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PubMed2003

Heartbeat control in leeches. II. Fictive motor pattern.

The rhythmic beating of the tube-like hearts in the medicinal leech is driven and coordinated by rhythmic activity in segmental heart motor neurons. The motor neurons are controlled by rhythmic inhibitory input from a network of heart interneurons that compose the heartbeat central pattern generator. In the preceding paper, we described the constriction pattern of the hearts in quiescent intact animals and showed that one heart constricts in a rear-to-front wave (peristaltic coordination mode), while the other heart constricts in near unison over its length (synchronous coordination mode) and that they regularly switch coordination modes. Here we analyze intersegmental and side-to-side-coordination of the fictive motor pattern for heartbeat in denervated nerve cords. We show that the intersegmental phase relations among heart motor neurons in both coordination modes are independent of heartbeat period. This finding enables us to combine data from different experiments to form a detailed analysis of the relative phases, duty cycle, and intraburst spike frequency of the bursts of the segmental heart motor neurons. The fictive motor pattern and the constriction pattern seen in intact leeches closely match in their intersegmental and side-to-side coordination, indicating that sensory feedback is not necessary for properly phased intersegmental coordination. Moreover, the regular switches in coordination mode of the fictive motor pattern mimic those seen in intact animals indicating that these switches likely arise by a central mechanism.

Action Potentials↗
PubMed2005

Calcium dynamics and compartmentalization in leech neurons.

Calcium dynamics in leech neurons were studied using a fast CCD camera. Fluorescence changes (DeltaF/F) of the membrane impermeable calcium indicator Oregon Green were measured. The dye was pressure injected into the soma of neurons under investigation. DeltaF/F caused by a single action potential (AP) in mechanosensory neurons had approximately the same amplitude and time course in the soma and in distal processes. By contrast, in other neurons such as the Anterior Pagoda neuron, the Annulus Erector motoneuron, the L motoneuron, and other motoneurons, APs evoked by passing depolarizing current in the soma produced much larger fluorescence changes in distal processes than in the soma. When APs were evoked by stimulating one distal axon through the root, DeltaF/F was large in all distal processes but very small in the soma. Our results show a clear compartmentalization of calcium dynamics in most leech neurons in which the soma does not give propagating action potentials. In such cells, the soma, while not excitable, can affect information processing by modulating the sites of origin and conduction of AP propagation in distal excitable processes.

Action Potentials↗
PubMed2002

Activity-dependent increase of the AHP amplitude in T sensory neurons of the leech.

We identified a new form of activity-dependent modulation of the afterhyperpolarization (AHP) in tactile (T) sensory neurons of the leech Hirudo medicinalis. Repetitive intracellular stimulation with 30 trains of depolarizing impulses at 15-s inter-stimulus interval (ISI) led to an increase of the AHP amplitude (~60% of the control). The enhancement of AHP lasted for >/=15 min. The AHP increase was also elicited when a T neuron was activated by repetitive stimulation of its receptive field. The ISI was a critical parameter for the induction and maintenance of AHP enhancement. ISI duration had to fit within a time window with the upper limit of 20 s to make the training effective to induce an enhancement of the AHP amplitude. After recovery from potentiation, AHP amplitude could be enhanced once again by delivering another training session. The increase of AHP amplitude persisted in high Mg(2+) saline, suggesting an intrinsic cellular mechanism for its induction. Previous investigations reported that AHP of leech T neurons was mainly due to the activity of the Na(+)/K(+) ATPase and to a Ca(2+)-dependent K(+) current (I(K/Ca)). In addition, it has been demonstrated that serotonin (5HT) reduces AHP amplitude through the inhibition of the Na(+)/K(+) ATPase. By blocking the I(K/Ca) with pharmacological agents, such as cadmium and apamin, we still observed an increase of the AHP amplitude after repetitive stimulation, whereas 5HT application completely inhibited the AHP increment. These data indicate that the Na(+)/K(+) ATPase is involved in the induction and maintenance of the AHP increase after repetitive stimulation. Moreover, the AHP increase was affected by the level of serotonin in the CNS. Finally, the increase of the AHP amplitude produced a lasting depression of the synaptic connection between two T neurons, suggesting that this activity-dependent phenomenon might be involved in short-term plasticity associated with learning processes.

Algorithms↗
PubMed1976

Sensory neurons in leech central nervous system: changes in potassium conductance an excitation threshold.

1. The sensory neurons in the leech central nervous system differ in their accommodation to linearly rising currents. Advantage was taken of these differences to study the ionic mechanism of accommodation in single pairs of N (noxious), P (pressure), and T (touch) cells. 2. Nonlinearities in membrane-potential changes and current-voltage relationships with square-wave and ramp currents are more pronounced in P and T cells than in N cells. The accommodation coefficients increase in conditions that reflect this delayed rectification. When rectification is absent, the accommodation coefficients depart from unity only slightly or not at all. 3. Accommodation coefficients remain unchanged when half of the chloride in the bathing medium is replaced by sulfate. Accommodation coefficients become greater when the extracellular potassium concentration is reduced from 4 to 0 mM, and decrease when the concentration is raised to 8 mM. The membrane potential changes by only a few millivolts. 4. As extracellular potassium concentration is increased, the action potential is lengthened and the maximal rate of fall of the action potential is reduced. With concentrations greater than 4 mM these relationships are linear, but depart from linearity at lower concentrations. The amplitude of the undershoot decreases linearly as the extracellular potassium concentration increases from 4 to 16 mM, and increases non-linearly at concentrations below 4 mM. 5. The rapid accommodation of leech neurons is based primarily on an increased potassium conductance. The possibility is considered that concentration changes like those produced experimentally may occur naturally, affecting integrative processes in the central nervous system.

Action Potentials↗
PubMed1977

Extrasynaptic receptors on cell bodies of neurons in central nervous system of the leech.

1. A systematic study has been made of the sensitivity of identified sensory and motoneurons in the leech central nervous system to chemical transmitter substances. 2. The following substances elicited responses from the cell bodies of individual neurons: acetylcholine, 5-hydroxytryptamine, gamma-aminobutyric acid, glutamic acid, glycine, dopamine, and norepinephrine. Since the cell bodies of leech neurons are free of synapses, the receptors that give rise to these responses are extrasynaptic. 3. Sensory and motoneurons of different function had characteristic complements of extrasynaptic receptors. For example, mechanosensory cells responding to light touch, to pressure, and to noxious stimuli could be distinguished by their responses to iontophoretically applied compounds. For one of these modalities (nociceptive), neurons with different receptive fields but otherwise similar properties had markedly distinct extrasynaptic receptors. The possible significance of extrasynaptic receptors is discussed.

Animals↗
PubMed1977

Synthesis of acetylcholine by excitatory motoneurons in central nervous system of the leech.

1. A study was made of the synthesis of acetylcholine (ACh) and other transmitters by the cell bodies of functionally identified neurons in leech segmental ganglia. 2. Choline acetyltransferase, the synthetic enzyme for ACh, was detected in excitatory motoneurons but not in mechanosensory cells or Retzius cells. The ability of motoneurons to synthesize ACh was also demonstrated by their accumulation of [3H]ACh following incubation of segmental ganglia with [3H]choline. [3H]ACh was not detected in the other cell types. When eserine was included in [3H]choline incubations, the amount of [3H]ACh in motoneurons increased severalfold and small amounts of [3H]ACh (1% that in motor cells) appeared in extracts of sensory and Retzius cells. 3. In addition to [3H]ACh segmental ganglia synthesized [3H]5-HT, [3H]gamma-aminobutyric acid, [3H]dopamine, and [3H]octopamine from exogenous, labeled precursors. None of these labeled transmitters was detected in identified neurons except [3H]5-HT, which was found in Retzius cells. 4. These results provide biochemical evidence that excitatory motoneurons in the leech are cholinergic, but leave open the identity of the sensory transmitter(s).

Acetylcholine↗
PubMed1979

Identifiable neurons controlling penile eversion in the leech.

1. This paper describes the neuroanatomy and electrophysiology of motor neurons causing penile eversion in the leech. 2. The male organ is innervated by ganglia 5 and 6 of the 34 ganglia in the leech brain through special sex nerves deriving from anterior roots. These sex ganglia have at least 200 more neurons than the other midbody ganglia. Many of the extra neurons are involved in reproductive behavior. 3. Two pairs of motor neurons on the ventral side of ganglion 6, named rostral and lateral neurons, are the only ones that elicit full penile eversion. Evidence that the lateral and rostral neurons are, in fact, motor neurons comes from HRP and electrophysiological studies. HRP injections reveal that each neuron's single primary axon grows into the sex nerve. Electrophysiological evidence is twofold: a) action potentials of lateral and rostral cells can still contract the genitalia after the neurons are deafferented from chemical synaptic input in the ganglion by high Mg2+, b) their action potentials are followed by junction potentials in male organ muscle fibers.

Animals↗
PubMed1985

Intersegmental coordination of the leech swimming rhythm. I. Roles of cycle period gradient and coupling strength.

The isolated leech nervous system generates a metachronally coordinated rhythmic output that is the neuronal correlate of swimming activity. We investigated two factors that contribute to intersegmental coordination: the swim-cycle periods expressed by segmental ganglia and the strength of neuronal coupling between ganglia. To determine the regional variation in swim-cycle periods, we severed both of the lateral intersegmental connectives. We left intact the median connective, which conveys tonic excitation but little phasic information. We obtained a reduction in intersegmental coupling strength by severing a single lateral intersegmental connective. Cycle periods were manipulated by cooling restricted sections of the nerve cord. Our experiments revealed an anterior-posterior gradient of cycle periods in ganglia of the isolated nerve cord; that is, chains of ganglia obtained from the anterior nerve cord exhibited longer cycle periods than those obtained from the posterior end of the cord. This gradient extends posteriorly to approximately ganglion 12 and may reverse posterior to ganglion 13. Increasing local cycle periods by cooling restricted sections of the nerve cord caused delay in activity cycles in the cooled ganglia, relative to the cycles of ganglia at the control temperature. This finding demonstrates that the observed gradient in cycle period provides for smaller intersegmental phase lags than would occur if there were no period gradients. Reduction of coupling strength by severing a lateral connective led to altered phase relationships across the lesion, both at the motor and oscillator levels. For those ganglion chains in which the anterior ganglia had greater periods, the reduced coupling led to reduced or even reversed phase relationships across the lesion but left unchanged the phase lag between the ends of the chain. In contrast, reduced coupling between halves of a chain in which the posterior ganglia had greater cycle periods led to increased phase lags across the lesion and between the ends. These altered phase relationships arise from a relative increase in the contribution of period differences when coupling strength is decreased. We conclude that the anterior-to-posterior progression of neuronal activity in the isolated leech nerve cord during swimming activity is provided by the intersegmental coupling signals. Furthermore, the period gradient expressed in our preparations acts to provide for smaller phase lags than would be generated by these coupling signals in the absence of such a gradient.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Intersegmental coordination of the leech swimming rhythm. II. Comparison of long and short chains of ganglia.

Preparations of the nearly isolated leech nerve cord containing as few as two ganglia are sufficient to generate intersegmentally coordinated swim oscillations, provided that they receive tonic excitation from other segments via the median connective (Faivre's nerve). Due to their greatly reduced complexity, these preparations should provide useful experimental models of neuronal coordination. As a step in the development of such models, we have characterized the intersegmental coordination of nerve-cord chains ranging from 2 to 18 ganglia in length. We found that increases in swim-cycle period give rise to increases in intersegmental delay between homologous motoneuron bursts. Thus the intersegmental phase relationships are nearly independent of period. The relationship between intersegmental delay and period is approximately linear and extrapolates to intersect the period axis at approximately 0.3 s. This value is in close agreement with the analogous measure derived from tension measurements in the intact swimming leech. Chain length (number of connected ganglia in a preparation) has a pronounced influence on the magnitude of intersegmental phase lag. The longest chains (18 ganglia) exhibited phase lags of approximately 8 degrees per segment, whereas for pairs of ganglia the phase lag was approximately 40 degrees per segment. This dependence of phase lags on chain length was apparent at both the motor and oscillator levels. The intersegmental phase lag is not the same in all parts of the nerve cord. Rather, it increases steadily toward the posterior end of the chain, providing a deceleration in the rearward progression of the metachronal activity. The rearward increase in intersegmental phase lag is paralleled by a propensity of chains taken from more posterior sections of the nerve cord to exhibit larger phase lags. That is, there appears to be a phase-lag gradient intrinsic to the nerve cord to account for the deceleration of activity. The anterior and posterior ends of an isolated nerve cord continue to exhibit phase-locked bursting when an intervening section of five ganglia is bathed in elevated Mg2+ saline. Thus, information sufficient to coordinate oscillations in separate ganglia travels at least six segments. The phase lag across the blocked section is reduced but within each unblocked section is increased so that the phase lag between extreme ends is nearly unchanged. This altered burst pattern is due to a combination of synaptic block in segmental ganglia and conduction block in through-fibers.

Animals↗
PubMed1991

Synchronized oscillatory activity in leech neurons induced by calcium channel blockers.

1. Leech ganglia were superfused with salines in which Ca2+ was replaced with equimolar concentrations of Co2+, Ni2+, or Mn2+. These salines elicited rhythmic membrane potential oscillations with cycle periods ranging from 8 to 25 s in all neurons examined within the ventral nerve cord. 2. Rhythmic activity consisted of a rapid depolarization to a prolonged (3-6 s) plateau level, followed by a rapid repolarization. Each depolarization elicited a burst of action potentials. Peak-to-trough amplitudes of the plateau depolarizations were up to 40 mV in some cells. The plateau depolarizations were separated by slowly depolarizing ramp potentials. 3. Oscillations in all neurons were synchronized (in phase) both within individual ganglia and between ganglia linked by connective nerves. Rhythmic activity in isolated ganglia persisted after the interposed connective nerves were cut. 4. The occurrence of oscillatory activity was strongly correlated with the block of chemical synaptic transmission. 5. Electrotonic interactions persisted during oscillatory activity and may be one mechanism by which oscillations are synchronized. 6. The phase of rhythmic impulse bursts monitored with extracellular electrodes could be reset by electrical stimulation of connective nerves but not by injection of current pulses into individual neurons. Phase reset appeared to occur within one cycle and to a fixed phase point (plateau termination). 7. Oscillatory activity was eliminated by 75-100% reductions of [Na+]o (Na+ replaced with N-methyl-D-glucamine). Smaller reductions of Na+ (by 25-50%) increased the cycle period of oscillations. 8. The Na(+)-K+ pump inhibitors ouabain and strophanthidin disrupted oscillations. Cells were depolarized by approximately 20 mV and fired tonically. After the initial washout of the inhibitors, cells repolarized and became quiescent. After several minutes of continued washing, oscillatory activity resumed. 9. A conceptual model is proposed to explain the mechanisms underlying oscillatory activity induced by Ca2+ channel blockers. According to this model, depolarizing plateaus are generated by a noninactivating Na+ conductance. Na+ influx during the plateau leads to an increase in [Na+]i, which activates an electrogenic Na(+)-K+ pump that contributes to plateau termination. 10. A quantitative computer simulation incorporating six types of currents (capacity, outward rectifying potassium, inward rectifying potassium, sodium, leakage, and an electrogenic sodium pump) demonstrates the plausibility of the conceptual model. 11. These data suggest that a novel Na(+)-based mechanism for membrane potential oscillation is revealed by blockade of Ca2+ channels in leech ganglia.

Animals↗
PubMed1995

Regulation of the segmental swim-generating system by a pair of identified interneurons in the leech head ganglion.

1. The aim of this study was to identify neurons that modulate activity of segmental swim gating interneurons. We found a pair of bilaterally symmetrical interneurons, cells SE1, whose activity level directly influences three groups of segmental neurons associated with generating swimming in the medicinal leech. 2. The somata of cells SE1 are located on the dorsal surface of the subesophageal ganglion. Their axons extend most, if not the entire, length of the ventral nerve cord and appear to make identical connections with the same group of swim-generating neurons in all segmental ganglia. 3. Cells SE1 excite monosynaptically all segmental swim gating interneurons, cells 204, examined. The level of excitation in cell 204 is directly correlated with the firing frequency of cell SE1. In most quiescent preparations (when the preparation is not swimming) hyperpolarization of a single cell SE1 eliminates all excitatory synaptic input to cells 204. 4. Cells SE1 excite monosynaptically three swim oscillatory interneurons, cells 115, 28, and 208. The strength of the connection from cell SE1 to cell 115 is stronger than the connection from cell SE1 to either cells 28 or 208. The level of excitation in cell 115 is directly correlated with the firing frequency of cell SE1. In most quiescent preparations, hyperpolarization of a single cell SE1 eliminates all excitatory synaptic input to cell 115 but has only a minor effect on the level of activity in cells 208 and 28. 5. Due most likely to the strong and direct connections cells SE1 have with swim gating and oscillatory interneurons, brief stimulation of cell SE1 can elicit swimming. Swimming generally occurs within 1 s after stimulation of cell SE1. During swimming, the membrane potential of cell SE1 depolarizes by 2-5 mV, and its firing frequency increases. Brief depolarization of cell SE1 during swimming reliably shifts the phase of the swimming rhythm, whereas longer periods of depolarization increase both swim period and burst duration. 6. Excitatory motor neurons to the dorsal longitudinal muscles, cells 3, 5, and 7, are strongly excited by stimulation of cell SE1. The firing frequency of cell 3 is positively correlated with the firing frequency of cell SE1. 7. The results of this study indicate that cells SE1 can modulate the level of excitation in three groups of neurons associated with generating leech swimming.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1996

Properties of the nociceptive neurons of the leech segmental ganglion.

1. The electrical responses of nociceptive (N) lateral and N medial neurons of the leech segmental ganglion to mechanical, chemical, and thermal stimulation of the skin were studied in a superfused ganglion-body wall preparation. 2. Mechanical indentation of the skin > 10 mN evoked in both types of cells a sustained discharge of impulses; afterdischarge was often observed with suprathreshold stimulations. 3. Application to the cutaneous receptive area of 10-100 mM acetic acid or of NaCI crystals and solutions also elicited a firing response in N medial and N lateral cells. In contrast, capsaicin applied to the skin (3.3 x 10(-5) to 3.3 x 10(-2) M) excited N lateral but not N medial neurons. Likewise, impulse discharges were obtained when capsaicin was applied to the cell bodies of N lateral but not of N medial neurons. 4. In both types of N neurons, heating of the skin above 39 degrees C evoked a discharge of impulses whose frequency was roughly proportional to temperature values. 5. Application of repeated suprathreshold heating cycles at 10-min intervals enhanced the impulse frequency of the response (sensitization). Shorter time intervals between heating cycles depressed the response to heat. Sensitization could not be obtained by equivalent soma depolarizations obtained by intracellular current injection. 6. Impulse discharges evoked by irritant agents were also augmented by previous application of noxious heat. 7. N lateral neurons fired in response to low-pH solutions and capsaicin directly applied onto the ganglion. N medial neurons responded inconsistently to acid and were insensitive to capsaicin. Action potentials evoked in N lateral cells by capsaicin had a slow rise, a prominent hump, and a prolonged afterhyperpolarization. 8. It is concluded that N neurons of the leech segmental ganglion respond to different modalities of noxious stimuli applied to their peripheral receptive fields and develop sensitization after repeated noxious stimulation. These properties are typical of mammalian polymodal nociceptors; thus N neurons may be a simple model for analysis of membrane mechanisms associated with polymodality of nociceptive neurons.

Action Potentials↗
PubMed1996

Distribution and functional properties of glutamate receptors in the leech central nervous system.

1. The effect of kainate and other glutamatergic agonists on the membrane potential (Em), the intracellular Na+ activity (aNai), and the intracellular free Ca2+ concentration ([Ca2+]i) of identified leech neurons and neuropile glial cells was measured with conventional and ion-sensitive microelectrodes, as well as with the use of the iontophoretically injected fluorescent indicators sodium-binding benzofuran isophthalate and Fura-2. 2. In Retzius neurons, AE, L, 8, and 101 motoneurons, and in the unclassified 50 neurons (Leydig cells) and AP neurons, as well as in neuropile glial cells, bath application of 100 microM kainate evoked a marked membrane depolarization and an increase in aNai and [Ca2+]i. The kainate-induced aNai increase persisted in solutions with high Mg2+ concentration in which synaptic transmission is blocked. 3. A membrane depolarization as well as an increase in aNai and [Ca2+]i was also evoked by L-glutamate, quisqualate, and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). The agonist-induced [Ca2+]i increase was inhibited by 6,7-dinitroquinoxaline-2,3-dione (DNQX). 4. In Ca(2+)-free solution, the kainate-induced [Ca2+]i increase was abolished in the neurons and in neuropile glial cells, whereas membrane depolarization and aNai increase were unchanged. In Na(+)-free solution, kainate had no effect on Em, aNai, or [Ca2+]i in the neurons. 5. In the mechanosensory T, P, and N neurons, kainate induced considerably smaller membrane depolarizations than in the other neurons or in neuropile glial cells, and it had no significant effect on aNai or [Ca2+]i. 6. It is concluded that in leech segmental ganglia the majority of the neurons and the neuropile glial cells, but probably not the mechanosensory neurons, possess glutamate receptors of the AMPA-kainate type. In the neurons, the [Ca2+]i increase caused by glutamatergic agonists is due to Ca2+ influx through voltage-dependent Ca2+ channels that are activated by the agonist-induced membrane depolarization.

Animals↗
PubMed1996

Sodium-dependent plateau potentials in cultured Retzius cells of the medicinal leech.

1. Individual leech Retzius (Rz) cells were removed from mid-body ganglia and plated in cell culture on concanavalin A or polylysine. Experiments on the majority of cells were performed after 6-11 days in culture. Isolated Rz cells were superfused with normal leech saline (NS), cobalt saline (Ca2+ replaced with Co2+), or one of a variety of other modified salines. 2. Prolonged plateau potentials (PPs) with durations ranging from several seconds to nearly 2 min were evoked in isolated Rz cells in response to 1-s depolarizing current pulses delivered under discontinuous current clamp. Some PPs terminated spontaneously while others were terminated with hyperpolarizing current pulses. PPs were associated with a dramatic increase in the input conductance of the neuron. The PP decayed slightly over time, and this decay was accompanied by a small decrease in the input conductance. 3. PP duration was enhanced by penetrating cells with electrodes containing tetraethylammonium (TEA) and by bathing cells in Co2+ saline, but PPs were evoked also in NS and using electrodes without TEA. The effects of TEA and Co2+ saline suggest that voltage-dependent and especially calcium-dependent outward currents normally suppress plateau formation. 4. PPs occurred most reliably in neurons with extensive neurite sprouting. Isolated somata with few or no neurites usually failed to express PP, although there were several exceptions to this trend. 5. PPs persisted when Ca2+ was replaced with either of the calcium channel blockers Co2+, Ni2+, or Mn2+, when 200 microM Cd2+ was added to normal saline, or when Na+ was replaced with Li+. In contrast, PPs were eliminated rapidly when Na+ was replaced with N-methyl-D-glucamine. 6. Isolated Rz cells also expressed repetitive PPs either spontaneously or in response to injection of sustained depolarizing current. Spontaneous repetitive PPs were suppressed by hyperpolarizing current. Repetitive PPs in isolated Rz cells are similar in many respects to the bursting electrical activity induced by Co2+ saline in Rz and other neurons in intact ganglia. 7. The ionic dependence and prolonged duration of PPs suggest that these responses are generated by a persistent voltage-dependent Na+ current. A quantitative computer simulation of PPs was achieved using a depolarization-activated Na+ conductance with very slow inactivation. Repetitive PPs were simulated by addition of a slow outward current in the form of an electrogenic pump.

Animals↗
PubMed1997

Coactivation of putative octopamine- and serotonin-containing interneurons in the medicinal leech.

Possible interactions between octopamine-immunoreactive (IR) and serotonergic neurons in the CNS of the medicinal leech were investigated. Simultaneous intracellular recordings of serotonin-containing neurons (either the Retzius neuron or cell 21) and the dorsolateral octopamine-IR (DLO) neuron demonstrated that both sets of neurons are coactive at times. Depolarization of either serotonergic cell 21 or the Retzius neuron did not alter the membrane potential of the DLO. Similarly, depolarization of the DLO did not affect the serotonergic neurons examined. Because it was found that the DLO and either the serotonergic cell 21 or Retzius neuron were at times coactive, we looked for possible sources of common excitatory inputs. The centrally located pressure (P)- and touch (T)-sensitive mechanosensory neurons excited the DLOs through a polysynaptic pathway. Stimulation of nociceptive (N) mechanosensory neurons did not cause a measurable depolarization in the membrane potential of the DLO. Through simultaneous recordings of the DLO, cell 21, and a particular identified mechanosensory neuron, it was demonstrated that activity in the T or P cells can excite both serotonergic cell 21 and the octopamine-IR DLO. These findings indicate that, in many instances, both serotonin and octopamine, biogenic amines with neuromodulatory actions in many different invertebrates, may be released simultaneously in the leech.

Animals↗
PubMed1999

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.

Animals↗
PubMed2001

Indirectly gated Cl(-)-dependent Cl(-) channels sense physiological changes of extracellular chloride in the leech.

The maintenance of ion homeostasis requires adequate ion sensors. In leeches, 34 nephridial nerve cells (NNCs) monitor the Cl(-) concentration of the blood. After a blood meal, the Cl(-) concentration of leech blood triples and is gradually restored to its normal value within 48 h after feeding. As previously shown in voltage-clamp experiments, the Cl(-) sensitivity of the NNCs relies on a persistent depolarizing Cl(-) current that is turned off by an increase of the extracellular Cl(-) concentration. The activation of this Cl(-)-dependent Cl(-) current is independent of voltage and of extra- and intracellular Ca(2+). The transduction mechanism is now characterized on the single-channel level. The NNC's sensitivity to Cl(-) is mediated by a slowly gating Cl(-)-dependent Cl(-) channel with a mean conductance of 50 pS in the cell-attached configuration. Gating of the Cl(-) channel is independent of voltage, and channel activity is independent of extra- and intracellular Ca(2+). Channel activity and the macroscopic current are reversibly blocked by bumetanide. In outside-out patches, changes of the extracellular Cl(-) concentration do not affect channel activity, indicating that channel gating is not via direct interaction of extracellular Cl(-) with the channel. As shown by recordings in the cell-attached configuration, the activity of the channels under the patch is instead governed by the Cl(-) concentration sensed by the rest of the cell. We postulate a membrane-bound Cl(-)-sensing receptor, which-on the increase of the extracellular Cl(-) concentration-closes the Cl(-) channel via a yet unidentified signaling pathway.

Animals↗
PubMed1979

Neural control of heartbeat in the leech and in some other invertebrates.

The heartbeat of the leech Hirudo consists of the contractile rhythm of the circular muscles in the wall of a bilateral pair of celomic sinuses, the heart tubes, that run the length of the leech body. The constriction cycles of the segmental heart-tube sections are coordinated so that on one body side they constrict in a rear-to-front progression (peristalsis), while on the other side they constrict nearly in concert (nonperistalsis). Spontaneous right-left reciprocal transitions between peristaltic and nonperistaltic coordination modes occur every few dozen heartbeat cycles. The constriction of each segmental heart-tube section is controlled via excitatory synapses by a rhythmically active heart motor neuron, or HE cell, of which 17 bilateral pairs are iterated in segmental ganglia of the ventral nerve cord. The activity rhythm of the HE cell ensemble is in turn controlled via inhibitory synapses by a rhythmically active heart interneuron, the HN cell, of which seven bilateral pairs are iterated in the rostral segmental ganglia. The HN heart interneuron owes its activity rhythm to an endogenous polarization cycle, and the cycles of all members of the HN cell ensemble are locked into an appropriate phase relation thanks to their mutual interconnection via excitatory and inhibitory synaptic connections. The observed activity pattern and identified synaptic connections of HE cells and HN cells can account not only for the generation of the two bilaterally asymmetric heartbeat coordination modes but also for the right-left coordination mode transitions. In contrast to the heartbeat of Hirudo, the beat of the single-chambered heart of the lobsters Panulirus and Homarus is controlled by a set of nine rhythmically active neurons that make up the cardiac ganglion. Of these, five larger cells are heart motor neurons that innervate the heart muscle fibers via excitatory synapses. The remaining four smaller neurons of the cardiac ganglion are interneurons that provide excitatory input to each other and to the heart motor neurons. Although all the neurons of the cardiac ganglion appear capable of producing their own endogenous polarization rhythm, it is currently believed that one of the interneurons acts as a pacemaker for the whole ensemble of interneurons and motor neurons. The beat of the two-chambered heart of the marine snail Aplysia is generated by yet an entirely different mechanism. Here, the basic contractile rhythm of the heart is due to an endogenous polarization cycle of the heart muscle fibers. That myogenic rhythm is controlled and modulated by a set of cardiovascular motor neurons located in the abdominal ganglion, some of which make excitatory and others of which make inhibitory connections with the heart muscle fibers. The activity of these cardiovascular motor neurons is controlled by three types of heart interneurons via both inhibitory and excitatory connections. The interneurons are in turn interconnected in a manner that prevents the simultaneous activation of antagonistic cardiac motor acts...

Action Potentials↗