© 2001 by The Society for Integrative and Comparative Biology
Mechanism of Postinhibitory Rebound in Molluscan Neurons1
1 Pacific Biomedical Research Center, 1993 East-West Road, University of Hawaii, Honolulu, Hawaii 96822
2 Hopkins Marine Station, Stanford University, Pacific Grove, California 93950
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Postinhibitory rebound (PIR) is an intrinsic property of many neurons but the underlying mechanism is not well understood. We studied PIR and its relationship to spike adaptation in B-cells isolated from the buccal ganglia of Aplysia. These neurons exhibit PIR following inhibitory synaptic input and following direct membrane hyperpolarization. Hyperpolarizing and depolarizing voltage clamp pulses from the resting potential evoke slow changes in membrane current that persist in the form of tail currents following the pulses. A subtraction method was used to isolate slow tail currents for study. Current-voltage measurements indicate that slow outward tail currents following depolarizing pulses result from increases in membrane conductance, while inward tail currents following hyperpolarizations to –50 and –60 mV result from conductance decreases. The reversal potential of both outward and inward tail current is between –60 and –70 mV. Tail currents activated by pulses more positive than –60 mV are sensitive to the external K+ concentration and blocked by injection of Cs+ and TEA. When Ca2+ influx is prevented by bathing cells in Ca2+ free saline or by adding Co2+ or Ni2+, the tail currents are reduced but a significant fraction of the current is insensitive to these treatments. More negative conditioning pulses activate a second component of inward tail current that is weakly sensitive to K+ but more strongly effected by substitution of N-methyl glucamine or Li+ for external Na+. We conclude that both PIR and adaptation result from slow changes in a voltage dependent, non-inactivating K+ conductance that is active at voltages near the resting potential and is not tightly coupled to Ca2+ influx. In addition, a second inward current is activated by large hyperpolarizing pulses that results from an increase in Na+ and K+ conductance. This second process is likely to contribute to PIR under particular circumstances.
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INTRODUCTION |
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The term postinhibitory rebound (PIR) refers to a period of increased neuronal excitability following the cessation of inhibition. PIR is an intrinsic property of many central nervous system neurons and in some cases it results in a burst of action potentials that follows directly after a purely inhibitory synaptic input. Because of this, it has been suggested that PIR may contribute to the maintenance of oscillatory activity in neural networks that are characterized by mutual inhibitory connections, like those involved in locomotor behaviors. In addition, PIR is often included as an element in computational models of neural networks that involve mutual inhibition (Perkel and Maloney, 1974
; Perkel, 1976; Roberts et al., 1990). In light of this interest, it is surprising how little is known about the cellular mechanisms responsible for PIR.
Kuffler and Eyzaguirre (1955) concluded that PIR in crayfish stretch receptor neurons is caused by recovery from adaptation during the course of inhibitory hyperpolarization. One feature of that system is that PIR only occurs if the hyperpolarization is imposed on a background of excitation, caused in this case by stretch. They also found that PIR can be elicited in the stretch receptor by hyperpolarizing current pulses. This was an important finding because it showed that PIR is an intrinsic property of the postsynaptic neuron, related to the membrane potential change associated with inhibition but independent of transmitter receptors or presynaptic properties. The latter conclusion has stood the test of time, marking PIR as a robust property of CNS neurons in a wide variety of contexts (Gerasimov et al., 1966; Hartline and Gassie, 1979; Kandel and Spencer, 1961; Kater, 1974; Selverston et al., 1976; Barrio et al., 1994).
We studied the mechanism of PIR in identified neurons isolated from the buccal ganglia of Aplysia. In voltage clamp experiments these cells exhibit slowly decaying inward tail currents after hyperpolarizing voltage steps that have the correct time course and amplitude to explain PIR. A subtraction method was used to isolate slow tail currents for study. The inward tail current after a hyperpolarizing voltage step results primarily from slow recovery of a K+ conductance from a low level attained during hyperpolarization to a higher level appropriate to the resting potential. The slow K+ current is voltage dependent and largely insensitive to Ca2+ influx. A second process, a non-selective cation current that is activated during more negative voltage steps, also appears to contribute to PIR in some neurons.
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Specimens of Aplysia californica were obtained from Sea Life Supply (Sand City, CA) and kept in flowing sea water at 12–18°C. Buccal ganglia were removed and desheathed and clusters containing B-cells alone or B-cells in combination with A-cells were isolated by undercutting with iridectomy scissors. The normal saline contained (mM); 470 NaCl, 10 KCl, 50 MgCl2, 10 CaCl2, and 5 Hepes, pH 7.5. Ion substitutions appropriate to individual experiments are described in the text or figure legends. The temperature in all experiments was 15°C.
We used standard methods for two microelectrode current clamp and voltage clamp. Voltage electrodes were filled with 3 M KCl (10–25 M
) while current electrodes were filled with 2 M K-D-gluconic acid (30–100 M) to minimize Cl– loading. Tail currents were low pass filtered at 20 or 80 Hz prior to digitization at a sampling rate of 250 Hz. Iontophoretic injections of Cs+ and tetraethylammonium (TEA) employed a third pipette and were done under voltage clamp so that the membrane potential did not change during injection. Cs+ pipettes were filled with 8 M CsCl (resistance 5–20 M) or 3.2 M CsSO4 (resistance 15–25 M). TEA pipettes were filled with 2.5 M TEA.Cl (resistance 40–90 M).
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PIR in buccal ganglion B-cells
We studied PIR and its relationship to spike adaptation in neurons B3 and B6–B10 (B-cells) from the buccal ganglia of Aplysia californica using current clamp and voltage clamp methods. In each buccal ganglion there are two identified interneurons, called A-cells by Fiore and Meunier (1979)
or B4 and B5 by Gardner (1971a, b), that produce ipsp's in many of the B-cells. A surgical method was used to isolate clusters containing both A-cells and B-cells. This allowed us to examine the response of B-cells to inhibitory synaptic input (Fig. 1A). An individual A-cell was driven to fire a burst of spikes while the hyperpolarization due to summation of ipsp's was recorded in a B-cell. After the stimulus ends the B-cell responds with a postinhibitory rebound that lasts more than a minute (Fig. 1B). It is evident from this experiment that B-cells respond to the cessation of synaptic inhibition with a long lasting period of increased excitability and that this response does not involve excitatory synaptic drive from other sources.
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Direct hyperpolarization is followed by PIR in fully isolated B-cells. Six examples are shown in Figure 1C. Each cell exhibits a prolonged increase in firing frequency after the end of a hyperpolarizing current pulse. Some cells begin to fire immediately after the pulse while others begin to fire after a delay that is due to activation of IA (Connor and Stevens, 1971
). All of the initially silent neurons in Figure 1C had rest potentials within 5 mV of –40 mV. Neurons with rest potentials more negative than approximately –50 mV did not exhibit PIR. The strength of PIR depends on the amplitude and duration of the preceding hyperpolarization (Fig. 2). Hyperpolarizing pulses of 1 sec or less usually evoke only a single action potential in a fashion similar to anode-break excitation, a property predicted by the Hodgkin and Huxley model. Increasing pulse durations result in PIR responses with greater maximum firing frequencies and longer decay times (Fig. 2C, D). In addition to PIR, B-cells exhibit spike frequency adaptation during maintained depolarization (Fig. 3). The time course of spike frequency adaptation and the decay of PIR are remarkably similar which suggests that PIR and adaptation may be manifestations of a single underlying mechanism.
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Voltage clamp studies
In these experiments we are interested in small changes in membrane current in a voltage range close to either the resting potential or the mean interspike voltage in a firing cell. The holding voltage (Vh), therefore, is set near the resting potential (the voltage where Im = 0) and membrane currents are recorded in response to voltage pulses to between –80 and –20 mV. This is the integrative voltage range, the range between the K+ equilibrium potential and spike threshold. On return to Vh after such a step, one observes a tail current that decays back to the steady level of holding current at Vh (Fig. 4A). These slow tail currents are indicative of the mechanism responsible for both PIR and adaptation. Their slow kinetics, and the voltage range in which they operate set them apart from the much faster conductance changes responsible for action potentials.
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We used a subtraction method to isolate slow tail currents for study. Tail currents following 1 sec conditioning pulses were subtracted from the tail currents measured after longer pulses to the same voltage. The difference tail current represents the current component that is activated in the interval between 1 sec and the duration of the longer pulse. Examples are shown for conditioning pulses to –80 and –20 mV in Figure 4B. This method eliminates contributions from IA and all of the other rapidly decaying components of tail current.
A series of voltage clamp pulses is applied to generate families of difference tail currents (Fig. 5A). The amplitudes of difference tail currents are plotted against conditioning potential to obtain a measure of the sensitivity of slow tail current to membrane voltage (Fig. 5B). Depolarizing conditioning pulses activate outward tail currents and hyperpolarizing pulses activate inward tail currents on return to Vh and, therefore, the I(V) curve crosses the zero current axis at the holding potential, –40 mV in this example. The I(V) curves measured in different neurons varied in detail but had several features in common. When tail currents were measured at –40 mV, the slopes of these curves exhibit maxima in the membrane potential ranges between –80 and –60 mV and between –30 and –20 mV. Conditioning pulses in these ranges activate the largest tail currents. The amplitudes of difference tail currents activated by hyperpolarizations to –50 and –60 mV tend to saturate, so the curves representing 10 and 30 sec pulses converge in that region. On the other hand, the amplitudes of difference tail currents activated by depolarizing conditioning pulses continue to increase as the pulse is lengthened from 10 to 30 sec.
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We consistently found that inward tail currents following hyperpolarizing voltage pulses are very sensitive to the holding potential, Vh. Hyperpolarizing pulses from a holding potential of –60 mV do not result in inward tail currents. Inward tail currents begin to appear at holding potentials near –40 mV and increase in amplitude as the holding potential is made more positive. This finding gave an important clue as to the mechanism responsible for slow tail currents and is addressed again in the Discussion.
The difference method was used to measure the time course of recovery of slow tail currents. The outward tail current after a conditioning pulse to –20 mV falls to one half its initial amplitude (t1/2) in 7.2 ± 3.9 sec (mean ± SD, n = 22; range 1.6 to 17 sec) and fully recovers in 44 ± 17 sec (n = 22; range 20 to 78 sec). The value of t1/2 for the slow inward tail current after a conditioning pulse to –60 mV was 2.8 ± 3.9 sec (n = 24; range 0.2 to 15 sec) and the current fully recovered in 31 ± 18 sec (n = 24; range 3.4 to 75 sec).
Conductance change and reversal potential
The conductance change and reversal potential of tail currents activated by depolarizing and hyperpolarizing conditioning pulses were measured using a voltage clamp method developed by Johnson (1985). It involves subtracting the instantaneous I(V) relationship measured in control conditions from an instantaneous I(V) relationship measured under experimental conditions to obtain the I(V) relationship of the conductance under study. The slope of the resulting curve indicates whether a conductance increase (positive slope) or decrease (negative slope) is involved and the point where the curve crosses the voltage axis gives the reversal potential of the current.
When an I(V) curve obtained in the absence of a conditioning pulse is subtracted from an I(V) curve obtained during the outward tail current following a depolarizing conditioning pulse, the difference I(V) relationship has positive slope and crosses the voltage axis between –60 and –70 mV (Fig. 6). This indicates that outward tail currents result from a conductance increase to an ion that has a negative equilibrium potential. In contrast, when the control I(V) curve is measured following a 1 sec conditioning pulse to –50 mV and the experimental I(V) curve is measured after a 10 sec hyperpolarization to –50 mV, the difference I(V) curve again reverses near –70 mV but it has a negative slope. This shows that the current activated by the 10 sec hyperpolarization is caused by a conductance decrease.
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Potassium sensitivity
When the holding potential is –40 mV, depolarizing conditioning pulses are followed by slow outward tail currents and hyperpolarizing conditioning pulses are followed by slow inward tail currents. Hyperpolarizing conditioning pulses also elicit a transient outward current on return to –40 mV due to partial activation of IA. Each of these tail current components is reversed when the external K+ concentration is raised from 10 to 100 mM. Assuming an internal K+ concentration of 250 mM (Sato et al., 1968
), the expected value for the K+ equilibrium potential is –81 mV in 10 mM K+ and –23 mV in 100 mM K+ saline. The reversal of tail currents recorded at –40 mV in the presence of 100 mM K+ shows that these currents are due primarily to changes in K+ conductance.
The subtraction method was used to isolate slow components of membrane current and construct I(V) curves as before. Figure 7A show that the apparent sensitivity of difference tail currents to increased external potassium depends on the conditioning potential at which the tail currents are activated. The independence relation (Hodgkin and Huxley, 1952) was used to predict changes in the amplitude of a K+ current resulting from alterations in the K+ concentration of the bathing saline and the results are plotted as dotted lines in Figure 7A, B. Difference tail currents, measured at –40 mV and activated by conditioning pulses between –60 and –20 mV, are reversed in 100 mM K+ saline as predicted. However, the difference tail current activated by a conditioning pulse to –70 mV is not reversed and, therefore, exhibits considerably less apparent K+ sensitivity.
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3 x 10–10 mol of Cs+ from a 100 mM CsSO4 microelectrode. The injection resulted in a greater than 3-fold increase in spike duration (estimated internal Cs+ concentration: 74 mM for a 200 µm diameter cell and 595 mM for a 100 µm diameter cell assuming a pipette transfer coefficient of 0.5). D). Effects of TEA and Ca = free saline on difference tail currents (30 sec conditioning pulses, tails measured 1 sec after return to –40 mV). Open squares: Difference tail currents in normal saline. Triangles: Currents in Ca-free saline (Mg2+ replaces Ca2+, 1 mM EGTA added}. Ca-free saline had little effect on inward or outward tail currents. Closed squares: Currents measured in Ca2+ free saline after iontophoretic injection of A similar difference in apparent K+ sensitivity is observed when external K+ is reduced from 10 to 1 mM (Fig. 7B). Difference tail currents activated at conditioning potentials more positive than –60 mV are increased in low K+ saline but difference tail currents activated at more negative potentials are decreased. This results in a cross-over in the I(V) curve. It appears that large hyperpolarizations activate a slow current that has a different sensitivity to K+ than the current activated at less negative potentials and this suggests that a second process in involved.
Intracellular injection of Cs+, a K+ channel blocker, markedly reduced difference tail currents activated by depolarizing conditioning pulses and by hyperpolarizing conditioning pulses in the range –40 to –60 (Fig. 7C). This provides additional evidence that the tail current activated in this voltage range is a potassium current. On the other hand, a significant fraction of the tail currents activated by pulses to –70 and –80 mV is resistant to Cs+, indicating that tail currents activated by pulses to more negative voltages involve some additional process.
TEA was used to further explore the tail currents activated in these two voltage ranges (Fig. 7D). The amplitudes of difference tail currents after conditioning pulses to voltages between –60 and –20 mV are reduced following intracellular injection of TEA. Sensitivity to TEA is observed even in cells that exhibit little change in tail current amplitude following removal of Ca2+ from the external medium (Fig. 7D). These data support the conclusion that tail currents activated at voltages positive to –60 mV result primarily from changes in K+ current and in addition make it unlikely that Ca-dependent-K current contributes significantly to the TEA sensitive component.
Sodium sensitivity
Tail currents are sensitive to replacement (96%) of external Na+ with N-methyl glucamine (Fig. 8A). The tail currents activated by conditioning pulses more negative than –50 mV are reversed, while tail currents activated by more depolarized conditioning potentials are reduced. Recovery on return to normal saline is greater for the currents activated by large hyperpolarizations than for the K+ sensitive component activated by more positive conditioning pulses. We can think of two interpretations of this result; either Na+ contributes to the tail current across all voltage ranges, or the K+ sensitive component is activated by depolarization from Vh is partially blocked by the application of N-methyl glucamine. When Li+ was substituted for external Na+ channels, difference tail currents activated by conditioning pulses more negative than –60 mV were reversed but tail currents activated by more depolarizing pulses were little changed (Fig. 8B). Again, it appears that a second current that is sensitive to external Na+ and Li+ is activated by strongly negative conditioning pulses.
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Calcium sensitivity
The possible contribution of Ca2+ influx to difference tail currents was tested by bathing cells in Ca-free saline containing EGTA or by adding the Ca2+ channel blockers Co2+ or Ni2+. Each of these treatments has been shown to reduce or eliminate the internal Ca2+ concentration change responsible for activating the Ca-dependent-K current, IC (Thompson, 1977
). Difference tail currents in the majority of B-cells appear to be only partially dependent on Ca2+ influx (Fig. 8C, D) and in some B-cells the tail currents are completely insensitive to Ca-free saline (Fig. 7D).
Chloride sensitivity
When the Cl– concentration was reduced by replacing two-thirds of external NaCl with Na.gluconate, tail currents activated by conditioning pulses in the range –60 to –20 mV increased (Fig. 9). This was particularly apparent for inward tail currents after hyperpolarizing conditioning pulses. Part of this effect may be due to the positive shift in Vh expected in low Cl– saline. In addition, a Cl– conductance has been described in Aplysia neurons that appears after the cells have been loaded with Cl– or exposed to reduced external Cl– (Chesnoy-Marchais, 1983). This novel Cl– current could contribute to tail currents measured under the conditions of our experiment since it is activated by hyperpolarizing pulses and has slow kinetics that give rise to slowly decaying tail currents. It is unlikely, however, that the Cl– conductance that appears under these conditions will contribute to tail currents measured in normal saline. This is because the Cl– current will produce an inward tail only when Vh is more negative than the Cl– equilibrium potential. Gardner (1971a) estimated the reversal potential of the Cl– dependent IPSP in B-cells to be approximately –75 to –65 mV. In normal saline, therefore, ECl is more negative than the voltage where we measure tail currents.
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The inward tail currents we measured are two to three orders of magnitude smaller than the currents that typically occur during an action potential. Nevertheless, they have a strong influence on firing because the resting membrane conductance is so very low. For example in one cell the maximum amplitude of the slow inward tail current at –40 mV was –0.25 nA. In the same cell studied under current clamp, –0.25 nA completely silenced the neuron and +0.25 nA caused a fourfold increase in firing rate.
Our results suggest that at least two different slow ionic currents contribute to the tail currents recorded in B-cells following long lasting voltage clamp pulses in the integrative voltage range. They can be distinguished by their sensitivity to membrane potential, extracellular ion concentrations, and intracellular K+ current blockers. One current exhibits voltage dependence in the membrane potential range from –60 to –20 mV (and probably to more positive voltages as well), while the other is sensitive to voltage over a more negative, but overlapping membrane potential range from –50 to below –80 mV.
Depolarizing conditioning pulses from the resting potential activate a slow outward current that is associated with an increase in membrane conductance, while hyperpolarizing conditioning pulses to –50 and –60 mV activate a slow inward current that is associated with a decrease in conductance. The reversal potential for both inward and outward tail currents is approximately –70 mV. Both currents are sensitive to changes in external K+ and both are blocked by intracellular Cs+ and TEA. All the data suggest that changes in membrane K+ conductance are involved and the simplest interpretation is that B-cells possess a non-inactivating K+ conductance that responds slowly to membrane potential changes, that this conductance is minimally activated at –60 mV, and that the conductance increases with depolarization.
The inward current will produce a change in firing frequency only if the current drives the membrane potential beyond threshold. The observation that B-cells rarely exhibit PIR if the resting potential is more negative than about –50 mV can be explained by considering two properties of the inward tail current. First, the small amplitudes of inward tail currents suggest that PIR is most likely to occur if the resting potential is near spike threshold. In addition, the amplitude of the inward current decreases as the holding voltage Vh is made more negative. Thus, more negative resting potentials would be associated with smaller inward currents following hyperpolarization. These two features of the inward tail current are sufficient to explain the resting potential dependence of the PIR responses.
Application of Ca2+ free saline and Ca2+ current blockers had a minimal effect on tail currents. This indicates that IC, the Ca-dependent-K current, does not contribute significantly to slow tail currents, at least not at a temperature of 15°C. Tail currents were reduced in some cells by Ca2+ channel blockers, but not in the majority of cells and it is clear that a significant fraction of the slow K+ current in B-cells is insensitive to changes in Ca2+ influx (Thompson et al., 1986).
A Ca2+ independent K+ current with slow kinetics has been described in molluscan neurons that shares a number of properties with the slow K+ current in B-cells (Thompson et al., 1986; Brodwick and Junge, 1972; Huguenard et al., 1985; Johnson, 1985; Partridge and Stevens, 1976). Two previous studies reported partial reduction in current amplitude following inhibition of Ca2+ influx (Huguenard et al., 1985; Johnson, 1985). Interestingly, the activation voltage dependence of the slow K+ current in Aplysia neurons appears to be shifted in the depolarizing direction by approximately 20 mV compared to the current observed in nudibranch neurons (Partridge and Stevens, 1976). We measured a reversal potential of app. –70 which is in the range of published reversal potentials for the slow K+ current (–77 to –60 mV) estimated from the apparent reversal of the membrane potential response (Brodwick and Junge, 1972), from the apparent reversal of tail currents (Huguenard et al., 1985; Partridge and Stevens, 1976), and from the reversal of subtracted I(V) relationships (Johnson, 1985). It is likely that the potassium current we measured is the same as that described previously.
The large increase in inward tail currents observed when external Cl– is lowered is difficult to explain. It is possible that reducing Cl–, or adding gluconate, has a pharmacological effect on the slow K+ current. However, such an effect would have to be voltage dependent because outward tail currents are much less affected than inward tail currents. Also, Huguenard et al. (1985) reported that replacing 90% of external Cl– with sulfate did not affect the depolarization induced outward tail currents in the neurons they studied. Alternatively, the currents observed in low Cl– may be contaminated by the Cl– current described by Chesnoy-Marchais (1983). Whatever the cause, it is not likely that a Cl– current contributes to PIR under normal circumstances since a Cl– current will only produce an inward tail current if the Cl– reversal potential is more positive than the resting potential and this is not normally the case in B-cells.
The slow inward current activated at more negative voltages
The second component of slow current in B-cells is activated by conditioning pulses to more negative potentials. There are two important features of this current to consider. First, because the current is inward at Vh = –40 mV, and reverses at that voltage when external Na+ is reduced, it would seem that Na+ flux contributes to the current. Johnson (1985) studied a similar current in a different group of Aplysia cells and found that it results from a conductance increase and has a reversal potential more positive than 0 mV. This may be a non-selective cation current that is activated by rather strong hyperpolarizations and has slow intrinsic kinetic. The activation curve for this process appears to overlap with the negative end of the integrative voltage range is some cells, so PIR in those cells would result from both recovery from adaptation and the slow decay of an additional inward current. Since the slow inward current is associated with a conductance increase and it has a relatively positive reversal potential, activation of this current by long-lasting inhibition should be self-limiting. This is not the case with the slow K+ current because it is associated with a decrease in membrane conductance. Thus, IPSPs are not shunted by its activation. Also, hyperpolarization reduces the K+ driving force so significant inward current development associated with the decrease in K+ conductance may not be present during inhibition.
The relationship between PIR and adaptation
Our results provide evidence that PIR and spike adaptation are two manifestations of the same process. The decay kinetics of PIR are strikingly similar to the kinetics of adaptation. Neurons that do not adapt lack inward tail currents after conditioning pulses in the integrative voltage range. Both inward and outward slow tail currents are caused primarily by changes in K+ current; PIR is associated with a decrease in K+ conductance, while adaptation results from slow activation of a K+ conductance. The simplest conclusion is that PIR is the result of recovery from tonic adaptation during membrane hyperpolarization. This is consistent with the fact that the slow inward current that underlies PIR is only seen if the holding voltage is more positive than –60 mV. According to our idea, –60 mV must be very near the bottom of the activation curve for the slow K+ current. Kuffler and Eyzaguirre (1955) reached the same conclusion in their study of slowly adapting stretch receptor neurons in crayfish (see also, Barrio et al., 1994).
Functional role of PIR
The functional importance of PIR is most clearly shown in studies of oscillating neural activity in networks operating primarily through inhibitory synaptic interactions (Anderson et al., 1964; Hartline and Gassie, 1979; Kater, 1974; Satterlie, 1985; Selverston et al., 1983). The idea that emerged from these studies is that PIR provides recurrent excitation in networks of reciprocally inhibitory neurons that acts on a cycle-to-cycle basis to help to maintain oscillation. This idea was successfully demonstrated in modeling studies by Perkel and Malloney (1974) and it has drawn a large number of supporters, too many to list here. PIR has been observed in oscillating neural circuits subserving feeding, locomotion, cardiac function, gastric function, respiration, and singing, with examples drawn from all the major phyletic groups. It is obviously an important property of central neurons that may be involved in a variety of integrative activities. The importance of PIR for network oscillation was perhaps most firmly established in the lobster stomatogastric ganglion. All of the neurons in this ganglion exhibit PIR to some degree (Selverston et al., 1976) and experimental studies combined with a computer model of the network indicate that variability in the temporal characteristics of PIR in different elements contributes to the exact phase relationships observed in the motoneuron output (Hartline and Gassie, 1979).
What is the evidence that PIR provides a source of driving energy that helps to maintaining oscillators in such a network? We propose that PIR results from the removal of tonic adaptation, it is not so much an energy source but rather an energy revealment. The source would have to be tonic excitatory drive that puts the cell into a partially adapted state and we have noted that the cells used in our study and many of the others mentioned in the literature all seem to require a slight depolarization to become tonically adapted before than can express PIR. If the theory holds that PIR is due to a removal of adaptation, then the source of driving energy in many neuronal oscillators remains unknown. In our view PIR and adaptation are present to provide a slow transform, to shape the output of the oscillator into a behaviorally meaningful time frame. The sources of driving energy in a system like this can be anything that shifts the rest potential of key neurons into the adapted state. This might be caused by neuromodulators, internal biochemical events, ionic environment, tonic excitation, etc. In this view, the rest potential of neurons in the core oscillator becomes a sort of switch that can be used to gate oscillation by engaging or disengaging PIR.
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ACKNOWLEDGMENTS |
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We thank the staff of the Hopkins Marine Station for generous support and Dr. Jon Johnson for sharing his expertise and his unpublished data. This study was supported by NSF grant IBN-9514421. The Symposium was supported by National Science Foundation Grant 1BN 990 5990.
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FOOTNOTES |
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1 From the Symposium Swimming in Opisthobranch Mollusks: Contributions to Control of Motor Behavior presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2000, at Atlanta, Georgia.
2 Corresponding author; E-mail: stuartt{at}leland.stanford.edu
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References |
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