Functions of the LE sensory neurons inAplysia

Mechanosensory neurons which innervate the siphon and have their cell bodies in the LE cluster of the abdominal ganglion ofAplysia have revealed many cellular and molecular processes that may play general roles in learning and memory. It was initially suggested that these cells are largely responsible for triggering the gill-withdrawal reflex evoked by weak siphon stimulation, and that most of this effect is mediated by their monosynaptic connections to gill motor neurons. This implied a simple link between plasticity at these synapses and modifications of the reflex during learning. We review more recent studies from several laboratories showing that the LE cells are not activated by very weak tactile stimuli that elicit the gill-withdrawal reflex, and that an unidentified population of siphon sensory neurons has lower mechanosensory thresholds and produces shorter latency responses. Furthermore, the direct connections between LE cells and gill motor neurons make a minor contribution when the reflex is elicited in pinned siphon preparations by light stimuli that weakly activate the LE cells. Because weak mechanical stimulation of the unrestrained siphon causes little or no LE cell activation, it is unlikely that, under natural conditions, sensitization or conditioning of reflex responses elicited by light siphon touch depends upon plasticity of LE cell synapses onto either motor or interneurons. The LE cells appear to function as nociceptors because they are tuned to noxious stimuli and, like mammalian nociceptors, show peripheral sensitization following nociceptive activation. This sensitization and the profound activity-dependent potentiation of LE synapses indicate that LE cell contributions to defensive reflexes should be largest during and after intense activation of the LE cells by noxious stimulation (with the LE cell plasticity contributing to long-lasting memory of peripheral injury). The LE sensory neurons offer special opportunities for direct tests of this and other hypotheses about specific mnemonic functions of fundamental mechanisms of neural plasticity.     

Decline in the Recovery from Synaptic Depression in Heavier Aplysia Results from Decreased Serotonin-Induced Novel PKC Activation

The defensive withdrawal reflexes of Aplysia are important behaviors for protecting the animal from predation. Habituation and dishabituation allow for experience-dependent tuning of these reflexes and the mechanisms underlying these forms of behavioral plasticity involve changes in transmitter release from the sensory to motor neuron synapses through homosynaptic depression and the serotonin-mediated recovery from depression, respectively. Interestingly, dishabituation is reduced in older animals with no corresponding change in habituation. Here we show that the cultured sensory neurons of heavier animals (greater than 120g) that form synaptic connections with motor neurons have both reduced recovery from depression and reduced novel PKC Apl II activation with 5HT. The decrease in the recovery from depression correlated better with the size of the animal than the age of the animal. Much of this change in PKC activation and synaptic facilitation following depression can be rescued by direct activation of PKC Apl II with phorbol dibutyrate, suggesting a change in the signal transduction pathway upstream of PKC Apl II activation in the sensory neurons of larger animals.     

Gene expression profiling of cranial sensory ganglia that transmit food intake stimuli

Peripheral cranial sensory nerves projecting into the oral cavity receive food intake stimuli and transmit sensory signals to the central nervous system. They are derived from four cranial sensory ganglia, trigeminal, geniculate, petrosal, and nodose ganglia, each of which contains multiple kinds of sensory neurons with different cell morphologies and neuronal properties. We investigated the complex properties of these neurons from the viewpoint of gene expression using DNA microarrays. The 498 genes were selected from a total of 8,740 genes as showing tissue-dependent expression on the microarray by hierarchical cluster analysis, in which several genes known to be differentially expressed in cranial sensory ganglia are included. This suggests that DNA microarray cluster analysis revealed a number of characteristic genes for sensory neurons in these ganglia. Among the selected 498 genes, 44 genes are associated with neurotransmission, such as neuropeptides, their receptors, and vesicle transport, and 26 are ion channels regulating membrane potentials. The identification of a number of genes related directly to neural properties indicates that these sensory ganglia contain heterogeneous types of neurons with different neural properties.     

Development of the gin trap reflex inManduca sexta: a comparison of larval and pupal motor responses

1. Responses of motor neurons in larvae and pupae of Manduca sexta to stimulation of tactile sensory neurons were measured in both semi-intact, and isolated nerve cord preparations. These motor neurons innervate abdominal intersegmental muscles which are involved in the production of a general flexion reflex in the larva, and the closure reflex of the pupal gin traps. 2. Larval motor neurons respond to stimulation of sensory neurons innervating abdominal mechanosensory hairs with prolonged, tonic excitation ipsilaterally, and either weak excitation or inhibition contralaterally (Figs. 4A, 6). 3. Pupae respond to tactile stimulation of mechanosensory hairs within the gin traps with a rapid closure reflex. Motor neurons which innervate muscles ipsilateral to the stimulus exhibit a large depolarization, high frequency firing, and abrupt termination (Figs. 2, 4B). Generally, contralateral motor neurons fire antiphasically to the ipsilateral motor neurons, producing a characteristic triphasic firing pattern (Figs. 7, 8) which is not seen in the larva. 4. Pupal motor neurons can also respond to sensory stimulation with other types of patterns, including rotational responses (Fig. 3A), gin trap opening reflexes (Fig. 3B), and 'flip-flop' responses (Fig. 9). 5. Pupal motor neurons, like larval motor neurons, do not show oscillatory responses to tonic current injection, nor do motor neurons of either stage appear to interact synaptically with one another. Most pupal motor neurons also exhibit i-V properties similar to those of larval motor neurons (Table 1; Fig. 10). Some pupal motor neurons, however, show a marked non-linear response to depolarizing current injection (Fig. 11).     

Mucosal stimulation activates secretomotor neurons via long myenteric pathways in guinea pig ileum

This study examined whether mucosal stimulation activates long secretomotor neural reflexes and, if so, how they are organized. The submucosa of in vitro full thickness guinea pig ileal preparations was exposed in the distal portion and intracellular recordings were obtained from electrophysiologically identified secretomotor neurons. Axons in the intact mucosa of the oral segment were stimulated by a large bipolar stimulating electrode. In control preparations, a single stimulus pulse evoked a fast excitatory postsynaptic potential (EPSP) in 86% of neurons located 0.7-1.0 cm anal to the stimulus site. A stimulus train evoked multiple fast EPSPs, but slow EPSPs were not observed. To examine whether mucosal stimulation specifically activated mucosal sensory nerve terminals, the mucosa/submucosa was severed from the underlying layers and repositioned. In these preparations, fast EPSPs could not be elicited in 89% of cells. Superfusion with phorbol dibutyrate enhanced excitability of sensory neurons and pressure-pulse application of serotonin to the mucosa increased the fast EPSPs evoked by mucosal stimulation, providing further evidence that sensory neurons were involved. To determine whether these reflexes projected through the myenteric plexus, this plexus was surgically lesioned between the stimulus site and the impaled neuron. No fast EPSPs were recorded in these preparations following mucosal stimulation whereas lesioning the submucosal plexus had no effect. These results demonstrate that mucosal stimulation triggers a long myenteric pathway that activates submucosal secretomotor neurons. This pathway projects in parallel with motor and vasodilator reflexes, and this common pathway may enable coordination of intestinal secretion, blood flow, and motility.     

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex.The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.     

Widespread anatomical projections of the serotonergic modulatory neuron,CB1, inAplysia

Although sensitization-related changes in the neural circuitry of withdrawal reflexes inAplysia are well studied, relatively few studies address the organization of the modulatory components of sensitization. In particular, it is not known whether individual modulatory loci can simultaneously influence multiple reflex circuits. There is, however, evidence that a single modulatory transmitter, serotonin, plays a pivotal role in facilitating different reflex circuits during sensitization. Furthermore, it is known that activation of a pair of serotonergic neurons, the CB1s, produces heterosynaptic facilitation of the sensorimotor connections of one of these reflex circuits. These data together raise the possibility that the CB1s may produce sensitizing changes in the neural elements of multiple reflex systems simultaneously. In the present study, we utilized immunocytochemistry and intracellular labeling to obtain anatomical evidence of CB1's possible role in modulating multiple reflex circuits. We found that two distinct neurons satisfy previously published physiological criteria for CB1. One of these, CB1, is immunoreactive to serotonin. The second cell, here named CB2, has a different neuroanatomy and is not serotonin immunoreactive. Focusing on CB1, we found (1) profuse fine processes given off by its axons in the posterior neuropil of the cerebral ganglion, (2) extensive branching and fine processes in the pleural ganglion, and (3) a branch of CB1 that projects into the pedal ganglion. These three observations are consistent with the hypothesis that, in addition to its already established role in modulating the siphon withdrawal circuit, CB1 may also modulate synaptic connections between (1) the sensory and motor neurons of the tentacle withdrawal reflex (2) the sensory neurons and interneurons of the tail and tail-elicited siphon withdrawal reflex, and (3) the sensory and motor neurons of the tail withdrawal reflex. These observations support further physiological investigations of a possible global role of CB1 in modulating the tail and tentacle withdrawal reflexes.     

Inputs from intrinsic primary afferent neurons to nitric oxide synthase-immunoreactive neurons in the myenteric plexus of guinea pig ileum

Nitric oxide synthase (NOS) immunoreactivity occurs in two groups of neurons in the guinea pig small intestine: descending interneurons that are also immunoreactive for choline acetyltransferase (ChAT), and inhibitory motor neurons that lack ChAT immunoreactivity. Interneurons that are involved in local reflexes would be expected to have inputs from intrinsic primary afferent (sensory) neurons, most of which are calbindin-immunoreactive. We examined this possibility using triple staining for NOS, ChAT and calbindin immunoreactivity and investigated the relationships between calbindin-immunoreactive varicosities and the cell bodies of NOS-immunoreactive neurons, using high-resolution confocal microscopy and electron microscopy. By confocal microscopy, we found that the cell bodies of ChAT/NOS interneurons received 84 +/- 23 (mean +/- SD) direct appositions from calbindin-immunoreactive varicosities and that the cell bodies of NOS-inhibitory motor neurons received 82 +/- 20 appositions. Electron-microscopic examination of the relations of 265-calbindin-immunoreactive varicosities, at distances within the resolution of the confocal microscope (300 nm), to 30 NOS-immunoreactive nerve cells indicated that 84% formed close contacts or synapses and 16% were separated from neurons by thin glial cell processes. Thus, each NOS-immunoreactive nerve cell receives about 70 synaptic inputs or close contacts from the calbindin-immunoreactive varicosities of intrinsic primary afferent neurons. It is concluded that there are monosynaptic reflex connections in which intrinsic primary afferent neurons synapse directly with motor neurons and di- or poly-synaptic reflexes in which ChAT- and NOS-immunoreactive neurons are interneurons, interposed between intrinsic primary afferent neurons and NOS-inhibitory neurons.     

Integrative properties of parietal cortex neurons of rabbits during the switching of heterogeneous classical conditioned reflexes

The impulsive activity of parietal cortex neurons of rabbits during the elaboration and specialisation of switching-over of alimentary and defensive conditioned reflexes has been studied. There are changes of parietal cortex neurons activity correlated with the stages of switching-over elaboration. The significant differences of the background neurons activity and neurons response to the conditional signal during alimentary and defensive conditioned reflexes have been established. The dominant motivation influences the neurons activity by modulation of background and conditioned activity of neurons. The integrative properties of neurons investigated in switching-over situation are revealed in their ability to summarize the excitation coming to the same sensory input with excitation from different motivational and reinforcement systems.     

Intersegmental interneurons serving larval and pupal mechanosensory reflexes in the moth Manduca sexta

1. Intersegmental interneurons (INs) that participate in the larval bending reflex and the pupal gin trap closure reflex were identified in the isolated ventral nerve cord of Manduca sexta. INs 305, 504, and 703 show qualitatively different responses in the pupa than in the larva to electrical stimulation of sensory neurons that are retained during the larval-pupal transition to serve both reflexes. Action potentials produced by current injected into the 3 interneurons excite motor neurons that are directly involved in the larval and pupal reflexes. The excitation of the motor neurons is not associated with EPSPs at a fixed latency following action potentials in the interneurons, and thus there do not seem to be direct synaptic connections between the interneurons and the motor neurons. 2. IN 305 (Fig. 2) has a lateral soma, processes in most of the dorsal neuropil ipsilateral to the soma, and a crossing neurite that gives rise to a single contralateral descending axon. IN 305 is excited by stimulation of the sensory nerve ipsilateral to its soma in the larva and the pupa. Stimulation of the sensory nerve contralateral to its soma produces an inhibitory response in the larva, but a mixed excitatory/inhibitory response to the identical stimulus in the pupa. 3. IN 504 (Fig. 3) has a lateral soma, processes throughout most of the neuropil ipsilateral to the soma, and a crossing neurite that bifurcates to give rise to a process extending to the caudal limit of the neuropil and an ascending axon. IN 504 is excited by stimulation of the sensory nerve ipsilateral to its soma in both larvae and pupae, while the response to stimulation of the sensory nerve contralateral to its soma is inhibitory in the larva but mixed (excitatory/inhibitory) in the pupa. 4. IN 703 has a large antero-lateral soma, a neurite that extends across to the contralateral side giving rise to processes located primarily dorsally in both ipsilateral and contralateral neuropils, and two axons that ascend and descend in the connectives contralateral to the soma (Fig. 4). IN 703 responds to stimulation of the sensory nerves on either side of the ganglion, but the form of the response changes during the larval-pupal transition. In the larva, the response consists of very phasic (0-2 spikes) excitation, but in the pupa there is a prolonged excitation that greatly outlasts the stimulus (Fig. 6).(ABSTRACT TRUNCATED AT 400 WORDS)     

The modulation of sensory transmission through the medullary dorsal horn during cortically driven mastication

During mastication, reflexes are modulated and sensory transmission is altered in interneurons and ascending pathways of the rostral trigeminal sensory complex. The current experiment examines the modulation of sensory transmission through the most caudal part of the trigeminal sensory system, the medullary dorsal horn, during fictive mastication produced by cortical stimulation. Extracellular single unit activity was recorded from the medullary dorsal horn, and multiple unit activity was recorded from the trigeminal motor nucleus in anesthetized, paralyzed rabbits. The masticatory area of sensorimotor cortex was stimulated to produce rhythmic activity in the trigeminal motor nucleus (fictive mastication). Activity in the dorsal horn was compared in the presence and absence of cortical stimulation. Fifty-two percent of neurons classified as low threshold and 83% of neurons receiving noxious inputs were influenced by cortical stimulation. The cortical effects were mainly inhibitory, but 21% of wide dynamic range and 6% of low threshold cells were excited by cortical stimulation. The modulation produced by cortical stimulation, whether inhibitory or excitatory, was not phasically related to the masticatory cycle. It is likely that, when masticatory movements are commanded by the sensorimotor cortex, the program includes tonic changes in sensory transmission through the medullary dorsal horn.     

Regulation of afferent transmission in the feeding circuitry of Aplysia

Although feeding in Aplysia is mediated by a central pattern generator (CPG), the activity of this CPG is modified by afferent input. To determine how afferent activity produces the widespread changes in motor programs that are necessary if behavior is to be modified, we have studied two classes of feeding sensory neurons. We have shown that afferent-induced changes in activity are widespread because sensory neurons make a number of synaptic connections. For example, sensory neurons make monosynaptic excitatory connections with feeding motor neurons. Sensori-motor transmission is, however, regulated so that changes in the periphery do not disrupt ongoing activity. This results from the fact that sensory neurons are also electrically coupled to feeding interneurons. During motor programs sensory neurons are, therefore, rhythmically depolarized via central input. These changes in membrane potential profoundly affect sensori-motor transmission. For example, changes in membrane potential alter spike propagation in sensory neurons so that spikes are only actively transmitted to particular output regions when it is behaviorally appropriate. To summarize, afferent activity alters motor output because sensory neurons make direct contact with motor neurons. Sensori-motor transmission is, however, centrally regulated so that changes in the periphery alter motor programs in a phase-dependent manner.     

Sensory responses and movement-related activities in extrinsic neurons of the cockroach mushroom bodies

We have previously reported that most units in the input regions of the cockroach mushroom bodies have activities related to sensory inputs, while the majority of units in the output regions are related to movements of the animal. In the present study, we were able to attain a more satisfactory isolation of single units by using thinner wires and further characterize the activities of units in the mushroom body output regions. Forty-one units recorded here were classified into three types: sensory, movement-related, and sensori-motor units. Different units from each group exhibited a great variety in activities. Some movement-related and sensori-motor units exhibited activity preceding the onset of movements. We propose that the mushroom body participates in the integration of a variety of sensory and motor signals, possibly for initiating and maintaining motor action. While different neurons displayed a great diversity of responses, the activities of multiple neurons recorded simultaneously exhibited similar, but not identical, responses. These neurons appeared to locate adjacent to each other and may represent a cluster of extrinsic neurons that act synergistically to transmit a specific set of mushroom body output signals. Accepted: 5 May 1999     

The fate of specific motoneurons and sensory neurons of the pregenital abdominal segments inTenebrio molitor (Insecta : Coleoptera) during metamorphosis

The anatomy and innervation of the lateral external muscle and sensory cells located in the ventral region of pregenital abdominal segments were examined at the larval and adult stages ofTenebrio molitor (Coleoptera). All seven muscles located in this region degenerate during the pupal stage, whilst only the lateral external median (lem) appears in the adult. Backfillings of the motor nerve innervating this muscle reveal that, at both larval and adult stages, it is innervated by ten neurons. Intracellular records from the muscle fibres show that two neurons are inhibitory, and at least five are excitatory. There are also two unpaired neurons. A variety of sensory organs are located in the ventral region of the larvae, whilst only campaniform sensilla are found in the adult. At both stages, the innervation pattern of the sensory nerve branches is very similar. Also, the central projections of the sensory cells occupy similar neuropilar areas. Finally, prolonged intracellular records from the lem muscle revealed that, at the larval stage, it participates only in segmental or intersegmental reflexes, whilst in the adult it has a primary expiratory role in ventilation. The results show that extensive changes occur in the number of muscles located in the ventral region of the pregenital abdominal segments, as well as in the arrangement and number of sensory neurons, in the structure of the exoskeleton, and even in the central nervous system. In contrast, only minor changes are observed in the sensory and motor nerve branches, in the sensory projections, and in the number and the location of the motoneurons innervating the lateral external median muscle. Correspondence to: G. Theophilidis     

The anatomy and motor nerve distribution of the eye muscles in the crayfish

Summary The gross anatomy of the muscles in the crayfish compound eye and the distribution of brain oculomotor neurons were studied by a variety of anatomical and physiological techniques. There are 11 major muscles in each eye. These vary considerably in size and influence upon eye movements and in their source of motor innervation. Muscles that cause defensive eyestalk withdrawal are controlled by axons of a giant motor neuron cluster. Muscles that move the eyecup in vertical planes are innervated by cells of an anterior motor cluster, as well as by cells in the medulla terminalis. Muscles which move the eyecup horizontally are supplied by neurons of the lateral motor cluster. The separation of the oculomotor system into different neuronal groups that supply different sets of muscles thus reflects functional specializations of the component divisions.I am grateful to Mr. Gene Lorton for his technical assistance with some phases of this work. I also thank Sharon Greene for executing the illustration in Figure 1 and Susan Suarez for illustrating Figure 2. This work was supported by USPHS research grant NS04989.     

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.     

Target-dependent morphological segregation of Aplysia sensory outgrowth in vitro.

The adult nervous system is characterized by partial or complete morphological segregation of terminals from different afferent neurons innervating the same postsynaptic target. This segregation is thought to result, in part, from competition between the afferent terminals. To explore the role of the target cell in the spatial distribution of presynaptic inputs, the sensory neurons of Aplysia were cultured either with or without a common target motor neuron. In the presence of a common target, the outgrowth from two different sensory neurons tends to occupy separate postsynaptic regions. When cultured without a target motor neuron, processes from different sensory neurons do not segregate, but rather grow freely along one another. Thus, morphological segregation of sensory outgrowth requires interaction with a target neuron and may reflect competition between presynaptic terminals for a limited number of synaptic sites on the motor neuron, or for a postsynaptic trophic factor.