Journal de physiologie最新文献

This paper gives a short historical summary of how the second messenger concept, introduced by Earl Sutherland some three decades ago, has been refined and applied by neurobiologists to account for long-lasting changes in the membrane properties of certain neurons. Such refinements in the second messenger hypothesis have application to two specific long-lasting changes in neurons in Aplysia. In the bag cell neuroendocrine system, a brief synaptic input induces an afterdischarge lasting about 30 minutes. Both cAMP-dependent and Ca2+ and phospholipid-dependent protein kinases are activated by the synaptic input and a variety of potassium and calcium channels are modulated. In the eye of Aplysia a spontaneous circadian modulation of ion channels takes place over a twenty-four hour period. In addition phase shifts of this circadian oscillator are mediated, for light by cGMP and for serotonin by cAMP. The circadian oscillator, unlike the bag cell afterdischarge mechanism, is sensitive to ionizing radiation as well as to transcriptional inhibitors. Evidence is presented that specific proteins are synthesized at different times in the circadian cycle. One of these proteins (m.w. 41.9, pI 5.5) accumulates linearly with time of day, resembling a sawtooth oscillator. This protein may be the driver for the circadian oscillation itself. The role of second messengers in various forms of plasticity in neuronal systems (sensitization, long-term potentiation, long-term depression, "learning") may just be part of a very widespread mechanism by which neurons and other cells can generate long-lasting changes in membrane and other cellular properties with brief inputs (synaptic, hormonal) that are of some special adaptive value to the organism.

1.) By extracellular and intracellular recordings of the red nucleus (RN) cell activity, we investigated enhancement of signaling effectiveness at the cortico-rubral synapses underlying the establishment of classical conditioning mediated by RN in the cat. The classical conditioning of forelimb flexion was produced by pairing the conditioned stimulus (CS) to the cerebral peduncle (CP) with the unconditioned stimulus (US) to the forelimb skin at an interval of 100 msec for about a week. 2.) The increased responsiveness of RN cells to the CS was correlated with acquisition of the conditioned forelimb flexion, i.e. RN cells responded to the CS with higher firing probability in the animals which received the paired conditioning than those in the animals which received the CS alone or pairing of the CS and the US at random intervals or those in the naive animals which did not receive any training. 3.) Monosynaptic excitation of RN cells in response to the single pulse to CP was most enhanced in the animals which received the paired conditioning. By contrast, response of RN cells, as well as the behavioral response, induced by stimulation of the cerebellar interpositus nucleus (IP) was not enhanced after the paired conditioning. The difference between the responses to the stimulation of CP and IP suggested that the primary site of neuronal change is the cortico-rubral synapses. 4.) In the animals that received the paired conditioning, the excitatory postsynaptic potentials (EPSPs) induced by stimulation of CP had fast-rising components superimposed on the normal slow-rising EPSPs. On the other hand, most of the CP-EPSPs recorded in the naive animals showed a slow time course. The slow time course of the CP-EPSPs has been attributed to the peripheral localization of the cortico-rubral synapses on the dendrites of RN cells. 5.) The electrotonic length of RN cells in the animals which received the paired conditioning was not shorter than that in the naive animals. Therefore, it was suggested that the appearance of the fast-rising component in the CP-EPSPs is cause by formation of the new cortico-rubral synapses on proximal portions of the soma-dendritic membrane of RN cells. 6.) Since it has been established that new synapses formed by collateral sprouting are retained for more than several months, the formation of new synaptic connections could underlie long-lasting behavioral modification.

1.) This paper further explores the feasibility of a model for long-term memory (Lisman, 1985; Lisman and Goldring, 1988). According to this model, the value of synaptic efficacy of individual synapses is stored locally by the group of Ca2+/calmodulin-dependent protein kinase II molecules contained within the post-synaptic density. 2.) Calculations presented in a previous paper indicate that it is feasible for these kinase molecules to encode information with the stability required for long-term memory. These calculations were based on the assumption that the 30 phosphorylation sites on the enzyme are phosphorylated in a serial fashion, i.e. that the sites are not independent. This paper presents similar calculations based on the alternative assumption that the sites are phosphorylated independently. 3.) Kinase molecules that have been switched "on" undergo a continuous process of dephosphorylation and rephosphorylation. Estimates of the energy consumed by this cycle indicate that the energy required would not make unreasonable demands on neuronal metabolism. The necessity of energy consumption by molecular switches is discussed. 4.) The rate of intramolecular autophosphorylation of the Ca2+/calmodulin dependent kinase implies that the cytoplasmic free Ca2+ concentration must stay elevated for several seconds in order for kinase molecules to switch "on". How such a sustained rise in Ca2+ might be achieved is discussed. 5.) An alternative view is that long-term memory storage involves a change in gene expression. The principal evidence supporting this view is the effect of protein synthesis inhibitors on memory. A model is presented showing that the observed effect of protein synthesis inhibitors does not necessarily imply that information storage is at the level of gene expression. 6.) The distinction between "presynaptic" and "postsynaptic" associative learning mechanisms is discussed. It is concluded that this distinction can be misleading and that the possibility that the underlying mechanisms are similar should not be excluded.

1.) Application of serotonin to Aplysia sensory neurons can result in facilitated synaptic transmission, both short- and long-term. This facilitation is likely to be produced by a complex set of molecular mechanisms: serotonin activates adenylate cyclase, increasing cAMP and protein kinase (Cedar and Schwartz, 1972); serotonin also changes the subcellular distribution of the Ca2+/calmodulin-dependent protein kinase (Saitoh and Schwartz, 1983). Recently, phorbol esters also have been shown to produce facilitation. We have therefore investigated how protein kinase C (PKC) participates in serotonin-mediated synaptic facilitation. 2.) We found that the Aplysia genome encodes PKC, which is expressed in nervous tissue as at least two abundant transcripts (about 0.003% of the total message). Its inferred amino acid sequence is 85% homologous to that of enzymes from mammals and Drosophila, and over 95% homologous if compared to both. The specific activity of the Aplysia kinase is comparable to that found in rat brain, with similar reaction parameters and dependencies on phosphatidylserine (PS), Ca2+, diacylglycerol and phorbol esters. While PKC is found on neuronal membrane in the basal state, the PKC activators, Ca2+ and phorbol esters, further translocate the kinase to membrane in crude extracts of neuronal tissue. The amounts of membrane-bound PKC, as determined by 3H-phorbol-ester binding, are greatest in neuropil and nerve. 3.) Exposure of sensory neurons and their terminals in Aplysia pleural-pedal ganglia to facilitating doses of either phorbol ester or serotonin results in the translocation of PKC from cytosol to membrane, activating the enzyme. cAMP does not produce this translocation.(ABSTRACT TRUNCATED AT 250 WORDS)

The olfactory system is favorable for studying mechanisms of development, plasticity and regeneration. Monoclonal antibodies have been generated which differentially stain olfactory axons and can identify their earliest trajectories in the fetal rat. The developing olfactory pathway also shows differential metabolic activity, as revealed by the 2-deoxyglucose method, and these patterns show plasticity as judged by both physiological and behavioral measures. The sensory neurons undergo dieback and neurogenesis following axonal transection; electrophysiological methods are being used to reveal the membrane mechanisms underlying this unique capacity.

In order to study the role of the renal pelvis on urea sparing in sheep fed low protein diets, the pelvis was perfused through the ureter with 1M and 3M urea solutions. Eight ewes were used: four on a regular diet (total nitrogen 188.7 g.kg-1 dry matter) and the other four on a low protein diet (total nitrogen 109.4 g.kg-1 dry matter). On each animal, perfusions were performed on one kidney; the other one was kept as a control. Fractional excretion of urea (TEu) and urea (Cu), inulin, para-aminohippurate and osmolar clearances, were determined during five experimental periods of 30 min each (T = control, 1M = perfusion with 1M urea solution, R1 = first period of recovery, 3M = perfusion with 3M urea solution, R2 = second period of recovery). 1. During control periods sheep on low protein diet have a greater capacity of urea retention than sheep on regular diet, under antidiuretic conditions (inulin U/P = 200). The following data (means +/- S.D.) are all reduced in animals on low protein diet: TEu by 36% (0.38 +/- 0.19 vs. 0.59 +/- 0.28 for normal protein sheep, p less than 0.05), Cu by 55% (0.50 +/- 0.19 vs. 1.15 +/- 0.49 ml.min-1.kg-1 for normal sheep, p less than 0.01) and amount of urea excreted by 80% (2.1 +/- 0.7 vs. 10.4 +/- 2.7 mg.min-1 for normal sheep, p less than 0.01). 2. The linear regression analysis of the relationship between tubular reabsorption of urea and its filtered amount shows that the capacity of urea retention is significantly higher in low protein sheep and that the difference between the two groups is greater as the filtered amount increases. Following 1M and 3M perfusions, the capacity of urea reabsorption by the perfused kidneys is significantly decreased in low protein animals whereas there is no change in the normal ones. The result is that perfused kidneys of the low protein sheep increase the amount of urea excreted during these periods: urine concentration of urea (Uu) increases by 55% during R1 and by 144% during R2, TEu increases by 60% during R1 and by 147% during R2 and Cu increases by 40% during R1 and by 95% during R2, without any variation of urine flow rate. These changes could be understood, provided that an important transfer of the perfused urea to the renal medulla in the low protein sheep would reduce the concentration gradients which enhance urea passive reabsorption from the collecting ducts.(ABSTRACT TRUNCATED AT 250 WORDS)

Classical conditioning of the gill withdrawal reflex can be demonstrated in two different in vitro Aplysia preparations. The data obtained show that as conditioning of the gill withdrawal reflex proceeds there are changes in synaptic efficacy at the central sensory-motor neurone synapse. These changes in synaptic efficacy, however, are not necessary nor are they sufficient for the observed changes in gill reflex behaviour. Changes must be occurring at other loci within the nervous system to mediate the associative learning. We hypothesized, based on data obtained from one type of in vitro preparation, that changes occur in the ability of the motor neurone to elicit a gill withdrawal response as a result of classical conditioning training. In order to test this hypothesis we depolarized an identified gill motor neurone before and after classical conditioning and found that the motor neurone's ability to elicit a gill movement was facilitated following classical conditioning training. In control preparations that received an explicitly unpaired stimulus paradigm (which does not lead to classical conditioning of the reflex) there was a decrease in the efficacy of a gill motor neurone to elicit a gill withdrawal response. There are a number of possible sites within the integrated central (CNS) and peripheral (PNS) nervous systems where changes could occur to bring about the alterations in motor neurone efficacy. Our results suggest that changes in neuronal activity which underlie learning occur at multiple sites within the nervous system and that a complete understanding of the mechanisms of associative learning can only be obtained when all of these sites are taken into account.

1.) This review considers factors that produce systematic variations in the amplitude of monosynaptic group Ia EPSPs in triceps surae alpha-motoneurons belonging to different motor unit types. 2.) Anatomical studies using horseradish peroxidase to label functionally-identified group Ia afferents and motoneurons postsynaptic to them, and combined anatomicalelectro-physiological studies of type-identified alpha-motoneurons, have constrained some of the factors that produce variations in peak Ia EPSP amplitude in different cells. 3.) Computer modeling studies based on these experimental data, together with other evidence in the literature, suggest that the major factor that produces systematic variation in Ia EPSP amplitudes in type FF, FR, and S motoneurons is a corresponding variation in the density of active group Ia synapses. 4.) Although EPSP amplitudes are also affected by the relative conductance of the somatic membrane, as reflected in the dendritic-to-somatic conductance ratio, it is possible that at least some of this influence is an artifact produced by microelectrode penetration.

After a control experiment under initial normal hydration (N), five healthy unacclimated subjects were studied to investigate the effects of initial hypo- and hyperhydration on cardiovascular and thermo-regulatory responses to prolonged intermittent exercise in the heat (To = 36 degrees C; Tdp = 10 degrees C; Va = 0.6 m.s-1). Prior hydrohydration (O) was obtained by diuretics and prior hyperhydration (R) by ingestion of 0.5 L of isotonic (ISO) electrolyte sucrose solution 30 min before the experiments (4 h) started. Exercise (70 W) lasted 3 hours, and was periodically interrupted by resting periods (5-10 min). Three dehydration (D) runs were thus performed under the three initial hydration states (O,N,R) without fluid replacement during the exercise period. Four additional rehydration runs were carried out: 2 in each initial hydration level (O, R) included ingestion (at 36 degrees C) of water or ISO-solution during the first 3 hours. Physiological measurements were continuously recorded and hourly blood samples (15 ml) were obtained. Results showed that dehydration increased core temperature and heart rate and provoked blood hypovolemia and hyperosmolarity, the latter being somewhat prevented by prior ISO-ingestion. Dehydration reduced significantly the overall sweat rate only in hypohydrated subjects and the large hyperosmolarity seemed to be responsible for this. The significant Tcore rise during dehydration is unlikely to be the result of a decrease in evaporative heat transfer, which was found only in the case of initial hypohydration. Rehydration during exercise with water or ISO-solution induced different dynamic responses depending on the initial hydration level, but it never restored plasma volume.(ABSTRACT TRUNCATED AT 250 WORDS)

1. The mechanism which enables activation of the nicotinic receptors to modify the synthesis of RNA was investigated in the incubated superior cervical ganglion of the rat. RNA labelling with [5-3H] uridine was used in order to screen the effects of varying the ionic environment and altering Na+ channels on the sequence of the three changes in ganglionic RNA synthesis induced by stimulation, i.e. a) an initial decrease, b) an increase during the late stages of stimulation, c) another increase taking place after the end of stimulation. These successive variations were obtained by either repetitive excitation of the preganglionic nerve, or application of ACh or carbachol to the ganglion. 2. The three changes in RNA synthesis mediated by ACh or carbachol were prevented when the KCl concentration of the medium was increased up to 37 mM or when NaCl of the medium was replaced with Tris or sucrose. This confirmed previous indications that the sequence of activity-induced changes is initiated by the transmembrane ionic fluxes mediated by nicotinic activation and not by depolarization per se. 3. Application of aconitine to resting ganglia for 1 h decreased the RNA synthesis to the same extent as a 1 h preganglionic or ACh stimulation. This effect was prevented by a concomitant application of tetrodotoxin (TTX) which also restored the ability of carbachol to modify RNA synthesis. This suggested that the initial decrease in RNA synthesis is caused by the increase in [Na+]i which seems to interfere directly with the transcription process. 4. The increase of RNA synthesis occurring during the late stages of synaptic activation was selectively inhibited by replacing Cl- of the medium with SO4-. On the other hand, the post-stimulation increase was selectively inhibited when the generation of after-hyperpolarization resulting from the electrogenic extrusion of Na+ was prevented by substituting LiCl for NaCl. This indicated that increases in RNA synthesis during and after the stimulation are triggered by different ionic events. 5. An induction of RNA synthesis was obtained, without previous activation of the nicotinic receptors, by incubating the ganglia at rest in conditions which entail the generation of an hyperpolarization resulting from the activation of the Na(+)-K- pump, i.e. low external KCl as well as application of TTX following an aconitine treatment. However, in these cases, the increase in RNA synthesis was delayed by about 2 h as compared to that observed after the end of nicotinic activation.(ABSTRACT TRUNCATED AT 400 WORDS)