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E R Kandel

Publications and source records attributed to E R Kandel.

At least 199 records · Page 11Linked to original sources
PubMed1987

Biochemical correlates of short-term sensitization in Aplysia: temporal analysis of adenylate cyclase stimulation in a perfused-membrane preparation.

During short-term sensitization, a simple form of nonassociative learning in Aplysia, the presentation of a single brief noxious stimulus results in enhancement of the defensive withdrawal reflex lasting minutes to tens of minutes. This behavioral plasticity involves presynaptic facilitation of synaptic transmission from the mechanosensory neurons that mediate the reflex to their central target cells. This facilitation is due to cAMP-dependent protein phosphorylation. To determine whether the time course of presynaptic facilitation might be due to a persistent increase in activity of adenylate cyclase (EC 4.6.1.1) itself, persistence of the transmitter, or yet other processes, we developed a perfused-membrane method to analyze the time course of activation of adenylate cyclase by transient stimuli. After stimulation by a pulse of stimulatory transmitter, activation of adenylate cyclase decayed within 60 sec. This finding indicates that the enzyme does not remain persistently active in the absence of transmitter and suggests that short-term retention is likely to be due to other mechanisms. Possible additional mechanisms include continued activation of the cyclase by transmitter, cellular factors extrinsic to the cyclase that prolong the time course of its activation, and persistence of processes downstream from the cyclase.

Adenylyl Cyclases↗
PubMed1987

Serotonin produces long-term changes in the excitability of Aplysia sensory neurons in culture that depend on new protein synthesis.

When isolated and grown in cell culture, the sensory and motor neurons of the gill withdrawal reflex of Aplysia readily form synaptic connections. Repeated exposures to 5-HT cause facilitation of the synaptic connections between co-cultured sensory and motor neurons lasting at least 24 hr. As a first step toward understanding the locus and the mechanisms underlying this long-term synaptic facilitation, we have examined the membrane excitability of the isolated presynaptic sensory neurons grown alone in dissociated cell culture. Four repeated applications of 1 microM 5-HT caused a significant increase in the excitability of sensory neurons, lasting at least 24 hr. This resembles the short-term changes in excitability seen in response to a single application of 5-HT. Unlike the short-term effect, this long-lasting change was blocked by exposure of the cells during the 5-HT treatment to 10 microM anisomycin, an inhibitor of protein synthesis. Thus, like the synaptic facilitation, the long-term change in excitability of the isolated presynaptic neurons differs from the short-term in requiring the synthesis of new protein. This finding suggests that the sensory neuron uses gene products to modulate membrane currents in its long-term response to repeated external stimuli that are not required in its short-term response to a single stimulus.

Action Potentials↗
PubMed1986

A critical period for macromolecular synthesis in long-term heterosynaptic facilitation in Aplysia.

Both long-term and short-term sensitization of the gill and siphon withdrawal reflex in Aplysia involve facilitation of the monosynaptic connections between the sensory and motor neurons. To analyze the relationship between these two forms of synaptic facilitation at the cellular and molecular level, this monosynaptic sensorimotor component of the gill-withdrawal reflex of Aplysia can be reconstituted in dissociated cell culture. Whereas one brief application of 1 microM serotonin produced short-term facilitation in the sensorimotor connection that lasted minutes, five applications over 1.5 hours resulted in long-term facilitation that lasted more than 24 hours. Inhibitors of protein synthesis or RNA synthesis selectively blocked long-term facilitation, but not short-term facilitation, indicating that long-term facilitation requires the expression of gene products not essential for short-term facilitation. Moreover, the inhibitors only blocked long-term facilitation when given during the serotonin applications; the inhibitors did not block the facilitation when given either before or after serotonin application. These results parallel those for behavioral performance in vertebrates and indicate that the critical time window characteristic of the requirement for macromolecular synthesis in long-term heterosynaptic facilitation is not a property of complex circuitry, but an intrinsic characteristic of specific nerve cells and synaptic connections involved in the long-term storage of information.

Amanitins↗
PubMed1986

Development of plastic mechanisms related to learning at identified chemical synaptic connections in Aplysia.

In the marine snail Aplysia californica learned changes in behavior have been traced to alterations in synaptic efficacy. With the ability to raise the animals in the laboratory, we have explored the development of four types of plastic mechanisms at identified synapses: post-tetanic potentiation and pre-synaptic inhibition, which do not as yet have known behavioral functions, and homosynaptic depression, the cellular mechanism of short-term habituation, and heterosynaptic facilitation, the basis of short-term sensitization. Homosynaptic depression and pre-synaptic inhibition are present early in juvenile life. In contrast, post-tetanic potentiation and heterosynaptic facilitation appear only later, after a discrete interval. The step-wise ontogeny of synaptic plastic mechanisms in Aplysia parallels the gradual emergence of behavior in successive developmental stages. Interestingly, senescence reverses the developmental sequence for habituation and sensitization mechanisms. To pursue further an understanding of the relationship between synapse formation and plasticity underlying learning it will be necessary to extend these studies to dissociated cell culture where mechanisms can be explored on the molecular level. Cell culture may also permit examination of the development of cellular mechanisms underlying classical and operant conditioning which may clarify differences between associative and non-associative mechanisms.

Animals↗
PubMed1986

The growth cones of Aplysia sensory neurons: Modulation by serotonin of action potential duration and single potassium channel currents.

Serotonin (5-HT) closes a specific K channel ("S") in the cell body of Aplysia sensory neurons, resulting in a slow excitatory postsynaptic potential and spike broadening. To determine whether the S channel is present and can be modulated in processes of the neuron other than the cell body, we studied the effects of 5-HT on growth cones of sensory neurons in culture by using the patch-clamp technique. Simultaneous application of 5-HT to the cell body and to the growth cones of sensory neurons produced, in both, a slow depolarization of approximately 5 mV. Also, 5-HT produced a lengthening of the duration of action potential in the growth cone and cell body by 20-30%. Similar effects were observed in isolated growth cones that had been severed from the rest of the neuron, implying that the growth cones contain all the molecular components (i.e., receptors, channels, cAMP cascade) necessary for 5-HT action. Cell-attached patch-clamp recordings demonstrated the presence of S channels in sensory neuron growth cones. Application of serotonin to the bath produced long-lasting all-or-none closures of these channels in a manner identical to the previously characterized action of 5-HT in the cell body. Thus, channel modulation is not restricted to the cell body and probably occurs throughout the sensory neuron. This strengthens the view that S-channel modulation may also occur at the sensory neuron presynaptic terminal, where it could play a role in the presynaptic facilitation produced by 5-HT.

Journal Article↗
PubMed1986

Facilitatory transmitters and cAMP can modulate accommodation as well as transmitter release in Aplysia sensory neurons: Evidence for parallel processing in a single cell.

Presynaptic facilitation of transmission from sensory to motor neurons contributes significantly to behavioral sensitization of defensive withdrawal reflexes in Aplysia. Presynaptic facilitation is associated with a decrease in the serotonin-sensitive K(+) conductance. This decrease broadens the presynaptic action potential. In addition, the procedures that cause facilitation-stimulation of the connective (the pathway from the tail and head), application of modulatory transmitters, or injection of cAMP-also increase the excitability of the sensory neurons as tested with intracellular depolarizing pulses injected into the cell body. The increased excitability is reflected in a decreased threshold for generating action potentials and a reduction in accommodation to prolonged constant current stimuli. By influencing the excitability of the peripheral processes of the sensory neurons, stimulation of the connectives or serotonin also produces a small enhancement of the response of the sensory neurons to a tactile stimulus applied to the siphon. The excitability changes appear to result, at least in part, from the same cellular mechanisms that lead to broadening of the action potential, a cAMP-mediated closure of K(+) channels. Therefore, these findings indicate that the same class of mechanisms can, in principle, have a dual action and provide further evidence for parallel processing in the modulation of transmitter release from a single neuron.

Journal Article↗
PubMed1986

Action-potential duration and the modulation of transmitter release from the sensory neurons of Aplysia in presynaptic facilitation and behavioral sensitization.

Presynaptic facilitation of transmitter release from Aplysia sensory neurons is an important contributor to behavioral sensitization of the gill and siphon withdrawal reflex. The enhanced release is accompanied by reduction of the serotonin-sensitive S current in the sensory neurons and a consequent increase in duration of the presynaptic action potential (ranging from 10% to 30%). We find that changes of similar magnitude in the duration of depolarizing voltage-clamp steps in sensory neurons in intact abdominal ganglia yield increases in synaptic potentials of 45-120%. In dissociated cell culture, these changes lead to increases of 25-60% in the synaptic potential. Prolongation of presynaptic depolarization using voltage clamp or prolongation of the duration of the action potential by K(+)-channel blockers leads to prolongation of the time-to-peak of the synaptic potentials; similar changes in time-to-peak occur during presynaptic facilitation. The time-to-peak is not changed by homosynaptic depression or by changing the Ca(2+) concentration, procedures that alter release without changing the duration of the action potential. Preventing the spike from broadening by voltage clamping the presynaptic neuron substantially reduces or blocks the facilitation. These results suggest that broadening of the action potential during facilitation is a causal factor in the enhancement of transmitter release.

Journal Article↗
PubMed1986

Additional component in the cellular mechanism of presynaptic facilitation contributes to behavioral dishabituation in Aplysia.

Sensitization of defensive gill and siphon withdrawal reflexes in Aplysia results, in part, from presynaptic facilitation of transmitter release from mechanoreceptor sensory neurons that innervate the siphon skin and synapse with interneurons and motor neurons. Presynaptic facilitation also can be elicited by application of serotonin. This facilitation is associated with two phenomena, a prolongation of the presynaptic action potential resulting from a decrease in a specific K+ current and an enhancement of the Ca2+ transients elicited by depolarization. Previous work has shown that prolongation of the action potential enhances synaptic transmission at normal levels of release. Here we report that an additional set of processes also contributes to facilitation. When repeated activation of the sensory neurons induces profound homosynaptic depression, prolonging the duration of action potentials (or of depolarizing commands under voltage clamp) has little effect on transmitter release. Nonetheless, serotonin is still capable of enhancing release. Since homosynaptic depression underlies the behavioral process of habituation, the second set of processes, by counteracting the consequences of the depression, seems to mediate the effects of dishabituation in the sensory neuron. Prolongation of the action potential by closure of the K+ channel seems to mediate the effects of sensitization.

Action Potentials↗
PubMed1986

Presynaptic inhibition produced by an identified presynaptic inhibitory neuron. I. Physiological mechanisms.

We have examined the synaptic conductance mechanisms underlying presynaptic inhibition in Aplysia californica in a circuit in which all the neural elements are identified cells (Fig. 1). L10 makes connections to identified follower cells (RB and left upper quadrant cells, L2-L6). These connections are presynaptically inhibited by stimulating cells of the L32 cluster (4). L32 cells produce a slow inhibitory synaptic potential on L10. This inhibitory synaptic potential is associated with an apparent increased membrane conductance in L10. Both the inhibitory postsynaptic potential (IPSP) and the conductance increase are voltage dependent; the IPSP could not be reversed by hyperpolarizing the membrane potentials to - 120 mV. The hyperpolarization of L10 induced by L32 reduces the transmitter output of L10 and thereby contributes to presynaptic inhibition. However, this hyperpolarization accounts for about 30% of the effect because presynaptic inhibition can still be observed even when the hyperpolarization of L10 by L32 is prevented by voltage clamping. When L10 is voltage clamped, stimulation of L32 produces a slow outward synaptic current associated with an apparent increased conductance. Both the synaptic current and conductance change measured under clamp are voltage dependent, and the outward current could not be reversed. This synaptic current is not mediated by an increase in C1- conductance. It is sensitive to external K+ concentration, especially at hyperpolarized membrane potentials. With L10 under voltage clamp, stimulation of L32 also reduces a slow inward current in L10. This current has time and voltage characteristics similar to those of the Ca2+ current. Presynaptic inhibition is still produced by L32 when L10 is voltage clamped, and transmitter release is elicited by depolarizing voltage-clamp pulses. This component of presynaptic inhibition, which accounts for approximately 70% of the inhibition, appears to be due to a decrease in the Ca2+ current in the presynaptic neuron.

Abdomen↗
PubMed1986

Presynaptic inhibition produced by an identified presynaptic inhibitory neuron. II. Presynaptic conductance changes caused by histamine.

We have examined the morphology and pharmacology of the L32 neurons, identified cells that mediate presynaptic inhibition in the Aplysia abdominal ganglion, to gain insight into the putative transmitter released by the L32 cells. We analyzed the fine structure of the synaptic release sites of L32 cells stained with horseradish peroxidase. Each varicosity of L32 was found to contain two general classes of vesicles. One class of vesicles is large (mean long diameter of 98 nm) and contains an electron-dense core that typically filled or nearly filled each vesicle profile. The second class of vesicles is smaller (mean long diameter of 67 nm) and relatively electron lucent. The size, distribution, and morphology of the vesicle population in L32's terminals was similar to that described at the synapses of the identified histaminergic neuron C2 in Aplysia (2). These morphological observations suggested that L32 cells might be histaminergic. Among the various putative transmitters tested, histamine was most effective in mimicking the postsynaptic effects of L32 cells onto L10, and onto other follower cells of L32 in the abdominal ganglion. Histamine also caused inhibition of transmitter output from L10. Both the IPSP produced by L32 in L10 and the response of L10 to histamine could be reversibly blocked by cimetidine, a histamine antagonist in Aplysia (14). These results support, but do not establish the identification of histamine as the putative transmitter of L32 cells. Histamine mimics the action of L32 in mediating presynaptic inhibition allowing us to examine in more detail the conductance changes in L10 underlying presynaptic inhibition. Voltage-clamp analysis revealed that histamine blocked the voltage-dependent Ca2+ current and increased a voltage-dependent K+ current in L10, much as did L32. Both of these changes are likely to act synergistically to inhibit transmitter release. Reduction of Ca2+ current in L10 would directly inhibit transmitter release from L10 directly by decreasing the amount of Ca2+ entering during spike depolarization. The increase in K+ current would act indirectly to reduce transmitter release from L10, by hyperpolarizing L10 and decreasing the amplitude and duration of spikes in L10, as well as reducing the steady-state Ca2+ influx. These results support the idea that in Aplysia presynaptic inhibition is caused primarily by a direct transmitter-mediated reduction in presynaptic Ca2+ current and secondarily by a hyperpolarization of the presynaptic neuron due to a transmitter-mediated increase in a K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

Abdomen↗
PubMed1986

Cell and molecular analysis of long-term sensitization in Aplysia.

We have found that one cellular locus for the storage of the memory underlying short-term sensitization of the gill and siphon withdrawal reflex in Aplysia is the set of monosynaptic connections between the siphon sensory cells and the gill and siphon motor neurons. These connections also participate in the storage of memory underlying long-term sensitization. In animals that have undergone long-term sensitization, the amplitudes of the monosynaptic connections are significantly larger (2.2x) than the ones in control animals. To study the mechanisms of onset and retention of long-term synaptic facilitation that underly long-term sensitization and the role of protein synthesis in long-term memory, we have developed two types of reduced preparations: the intact reflex isolated from the remainder of the animal, and a dissociated cell culture system in which the monosynaptic component (sensory neurons and motor neurons) of the neuronal circuit mediating the withdrawal reflex is reconstituted. We found that protein synthesis inhibitors, such as anisomycin or emetine, and RNA synthesis inhibitors, such as actinomycin D or alpha-amanitin, blocked long-term facilitation without interfering with short-term facilitation. These results suggest that the acquisition of long-term memory may require the expression of genes and the synthesis of proteins not needed for short-term memory.

Amanitins↗
PubMed1986

Effects of interstimulus interval and contingency on classical conditioning of the Aplysia siphon withdrawal reflex.

The siphon withdrawal reflex of Aplysia undergoes differential classical conditioning with cutaneous stimulation of the siphon or mantle shelf as the discriminative conditioned stimuli (CS+ and CS-) and shock to the tail as the unconditioned stimulus (US). The reflex has proved to be useful for analyzing the neural mechanisms of conditioning. To test the generality of this experimental system, we have begun to compare the properties of conditioning in Aplysia with those of conditioning in vertebrates. We first examined the effect of the interstimulus interval (ISI) by varying the time between presentation of the CS+ and the US in different groups of animals. Significant differential conditioning was obtained when the onset of the CS+ preceded the onset of the US by 0.5 sec, and marginal conditioning was obtained when the ISI was 1.0 sec. By contrast, no significant conditioning occurred when the CS+ preceded the US by 2, 5, or 10 sec, when the onsets of the stimuli were simultaneous, or when US onset preceded the CS+ by 0.5, 1.0, or 1.5 sec (backward conditioning). We next examined the effect of contingency by giving one group of animals normal differential conditioning, and a second group the same training but with additional USs inserted between the paired trials. Presentation of these additional USs reduced the degree to which the US was contingent on the CS+, but did not change the number of pairings. Animals receiving normal training again showed significant conditioning, whereas animals receiving additional USs showed no conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)

Aplysia↗
PubMed1985

Monosynaptic connections made by the sensory neurons of the gill- and siphon-withdrawal reflex in Aplysia participate in the storage of long-term memory for sensitization.

We have found that in the gill- and siphon- withdrawal reflex of Aplysia, the memory for short-term sensitization grades smoothly into long-term memory with increased amounts of sensitization training. One cellular locus for the storage of the memory underlying short-term sensitization is the set of monosynaptic connections between the siphon sensory cells and the gill and siphon motor neurons. We have now also found that these same monosynaptic connections participate in the storage of the memory underlying long-term Sensitization. We examined the amplitudes of the direct synaptic connections produced by siphon sensory neurons on the gill motor neuron L7 in nervous systems removed from control and from long-term sensitized animals 1 day after the end of long-term sensitization training. The connections were significantly larger in long-term sensitized animals than in control animals. The finding that long-term memory occurs at the same synaptic locus as the short-term memory should facilitate study of the cellular and molecular mechanisms involved in the conversion of short-term to long-term memory.

Journal Article↗
PubMed1985

Distribution of serotonin-immunoreactive cell bodies and processes in the abdominal ganglion of mature Aplysia.

Sensitization of the gill withdrawal reflex in Aplysia californica is an elementary form of learning, in part resulting from presynaptic facilitation of the LE mechanoreceptor neurons of the abdominal ganglion. It has previously been established that either application of serotonin or direct stimulation of a group of facilitatory neurons, the L29 cells of the abdominal ganglion, can simulate the effect of physiological stimulation in producing presynaptic facilitation. Because the evidence that serotonin serves as a facilitatory transmitter was indirect, we examined the distribution of serotonin-immunoreactive fibers and cell bodies in the abdominal ganglion in order to answer two questions: (1) do the sensory neurons receive serotonergic innervation and (2) are the L29 cells serotonergic? We observed two distinctive patterns of serotonergic innervation within the ganglion, sparse and dense. The sparse pattern is correlated with a serotonin-stimulated increase in cAMP in identified target cells, while the dense innervation is not. We found a sparse distribution of serotonin-immunoreactive fibers with varicosities close to both cell bodies and processes of identified LE sensory cells. It therefore is likely that the sensory neurons do receive serotonergic innervation. We also mapped the population of serotonergic neuronal cell bodies in the ganglion, and found five clusters of neurons. Cells in one of these clusters, the identified RB neurons, had previously been shown to synthesize serotonin from tryptophan and to contain the neurotransmitter in high concentration. Identified L29 facilitator cells marked by injection with Lucifer Yellow do not contain serotonin immunoreactivity and therefore evidently are not a source of serotonergic input onto sensory cells.

Animals↗