Differential classical conditioning of a defensive withdrawal reflex in Aplysia californica

The defensive siphon and gill withdrawal reflex of Aplysia is a simple reflex mediated by a well-defined neural circuit. This reflex exhibits classical conditioning when a weak tactile stimulus to the siphon is used as a conditioned stimulus and a strong shock to the tail is used as an unconditioned stimulus. The siphon withdrawal component of this reflex can be differentially conditioned when stimuli applied to two different sites on the mantle skin (the mantle shelf and the siphon) are used as discriminative stimuli. The differential conditioning can be acquired in a single trial, is retained for more than 24 hours, and increases in strength with increased trials. Differential conditioning can also be produced within the field of innervation of a single cluster of sensory neurons (the LE cluster) since two separate sites on the siphon skin can serve as discriminative stimuli. The finding that two independent afferent inputs that activate a common set of interneurons and motor neurons can be differentially conditioned restricts the possible cellular loci involved in the associative learning.     

Long-term habituation of a defensive withdrawal reflex in aplysia

A tactile stimulus to the siphon of Aplysia produces a defensive withdrawal reflex consisting of contraction of the siphon, the gill, and the mantle shelf. We studied long-term habituation of this reflex using two types of preparations, one focusing on the siphon component and the other on the gill component of the reflex. Siphon withdrawal, studied in unrestrained animals, showed marked habituation within a single ten-trial training session. Five daily training sessions produced habituaton that built up across days and lasted for at least 3 weeks. Furthermore, spaced training produced significantly longer lasting habituation than massed training. Gill withdrawal, studied in a restrained animal, also showed long-term retention of habituation. Since the neural circuitry of gill withdrawal is relatively well understood, it may be possible to study the cellular mechanisms underlying a long-term behavioral modification.     

Long-term sensitization in Aplysia: biophysical correlates in tail sensory neurons

A fundamental problem in the cellular analysis of learning and memory is the identification of the neuronal substrates of long-term information storage and their relation to short-term cellular alterations. In this report, biophysical correlates of long-term sensitization of a simple withdrawal reflex in the mollusc Aplysia were examined. A voltage-clamp analysis of the sensory neurons that control the reflex, 24 hours after sensitization training, revealed a significant reduction in net outward current. The results indicate that one mechanism for the storage of long-term sensitization is the regulation of membrane currents that influence the characteristics of the action potential and the excitability of individual neurons. The results also provide insights into the relation between short- and long-term sensitization in that the biophysical loci involved in the storage of long-term sensitization appear similar to those involved in short-term sensitization.     

Behavioral dissociation of dishabituation, sensitization, and inhibition in Aplysia

Three forms of nonassociative learning (habituation, dishabituation, and sensitization) have commonly been explained by a dual-process view in which a single decrementing process produces habituation and a single facilitatory process produces both dishabituation and sensitization. A key prediction of this view is that dishabituation and sensitization should always occur together. However, we show that dishabituation and sensitization, as well as an additional process, inhibition, can be behaviorally dissociated in Aplysia by (i) their differential time of onset, (ii) their differential sensitivity to stimulus intensity, and (iii) their differential emergence during development. A simple dual-process view cannot explain these results; rather, a multiprocess view appears necessary to account for nonassociative learning in Aplysia.     

Long-term facilitation in Aplysia involves increase in transmitter release

In a variety of vertebrates and invertebrates, long-lasting enhancement of synaptic transmission contributes to the storage of memory lasting one or more days. However, it has not been demonstrated directly whether this increase in synaptic transmission is caused by an enhancement of transmitter release or an increase in the sensitivity of the postsynaptic receptors. These possibilities can be distinguished by a quantal analysis in which the size of the miniature excitatory postsynaptic potential released spontaneously from the presynaptic terminal is used as a reference. By means of microcultures, in which single sensory and motor neurons of Aplysia were plated together, miniature excitatory postsynaptic potentials attributable to the spontaneous release of single transmitter quanta from individual presynaptic neurons were recorded and used to analyze long-term facilitation induced by repeated applications of 5-hydroxytryptamine. The results indicate that the facilitation is caused by an increase in the number of transmitter quanta released by the presynaptic neuron.     

Long-term synaptic changes produced by a cellular analog of classical conditioning in Aplysia

A change in synaptic strength arising from the activation of two neuronal pathways at approximately the same time is a form of associative plasticity and may underlie classical or Pavlovian conditioning. A cellular analog of a classical conditioning protocol produces short-term associative plasticity at the connections between sensory and motor neurons in Aplysia. A similar training protocol produced long-term (24-hour) enhancement of excitatory postsynaptic potentials (EPSPs). EPSPs produced by sensory neurons in which activity was paired with a reinforcing stimulus were significantly larger than unpaired controls 24 hours after training. Thus, associative plasticity at the sensory to motor neuron connection can occur in a long-term form in addition to the short-term form. In this system, it should be possible to analyze the molecular mechanisms underlying long-term associative plasticity and classical conditioning.     

Neuronal mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia

The cellular mechanisms of habituation and dishabituation of the gill-withdrawal reflex in Aplysia were studied with an isolated abdominal ganglion connected to a piece of skin from the tactile receptive field of the reflex. By obtaining simultaneous intracellular recordings from both the sensory neurons and one of the main identified motor neurons, we have been able to reduce the reflex to its monosynaptic components. The monosynaptic excitatory postsynaptic potentials showed a profound low-frequency depression when repeatedly elicited and showed heterosynaptic facilitation after application of a strong stimulus to another pathway. Thus, both habituation and dishabituation can be explained in part and perhaps entirely by changes in the efficacy of specific excitatory synapses.     

Neuronal correlates of habituation and dishabituation of the gill-withdrawal reflex in Aplysia

We have examinived the nieural correlates of habittuatiotn atid dishabitiuation of tlhe gill-withdrwal reflex in Aplysia. We obtained intracelllular recordings from identified gill motor neurons in the abdominal ganglionz of a semi-intact preparation of Aplysia wlhile we simultaneously recorded behavior responises of the gill. Habituation and dishabituation were not due to peripheral changes in either the sensory receptors or the gill musculature butt were caused by changes in the amplitlude of the excitatory synaptic potentials produced at the gill motor neurons.     

Proteins in a nonvenomous defensive secretion: biosynthetic significance

In common with many arthropods, the true bug, Leptoglossus phyllopus, when disturbed, emits a two-phase secretion that consists of an organic phase and an aqueous phase. The organic phase is a mixture of highly reactive low-molecular-weight compounds, analogous to those produced by other arthropods, and is deterrent to many kinds of predators. The aqueous phase, heretofore ignored in most analyses of arthropod defensive secretions, contains proteins. Even though the secretion is not injected, the proteins enzymatically catalyze the derivation of the most reactive components within the impermeable cuticular storage reservoir and, thus, constitute part of the defensive system that appears to be commonly used by arthropods producing irritating chemicals.     

防御酶在水稻抗瘟性中的作用

用稻瘟病菌菌株193(ZB25)对湘资3150(XZ3150),Tetep,关东51,南京11,CO39和丽江新团黑谷6份水稻材料进行苗期抗性和过氧化物酶POD,多酚氧化酶PPO,苯丙氨酸解氨酶PAL和抗坏血酸过氧化物酶APX等防御酶活性测定.结果表明,XZ3150,Teteo抗病性较好,叶瘟发病级数为0级和1级,关东51叶瘟发病级数为2级,为害叶面积约1%,南京11,CO39和丽江新团黑谷发病较重.抗病水稻的POD,PPO,PAL活性显著高于感病水稻.接种后抗病水稻中该3种酶活性波动不明显,而感病水稻中该3种酶活性波动较大,240 h后,其酶值甚至低于接种前的水平,该3种酶活性与水稻抗性呈正相关;接种后,抗病水稻中APX活性增加较早,活性增强持续时间也较长;而感病水稻的APX活性增加较迟且其波动较小.     

Central and peripheral control of gill movements in Aplysia

Two types of gill contraction in Aplysia were used to study the relation of peripheral and central pathways in controlling behavioral responses in a mollusk. A weak or moderate tactile stimulus to the mantle elicits gill contraction (gill-withdrawal reflex) as a component of a more extensive withdrawal response; a stimulus applied directly to the gill elicits a localized response of the gill pinnule (pinnule response). Central pathways through the abdominal ganglion are both necessary and sufficient for the gill-withdrawal reflex, and motor neuron L7 makes direct connections with gill muscles, without engaging the peripheral plexus. Peripheral pathways are necessary and sufficient for the pinnule response. As a result of the independence of peripheral and central pathways, habituation by repeated tactile stimulation of one pathway does not affect the responsiveness of the other pathway.     

Activity-dependent enhancement of presynaptic inhibition in Aplysia sensory neurons

Tail shock produces transient presynaptic inhibition and longer lasting presynaptic facilitation of the siphon sensory neurons in Aplysia. The facilitation undergoes activity-dependent enhancement that is thought to contribute to classical conditioning of the gill- and siphon-withdrawal reflex. Inhibition of the sensory neurons has now also been shown to undergo activity-dependent enhancement when action potential activity in the sensory neurons is paired with inhibitory transmitter. This effect appears to involve an amplification of the same cellular mechanisms that are involved in normal presynaptic inhibition. These results suggest that activity-dependent enhancement may be a general type of associative cellular mechanism.     

Neuronal controls of a behavioral response mediated by the abdominal ganglion of Aplysia

Tactile stimulation of the siphon and mantle shelf in Aplysia causes a characteristic withdrawal response of the external organs of the mantle cavity. A similar response also occurs spontaneously. Both responses are mediated by the abdominal ganglion and therefore provide an opportunity for correlating cellular functioning and behavior in a relatively simple and well-studied neuronal system. The withdrawal responses are controlled by five identified motor cells which receive two types of synaptic inputs. One set of excitatory connections, activated by tactile stimulation of the siphon and mantle shelf, mediates the defensive withdrawal reflex. A second set of connections is activated by a spontaneous burst of activity in a group of closely coupled interneurons which are excitatory to some of the motor cells and inhibitory to the others. This second set of connections mediates the spontaneous withdrawal response. These two inputs can therefore switch the same population of motor cells from a simple reflex to a more complex, internally organized response.     

Low internal conductivity of Aplysia neuron somata

The internal conductivity of Aplysia neuron somata was measured by passing constant current pulses across a calibrated four-electrode array. The intracellular medium is less than one-tenth as conductive as seawater. The low conductivity probably results from structured cell water since ions are present in quantity and do not appear to be bound.     

Emergence of posttetanic potentiation as a distinct phase in the differentiation of an identified synapse in Aplysia

The developmental time course of posttetanic potentiation was studied at an identified chemical synapse. In stage 11 juveniles (3 weeks after metamorphosis), the synaptic connections made by cholinergic neuron L10 onto postsynaptic neurons L2 to L6 were present but showed no posttetanic potentiation. In stage 13 adults (12 weeks after metamorphosis), the same tetanus resulted in an increase of 300 percent in the synaptic potential. A similar pattern was observed at two other identified synapses in the abdominal ganglion. Thus, the initial steps in synapse formation do not include the expression of this plastic capability. Rather, at least 10 weeks is required between the onset of synaptic function and the final expression of mature synaptic properties.     

Identification of a peptide specific for Aplysia sensory neurons by PCR-based differential screening.

In order to identify genes specific for the sensory neurons of Aplysia, a miniaturized differential screening method based on the polymerase chain reaction and applicable to small amounts of tissue was used. One messenger RNA was isolated that is expressed in every mechanoreceptor sensory cluster of the Aplysia central nervous system. This messenger RNA encodes a peptide that seems to function as an inhibitory cotransmitter. The peptide selectively inhibits certain postsynaptic cells but not others and thereby allows the sensory neurons to achieve target-specific synaptic actions.     

延边地区水稻冷害及其防御技术

根据统计资料和实地调查结果,阐明了延边地区低温冷害的现状及其防御技术．近年来延边地区示范推广优质、抗冷水稻新品种“吉粳81号”（“品星1号”）,大大减轻了低温冷害的影响．     

Choline acetyltransferase: regional distribution in the abdominal ganglion of Aplysia

Using a new microassay, we have determined the properties and the regional distribution of choline acetyltransferase in the abdominal ganglion of Aplysia. Enzyme concentrations in homogenates of groups of cells and in single identified cells indicate that neurons which function as neurosecretory cells, and which do not form chemical synapses with other cells or with peripheral structures, have little or no ability to synthesize acetylcholine; neurons which are involved in visceromotor integrations, and which connect with each other or with the periphery, have a substantial concentration of the enzyme.