15°C. The conformations of the sugars were determined using coupling constants and time-dependent NOE measurements. The J-splitting patterns of the H1, H2 and H2 resonances show that the sugars exist as mixtures of conformations near C2 endo (south) and C3 endo (north). The population of the south conformation was larger for the purines than for the pyrimidines. The overall tumbling time of the molecule in ²H₂O was determined from measurements of the cross relaxation rate constant for the H6-H5 vectors of the two cytosine residues. Order parameters were determined for the H1-H2, H2-H2 and H2-H3 vectors from measurements of cross relaxation rate constants, making use of multi-spin analysis of the NOE build up rates. These order parameters are weakly dependent of the base sequence, and except for the terminal Cyt 1 residue, the H2-H2 and H2-H3 vectors are near unity, indicating the absence of rapid pseudorotation on the nanosecond time scale. However, the order parameter for the H1-H2 vector is significantly smaller than expected for rapid pseudorotation indicating the presence of other motions of the sugars. This motion must be about an effective axis parallel to the H2-H vector, and to occur with an angular fluctuation of about 30°.The results show that to obtain highly refined structures for nucleic acids by NMR the effects of spin diffusion and motional averaging cannot be ignored.Some of this work was presented as a poster at the 30^th Experimental NMR Conference at Asilomar, California 1989     

Residual motion of hemoglobin-bound spin labels and protein dynamics: viscosity dependence of the rotational correlation times

The residual motion of spin labels bound to cysteine 93 and to lysines of methemoglobin has been studied by electron paramagnetic resonance spectroscopy. To separate the influences of the solvent and the protein environment of the label fluctuations, the correlation times, , were analyzed as a function of temperature for fixed solvent viscosities, . Results show that over a wide range of viscosity the dependence of on may be empirically described by a power law ^k . The exponent k depends strongly on the location of the label on the protein surface. If one regards the spin labels as artificial amino acid side chains, characteristic values of correlation times and amplitudes of the rotational motion at the surface can be given. For =1 cP and T=297 K the correlation time of the labels bound to lysines is found to be =9 · 10^–10 s and the rotational diffusion is nearly isotropic. The spin label bound to cysteine 93 occupies a protein pocket, its rotational motion is therefore restricted. The correlation time of the label motion within a limited motion cone of semi angle =30° ± 3° is found to be =1.3 · 10^–9 s for =1 cP and T=297 K.     

Conformational dynamics of the anticodon loop in yeast tRNAPhe as sensed by the fluorescence of wybutine

Conformational and dynamic properties of the anticodon loop of yeast tRNA^Phe were investigated by analyzing the time resolved fluorescence of wybutine serving as a local structural probe adjacent to the anticodon GmAA on its 3 side. The influence of Mg²⁺, important for stabilizing the tertiary structure of tRNA, and of the complementary anticodon s²UUC of E. coli tRNA ₂ ^Glu were investigated.Fluorescence lifetimes and anisotropies were measured with ps time resolution using time correlated single photon counting and a mode locked synchronously pumped and frequency doubled dye laser as excitation source. From the analysis of lifetimes () and rotational relaxation times ( _R ) we conclude that wybutine occurs in various structural states: (i) one stacked conformation where the base has no free mobility and the only rotational motion reflects the mobility of the whole tRNA molecule (=6 ns, _R =19 ns), (ii) an unstacked conformation where the base can freely rotate (=100 ps, _R = 370 ps) and (iii) an intermediary state (=2 ns, _R = 1.6 ns).Under biological conditions, i. e. in the presence of Mg²⁺ and neutral salts, wybutine is found in a stacked and immobile state which is consistent with the crystallographic picture. In the presence of the complementary codon however, as exemplified by the E. coli-tRNA ₂ ^Glu anticodon, our analysis indicates that the codon-anticodon complex exists in an equilibrium of structural states with different rotational mobility of wybutine. The conformation with wybutine freely mobile is the predominant one and suggests that this conformation of the codon-anticodon structure differs from the canonical 3–5 stack.     

State and time delay estimation of continuous microorganisms cultivation

A continuous fermentation model taking into account the culture memory is used for a state estimation design. The influence of the culture memory on the process dynamics is accounted for by a time delay parameter. The proposed procedure of on-line state estimation in the case when the delay has a constant value is based on the extended Kalman observer. The case when the delay parameter is evaluated on-line is also considered. An adaptive state and parameter algorithm on the base of the extended Kalman filter is proposed. The theoretical results are applied to continuous culture for growth of a strain of Saccharomyces cerevisiae.List of Symbols X, S mg/l Biomass concentration and substrate concentration respectively - S ⁰ mg/l Feed substrate concentration - Z mg/l Past substrate concentration - µ h^–1 Specific growth rate taking into account culture memory - h^–1 Specific consumption rate - h Time delay parameter denoting culture memory - D h^–1 Dilution rate - State variables vector - W _ij Gain coefficient for on-line state and parameter estimation - F Substrate feed rate vector - () Gain coefficient matrix - R Square symmetric Riccati matrix - K Matrix of coefficients - K(t) Delay kernel taking account of culture memory - Denote an estimation value The partial support by Bulgarian National Science Research Foundation under Grant SRTS 428/94 Modeling and Control of Fermentation Processes Taking the Memory Effect into Account is gratefully acknowledged.     

Internal motions of apo-neocarzinostatin as studied by 13C NMR methine relaxation at natural abundance

Summary Dynamics of the backbone and some side chains of apo-neocarzinostatin, a 10.7 kDa carrier protein, have been studied from ¹³C relaxation rates R1, R2 and steady-state ¹³C-{¹H} NOEs, measured at natural abundance. Relaxation data were obtained for 79 nonoverlapping C resonances and for 11 threonine C single resonances. Except for three C relaxation rates, all data were analysed from a simple two-parameter spectral density function using the model-free approach of Lipari and Szabo. The corresponding C–H fragments exhibit fast (_e < 40 ps) restricted libration motions (S²=0.73 to 0.95). Global examination of the microdynamical parameters S² and _e along the amino acid sequence gives no immediate correlation with structural elements. However, different trends for the three loops involved in the binding site are revealed. The -ribbon comprising residues 37 to 47 is spatially restricted, with relatively large _e values in its hairpin region. The other -ribbon (residues 72 to 87) and the large disordered loop ranging between residues 97–107 experience small-amplitude motions on a much faster (picosecond) time scale. The two N-terminal residues, Ala¹ and Ala², and the C-terminal residue Asn¹¹³, exhibit an additional slow motion on a subnanosecond time scale (400–500 ps). Similarly, the relaxation data for eight threonine side-chain C must be interpreted in terms of a three-parameter spectral density function. They exhibit slower motions, on the nanosecond time scale (500–3000 ps). Three threonine (Thr⁶⁵, Thr⁶⁸, Thr⁸¹) side chains do not display a slow component, but an exchange contribution to the observed transverse relaxation rate R2 could not be excluded at these sites. The microdynamical parameters (S², _e and R2_ex) or (S _infslow ^sup2 , S _inffast ^sup2 and _slow) were obtained from a straightforward solution of the equations describing the relaxation data. They were calculated assuming an overall isotropic rotational correlation time _e for the protein of 5.7 ns, determined using standard procedures from R2/R1 ratios. However, it is shown that the product (1–S²)× _e is nearly independent of _e for residues not exhibiting slow motions on the nanosecond time scale. In addition, this parameter very closely follows the heteronuclear NOEs, which therefore could be good indices for local fast motions on the picosecond time scale.     

Carotenogenesis and asparagine inLeptosphaeria michotii (West) Sacc.

Summary InLeptosphaeria michotii U¹⁴C-asparagine was incorporated into the coloured carotenoids, the synthesis of which carried on till day 8. The pigment turnover, obvious from day 6, was not modified by the light conditions used.Nicotine (0.25 to 4.5 mM) has been used to study carotenogenesis and sporulation rhythm regulation inL. michotii fed with asparagine 2.6 mM. Control cultures contained in darkness -carotene only and in continuous light -carotene 98% and lycopene 2%. The mold receiving nicotine 0.25 mM in darkness contained -carotene 98% and lycopene 2%. For nicotine 0.5 mM and upwards -carotene decreased, lycopene increased and -carotene appeared, the balance between these pigments also depending on the light conditions. Whereas period length () of the sporulation rhythm increased from one cycle to the next in control cultures in darkness, it was stabilized either by continuous light ( 27 h) or by nicotine 0.25 mM ( 30 h). For nicotine 0.5 mM sporulation was uniform in darkness or in light.     

Existence and uniqueness of a sharp travelling wave in degenerate non-linear diffusion Fisher-KPP equations

In this paper we use a dynamical systems approach to prove the existence of a unique critical value c ^* of the speed c for which the degenerate density-dependent diffusion equation u _ct = [D(u)u _x ] _x + g(u) has: 1. no travelling wave solutions for 0 < c < c ^*, 2. a travelling wave solution u(x, t) = (x - c ^* t) of sharp type satisfying (– ) = 1, () = 0 ^*; '(^*–) = – c ^*/D'(0), '(^*+) = 0 and 3. a continuum of travelling wave solutions of monotone decreasing front type for each c > c ^*. These fronts satisfy the boundary conditions (– ) = 1, '(– ) = (+ ) = '(+ ) = 0. We illustrate our analytical results with some numerical solutions.     

A comparative study of the backbone dynamics of two closely related lipid binding proteins: Bovine heart fatty acid binding protein and porcine ileal lipid binding protein

The backbone dynamics of bovine heart fatty acid binding protein (H-FABP) and porcine ileal lipid binding protein (ILBP) were studied by 15N NMR relaxation (T1 and T2) and steady state heteronuclear 15N{1H} NOE measurements. The microdynamic parameters characterizing the backbone mobility were determined using the model-free approach. For H-FABP, the non-terminal backbone amide groups display a rather compact protein structure of low flexibility. In contrast, for ILBP an increased number of backbone amide groups display unusually high internal mobility. Furthermore, the data indicate a higher degree of conformational exchange processes in the sec-msec time range for ILBP compared to H-FABP. These results suggest significant differences in the conformational stability for these two structurally highly homologous members of the fatty acid binding protein family.     

Estimating the time scale of chemical exchange of proteins from measurements of transverse relaxation rates in solution

Chemical (conformational) exchange on the ms-s time scale is reliably identified by the observation of transverse relaxation rates, R_ex, that depend upon the strength of the effective field (_1eff=B_1eff) used in spin lock or CPMG experiments. In order to determine if the exchange correlation time, _ex, is the fast or slow limit, measurements of (i) signal line shape and (ii) temperature dependence of R_ex have been commonly used in studies of stable, small molecules. However, these approaches are often not applicable to proteins, because sample stability and solubility, respectively, limit the temperature range and signal sensitivity of experiments. Herein we use a complex, but general, two-site exchange equation to show when the simple fast exchange equations for R_ex are good approximations, in the case of proteins. We then present a simple empirical equation that approximately predicts R_ex in all exchange regimes, and explains these results in a clear, straightforward manner. Finally we show how one can reliably determine whether _ex is in the fast or slow exchange limit.     

Heritable variation in the phaseolin protein of nondomesticated common bean,Phaseolus vulgaris L.

Summary Crude protein extracts from single seeds of nondomesticated Mexican bean accessions were analysed by SDS polyacrylamide gel electrophoresis for variability in phaseolin protein. Six new phaseolin types; M1, M2, M3, M4, M5, M6, which contained polypeptides within the same range of molecular weights (51,000 to 45,000 daltons) as occur in the S, T and C phaseolin types of cultivated beans were identified. No T and C types were found among the non-domesticated Mexican accessions, and the S type occurred in less than 7% of the seeds screened. Genetic analyses of F₂ progenies from crosses between Sanilac (S), and five of the M types showed that each M phaseolin phenotype was allelic to the S type and expressed codominantly.     

Holling's “hungry mantid” model for the invertebrate functional response considered as a Markov process. Part I: The full model and some of its limits

In this paper we give an analytical reformulation of Holling's (1966) simulation model for invertebrate predatory behaviour. To this end we represent a population of predators as a frequency distribution over a space of (physiological) states. The functional response of a predator is calculated from the (stable) equilibrium distribution of its state as a function of prey density.Starting from the general model various other models are obtained by limit processes, some of them new and some of them old. The more interesting of which will be studied in further papers in this series.List of Notation a rate constant of digestion - b maximum of rate constant of prey encounter in the mantid - b maximum pursuit duration in the mantid (_p(0)) - c satiation threshold for search - c satiation threshold for pursuit in the mantid: c=c(b-D_s/v)/b - D _m maximum sighting distance - D _p pursuit distance - D _s strike distance - expectation operator - f, f ₀ rate of change of satiation during search - f ₁ rate of change of satiation during prey handling - F functional response: number of prey eaten per unit of time by one predator - g rate constant of effective prey encounter in the gobbler and sucker - g₀ rate constant of prey encounter - g₁ probability of no prey loss from pursuit - g₂ probability of no prey escaping during pursuit - H Holling secretary correction factor in the sucker: fraction of the time spent searching - k _R density of R - k_T probability density of maximum prey handling time - K probability that maximum prey handling time is _e, i.e. pursuit duration is zero - K _R distribution function of R - N number of prey caught - p (marginal) density of S - p₀ density of S in search - p₁ simultaneous density of S and T - P probability - p ₁ marginal density of S in handling prey - q probability of strike success - R ratio of realized to maximum sighting distance - s, S satiation - satiation axis - t time - handling time axis - u eating speed - U homogeneous(0,1) random variable - v pursuit speed - V exponential(1) random variable - w prey weight - W exponential(_m) random variable - x prey density - ratio of maximum successful pursuit duration to meal duration (_pm/_e) - _pm - relative duration of successful pursuit (_p/_pm) - ratio of shortest to largest sighting distance - x_e - time already spent handling a prey item - rate of prey loss during prey handling - prey escape rate during pursuit - prey biomass density (xw) - , T maximum time still to be spent handling a prey item - _e meal duration - _m maximum handling time ( _e+ _p) - _p duration of successful pursuit - _pm maximum duration of successful pursuit (_p(0)) - hazard rate - _m maximum of hazard rate - scaled functional response (wF) - minimal i-state space     

Effect of Gamma Irradiation on Embryogenic Avocado Cultures and Somatic Embryo Development

Embryogenic avocado cultures were exposed to ionizing irradiation in order to determine its effect on proliferation and subsequent somatic embryo development. The approximate PD₅₀ as determined by linear regression is 35 Gy 2 weeks after irradiation for Fuerte 2.11.1 and 4 weeks after irradiation for T362 2.11.1. Irradiation of embryogenic cultures did not significantly affect the number of early stage Fuerte 2.11.1 somatic embryos that developed directly from irradiated cultures; however, 10–50 Gy inhibited somatic embryo development. Irradiation of T362 2.11.1 embryogenic cultures at 25–50 Gy inhibited the number of intermediate and mature stages of somatic embryos that developed directly from irradiated cultures, and 50 Gy inhibited somatic embryo maturation. Inhibition of somatic embryo development could be partially offset by proliferation of irradiated embryogenic cultures as suspensions. Irradiation up to 10 Gy significantly increased the number of mature Fuerte 2.11.1 somatic embryos that developed from suspension cultures. Irradiation with doses up to 25 Gy stimulated development of heart stage T362 2.11.1 somatic embryos; however, mature somatic embryo development was suppressed at dosages of 10 Gy and greater.     

Potential induced in a spherical cell by an intracellular point source and an extracellular point sink

Summary The potential is calculated for all time, inside and outside a spherical cell for a point source of current inside the cell and a point sink located a finite distance outside the cell. The source and sink are step functions in time. An eigenfunction expansion is obtained, valid for arbitrary =_m a/_i , where _i and _m are the conductivities inside the cell and in the membrane, respectively, a is the cell radius and the membrane thickness. For small , the eigenfunction expansion is expanded in powers of . The time dependence of the potential contains transients with two widely differing time constants =C_m a/_i, where C_m is the membrane surface capacitance, and _m=/. Closed-form expressions are obtained for the two leading terms, for small , after the rapid transient is over. The remaining time dependence is only in the potential inside the cell, and is a simple exponential increase, independent of position within the cell. It is found that the transmembrane potential is insensitive to the location of the extracellular sink at long times, but not at short times. The dependence of the potential on location of source, sink, and observer is studied for long times after the quick transients are over. A uniqueness theorem is derived for the solution to Laplace's equation for the membrane boundary condition.This work was supported in part by NSF Grant No. GB-24965. Dr. Peskoff is the recipient of NIH Special Research Fellowship No. 1F03 GM 55849-01. Mr. Ramirez is a Ford Foundation Pre-doctoral Fellow.     

Mutations in a sequence near the N-terminus of the small subunit alter the CO2/O2specificity factor for ribulose bisphosphate carboxylase/oxygenase

Seven unique substitutions have been introduced by site-directed mutagenesis into the first conserved region of the small subunit of ribulose bisphosphate carboxylase/oxygenase from Anacystis nidulans 6301. After expression of each large, altered-small subunit gene tandem in Escherichia coli, two substitutions in the small subunit tyr17asp17 (Y17D) and arg10gly10 (R10G) result in little or no carboxylase activity. For the latter substitution, no L₈S₈enzyme complex could be detected suggesting that this mutation prevents assembly. Mutant enzymes containing the following substitutions R11G, T14A, S16A, Y17D and P19A have CO₂/O₂specificity factors ( values) of 40, 35, 18, 39 and 44, respectively, compared with that of 44 for wild-type recombinant enzyme whereas P20A has full carboxylase activity and a value of 55.     

Average phase difference theory and 1∶1 phase entrainment in interlimb coordination

The dynamics of coupled biological oscillators can be modeled by averaging the effects of coupling over each oscillatory cycle so that the coupling depends on the phase difference between the two oscillators and not on their specific states. Average phase difference theory claims that mode locking phenomena can be predicted by the average effects of the coupling influences. As a starting point for both empirical and theoretical investigations, Rand et al. (1988) have proposed d/dt= — K sin ), with phase-locked solutions =arcsin( /K), where is the difference between the uncoupled frequencies and K is the coupling strength. Phase-locking was evaluated in three experiments using an interlimb coordination paradigm in which a person oscillates hand-held pendulums. was controlled through length differences in the left and right pendulums. The coupled frequency _c was varied by a metronome, and scaled to the eigenfrequency _v of the coupled system K was assumed to vary inversely with _c. The results indicate that: (1) and K contribute multiplicatively to (2) =0 or = regardless of K when =0; (3) 0 or regardless of when K is large (relative to ); (4) results (1) to (3) hold identically for both in phase and antiphase coordination. The results also indicate that the relevant frequency is _c/_v rather than _c. Discussion high-lighted the significance of confirming =arcsin(/K) for more general treatments of phase-locking, such as circle map dynamics, and for the 11 phase-entrainment which characterizes biological movement systems.     

Application of the Criterion of Biological Time (τ n /τ0) to the Studies of Embryogenesis in Birds

It was shown on the example of chick embryo that the number ₀ ( _n /₀) can be recommended as a measure of biological time and, for this purpose, the duration of the minimal mitotic cycle during synchronous cleavage divisions should be determined (in minutes) in various avian species. Based on the preliminary data, one can propose the comparability and similarity of the temporal programs of gastrulation and somitogenesis in the chick embryo and embryos of some fish and amphibians.     

Die Aktivitätsperiodik von zwei tagaktiven Nagern,Funambulus palmarum undEutamias sibiricus,unter Dauerlichtbedingungen

Zusammenfassung Bei, in kleinen Käfigen mit Zugang zu einer Lauftrommel, einzeln gehaltenen Tieren der beiden ArtenFunambulus palmarum (Palmenhörnchen) undEutamias sibiricus (asiatisches Streifenhörnchen) wurde der Einfluß verschiedener Beleuchtungsstärke unter Dauerlichtbedingungen auf die circadiane Aktivitätsperiodik untersucht.4 Palmenhörnchen wurden in verschiedenen Beleuchtungsstärken von nicht meßbarer Intensität bis etwa 1000 Lux gemessen. Von etwa 1 Lux bis 1000 Lux war die Periodendauer () positiv mit der Beleuchtungsstärke korreliert. Über den gesamten Bereich der untersuchten Lichtintensitäten war die Aktivitätsmenge positiv mit dem Logarithmus der Beleuchtungsstärke (logI _LL ) korreliert. Es bestand jedoch ebenso wie zwischen und logI _LL keine lineare Beziehung zwischen beiden Größen (Fig. 1). Das Verhältnis Aktivitätszeit zu Ruhezeit (:) war bei allen untersuchten Tieren weder mit noch mit der Aktivitätsmenge korreliert.Von insgesamt 15 unter Dauerlichtbedingungen zwischen 0,01 und 100 Lux gemessenen asiatischen Streifenhörnchen zeigten nur 7 Tiere circadiane Perioden der Aktivität. Infolge der großen Streuung der Aktivitätsanfänge und -enden sowie des Aktivitätsmusters konnten a und : bei den meisten Tieren nicht bestimmt werden. Die -Werte streuten um einen mittleren Wert von 23,6 Std; sie zeigten jedoch keine regelhafte Abhängigkeit von der Beleuchtungsstärke (Fig. 5).Bei einzelnen Tieren beider Arten wurde bei Beleuchtungsstärken von etwa 3–4 Lux eine Aufspaltung (temporäre Dissoziation) einzelner Aktivitätskomponenten mit verschiedenen Frequenzen beobachtet (Figs. 2 und 4).Aufgrund der unterschiedlichen Abhängigkeit der Aktivitätsparameter (Aktivitätszeit und Aktivitätsmenge) von bei verschiedenen Tiergruppen wird vermutet, daß das Licht sowohl verschiedene Parameter der circadianen Aktivitätsperiodik als auch einzelne Komponenten des der Aktivität zugrunde liegenden circadianen Systems unabhängig voneinander beeinflußt. Tag- und nachtaktive Säuger unterscheiden sich im wesentlichen nur in den kritischen Beleuchtungsstärken, d.h. den Beleuchtungsstärken, bei welchen die kürzesten Perioden gemessen werden (Fig. 6).

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The activity rhythm of two day-active rodents,Funambulus palmarum andEutamias sibiricus, in continuous illumination

Summary Palm squirrels (Funambulus palmarum) and Sibirian chipmunks (Eutamias sibiricus) were kept individually in small cages attached to a running wheel. Under continuous illumination the influence of different light intensities on the circadian rhythm of activity was measured.Four palm squirrels were exposed to light intensities between unmeasurable intensity (total darkness) and about 1000 lux. Between 1 and 1000 lux the period length () of the circadian rhythm of activity was positively correlated with light intensity. The amount of activity per period showed a positive relationship with light intensity over the range of intensities tested (Fig. 1). In all 4 animals the ratio activity-time to rest-time (: ) was neither correlated with the amount of activity per period nor with .From a total of 15 Sibirian chipmunks tested under continuous illumination between 0.01 and 100 lux only 7 animals showed circadian rhythms of activity. As a consequence of the great variability of the beginning and the end of activity as well as in the temporal activity pattern, a (activity time) and : could not be determined in most animals of this species. The values of varied around a mean of 23.6 hr, but did not change as a function of light intensity (Fig. 5).In some individuals of both species a splitting or temporary dissociation of single components of activity with different frequencies was observed at light intensities of about 3 to 4 lux (Figs. 2 and 4).On the basis of the different relationships between the activity functions (amount of activity and activity-time) and in various groups of animals (cf. day-active mammals and birds), it is assumed that light acts separately on the parameters of the circadian activity rhythm as well as on single components of the circadian system underlying activity. Day- and night-active mammals differ essentially only in respect to their critical light intensities, i.e. the light intensities at which the shortest circadian periods are measured (Fig. 6).

     

End plate currents at the physiological level of quantal secretion and after potentiation of transmitter release by 4-aminopyridine

The role of postysynaptic potentiation (PSP) and asynchronous secretion of acetylcholine (ACh) in the generation of multiquantal currents and end plate currents (EPC) was investigated under voltage clamp conditions in transected sartorius muscle of the frog before and after 4-aminopyridine (4-AP) treatment. Compared with miniature EPC (MEPC), showing an average quantum content of 249, multiquantal EPC has a larger amplitude, longer rise-time, and longer decay-time (_epc). Magnesium ions (6–10 mM) reduce the amplitude and _spec of EPC without affecting its rise-time. Rhythmical stimulation (10 Hz for 60 sec) results in reduced amplitude and but increased rise-time of EPC. D-turbocurarine (5×10^–7 M) and -bungarotoxin (1×10^–5 gm/ml) diminishes the difference between _epc and _mepc. In the presence of 4-AP, all these effects are much more pronounced. It is proposed that asynchronous secretion of ACh from motor nerve teminals causes prolongation of the rise-time and reduction of the amplitude of EPC but has little or no effect upon the decay rate of EPC. The slow decay of multiquantal EPC, both in the absence and in the presence of 4-AP, is almost entirely due to postynaptic interaction of ACh quanta, i.e., PSP.Kurashov Meidcal Institute, Kazhan. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 48–56, January–February, 1991.