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PubMed1995

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.

Animals↗
PubMed1995

Diversity and modulation of ionic conductances in leech neurons.

A complete understanding of animal behavior at the cellular level requires detailed information on the intrinsic biophysical properties of neurons, muscles, and the synaptic connections they make. In the past 10 to 15 years, electrophysiological studies of leech neurons have revealed a diverse array of voltage-gated ionic conductances distinguished by their pharmacological sensitivity to classic ion channel blockers. Voltage-clamp studies have provided new information about the kinetics and voltage-dependence of Na+ conductances, several K+ currents, including IA, IK and IK(Ca.), and high- and low-voltage-gated Ca2+ conductances. These studies showed that the action potentials of most leech neurons result from the usual sequence of permeability changes to Na+, K+, and Ca2+ ions. They also added insight as to the role played by particular combinations of conductances in providing individual neurons with electrical properties appropriate for the particular information they encode. Evidence is accumulating on the modulatory actions fo endogenous neurotransmitters such as FMRFamide, serotonin, and octopamine on motor behaviors in the animal. Parallel studies suggest that changes in behavior can be explained, at least in part, by the alteration of firing patterns of selected neurons and muscles resulting from modulation of multiple ion conductances. This makes the leech exceptionally attractive for neuroethological studies because it is one of the simplest organisms in which the methods of psychology and neurobiology can be combined. Information gathered from this animal will therefore increase our understanding regarding general principles underlying the cellular basis of behavior.

Animals↗
PubMed1995

What we have learned from the study of learning in the leech.

The use of invertebrate preparations has contributed greatly to our understanding of the neural basis of learning. The leech is especially useful for studying behavioral changes and their underlying neuronal mechanisms. Learning in the leech is essentially identical to that found in other animals, both vertebrate and invertebrate. Using anatomical and physiological techniques on leeches as they learn, we have begun to characterize the properties of individual neurons and neuronal networks that play a role in learning. We have been able to show two neuronal mechanisms that have not been previously associated with associative conditioning. The first has to do with the importance of contingency: one stimulus [the conditional stimulus (CS)] becomes associated with a second stimulus [the unconditional stimulus, (US)] in proportion to the ability of the CS to predict the US. We have found that important properties for encoding predictability, such as circuit reconfiguration, may lie in the US pathway. The firing of the serotonergic Retzius cells is taken as the US; consistent CS prediction of a US prevents "dropout" of a critical component of one US pathway. Throughout training, predicted USs continue to elicit a barrage of action potentials in these cells. Recurring unpredicted USs degrade both the learning and the response of the Retzius cell to the US. A second insight is that at least two US pathways contribute to learning, the Retzius cell pathway and the nociceptive (N) cell pathway. This second pathway persists after the elimination of the Retzius cell pathway. The observation of multiple US pathways raises a host of issues concerning CS-US convergence and the functional significance of distinct US pathways, and our results are discussed in terms of implications to current models of learning.

Animals↗
PubMed1997

Conserved anterior boundaries of Hox gene expression in the central nervous system of the leech Helobdella.

Molecular developmental studies of fly and mouse embryos have shown that the identity of individual body segments is controlled by a suite of homeobox-containing genes called the Hox cluster. To examine the conservation of this patterning mechanism in other segmented phyla, we here describe four Hox gene homologs isolated from glossiphoniid leeches of the genus Helobdella. Based on sequence similarity and phylogenetic analysis, the leech genes Lox7, Lox6, Lox20, and Lox5 are deemed to be orthologs of the Drosophila genes lab, Dfd, Scr, and Antp, respectively. Sequence similarities between Lox5 and Antp outside the homeodomain and phylogenetic reconstructions suggest that the Antennapedia family of Hox genes (as defined by Bürglin, 1994) had already expanded to include at least two discrete Antp and Ubx/abdA precursors prior to the annelid/arthropod divergence. In situ hybridization reveals that the four Lox genes described in this study are all expressed at high levels within the segmented portion of the central nervous system (CNS), with variable levels of expression in the segmental mesoderm. Little or no expression was seen in peripheral ectoderm or endoderm, or in the unsegmented head region (prostomium). Each Lox gene has a distinct anterior expression boundary within one of the four rostral segments, and the anterior-posterior (AP) order of these expression boundaries is identical to that reported for the orthologous Hox gene products in fly and mouse. This finding supports the idea that the process of AP axis differentiation is conserved among the higher metazoan phyla with respect to the regional expression of individual Hox genes along that axis. One unusual feature of leech Hox genes is the observation that some genes are only expressed during later development -- beginning at the time of terminal cell differentiation -- whereas others begin expression at a much earlier stage, and their RNA ceases to be detectable shortly after the onset of expression of the 'late' Hox genes. The functional significance of this temporal disparity is unknown, but it is noteworthy that only the two 'early' Hox genes display high levels of mesodermal expression.

Amino Acid Sequence↗
PubMed1996

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.

Animals↗
PubMed1995

Neuronal factors influencing the decision to swim in the medicinal leech.

The initiation of leech (Hirudo medicinalis) swimming in isolated segmental nerve cord preparations requires only excitation of segmental swim gating and swim oscillatory interneurons. However, several observations indicate that when the entire isolated central nervous system (head ganglion through tail ganglion) is used, neuronal inputs from the head ganglion other than excitatory inputs to the segmental swim-generating network influence whether swimming results in response to a given stimulus. In this study, experiments were performed to demonstrate that the initiation of swimming is controlled by two parallel pathways emanating from the head ganglion that have opposite effects on the segmental swim-generating network. One pathway, the swim-activating system, excites the segmental swim-generating network, while the other pathway, the swim-inactivating system, suppresses it. The balance between the effects that the swim-activating and inactivating systems have on the segmental swim-generating network determines whether swimming occurs. Moreover, we identified a pair of interneurons, cells SIN1, in the leech head ganglion whose spiking activity must be suppressed in order for swimming to be initiated since their activity is incompatible with swimming. Depolarization of cell SIN1 during swimming indirectly inhibits segmental swim-gating interneurons and terminates ongoing swimming activity. Thus, cells SIN1 are most likely part of the swim-inactivating system in the leech head ganglion.

Animals↗
PubMed1995

Genesis of segmental identity in the leech nervous system.

The Antennapedia-class homeobox genes are likely to play a role in the specification of neuronal identities in invertebrates. The leech Hirudo medicinalis, a species well-suited for the analysis of these genes at the level of identified neurons, contains homologs of many Antennapedia-class and related genes. The expression pattern in the central nervous system of four leech homebox genes was examined in detail. Lox1 is expressed during early gangliogenesis in one pair of transient neurons present in every segment and, at later stages, in 15-20 pairs of neurons per segment. Lox2 is expressed in 25-30 pairs of neurons repeated in the posterior two-thirds of the midbody. Lox4 is present in 20-30 pairs of iterated neurons in the posterior half of the midbody, and in smaller subset of them in more anterior ganglia. Lox6 is expressed in 15-20 pairs of neurons of the third subesophageal neuromere and in fewer cells of more posterior ganglia. The subsets of neurons that express these homeobox genes are different but overlapping. Combinations of Lox genes could in theory generate enough variability to specify all central neurons in a leech ganglion.

Animals↗
PubMed1993

Modulation of swimming behavior in the medicinal leech. I. Effects of serotonin on the electrical properties of swim-gating cell 204.

The effects of serotonin on the electrical properties of swim-gating neurons (cell 204) were examined in leech (Hirudo medicinalis) nerve cords. Exposure to serotonin decreased the threshold current required to elicit swim episodes by prolonged depolarization of an individual cell 204 in isolated nerve cords. This effect was correlated with a more rapid depolarization and an increased impulse frequency of cell 204 in the first second of stimulation. In normal leech saline, brief depolarizing current pulses (1 s) injected into cell 204 failed to elicit swim episodes. Following exposure to serotonin, however, identical pulses consistently evoked swim episodes. Thus, serotonin appears to transform cell 204 from a gating to a trigger cell. Serotonin had little effect on the steady-state current-voltage relation of cell 204. However, serotonin altered the membrane potential trajectories in response to injected current pulses and increased the amplitude of rebound responses occurring at the offset of current pulses. These changes suggest that serotonin modulates one or more voltage dependent conductances in cell 204, resulting in a more rapid depolarization and greater firing rate in response to injected currents. Thus, modulation of intrinsic ionic conductances in cell 204 may account in part for the increased probability of swimming behavior induced by serotonin in intact leeches.

Animals↗
PubMed1994

Modulation of swimming behavior in the medicinal leech. IV. Serotonin-induced alteration of synaptic interactions between neurons of the swim circuit.

Serotonin enhances the expression of swimming in the medicinal leech Hirudo medicinalis. These two reports examine the physiological causes underlying this modulation. The initial paper (Mangan et al. 1994) demonstrated that serotonin enhanced the participation of inhibitory swim motor neurons (MNs) in the generation of the swimming rhythm in the isolated nerve cord. In experiments reported here, we examined whether synaptic interactions between neurons of the swim circuit are altered by serotonin. Following exposure to 50 microM serotonin, pairwise intracellular recording revealed the presence of a time-dependent synaptic decrement. Synaptic decrement was characterized by: 1) a substantial decline in synaptic inhibition (half-decay time about 0.4 s) during constant presynaptic excitation; 2) a reduced half-time of recovery from synaptic inhibition; and 3) a strong dependence on the presynaptic neuron's membrane potential. We found little alteration in the physiology of synaptic transmission involving MNs following amine depletion in leech nerve cords. We propose that alterations in synaptic interactions resulting from exposure to elevated serotonin levels, coupled with the changes in MN cellular properties described earlier, are crucial to the increased efficacy of MNs in participating in generating and expressing the leech swimming rhythm.

Animals↗
PubMed1983

The cytoskeleton of microvilli of leech photoreceptors. A stable bundle of actin microfilaments.

The microvilli of leech photoreceptors have diameters in the range of 60-100 nm. Each contains a bundle of microfilaments extending into the photoreceptor soma as a rootlet (Walz 1979 b). Apparent thicknesses of individual filaments are estimated to be 4-5 nm, consonant with those of identified actin filaments in the basement membranes of blowflies (Blest and De Couet 1983). Frozen sections of leech photoreceptors labelled with antibodies against scallop actin exhibited strong binding to the rootlet region but not to the microvilli, which are destroyed by the severe saponin or acetone extraction needed to permeabilise the preparation. NBD-phallacidin binds strongly but non-specifically to the photoreceptors and does not allow positive identification of F-actin. The cytoskeletons of the microvilli and rootlets are adequately preserved by conventional routines of fixation, and similar results were obtained when retinae were pretreated with either 0.5 mM Ca2+, 10 mM EGTA, 2 mM Ep-475 (a specific inhibitor of thiol proteases) or 2 mM Ep-475 combined with 0.5 mM Ca2+. Unlike the axial cytoskeletons of arthropod photoreceptor microvilli, those of the leech are stable to cellular insult.

Actins↗
PubMed1980

Fluorescence marking of neuropile glial cells in the central nervous system of the leech Hirudo medicinalis.

Neuropile glial (NG) cells in the central nervous system of the medicinal leech, Hirudo medicinalis L., were studied by histological and intracellular electrophysiological methods. Potential profiles of single leech ganglia were mapped by advancing an electrolyte-filled microelectrode into the ganglion as far as the NG cell. A small negative potential usually appeared during or immediately after penetration of the ganglion sheath. Most of the ganglia in the chain (ganglia 1-4 and 7-21) have Retzius-cell-bodies of normal size; in these, the potential associated with the ganglion sheath was followed by a jump to a more negative potential. Superimposed action potentials were associated with entry of the electrode into a Retzius cell. When the electrode tip passed out of the cell into the center of the ganglion, another potential change was observed, namely that to the membrane potential of the anterior NG cell. This membrane potential averaged -60.2mV and ranged from -50 to -73mV. In ganglia 5 and 6 the Retzius-cell-bodies are particularly small, and no changes of potential associated with these cells were observed; the first potential to appear after the electrode passed through the sheath of the ganglion was the membrane potential of the NG cell. Potential profiles like those of ganglia 5 and 6 are recorded in the posterior parts of all ganglia. Potential profiles of single leech ganglia were also recorded with microelectrodes filled with the fluorescent dye Procion Yellow M4-RAN. When the presumed membrane potential of an NG cell appeared, the dye was injected into the gaglion. Subsequent histological examination with the fluorescence microscope revealed that all of the dye was contained in NG cells.

Animals↗
PubMed1993

Ion transport across leech integument. I. Electrogenic Na+ transport and current fluctuation analysis of the apical Na+ channel.

The dorsal skin of the leech Hirudo medicinalis was used for electrophysiological measurements performed in Ussing chambers. The leech skin is a tight epithelium (transepithelial resistance = 10.5 +/- 0.5 k omega.cm-2) with an initial short-circuit current of 29.0 +/- 2.9 microA.cm-2. Removal of Na+ from the apical bath medium reduced short-circuit current about 55%. Ouabain (50 mumol.l-1) added to the basolateral solution, depressed the short-circuit current completely. The Na+ current saturated at a concentration of 90 mmol Na+.l-1 in the apical solution (KM = 11.2 +/- 1.8 mmol.l-1). Amiloride (100 mumol.l-1) on the apical side inhibited ca. 40% of the Na+ current and indicated the presence of Na+ channels. The dependence of Na+ current on the amiloride concentration followed Michaelis-Menten kinetics (Ki = 2.9 +/- 0.4 mumol.l-1). The amiloride analogue benzamil had a higher affinity to the Na+ channel (Ki = 0.7 +/- 0.2 mumol.l-1). Thus, Na+ channels in leech integument are less sensitive to amiloride than channels known from vertebrate epithelia. With 20 mmol Na+.l-1 in the mucosal solution the tissue showed an optimum amiloride-inhibitable current, and the amiloride-sensitive current under this condition was 86.8 +/- 2.3% of total short-circuit current. Higher Na+ concentrations lead to a decrease in amiloride-blockade short-circuit current. Stimulation of the tissue with cyclic adenosine monophosphate (100 mumol.l-1) and isobutylmethylxanthine (1 mmol.l-1) nearly doubled short-circuit current and increased amiloride-sensitive Na+ currents by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiloride↗
PubMed1992

Glutamate-like immunoreactivity in the leech central nervous system.

Using a monoclonal antibody for glutamate the distribution was determined of glutamate-like immunoreactive neurons in the leech central nervous system (CNS). Glutamate-like immunoreactive neurons (GINs) were found to be localized to the anterior portion of the leech CNS: in the first segmental ganglion and in the subesophageal ganglion. Exactly five pairs of GINs consistently reacted with the glutamate antibody. Two medial pairs of GINs were located in the subesophageal ganglion and shared several morphological characteristics with two medial pairs of GINs in the first segmental ganglion. An additional lateral pair of GINs was also located in segmental ganglion 1. A pair of glutamate-like immunoreactive neurons, which are potential homologs of the lateral pair of GINs in segmental ganglion 1, were occasionally observed in more posterior segmental ganglia along with a selective group of neuronal processes. Thus only a small, localized population of neurons in the leech CNS appears to use glutamate as their neurotransmitter.

Animals↗
PubMed1988

A model for intersegmental coordination in the leech nerve cord.

The neuronal circuits that generate swimming movements in the leech were simulated by a chain of coupled harmonic oscillators. Our model incorporates a gradient of rostrocaudally decreasing cycle periods along the oscillator chain, a finite conduction delay for coupling signals, and multiple coupling channels connecting each pair of oscillators. The interactions mediated by these channels are characterized by sinusoidal phase response curves. Investigations of this model were carried out with the aid of a digital computer and the results of a variety of manipulations were compared with data from analogous physiological experiments. The simulations reproduced many aspects of intersegmental coordination in the leech, including the findings that: 1) phase lags between adjacent ganglia are larger near the caudal than the rostral end of the leech nerve cord; 2) intersegmental phase lags increase as the number of ganglia in nerve cord preparations is reduced; 3) severing one of the paired lateral connective nerves can reverse the phase lag across the lesion and 4) blocking synaptic transmission in midganglia of the ventral nerve cord reduces phase lags across the block.

Animals↗
PubMed2001

Voltage-induced activation of mechanosensitive cation channels of leech neurons.

The voltage dependence of stretch-activated cation channels in leech central neurons was studied in cell-free configurations of the patch-clamp technique. We established that stretch-activated channels excised from identified cell bodies of desheathed ganglia, as well as from neurons in culture, were slowly and reversibly activated by depolarizing membrane potentials. Negative pressure stimuli, applied to the patch pipette during a slow periodical modulation of membrane potential, enhanced channel activity, whereas positive pressures depressed it. Voltage-induced channel activation was observed, with soft glass pipettes, both in inside-out and outside-out membrane patches, at negative and positive reference potentials, respectively. The results presented in this study demonstrate that membrane depolarization induces slow activation of stretch-activated channels of leech central neurons. This phenomenon is similar to that found in Xenopus oocytes, however, some peculiar features of the voltage dependence in leech stretch-activated channels indicate that specific membrane-glass interactions might not necessarily be involved. Moreover, following depolarization, stretch-activated channels in membrane patches from neurons in culture exhibited significantly shorter delay to activation (sec) than their counterparts from neurons of freshly isolated ganglia (hundreds of sec).

Animals↗
PubMed1996

Single K+ channels in embryonic leech ganglion cells.

We investigated the properties of single K+ channels in the soma membrane of embryonic leech ganglion cells using the patch-clamp technique. We compared these K+ channels with the K+ channels found previously in Retzius neurons of the adult leech. In ganglion cells of 9- to 15-day-old embryos we characterized eight different types of K+ channels with mean conductances of 21, 55, 84, 111, 122, 132, 149 and 223 pS. The 55 pS and 84 pS channels showed flickering and were active for less than 2 min after excising the patch. The 111 pS channel was an outward rectifier, and the open state probability (po) decreased in the inside-out configuration when the Ca2+ concentration was raised from pCa 7 to pCa 3. The 122 pS channel also showed outward rectification. This type of channel was activated after changing from the cell-attached to the inside-out configuration and it did not inactivate during more than 30 min. The po was Ca2+- and voltage-insensitive. One hundred micron glibenclamide reversibly reduced po. The 132 pS channel was an outward rectifier and was Ca2+-insensitive. The 149 pS channel inactivated in the inside-out configuration. The 149- and the 223 pS channel showed inward rectification. The 111 pS channel had similar properties to the Ca2+-dependent K+ channel and the 122 pS channel resembled the ATP-inhibited K+ channel found previously in Retzius neurons of the adult leech.

Adenosine Triphosphate↗
PubMed2002

Pressure sensation by an anterior pagoda neuron population distributed in multiple ganglia in the leech, Whitmania pigra.

Sensory processing of pressure signals in the central nervous system of the leech, Whitmania pigra, was studied through the interaction between pressure sensory neurons and anterior pagoda neurons. The responses of anterior pagoda neurons to one pulse or a train of pulses in pressure sensory neurons were characterized by the latency and amplitude of excitatory postsynaptic potentials. Here we show that each pressure sensory neuron is able to activate all the anterior pagoda neurons throughout the leech central nervous system. The response patterns of all anterior pagoda neurons were appropriate to the pressure location: in the longitudinal direction the anterior pagoda neuron further away from the pressure sensory neuron had a smaller response with longer latency; inside each ganglion, the anterior pagoda neuron on the contralateral side had a larger response with shorter latency than that on the ipsilateral side. All anterior pagoda neurons excited by pressure sensory neurons comprised a parallel system in which each anterior pagoda neuron was independent from the others. The location information of pressure stimuli was represented through the response of all 40 anterior pagoda neurons covering the whole leech body with a specific pattern of latency and amplitude.

Animals↗
PubMed1998

Effects of transition metal ions on spontaneous electrical activity and chemical synaptic transmission of neurons in the medicinal leech.

Leech neurons exposed to salines containing inorganic Ca(2+)-channel blockers generate rhythmic bursts of impulses. According to an earlier model, these blockers unmask persistent Na+ currents that generate plateau-like depolarizations, each triggering a burst of impulses. The resulting increase in intracellular Na+ activates an outward Na+/K+ pump current that contributes to burst termination. We tested this model by examining systematically the effects of six transition metal ions (Co2+, Ni2+, Mn2+, Cd2+, La3+, and Zn2+) on the electrical activity of neurons in isolated leech ganglia. Each ion induced bursting activity, but the amplitude, form, and persistence of bursting differed with the ion used and its concentration relative to Ca2+. All ions tested suppressed chemical synaptic transmission between identified motor neurons, consistent with block of voltage-dependent Ca2+ currents in these cells. In addition, a strong correlation between suppression of synaptic transmission and burst amplitudes was obtained. Finally, burst duration was increased and the rate of repolarization decreased in reduced K+ saline, as expected for pump-dependent repolarization. These results provide further support for the hypothesis that a novel form of oscillatory electrical activity driven by persistent Na+ currents and the Na+/K+ pump occurs in leech ganglia exposed to Ca(2+)-channel blockers.

Animals↗