Facilitation by acetylcholine of tetrodotoxin-resistant spikes in rat hippocampal pyramidal cells

The electrical activity of hippocampal pyramidal cells was studied in slice cultures during blockade of the regenerative Na currents. In the presence of tetrodotoxin, these neurones had a mean resting potential of -68 mV, a membrane input resistance of 87 M omega and displayed marked non-linearities in their current voltage relationship. In response to depolarizing stimuli, pyramidal cells generated action potentials of small amplitude, slow rise and long duration. These tetrodotoxin-resistant spikes were abolished by calcium conductance blockers such as cobalt and cadmium ions. Acetylcholine applied to the bath or by iontophoresis depolarized pyramidal cells, elicited spontaneous tetrodotoxin-resistant spikes and facilitated spiking evoked by depolarizing rectangular current pulses or a current ramp. The effects of acetylcholine were not only slow in onset, but also prolonged; they were completely reversible and sensitive to atropine and calcium-antagonists such as cadmium and cobalt ions which, respectively, reduced and abolished these effects. After hyperpolarizations following injection of depolarizing current pulses were suppressed by acetylcholine and often transformed into depolarizing afterpotentials. Acetylcholine had no effect on voltage-independent conductances as determined by application of hyperpolarizing current pulses. These results could be explained by inhibition of the voltage-dependent K+-current, i.e. the M current (blockade of the calcium current could remove any depolarizing influence resulting from M current inhibition) or by a direct activation of a voltage-dependent calcium current by muscarinic agonists.     

Multiple actions of acetylcholine on hippocampal pyramidal cells in organotypic explant cultures

Hippocampal cultures were prepared from 7- to 10-day-old rats by means of the roller-type technique. The preservation of the characteristic hippocampal cytoarchitecture allowed, after many weeks in vitro, impalement of pyramidal cells by microelectrodes under visual control. Application of 10(-7) to 10(-5) M acetylcholine to the bath depolarized hippocampal pyramidal cells, strongly increased their rate of firing and induced paroxysmal depolarization shifts. This depolarizing action was accompanied by a reduction in the amplitude of evoked postsynaptic potentials. Whereas it was not clear whether the decrease in the amplitude of the excitatory postsynaptic potentials was only a result of membrane depolarization, acetylcholine clearly and reversibly reduced the potency of evoked inhibitory postsynaptic potentials. Iontophoresis of acetylcholine to the perisomatic region of pyramidal neurons, like acetylcholine applied to the bath, increased their firing rate and powerfully decreased the amplitude and duration of spontaneous and evoked inhibitory postsynaptic potentials. In contrast, iontophoresis of acetylcholine in the pyramidal cell layer at a distance from the recorded neuron generated a hyperpolarizing response associated with a reduction in firing rate. At high current strength, the initial hyperpolarization was (often) followed by a paroxysmal depolarization shift. High frequency electrical stimulation with electrodes located close to the acetylcholine pipette in the pyramidal cell layer (i.e. about 1 mm away from the recorded neuron) mimicked the acetylcholine effect. Resistance measurements indicated that membrane input resistance was decreased in the majority of cells during application of acetylcholine. This decrease in membrane resistance may result from a direct action of acetylcholine or from an increased synaptic activity. Synaptic alterations induced by acetylcholine were quick in onset and in recovery, while the increase in the rate of firing occurred somewhat later. Atropine (10(-5) M), which had no significant action by itself, completely abolished the action of acetylcholine applied to the bath or by iontophoresis. In contradistinction, naloxone did not influence the acetylcholine effects, although opiates and opioid peptides produce paroxysmal depolarization shifts in pyramidal cells which resemble those induced by acetylcholine. Addition of 8-16 mM magnesium to the bathing solution or exposure of the cultures to a calcium-free solution containing 1 mM cobalt abolished the effects of acetylcholine. In the presence of 10(-6) g/ml tetrodotoxin, 10(-5) M acetylcholine decreased the membrane input resistance of pyramidal cells, reduced their threshold for the generation of tetrodotoxin-resistant spikes and generated paroxysmal depolarization shifts in a proportion of pyramidal cells...     

Intracellular observations on the disinhibitory action of acetylcholine in the hippocampus

Intracellular recording (with KCl microelectrodes) from CA1 and CA3 hippocampal neurons in rats under urethane anaesthesia has revealed two kinds of facilitatory actions of acetylcholine (applied microiontophoretically). One was a mild depolarization (mean + 12 mV) accompanied by a rise in input resistance (mean + 14%). The reversal potential for this effect was much more negative than the resting potential, and it differed from the reversal potential of the inhibitory synaptic potential by a mean of 65 mV. It was therefore concluded that one action of acetylcholine tends to reduce K conductance, as in neocortical neurons. The second effect is a reduction in potency of inhibitory synaptic potentials—evoked by fimbrial or entorhinal stimulation—made evident by a 62% average reduction in the conductance increase recorded near the peak of inhibitory potentials.Since acetylcholine did not depress the inhibitory potency of iontophoretic applications of γ-aminobutyrate, it was concluded that acetylcholine must reduce the release of γ-aminobutyrate either by a direct action on inhibitory terminals or by inhibition of inhibitory interneurons. The former appears more likely.     

The effect of acetylcholine on the function and structure of the developing mammalian neuromuscular junction

Isolated soleus muscles from rats aged 9–12 days were exposed to acetylcholine for 2 h in normal Krebs solution. This treatment caused changes in the ultrastructural appearance of the neuro-muscular junction and a significant reduction of axon profiles per endplate. Nevertheless, most neuro-muscular junctions remained functional, since the ratio of the indirectly to directly elicited contraction was not reduced.If muscles were exposed to acetylcholine in Krebs solutions containing 12m M Ca²⁺ instead of the normal 1.9 m M, the ultrastructural changes produced by acetylcholine were more severe, and the number of axon terminals per endplate was further reduced so that many endplates became completely denervated. This was also reflected in the impaired function of the nerve-muscle preparation; the ratio of the indirectly to directly elicited contraction decreased and about 40% of the muscles fibres became functionally denervated. Addition of curare to the incubating medium prevented the functional deterioration of the preparation.Addition of the protease inhibitors leupeptin and pepstatin protected the nerve terminals from the damaging effects of acetylcholine in Krebs solution containing 12m M Ca²⁺ and the number of axon profiles per endplate remained normal. The functional deterioration was also much reduced when protease inhibitors were included in the incubation medium.These results suggest that acetylcholine causes the activation and release of proteolytic enzymes in developing muscles. The response is mediated by calcium and may have a role in the removal of superfluous nerve-muscle contacts during development.     

Catecholamine uptake into isolated adrenal chromaffin cells: inhibition of uptake by acetylcholine

We have investigated the process of catecholamine uptake in guinea-pig chromaffin cells. Isolated guinea-pig chromaffin cells accumulate [3H]norepinephrine and [3H]epinephrine by a saturable transport system. Catecholamine uptake is dependent upon temperature, energy, and extracellular Na+. The apparent KmS for norepinephrine and epinephrine transport are approximately 1 and 3.5 microM, respectively; the transport maximum (Vmax) for both compounds is about 100 pmol/min/mg protein. The uptake of norepinephrine into chromaffin cells is inhibited by imipramine (Ki = 50 nM) and by desmethylimipramine (IC50 = 20 nM). In both its substrate specificity and its sensitivity to pharmacological inhibition, the catecholamine uptake system in chromaffin cells is similar to the catecholamine transport system previously described in sympathetic neurons. Decreasing external Na+ from 130 to 19 mM increases the apparent Km for norepinephrine to 2.8 microM. Decreasing external norepinephrine increases the Na+ concentration required for half-maximal transport. Agents that depolarize chromaffin cells, such as acetylcholine and veratridine, significantly inhibit [3H]norepinephrine uptake. This decrease in uptake is due to an increase in the apparent Km for norepinephrine. The inhibition of [3H]norepinephrine uptake by depolarizing agents cannot be accounted for by the preferential release of newly-accumulated [3H]norepinephrine, or by the competitive inhibition of [3H]norepinephrine uptake by secreted catecholamines. The inhibition of catecholamine uptake by depolarizing agents suggests that the transport system may be regulated by the membrane potential. Norepinephrine and epinephrine that are spontaneously released from the adrenal medulla may be recaptured in vivo. The inhibition of transport by acetylcholine may prevent the re-uptake of catecholamine released during the physiological stimulation of secretion.     

Ionic dependence of acetylcholine equilibrium potential of acinar cells in mouse submaxillary gland

The ionic dependence of the acetylcholine equilibrium potential (E_ACh) of acinar cells in which acetylcholine (ACh) induced hyperpolarization under control conditions was investigated using intracellular micro-electrode recording in superfused segments of mouse submaxillary gland. For measurements of E_ACh two micro-electrodes were inserted into neighbouring communicating cells, direct current was passed through one of these electrodes and E_ACh was determined by plotting the relation between the size of the ACh potential and the resting potential. ACh was applied by micro-iontophoresis.A complex potential change was induced by ACh when the membrane potential was set at high levels (–50 –80 mV). The appearance of complex responses dependend on the external [Na]. A severe reduction in external Na concentration abolished the appearance of complex responses, whereas alterations of external K concentration had no such effect. The results indicate that a depolarizing component separate from the hyperpolarizing component exists even in acinic in which ACh only evokes hyperpolarization under control conditions. Intracellular injection of TEA ions converted the ACh evoked potential change from hyperpolarization to depolarization in acini superfused with solutions containing Na in concentrations between 50 and 135 mM. However, the conversion was never obtained using solutions with low Na concentration (12.5 mM).The mean E_ACh was –60 mV under normal conditions. E_ACh was made more negative (5 mV) by a reduction in external Na concentration from 135 to 12.5 mmol·l^–1. E_ACh was influenced by alterations of external K concentration, particularly when combined with reduction in external Na concentration. Alteration of K concentration from 2 to 20 mmol·l^–1 shifted E_ACh to more positive values by about 40 mV. E_ACh in acini treated with TEA was about –28 mV in control solution (Na: 135 mmol·l^–1) and –35 mV in a low Na concentration (50 mmol·l^–1).Assuming that the response in submaxillary gland acinar cells to ACh under control condition is composed of two different kinds of potential changes (depolarization and hyperpolarization), the ionic basis of each of the potential changes and a possible explanation for the mechanism of ACh are discussed.     

Effects of histamine on hippocampal pyramidal cells of the rat in vitro

Summary The actions of bath applied histamine on CA1 pyramidal cells were investigated in hippocampal slices of the rat. Histamine caused a) a slight depolarization but no significant change in resting membrane conductance; b) an abbreviation of long afterhyperpolarizations after single action potentials, bursts of action potentials or TTX resistant spikes; c) a loss of accommodation of firing. In the presence of TEA or barium, histamine prolonged and increased the size and number of the slow TTX resistant spikes. A depolarizing plateau which follows such spikes was also increased by histamine. Evoked synaptic potentials were unaffected by histamine, but the population spike was increased. The frequency of spontaneous chloride dependent potentials, which reflect interneurone firing, was also increased. These effects considerably outlasted histamine application and were mimicked by the H₂-agonist impromidine but not the H₁-agonist thiazolethylamine, and blocked by the H₂-antagonists cimetidine and metiamide but not the H₁-antagonists mepyramine or the beta-antagonist propranolol. It is concluded that histamine, by activating H₂-receptors, antagonizes a calcium mediated potassium conductance in hippocampal pyramidal cells without affecting calcium current. By this mechanism histaminergic afferent fibres could effectively regulate cortical responsiveness by selectively potentiating large excitatory inputs of target neurones.     

Attenuation of glutamate-action,excitatory postsynaptic potentials,and spikes by intracellular QX 222 in hippocampal neurons

The effects of intracellular applications of QX 222, a quaternary analogue of lidocaine, were investigated in CA1 neurons of in vitro hippocampal slices of guinea-pig brain. QX 222 produced a strong depression of spontaneous, electrically- (by current injection) or orthodromically-evoked action potentials. These dose-dependent effects were characterized by a reduction in the rate of rise and amplitude of spikes, presumed to be mediated by a Na⁺-conductance. Although resting membrane conductance tended to diminish with prolonged applications of QX 222, marked changes in resting potential generally were not observed. The threshold for eliciting spikes by intracellular injection of depolarizing current was increased greatly by QX 222, reflecting the impairment of Na⁺ -electrogenesis of spikes. The reduction of action potential amplitude by QX 222 may be partly attributable to enhanced inactivation of Na⁺-channels because brief depolarizing pulses preceded by strong tonic hyperpolarization, elicited spikes at a lower threshold and of considerably larger amplitude than in the absence of such tonic hyperpolarization. These observations on recovery are compatible with a removal of sodium inactivation. However, this experimental paradigm of current injection also might be expected to remove QX 222 molecules from their blocking sites at the inner end of Na⁺-channels. When spikes were abolished by QX 222, the depolarization evoked with application of S-glutamate by pressure ejection from an extracellular micropipette positioned close to the neuron was attenuated. This reversible blockade was reproducible in the 14 neurons where the interactions of QX 222 and glutamate were examined systematically. Excitatory postsynaptic potentials, evoked by stimulation of strata oriens or radiatum, were reduced in a similar manner by internal QX 222.These data confirm previous observations that voltage-dependent Na⁺-channels mediating spike genesis in CA1 neurons can be blocked by internal QX 222. However, QX 222 also apparently interferes with the functions of Na⁺ -channels activated by glutamate-receptor interaction or by receptor interactions with neurotransmitter(s) associated with certain excitatory postsynaptic potentials in CA1 neurons.     

The nature of the membrane potential of glial cells associated with the medial giant axon of the crayfish

Electrophysiological, pharmacological and electron microscopic methods were used to characterize the satellite glial (Schwann-like) cells associated with the medial giant axon of the crayfish, Procambarus clarkii. The satellite glial cell layer surrounding the axon is formed by 15–20 cells deeply interdigitated into each other, forming a vast system of intercellular channels which communicate the axo-glial space with the external medium. The satellite cell layer varies from 0.2–3 μm in thickness. Membrane potentials of the giant axon and the satellite glial cells were monitored before, during and after treatment with ouabain and a variety of cholinergic agonists and antagonists. The membrane potentials of 63 control satellite cells averaged?42.6 ± 0.6 mV. The intracellular localization of the glial cell potential difference was corroborated by lithium carminate marking of the microelectrode tip recording site. Superfusion of satellite cells with 10^?7m carbachol, nicotine or acetylcholine caused a 15 to 20 mV hyperpolarization from resting level. Muscarine (10^?6m) had no effect on the glial cell potential. The effect of nicotine was prevented or reversed by-tubocurarine (10^?9m). The effects of cholinomimetics were reversed by washing the cells in drug-free solution. None of these agents had an obvious effect on axon membrane potential or action potential generation at the concentration used in this study. Ouabain (10^?3m) also caused a rapid hyperpolarization of the glial cells. The effect lasted 15–18 min after which the membrane rapidly depolarized.The results suggest that (1) the satellite glial cell of the crayfish giant axon system may be studied simultaneously with the axon using electrophysiological techniques; (2) the satellite glial cell membrane appears to have typical acetylcholine receptors of the nicotinic type; (3) the membrane potential of the glial cell is sensitive to ouabain and (4) properties of the glial cell associated with the giant axon of the crayfish are similar to those of the Schwann cell of the tropical squid Sepioteuthis sepioidea.     

A Single Gene Mutation that Affects a Potassium Conductance and Resting Membrane Potential in Paramecium

A new mutant of Paramecium letraurelia has been isolated with a profound defect in the regulation of membrane potential. This mutant, restless, hyperpolarizes as a potassium electrode below 8 mM external K ⁺ whereas wild-type cells can maintain a constant resting cell potential independent of low external K ⁺ concentration, restless dies in solutions of low K⁺ concentration in which wild-type can survive indefinitely. restless is not allelic to mutations that affect the depolarization-dependent Ca^{2 +} current, the Ca^{2 +} -activated K ⁺ current, and the Ca^{2 +} -activated Na ⁺ current. The results suggest that restless is a new class of mutant affecting a K ⁺ conductance hitherto not characterized genetically in Paramecium.     

Phase shift of subthreshold theta oscillation in hippocampal CA1 pyramidal cell membrane by excitatory synaptic inputs

Hippocampal CA1 neurons receive multiple rhythmical inputs with relatively independent phases during theta activity. It, however, remains to be determined how these multiple rhythmical inputs affect oscillation properties in membrane potential of the CA1 pyramidal cell. In order to investigate oscillation properties in the subthreshold membrane potential, we generated oscillations in the membrane potential of the CA1 pyramidal cells in rat hippocampal slices in vitro with a sinusoidal current injection into the pyramidal soma at theta band frequencies (4–7 Hz), and analyzed effect of rhythmically excitatory synaptic inputs. The Schaffer collaterals were stimulated with a cyclic Gaussian stimulation method, whose pulse intervals were distributed at 10 pulses/cycle (5 cycles/s). We found that the cyclic Gaussian stimulations induced membrane potential oscillations and their phase delays from the mean of the pulse distribution were dependent on membrane potential oscillation amplitude. We applied four pairs of cyclic Gaussian stimulations and somatic sinusoidal current stimulations at the same frequency (5 Hz) with varying phase differences (−π/2, 0, π/2, π rad). The paired stimulations induced phase distributions of the oscillation in the membrane potential, which showed a dependency on an increasing membrane potential oscillation amplitude response to cyclic Gaussian stimulation. This membrane potential dynamic was exhibited by the mixture of the membrane potential oscillation-amplitude-dependent phase delay and the linear summation of the two sinusoidal waves. These suggest that phases of the membrane potential oscillation are modulated by excitatory synaptic inputs. This phase-modulation by excitatory synaptic inputs may play a crucial role for memory operation in the hippocampus.     

Membrane potential and impedance changes in hippocampal pyramidal cells during theta rhythm

Summary Intracellular recordings were made from hippocampal pyramidal cells identified by their depths and their responses to commissural stimulation. Recordings were made during spontaneous bouts of hippocampal theta rhythm in urethane anesthetized rats. Membrane potentials (V _m) of pyramidal cells varied with the phase of the theta rhythm, that is, there was an intracellular theta rhythm. The changes in V _m averaged about 2 mV peak to peak. Averaged intracellular theta waves showed that CA1 pyramids were most depolarized at the time of the positive peak of the extracellular theta rhythm recorded in (and superficial to) the CA1 pyramidal cell layer (CA1 theta). Peak depolarizations for CA3/4 pyramids were more broadly distributed, but occurred mainly in the interval just before the positive peak to just before the negative peak of the CA1 theta. Input impedance minima that were measurable at frequencies as high as 100 Hz occurred at about the same phases of the extracellular theta rhythm as the peak depolarizations (positive-going zero crossing to negative-going zero crossing of the CA1 theta). Such impedance changes imply conductance changes on the soma. The magnitude and localization of the conductance changes suggests that somatic IPSPs make major contributions to the intracellular theta rhythm. The phase relation between the intracellular and extracellular theta rhythms could be reversed by long duration current pulses that depolarized the cells slightly. This implies that either the intracellular theta-related IPSPs are depolarizing potential changes, or that they occur simultaneously with EPSPs. The phase of the intracellular theta rhythm was generally unaffected by long duration hyperpolarizing current pulses. Chloride leakage that reversed the evoked IPSPs usually had no effect on the phase of the intracellular theta rhythm, although in one case it appeared to cause its amplitude to increase.     

Cholinergic suppression of glutamatergic synaptic transmission in hippocampal region CA3 exhibits laminar selectivity: Implication for hippocampal network dynamics

Acetylcholine may help set the dynamics within neural systems to facilitate the learning of new information. Neural models have shown that if new information is encoded at the same time as retrieval of existing information that is already stored, the memories will interfere with each other. Structures such as the hippocampus have a distinct laminar organization of inputs that allows this hypothesis to be tested. In region CA1 of the rat (Sprague Dawley) hippocampus, the cholinergic agonist carbachol (CCh) suppresses transmission in stratum radiatum (SR), at synapses of the Schaffer collateral projection from CA3, while having lesser effects in stratum lacunosum-moleculare (SLM), the perforant path projection from entorhinal cortex (Hasselmo and Schnell, 1994). The current research extends support of this selectivity by demonstrating laminar effects in region CA3. CCh caused significantly greater suppression in SR than in SLM at low concentrations, while the difference in suppression was not significant at higher concentrations. Differences in paired-pulse facilitation suggest presynaptic inhibition substantially contributes to the suppression and is highly concentration and stratum dependent. This selective suppression of the recurrent excitation would be appropriate to set CA3 dynamics to prevent runaway modification of the synapses of excitatory recurrent collaterals by reducing the influence of previously stored associations and allowing incoming information from the perforant path to have a predominant influence on neural activity.     

The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

The present study was designed to elucidate the effects of sodium-coupled transport on the electrical properties of proximal tubule cells in the isolated perfused frog kidney. Cable analysis techniques have been employed to determine the resistance of the luminal and peritubular cell membranes in parallel (R _m) and the apparent ratio of the luminal over the peritubular cell membrane resistance (VDR). Furthermore, the sensitivity of the potential difference across the peritubular cell membrane (PD_pt) to 6-fold increases of peritubular potassium concentration (PD_k) was taken as a measure of the relative potassium conductance of this membrane. In the absence of luminal phenylalanine, PD_pt amounts to –60±1 mV (n=90),R _m to 36±3 k cm (n=22), VDR to 1.81±0.14 (n=20), and PD_k to 15.0±0.9 mV (n=25). The application of 10 mmol/l phenylalanine replacing 10 mmol/l raffinose leads to a rapid (within 30 s) depolarisation of PD_pt to 50±5% of its control value and to a delayed (within 12 min) recovery to 95±5% of control. The rapid depolarisation is associated with a decline ofR _m and VDR, indicating a decrease mainly of the luminal cell membrane resistance. During recovery of PD_pt there is a parallel increase of VDR and a further decline ofR _m pointing to a decline of the basolateral cell membrane resistance. PD_k is decreased during rapid depolarisation but increases again during the recovery phase. Thus, phenylalanine initially decreases but then increases above control the apparent potassium conductance. Removal of phenylalanine leads to a transient hyperpolarisation and increased apparent potassium conductance. If a cell is depolarised by current injection into a neighbouring cell, a similar decrease of PD_k is observed which shows also a similar recovery (partial repolarisation) despite continued injection of constant current. The data point to a potential-dependent peritubular K⁺-conductance (of the inwardly rectifying type) and to a regulatory increase within some ten minutes, when the cell is depolarised either by sodium entry across the luminal cell membrane or by current injection into a neighbouring cell.     

Influence of potassium depletion on potassium conductance in proximal tubules of frog kidney

In order to test for the contribution of intracellular potassium activity to the link of sodium/potassium-ATPase activity and potassium conductance, studies with conventional and potassium selective microelectrodes were performed on proximal tubules of the isolated perfused frog kidney. The peritubular transference number for potassium (t _k), i.e., the contribution of peritubular slope potassium conductance to the slope conductance of the cell membranes (luminal and peritubular), was estimated from the influence of peritubular potassium concentration on the potential difference across the peritubular cell membrane (PD _pt). During control conditions,PD _pt is –65±1 mV, intracellular potassium activity (K _i) 57±2 mmol/l andt _k 0.41±0.05. The resistance in parallel of the luminal and peritubular cell membranes (R _m) is 44±4 kcm, the resistance of the cellular cable (R _c) 137±13 M/cm. When the cells are exposed 10 min to potassium free perfusates (series I),PD _pt increases by –28±3 mV within 2 min and then decreases gradually to approach the control value within 10 min.K _i decreases by 22±3 mmol/l andR _c increases by 35±10%. After a transient decrease,R _m increases by 36±9%. Readdition of peritubular potassium leads to a transient increase ofPD _pt, a gradual decrease ofR _m andR _c as well as a gradual increase ofK _i t _k recovers only slowly to approach 65±8% of control value within 3 and 79±10% within 6 min. When the cells are exposed 10 min to potassium free perfusates containing 1 mmol/l barium (series II),PD _pt depolarizes by +28±4 mV andK _i decreases by 7±1 mmol/l within 10 min. Within 2 min of reexposure to control perfusatesPD _pt approaches the control value.t _k recovers significantly faster than in series I and approaches 92±8% of control value within 3 min and 107±8% within 6 min reexposure to control perfusates. In conclusion, the effect of potassium free perfusates on peritubular potassium conductance depends on the degree of potassium depletion of the cell.     

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 I_k and partial inactivation of I_Ca 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.     

Electrotonic and dye coupling in hippocampal CA1 pyramidal cells in vitro

The presence of electrotonic and dye coupling in region CA1 of the guinea-pig hippocampus was investigated in the in vitro hippocampal slice preparation. No electrotonic coupling potentials were observed in simultaneous recordings from 101 pairs of pyramidal cells. Also, no electrotonically-coupled short latency depolarizations were observed in more than 75 pyramidal cells in response to antidromic activation of the pyramidal cell population, either in normal bathing medium or in medium with lowered Ca²⁺ concentration and added Mn²⁺. When the fluorescent dye Lucifer Yellow was injected into pyramidal cell somas, spread of the dye to other cells (dye coupling) was often observed. Injection of Lucifer Yellow into the dendrites of these neurons resulted in many fewer cases of dye coupling.The failure to find electrophysiological evidence of electrotonic coupling among CA1 pyramidal cells suggests that such coupling is not a functionally important feature of this area of the CNS. The lack of electrophysiological evidence of coupling combined with the observation that the site of Lucifer Yellow injection influences the extent of dye coupling further suggests that at least part of the observed dye coupling may be artifactual. Electrotonic coupling may exist in a small percentage of hippocampal pyramidal cells. However, it is not clear that this small amount of coupling is either necessary or sufficient for the synchronization of neural activity as has been hypothesized to occur during epileptogenesis.     

Determination of cell membrane resistance in cultured renal epithelioid (MDCK) cells: effects of cadmium and mercury ions

Previous studies have indicated that the cell membrane of Madin Darby Canine Kidney (MDCK) cells is hyperpolarized by a number of hormones and trace elements, in parallel with an enhancement of potassium selectivity. Without knowledge of the cell membrane resistance (R _m), however, any translation of potassium selectivity into potassium conductance remains equivocal. The present study was performed to determine the R _m of MDCK cells by cellular cable analysis. To this end, three microelectrodes were impaled into three different cells of a cell cluster; current was injected via one microelectrode and the corresponding voltage deflections measured by the other two microelectrodes. In order to extract the required specific resistances, the experimental data were analysed mathematically in terms of an electrodynamical model derived from Maxwell's equations. As a result, a mean R _m of 2.0±0.2 kcm² and an intercellular coupling resistance (R _c) of 6.1±0.8 M were obtained at a mean potential difference across the cell membrane of -47.0±0.6 mV. An increase of the extracellular K⁺ concentration from 5.4 to 20 mmol/l depolarized the cell membrane by 16.2±0.5 mV and decreased R _m by 30.6±3.0%; 1 mmol/l barium depolarized the cell membrane by 20.1±1.1 mV and increased R _m by 75.9±14.3%. Omission of extracellular bicarbonate and carbon dioxide at constant extracellular pH caused a transient hyperpolarization (up to –60.4±1.4 mV), a decrease of R _m (by 75±4.5%) and a decrease of R _c (by 23.1±8.4%). The changes in R _m and R _c were probably the result of intracellular alkalosis. Cadmium ions (1 mol/l) led to a sustained, reversible hyperpolarization (to –64.8±1.3 mV) and to a decrease of R _m (by 77.0±2.7%); mercury ions (1 mol/l) cause a sustained hyperpolarization (to –60.1±1.2 mV) and a decrease of R _m (by 76.3±3.9%). Neither manoeuvre significantly altered R _c. We have previously shown that both cadmium and mercury hyperpolarize the cell membrane potential and increase its potassium selectivity; the decrease of the R _m observed in the present study indicates that these effects are due to an increase of the potassium-selective conductance of the cell membrane.     

Voltage-gated ion channels in dendrites of hippocampal pyramidal neurons

The properties and distribution of voltage-gated ion channels contribute to electrical signaling in neuronal dendrites. The apical dendrites of CA1 pyramidal neurons in hippocampus express a wide variety of sodium, calcium, potassium, and other voltage-gated channels. In this report, we provide some new evidence for the role of the delayed-rectifier K⁺ channel in shaping the dendritic action potential at different membrane potentials.     

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.