Accumulation of an apoplastic solute in the guard-cell wall is sufficient to exert a significant effect on transpiration in Vicia faba leaflets

Accumulation of recently photosynthesized sucrose in the guard‐cell wall is the empirical foundation for a hypothesis that links the rates of photosynthesis, translocation, and transpiration (Plant Physiology 114, 109–118). Critical assumptions of this hypothesis were tested by use of Vicia faba, an apoplastic phloem loader. Following measurements of the leaflet‐apoplastic‐water volume (by P–V isotherm analysis) and the guard‐cell wall volume (by 3‐D analysis), intact leaflets were fed dilute solutions of mannitol, an impermeant non‐toxic osmolyte. Even at bulk‐leaflet mannitol concentrations that would have only a negligible osmotic effect on stomata, transpiration at constant temperature, water‐vapour pressure, air movement and irradiance was diminished up to 25%, compared with controls. This effect on transpiration, a manifestation of smaller stomatal aperture size, was explained by accumulation of mannitol, up to 350 mol m ^{? 3}, in the estimated aqueous volume of the guard‐cell wall. The conclusion is that mannitol, a xenobiotic with structural similarity to sucrose, can move throughout the apoplast of a transpiring leaflet and accumulate in an osmotically significant concentration in the guard‐cell wall. These data therefore provide support for a new role for sucrose as a signal metabolite that integrates essential functions of the whole leaf. In addition, the results raise questions about the physiological or experimental accumulation of other guard‐cell‐targeted apoplastic solutes such as plant growth regulators, particularly abscisic acid, and ions.     

The antidromic activation of tectal neurons by electrical stimuli applied to the caudal medulla oblongata in the toad,Bufo bufo L.

In order to specify the tectal projection to the bulbar/spinal regions, the antidromic responses of the physiologically identified tectal neurons as well as the gross antidromic field responses in the optic tectum to electrical stimuli applied to the caudal medulla were examined in the paralyzed common toad, Bufo bufo. The antidromic field potential was recorded in the optic tectum in response to electrical stimuli applied to the ventral paramedian portion of the contralateral caudal medulla (where the crossed tecto-spinal pathway of Rubinson (1968) and Lázár (1969) runs), but generally not when they were applied to various parts of the ipsilateral caudal medulla. The antidromic field potential was largest at the superficial part of Layer 6 or at the border between Layers 6 and 7 of the optic tectum, indicating that neurons in these layers project to the contralateral caudal medulla. Mapping experiments of the antidromic field potential over the optic tectum showed that the antidromic field potential was recorded mainly in the lateral part of it, indicating that this part of the optic tectum is the main source of projection neurons to the contralateral caudal medulla. Various classes of tectal neurons as well as retinal ganglion neurons were identified from the characteristics of the response properties to moving visual stimuli and the properties of the receptive fields. Of these, the Class T1, T2, T3, T4, T5(1), T5(2), T5(3), and T5(4) tectal neurons were activated antidromically by stimuli applied to the contralateral caudal medulla. Only a limited proportion of the Class T5(1) neurons was activated antidromically by stimuli applied to the ipsilateral caudal medulla. On the other hand, the Class T7 and T8 neurons, as well as the Class R2, R3, and R4 retinal neurons, were not activated antidromically by stimuli applied to the caudal medulla of either side. These results suggest a possibility that these tectal neurons which project to the medullary regions form the substrate of the sensorimotor interfacing and contribute to the initiation or coordination of the visually guided behavior, such as prey-catching.     

Light chain gene conversion continues at high rate in an ALV-induced cell line.

We have analyzed immunoglobulin light chain sequences from avian leukosis virus (ALV) induced bursal and metastatic tumors and from cell lines derived from these tumors. Sequence data presented demonstrate that ALV-induced tumors and one cell line (DT40) derived therefrom continue to diversify their light chain genes outside of the bursal environment. Diversification within these tumor cells seems to occur by gene conversion events comparable with those observed in bursal B cells. Sequence analysis of spontaneously arising surface immunoglobulin negative subclones of the DT40 cell line revealed frameshifts within the rearranged light chain genes which most likely resulted from non-functional recombination events. Superimposed gene conversion events can repair these frameshifts leading to re-expression of surface immunoglobulin.     

Der Einfluß von Störreizen auf die Antwortbereitschaft bei der Richtbewegung der Erdkröte (Bufo bufo L.)

Löst man bei der Erdkröte (Bufo bujo L.) die Richtbewegung (Taxis) als 1. Teilglied der Beutefanghandlung mit einer bewegten Beuteattrappe unter konstantem Darbietungswinkel auf einem Auge wiederholt aus, so sinkt die Richtbereitschaft langsam ab (Reizserie). Innerhalb einer Reizserienfolge (mehrere Reizserien, getrennt durch Erholungspausen konstanter Dauer) vermindert sich die Anzahl der Richtbewegungen je Reizserie annähernd exponentiell. Es wurde geprüft, in welcher Weise der Verlauf der taxisspezifischen Ermüdung durch optische, akustische und taktile Reize gestört werden kann. Optische Störreize waren Bewegungsreize (bewegte Beuteattrappe) auf das andere Auge oder auf eine andere Retinapartie. Als akustisches Störsignal diente eine elektrische Klingel. Taktile Störreize waren Borstenreize (punktförmige Reize) oder Wasser (Flächenreize).

• 1 Optische und taktile Reize während einer Erholungspause förderten die Erholung der zuvor ermüdeten Richtbewegung (Bahnung bzw. Aufmerksam-keitssteigerung).
• 2 Akustische und taktile Störreize während einer Reizserie konnten die Richtbereitschaft anhaltend oder kurzfristig unterdrücken (Aufmerksamkeitsverschiebung). Im ersten Fall gewöhnte sich die Kröte an den Störreiz relativ langsam; dann nahm die Anzahl der Richtbewegungen ab. Im zweiten Fall gewöhnte sie sich schnell an ihn, und die Phase maximaler Richtbereitschaft wurde innerhalb der Reizserie nur zeitlich verschoben.

     

Edge preference of retinal and tectal neurons in common toads (Bufo bufo) in response to worm-like moving stripes: the question of behaviorally relevant ‘position indicators’

Summary Previous experiments have shown that during prey-catching behavior (orienting, snapping) in response to a worm-like moving stripe common toads.Bufo bufo (L.) exhibit a contrast-and direction-dependent edge preference. To a black (b) stripe moving against a white (w) background (b/w), they respond (R^*) preferably toward the leading (l) rather the trailing (t) edge (R _l ^* > R _t ^* ), thus displaying head preference. If the contrastdirection is reversed (w/b), the stripe's trailing edge is preferred (R _l ^* < R _t ^* ), hence showing tail preference. In the present study, neuronal activities of retinal classes R2 and R3 and tectal classes T5(2) and T7 have been extracellularly recorded in response to leading and trailing edges of a 3 ° × 30 ° stripe simulating a worm and traversing the centers of their excitatory receptive fields (ERF) horizontally at a constant angular velocity in variable movement direction (temporo-nasal or naso-temporal).The behavioral contrast-direction dependent edge preferences are best resembled by the responses (R) of prey-selective class T5(2) neurons (R_l R_t=101 for b/w, 0.31 for w/b) and T7 neurons (R_lR_t=61 for b/w, 0.41 for w/b); the T7 responses may be dendritic spikes. This property can be traced back to off-responses dominated retinal class R3 neurons (R_lR_t=61 for b/w, 0.51 for w/b), but not to class R2 (R_lR_t =1.21 for b/w and 0.91 for w/b). The respective edge preference phenomena are independent of the direction of movement.When stimuli were moved against a stationary black-white structured background, the head preference to the black stripe and the tail preference to the white stripe were maintained in class R3, T5(2), and T7 neurons. If the stripe traversed the ERF together with the structured background in the same direction at the same velocity, the responses of tectal class T5(2) and T7 neurons were strongly inhibited, particularly in the former. Responses of retinal R2 neurons in comparable situations could be reduced by about 50%, while class R3 neurons responded to both the stimulus and the moving background structure.The results support the concept that the prey feature analyzing system in toads applies principles of (i) parallel and (ii) hierarchial information processing. These are (i) divergence of retinal R3 neuronal output contributes to stimulus edge positioning and (in combination with R2 output) area evaluation intectal neurons and to stimulus area evaluation and (in combination with R4 output) sensitivity for moving background structures inpre tectal neurons; (ii) convergence of tectal excitatory and pretectal inhibitory inputs specify the property of prey-selective tectal T5(2) neurons which are known to project to bulbar/spinal motor systems.Abbreviations ERF excitatory receptive field - IRF inhibitory receptive field - N nasal - T temporal - R _w response to a worm-like stripe moving in the direction of its longer axis - R _A response to an antiworm-like stripe whose longer axis is oriented perpendicular to the direction of movement - R _l response to the leading edge of a worm-like moving stripe - R _t response to the trailing edge of a worm-like moving stripe - b/w black stimulus against a white background - w/b white stimulus against a black background - sm structured moving background - ss structured stationary background - u minimal structure width of a structured background consisting of rectangular black and white patches in random distribution - HRP horseradish peroxidase     

The dopamine D2 receptor: two molecular forms generated by alternative splicing.

Cloned human dopamine D2 receptor cDNA was isolated from a pituitary cDNA library and found to encode an additional 29 amino acid residues in the predicted intracellular domain between transmembrane regions 5 and 6 relative to a previously described rat brain D2 receptor. Results from polymerase chain reactions as well as in situ hybridization revealed that mRNA encoding both receptor forms is present in pituitary and brain of both rat and man. The larger form was predominant in these tissues and, as shown in the rat, expressed by dopaminergic and dopaminoceptive neurons. Analysis of the human gene showed that the additional peptide sequence is encoded by a separate exon. Hence, the two receptor forms are generated by differential splicing possibly to permit coupling to different G proteins. Both receptors expressed in cultured mammalian cells bind [3H]spiperone with high affinity and inhibit adenylyl cyclase, as expected of the D2 receptor subtype.     

Concepts in vertebrate neuroethology

Neuroethology is concerned with the analysis of the neural substrates and mechanisms that underlie behaviour. In 1951, Tinbergen implied the following goals of Ethophysiology, today known as Neuroethology: (1) recognition and localization of behaviourally meaningful stimuli, e.g. key stimuli; (2) sensorimotor interfacing and feedback interactions; (3) modulations according to internal states and acquired information; (4) motor pattern generation; and (5) ontogenetic and phylogenetic aspects. Using the toad's (Bufo bufo) visually-guided prey-catching behaviour as an example, experimental strategies and concepts of vertebrate Neuroethology can be demonstrated: (i) visual space is mapped in the brain in a multiple manner; (ii) specification of neurons results from inhibitory and/or excitatory interactions among and within space-mapped neuronal networks; (iii) specialized neurons, e.g. those with stimulus recognition and localization properties, show information processing that takes place in functional units of cell assemblies (so-called ‘neuronal machines’ or ‘brain chips’); (iv) activation of motor pattern generators for different behavioural actions may require coincidence of inputs from different combinations of specialized neurons that serve as command elements and together form a command system; (v) a command system can be regarded as a sensorimotor interface fulfilling tasks with respect to (a) visual pattern recognition and localization, (b) command functions, by initiating the activation of the motor pattern generation system, (c) motor pattern generation through participation in the temporal sequence of the positive feedback from the motor system, and (d) integration of modulatory inputs according to internal states; (vi) the motor pattern generation circuitry consists of a neuronal network capable of producing a consistent distribution of excitation and inhibition and whose output has privileged access to the required motoneuronal pools; (vii) the basic principles of prey selection and motor pattern generation of prey capture emerge in the post-metamorphic juvenile toad with the transition to land and without previous prey experience. During early ontogeny the acuity of sensory discrimination and the performance of motor patterns are subject to maturation paralleled by neural differentiations in the bbrain. performance of motor patterns are subject to maturation paralleled by neural differentiations in the brain. those are programmed and, in amphibians, presumably less dependent on the interaction with the environment than in mammals. This of course does not exclude the possibility of modulating, modifying, specifying and/or extending the visual pattern recognition system of toads due to internal states and individual experience.