Simona Ginsburg

The evolution of associative learning: A factor in the Cambrian explosion

The Cambrian explosion is probably the most spectacular diversification in evolutionary history, and understanding it has been a challenge for biologists since the time of Darwin. We propose that one of the key factors that drove this great diversification was associative learning. Although the evolutionary emergence of associative learning required only small modifications in already existing memory mechanisms and may have occurred in parallel in several groups, once this type of learning appeared on the evolutionary scene, it led to extreme diversifying selection at the ecological level: it enabled animals to exploit new niches, promoted new types of relations and arms races, and led to adaptive responses that became fixed through genetic accommodation processes. This learning-based diversification was accompanied by neurohormonal stress, which led to an ongoing destabilization and re-patterning of the epigenome, which, in turn, enabled further morphological, physiological, and behavioral diversification. Our hypothesis combines several previous ideas about the dynamics of the Cambrian explosion and provides a unifying framework that includes both ecological and genomic factors. We conclude by suggesting research directions that would clarify the timing and manner in which associative learning evolved, and the effects it had on the evolution of nervous systems, genomes, and animal morphology.

The co-evolution of language and emotions

We argue that language evolution started like the evolution of reading and writing, through cultural evolutionary processes. Genuinely new behavioural patterns emerged from collective exploratory processes that individuals could learn because of their brain plasticity. Those cultural–linguistic innovative practices that were consistently socially and culturally selected drove a process of genetic accommodation of both general and language-specific aspects of cognition. We focus on the affective facet of this culture-driven cognitive evolution, and argue that the evolution of human emotions co-evolved with that of language. We suggest that complex tool manufacture and alloparenting played an important role in the evolution of emotions, by leading to increased executive control and inter-subjective sensitivity. This process, which can be interpreted as a special case of self-domestication, culminated in the construction of human-specific social emotions, which facilitated information-sharing. Once in place, language enhanced the inhibitory control of emotions, enabled the development of novel emotions and emotional capacities, and led to a human mentality that departs in fundamental ways from that of other apes. We end by suggesting experimental approaches that can help in evaluating some of these proposals and hence lead to better understanding of the evolutionary biology of language and emotions.

Epigenetic learning in non-neural organisms

Learning involves a usually adaptive response to an input (an external stimulus or the organism℉s own behaviour) in which the input-response relation is memorized; some physical traces of the relation persist and can later be the basis of a more effective response. Using toy models we show that this characterization applies not only to the paradigmatic case of neural learning, but also to cellular responses that are based on epigenetic mechanisms of cell memory. The models suggest that the research agenda of epigenetics needs to be expanded.

The activation enthalpies for ion conductance systems in lipid bilayer membranes

SummaryThe activation enthalpies for various ion transport mechanisms through lipid bilayer membranes were measured using a feedback control system that allowed accurate control of temperature over the range 17 to 45°C. In all cases (valinomycin, monactin, dibenzo-18-crown-6, benzo-15-crown-5, gramicidin A and alamethicin), the results give a simple straight line when presented as an Arrhenius plot, so that a single activation enthalpy characterizes the overall process in each case. The activation enthalpies for valinomycin and monactin are high and are similar to those expected for an electrostatic (“image”) barrier in the hydrocarbon phase. The interpretation of the energies for the pore-formers is more complex. Alamethicin gives a very large activation energy that probably represents the energy for a co-operative process involved in the formation of each conducting unit. The results for gramicidin A are used, together with those of Hladky and Haydon (Biochim. Biophys. Acta,274: 294, 1972) to calculate limits on the values of the energy of formation of a channel and the activation energy for formation of a channel.

Beef liver esterase: II. Kinetic properties☆

Abstract The kinetic parameters, k cat and K M , in beef liver esterase-catalyzed hydrolysis were determined for about 100 substrates, which can be classified in several groups: (1) In the ethyl ester series of fatty acids K M decreases with elongation of the acid, while k cat has a maximum value with pentanoate. (2) Alkyl acetates are better substrates as the alkyl moiety is longer, whereas esters with branched alkyl groups become worse substrates. (3) Aryl esters are very good substrates. (4) Esters of dicarboxylic acids are good substrates, but only one ester group is cleaved by the enzyme. Fumarate diester is susceptible to esterase hydrolysis, while maleate is not. (5) Esters of hydrophobic amino acids are very good substrates; the enzyme is not stereoselective and both the l and d stereoisomers are readily hydrolyzed. Branching at the β-carbon atom leads to loss of activity, and blocking of the amino group abolishes it. Fluoride ion and dl -malate esters are potent competitive inhibitors of the enzymic reaction. The optimal pH was found to lie between 8 and 8.5. The reaction rate increased between 5 and 40 °C then dropped sharply. The activity decreased at high salt concentration.

The difference in shape of spontaneous and uniquantal evoked synaptic potentials in frog muscle.

1. Spontaneous and stimulation‐induced uniquantal synaptic activity at the frog cutaneous pectoris muscle, treated with neostigmine, was recorded by focal extracellular microelectrodes. A monoexponential curve was fitted to the decay of each synaptic response. 2. A highly significant positive relationship was found between the amplitude and the decay time constant of spontaneous extracellular miniature endplate potentials (MEPPs(o)), whereas the relationship displayed by evoked uniquantal extracellular endplate potentials (EPPs(o)) was only slightly greater than zero. 3. The difference did not stem from changes in the muscle membrane conductance or from inclusion of outstanding MEPPs(o) formed as a result of the block of acetylcholinesterase. 4. The dependence of the rise time on the amplitude was also stronger in MEPPs(o) than in EPPs(o). 5. In the absence of neostigmine, MEPPs(o) exhibited a positive correlation between decay time constant and amplitude, while EPPs(o) did not show such a correlation. 6. In view of previously published models of transmitter release, it is suggested that spontaneous secretion of quanta occurs both within and outside the active zones facing postsynaptic areas of variable receptor density.

A bursting potassium channel in isolated cholinergic synaptosomes of Torpedo electric organ.

1. Pinched‐off cholinergic nerve terminals (synaptosomes) prepared from the electric organ of Torpedo ocelata were fused into large structures (greater than 20 microns) using dimethyl sulphoxide and polyethylene glycol 1500, as previously described for synaptic vesicles from the same organ. 2. The giant fused synaptosomes were easily amenable to the patch clamp technique and 293 seals with a resistance greater than 4 G omega were obtained in the ‘cell‐attached’ configuration. In a large fraction of the experiments, an ‘inside‐out’ patch configuration was achieved. 3. Several types of unitary ionic currents were observed. This study describes the most frequently observed single‐channel activity which was found in 247 out of the 293 membrane patches (84.3%). 4. The single‐channel current‐voltage relation was linear between ‐60 and 20 mV and showed a slope conductance of 23.8 +/‐ 1.3 pS when the pipette contained 350‐390 mM‐Na+ and the bath facing the inside of the synaptosomal membrane contained 390 mM‐K+. 5. From extrapolated reversal potential measurements, it was concluded that this channel has a large selectivity for K+ over Na+ (70.4 +/‐ 11.5, mean +/‐ S.E.M.). Chloride ions are not transported significantly through this potassium channel. 6. This potassium channel has a low probability of opening. The probability of being in the open state increases upon depolarization and reaches about 1% when the inside of the patch is 20 mV positive compared to the pipette side. 7. The mean channel open time increases with depolarization; thus the product current x time (= charge) also increases upon depolarization, showing properties of an outward rectifier. 8. The potassium channel in the giant synaptosome membrane has a bursting behaviour. Open‐time distribution, closed‐time distribution and a Poisson analysis indicate that the minimal kinetic scheme requires one open state and three closed states.

Use of current-voltage diagrams in locating peak energy barriers in cell membranes

SummaryThe current-voltage relations obtained by integrating the Nernst-Planck equations for a variety of energy profiles are obtained. A simple and approximate method for comparing these relations is described. The method is based on using a linearized transform of current-voltage relations for an Eyring single barrier model. A parameter, γ, related to the location of the single barrier in the Eyring model, and to the shape of the barrier in other models, is readily obtained from the slopes of the linearized relations. It is then a simple matter to determine whether a given current-voltage relation allows discrimination between any particular energy profiles. The results show that the equivalent Eyring model does not always place the peak energy barrier in the same position as other models and that quite large errors in the assignment of position may be made if such a model is used. The results are also used to test the ability of some experimental current-voltage diagrams to discriminate between various energy profiles.

Shaping the learning curve: epigenetic dynamics in neural plasticity

A key characteristic of learning and neural plasticity is state-dependent acquisition dynamics reflected by the non-linear learning curve that links increase in learning with practice. Here we propose that the manner by which epigenetic states of individual cells change during learning contributes to the shape of the neural and behavioral learning curve. We base our suggestion on recent studies showing that epigenetic mechanisms such as DNA methylation, histone acetylation, and RNA-mediated gene regulation are intimately involved in the establishment and maintenance of long-term neural plasticity, reflecting specific learning-histories and influencing future learning. Our model, which is the first to suggest a dynamic molecular account of the shape of the learning curve, leads to several testable predictions regarding the link between epigenetic dynamics at the promoter, gene-network, and neural-network levels. This perspective opens up new avenues for therapeutic interventions in neurological pathologies.

The Transition to Minimal Consciousness through the Evolution of Associative Learning

The minimal state of consciousness is sentience. This includes any phenomenal sensory experience – exteroceptive, such as vision and olfaction; interoceptive, such as pain and hunger; or proprioceptive, such as the sense of bodily position and movement. We propose unlimited associative learning (UAL) as the marker of the evolutionary transition to minimal consciousness (or sentience), its phylogenetically earliest sustainable manifestation and the driver of its evolution. We define and describe UAL at the behavioral and functional level and argue that the structural-anatomical implementations of this mode of learning in different taxa entail subjective feelings (sentience). We end with a discussion of the implications of our proposal for the distribution of consciousness in the animal kingdom, suggesting testable predictions, and revisiting the ongoing debate about the function of minimal consciousness in light of our approach.