Dendritic Ca(2+)-activated K(+) conductances regulate electrical signal propagation in an invertebrate neuron

Activity-dependent changes in the short-term electrical properties of neurites were investigated in the anterior pagoda (AP) cell of leech. Imaging studies revealed that backpropagating Na(+) spikes and synaptically evoked EPSPs caused Ca(2+) entry through low-voltage-activated Ca(2+) channels that are distributed throughout the neurites. Voltage-clamp recordings from the soma revealed a TEA-sensitive outward current that was reduced when Ca(2+) entry was blocked with Co(2+) or when the intracellular concentration of free Ca(2+) was reduced by a high-affinity Ca(2+) buffer. Ca(2+) released in the neurite from a caged Ca(2+) compound caused a hyperpolarization of the membrane potential. These data imply that the AP cell expresses Ca(2+)-activated K(+) conductances, and that these conductances are present in the neurites. When the Ca(2+)-activated K(+) current was reduced through the block of Ca(2+) entry, backpropagating Na(+) spikes and synaptically evoked EPSPs increased in amplitude. Hence, the activity-dependent changes in the intracellular [Ca(2+)] together with the Ca(2+)-activated K(+) conductances participate in the regulation of dendritic signal propagation.

Relative roles of the S cell network and parallel interneuronal pathways in the whole-body shortening reflex of the medicinal leech

The whole-body shortening reflex of the medicinal leech Hirudo medicinalis is a withdrawal response produced by anterior mechanical stimuli. The interneuronal pathways underlying this reflex consist of the S cell network (a chain of electrically coupled interneurons) and a set of other, parallel pathways. We used a variety of techniques to characterize these interneuronal pathways further, including intracellular stimulation of the S cell network, photoablation of the S cell axon, and selective lesions of particular connectives (the axon bundles that link adjacent ganglia in the leech nerve cord). These experiments demonstrated that the S cell network is neither sufficient nor necessary for the production of the shortening reflex. The axons of the parallel pathways were localized to the lateral connectives (whereas the S cell axon runs through the medial connective). We used physiological techniques to show that the axons of the parallel pathways have a larger diameter in the anterior connective and to demonstrate that the parallel pathways are activated selectively by anterior mechanosensory stimuli. We also presented correlative evidence that the parallel pathways, along with activating motor neurons during shortening, are responsible for inhibiting a higher-order "command-like" interneuron in the neuronal circuit for swimming, thus playing a role in the behavioral choice between swimming and shortening.

Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic gain is mediated by an "inward rectifier" K(+) current

Dendritic processing of glutamatergic synaptic inputs was investigated in the anterior pagoda cell of leech. We observed that below spike threshold, the amplitude of individual EPSPs decreased with hyperpolarization and that simultaneous stimulation of pairs of synaptic inputs leads to the supralinear summation of EPSPs. Voltage-clamp measurements revealed a hyperpolarization-activated, Ba(2+)-sensitive, fast, noninactivating K(+) conductance that depends on the external [K(+)]. These features are those of an "inward rectifier," Kir. Microsurgery experiments, in combination with electrophysiological measurements, revealed an inhomogeneous spatial distribution of the Kir conductance. Furthermore, on surgical removal of the neurites that contain the Kir conductance, the amplitude of EPSPs from the remaining synaptic inputs increased with hyperpolarization. A model cell, with the Kir conductance as the sole voltage-dependent conductance, reproduced qualitatively the observed voltage dependence of individual EPSPs as well as the supralinear summation of EPSP pairs.

Identification of neural circuits by imaging coherent electrical activity with FRET-based dyes

We show that neurons that underlie rhythmic patterns of electrical output may be identified by optical imaging and frequency-domain analysis. Our contrast agent is a two-component dye system in which changes in membrane potential modulate the relative emission between a pair of fluorophores. We demonstrate our methods with the circuit responsible for fictive swimming in the isolated leech nerve cord. The output of a motor neuron provides a reference signal for the phase-sensitive detection of changes in fluorescence from individual neurons in a ganglion. We identify known and possibly novel neurons that participate in the swim rhythm and determine their phases within a cycle. A variant of this approach is used to identify the postsynaptic followers of intracellularly stimulated neurons.

Glial responses during evoked behaviors in the leech

Glial cells can respond with membrane potential changes during electrically stimulated neuronal activity (Kuffler, Proc R Soc Lond B 168:1-21, 1967; Orkand, Oxford University Press, 1995). Their role in contributing to, or controlling, neural circuits underlying behaviors, however, is completely unknown. We have used semi-intact preparations of the leech Hirudo medicinalis, where behaviors can be elicited and monitored (Kristan et al., J Neurobiol 27:380-389, 1995), to record membrane responses of identified glial cells during whole-body shortening and during fictive swimming. Giant glial cells are located in the neuropil of segmental ganglia, where neuronal axons and dendrites establish numerous synaptic contacts (Coggeshall and Fawcett, J Neurophysiol 27:229-289, 1964). We report here that these glial cells hyperpolarize when the whole-body-shortening response is evoked but not during fictive swimming. To our knowledge, this is the first report that associates a specific behavior with glial cell responses.

Development of spontaneous and evoked behaviors in the medicinal leech

ABSTRACT The ontogeny of behavior in an organism must reflect developmental events in the nervous system, and it thus provides a noninvasive measure of neuronal development. This approach may be particularly fruitful in the medicinal leech because the neuronal basis of several behaviors has been characterized in adult leeches, providing a rich background against which behavioral development can be interpreted. We have investigated the order in which behaviors arise during the period of embryonic development and have determined the time at which each behavior is first expressed. Some behaviors, such as lateral ridge formation, germinal plate bending, spiral twisting, and sidewinding, were produced spontaneously by embryos. Others, such as shortening, circumferential indentation, local bending, and elongation, occurred only when they were elicited by weak mechanical stimulation. Such stimulation rarely evoked a behavioral response in young embryos (at 45% of the time required for complete embryonic development, 45% ED), but by 80% ED embryos responded to nearly 100% of the stimuli presented. In embryos older than 50% ED, the behavior most frequently evoked by stimulation of the anterior end, the posterior end, or the rear sucker was shortening. Stimulation of the midbody usually evoked behavior other than shortening, illustrating that the body was behaviorally compartmentalized, at least in part. Some behaviors observed during embryogenesis are never seen in adult leeches. For example, in response to stimulation of the midbody, young embryos produced a behavior that we have called "circumferential indentation," whereas older embryos produced local bending, a response previously described for adults. The switch from circumferential indentation to local bending may signal the formation of new synaptic connections.

Staging of middle and late embryonic development in the medicinal leech, Hirudo medicinalis

We present a description of the last half of embryonic development in the European medicinal leech, Hirudo medicinalis, based entirely on externally visible morphological features, and establish reliably observable stages during that development. Embryogenesis, from the time fertilized eggs are deposited in an eggcase (called a cocoon) to the emergence of juveniles from the cocoon, takes approximately 4 weeks at room temperature. The stages described in this paper extend from the completion of segmentation to the appearance of the final bands of pigmentation. Developmental stages are expressed as percentages of total embryonic developmental time. This staging table was constructed for embryos kept at 20 degrees C. In addition, the development of animals kept at 17 degrees C or at 24 degrees C was compared with those held at 20 degrees C. Development proceeds more quickly at higher temperatures. Because development in embryos held at higher or lower temperatures was linearly related to the stages determined for embryos held at 20 degrees C, the rate of development at any intermediate temperature can be predicted from the staging table at 20 degrees C by simple multiplication.

Representation of touch location by a population of leech sensory neurons

To form accurate representations of the world, sensory systems must accurately encode stimuli in the spike trains of populations of neurons. The nature of such neuronal population codes is beginning to be understood. We characterize the entire sensory system underlying a simple withdrawal reflex in the leech, a bend directed away from the site of a light touch. Our studies show that two different populations of mechanosensory neurons each encode touch information with an accuracy that can more than account for the behavioral output. However, we found that only one of the populations, the P cells, is important for the behavior. The sensory representation of touch location is based on the spike counts in all of the four P cells. Further, fewer than three action potentials in the P cell population, occurring during the first 100 ms of a touch stimulus, may be required to process touch location information to produce the appropriately directed bend.

Long-lasting depolarization of leech neurons mediated by receptors with a nicotinic binding site

The serotonergic Retzius neurons of the leech midbody ganglia respond in a complex manner to pressure pulses of acetylcholine (ACh) applied onto their soma with a fast depolarization followed by a slower hyperpolarization and an additional delayed long-lasting depolarization. The delayed depolarization is the subject of the present study. The delayed depolarization could be elicited by long (>1 s) ACh pressure pulses or by short pulses (10 ms) of carbachol, nicotine and DMPP, but not by muscarinic agonists. It was inhibited by bath application of nicotine (10-100 micromol l-1), strychnine (100 micromol l-1) and atropine (10-100 micromol l-1). Nicotinic antagonists that blocked the fast depolarization and the slow hyperpolarization (100 micromol l-1 mecamylamine and d-tubocurarine) did not affect the delayed depolarization induced by carbachol. Partial replacement of the extracellular Na+ by glucamine caused a decrease in the amplitude of the response and a shift of its reversal potential to more negative values. Carbachol pulses applied to Retzius neurons of the ganglia innervating the reproductive segments elicited delayed depolarizations of much smaller amplitude than the ones recorded in Retzius neurons from standard segments. The delayed depolarization could be elicited by the application of short agonist pulses onto different loci over the surface of the ganglion, at a distance from the soma. Isolated cultured Retzius neurons did not exhibit the delayed depolarization although they readily expressed the earlier phases of the complex cholinergic response. Carbachol pulses applied to the soma of other neurons in the leech ganglion produced a variety of specific responses.The results suggest that the delayed depolarization was produced by the activation of a cationic conductance mediated by receptors with a pharmacological profile similar to that of the <IMG src="/images/symbols/&agr ;.gif" WIDTH="9" HEIGHT= "12" ALIGN="BOTTOM" NATURALSIZEFLAG="3">9 nicotinic receptors and was not a byproduct of the early phases of the cholinergic response. The response seemed to be initiated in the extensive neuropilar processes of the Retzius cell, enabling a persistent excitatory signal.

Long-lasting depolarization of leech neurons mediated by receptors with a nicotinic binding site

The serotonergic Retzius neurons of the leech midbody ganglia respond in a complex manner to pressure pulses of acetylcholine (ACh) applied onto their soma with a fast depolarization followed by a slower hyperpolarization and an additional delayed long-lasting depolarization. The delayed depolarization is the subject of the present study. The delayed depolarization could be elicited by long (> 1 s) ACh pressure pulses or by short pulses (10 ms) of carbachol, nicotine and DMPP, but not by muscarinic agonists. It was inhibited by bath application of nicotine (10-100 mumol l-1), strychnine (100 mumol l-1) and atropine (10-100 mumol l-1). Nicotinic antagonists that blocked the fast depolarization and the slow hyperpolarization (100 mumol l-1 mecamylamine and d-tubocurarine) did not affect the delayed depolarization induced by carbachol. Partial replacement of the extracellular Na+ by glucamine caused a decrease in the amplitude of the response and a shift of its reversal potential to more negative values. Carbachol pulses applied to Retzius neurons of the ganglia innervating the reproductive segments elicited delayed depolarizations of much smaller amplitude than the ones recorded in Retzius neurons from standard segments. The delayed depolarization could be elicited by the application of short agonist pulses onto different loci over the surface of the ganglion, at a distance from the soma. Isolated cultured Retzius neurons did not exhibit the delayed depolarization although they readily expressed the earlier phases of the complex cholinergic response. Carbachol pulses applied to the soma of other neurons in the leech ganglion produced a variety of specific responses. The results suggest that the delayed depolarization was produced by the activation of a cationic conductance mediated by receptors with a pharmacological profile similar to that of the alpha 9 nicotinic receptors and was not a byproduct of the early phases of the cholinergic response. The response seemed to be initiated in the extensive neuropilar processes of the Retzius cell, enabling a persistent excitatory signal.

Quantitative analysis of a directed behavior in the medicinal leech: implications for organizing motor output

The local bend is a directed behavior produced by the leech, Hirudo medicinalis, in response to a light touch. Contraction of longitudinal muscles near the touched location results in a bend directed away from the stimulus. We quantify the relationship between the location of touch around the body perimeter and the behavioral output by using video analysis, muscle tension measurements, and electromyography. On average, the direction of the behavioral output differed from the touch location by <8% of the total body perimeter. We discuss our results in the context of two contrasting behavioral strategies: a Continuous strategy, in which the local bend is directed exactly opposite to stimulus location, and a Categorical strategy, in which there are four distinct bend directions, each elicited by stimuli given in a single quadrant of the body perimeter. To distinguish between these strategies, we delivered two competing stimuli simultaneously. The resulting behavioral output is best described by an average of the effects of each stimulus given alone and thus provides support for the Continuous strategy. We also use a simple model, based on anatomical and physiological data, to predict the responses of the known motor neurons to different stimulus locations. The model shows that the activation of two of the motor neurons (D and V) is inconsistent with a Categorical strategy. However, these neurons are known to be active during the local bend behavior. This result, along with our experimental observations, suggests that the local bend network uses a Continuous strategy to encode stimulus location and produce directed behavioral output.

Behavioral hierarchy in the medicinal leech, Hirudo medicinalis: feeding as a dominant behavior

The effect of feeding behavior on other behaviors (swimming, crawling and shortening) was investigated in the leech, Hirudo medicinalis. The stimulus locations and intensities required to produce mechanically elicited behaviors were first determined in the non-feeding leech. Stimuli were delivered while the leech was in various body positions to determine whether stimulus location affected behavioral response. Response thresholds were determined for the mechanically elicited behaviors. The same stimuli were then applied to feeding leeches to determine if response thresholds had changed. A solution with NaCl and arginine was used to elicit feeding. The same sets of stimuli were applied at intervals for an hour after feeding, to determine the duration of feeding-induced changes in behavior. Depending on the body position and stimulus location, stimuli produced different combinations of behaviors that included shortening, swimming and crawling. Anterior stimuli generally elicited shortening, whereas posterior stimuli generally elicited crawling and swimming, with swimming more likely to ventral stimulation than to dorsal stimulation. Having the front sucker attached changed these behavioral patterns. During feeding, the response thresholds changed dramatically, from 3-5 V to greater than 9 V. This increase in threshold began with the start of feeding, even before ingestion commenced. Suppression of the behaviors lasted up to 1 h after the end of feeding, with the effect on swimming being the most pronounced and longest lasting.

A neuronal network for computing population vectors in the leech

The correlation of neuronal activity with sensory input and behavioural output has revealed that information is often encoded in the activity of many neurons across a population, that is, a neural population code is used. The possible algorithms that downstream networks use to read out this population code have been studied by manipulating the activity of a few neurons in a population. We have used this approach to study population coding in a small network underlying the leech local bend, a body bend directed away from a touch stimulus. Because of the small size of this network we are able to monitor and manipulate the complete set of sensory inputs to the network. We show here that the population vector formed by the spike counts of the active mechanosensory neurons is well correlated with bend direction. A model based on the known connectivity of the identified neurons in the local bend network can account for our experimental results, and is suitable for reading out the neural population vector. Thus, for the first time to our knowledge, it is possible to link a proposed algorithm for neural population coding with synaptic and network mechanisms in an experimental system.

Population coding and behavioral choice

Many individual behavioral acts are produced by the combined activity of large populations of broadly tuned neurons, and the neuronal populations for different behaviors can overlap. Recent experiments monitoring and manipulating neuronal activity during behavioral decisions have begun to shed light on the mechanisms that enable overlapping populations of neurons to generate choices between categorically distinct behaviors.

The neuronal basis of the behavioral choice between swimming and shortening in the leech: control is not selectively exercised at higher circuit levels

Swimming and the whole-body shortening reflex are two incompatible behaviors performed by the medicinal leech Hirudo medicinalis. We set out to examine the neuronal basis of the choice between these behaviors, taking advantage of the fact that the neuronal circuit underlying swimming is relatively well understood. The leech swim circuit is organized hierarchically and contains three interneuronal levels, including two upper levels of "command-like" neurons. We tested the responses of the swim circuit neurons to stimuli that produced shortening, using reduced preparations in which neurophysiological recording could be performed while behaviors were elicited. We found that the majority of the swim circuit neurons, including most of the command-like cells and all of the cells at the highest hierarchical level of the circuit, were excited by stimuli that produced shortening as well as by stimuli that produced swimming. Only a subset of neurons, at levels below the top, were inhibited during shortening; these included one of the command-like cells and an oscillator cell (an interneuron that is part of the central pattern generator for swimming). These results imply that the control of the choice between swimming and shortening is not exercised selectively at the higher levels of the swim circuit.

A model of the hydrostatic skeleton of the leech

A mathematical model of the hydrostatic skeleton of the leech has been developed to predict the shape of and internal pressure within the animal in response to a given pattern of motor neuron activity in different behaviors. The model incorporates experimental data on: the dimensions of the animal at behavioral extremes, the passive properties of the tissues, the active length-tension behavior of the muscles in response to neural activation, the relations between firing frequencies and forces developed by the muscles. The model is based on three general assumptions: (i) the cross-sectional geometry of each segment is elliptical, (ii) the volume of each segment remains constant during movement, (iii) the shape of the animal reflects dimensions that minimize the total potential energy. Presently the model is implemented to simulate the vermiform elongation of the leech, predicting the shape and the pressure changes during behavior. The results are in good agreement with the experimental measurements. The pattern of motor neuronal activity was determined by the known intersegmental travel time and estimated delay time between relaxation of the longitudinal muscles and the activation of the circular muscles. The anesthetized state of the leech was taken as the reference state for the model in which the active and passive stresses are zero.

An increase in activity of serotonergic Retzius neurones may not be necessary for the consummatory phase of feeding in the leech Hirudo medicinalis

During the consummatory phase of feeding, in which blood is ingested, medicinal leeches display a characteristic set of behaviours: they extend their jaws, are less responsive to sensory input, produce mucus, relax the body wall and exhibit waves of peristalsis that can run the length of the body. Earlier reports suggested that this pattern of behaviour is orchestrated by serotonin released from Retzius cells in response to the appropriate sensory stimulation of the lip. We have developed a semi-intact preparation in which only the nervous system in the posterior half of the leech was exposed. The front half of the leech was free to explore, bite through and feed until satiated from a blood-filled sausage casing while continuous intracellular and extracellular recordings were made from identified cells and the nerve roots of the exposed segments. Prior to attachment of the animal to the feeding device, the firing frequency of the Retzius cell increased transiently during spontaneous movements or tactile stimuli to its front or posterior end. In contrast, Retzius cell activity decreased after the anterior sucker attached to the membrane of the feeding device at about the time when ingestion was initiated. Increased activity of Leydig cells, which are known to modulate several circuits in the leech, was also associated with exploration. However, unlike that of Retzius cells, the activity of Leydig cells increased significantly following the onset of consumption. These results suggest that increased activity of Retzius cells in midbody ganglia is not a prerequisite for the consummatory phase of feeding and raises questions regarding the role of serotonin in regulating this behaviour.

Mapping motor neurone activity to overt behaviour in the leech: internal pressures produced during locomotion

Several behaviour patterns have been studied in the leech at both the kinematic and neuronal levels. However, very little is known about how patterns of motor neurone activity map to actual movements. Internal pressure is an essential biomechanical property in this process, being responsible for producing the rigidity and posture that allow the directed delivery of forces produced by muscle contraction. To obtain a better understanding of the biomechanical processes involved in movement of the leech, we have measured the internal pressure of the animal by placing catheters through the body wall and into the gut of intact animals showing normal patterns of behaviour. Each type of behaviour had a characteristic pressure waveform. The elongation phase of crawling produced a rapid increase in pressure that peaked when midbody segments were maximally elongated. The pressure produced during the contraction phase of crawling depended on the type of crawl, only inchworm crawling producing a second peak. Whole-body shortening in response to a head poke also produced a pressure peak, but it had a faster rise time. Swimming produced the largest pressure, which was marked by a large sustained increase that fluctuated phasically with undulations of the body. Dual pressure recordings using two catheters demonstrated that pressure was not uniform along the length of the leech, indicating that the body cavity is functionally compartmentalised. Injecting fluid into the gut via a recording catheter allowed us to determine the effects of increasing internal volume on pressure. In line with previous predictions made using an abstract biomechanical model of the leech hydroskeleton, we found that an increase in the volume caused a reduction in the pressure. We are in the process of constructing a more realistic biomechanical model of the leech, based on actual data reported elsewhere. The results in this paper will provide key tests for refining these models.

Mapping motor neuron activity to overt behavior in the leech. I. Passive biomechanical properties of the body wall

As an initial step in constructing a quantitative biomechanical model of the medicinal leech (Hirudo medicinalis), we determined the passive properties of its body wall over the physiological range of dimensions. The major results of this study were: 1. The ellipsoidal cross section of resting leeches is maintained by tonic muscle activation as well as forces inherent in the structure of the body wall (i.e., residual stress). 2. The forces required for longitudinal and circumferential stretch to maximum physiological dimensions were similar in magnitude. Cutting out pieces of body wall did not affect the passive longitudinal or circumferential properties of body wall away from the edges of the cut. 3. The strain (i.e., the percentage change in dimension of different body segments when subject to the same force was identical, despite differences in muscle cross-sections. 4. Serotonin, a known modulator of tension in leech muscles, affected passive forces at all physiological muscle lengths. This suggests that the longitudinal muscle is responsible for at least part of the passive tension of the body wall. 5. We propose a simple viscoelastic model of the body wall. This model captures the dynamics of the passive responses of the leech body wall to imposed step changes in length. Using steady-state passive tensions predicted by the viscoelastic model we estimate the forces required to maintain the leech at any given length over the physiological range.

The whole-body shortening reflex of the medicinal leech: motor pattern, sensory basis, and interneuronal pathways

The leech whole-body shortening reflex consist of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations - semi-intact, body wall, and isolated nerve cord - to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a 'fast conducting pathway' in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.

Widespread mechanosensory activation of the serotonergic system of the medicinal leech

The serotonergic system of the medicinal leech comprises a small number of iterated, identified neurons, of which the Retzius (Rz) neurons are major components. Activity in pressure mechanosensory (P) cells sufficient to elicit locomotory and defensive behaviors also excites Rz neurons. We characterized the interactions between P and Rz neurons within the ganglion and at different distances along the nerve cord. 2. Within a ganglion 1) P cells excited both Rz neurons, electrically close to the site of electrical coupling between the Rz neurons; 2) each of the four P cells had similar effects on the Rz neurons; and 3) homologous contralateral P cells shared interneuronal pathways. These data show that P cells provide nearly identical bilateral information onto Rz neurons. 3. Along the nerve cord 1) every P cell excited Rz neurons in ganglia anterior and posterior to the site of stimulation; 2) the signal was carried the entire length of the nerve cord along interneuronal pathways with similar overall (but regionally different) conduction velocities in the two directions; 3) the amplitude of the Rz responses was smaller as the distance to the activated P cell increased; 4) the rate of change of the amplitude along the cord was larger when the signal traveled from front-to-back than in the opposite direction. 4. These data shows that mechanosensory input from any segment could excite Rz neurons along the cord, in proportion to the intensity of the stimulus.

Using reflexive behaviors of the medicinal leech to study information processing

The interneuronal network that produces local bending in the leech is distributed, in the sense that most of the interneurons involved are activated in all forms of local bending, even those in which their outputs would produce inappropriate movements. Such networks have been found to control a number of different behaviors in a variety of animals. This article reviews three issues: the physiological and modeling observations that led to the conclusion that local bending in leeches is controlled by a distributed system; what distributed processing means for this and other behaviors; and why the leech interneuronal network may have evolved to be distributed in the first place.