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.     

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.     

Habituation and dishabituation: interactions between peripheral and central nervous systems in Aplysia

The withdrawal response of the isolated siphon of Aplysia habituates to a light stimulus and dishabituates to a tactile stimulus, and vice versa, with or without connections to the central nervous system. The peripheral nervous system can dishabituate or enhance the response mediated by the central nervous system and vice versa. Normally the adaptive siphon withdrawal response of the intact animal must be mediated by the integrated activity of the peripheral and central nervous systems.     

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 of a defensive withdrawal reflex in Aplysia

When a weak tactile stimulus is applied to the siphon of Aplysia californica, the animal withdraws the siphon between the parapodia. This defensive withdrawal reflex can be facilitated (sensitized) if the animal is previously given 4 days of training, consisting of four brief noxious stimuli each day. The sensitization of this reflex can last for up to 3 weeks after training and is mediated by the abdominal ganglion which also mediates long-term habituation. This preparation may provide a system for analyzing the neural mechanism of long-term behavioral modifications of complexity which is intermediate between habituation and associative learning.     

Acquisition and retention of long-term habituation in Aplysia: correlation of behavioral and cellular processes

To examine the cellular mechanisms responsible for transition from a short-term to a long-term behavioral modification, a rapid training procedure was developed for producing long-term habituation of the defensive withdrawal of gill and siphon in Aplysia. Four ten-trial training sessions, with 1(1/2)-hour intersession intervals, produced habituation that was retained for more than 1 week. This 5-hour procedure could be applied to a test system in the isolated abdominal ganglion where the cellular changes accompanying the acquisition of long-term habituation can be examined. During acquisition, intracellular recordings were obtained from L7, a major gill and siphon motor neuron, and the pattern of stimulation used in the behavioral experiments was applied to an afferent nerve. Acquisition was associated with a progressive decrease in the complex excitatory synaptic potential produced in L7 by afferent nerve stimulation. When retention was tested 24 hours later, the synaptic decrement was still evident. Thus, a behaviorally meaningful stimulus sequence, consisting of only 40 patterned stimuli, leads to changes in synaptic effectiveness lasting one or more days in a neural pathway involved in short-term habituation of this reflex.     

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.     

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.     

Operant reward learning in Aplysia: neuronal correlates and mechanisms

Operant conditioning is a form of associative learning through which an animal learns about the consequences of its behavior. Here, we report an appetitive operant conditioning procedure in Aplysia that induces long-term memory. Biophysical changes that accompanied the memory were found in an identified neuron (cell B51) that is considered critical for the expression of behavior that was rewarded. Similar cellular changes in B51 were produced by contingent reinforcement of B51 with dopamine in a single-cell analog of the operant procedure. These findings allow for the detailed analysis of the cellular and molecular processes underlying operant conditioning.     

Habituation and dishabituation in the absence of a central nervous system

Habituation and dishabituation have been observed in a semi-intact Aplysia preparation in which the central nervous system is removed. The amplitude of withdrawal responses in the gill decreases in proportion to the rate of water drops applied (one drop per 0.5 minute to one drop per 2.5 minutes at 15 degrees C). The effects of habituation last for at least 2 hours. A dishabituated response is elicited by stopping the water drops or electrically stimulating the preparation. Furthermore, the gill contains nerve cell bodies, and habituation and dishabituation appear to be properties of these peripheral neurons.     

Bilateral symmetry and interneuronal organization in the buccal ganglia of Aplysia

Principles of functional organization of the bilaterally symmetric buccal ganglia of Aplysia were studied in 20 identified cells used as a reference population. Four of the identified cells (two in each ganglion) are multiaction interneurons, each of which innervates six identified ipsilateral follower cells, mediating cholinergic excitation to one cell and cholinergic inhibition to five others. Bilateral coordination is effected by common inputs to all four interneurons. Ipsilateral pairs of interneurons are electrotonically coupled and produce identical synaptic actions on their common follower population. This apparent redundancy of interneuronal action leads to feed-forward summation, eliciting amplified synaptic output from each interneuron pair.     

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.     

Phycomyces: habituation of the light growth response

Phycomyces sporangiophores respond to four distinct physical stimuli: gravity, light, stretch, and an avoidance stimulus. Saturating the organism with a light stimulus so that it does not respond to any additional light program does not decrease its ability to respond to an avoidance stimulus. This demonstrates that the organism has the potential to respond after a saturating light stimulus and that the an avoidance stimulus acts at some point past or parellel or parellel to the light-receiving mechanism.     

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.     

Studying neural organization in Aplysia with the scanning electron microscope

Preliminary attempts have been made to employ the scanning electron microscope in the mapping of a simple nervous system, the abdominal ganglion of Aplysia californica. Early results are encouraging: neuronal fibers have been identified and traced over relatively long distances from their cell bodies to structures tentatively identified as synapses and to structures tentatively identified as electrotonic connections.     

Neuronal activity in the lateral intraparietal area and spatial attention

Although the parietal cortex has been implicated in the neural processes underlying visual attention, the nature of its contribution is not well understood. We tracked attention in the monkey and correlated the activity of neurons in the lateral intraparietal area (LIP) with the monkey's attentional performance. The ensemble activity in LIP across the entire visual field describes the spatial and temporal dynamics of a monkey's attention. Activity subtending a single location in the visual field describes the attentional priority at that area but does not predict that the monkey will actually attend to or make an eye movement to that location.     

棉铃虫幼虫对印楝素的味觉适应性行为及其生理机制

采用行为试验双选法和电生理顶端刺激法研究不同取食经历的棉铃虫5龄幼虫对印楝素的味觉适应性行为。行为试验结果表明,正常人工饲料饲养幼虫对印楝素敏感,表现为味觉抑制行为,而使用含有低浓度印楝素的人工饲料饲养的幼虫对其不敏感。电生理顶端刺激法结果表明,不同取食经历的棉铃虫幼虫的侧栓椎感受器中的取食抑制素神经元对印楝素反应不同。说明棉铃虫幼虫对印楝素有味觉适应性行为,且这种适应性行为与相关味觉神经元的脱敏有关。     

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.