Conductance changes, an electrogenic pump and the hyperpolarization of leech neurones following impulses

Tonic Stimulation of Dorsal Root Ganglion Results in Progressive Decline in Recruitment of Aα/β-Fibers in Rats

OBJECTIVES

In this study, we aimed to characterize the recruitment and maintenance of action potential firing in Aα/β-fibers generated during tonic dorsal root ganglion stimulation (DRGS) applied over a range of clinically relevant stimulation parameters.

MATERIALS AND METHODS

We delivered electrical stimulation to the L5 dorsal root ganglion and recorded antidromic evoked compound action potentials (ECAPs) in the sciatic nerve during DRGS in Sprague Dawley rats. We measured charge thresholds to elicit ECAPs in Aα/β-fibers during DRGS applied at multiple pulse widths (50, 150, 300, 500 μs) and frequencies (5, 20, 50, 100 Hz). We measured the peak-to-peak amplitudes, latencies, and widths of ECAPs generated during 180 seconds of DRGS, and excitation threshold changes to investigate potential mechanisms of ECAP suppression.

RESULTS

Tonic DRGS produced ECAPs in Aα/β-fibers at charge thresholds below the motor threshold. Increasing the pulse width of DRGS led to a significant increase in the charge required to elicit ECAPs in Aα/β-fibers, while varying DRGS frequency did not influence ECAP thresholds. Over the course of 180 seconds, ECAP peak-to-peak amplitude decreased progressively in a frequency-dependent manner, where 5- and 100-Hz DRGS resulted in 22% and 87% amplitude reductions, respectively, and ECAP latencies increased from baseline measurements during DRGS at 10, 20, 50, and 100 Hz. Regardless of DRGS frequency, ECAP amplitudes recovered within 120 seconds after turning DRGS off. We determined that ECAP suppression may be attributed to increasing excitation thresholds for individual fibers during DRGS. Following 180 seconds of DRGS, an average of 7.33% increase in stimulation amplitude was required to restore the ECAP to baseline amplitude.

CONCLUSIONS

DRGS produces a progressive and frequency-dependent reduction in ECAP amplitude that occurs within and above the frequency range used clinically to relieve pain. If DRGS-mediated analgesia relies on Aβ-fiber activation, then the frequency or duty cycle of stimulation should be set to the lowest effective level to maintain sufficient activation of Aβ-fibers.

Initial Variability and Time-Dependent Changes of Neuronal Response Features Are Cell-Type-Specific

Different cell types are commonly defined by their distinct response features. But several studies proved substantial variability between cells of the same type, suggesting rather the appraisal of response feature distributions than a limitation to "typical" responses. Moreover, there is growing evidence that time-dependent changes of response features contribute to robust and functional network output in many neuronal systems. The individually characterized Touch (T), Pressure (P), and Retzius (Rz) cells in the medicinal leech allow for a rigid analysis of response features, elucidating differences between and variability within cell types, as well as their changes over time. The initial responses of T and P cells to somatic current injection cover a wide range of spike counts, and their first spike is generated with a high temporal precision after a short latency. In contrast, all Rz cells elicit very similar low spike counts with variable, long latencies. During prolonged electrical stimulation the resting membrane potential of all three cell types hyperpolarizes. At the same time, Rz cells reduce their spiking activity as expected for a departure from the spike threshold. In contrast, both mechanoreceptor types increase their spike counts during repeated stimulation, consistent with previous findings in T cells. A control experiment reveals that neither a massive current stimulation nor the hyperpolarization of the membrane potential is necessary for the mechanoreceptors' increase in excitability over time. These findings challenge the previously proposed involvement of slow K+-channels in the time-dependent activity changes. We also find no indication for a run-down of HCN channels over time, and a rigid statistical analysis contradicts several potential experimental confounders as the basis of the observed variability. We conclude that the time-dependent change in excitability of T and P cells could indicate a cell-type-specific shift between different spiking regimes, which also could explain the high variability in the initial responses. The underlying mechanism needs to be further investigated in more naturalistic experimental situations to disentangle the effects of varying membrane properties versus network interactions. They will show if variability in individual response features serves as flexible adaptation to behavioral contexts rather than just "randomness".

Effects of calcium-activated potassium channel modulators on afterhyperpolarizing potentials in identified motor and mechanosensory neurons of the medicinal leech

Calcium-activated potassium (K_Ca) channels contribute to multiple neuronal properties including spike frequency and afterhyperpolarizing potentials (AHPs). K_Ca channels are classified as K_Ca1.1, K_Ca2, or K_Ca3.1 based on single-channel conductance and pharmacology. Ca²⁺-dependent AHPs in vertebrates are categorized as fast, medium, or slow. Fast and medium AHPs are generated by K_Ca1.1 and K_Ca2 channels, respectively. The K_Ca subtype responsible for slow AHPs is unclear. Prolonged, Ca²⁺-dependent AHPs have been described in several leech neurons. Unfortunately, apamin and other K_Ca blockers often prove ineffective in the leech. An alternative approach is to utilize K_Ca modulators, which alter channel sensitivity to Ca²⁺. Vertebrate K_Ca2 channels are targeted selectively by the positive modulator CyPPA and the negative modulator NS8593. Here we show that AHPs in identified motor and mechanosensory leech neurons are enhanced by CyPPA and suppressed by NS8593. Our results indicate that K_Ca2 channels underlie prolonged AHPs in these neurons and suggest that K_Ca2 modulators may serve as effective tools to explore the role of K_Ca channels in leech physiology.

The Effect of Spinal Cord Stimulation Frequency on the Neural Response and Perceived Sensation in Patients With Chronic Pain

Background

The effect of spinal cord stimulation (SCS) amplitude on the activation of dorsal column fibres has been widely studied through the recording of Evoked Compound Action Potentials (ECAPs), the sum of all action potentials elicited by an electrical stimulus applied to the fibres. ECAP amplitude grows linearly with stimulus current after a threshold, and a larger ECAP results in a stronger stimulus sensation for patients. This study investigates the effect of stimulus frequency on both the ECAP amplitude as well as the perceived stimulus sensation in patients undergoing SCS therapy for chronic back and/or leg pain.

Methods

Patients suffering with chronic neuropathic lower-back and/or lower-limb pain undergoing an epidural SCS trial were recruited. Patients were implanted according to standard practice, having two 8-contact leads (8 mm inter-electrode spacing) which overlapped 2–4 contacts around the T9/T10 interspace. Both lead together thus spanning about three vertebral levels. Neurophysiological recordings were taken during the patient’s trial phase at two routine follow-ups using a custom external stimulator capable of recording ECAPs in real-time from all non-stimulating contacts. Stimulation was performed at various vertebral levels, varying the frequency (ranging from 2 to 455 Hz) while all other stimulating variables were kept constant. During the experiments subjects were asked to rate the stimulation-induced sensation (paraesthesia) on a scale from 0 to 10.

Results

Frequency response curves showed an inverse relationship between stimulation sensation strength and ECAP amplitude, with higher frequencies generating smaller ECAPs but stronger stimulation-induced paraesthesia (at constant stimulation amplitude). Both relationships followed logarithmic trends against stimulus frequency meaning that the effects on ECAP amplitude and sensation are larger for smaller frequencies.

Conclusion

This work supports the hypothesis that SCS-induced paraesthesia is conveyed through both frequency coding and population coding, fitting known psychophysics of tactile sensory information processing. The inverse relationship between ECAP amplitude and sensation for increasing frequencies at fixed stimulus amplitude questions common assumptions of monotonic relationships between ECAP amplitude and sensation strength.

Non-synaptic Plasticity in Leech Touch Cells

The role of Na⁺/K⁺-pumps in activity-dependent synaptic plasticity has been described in both vertebrates and invertebrates. Here, we provide evidence that the Na⁺/K⁺-pump is also involved in activity-dependent non-synaptic cellular plasticity in leech sensory neurons. We show that the resting membrane potential (RMP) of T cells hyperpolarizes in response to repeated somatic current injection, while at the same time their spike count (SC) and the input resistance (IR) increase. Our Hodgkin–Huxley-type neuron model, adjusted to physiological T cell properties, suggests that repetitive action potential discharges lead to increased Na⁺/K⁺-pump activity, which then hyperpolarizes the RMP. In consequence, a slow, non-inactivating current decreases, which is presumably mediated by voltage-dependent, low-threshold potassium channels. Closing of these putative M-type channels due to hyperpolarization of the resting potential increases the IR of the cell, leading to a larger number of spikes. By this mechanism, the response behavior switches from rapidly to slowly adapting spiking. These changes in spiking behavior also effect other T cells on the same side of the ganglion, which are connected via a combination of electrical and chemical synapses. An increased SC in the presynaptic T cell results in larger postsynaptic responses (PRs) in the other T cells. However, when the number of elicited presynaptic spikes is kept constant, the PR does not change. These results suggest that T cells change their responses in an activity-dependent manner through non-synaptic rather than synaptic plasticity. These changes might act as a gain-control mechanism. Depending on the previous activity, this gain could scale the relative impacts of synaptic inputs from other mechanoreceptors, versus the spike responses to tactile skin stimulation. This multi-tasking ability, and its flexible adaptation to previous activity, might make the T cell a key player in a preparatory network, enabling the leech to perform fast behavioral reactions to skin stimulation.

Na+/K+-pump and neurotransmitter membrane receptors

Na⁺/K⁺-pump is an electrogenic transmembrane ATPase located in the outer plasma membrane of cells. The Na⁺/K⁺-ATPase pumps 3 sodium ions out of cells while pumping 2 potassium ions into cells. Both cations move against their concentration gradients. This enzyme's electrogenic nature means that it has a chronic role in stabilizing the resting membrane potential of the cell, in regulating the cell volume and in the signal transduction of the cell. This review will mainly consider the role of the Na⁺/K⁺-pump in neurons, with an emphasis on its role in modulating neurotransmitter receptor. Most of the literature on the modulation of neurotransmitter receptors refers to the situation in the mammalian nervous system, but the position is likely to be similar in most, if not all, invertebrate nervous systems.

Differential contributions of Ca2+ -activated K+ channels and Na+ /K+ -ATPases to the generation of the slow afterhyperpolarization in CA1 pyramidal cells

In many types of CNS neurons, repetitive spiking produces a slow afterhyperpolarization (sAHP), providing sustained, intrinsically generated negative feedback to neuronal excitation. Changes in the sAHP have been implicated in learning behaviors, in cognitive decline in aging, and in epileptogenesis. Despite its importance in brain function, the mechanisms generating the sAHP are still controversial. Here we have addressed the roles of M-type K+ current (IM ), Ca2+ -gated K+ currents (ICa(K) 's) and Na+ /K+ -ATPases (NKAs) current to sAHP generation in adult rat CA1 pyramidal cells maintained at near-physiological temperature (35 °C). No evidence for IM contribution to the sAHP was found in these neurons. Both ICa(K) 's and NKA current contributed to sAHP generation, the latter being the predominant generator of the sAHP, particularly when evoked with short trains of spikes. Of the different NKA isoenzymes, α1 -NKA played the key role, endowing the sAHP a steep voltage-dependence. Thus normal and pathological changes in α1 -NKA expression or function may affect cognitive processes by modulating the inhibitory efficacy of the sAHP.

Na(+)/K(+) pump interacts with the h-current to control bursting activity in central pattern generator neurons of leeches

The dynamics of different ionic currents shape the bursting activity of neurons and networks that control motor output. Despite being ubiquitous in all animal cells, the contribution of the Na(+)/K(+) pump current to such bursting activity has not been well studied. We used monensin, a Na(+)/H(+) antiporter, to examine the role of the pump on the bursting activity of oscillator heart interneurons in leeches. When we stimulated the pump with monensin, the period of these neurons decreased significantly, an effect that was prevented or reversed when the h-current was blocked by Cs(+). The decreased period could also occur if the pump was inhibited with strophanthidin or K(+)-free saline. Our monensin results were reproduced in model, which explains the pump's contributions to bursting activity based on Na(+) dynamics. Our results indicate that a dynamically oscillating pump current that interacts with the h-current can regulate the bursting activity of neurons and networks.

A mechanism for spike frequency adaptation

1. Spike frequency adaptation was studied in large neurones of the marine molluscs Archidoris montereyensis and Anisodoris nobilis. These cells respond to a current step with a rapid rise in spike frequency followed by a gradual decline to a new steady level.2. An exponentially declining current, I(s), was measured when the cell was voltage clamped following an adapting spike train. The initial amplitude of this current depended on the preceding number of spikes and on the voltage to which the cell was clamped. A reversal potential (V(s)) for this current was obtained by clamping to various potentials following a spike train. The time constant (tau(s)) of decay of the current was dependent upon the clamping potential.3. Clamping the membrane potential to a constant test level from various initial levels initiates an exponentially decaying current of similar time constant. The voltage dependence of the steady-state conductance (g(s)a(s)(V, infinity)) associated with this current was determined using this technique.4. Equations for neural repetitive firing (Connor & Stevens, 1971c) were modified by the addition of a term describing these slow membrane currents: [Formula: see text]. The solution to the modified equation was in good agreement with the spike frequency adaptation observed in these cells.

Suppression of spikes during posttetanic hyperpolarization in auditory neurons: the role of temperature, I(h) currents, and the Na(+)-K(+)-ATPase pump

In vivo recordings from postsynaptic neurons in the medial nucleus of the trapezoid body (MNTB), an auditory brain stem nucleus, show that acoustic stimulation produces a burst of spikes followed by a period of hyperpolarization and suppressed spiking activity. The underlying mechanism for this hyperpolarization and reduced spiking is unknown. Furthermore, the mechanisms that control excitability and resting membrane potential are not fully determined for these MNTB neurons. In this study we investigated the excitability of principal neurons from the MNTB after high-frequency afferent fiber stimulation, using whole cell recordings from postnatal day 15-17 rat brain stem slices. We found that Na(+)-K(+)-ATPase activity mediates a progressive hyperpolarization during a prolonged tetanic train and a posttetanic hyperpolarization (PTH) at the end of the train, when postsynaptic action potentials failed to fire. Raising the temperature to more physiological levels (from 22 to 35°C) depolarized the resting membrane potential of both presynaptic and postsynaptic cells and decreased the latency of action potential firing during PTH. Higher temperatures also reduced the presynaptic calyx action potential failure rates by 50% during presynaptic PTH, thus increasing the safety-factor for presynaptic spiking. The effect of temperature on hyperpolarization-activated cation current (I(h)) is reflected in the resting potential at both pre- and postsynaptic neurons. We thus propose that temperature-sensitive Na(+)-K(+)-ATPase activity and I(h) contribute to set the resting membrane potential and produce a brief period of suppressed spiking (or action potential failures) after a prolonged high-frequency afferent tetanus.

Functional properties and cell type specific distribution of I(h) channels in leech neurons

The hyperpolarisation-activated cation current (I(h)) has been described in many vertebrate and invertebrate species and cell types. In neurons, I(h) is involved in rhythmogenesis, membrane potential stabilisation and many other functions. In this work, we investigate the distribution and functional properties of I(h) in identified leech neurons of intact segmental ganglia. We found I(h) in the mechanosensory touch (T), pressure (P) and noxious (N) neurons, as well as in Retzius neurons. The current displayed its largest amplitude in P neurons and we investigated its biophysical and pharmacological properties in these cells. I(h) was half-maximally activated at -65 mV and fully activated at -100 mV. The current mutually depended on both Na(+) and K(+) with a permeability ratio p(Na)/p(K) of ∼0.21. The reversal potential was approximately -35 mV. The time course of activation could be approximated by a single time constant of ∼370 ms at -60 mV, but required two time constants at -80 mV of ∼80 and ∼560 ms. The current was half-maximally blocked by 0.3 mmol l(-1) Cs(+) but was insensitive to the bradycardic agent ZD7288. The physiological function of this channel could be a subtle alteration of the firing behaviour of mechanosensory neurons as well as a stabilisation of the resting membrane potential.

Sodium-dependent potassium channels in leech P neurons

In leech P neurons the inhibition of the Na(+)-K(+) pump by ouabain or omission of bath K(+) leaves the membrane potential unaffected for a prolonged period or even induces a marked membrane hyperpolarization, although the concentration gradients for K(+) and Na(+) are attenuated substantially. As shown previously, this stabilization of the membrane potential is caused by an increase in the K(+) conductance of the plasma membrane, which compensates for the reduction of the K(+) gradient. The data presented here strongly suggest that the increased K(+) conductance is due to Na(+)-activated K(+) (K(Na)) channels. Specifically, an increase in the cytosolic Na(+) concentration ([Na(+)](i)) was paralleled by a membrane hyperpolarization, a decrease in the input resistance (R(in)) of the cells, and by the occurrence of an outwardly directed membrane current. The relationship between R(in) and [Na(+)](i) followed a simple model in which the R(in) decrease was attributed to K(+) channels that are activated by the binding of three Na(+) ions, with half-maximal activation at [Na(+)](i) between 45 and 70 mM. At maximum channel activation, R(in) was reduced by more than 90%, suggesting a significant contribution of the K(Na) channels to the physiological functioning of the cells, although evidence for such a contribution is still lacking. Injection experiments showed that the K(Na) channels in leech P neurons are also activated by Li(+).

Remembering that things have changed: a review of the cellular mechanisms of memory re-consolidation in the day-old chick

It has been one of the unshakeable orthodoxies of memory research that memory is initially laid down in a labile form for a short period following the experience and that over time the memory is "fixed" or "consolidated" into the physical structure of the brain. Over the last decade a large body of data has gathered which demonstrates that a "consolidated" memory can be returned to a labile state following retrieval of material from the store, which can then be re-consolidated, incorporating the newly acquired information into the representation of the world. The process of re-consolidation thus provides a sensible means for the crucial process of memory updating to occur. The paper focuses on pharmaco-behavioural experiments undertaken in our laboratories as well as in those of other groups which use the day-old chick as subject and the passive avoidance learning (PAL) task to examine the behavioural and metabolic parameters of re-consolidation. The data indicate that the consolidation and the re-consolidation processes are similar but not identical physiological processes. The re-processing of the memory following a re-consolidation involves each of the glutamatergic, adrenergic and dopaminergic neurotransmitter systems as well as re-activation of protein synthesis associated with the respective traces. In the chick model system, the ability to undertake re-consolidation is transient, and is observed only for a maximum of 24-48 h following the initial training event. Controversy persists as to whether the re-consolidated memory represents a new memory or whether it is a modification of the original memory processing.

Sodium pumps adapt spike bursting to stimulus statistics

Pump activity is a homeostatic mechanism that maintains ionic gradients. Here we examined whether the slow reduction in excitability induced by sodium-pump activity that has been seen in many neuronal types is also involved in neuronal coding. We took intracellular recordings from a spike-bursting sensory neuron in the leech Hirudo medicinalis in response to naturalistic tactile stimuli with different statistical distributions. We show that regulation of excitability by sodium pumps is necessary for the neuron to make different responses depending on the statistical context of the stimuli. In particular, sodium-pump activity allowed spike-burst sizes and rates to code not for stimulus values per se, but for their ratio with the standard deviation of the stimulus distribution. Modeling further showed that sodium pumps can be a general mechanism of adaptation to statistics on the time scale of 1 min. These results implicate the ubiquitous pump activity in the adaptation of neural codes to statistics.

Inhibition of Na+/K+ ATPase potentiates synaptic transmission in tactile sensory neurons of the leech

Increasing evidence indicates that modulation of Na(+)/K(+) ATPase activity is involved in forms of neuronal and synaptic plasticity. In tactile (T) neurons of the leech Hirudo medicinalis, Na(+)/K(+) ATPase is the main determinant of the afterhyperpolarization (AHP), which characterizes the firing of these mechanosensory neurons. Previously, it has been reported that cAMP (3',5'-cyclic adenosine monophosphate), which mediates the effects of serotonin (5HT) in some forms of learning in the leech, negatively modulates Na(+)/K(+) ATPase activity, thereby reducing the AHP amplitude in T neurons. Here, we show that a transient inhibition of Na(+)/K(+) ATPase can affect the synaptic connection between two ipsilateral T neurons. Bath application of 10 nm dihydroouabain (DHO), an ouabain analogue, causes an increase in the amplitude of the synaptic potential (SP) recorded in the postsynaptic element when a test stimulus is applied in the presynaptic neuron. Iontophoretic injection of cAMP into the presynaptic T neuron also produces an increase of SP. Simulations carried out by using a computational model of the T neuron suggest that a reduction of the pump rate and a consequent depression of the AHP might facilitate the conduction of action potentials to the synaptic terminals. Moreover, nearly intact leeches injected with 10 nm DHO respond with a swimming episode more quickly to an electrical stimulation, which selectively activates T neurons exhibiting sensitization of swimming induction. Collectively, our results show that inhibition of Na(+)/K(+) ATPase is critical for short-term plasticity.

Postactivation inhibition of spontaneously active neurosecretory neurons in the medicinal leech

Spontaneously active neurosecretory neurons in vertebrate and invertebrate nervous systems share similarities in firing frequencies, spike shapes, inhibition by the transmitters they themselves release and postactivation inhibition, an intensity-dependent period of suppressed spontaneous generation of action potentials following phases of high-frequency activity. High-frequency activation of spontaneously active serotonin-containing Retzius cells in isolated ganglia of the leech Hirudo medicinalis induced prolonged membrane hyperpolarisations causing periods of postactivation inhibition of up to 33 s. The duration of the inhibitory periods was directly related to both the number and rate of spikes during activation and was inversely proportional to a cell's spontaneous firing frequency. The periods of postactivation inhibition remained unaffected by both serotonin depletion through repeated injections of 5,7-dihydroxytryptamine and suppressing the afterhyperpolarisation following each action potential with tetraethylammonium (TEA), iberiotoxin or charybdotoxin, suggesting that neither autoinhibition by synaptic release of serotonin nor calcium-activated potassium channels contribute to the underlying mechanism. In contrast, the postactivation inhibitory period was significantly affected both by differential electrical stimulation of the same Retzius cells via microelectrodes filled with molar concentrations of either Na(+)-acetate or K(+)-acetate, and by partial inhibition of Na(+)/K(+)-ATPase with ouabain. Thus, postactivation inhibition in Retzius cells results from prolonged hyperpolarising activity of Na(+)/K(+)-ATPase stimulated by the accumulation of cytosolic Na(+ )during phases of high-frequency spike activity.

Activity-dependent suppression of spontaneous spike generation in the Retzius neurons of the leech Hirudo medicinalis L

We report on factors affecting the spontaneous firing pattern of the identified serotonin-containing Retzius neurons of the medicinal leech. Increased firing activity induced by intracellular current injection is followed by a 'post-stimulus-depression' (PSD) without spiking for up to 23 s. PSD duration depends both on the duration and the amplitude of the injected current and correlates inversely with the spontaneous spiking activity. In contrast to serotonin-containing neurons in mammals, serotonin release from the Retzius cells presumably does not mediate the observed spike suppression in a self-inhibitory manner since robust PSD persists after synaptic isolation. Moreover, single additional spikes elicited at specific delays after spontaneously occurring action potentials are sufficient to significantly alter the firing pattern. Since sub-threshold current injections do not affect the ongoing spiking pattern and PSD persists in synaptically isolated preparations our data suggest that PSD reflects an endogenous and 'spike-dependent' mechanism controlling the spiking activity of Retzius cells in a use-dependent way.

A 3-synapse positive feedback loop regulates the excitability of an interneuron critical for sensitization in the leech

Sensitization of reflexive shortening in the leech has been linked to serotonin (5-HT)-induced changes in the excitability of a single interneuron, the S cell. This neuron is necessary for sensitization and complete dishabituation of reflexive shortening, during which it contributes to the sensory-motor reflex. The S cell does not contain 5-HT, which is released primarily from the Retzius (R) cells, whose firing enhances S-cell excitability. Here, we show that the S cell excites the R cells, mainly via a fast disynaptic pathway in which the first synapse is the electrical junction between the S cell and the coupling interneurons, and the second synapse is a glutamatergic synapse of the coupling interneurons onto the R cells. The S cell-triggered excitatory postsynaptic potential in the R cell diminishes and nearly disappears in elevated concentrations of divalent cations because the coupling interneurons become inexcitable under these conditions. Serotonin released from the R cells feeds back on the S cell and increases its excitability by activating a 5-HT7-like receptor; 5-methoxytryptamine (5-MeOT; 10 microM) mimics the effects of 5-HT on S cell excitability, and effects of both 5-HT and 5-MeOT are blocked by pimozide (10 microM) and SB-269970 [(R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol] (5 microM). This feedback loop may be critical for the full expression of sensitization of reflexive shortening.

Cocaine elicits action potential bursts in a central snail neuron: The role of delayed rectifying K+ current

The effects of cocaine were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials and bath application of cocaine (0.3-1 mM) reversibly elicited action potential bursts of the central RP4 neuron in a concentration-dependent manner. The action potential bursts were not blocked when neurons were immersed in high-Mg(2+)solution, Ca(2+)-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, or sulpiride. Similarly, the action potential bursts were not abolished by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester or anisomycin. Injection of hyperpolarizing current at an intensity of greater than 2 nA effectively suppressed the cocaine-elicited action potential bursts and no postsynaptic potentials were observed under these conditions. These results suggest that the generation of action potential bursts elicited by cocaine was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic or dopaminergic receptors of the excitable membrane, or (3) the cAMP second messengers and new protein synthesis of the RP4 neuron. Notably, the induction of action potential bursts was blocked by pretreatment with 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione. Voltage-clamp studies conducted on the RP4 neuron revealed that cocaine at 0.3 mM decreased (1) the Ca(2+) current, (2) the delayed rectifying K(+) current, (3) the fast-inactivating K(+) current and (4) the Ca(2+)-activated K(+) current, but had no remarkable effects on the Na(+) current. Perfusion with Ca(2+)-free solution, which may abolish the Ca(2+) current and Ca(2+)-activated K(+) current, did not cause any bursts of action potentials in control RP4 neurons. Application of 4-aminopyridine, an inhibitor of fast-inactivating K(+) current, and paxilline, an inhibitor of Ca(2+)-activated K(+) current, failed to elicit action potential bursts, whereas tetraethylammonium chloride, a blocker of Ca(2+)-activated K(+) current and delayed rectifying K(+) current, and tacrine, an inhibitor of delayed rectifying K(+) current, successfully elicited action potential bursts. Further, while 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione did not affect the delayed rectifying K(+) current of the RP4 neuron, 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione decreased the inhibitory effect of cocaine on the delayed rectifying K(+) current. It is concluded that cocaine elicits action potential bursts in the central snail RP4 neuron and that the effect is closely related to the inhibitory effects on the delayed rectifying K(+) current.

Role for calcium signaling and arachidonic acid metabolites in the activity-dependent increase of AHP amplitude in leech T sensory neurons

Previous studies have revealed a new form of activity-dependent modulation of the afterhyperpolarization (AHP) in tactile (T) neurons of the leech Hirudo medicinalis. The firing of T cells is characterized by an AHP, which is mainly due to the activity of the Na+/K+ ATPase. Low-frequency repetitive stimulation of T neurons leads to a robust increment of the AHP amplitude, which is correlated with a synaptic depression between T neuron and follower cells. In the present study, we explored the molecular cascades underlying the AHP increase. We tested the hypothesis that this activity-dependent phenomenon was triggered by calcium influx during neural activity by applying blockers of voltage-dependent Ca2+ channels. We report that AHP increase requires calcium influx that, in turn, induces release of calcium from intracellular stores so sustaining the enhancement of AHP. An elevation of the intracellular calcium can activate the cytosolic isoforms of the phosholipase A2 (PLA2). Therefore we analyzed the role of PLA2 in the increase of the AHP, and we provide evidence that not only PLA2 but also the recruitment of arachidonic acid metabolites generated by the 5-lipoxygenase pathway are necessary for the induction of AHP increase. These data indicate that a sophisticated cascade of intracellular signals links the repetitive discharge of T neurons to the activation of molecular pathways, which finally may alter the activity of critical enzymes such as the Na+/K+ ATPase, that sustains the generation of the AHP and its increase during repetitive stimulation. These results also suggest the potential importance of the poorly studied 5-lipoxygenase pathway in forms of neuronal plasticity.

Slow adaptation in spider mechanoreceptor neurons

Slow adaptation of action potential firing is a common but poorly understood property of sensory neurons. We quantified slow adaptation in a cuticular mechanoreceptor organ of the spider, Cupiennius salei, by stimulating with continuous pseudorandom mechanical displacements while recording action potentials intracellularly from the cell bodies. Firing rate declined over a period of several minutes before reaching a steady level at about half the initial rate. This slow adaptation was fitted by an exponential decay with mean time constant of 18.5 s. Recovery from slow adaptation was also fitted by an exponential process, but with a longer time constant of 167 s. The receptor potential produced by the same stimulation protocol did not change its amplitude or dynamics, showing that slow adaptation occurs during action potential encoding from the receptor potential. Experiments with chemical blockers of calcium entry or the known potassium currents failed to reduce the slow adaptation. The Na+/K+ pump blocker Ouabain decreased the time constant of slow adaptation, suggesting that ion accumulation is involved. In some experiments, a second class of small action potentials were observed, which were tentatively attributed to failed conduction from the sensory dendrite through the soma to the axon.

Acetyl-l-carnitine induces a sustained potentiation of the afterhyperpolarization

Acetyl-L-carnitine is known to improve many aspects of the neural activity even if its exact role in neurotransmission is still unknown. This study investigates the effects of acetyl-L-carnitine in T segmental sensory neurons of the leech Hirudo medicinalis. These neurons are involved in some forms of neural plasticity associated with learning processes. Their physiological firing is accompanied by a large afterhyperpolarization that is mainly due to the Na+/K+ ATPase activity and partially to a Ca2+ -dependent K+ current. A clear-cut hyperpolarization and a significant increase of the afterhyperpolarization have been recorded in T neurons of leeches injected with 2 mM acetyl-L-carnitine some days before. Acute treatments of 50 microM acetyl-L-carnitine induced similar effects in T cells of naive animals. In the presence of apamin, a pharmacological blocker of Ca2+ -dependent K+ channel, acetyl-L-carnitine still enhanced the residual afterhyperpolarization, suggesting an effect of the drug on the Na+/K+ATPase. Acetyl-L-carnitine also increased the hyperpolarization induced by intracellular injection of Na+ ions. Therefore, acetyl-L-carnitine seems to be able to exert a positive sustained effect on the Na+/K+ ATPase activity in leech T sensory neurons. Moreover, in these cells, widely arborized, the afterhyperpolarization seems to play an important role in determining the action potential transmission at neuritic bifurcations. A computational model of a T cell has been previously developed considering detailed data for geometry and the modulation of the pump current. Herein, we showed that to a larger afterhyperpolarization, due to the acetyl-L-carnitine-induced effects, corresponds a decrement in the number of action potentials reaching synaptic terminals.

Memory retrieval in the day-old chick: a psychobiological approach

This review integrates a series of studies conducted examining memory retrieval processes in the day-old chick. On the basis of these studies it is proposed that two processes are activated following retrieval of a memory. The first is an immediate memory recall or retrieval mechanism responsible for the chick's ability to remember the information and respond appropriately to the stimulus. The second process is activated following the completion of the first immediate retrieval phase. Further, it is proposed that the function of this secondary phase may be to allow for the modification of a memory undergoing storage processes. It is proposed that the processes of memory formation and memory retrieval are parallel at a cellular level, but at the functional level of information transfer they are interdependent.

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.

Caulerpenyne, a toxin from the seaweed Caulerpa taxifolia, depresses afterhyperpolarization in invertebrate neurons

The massive invasion of the Mediterranean Sea by the tropical seaweed Caulerpa taxifolia (Vahl) C. Agardh has stimulated several investigations in order to test the environmental risk from an ecotoxicological point of view. The studies carried out on various experimental models have shown that caulerpenyne, the major metabolite synthesized by the seaweed, affects several cellular and molecular targets. In addition, neurological disorders have been reported in patients who accidentally ate C. taxifolia, but no evidence about the potential effects of the seaweed and of its metabolites on nerve cells were up to now available. Herein we describe that caulerpenyne modifies the electrical properties of touch mechanosensory cells of the leech Hirudo medicinalis. The physiological firing of these cells causes an afterhyperpolarization that is mainly due to the activity of the Na+/K+-ATPase and to a lesser extent to a calcium-dependent potassium current. Caulerpenyne depressed this afterhyperpolarization; the effect was dose-dependent and partially reversible. Experiments have been carried out in order to understand the mechanism through which caulerpenyne reduced the afterhyperpolarization. The action of the biotoxin has been tested in the presence of pharmacological blockers of calcium-dependent potassium channels such as cadmium and apamin. In these experimental conditions, caulerpenyne still reduced the residual afterhyperpolarization, suggesting a direct effect of the toxin on the Na+/K+-ATPase. In order to test this hypothesis, we have performed experiments where the Na+/K+-ATPase was activated by the intracellular injection of sodium and where also its basal activity was modified as well. From the data collected we suggest that caulerpenyne inhibits both the basal and the sodium-induced activity of the Na+/K+-ATPase in leech touch neurons.

Cyclosporin A, FK506 and rapamycin produce multiple, temporally distinct, effects on memory following single-trial, passive avoidance training in the chick

Few studies have used a pharmaco-behavioural methodology to directly investigate roles for the calcium-dependent protein phosphatase calcineurin (CaN) in memory formation, due partly to the absence of specific inhibitory agents. A number of drugs with different inhibitory profiles were used to examine this issue in groups of chicks trained on a single-trial, passive-avoidance task. Bilateral intracranial administration of the immunosuppressants FK506 and cyclosporin A (CyA) led to two temporally distinct effects, distinguished by the concentration of drug required and the effective time of administration relative to training. In addition to inhibiting CaN, CyA and FK506 inhibit distinct classes of peptidyl prolyl-cis/trans-isomerases (PPIases). Other agents known to inhibit these enzymes, including the Map kinase inhibitor Rapamycin, also induced memory deficits in a complex, dose- and time-of-administration-dependent, manner. The data fail to conclusively implicate CaN in memory formation, but are consistent with proposals that a phosphatase cascade may participate in an early stage of information storage. PPIases may be required at a later stage to catalyse the folding of new or translocated proteins, the synthesis of which is required for formation of long-term memory, although other possible explanations for the data remain to be investigated.

Coding and adaptation during mechanical stimulation in the leech nervous system

The experiments described here were designed to characterise sensory coding and adaptation during mechanical stimulation in the leech (Hirudo medicinalis). A chain of three ganglia and a segment of the body wall connected to the central ganglion were used. Eight extracellular suction pipettes and one or two intracellular electrodes were used to record action potentials from all mechanosensory neurones of the three ganglia. When the skin of the body wall was briefly touched with a filament exerting a force of about 2 mN, touch (T) cells in the central ganglion, but also those in adjacent ganglia (i.e. anterior and posterior), fired one or two action potentials. However, the threshold for action potential initiation was lower for T cells in the central ganglion than for those in adjacent ganglia. The timing of the first evoked action potential in a T cell was very reproducible with a jitter often lower than 100 us. Action potentials in T cells were not significantly correlated. When the force exerted by the filament was increased above 20 mN, pressure (P) cells in the central and neighbouring ganglia fired action potentials. Action potentials in P cells usually followed those evoked in T cells with a delay of about 20 ms and had a larger jitter of 0.5-10 ms. With stronger stimulations exceeding 50 mN, noxious (N) cells also fired action potentials. With such stimulations the majority of mechanosensory neurones in the three ganglia fired action potentials. The spatial properties of the whole receptive field of the mechanosensory neurones were explored by touching different parts of the skin. When the mechanical stimulation was applied for a longer time, i.e. 1 s, only P cells in the central ganglion continued to fire action potentials. P cells in neighbouring ganglia fully adapted after firing two or three action potentials.P cells in adjacent ganglia, having fully adapted to a steady mechanical stimulation of one part of the skin, fired action potentials following stimulation of a different region of the skin. These results indicate that a brief and localised stimulation of the skin can activate more than a dozen different mechanosensory neurones in the three ganglia and after 100 ms of steady stimulation many of these mechanosensory neurones stop firing action potentials and fully adapt. Adaptation occurs primarily at the nerve endings and mechanosensory neurones can quickly respond to mechanical stimulation at a different location on the skin.

Dendritic calcium accumulation associated with direction-selective adaptation in visual motion-sensitive neurons in vivo

Motion adaptation in directionally selective tangential cells (TC) of the fly visual system has previously been explained as a presynaptic mechanism. Based on the observation that adaptation is in part direction selective, which is not accounted for by the former models of motion adaptation, we investigated whether physiological changes located in the TC dendrite can contribute to motion adaptation. Visual motion in the neuron's preferred direction (PD) induced stronger adaptation than motion in the opposite direction and was followed by an afterhyperpolarization (AHP). The AHP subsides in the same time as adaptation recovers. By combining in vivo calcium fluorescence imaging with intracellular recording, we show that dendritic calcium accumulation following motion in the PD is correlated with the AHP. These results are consistent with a calcium-dependent physiological change in TCs underlying adaptation during continuous stimulation with PD motion, expressing itself as an AHP after the stimulus stops. However, direction selectivity of adaptation is probably not solely related to a calcium-dependent mechanism because direction-selective effects can also be observed for fast moving stimuli, which do not induce sizeable calcium accumulation. In addition, a comparison of two classes of TCs revealed differences in the relationship of calcium accumulation and AHP when the stimulus velocity was varied. Thus the potential role of calcium in motion adaptation depends on stimulation parameters and cell class.

Time course of post-excitatory effects separates afferent human C fibre classes

1. To study post-excitatory changes of conduction velocity, action potentials were recorded from 132 unmyelinated nerve fibres (C fibres) in cutaneous fascicles of the peroneal nerve using microneurography in healthy human subjects. The 'marking' technique was used to assess responsiveness to mechanical and heat stimuli or sympathetic reflex provocation. 2. C fibres were classified into three major classes: mechano-responsive afferent (n = 76), mechano-insensitive afferent (n = 48) and sympathetic efferent C fibres (n = 8). 3. During regular stimulation at 0.25 Hz, conditioning pulses were intermittently interposed. Changes of conduction velocity were assessed for different numbers of conditioning impulses and varying interstimulus intervals (ISIs). For all three fibre classes the latency shift following conditioning pulses at an ISI of 1000 ms increased linearly with their number (n = 1, 2 and 4). However, the absolute degree of conduction velocity slowing was much higher in the 32 mechano-insensitive fibres as compared with 56 mechano-responsive or 8 sympathetic fibres. 4. Single additional pulses were interposed at different ISIs from 20 to 2000 ms. For 20 mechano-responsive fibres conduction velocity slowing increased with decreasing ISI (subnormal phase). In contrast, for 16 mechano-insensitive C fibres the conduction velocity slowing decreased with shorter ISIs, and at values lower than 417 +/- 49 ms (mean +/- s.e.m.) the conduction velocity of the conditioned action potential was faster than before (conduction velocity speeding). This supernormal phase had its maximum at 69 +/- 10 ms. 5. In this study we provide, for the first time, direct evidence of relative supernormal conduction in human mechano-insensitive C fibres. The implications for temporal coding in different afferent C fibre classes are discussed.

Neurotoxic effects of caulerpenyne

1. In this paper the authors tested the effect of caulerpenyne (CYN), a sesquiterpene synthesized by the green alga Caulerpa taxifolia onto the central nervous system of the leech Hirudo medicinalis. Investigations have been performed with three different approaches: neuroethological, electrophysiological and neurochemical techniques. 2. CYN application mimics the effect of a nociceptive stimulation (brushing), eliciting a clear-cut potentiation of the animal swim response to the test stimulus (non associative learning process such as sensitization). This effect is similar to that one induced by the endogenous neurotransmitter serotonin (5HT). 3. CYN strongly reduces the after-hyperpolarization (AHP) recorded from T sensory neurons. This effect overlaps that one produced by 5HT, but it is not affected by the serotonergic antagonist methysergide. 4. The decrease of AHP amplitude due to CYN application is observed also in presence of apamin, a blocking agent of Ca++-dependent K+ channels, suggesting that CYN is acting through the inhibition of the Na+/K+ electrogenic pump. 5. The depression of the AHP driven by CYN is not prevented by application of MDL 12330A, an adenylate cyclase inhibitor. On the other hand MDL 12330A counteracts the reduction of AHP due to 5HT application. 6. Incubation of the leech central nervous system with CYN induces the phosphorylation of proteins of 29, 50, 66 and 100 kDa. This pattern of phosphorylation is similar to that one elicited by 5HT treatment. 7. The data demonstrate that CYN exerts remarkable effects on leech neurons by acting onto specific molecular targets such as the Na+/K+ ATPase. This effect may influence important neural integrative functions and may explain the sensitizing action produced by the toxin on swim induction. Finally, caulerpenyne does not act through the pathways involved in the 5HT action, and its effect is not mediated by the second messenger cyclic AMP. The mechanism of action of CYN are still under investigations.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Action potential reflection and failure at axon branch points cause stepwise changes in EPSPs in a neuron essential for learning

In leech mechanosensory neurons, action potentials reverse direction, or reflect, at central branch points. This process enhances synaptic transmission from individual axon branches by rapidly activating synapses twice, thereby producing facilitation. At the same branch points action potentials may fail to propagate, which can reduce transmission. It is now shown that presynaptic action potential reflection and failure under physiological conditions influence transmission to the same postsynaptic neuron, the S cell. The S cell is an interneuron essential for a form of nonassociative learning, sensitization of the whole body shortening reflex. The P to S synapse has components that appear monosynaptic (termed "direct") and polysynaptic, both with glutamatergic pharmacology. Reflection at P cell branch points on average doubled transmission to the S cell, whereas action potential failure, or conduction block, at the same branch points decreased it by one-half. Each of two different branch points affected transmission, indicating that the P to S connection is spatially distributed around these branch points. This was confirmed by examining the locations of individual contacts made by the P cell with the S cell and its electrically coupled partner C cells. These results show that presynaptic neuronal morphology produces a range of transmission states at a set of synapses onto a neuron necessary for a form of learning. Reflection and conduction block are activity-dependent and are basic properties of action potential propagation that have been seen in other systems, including axons and dendrites in the mammalian brain. Individual branch points and the distribution of synapses around those branch points can substantially influence neuronal transmission and plasticity.

Functional attributes discriminating mechano-insensitive and mechano-responsive C nociceptors in human skin

Microneurography was used in healthy human subjects to record action potentials from unmyelinated nerve fibers (C units) in cutaneous fascicles of the peroneal nerve. Activity-dependent slowing (n = 96) and transcutaneous electrical thresholds (n = 67) were determined. Eight units were sympathetic efferents according to their responses to sympathetic reflex provocations. Mechano-heat-responsive C units (CMH) (n = 56) had thresholds to von Frey hair stimulation </=90 mN (6.5 bar). Mechano-insensitive C units (n = 32) were unresponsive to 750 mN (18 bar). Twenty-six mechano-insensitive units responded to heat (CH), and the remaining six units did not respond to physical stimuli but were proven to be afferent by their response to intracutaneous capsaicin (CM(i)H(i)). Mechano-insensitive units had significantly slower conduction velocity (0.81 +/- 0.03 m/sec), and CH units had higher heat thresholds (48.0 +/- 0.6 degrees C) compared with CMH units (1.01 +/- 0.01 m/sec; 40.7 +/- 0.4 degrees C). Transcutaneous electrical thresholds were <9 mA for CMH units and >35 mA for CH and CM(i)H(i) units. Activity-dependent slowing was much more pronounced in mechano-insensitive than in mechano-responsive units, without overlap. Sympathetic efferent C units showed intermediate slowing, significantly different from CMH, and completely separate from CH and CM(i)H(i) units. The activity-dependent slowing of conduction provides evidence for different membrane attributes of different classes of C fibers in humans.

Functional attributes discriminating mechano-insensitive and mechano-responsive C nociceptors in human skin

Microneurography was used in healthy human subjects to record action potentials from unmyelinated nerve fibers (C units) in cutaneous fascicles of the peroneal nerve. Activity-dependent slowing (n = 96) and transcutaneous electrical thresholds (n = 67) were determined. Eight units were sympathetic efferents according to their responses to sympathetic reflex provocations. Mechano-heat-responsive C units (CMH) (n = 56) had thresholds to von Frey hair stimulation </=90 mN (6.5 bar). Mechano-insensitive C units (n = 32) were unresponsive to 750 mN (18 bar). Twenty-six mechano-insensitive units responded to heat (CH), and the remaining six units did not respond to physical stimuli but were proven to be afferent by their response to intracutaneous capsaicin (CM(i)H(i)). Mechano-insensitive units had significantly slower conduction velocity (0.81 +/- 0.03 m/sec), and CH units had higher heat thresholds (48.0 +/- 0.6 degrees C) compared with CMH units (1.01 +/- 0.01 m/sec; 40.7 +/- 0.4 degrees C). Transcutaneous electrical thresholds were <9 mA for CMH units and >35 mA for CH and CM(i)H(i) units. Activity-dependent slowing was much more pronounced in mechano-insensitive than in mechano-responsive units, without overlap. Sympathetic efferent C units showed intermediate slowing, significantly different from CMH, and completely separate from CH and CM(i)H(i) units. The activity-dependent slowing of conduction provides evidence for different membrane attributes of different classes of C fibers in humans.

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

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

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

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

Autoinhibition of serotonin cells: an intrinsic regulatory mechanism sensitive to the pattern of usage of the cells

After periods of high-frequency firing, the normal rhythmically active serotonin (5HT)-containing neurosecretory neurons of the lobster ventral nerve cord display a period of suppressed spike generation and reduced synaptic input that we refer to as "autoinhibition." The duration of this autoinhibition is directly related to the magnitude and duration of the current injection triggering the high-frequency firing. More interesting, however, is that the autoinhibition is inversely related to the initial firing frequency of these cells within their normal range of firing (0.5-3 Hz). This allows more active 5HT neurons to resume firing after shorter durations of inhibition than cells that initially fired at slower rates. Although superfused 5HT inhibits the spontaneous firing of these cells, the persistence of autoinhibition in saline with no added calcium, in cadmium-containing saline, and in lobsters depleted of serotonin suggests that intrinsic membrane properties account for the autoinhibition. A similar autoinhibition is seen in spontaneously active octopamine neurons but is absent from spontaneously active gamma-aminobutyric acid cells. Thus, this might be a characteristic feature of amine-containing neurosecretory neurons. The 5HT cells of vertebrate brain nuclei share similarities in firing frequencies, spike shapes, and inhibition by 5HT with the lobster cells that were the focus of this study. However, the mechanism suggested to underlie autoinhibition in vertebrate neurons is that 5HT released from activated or neighboring cells acts back on inhibitory autoreceptors that are found on the dendrites and cell bodies of these neurons.

Mechanisms of afterhyperpolarization in lobster olfactory receptor neurons

In lobster olfactory receptor neurons (ORNs), depolarizing responses to odorants and current injection are accompanied by the development of an afterhyperpolarization (AHP) that likely contributes to spike-frequency adaptation and that persists for several seconds after termination of the response. A portion of the AHP can be blocked by extracellular application of 5 mM CsCl. At this concentration, CsCl specifically blocks the hyperpolarization-activated cation current (Ih) in lobster ORNs. This current is likely to be active at rest, where it provides a constant, depolarizing influence. Further depolarization deactivates Ih, thus allowing the cell to be briefly hyperpolarized when that depolarizing influence is removed, thus generating an AHP. Reactivation of Ih would terminate the AHP. The component of the AHP that could not be blocked by Cs+ (the Cs(+)-insensitive AHP) was accompanied by decreased input resistance, suggesting that this component is generated by increased conductance to an ion with an equilibrium potential more negative than the resting potential. The Cs(+)-insensitive AHP in current clamp and the underlying current in voltage clamp displayed a reversal potential of approximately -75 mV. Both EK and ECl are predicted to be in this range. Similar results were obtained with the use of a high Cl- pipette solution, although that shifted ECl from -72 mV to -13 mV. However, when EK was shifted to more positive or negative values, the reversal potential also shifted accordingly. A role for the Ca(2+)-mediated K+ current in generating the Cs(+)-independent AHP was explored by testing cells in current and voltage clamp while blocking IK(Ca) with Cs+/Co(2+)-saline. In some cells, the Cs(+)-independent AHP and its underlying current could be completely and reversibly blocked by Cs+/Co2+ saline, whereas in other cells some fraction of it remained. This indicates that the Cs(+)-independent AHP results from two K+ currents, one that requires an influx of extracellular Ca2+ and one that does not. Collectively, these findings indicate that AHPs result from three phenomena that occur when lobster ORNs are depolarized: 1) inactivation of the hyperpolarization-activated cation current, 2) activation of a Ca(2+)-mediated K+ current, and 3) activation of a K+ current that does not require influx of extracellular Ca2+. Roles of these processes in modulating the output of lobster ORNs are discussed.

Synaptic facilitation by reflected action potentials: enhancement of transmission when nerve impulses reverse direction at axon branch points

A rapid, reversible enhancement of synaptic transmission from a sensory neuron is reported and explained by impulses that reverse direction, or reflect, at axon branch points. In leech mechanosensory neurons, where one can detect reflection because it is possible simultaneously to study electrical activity in separate branches, action potentials reflected from branch points within the central nervous system under physiological conditions. Synapses adjacent to these branch points were activated twice in rapid succession, first by an impulse arriving from the periphery and then by its reflection. This fast double-firing facilitated synaptic transmission, increasing it to more than twice its normal level. Reflection occurred within a range of resting membrane potentials, and electrical activity produced by mechanical stimulation changed membrane potential so as to produce and cease reflection. A compartmental model was used to investigate how branch-point morphology and electrical activity contribute to produce reflection. The model shows that mechanisms that hyperpolarize the membrane so as to impair action potential propagation can increase the range of structures that can produce reflection. This suggests that reflection is more likely to occur in other structures where impulses fail, such as in axons and dendrites in the mammalian brain. In leech sensory neurons, reflection increased transmission from central synapses only in those axon branches that innervate the edges of the receptive field in the skin, thereby sharpening spatial contrast. Reflection thus allows a neuron to amplify synaptic transmission from a selected group of its branches in a way that can be regulated by electrical activity.

Complex roles of glutamate in the Gibbs-Ng model of one-trial aversive learning in the new-born chick

Glutamate is the most widespread excitatory transmitter in the CNS and is probably involved in LTP, a neural phenomenon which may be associated with learning and memory formation. Intracerebral injection of large amounts of glutamate between 5 min and 2.5 min after passive avoidance learning in young chicks inhibits short-term memory, which occurs between 0 and 10 min post-learning in a three-stage model of memory formation first established by Gibbs and Ng(25) [Physiol. Behav. 23:369-375; 1979]. This effect may be attributed to non-specific excitation. Blockade of glutamate uptake by L-aspartic and beta-hydroxamate also abolishes this stage of memory, provided the drug is administered within 2.5 min of learning. Interference with either production of percursors for transmitter glutamate in astrocytes or with glutamate receptors is also detrimental to memory formation, but the effects appear much later. After its release from glutamatergic neurons, glutamate is, to a large extent, accumulated into astrocytes where it is converted to glutamine, which can be returned to glutamatergic neurons and reutilized for synthesis of transmitter glutamate, and partly oxidized as a metabolic substrate. The latter process leads to a net loss of transmitter glutamate which can be compensated for by de novo synthesis of a glutamate precursor alpha-ketoglutarate (alpha KG) in astrocytes, a process which is inhibited by the astrocyte-specific toxin fluoroacetate (R. A. Swanson, personal communication). Intracerebral injection of this toxin abolishes memory during an intermediate stage of memory processing occurring between 20 and 30 min post-training (50) [Cog. Brain Res, 2:93-102; 1994]. Injection of methionine sulfoximine (MSO), a specific inhibitor of glutamine synthetase, which interferes with the re-supply of transmitter glutamate to neurons by inhibition of glutamine synthesis in astrocytes, has a similar effect. This effect of MSO is prevented by intracerebral injection of glutamate, glutamine, or a combination and alpha KG and alanine. MSO must be administered before learning, but does not interfere with acquisition since short-term memory remains intact. Administration of either the NMDA antagonist AP5, the AMPA antagonist DNQX, or the metabotropic receptor antagonist MCPF, also induces amnesia. Memory loss in each case does not occur until after 70 min post-training, during a protein synthesis-dependent long-term memory stage which begins at 60 min following learning. However, to be effective, AP5 must be administered within 60 s following learning, MCPG before 15 min post-learning, and DNQX between 15 and 25 min after learning. Together, these findings suggest that learning results in an immediate release of glutamate, followed by a secondary release of this transmitter at later stages of processing of the memory trace, and that one or both of these increases in extracellular glutamate concentration are essential for the consolidation of long-term memory. Since both fluoroacetate and MSO act exclusively on glial cells, the findings also show that neuronal-glial interactions are necessary during the establishment of memory.

Modulation of conduction block in leech mechanosensory neurons

Conduction block is a mechanism of activity-dependent neuronal plasticity, but little is known about its possible neuromodulation. Extensive activity in leech touch (T), pressure (P), and nociceptive (N) mechanosensory neurons results in conduction block of their minor receptive fields. We have examined whether the duration of conduction block could be modulated by the serotonergic Retzius neurons or by application of serotonin (5-HT). Activation of one Retzius cell reduced the duration of conduction block in T and P cell posterior fields, but their anterior fields and N cell fields were unaffected. Perfusion with 5-HT had stronger effects, reducing the duration of conduction block in T, P, and lateral N cells in the posterior fields and either reducing or more often enhancing the expression of conduction block in anterior fields. The effects of 5-HT on posterior fields were blocked by the nonspecific 5-HT antagonist methysergide and were partly suppressed by the 5-HT2 antagonist ketanserin. To determine the site of 5-HT action, the central ganglion or peripheral skin was perfused independently. T and to a greater extent P cells showed a preferential sensitivity to application of 5-HT onto the central ganglion. Interestingly, medial N cells exhibited a progressive decrease in the duration of conduction block during repeated trials ("wind-up") that was unaffected by 5-HT. We conclude that secretion of 5-HT by the Retzius cells has a central modulatory effect on the duration of conduction block in T, P, and lateral N cells.

Modulation of conduction block in leech mechanosensory neurons

Conduction block is a mechanism of activity-dependent neuronal plasticity, but little is known about its possible neuromodulation. Extensive activity in leech touch (T), pressure (P), and nociceptive (N) mechanosensory neurons results in conduction block of their minor receptive fields. We have examined whether the duration of conduction block could be modulated by the serotonergic Retzius neurons or by application of serotonin (5-HT). Activation of one Retzius cell reduced the duration of conduction block in T and P cell posterior fields, but their anterior fields and N cell fields were unaffected. Perfusion with 5-HT had stronger effects, reducing the duration of conduction block in T, P, and lateral N cells in the posterior fields and either reducing or more often enhancing the expression of conduction block in anterior fields. The effects of 5-HT on posterior fields were blocked by the nonspecific 5-HT antagonist methysergide and were partly suppressed by the 5-HT2 antagonist ketanserin. To determine the site of 5-HT action, the central ganglion or peripheral skin was perfused independently. T and to a greater extent P cells showed a preferential sensitivity to application of 5-HT onto the central ganglion. Interestingly, medial N cells exhibited a progressive decrease in the duration of conduction block during repeated trials ("wind-up") that was unaffected by 5-HT. We conclude that secretion of 5-HT by the Retzius cells has a central modulatory effect on the duration of conduction block in T, P, and lateral N cells.