Depolarizing afterpotentials and burst production in molluscan pacemaker neurons

Robust Modulation of Integrate-and-Fire Models

By controlling the state of neuronal populations, neuromodulators ultimately affect behavior. A key neuromodulation mechanism is the alteration of neuronal excitability via the modulation of ion channel expression. This type of neuromodulation is normally studied with conductance-based models, but those models are computationally challenging for large-scale network simulations needed in population studies. This article studies the modulation properties of the multiquadratic integrate-and-fire model, a generalization of the classical quadratic integrate-and-fire model. The model is shown to combine the computational economy of integrate-and-fire modeling and the physiological interpretability of conductance-based modeling. It is therefore a good candidate for affordable computational studies of neuromodulation in large networks.

Persistent Ca2+ Current Contributes to a Prolonged Depolarization in Aplysia Bag Cell Neurons

Neurons may initiate behavior or store information by translating prior activity into a lengthy change in excitability. For example, brief input to the bag cell neurons of Aplysia results in an approximate 30-min afterdischarge that induces reproduction. Similarly, momentary stimulation of cultured bag cells neurons evokes a prolonged depolarization lasting many minutes. Contributing to this is a voltage-independent cation current activated by Ca2+ entering during the stimulus. However, the cation current is relatively short-lived, and we hypothesized that a second, voltage-dependent persistent current sustains the prolonged depolarization. In bag cell neurons, the inward voltage-dependent current is carried by Ca2+; thus we tested for persistent Ca2+ current in primary culture under voltage clamp. The observed current activated between −40 and −50 mV exhibited a very slow decay, presented a similar magnitude regardless of stimulus duration (10–60 s), and, like the rapid Ca2+ current, was enhanced when Ba2+ was the permeant ion. The rapid and persistent Ca2+ current, but not the cation current, were Ni2+ sensitive. Consistent with the persistent current contributing to the response, Ni2+ reduced the amplitude of a prolonged depolarization evoked under current clamp. Finally, protein kinase C activation enhanced the rapid and persistent Ca2+ current as well as increased the prolonged depolarization when elicited by an action potential-independent stimulus. Thus the prolonged depolarization arises from Ca2+ influx triggering a cation current, followed by voltage-dependent activation of a persistent Ca2+ current and is subject to modulation. Such synergy between currents may represent a common means of achieving activity-dependent changes to excitability.

Reduction of spike afterdepolarization by increased leak conductance alters interspike interval variability

Data from neurons in vivo have shown that spike output can often sustain episodes of high variability. Theoretical studies have indicated that the high conductance state of neurons brought on by synaptic activity can contribute to an increase in the variability of spike output by decreasing the integration timescale of the neuron. In the present work, we were interested in understanding how background synaptic conductance activity alters the interspike interval (ISI) variability of layer III pyramidal cells of the medial entorhinal cortex. We compared ISI variability in pyramidal cells as a result of synaptic current- or conductance-mediated membrane fluctuations. We found that the effects of background synaptic conductance activity on ISI variability depend on the neuron type. In pyramidal cells lacking spike frequency adaptation, the variability increased in relation to a comparable synaptic current stimulus. In contrast, in pyramidal cells displaying spike frequency adaptation, the synaptic conductance stimulus produced lower ISI variability. To understand this result, we constructed a phenomenological model that reproduced the basic properties of these neurons under control and increased leak conductance. We found that leak can change the properties of the neuron by acting as a bifurcation parameter that reduces the afterdepolarization (ADP) and decreases the slope (gain) of the frequency-current relationship, particularly for transient stimuli. A lower gain with the added leak causes a reduction in ISI variability. We conclude that the ability of a high conductance state to increase ISI variability cannot be generalized and can depend on the spike ADP dynamics expressed by the neuron.

Activity-Dependent Initiation of a Prolonged Depolarization in Aplysia Bag Cell Neurons: Role for a Cation Channel

The translation of prior activity into changes in excitability is essential for memory and the initiation of behavior. After brief synaptic input, the bag cell neurons of Aplysia californica undergo a nearly 30-min afterdischarge to release egg-laying hormone. The present study examines a prolonged depolarization in cultured bag cell neurons. A 5-Hz, 10-s action potential train elicited a depolarization of about 10 mV, which lasted ≤30 min and was reduced by calmodulin kinase inhibition. Very broad action potentials (resulting from TEA application) decreased prolonged depolarization amplitude, indicating that strong Ca2+ influx did not necessarily promote the response. The prolonged depolarization current ( IPD) was recorded after 5-Hz, 10-s trains of square voltage pulses of varying duration (10–150 ms). Despite Ca2+ influx increasing steadily with pulse duration, IPD was most reliably initiated at 100 ms, suggesting a Ca2+ window or limit exists for triggering IPD. Consistent with this, modestly broader action potentials, evoked by lengthening the train current-pulse duration, resulted in smaller prolonged depolarizations. With respect to the properties of IPD, it displayed a linear current–voltage relationship with a reversal potential of about −45 mV that was shifted to approximately −25 mV by lowering internal K+ or about −56 mV by lowering external Na+ and Ca2+. IPD was blocked by Gd3+, but was not antagonized by MDL-123302A, SKF-96365, 2-APB, tetrodotoxin, or flufenamic acid. Optimal Ca2+ influx may activate calmodulin kinase and a voltage-independent, nonselective cation channel to initiate the prolonged depolarization, thereby contributing to the afterdischarge and reproduction.

Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.

Action potential bursting in subicular pyramidal neurons is driven by a calcium tail current

Subiculum is the primary output area of the hippocampus and serves as a key relay center in the process of memory formation and retrieval. A majority of subicular pyramidal neurons communicate via bursts of action potentials, a mode of signaling that may enhance the fidelity of information transfer and synaptic plasticity or contribute to epilepsy when unchecked. In the present study, we show that a Ca(2+) tail current drives bursting in subicular pyramidal neurons. An action potential activates voltage-activated Ca(2+) channels, which deactivate slowly enough during action potential repolarization to produce an afterdepolarization that triggers subsequent action potentials in the burst. The Ca(2+) channels underlying bursting are located primarily near the soma, and the amplitude of Ca(2+) tail currents correlates with the strength of bursting across cells. Multiple channel subtypes contribute to Ca(2+) tail current, but the need for an action potential to produce the slow depolarization suggests a central role for high-voltage-activated Ca(2+) channels in subicular neuron bursting.

GABAergic excitatory synapses and electrical coupling sustain prolonged discharges in the prey capture neural network of Clione limacina

Afterdischarges represent a prominent characteristic of the neural network that controls prey capture reactions in the carnivorous mollusc Clione limacina. Their main functional implication is transformation of a brief sensory input from a prey into a lasting prey capture response. The present study, which focuses on the neuronal mechanisms of afterdischarges, demonstrates that a single pair of interneurons [cerebral A interneuron (Cr-Aint)] is responsible for afterdischarge generation in the network. Cr-Aint neurons are electrically coupled to all other neurons in the network and produce slow excitatory synaptic inputs to them. This excitatory transmission is found to be GABAergic, which is demonstrated by the use of GABA antagonists, uptake inhibitors, and double-labeling experiments showing that Cr-Aint neurons are GABA-immunoreactive. The Cr-Aint neurons organize three different pathways in the prey capture network, which provide positive feedback necessary for sustaining prolonged spike activity. The first pathway includes electrical coupling and slow chemical transmission from the Cr-Aint neurons to all other neurons in the network. The second feedback is based on excitatory reciprocal connections between contralateral interneurons. Recurrent excitation via the contralateral cell can sustain prolonged interneuron firing, which then drives the activity of all other cells in the network. The third positive feedback is represented by prominent afterdepolarizing potentials after individual spikes in the Cr-Aint neurons. Afterdepolarizations apparently represent recurrent GABAergic excitatory inputs. It is suggested here that these afterdepolarizing potentials are produced by GABAergic excitatory autapses.

Noradrenergic responses of neurones in the mediolateral part of the lateral septum: alpha1-adrenergic depolarization and rhythmic bursting activities, and alpha2-adrenergic hyperpolarization from guinea pig brain slices

Bath application of noradrenaline on neurones of class A, B and C within the mediolateral part of the lateral septum from guinea pig brain slices elicited depolarization (through alpha1-adrenoceptors) or hyperpolarization (through alpha2-adrenoceptors) and rhythmic bursting activities (through alpha1-adrenoceptors). A co-existence of these two types of adrenoceptors (alpha1 and alpha2) mediating opposite effects on membrane excitability was sometimes observed on the same neurone. Three types (I, II and III) of neurones were identified on the basis of their bursting properties during noradrenaline application. With the bursting activities persisting under tetrodotoxin and their frequency being sensitive to changes in membrane potential, these neurones could be considered as conditional bursters. An afterdepolarizing potential could promote burst initiation in the three types of neurones. Neuronal input resistance decreased at the afterdepolarizing potential peak. Application of low-Na+ medium blocked the generation of bursts and afterdepolarizing potentials induced by noradrenaline in the three types of neurones. Furthermore, the bursts were dependent on the presence of Ca(2+) in the medium in a subpopulation of neurones. We conclude that afterdepolarizing potentials and bursts induced by noradrenaline are generated by a cationic conductance largely permeable to Na+ in neurones of the mediolateral part of the lateral septum.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

Reduced outward K+ conductances generate depolarizing after-potentials in rat supraoptic nucleus neurones

1. Whole-cell patch clamp recordings were obtained from sixty-five rat supraoptic nucleus (SON) neurones in brain slices to investigate ionic mechanisms underlying depolarizing after-potentials (DAPs). When cells were voltage clamped around -58 mV, slow inward currents mediating DAPs (IDAP), evoked by three brief depolarizing pulses, had a peak of 17 +/- 1 pA (mean +/- S.E.M.) and lasted for 2.8 +/- 0.1 s. 2. No significant differences in the amplitude and duration were observed when one to three preceding depolarizing pulses were applied, although there was a tendency for twin pulses to evoke larger IDAP than a single pulse. The IDAP was absent when membrane potentials were more negative than -70 mV. In the range -70 to -50 mV, IDAP amplitudes and durations increased as the membrane became more depolarized, with an activation threshold of -65.7 +/- 0.7 mV. 3. IDAP with normal amplitude and duration could be evoked during the decay of a preceding IDAP. As frequencies of depolarizing pulses rose from 2 to 20 Hz, the times to peak IDAP amplitude were reduced but the amplitudes and durations did not change. 4. A consistent reduction in membrane conductance during the IDAP was observed in all SON neurones tested, and averaged 34.6 +/- 3.3%. Small hyperpolarizing pulses used to measure membrane conductances appeared not to disturb major ionic mechanisms underlying IDAP, since the slope and duration of IDAP with and without test pulses were similar. 5. The IDAP had an averaged reversal potential of -87.4 +/- 1.6 mV, which was close to the K+ equilibrium potential. An elevation in [K+]o reduced or abolished the IDAP, and shifted its reversal potential toward more positive levels. Perifusion of slices with 7.5-10 mM TEA, a K+ channel blocker, reversibly suppressed the IDAP. 6. Both Na+ and Ca2+ currents failed to induce an IDAP-like current during perifusion of slices with media containing high [K+]o or TEA. However, the IDAP was abolished by replacing external Ca2+ with Co2+, or replacing 82% of external Na+ with choline or Li+. Perifusion of slices with media containing 1-2 microM TTX also reduced IDAP by 55.5 +/- 9.0%. 7. These results suggest that the generation of DAPs in SON neurones mainly involves a reduction in outward K+ current(s), which probably has little or no inactivation and can be inhibited by [Ca2+]i transients, due to Ca2+ influx during action potentials and Ca2+ release from internal stores. Na+ influx might provide a permissive influence for Ca(2+)-induced reduction of K+ conductances and/or help to raise [Ca2+]i via reverse-mode Ca(2+)-Na+ exchange. Other conductances, making minor contributions to the IDAP, may also be involved.

Ca2+ release from internal stores: role in generating depolarizing after-potentials in rat supraoptic neurones

1. Influences of Ca2+ release from internal stores on the generation of depolarizing after-potentials (DAPs) were investigated in magnocellular neurones of rat supraoptic nucleus (SON) using whole-cell patch recording techniques in brain slices. 2. DAPs were recorded from more than half of the cells encountered, and following evoked single spikes had an amplitude of 3.00 +/- 0.19 mV (mean +/- S.E.M.) and lasted for 1.02 +/- 0.06 s. Their sizes usually increased with the number of preceding spikes, but could be reduced or eliminated when intervals between consecutive current pulses evoking tens of spikes were short. 3. DAPs were eliminated by removal of external Ca2+, and significantly reduced by bath application of nifedipine or omega-conotoxin. 4. Blockade of Ca2+ release from internal stores by perifusion with ryanodine or dantrolene, or direct diffusion of Ruthenium Red into cells suppressed DAP amplitudes by approximately 50% and shortened their durations. 5. Depletion of internal Ca2+ stores by perifusion with thapsigargin or cyclopiazonic acid also reduced DAP amplitudes by approximately 50% and eliminated phasic patterns of firing. 6. Caffeine, an agent known to enhance intracellular Ca2+ release, amplified DAPs and promoted phasic firing. 7. These results suggest that Ca2+ influx via high-voltage-activated Ca2+ channels in SON cells triggers ryanodine receptor-mediated Ca2+ release from internal stores. This process enhances DAPs and promotes phasic firing in SON cells, and would thus contribute to vasopressin release.

Calbindin-D28k: role in determining intrinsically generated firing patterns in rat supraoptic neurones

1. Physiological activation of rat supraoptic nucleus (SON) neurones leads to phasic firing in vasopressin neurones and fast, continuous firing in oxytocin neurones. Using whole-cell patch clamp methods in brain slices, we investigated the role of endogenous calbindin-D28k (calbindin) in determining these intrinsically generated patterns of firing. 2. Direct introduction of calbindin (0.1-0.2 mM) into twelve of twelve phasically firing neurones suppressed Ca(2+)-dependent depolarizing after-potentials (DAPs) and changed activity from phasic to continuous firing. Bovine calcium binding protein (0.3 mM), an analogue of calbindin, had similar effects on both DAPs and firing patterns in five of five cells tested. 3. Introduction of anti-calbindin antiserum (1:2000-5000) into thirteen of thirteen continuously firing neurones unmasked DAPs and converted continuous into phasic firing. Such effects could not be mimicked either by diffusion of normal rabbit serum or antibodies directed against glial fibrillary acidic protein or against neurophysin. 4. Immunocytochemical staining with antisera directed against calbindin revealed more intense staining in the dorsal, oxytocin-rich and less intense staining in the ventral, vasopressin-rich areas of the SON. 5. Elevated intracellular Ca2+ concentration ([Ca2+]i; 0.1 mM) induced DAPs and phasic firing in all twenty-nine SON cells recorded. During chelation of intracellular Ca2+ with (1.1-11 mM) BAPTA, fifty-eight of fifty-eight neurones recorded displayed regular continuous activity and had no DAPs. 6. These data suggest that firing activities in SON cells are dependent on [Ca2+]i and that calbindin, acting as an endogenous Ca2+ buffer, is involved in regulation of intrinsic firing patterns. It is likely that calcium binding proteins have a similar influence on the firing patterns of many neuronal types throughout the nervous system.

Response of neurons in the dorsal motor nucleus of the vagus to thyrotropin-releasing hormone

Autonomic motoneurons in the dorsal motor nucleus of the vagus (DMX) were recorded intracellularly in an in vitro slice preparation of the guinea pig brainstem. Bath-applied thyrotropin releasing hormone (TRH) (1-10 microM) induced a reversible depolarization of neurons that was typically accompanied by an increase in the spontaneous firing of the cells. In some cells, TRH induced rhythmic bursting activity. The TRH-induced depolarization occurred also in the presence of reduced Ca2+ and TTX. The response was dose-dependent over TRH concentrations of 0.1-10 microM. The TRH-induced depolarization was accompanied by an increase in input resistance. The reversal potential of this effect corresponded to that of K+. Our results indicate that TRH increases the excitability of DMX neurons by reducing a resting K+ conductance.

A novel tetrodotoxin-insensitive, slow sodium current in striatal and hippocampal neurons

Slow inward currents modulate neuronal firing patterns and may generate depolarizing afterpotentials (DAPs). We report a novel, slow Na+ current (INaS) in striatal and hippocampal neurons that can generate DAPs. INaS activated at depolarizations greater than -40 mV, was tetrodotoxin insensitive, and activated and deactivated slowly over hundreds of milliseconds. INaS was dependent upon extracellular Na+, but was not affected by 0 mM extracellular Ca2+ or by Ca2+ channel blockers (Mn2+, Cd2+, or Co2+). A tetrodotoxin-insensitive, Na(+)-dependent plateau potential that was likely generated by INaS was shown to underlie DAPs during intracellular recordings from hippocampal CA1 pyramidal neurons. Membrane depolarizations and DAPs generated by INaS may contribute to alterations in neuronal firing and epileptiform bursting.

Cerebral neurons underlying prey capture movements in the pteropod mollusc, Clione limacina. II. Afterdischarges

The pteropod mollusc Clione limacina is a highly specialized carnivore which feeds on shelled pteropods and uses, for their capture, three pairs of oral appendages, called buccal cones. Contact with the prey induces rapid eversion of buccal cones, which then become tentacle-like and grasp the shell of the prey. In the previous paper, a large group of electrically coupled, normally silent cells (A motoneurons) has been described in the cerebral ganglia of Clione. Activation of A neurons induces opening of oral skin folds and extrusion of the buccal cones. The present study continues the analysis of the electrical properties of A motoneurons. Brief intracellular stimulation of an A neuron can produce prolonged firing (afterdischarge), lasting up to 40 s, in the entire population of A neurons. After-discharge activity is based on an afterdepolarization evoked by an initial strong burst of A neuron spikes. The data suggest that this afterdepolarization represents excitatory synaptic input from unidentified neurons which in turn receive excitatory inputs from A neurons, thus organizing positive feedback. The main functional role of this positive feedback is the spread and synchronization of spike activity among all A neurons in the population. In addition, it serves to transform a brief excitatory input to A neurons into their prolonged and stable firing, which is required during certain phases of feeding behavior in Clione.

Excitatory effects of thyrotropin-releasing hormone on neurons within the nucleus ambiguus of adult guinea pigs

The electrophysiological effects of thyrotropin-releasing hormone (TRH) on neurons within the nucleus ambiguus (NA) of adult guinea pigs were studied using an in vitro brain stem slice preparation. In 0.01-1.0 micron TRH, NA neurons depolarized (25/39), expressed enhanced postinhibitory rebound (8/8 tested), or exhibited oscillations of the membrane potential (17/39). Because the amplitude of postinhibitory rebound in tetrodotoxin (TTX) at various membrane potentials was not altered by TRH, it suggests that TRH enhanced postinhibitory rebound indirectly by depolarizing the cell membrane. The membrane potential oscillations in NA neurons were persistent in TTX and their frequency was dependent on the membrane potential, suggesting that these oscillations were due to intrinsic membrane properties and not to synaptic inputs. The excitation of NA neurons in vitro by TRH suggests that endogenous TRH may modulate the activity of neurons involved in the regulation of respiratory and autonomic function.

Regenerative, all-or-none electrographic seizures in the rat hippocampal slice in Mg-free and physiological medium

All-or-none electrographic seizures (EGSs) were studied in hippocampal slices from young (21- to 38-day-old) rats in medium containing low (0 mM) or physiological (0.9 mM) levels of magnesium, with and without the GABAB agonist baclofen. Extracellular recording and stimulation were performed in stratum pyramidale and stratum radiatum of CA3, respectively. EGS activity was induced by exposure to low-Mg medium or by delivering repetitive stimulus trains in physiological Mg medium. After EGS activity had stabilized, the EGSs were tested for all-or-none behavior by varying the number of pulses in a train. An EGS was considered all-or-none if subthreshold stimulation produced no afterdischarge bursts, and if the EGS duration was largely independent of the number of suprathreshold stimulus pulses. According to this measure, EGSs in Mg-free + baclofen medium were all-or-none. EGSs evoked in physiological Mg medium were also all-or-none, although the threshold was higher, and the EGS duration lower, than in Mg-free medium. This all-or-none characteristic was observed whether the EGSs were induced by prior exposure to Mg-free medium or by repetitive stimulation, and in the presence and absence of baclofen. The all-or-none characteristic suggests that while the triggering mechanism for EGSs is strongly dependent on stimulus intensity, regenerative mechanisms--independent of stimulus intensity--are responsible for the maintenance of EGSs. EGSs are also terminated by mechanisms not dependent on stimulus intensity.

Burst generating and regular spiking layer 5 pyramidal neurons of rat neocortex have different morphological features

Intracellular recordings were obtained from pyramidal neurons in layer 5 of rat somatosensory and visual cortical slices maintained in vitro. When directly depolarized, one subclass of pyramidal neurons had the capacity to generate intrinsic burst discharges and another generated regular trains of single spikes. Burst responses were triggered in an all-or-none manner from depolarizing afterpotentials in most bursting neurons. Regular spiking cells responded to electrical stimulation of ascending afferents with a typical EPSP-IPSP sequence, whereas IPSPs were hard to detect in bursting cells. Orthodromic activation of the latter evoked a prominent voltage-dependent depolarization that could trigger a burst response. Intracellularly labelled bursting and regular spiking cells were located in layer 5b, but had distinctly different morphologies. Bursting neurons had a large pyramidal soma, a gradually emerging apical dendrite, and an extensive apical and basal dendritic tree. Their axonal collateral arborization was predominantly limited to layers 5/6. In contrast, regular spiking cells had a more rounded soma with abruptly emerging apical dendrite, a smaller dendritic arborization, and 2 to 8 ascending axonal collaterals that arborized widely in the supragranular layers. Both bursting and regular spiking cells had main axons that entered the subcortical white matter. These data show that some subgroups of pyramidal neurons within the deeper parts of layer 5 of rat cortex are morphologically and physiologically distinct and have different intracortical connections. Bursting cells presumably function to amplify and synchronize cortical outputs, whereas regular spiking output neurons provide excitatory feedback to neurons at all cortical levels and receive a more effective orthodromic inhibitory input. These data support the hypothesis that differences in gross neuronal structure, perhaps even the subtle differences that distinguish subclasses of neurons in a given lamina, are predictive of underlying differences in the type and distribution of ion channels in the nerve cell membrane and connections of cells within the cortical circuit.

Discharge induction in molluscan peptidergic cells requires a specific set of autoexcitatory neuropeptides

The peptidergic caudodorsal cells of the pond snail Lymnaea stagnalis generate long lasting discharges of synchronous spiking activity to release their products. During caudodorsal cell discharges a peptide factor is released which induces similar discharges in silent caudodorsal cells [Ter Maat A. et al. (1988) Brain Res. 438, 77-82]. To identify this factor, the electrophysiological effects of putative caudodorsal cell gene products, calfluxin, caudodorsal cell hormone, four alpha caudodorsal cell peptides and three beta caudodorsal cell peptides, were tested individually and in various combinations. Calfluxin, alpha caudodorsal cell peptide and beta 1 caudodorsal cell peptide each had no effect on membrane potential or excitability of the caudodorsal cells. All other caudodorsal cell peptides caused excitatory responses, but did not induce discharges. Instead, only a specific combination of four caudodorsal cell peptides, caudodorsal cell hormone and alpha caudodorsal cell peptide (1-11, 3-11 and 3-10), evoked caudodorsal cell discharges with similar characteristics to electrically evoked discharges. Incomplete versions of this combination failed to cause a discharge. In addition, antibodies to caudodorsal cell hormone or alpha caudodorsal cell peptide reduced caudodorsal cell excitability and prevented the generation of discharges by electrical stimulation. These results suggest that excitatory autotransmission caused by four caudodorsal cell peptides provides a means to amplify excitatory inputs, thus leading to the generation of the all-or-nothing caudodorsal cell discharge.

"Caged calcium" in Aplysia pacemaker neurons. Characterization of calcium-activated potassium and nonspecific cation currents

We have studied calcium-activated potassium current, IK(Ca), and calcium-activated nonspecific cation current, INS(Ca), in Aplysia bursting pacemaker neurons, using photolysis of a calcium chelator (nitr-5 or nitr-7) to release "caged calcium" intracellularly. A computer model of nitr photolysis, multiple buffer equilibration, and active calcium extrusion was developed to predict volume-average and front-surface calcium concentration transients. Changes in arsenazo III absorbance were used to measure calcium concentration changes caused by nitr photolysis in microcuvettes. Our model predicted the calcium increments caused by successive flashes, and their dependence on calcium loading, nitr concentration, and light intensity. Flashes also triggered the predicted calcium concentration jumps in neurons filled with nitr-arsenazo III mixtures. In physiological experiments, calcium-activated currents were recorded under voltage clamp in response to flashes of different intensity. Both IK(Ca) and INS(Ca) depended linearly without saturation upon calcium concentration jumps of 0.1-20 microM. Peak membrane currents in neurons exposed to repeated flashes first increased and then declined much like the arsenazo III absorbance changes in vitro, which also indicates a first-order calcium activation. Each flash-evoked current rose rapidly to a peak and decayed to half in 3-12 s. Our model mimicked this behavior when it included diffusion of calcium and nitr perpendicular to the surface of the neuron facing the flashlamp. Na/Ca exchange extruding about 1 pmol of calcium per square centimeter per second per micromolar free calcium appeared to speed the decline of calcium-activated membrane currents. Over a range of different membrane potentials, IK(Ca) and INS(Ca) decayed at similar rates, indicating similar calcium stoichiometries independent of voltage. IK(Ca), but not INS(Ca), relaxes exponentially to a different level when the voltage is suddenly changed. We have estimated voltage-dependent rate constants for a one-step first-order reaction scheme of the activation of IK(Ca) by calcium. After a depolarizing pulse, INS(Ca) decays at a rate that is well predicted by a model of diffusion of calcium away from the inner membrane surface after it has entered the cell, with active extrusion by surface pumps and uptake into organelles. IK(Ca) decays somewhat faster than INS(Ca) after a depolarization, because of its voltage-dependent relaxation combined with the decay of submembrane calcium. The interplay of these two currents accounts for the calcium-dependent outward-inward tail current sequence after a depolarization, and the corresponding afterpotentials after a burst

Calcium-activated chloride conductance in parasympathetic neurons of the rabbit urinary bladder

Intracellular recordings were made from vesical pelvic ganglion cells of the rabbit in a Krebs solution containing tetrodotoxin (1 microM). Experiments were carried out during complete suppression of the calcium-dependent potassium conductance by tetraethylammonium (greater than or equal to 20 mM) and/or intracellular injection of cesium ions. The action potential was followed by a depolarizing afterpotential which lasted for 0.3-10 s and had a peak amplitude of 5-20 mV at about -50 mV. The afterdepolarization (ADP) could not be observed when the preceding calcium-dependent action potential was blocked in a nominally calcium-free solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA) or total substitution of extracellular calcium ions with barium ions selectively blocked the ADP. The ADP, associated with an increased membrane conductance, reversed its polarity at -17 mV, when ganglion cells were impaled with microelectrodes filled with potassium chloride or cesium chloride. This reversal level was similar to that of the depolarization induced by gamma-aminobutyric acid. The reversal potential shifted to about -50 mV when acetate or sulphate were injected as counter anions. The peak amplitude and the total duration of the ADP was increased by substitution of external sodium chloride with sucrose or sodium isethionate. These results suggest that the ADP results from calcium entry during the spike and subsequent opening of chloride channels in parasympathetic neurons of the rabbit.

Dual inhibitory action of FMRFamide on neurosecretory cells controlling egg laying behavior in the pond snail

We describe here the electrophysiological characterization of a dual inhibitory action of FMRFamide (FMRFa, Phe-Met-Arg-Phe-NH2) on the caudodorsal cells (CDCs) of the pond snail Lymnaea stagnalis: (i) a transient hyperpolarizing response (H-response) and (ii) a suppression of the excitability of the cells, which lasted as long as the peptide was present. Both effects of FMRFa occurred in silent, excitable cells as well as discharging cells. The effects were reversible and dose-dependent in the range of 10(-9) to 10(-5) M. The H-response was not blocked by any of the antagonists to classical neurotransmitters that were tested. The reversal potential of the H-response was dependent on the [K+]o, which suggests that K+ is the major charge carrier in this response. 4-Aminopyridine (4-AP) blocked the H-response but did not affect the suppression of the excitability by FMRFa. This indicates that the effects of the peptide on these cells are independent. Experiments on the mechanism of the inhibition of the excitability indicated that FMRFa blocks the cAMP-dependent activation of the pacemaking mechanism of the CDCs. In experiments with isolated cells it was demonstrated that the actions of FMRFa are mediated directly through receptors on CDCs (H-response: ED50 = 10(-8) M). Finally, anti-FMRFa-positive varicosities and axons close to the somata, the axons and the neurohaemal endings of the CDCs were demonstrated immunocytochemically. The duality of the action of FMRFa on the neural activity of CDCs indicates its role of high priority in the regulation of egg laying behavior.

Calcium-activated inward spike after-currents in bursting neurone R15 of Aplysia

1. Slow inward and outward after-currents follow action potentials in the bursting pacemaker neurone, R 15, of Aplysia californica. These experiments were performed to examine the role of axo-dendritic calcium influx in activating these after currents. 2. Depolarizing voltage-clamp commands issued at the soma were used to elicit the after-currents. The earlier inward depolarizing after-current of DAC was followed by the hyperpolarizing after-current or HAC. The DAC and HAC appeared at a threshold following depolarizing commands in normal sea water, presumably due to triggering of action potentials in inadequately space-clamped axon. In 100 microM-tetrodotoxin (TTX), the after-currents were graded, increasing gradually in amplitude with increasing voltage or duration of the command. 3. After-current amplitudes varied with the holding potential through the range tested, -40 to -80 mV. DACs were maximum at -40 to -50 mV and decreased in amplitude with hyperpolarization. HACs were maximum at -40 mV and decreased with hyperpolarization to disappear between -70 and -80 mV. 4. The dependence of after-currents upon intracellular calcium accumulation during the depolarizing command was tested in several ways. Bathing R15 in 0 Ca2+-2 mM-EGTA (ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid) sea water eliminated the after-currents. Bathing in 1 mM-Ca2+ sea water reduced the DAC by 76% and the HAC by 87% following 10 ms long depolarizations to +40 mV. Application of Mn2+ (25 mM) or La3+ (5 mM) blocked the after-currents. Injection of EGTA intracellularly practically eliminated after-currents. Greatly prolonged depolarizations were required to elicit them after EGTA injection. Substitution of Ba2+ for Ca2+ also eliminated after-currents. 5. Sodium-free sea water eliminated the DAC. The HAC following brief (less than 30 ms) depolarizing commands was also eliminated in zero sodium, although longer commands were followed by an outward tail current. 6. Although the after-currents seemed dependent upon calcium influx, they were not suppressed by depolarizing commands whose voltage exceeded the calcium equilibrium potential at the soma as indicated by suppression of the calcium-activated potassium current, or IK(Ca), observed during the depolarization. However, if the extracellular calcium was lowered to 1 mM, large depolarizations did suppress the DAC. 7. Dopamine blocked the after-currents when applied to the axo-dendritic area but not when applied to the soma. Similarly, synaptic inhibition of long duration blocked the after-currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous bursting by rat supraoptic neuroendocrine cells is calcium dependent

1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasopressin release from axon terminals in the neurohypophysis. In the supraoptic nucleus of the coronal hypothalamic slice thirty-two of sixty-five m.n.c.s recorded intracellularly displayed repetitive bursting, either spontaneously or during a low level of tonic current injection. 2. Of the thirty-two repetitive bursters, twenty-four received no apparent patterned synaptic input and the phasic burst behaviour was voltage dependent. The evidence for these cells being bursting pace-makers and the underlying mechanism driving bursting were further investigated. 3. Phasic bursting by m.n.c.s is usually contingent upon two depolarizing events: a slow depolarization (s.d.) between bursts that brings the membrane potential to burst threshold, and the spike depolarizing after-potential (d.a.p.). One or several d.a.p.s can initiate a burst by summing to form a plateau potential which sustains firing. 4. Of eight phasic cells exposed to tetrodotoxin (TTX) and tonically depolarized with current injection, two cells retained the phasic burst pattern and underlying plateau potentials. Of the remaining six cells in TTX, three of four cells tested regained phasic firing with plateau potentials following the addition of Sr2+, a Ca2+ agonist. Evoked post-synaptic potentials were demonstrably blocked throughout TTX exposure, firmly establishing that some m.n.c.s are bursting pace-makers. 5. The s.d., d.a.p. and plateau potential were retained in TTX or low-Na+ saline, augmented in Sr2+ and blocked in low-Ca2+ saline. All three events were activated at membrane potentials depolarized from -70 mV but steadily inactivated with increasing hyperpolarization to -90 mV. The s.d. and d.a.p. apparently represented partial activation of the same process that drives a burst, the plateau potential. 6. Hyperpolarizing pulses of constant current revealed an apparent decrease in cell conductance underlying the s.d., d.a.p. and plateau potential which was not due to membrane rectification. The plateau potential was reduced in low Na+ and eliminated in low Ca2+. However, it remained relatively unaffected by altering the external K+ concentration and it did not reverse below -90 mV, suggesting a less important role for K+ movement relative to Ca2+ or Na+. A hyperpolarizing pulse during the s.d., d.a.p. or plateau potential probably momentarily inactivated inward Ca2+ current, causing the apparent conductance decrease.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-dependent spike after-current induces burst firing in magnocellular neurosecretory cells

Magnocellular neurosecretory cells (MNCs) were impaled in perfused explants of rat hypothalamus. Application of a voltage clamp after 1-5 current-evoked spikes revealed a tetrodotoxin-resistant, but Cd2+-sensitive inward current. From a threshold near -85 mV, the amplitude of this current increased as post-spike commands were made to more positive potentials. Following its activation, the current-voltage relation of the cell displayed a region of negative resistance which crossed the spike threshold. This Ca2+-dependent spike after-current can therefore induce and sustain burst firing in MNCs.

Thyrotropin-releasing hormone induces rhythmic bursting in neurons of the nucleus tractus solitarius

The nucleus tractus solitarius (NTS) contains neurons that are part of the central neuronal network controlling rhythmic breathing movements in mammals. Nerve terminals within the NTS show immunoreactivity to thyrotropin-releasing hormone (TRH), a neuropeptide that has potent stimulatory effects on respiration. By means of a brainstem slice preparation in vitro, TRH induced rhythmic bursting in neurons in the respiratory division of the NTS. The frequency of bursting was voltage-dependent and could be reset by short depolarizing current pulses. In the presence of tetrodotoxin, TRH produced rhythmic oscillations in membrane potential whose frequency was also voltage-dependent. These observations suggest that TRH modulates the membrane excitability of NTS neurons and allows them to express endogenous bursting activity.

Calcium-dependent action potentials in rat supraoptic neurosecretory neurones recorded in vitro

Intracellular recordings obtained from thirty-nine supraoptic nucleus neurones in perfused hypothalamic explants displayed a mean resting membrane potential of -69 mV and spike amplitude of 79 mV. Voltage-current plots were linear in the hyperpolarizing direction and revealed a mean slope resistance of 197 M omega. After Na+ channel blockade with tetrodotoxin (TTX; 0.3-16 microM), the voltage-current relationship did not change significantly for hyperpolarizing pulses. An increase in spike threshold in TTX permitted visualization of a reduction in slope resistance to depolarizing current pulses. This rectification was reduced by the addition of the Ca2+ channel blockers Cd2+, Co2+ or Mn2+. High threshold TTX-resistant spikes with amplitudes ranging between 25 and 64 mV were evoked in an all-or-none manner by brief intracellular current pulses applied above 1.0 Hz. Current pulses presented at lower frequencies (less than or equal to 0.5 Hz) evoked graded responses. In seventeen of nineteen cells, prolonged depolarizing current pulses elicited repetitive firing of TTX-resistant spikes with a progressive increase in their amplitude, rise and fall times and after-hyperpolarizations. TTX-resistant spikes were reversibly abolished when CaCl2 was replaced by equimolar amounts of EGTA or by the addition of either Cd2+, Co2+ or Mn2+ to the perfusion medium. In artificial medium containing EGTA, both the shoulder on the repolarization phase of the spike and the subsequent after-hyperpolarization were reversibly abolished. Tetraethylammonium (TEA; 2-5 mM) induced prolongation of mean action potential durations from 1.9 to 12.3 ms (nineteen cells); in TTX, TEA also prolonged the duration and increased the over-all peak amplitude of the TTX-resistant (Ca2+) spike. While TEA also enhanced the amplitude of the Na+ spike, no comparable prolongation in spike duration was observed. These data indicate that somatic action potentials of supraoptic nucleus cells arise from the co-activation of a low threshold Na+ conductance and a higher threshold Ca2+ conductance; the latter is expressed as a shoulder on the repolarization phase of the action potential.

Neural control of behavioral responses in the nudibranch mollusc Phestilla sibogae

The tropical aeoliacean nudibranch Phestilla sibogae, has a number of large and reidentifiable neurons in its centrally located cerebral-pedal-pleural ganglion complex. In studies involving nearly intact animal preparations, neurons were identified which control specific movements of the dorsal cerata, the oral veil tentacles, and the margins of the foot. Responses of neurons to mechanical vibrations in the environment, and responses to light not mediated by the eyes are described. Finally, a pair of large cerebral neurons are identified that are superficially similar in structure, location, and function to the metacerebral giants found in several other opisthobranchs. These neurons are electrically coupled and control stereotyped movements of the mouth. These anatomical and neurophysiological features, when coupled with the fact that the generation time of Phestilla (30 days) is comparable to that of Drosophila suggests that this nudibranch may prove useful in combined studies of neurophysiology, behavior and genetics.

Calcium-dependent inward current in Aplysia bursting pace-maker neurones

Depolarizing voltage-clamp pulses elicit a triphasic series of tail currents (phase I, II and III) in Aplysia burst-firing neurones L2-L6. The sequence and time course of the tail currents resemble slow changes in membrane potential which follow bursts in the unclamped cell. The phase II tail current is an inward current with a time course similar to that of the depolarizing after-potential (d.a.p.) which follows bursts in the unclamped cell. The phase II tail current is suppressed by depolarizing pulses which approach ECa, is blocked by Ca2+ current antagonists (Co2+ and Mn2+), and is blocked by intracellular injection of EGTA. The phase II tail current is not blocked by agents which block Na+-dependent action potentials, the Na+-Ca2+ exchange pump, or the Na+-K+ exchange pump. The phase II tail current is not blocked by the elimination of large outward K+ currents which can lead to extracellular K+ accumulation. Thus, the phase II tail current is not generated by any of these processes. The phase II tail current is reduced by about 60% following substitution of tetramethylammonium (TMA+) for external Na+, but is unaffected by reducing external Cl-. The phase II tail current is distinct from a persistent inward Ca2+ current which underlies the negative resistance region of the steady-state current--voltage relation of bursting cells. The persistent inward current is only slightly reduced by TMA+ substitution for Na+, and is enhanced by EGTA injection. Injection of Ca2+ into Aplysia bursting cells elicits a biphasic (inward-outward) current. The inward current can be observed in isolation after blocking the outward component (Ca2+-activated K+ current) with 50 mM-external tetraethylammonium. The Ca2+-elicited inward current has a reversal potential near -22 mV, and is non-selective for Na+, K+ and Ca2+. The reversal potential is unaffected by changes in Cl- and pH. The Ca2+- activated conductance is apparently voltage independent. We propose that the phase II tail current, and hence the d.a.p., is due to the Ca2+-dependent activation of a voltage-independent non-specific cationic conductance. This conductance participates in generating the depolarizing phase of bursting pace-maker activity.

Slow depolarizing and hyperpolarizing currents which mediate bursting in Aplysia neurone R15

Interruption of normal bursting activity by application of a voltage clamp reveals that action potentials in Aplysia neurone R15 are followed by two slow currents that long outlast the currents produced during the action potentials. Similar currents are seen following simulation of an action potential with a brief depolarizing pulse delivered under continuous voltage clamp. One of these currents, herein called ID, is an inward, or depolarizing current 0.5-5 nA in amplitude that reaches a peak 300-500 ms after the action potential. It produces the depolarizing after-potential that follows action potentials in this cell and is responsible also for the grouping together of action potentials into bursts. The second current, herein called IH, is an outward, or hyperpolarizing current 0.1-2 nA in amplitude that reaches a peak in 2-10 s and is still present for many tens of seconds following the action potential. IH mediates the interburst hyperpolarization. Both currents summate temporally during the burst. Despite changes in the amplitude and duration of action potentials during the burst, each action potential adds nearly constant increments to the summated amplitudes of ID and IH. The summated amplitude of ID grows during the first few action potentials and gives rise to the increased rate of depolarization and the increased firing rate seen during the first half of the burst. Due to its slower kinetics, IH summates throughout the burst until its summated amplitude is large enough to cause the cell to hyperpolarize, thereby bringing the burst to an end. When the normal burst is interrupted by application of the voltage clamp, the ID and IH current peaks are followed by a current which approaches a more negative steady-state level with a time course that consists of at least two phases. The first phase is exponential with a time constant of 15-30 s. Under continuous voltage clamp, the current following a train of depolarizing pulses returns to the holding current with a similar time course. These observations, together with time constants for IH that are longer than the interburst interval, suggest that IH is always partially activated during normal bursting. A computer simulation demonstrates that opposing inward and outward currents with different kinetics, i.e. ID and IH, are sufficient to give rise to bursting activity, in the absence of non-linear voltage-dependent conductances. Such voltage-dependent conductances, which are present in the normal cell, contribute to but are not necessary for bursting activity.

On the basis of delayed depolarization and its role in repetitive firing of Rohon-Beard neurones in Xenopus tadpoles

A delayed depolarization following the impulse can be recorded intracellularly from mature Rohon-Beard neurones in the spinal cord of Xenopus tadpoles, in response both to brief intracellularly injected current pulses and to antidromic stimulation. Evidence is presented suggesting that this delayed depolarization is unlikely to be due to the action of a chemical synapse, activation of a voltage-dependent conductance in the cell body, increased extracellular potassium, or electrotonic coupling. Hyperpolarization of the cell body during antidromic stimulation eliminates the action potential normally generated there, and reveals an impulse arising at some distance along a neurite. When an action potential is produced in the cell body, its repolarizing phase sculpts a delayed depolarization from this impulse in the neurite. The depolarization is enhanced by pressure applied to the neurites near the cell body, presumably by reducing the distal spread of current, and yields multiple action potentials. Although long current pulses usually produce only a single spike, small quantities of La3+ enhance the size of the depolarization and cause repetitive firing. The relation of impulse frequency to injected current shows a non-linearity consistent with the summation of the delayed depolarization and the depolarization by the injected current. The non-linearity is eliminated upon removal of delayed depolarization by hyperpolarizing current pulses injected after each impulse. The enhancement of the depolarization by La3+ is not the only cause of repetitive firing; La3+ also produces an effective reduction in conductance for outward currents. This depolarization may play a role in the normal firing behaviour of Rohon-Beard neurones; when repetitive firing results naturally in response to long current pulses the delayed depolarization is observed to be large.

A model of neuronal bursting using three coupled first order differential equations

We describe a modification to our recent model of the action potential which introduces two additional equilibrium points. By using stability analysis we show that one of these equilibrium points is a saddle point from which there are two separatrices which divide the phase plane into two regions. In one region all phase paths approach a limit cycle and in the other all phase paths approach a stable equilibrium point. A consequence of this is that a short depolarizing current pulse will change an initially silent model neuron into one that fires repetitively. Addition of a third equation limits this firing to either an isolated burst or a depolarizing afterpotential. When steady depolarizing current was applied to this model it resulted in periodic bursting. The equations, which were initially developed to explain isolated triggered bursts, therefore provide one of the simplest models of the more general phenomenon of oscillatory burst discharge.

A late slow depolarization unmasked in the presence of tetraethylammonium in neonatal rat sympathetic neurons in vitro

Neonatal rat superior cervical ganglia were mechanically dissociated, and the sympathetic neurons grown in dispersed cell cultures. Intracellular microelectrodes were used to study the effects of tetraethylammonium (TEA+), a blocker of outward K+ currents, on the excitable properties of these neurons. Addition of TEA+ to the perfusion media (TEA+-media) caused the resting potential to depolarize and the action potential to increase in duration. In TEA+-media (20-60 mM), a late delayed depolarization (LDD) followed the falling phase of the action potential with a delay of 1.5-2 s (n = 95). The LDD peak amplitude was in the range of 4-26 mV and the duration, to full return of the resting potential, was in the range of 18-90 s. For a given cell the amplitude and duration of the LDD were constant. The LDD was associated with a conductance increase. No LDD could be elicited in the presence of calcium channel blockers. Evidence was found for a Ca2+-dependence of the LDD: increasing the extracellular Ca2+ concentration caused increases in the amplitude and duration of the LDD. The significance of an endogenous LDD-like potential and possible explanations for the origin of the LDD are discussed.

Epileptogenic action of tungstic acid gel on cat lumbar motoneurons

Injection of tungstic acid gel (but not pH adjusted saline) into the cat lumbar ventral horn results in spontaneous, epileptiform activity consisting of waves of repetitive, high frequency, action potentials in motoneurons surrounding the injection site. In most motoneurons the action potentials are grouped in high frequency bursts composed of action potentials triggered from the delayed depolarization of the preceding action potential. The same kind of bursting can be triggered by intracellular current pulses, indicating that altered neuronal membrane properties are associated with the bursting activity. This type of bursting differs markedly from that seen in motoneurons during penicillin or strychnine-induced spinal seizures.

Electrotonic coupling and afterdischarges in the Light Green Cells: a comparison with two other cerebral ganglia neurosecretory cell types in the pond snail. Lymnaea stagnalis

Electrotonic coupling occurs between the growth hormone-producing Light Green Cells on the same but not the opposite side of the brain of Lymnaea. Long duration inhibitory post synaptic potentials occur spontaneously or can be evoked by nerve or connective stimulation. Afterdischarges, lasting for up to 1 min, are evoked by brief stimulation of the median lip nerves. Activity is confined to cells ipsilateral to the nerve being stimulated. The electrical properties of the Light Green Cells are compared with those of the Caudodorsal Cells and Canopy Cells.

Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism

Intracellular recordings from mammalian neuroendocrine cells showed that steady, injected currents can modify and block periodic spike bursts previously associated with increased neurohormone release. Spike afterpotentials could sum to form plateau potentials, which generated bursts and did not depend on axonal conduction or chemical synapses. Therefore, bursting involves a spike-dependent, positive-feedback mechanism endogenous to single neuroendocrine cells.

Simulation of intrinsic bursting in CA3 hippocampal neurons

Dendritic recordings from hippocampal pyramidal cells suggest that bursts of action potentials--riding on a depolarizing wave and terminating in a slow calcium-mediated spike--can be generated locally in the dendrites, as well as at the soma. These data necessitated revision of our earlier model in which bursts at the soma are generated by interaction of two spatially separated conductance systems--a fast-spike sodium mechanism at the soma and a slow-spike calcium mechanism on the apical dendrite. We have introduced into a model of the CA3 hippocampal neuron two experimentally testable concepts: voltage-dependent inactivation of Ik and partial inactivation of ICa by Ca2+ ion. With these mechanisms, the model accurately reproduces bursts generated in either soma or in the apical dendrites by sets of conductances all located in the same respective membrane region. The model is also capable of bursting repetitively in response to continuous stimulation.

Physiological basis of feeding behavior in Tritonia diomedea. III. Role of depolarizing afterpotentials

The nature and role of the depolarizing afterpotentials (DAPs) of buccal motoneurons of Tritonia diomedea were examined. Neuron B5 exhibits a DAP whose ionic dependence and modifiability by TEA and 4-AP suggest a similarity to the DAP previously described in pleural pacemaker neurons. Reduction of the DAP severely reduces the ability of these neurons to generate bursts of action potentials. Certain other motoneurons (B1 and b6) are reexcited by a slow DAP (SDAP) which appears to be of synaptic origin. It is concluded that DAPs, which are dependent upon motoneuron activity, contribute to the synthesis of motor output by the buccal ganglion.

The time courses of intracellular free calcium and related electrical effects after injection of CaCl2 into neurons of the snail, Helix pomatia

Controlled quantities of 100 mM aqueous CaCl2 solutions were pressure injected into voltage-clamped neurons with a resolution of 10(-11) 1. Ca2+-selective microelectrodes monitored the time course of changes in [Ca2+]i. At a membrane potential of -50 mV CaCl2 quantities in the range of 1% of the cell volume induced an inward current, associated with a conductance increase and having an equilibrium potential between -20 and +20 mV, which accompanied the rise in [Ca2+]i. An artifactual origin of the inward current by the injection procedure or by calcium screening of membrane sites could be excluded. The calcium-induced hyperpolarizing conductance, producing an outward current at -50 mV, followed the inward current and reached maximum during the late decline in [Ca2+]i. In most cases its development was separated from the inward current by an intermediate relative decrease of the membrane conductance. Neither of the two transient conductance increases showed a particular dependence on voltage. Renewed Ca2+ injection quickly decreased the calcium-induced hyperpolarizing conductance for several seconds. Ca2+ injections below 0.05% of the cell volume mostly produced pure outward currents or hyperpolarizing responses. Partial substitution of extracellular CaCl2 by NiCl2 decreased the hyperpolarizing response but not the initial inward current. The immediate effects of increased [Ca2+]i are activation of a depolarizing conductance and the partial block of the late hyperpolarizing conductance. The latter is probably produced through intermediate steps after increasing [Ca2+]i.

Soma spike of neuroendocrine bag cells of Aplysia californica

Soma action potentials of the neuroendocrine bag cells of Aplysia californica were studied with intracellular recording and current injection. Spikes in artificial sea water (ASW) were either graded with increasing depolarizing current pulses, or had a well-defined threshold. The latter spikes typically had faster rise times with larger overshoots and hyperpolarizing afterpotentials. Repetitive stimulation led to spike potentiation (SP), manifested as an increase in overshoot amplitude and duration of successive spikes in a train. SP was usually detectable at 0.5 Hz, and maximal between 0.8 and 4 Hz. Concomitant accommodation occurred rapidly at greater than or equal to 5 Hz. The increase in spike duration during SP resulted from a progressive enhancement of an inflection on the repolarizing phase. The inflection was dependent on membrane potential; small depolarizations (5-10 mV) enhanced it; hyperpolarization (less than 35 mV) reduced it. Solutions with O--Na+ (Tris-substituted) or O--Ca2+ (1 mM EGTA) revealed mixed Na+/Ca2+ spikes with variable degrees of Na+ versus Ca2+ dominance. Cd2+, Co2+, and Mn2+ reversibly abolished the inflection on the repolarizing phase, indicating that it is Ca2+ mediated; the spike was reduced irreversibly at higher concentrations. SP was generally reduced only if the spike was severely attenuated. It is proposed that SP results primarily from a voltage- and time-dependent potassium inactivation which then unmasks a calcium current. SP may play a role in augmenting the release of egg-laying hormone.