Voltage clamp study of fast excitatory synaptic currents in bullfrog sympathetic ganglion cells

Excitatory postsynaptic currents (EPSCs) have been studied in voltage- clamped bullfrog sympathetic ganglion B cells. The EPSC was small, rose to a peak within 1-3 ms, and then decayed exponentially over most of its time-course. For 36 cells at --50 mV (21-23 degrees C), peak EPSC size was --6.5 +/- 3.5 nA (mean +/- SD), and the mean decay time constant tau was 5.3 +/- 0.9 ms. tau showed a small negative voltage dependence, which appeared independent of temperature, over the range -- 90 to --30 mV; the coefficient of voltage dependence was --0.0039 +/- 0.0014 mV-1 (n = 29). The peak current-voltage relationship was linear between --120 and --30 mV but often deviated from linearity at more positive potentials. The reversal potential determined by interpolation was approximately --5 mV. EPSC decay tau had a Q10 = 3. The commonly used cholinesterase inhibitors, neostigmine and physostigmine, exhibited complex actions at the ganglia. Neostigmine (1 X 10(-5)M) produced a time-dependent slowing of EPSC decay without consistent change in EPSC size. In addition, the decay phase often deviated from a single exponential function, although it retained its negative voltage dependence. With 1 x 10(-6) M physostigmine, EPSC decay was slowed by the decay phase remained exponential. At higher concentrations of physostigmine, EPSC decay was markedly prolonged and was composed of at least two decay components. High concentrations of atropine (10(-5) to 10(-4) M) produced complex alterations in EPSC decay, creating two or more exponential components; one decay component was faster and the other was slower than that observed in untreated cells. These results suggest that the time-course of ganglionic EPSC decay is primarily determined by the kinetics of the receptor-channel complex rather than hydrolysis or diffusion of transmitter away from the postsynaptic receptors.     

Comparison of excitatory currents activated by different transmitters on crustacean muscle. I. Acetylcholine-activated channels

The properties of acetylcholine-activated excitatory currents on the gm1 muscle of three marine decapod crustaceans, the spiny lobsters Panulirus argus and interruptus, and the crab Cancer borealis, were examined using either noise analysis, analysis of synaptic current decays, or analysis of the voltage dependence of ionophoretically activated cholinergic conductance increases. The apparent mean channel open time (tau n) obtained from noise analysis at -80 mV and 12 degrees C was approximately 13 ms; tau n was prolonged e-fold for about every 100-mV hyperpolarization in membrane potential; tau n was prolonged e- fold for every 10 degrees C decrease in temperature. Gamma, the single- channel conductance, at 12 degrees C was approximately 18 pS and was not affected by voltage; gamma was increased approximately 2.5-fold for every 10 degrees C increase in temperature. Synaptic currents decayed with a single exponential time course, and at -80 mV and 12 degrees C, the time constant of decay of synaptic currents, tau ejc, was approximately 14-15 ms and was prolonged e-fold about every 140-mV hyperpolarization; tau ejc was prolonged about e-fold for every 10 degrees C decrease in temperature. The voltage dependence of the amplitude of steady-state cholinergic currents suggests that the total conductance increase produced by cholinergic agonists is increased with hyperpolarization. Compared with glutamate channels found on similar decapod muscles (see the following article), the acetylcholine channels stay open longer, conduct ions more slowly, and are more sensitive to changes in the membrane potential.     

Postsynaptic effects of nickel ions at the insect neuromuscular junction

The effect of extracellular nickel on the excitatory postsynaptic response at the insect neuromuscular junction was studied in the segmental muscle of the larval mealworm Tenebrio molitor. The response to L-glutamate applied iontophoretically (glutamate potential, GP) was potentiated in the presence of Ni2+ though the excitatory postsynaptic potential (EPSP) was reduced. It seems unlikely that Ni2+ acts at the same binding site as L-glutamate does since the value of the limiting slope of double logarithmic plots for the action of glutamate was increased in the presence of Ni2+. The potentiation of GP in the presence of Ni2+ cannot be ascribed to competition between Ni2+ and Ca2+ since GP amplitude did not show any dependence on the concentration of Ca2+. Nickel ions did not alter the reversal potential of excitatory postsynaptic current (EPSC) and glutamate current (GC) under the voltage clamp condition, whereas the amplitude of GC was potentiated in the presence of Ni2+. The time constant of the decay of EPSC showed a weak voltage dependency: the more depolarized the membrane, the more prolonged the time constant. In the presence of 1 mM Ni2+ the amplitude of miniature EPSCs (MEPSCs) increased and the half decay time was prolonged significantly. These results suggest that Ni2+ interacts with charged groups near the glutamate receptor-channel complex so that the kinetics of the channel are altered.     

The voltage dependence of the decay of the excitatory postsynaptic current and the effect of concanavalin A at the crayfish neuromuscular junction

In voltage clamped crayfish muscle fibers the time constant tau of decay of the EPSC was measured at different clamp potentials E. At 6 degrees C, the average potential dependence of tau is described by tau = 2.3 ms.eE/328 mV. tau was shorter in fast fibers than in slow ones. Concanavalin A supressed the potential dependence by tau, resulting in an increase in tau compared with the control, especially at high negative potentials.     

The sodium current underlying action potentials in guinea pig hippocampal CA1 neurons

Neurons were acutely dissociated from the CA1 region of hippocampal slices from guinea pigs. Whole-cell recording techniques were used to record and control membrane potential. When the electrode contained KF, the average resting potential was about -40 mV and action potentials in cells at -80 mV (current-clamped) had an amplitude greater than 100 mV. Cells were voltage-clamped at 22-24 degrees C with electrodes containing CsF. Inward currents generated with depolarizing voltage pulses reversed close to the sodium equilibrium potential and could be completely blocked with tetrodotoxin (1 microM). The amplitude of these sodium currents was maximal at about -20 mV and the amplitude of the tail currents was linear with potential, which indicates that the channels were ohmic. The sodium conductance increased with depolarization in a range from -60 to 0 mV with an average half-maximum at about -40 mV. The decay of the currents was not exponential at potentials more positive than -20 mV. The time to peak and half-decay time of the currents varied with potential and temperature. Half of the channels were inactivated at a potential of -75 mV and inactivation was essentially complete at -40 to -30 mV. Recovery from inactivation was not exponential and the rate varied with potential. At lower temperatures, the amplitude of sodium currents decreased, their time course became longer, and half-maximal inactivation shifted to more negative potentials. In a small fraction of cells studied, sodium currents were much more rapid but the voltage dependence of activation and inactivation was very similar.     

Voltage-dependent inactivation of T-tubular skeletal calcium channels in planar lipid bilayers

Single-channel properties of dihydropyridine (DHP)-sensitive calcium channels isolated from transverse tubular (T-tube) membrane of skeletal muscle were explored. Single-channel activity was recorded in planar lipid bilayers after fusion of highly purified rabbit T-tube microsomes. Two populations of DHP-sensitive calcium channels were identified. One type of channel (noninactivating) was active (2 microM +/- Bay K 8644) at steady-state membrane potentials and has been studied in other laboratories. The second type of channel (inactivating) was transiently activated during voltage pulses and had a very low open probability (Po) at steady-state membrane potentials. Inactivating channel activity was observed in 47.3% of the experiments (n = 84 bilayers). The nonstationary kinetics of this channel was determined using a standard voltage pulse (HP = -50 mV, pulse to 0 mV). The time constant (tau) of channel activation was 23 ms. During the mV). The time constant (tau) of channel activation was 23 ms. During the pulse, channel activity decayed (inactivated) with a tau of 3.7 s. Noninactivating single-channel activity was well described by a model with two open and two closed states. Inactivating channel activity was described by the same model with the addition of an inactivated state as proposed for cardiac muscle. The single-channel properties were compared with the kinetics of DHP-sensitive inward calcium currents (ICa) measured at the cellular level. Our results support the hypothesis that voltage-dependent inactivation of single DHP-sensitive channels contributes to the decay of ICa.     

Acute thyroid hormone promotes slow inactivation of sodium current in neonatal cardiac myocytes.

Sodium current (INa) inactivation kinetics in neonatal cardiac myocytes were analyzed using whole cell voltage clamp before and after acute treatments with thyroid hormone (3,5,3'-triiodo-L-thyronine, T3). In untreated neonatal myocytes, INa inactivation was predominantly mono-exponential, with 93 +/- 3% (S.D.; n = 9) of the peak amplitude decaying with a time constant, tau h1, of 1.8 +/- 0.5 ms at -30 mV. The remaining 7% of control INa decayed more slowly, with a time constant, tau h2, of 9.3 +/- 3.0 ms at -30 mV. The contribution of slowly-inactivating channels to peak current was increased from 7% to 43 +/- 27% within 5 min of exposure to 5-20 nM T3 (nine cells; P less than 0.005). The time constants for both the fast- and slow-inactivating components of peak current (tau h1 and tau h2) were not significantly changed by acute T3 treatment, nor was steady-state INa inactivation (h infinity) affected. Thyroid hormone action on sodium inactivation was partially reversible by lidocaine. These findings indicate that T3 acts at the neonatal cardiac cell membrane to promote slow inactivation kinetics in sodium channels.     

Activation and desensitization of glutamate-activated channels mediating fast excitatory synaptic currents in the visual cortex.

Brief glutamate applications to membrane patches, excised from neurons in the rat visual cortex, were used to assess the role of desensitization in determining the AMPA/kainate receptor-mediated excitatory postsynaptic current (EPSC) time course. A brief (1 ms) application of glutamate (1-10 mM) produced a response that mimicked the time course of miniature EPSCs (mEPSCs). Direct evidence is presented that the rate of onset of desensitization is much slower than the decay rate of the response to a brief application of glutamate, implying that the decay of mEPSCs reflects channel closure into a state readily available for reactivation. Rapid application of glutamate combined with nonstationary variance analysis provided an estimate of the single-channel conductance and open probability, allowing an approximation of the number of available channels at a single synaptic site.     

Kinetics of 9-aminoacridine block of single Na channels

The kinetics of 9-aminoacridine (9-AA) block of single Na channels in neuroblastoma N1E-115 cells were studied using the gigohm seal, patch clamp technique, under the condition in which the Na current inactivation had been eliminated by treatment with N-bromoacetamide (NBA). Following NBA treatment, the current flowing through individual Na channels was manifested by square-wave open events lasting from several to tens of milliseconds. When 9-AA was applied to the cytoplasmic face of Na channels at concentrations ranging from 30 to 100 microM, it caused repetitive rapid transitions (flickering) between open and blocked states within single openings of Na channels, without affecting the amplitude of the single channel current. The histograms for the duration of blocked states and the histograms for the duration of open states could be fitted with a single-exponential function. The mean open time (tau o) became shorter as the drug concentration was increased, while the mean blocked time (tau b) was concentration independent. The association (blocking) rate constant, kappa, calculated from the slope of the curve relating the reciprocal mean open time to 9-AA concentration, showed little voltage dependence, the rate constant being on the order of 1 X 10(7) M-1s-1. The dissociation (unblocking) rate constant, l, calculated from the mean blocked time, was strongly voltage dependent, the mean rate constant being 214 s-1 at 0 mV and becoming larger as the membrane being hyperpolarized. The voltage dependence suggests that a first-order blocking site is located at least 63% of the way through the membrane field from the cytoplasmic surface. The equilibrium dissociation constant for 9-AA to block the Na channel, defined by the relation of l/kappa, was calculated to be 21 microM at 0 mV. Both tau -1o and tau -1b had a Q10 of 1.3, which suggests that binding reaction was diffusion controlled. The burst time in the presence of 9-AA, which is the sum of open times and blocked times, was longer than the lifetime of open channels in the absence of drug. All of the features of 9-AA block of single Na channels are compatible with the sequential model in which 9-AA molecules block open Na channels, and the blocked channels could not close until 9-AA molecules had left the blocking site in the channels.     

Spike timing and reliability in cortical pyramidal neurons: effects of EPSC kinetics, input synchronization and background noise on spike timing

In vivo studies have shown that neurons in the neocortex can generate action potentials at high temporal precision. The mechanisms controlling timing and reliability of action potential generation in neocortical neurons, however, are still poorly understood. Here we investigated the temporal precision and reliability of spike firing in cortical layer V pyramidal cells at near-threshold membrane potentials. Timing and reliability of spike responses were a function of EPSC kinetics, temporal jitter of population excitatory inputs, and of background synaptic noise. We used somatic current injection to mimic population synaptic input events and measured spike probability and spike time precision (STP), the latter defined as the time window (Deltat) holding 80% of response spikes. EPSC rise and decay times were varied over the known physiological spectrum. At spike threshold level, EPSC decay time had a stronger influence on STP than rise time. Generally, STP was highest (6 ms) triggered spikes at lower temporal precision (>or=6.58 ms). We found an overall linear relationship between STP and spike delay. The difference in STP between fast and slow compound EPSCs could be reduced by incrementing the amplitude of slow compound EPSCs. The introduction of a temporal jitter to compound EPSCs had a comparatively small effect on STP, with a tenfold increase in jitter resulting in only a five fold decrease in STP. In the presence of simulated synaptic background activity, precisely timed spikes could still be induced by fast EPSCs, but not by slow EPSCs.     

Synaptic transmission at visualized sympathetic boutons: stochastic interaction between acetylcholine and its receptors.

Excitatory postsynaptic currents (EPSCs) were recorded with loose patch electrodes placed over visualized boutons on the surface of rat pelvic ganglion cells. At 34 degrees C the time to peak of the EPSC was about 0.7 ms, and a single exponential described the declining phase with a time constant of about 4.0 ms; these times were not correlated with changes in the amplitude of the EPSC. The amplitude-frequency histogram of the EPSC at individual boutons was well described by a single Gaussian-distribution that possessed a variance similar to that of the electrical noise. Nonstationary fluctuation analysis of the EPSCs at a bouton indicated that about 120 ACh receptor channels were available beneath boutons for interaction with a quantum of ACh. The characteristics of these EPSCs were compared with the results of Monte Carlo simulations of the quantal release of 9000 acetylcholine (ACh) molecules onto receptor patches of density 1400 microns-2 and 0.41 micron diameter, using a kinetic scheme of interaction between ACh and the receptors similar to that observed at the neuromuscular junction. The simulated EPSC generated in this way had temporal characteristics similar to those of the experimental EPSC when either the diffusion of the ACh is slowed or allowance is made for a finite period of transmitter release from the bouton. The amplitude of the simulated EPSC then exhibited stochastic fluctuations similar to those of the experimental EPSC.     

The properties of batrachotoxin-modified cardiac Na channels, including state-dependent block by tetrodotoxin

Batrachotoxin (BTX) modification and tetrodotoxin (TTX) block of BTX-modified Na channels were studied in single cardiac cells of neonatal rats using the whole-cell patch-clamp recording technique. The properties of BTX-modified Na channels in heart are qualitatively similar to those in nerve. However, quantitative differences do exist between the modified channels of these two tissues. In the heart, the shift of the conductance-voltage curve for the modified channel was less pronounced, the maximal activation rate constant, (tau m)max, of modified channels was considerably slower, and the slow inactivation of the BTX-modified cardiac Na channels was only partially abolished. TTX blocked BTX-modified mammalian cardiac Na channels and the block decreased over the potential range of -80 to -40 mV. The apparent dissociation constant of TTX changed from 0.23 microM at -50 mV to 0.69 microM at 0 mV. No further reduction of block was observed at potentials greater than -40 mV. This is the potential range over which gating from closed to open states occurred. These results were explained by assuming that TTX has a higher affinity for closed BTX-modified channels than for open modified channels. Hence, the TTX-binding rate constants are considered to be state dependent rather than voltage dependent. This differs from the voltage dependence of TTX block reported for BTX-modified Na channels from membrane vesicles incorporated into lipid bilayers and from amphibian node of Ranvier.     

Presynaptic adenosine A1 receptors regulate retinohypothalamic neurotransmission in the hamster suprachiasmatic nucleus

Adenosine has been implicated as a modulator of retinohypothalamic neurotransmission in the suprachiasmatic nucleus (SCN), the seat of the light-entrainable circadian clock in mammals. Intracellular recordings were made from SCN neurons in slices of hamster hypothalamus using the in situ whole-cell patch clamp method. A monosynaptic, glutamatergic, excitatory postsynaptic current (EPSC) was evoked by stimulation of the optic nerve. The EPSC was blocked by bath application of the adenosine A(1) receptor agonist cyclohexyladenosine (CHA) in a dose-dependent manner with a half-maximal concentration of 1.7 microM. The block of EPSC amplitude by CHA was antagonized by concurrent application of the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The adenosine A(2A) receptor agonist CGS21680 was ineffective in attenuating the EPSC at concentrations up to 50 microM. Trains of four consecutive stimuli at 25 ms intervals usually depressed the EPSC amplitude. However, after application of CHA, consecutive responses displayed facilitation of EPSC amplitude. The induction of facilitation by CHA suggested a presynaptic mechanism of action. After application of CHA, the frequency of spontaneous EPSCs declined substantially, while their amplitude distribution was unchanged or slightly reduced, again suggesting a mainly presynaptic site of action for CHA. Application of glutamate by brief pressure ejection evoked a long-lasting inward current that was unaffected by CHA at concentrations sufficient to reduce the evoked EPSC amplitude substantially (1 to 5 microM), suggesting that postsynaptic glutamate receptor-gated currents were unaffected by the drug. Taken together, these observations indicate that CHA inhibits optic nerve-evoked EPSCs in SCN neurons by a predominantly presynaptic mechanism.     

Steady-state and dynamic properties of cardiac sodium-calcium exchange. Sodium-dependent inactivation

Sodium-calcium exchange current was isolated in inside-out patches excised from guinea pig ventricular cells using the giant patch method. The outward exchange current decayed exponentially upon activation by cytoplasmic sodium (sodium-dependent inactivation). The kinetics and mechanism of the inactivation were studied. (a) The rate of inactivation and the peak current amplitude were both strongly temperature dependent (Q10 = 2.2). (b) An increase in cytoplasmic pH from 6.8 to 7.8 attenuated the current decay and shifted the apparent dissociation constant (Kd) of cytoplasmic calcium for secondary activation of the exchange current from 9.6 microM to < 0.3 microM. (c) The amplitude of exchange current decreased synchronously over the membrane potential range from -120 to 60 mV during the inactivation, indicating that voltage dependence of the exchanger did not change during the inactivation process. The voltage dependence of exchange current also did not change during secondary modulation by cytoplasmic calcium and activation by chymotrypsin. (d) In the presence of 150 mM extracellular sodium and 2 mM extracellular calcium, outward exchange current decayed similarly upon application of cytoplasmic sodium. Upon removal of cytoplasmic sodium in the presence of 2-5 microM cytoplasmic free calcium, the inward exchange current developed in two phases, a fast phase within the time course of solution changes, and a slow phase (tau approximately 4 s) indicative of recovery from sodium-dependent inactivation. (e) Under zero-trans conditions, the inward current was fully activated within solution switch times upon application of cytoplasmic calcium and did not decay. (f) The slow recovery phase of inward current upon removal of cytoplasmic sodium was also present under the zero-trans condition. (g) Sodium-dependent inactivation shows little or no dependence on membrane potential in guinea pig myocyte sarcolemma. (h) Sodium-dependent inactivation of outward current is attenuated in rate and extent as extracellular calcium is decreased. (i) Kinetics of the sodium-dependent inactivation and its dependence on major experimental variables are well described by a simple two-state inactivation model assuming one fully active and one fully inactive exchanger state, whereby the transition to the inactive state takes place from a fully sodium-loaded exchanger conformation with cytoplasmic orientation of binding sites (E1.3Ni).     

Comparison of excitatory currents activated by different transmitters on crustacean muscle. II. Glutamate-activated currents and comparison with acetylcholine currents present on the same muscle

The properties of glutamate-activated excitatory currents on the gm6 muscle from the foregut of the spiny lobsters Panulirus argus and interruptus and the crab Cancer borealis were examined using either noise analysis, analysis of synaptic current decays, or slow iontophoretic currents. The properties of acetylcholine currents activated in nonjunctional regions of the gm6 muscle were also examined. At 12 degrees C and -80 mV, the predominant time constant of power spectra from glutamate-activated current noise was approximately 7 ms and the elementary conductance was approximately 34 pS. At 12 degrees C and -80 mV, the predominant time constant of acetylcholine- activated channels was approximately 11 ms with a conductance of approximately 12 pS. Focally recorded glutamatergic extracellular synaptic currents on the gm6 muscle decayed with time constants of approximately 7-8 ms at 12 degrees C and -80 mV. The decay time constant was prolonged e-fold about every 225-mV hyperpolarization in membrane potential. The Q10 of the time constant of the synaptic current decay was approximately 2.6. The voltage dependence of the steady-state conductance increase activated by iontophoretic application of glutamate has the opposite direction of the steady-state conductance activated by cholinergic agonists when compared on the gm6 muscles. The glutamate-activated conductance increase is diminished with hyperpolarization. The properties of the marine crustacean glutamate channels are discussed in relation to glutamate channels in other organisms and to the acetylcholine channels found on the gm6 muscle and the gm1 muscle of the decapod foregut (Lingle and Auerbach, 1983).     

Intracellular regulation of neuronal nicotinic cholinergic receptors

Effects of substances affecting intracellular secondary messengers on the membrane currents evoked by ionophoretic application of acetylcholine (ACh currents) and on the excitatory postsynaptic currents (EPSC) evoked by single stimuli applied to preganglionic nerve fibres, were studied in neurones of the rat isolated superior cervical ganglion. Forskolin, the protein kinase A activator, and isobutyl-methyxanthine, the phosphodiesterase inhibitor, decreased the ACh currents. Neither forskolin nor isobutyl-methylxanthine affected the EPSC amplitude or the EPSC decay time constant. Phorbol ester, the protein kinase C activator, decreased the ACh current but did not affect either EPSC amplitude or the EPSC decay time constant. Thapsigargin, the intracellular calcium releaser, decreased the ACh current and the EPSC amplitude but did not affect the EPSC decay time constant. The data obtained suggest that nicotinic acetylcholine receptors (nAChRs) of ganglion neurones are not modulated through the pathways involving protein kinase A or protein kinase C. The nAChRs sensitivity to both exogenous and nerve-released acetylcholine is reduced by intracellular calcium without affecting kinetics of their ionic channels.     

Spontaneous and GABA-induced single channel currents in cultured murine spinal cord neurons

Intracellular and patch clamp recordings were made from embryonic mouse spinal cord neurons growing in primary cell culture. Outside-out membrane patches obtained from these cells usually showed spontaneous single channel currents when studied at the resting potential (-56 +/- 1.5 mV). In 18 out of 30 patches tested, spontaneous single channel activity was abolished by making Tris+ the major cation on both sides of the membrane. The remaining patches continued to display spontaneous single channel currents under these conditions. These events reversed polarity at a patch potential of 0 mV and displayed a mean single channel conductance of 24 +/- 1.2 pS. Application of the putative inhibitory transmitter gamma-aminobutyric acid (0.5-10 microM) to outside-out patches of spinal cord cell membrane induced single channel currents in 10 out of 15 patches tested. These channels had a primary conductance of 29 +/- 2.8 pS in symmetrical 145 mM Cl- solutions. Frequency distributions for the open times of these channels were well fit by the sum of a fast exponential term ("of") with a time constant tau of = 4 +/- 1.3 ms and a slow exponential term ("os") with a time constant tau os = 24 +/- 8.1 ms. Frequency distributions for channel closed times were also well fit by a double exponential equation, with time constants tau cf = 2 +/- 0.2 ms and tau cs = 62 +/- 20.9 ms.     

Dipyroxime as a blocker of ion channels activated by acetylcholine in rat skeletal muscle

The mechanism of cholinolytic action of dipyroxime--reactivator of the phosphorylated acetylcholinesterase were investigated in the rat diaphragm muscle by voltage-clamp technique. Dipyroxime reduced the amplitude and prolonged the decay of the miniature end-plate currents (MEPC) without affecting its exponential nature. Current-voltage relationship exhibited negative conduction in the hyperpolarized region. Dipyroxime increased the voltage dependence of the time constant of MEPC decay (the membrane potential alteration necessary for e-fold change of the decay time constant reduced from 80 to 35 mV). It was concluded that dipyroxime is a very fast blocker of the open end-plate channels.     

Quantal release of glutamate generates pure kainate and mixed AMPA/kainate EPSCs in hippocampal neurons

The relative contribution of kainate receptors to ongoing glutamatergic activity is at present unknown. We report the presence of spontaneous, miniature, and minimal stimulation-evoked excitatory postsynaptic currents (EPSCs) that are mediated solely by kainate receptors (EPSC(kainate)) or by both AMPA and kainate receptors (EPSC(AMPA/kainate)). EPSC(kainate) and EPSC(AMPA/kainate) are selectively enriched in CA1 interneurons and mossy fibers synapses of CA3 pyramidal neurons, respectively. In CA1 interneurons, the decay time constant of EPSC(kainate) (circa 10 ms) is comparable to values obtained in heterologous expression systems. In both hippocampal neurons, the quantal release of glutamate generates kainate receptor-mediated EPSCs that provide as much as half of the total glutamatergic current. Kainate receptors are, therefore, key players of the ongoing glutamatergic transmission in the hippocampus.     

Sodium and calcium currents in dispersed mammalian septal neurons

Voltage-gated Na+ and Ca2+ conductances of freshly dissociated septal neurons were studied in the whole-cell configuration of the patch-clamp technique. All cells exhibited a large Na+ current with characteristic fast activation and inactivation time courses. Half-time to peak current at -20 mV was 0.44 +/- 0.18 ms and maximal activation of Na+ conductance occurred at 0 mV or more positive membrane potentials. The average value was 91 +/- 32 nS (approximately 11 mS cm-2). At all membrane voltages inactivation was well fitted by a single exponential that had a time constant of 0.44 +/- 0.09 ms at 0 mV. Recovery from inactivation was complete in approximately 900 ms at -80 mV but in only 50 ms at -120 mV. The decay of Na+ tail currents had a single time constant that at -80 mV was faster than 100 microseconds. Depolarization of septal neurons also elicited a Ca2+ current that peaked in approximately 6-8 ms. Maximal peak Ca2+ current was obtained at 20 mV, and with 10 mM external Ca2+ the amplitude was 0.35 +/- 0.22 nA. During a maintained depolarization this current partially inactivated in the course of 200-300 ms. The Ca2+ current was due to the activity of two types of conductances with different deactivation kinetics. At -80 mV the closing time constants of slow (SD) and fast (FD) deactivating channels were, respectively, 1.99 +/- 0.2 and 0.11 +/- 0.03 ms (25 degrees C). The two kinds of channels also differed in their activation voltage, inactivation time course, slope of the conductance-voltage curve, and resistance to intracellular dialysis. The proportion of SD and FD channels varied from cell to cell, which may explain the differential electrophysiological responses of intracellularly recorded septal neurons.