Functional diversity of K+ currents in hippocampal pyramidal neurons

Sodium sensitivity of KNa channels in mouse CA1 neurons

Potassium channels play an important role regulating transmembrane electrical activity in essentially all cell types. We were especially interested in those that determine the intrinsic electrical properties of mammalian central neurons. Over 30 different potassium channels have been molecularly identified in brain neurons, but there often is not a clear distinction between molecular structure and the function of a particular channel in the cell. Using patch-clamp methods to search for single potassium channels in excised inside-out (ISO) somatic patches with symmetrical potassium, we found that nearly all patches contained non-voltage-inactivating channels with a single-channel conductance of 100-200 pS. This conductance range is consistent with the family of sodium-activated potassium channels (Slo2.1, Slo2.2, or collectively, K_Na). The activity of these channels was positively correlated with a low cytoplasmic Na⁺ concentration (2-20 mM). Cell-attached recordings from intact neurons, however, showed little or no activity of this K⁺ channel. Attempts to increase channel activity by increasing intracellular sodium concentration ([Na⁺]_i) with bursts of action potentials or direct perfusion of Na⁺ through a whole cell pipette had little effect on K_Na channel activity. Furthermore, excised outside-out (OSO) patches across a range of intracellular [Na⁺] showed less channel activity than we had seen with excised ISO patches. Blocking the Na⁺/K⁺ pump with ouabain increased the activity of the K_Na channels in excised OSO patches to levels comparable with ISO-excised patches. Our results suggest that despite their apparent high levels of expression, the activity of somatic K_Na channels is tightly regulated by the activity of the Na⁺/K⁺ pump.NEW & NOTEWORTHY We studied K_Na channels in mouse hippocampal CA1 neurons. Excised inside-out patches showed the channels to be prevalent and active in most patches in the presence of Na⁺. Cell-attached recordings from intact neurons, however, showed little channel activity. Increasing cytoplasmic sodium in intact cells showed a small effect on channel activity compared with that seen in inside-out excised patches. Blockade of the Na⁺/K⁺ pump with ouabain, however, restored the activity of the channels to that seen in inside-out patches.

D-type potassium channels normalize action potential firing between dorsal and ventral CA1 neurons of the mouse hippocampus

Specific memory processes and neurological disorders can be ascribed to different dorsoventral regions of the hippocampus. Recently, differences in the anatomical and physiological properties between dorsal and ventral hippocampal CA1 neurons were described for both the rat and mouse hippocampus and have greatly contributed to our understanding of these processes. While differences in the subthreshold properties were similar between rat and mouse neurons, differences in action potential output between dorsal and ventral neurons were strikingly less divergent in mouse compared with rat CA1 neurons. Here, we investigate the mechanism underlying the lack of difference in action potential firing between dorsal and ventral CA1 pyramidal neurons in mouse hippocampus. Consistent with rat, we found that ventral CA1 neurons had a more depolarized resting membrane potential and higher input resistance than dorsal CA1 neurons in the mouse hippocampus. Despite these differences, action potential output in response to current injection was not significantly different. We found that ventral neurons have a more depolarized action potential threshold compared with dorsal neurons and that threshold in ventral neurons was more sensitive to block of KV1 channels compared with dorsal neurons. Outside-out voltage-clamp recordings found that slowly inactivating K+ currents were larger in ventral CA1 neurons. These results suggest that, despite differences in subthreshold properties between dorsal and ventral CA1 neurons, action potential output is normalized by the differential functional expression of D-type K+ channels. NEW & NOTEWORTHY Understanding differences in neurons within a brain region is integral in the reliable interpretation of comparative studies. Our findings identify a novel mechanism by which D-type potassium channels normalize action potential firing between dorsal and ventral CA1 neurons of mouse hippocampus despite differences in subthreshold intrinsic properties. Action potential threshold in ventral neurons is influenced by a greater functional expression of D-type potassium channels resulting in a depolarized action potential threshold compared with dorsal hippocampus.

Sodium and potassium conductances in principal neurons of the mouse piriform cortex: a quantitative description

KEY POINTS

The primary olfactory (or piriform) cortex is a promising model system for understanding how the cerebral cortex processes sensory information, although an investigation of the piriform cortex is hindered by a lack of detailed information about the intrinsic electrical properties of its component neurons. In the present study, we quantify the properties of voltage-dependent sodium currents and voltage- and calcium-dependent potassium currents in two important classes of excitatory neurons in the main input layer of the piriform cortex. We identify several classes of these currents and show that their properties are similar to those found in better-studied cortical regions. Our detailed quantitative descriptions of these currents will be valuable to computational neuroscientists who aim to build models that explain how the piriform cortex encodes odours.

ABSTRACT

The primary olfactory cortex (or piriform cortex, PC) is an anatomically simple palaeocortex that is increasingly used as a model system for investigating cortical sensory processing. However, little information is available on the intrinsic electrical conductances in neurons of the PC, hampering efforts to build realistic computational models of this cortex. In the present study, we used nucleated macropatches and whole-cell recordings to rigorously quantify the biophysical properties of voltage-gated sodium (Na_V ), voltage-gated potassium (K_V ) and calcium-activated potassium (K_Ca ) conductances in two major classes of glutamatergic neurons in layer 2 of the PC, semilunar (SL) cells and superficial pyramidal (SP) cells. We found that SL and SP cells both express a fast-inactivating Na_V current, two types of K_V current (A-type and delayed rectifier-type) and three types of K_Ca current (fast-, medium- and slow-afterhyperpolarization currents). The kinetic and voltage-dependent properties of the Na_V and K_V conductances were, with some exceptions, identical in SL and SP cells and similar to those found in neocortical pyramidal neurons. The K_Ca conductances were also similar across the different types of neurons. Our results are summarized in a series of empirical equations that should prove useful to computational neuroscientists seeking to model the PC. More broadly, our findings indicate that, at the level of single-cell electrical properties, this palaeocortex is not so different from the neocortex, vindicating efforts to use the PC as a model of cortical sensory processing in general.

Delayed-rectifier K channels contribute to contrast adaptation in mammalian retinal ganglion cells

Retinal ganglion cells adapt by reducing their sensitivity during periods of high contrast. Contrast adaptation in the firing response depends on both presynaptic and intrinsic mechanisms. Here, we investigated intrinsic mechanisms for contrast adaptation in OFF Alpha ganglion cells in the in vitro guinea pig retina. Using either visual stimulation or current injection, we show that brief depolarization evoked spiking and suppressed firing during subsequent depolarization. The suppression could be explained by Na channel inactivation, as shown in salamander cells. However, brief hyperpolarization in the physiological range (5-10 mV) also suppressed firing during subsequent depolarization. This suppression was selectively sensitive to blockers of delayed-rectifier K channels (K(DR)). In somatic membrane patches, we observed tetraethylammonium-sensitive K(DR) currents that activated near -25 mV. Recovery from inactivation occurred at potentials hyperpolarized to V(rest). Brief periods of hyperpolarization apparently remove K(DR) inactivation and thereby increase the channel pool available to suppress excitability during subsequent depolarization.

A-type K+ current dominates somatic excitability of delayed firing neurons in rat substantia gelatinosa

Substantia gelatinosa neurons display three main types of intrinsic firing behavior: tonic, adapting, and delayed onset. Here, voltage-gated currents expressed by delayed firing neurons were studied in nucleated patches obtained in spinal cord slices of 3-5 weeks-old rats. Inward Na+ current was negligible under these conditions and was usually occluded by superposition of much larger outward currents. Two kinds of outward currents were found, an A-type (K(A) ) and delayed rectifier (K(DR) ) potassium currents. K(A) activated rapidly (<1.5 ms at >-20 mV) and operated at subthreshold membrane potentials; voltages of steady-state half-maximal activation and inactivation were -38.7 and -87.2 mV, respectively. Inactivation was biexponential with a dominant fast component (~90%, time constant ∼8 ms). K(DR) activated more slowly (<8 ms at >-20 mV), half-maximal activation was -23.6 mV, and decayed mono-exponentially with a time constant 70-110 ms. Maximal amplitudes of K(A) were almost 10-times larger than those of K(DR) , their respective densities were 8.5 and 0.97 μS μm⁻². Tetraethylammonium, 5 mM, blocked K(DR) but not K(A) , whereas both currents were depressed by 5 mM 4-aminopyridine. In current-clamp recordings, 4-action potential but not tetraethylammonium abolished firing delay suggesting the causative role of K(A) . Thus, the predominance of fast K(A) over other somatic currents is a distinctive feature of delayed firing neurons among all other types of substantia gelatinosa neurons and likely explains the appearance of their typical firing delay.

Potassium channel diversity within the muscular components of the feline lower esophageal sphincter

We hypothesized that regional differences in electrophysiological properties exist within the musculature of the feline lower esophageal sphincter (LES) and that they may potentially contribute to functional asymmetry within the LES. Freshly isolated esophageal smooth muscle cells (SMCs) from the circular muscle and sling regions within the LES were studied under a patch clamp. The resting membrane potential (RMP) of the circular SMCs was significantly more depolarized than was the RMP of the sling SMCs, resulting from a higher Na+ and Cl- permeability in circular muscle than in sling muscle. Large conductance Ca2+-activated K+ (BKCa) set the RMP at both levels, since specific BKCa inhibitors caused depolarization; however, BKCa density was greatest in the circular region. A significant portion of the outward current was due to non-BKCa, especially in sling muscle, and likely delayed rectifier K+ channels (KDR). There was a large reduction in outward current with 4-aminopyridine (4-AP) in sling muscle, while BKCa blockers had a limited effect on the voltage-activated outward current in sling muscle. Differences in BKCa:KDR channel ratios were also manifest by a leftward shift in the voltage-dependent activation curve in circular cells compared to sling cells. The electrophysiological differences seen between the circular and sling muscles provide a basis for their different contributions to LES activities such as resting tone and neurotransmitter responsiveness, and in turn could impart asymmetric drug responses and provide specific therapeutic targets.

Structure-Based Discovery of Potassium Channel Blockers from Natural Products

Potassium ion (K(+)) channels are attractive targets for rational drug design. Based upon a three-dimensional model of the eukaryotic K(+) channels, the docking virtual screening approach was employed to search the China Natural Products Database. Compounds were ranked according to the relative binding energy, favorable shape complementarity, and potential of forming hydrogen bonds with the K(+) channel. Four candidate compounds found by virtual screening were investigated by using the whole-cell voltage-clamp recording in rat dissociated hippocampal neurons. When applied extracellularly, compound 1 markedly depressed the delayed rectifier K(+) current (I(K)) and fast transient K(+) current (I(A)), whereas compounds 2, 3, and 4 exerted a more potent and selective inhibitory effect on I(K). Intracellular application of the four compounds had no effect on both the K(+) currents.

Two forms of electrical resonance at theta frequencies, generated by M-current, h-current and persistent Na+ current in rat hippocampal pyramidal cells

Coherent network oscillations in the brain are correlated with different behavioural states. Intrinsic resonance properties of neurons provide a basis for such oscillations. In the hippocampus, CA1 pyramidal neurons show resonance at theta (theta) frequencies (2-7 Hz). To study the mechanisms underlying theta-resonance, we performed whole-cell recordings from CA1 pyramidal cells (n = 73) in rat hippocampal slices. Oscillating current injections at different frequencies (ZAP protocol), revealed clear resonance with peak impedance at 2-5 Hz at approximately 33 degrees C (increasing to approximately 7 Hz at approximately 38 degrees C). The theta-resonance showed a U-shaped voltage dependence, being strong at subthreshold, depolarized (approximately -60 mV) and hyperpolarized (approximately -80 mV) potentials, but weaker near the resting potential (-72 mV). Voltage clamp experiments revealed three non-inactivating currents operating in the subthreshold voltage range: (1) M-current (I(M)), which activated positive to -65 mV and was blocked by the M/KCNQ channel blocker XE991 (10 microM); (2) h-current (I(h)), which activated negative to -65 mV and was blocked by the h/HCN channel blocker ZD7288 (10 microM); and (3) a persistent Na(+) current (I(NaP)), which activated positive to -65 mV and was blocked by tetrodotoxin (TTX, 1 microM). In current clamp, XE991 or TTX suppressed the resonance at depolarized, but not hyperpolarized membrane potentials, whereas ZD7288 abolished the resonance only at hyperpolarized potentials. We conclude that these cells show two forms of theta-resonance: "M-resonance" generated by the M-current and persistent Na(+) current in depolarized cells, and "H-resonance" generated by the h-current in hyperpolarized cells. Computer simulations supported this interpretation. These results suggest a novel function for M/KCNQ channels in the brain: to facilitate neuronal resonance and network oscillations in cortical neurons, thus providing a basis for an oscillation-based neural code.

Dynamics and diversity in interneurons: a model exploration with slowly inactivating potassium currents

Recent experimental and model work indicates that slowly inactivating potassium currents might play critical roles in generating population rhythms. In particular, slow (<1-4 Hz) rhythms recorded in the hippocampus correlate with oscillatory behaviors in interneurons in this frequency range. Limiting the ion channels to the traditional Hodgkin-Huxley sodium and potassium currents, a persistent sodium current, and a slowly inactivating potassium current, we explore the role of slowly inactivating conductances in a multi-compartmental interneuronal model. We find a rich repertoire of tonic and bursting behaviors depending on the distribution, density and kinetics of this conductance. Specifically, burst frequencies of appropriate frequencies could be obtained for certain distributions and kinetics of this conductance. Robust (with respect to injected currents) regimes of tonic firing and bursting behaviors are uncovered. In addition, we find a bistable tonic firing pattern that depends on the slowly inactivating potassium current. Therefore, this work shows ways in which different channel distributions and heterogeneities could produce variable signal outputs. We suggest that an understanding of the dynamical profiles of inhibitory neurons based on the density and distribution of their currents is helpful in dissecting out the complex roles played by this heterogeneous group of cells.

AMRP peptides modulate a novel K(+) current in pleural sensory neurons of Aplysia

Modulation of Aplysia mechanosensory neurons is thought to underlie plasticity of defensive behaviors that are mediated by these neurons. In the past, identification of modulators that act on the sensory neurons and characterization of their actions has been instrumental in providing insight into the functional role of the sensory neurons in the defensive behaviors. Motivated by this precedent and a recent report of the presence of Aplysia Mytilus inhibitory peptide-related (AMRP) neuropeptides in the neuropile and neurons of the pleural ganglia, we sought to determine whether and how pleural sensory neurons respond to the AMRPs. In cultured pleural sensory neurons under voltage clamp, AMRPs elicited a relatively rapidly developing, then partially desensitizing, outward current. The current exhibited outward rectification; in normal 10 mM K(+), it was outward at membrane potentials more positive than -80 mV but disappeared without reversing at more negative potentials. When external K(+) was elevated to 100 mM, the AMRP-elicited current reversed around -25 mV; the shift in reversal potential was as expected for a current carried primarily by K(+). In the high-K(+) solution, the reversed current began to decrease at potentials more negative than -60 mV, creating a region of negative slope resistance in the I-V relationship. The AMRP-elicited K(+) current was blocked by extremely low concentrations of 4-aminopyridine (4-AP; IC(50) = 1.7 x 10(-7) M) but was not very sensitive to TEA. In cell-attached patches, AMRPs applied outside the patch-thus presumably through a diffusible messenger-increased the activity of a K(+) channel that very likely underlies the macroscopic current. The single-channel current exhibited outward rectification, and the open probability of the channel decreased with hyperpolarization; together, these two factors accounted for the outward rectification of the macroscopic current. Submicromolar 4-AP included in the patch pipette blocked the channel by reducing its open probability without altering the single-channel current. Based on the characteristics of the AMRP-modulated K(+) current, we conclude that it is a novel current that has not been previously described in Aplysia mechanosensory neurons. In addition to this current, two other AMRP-elicited currents, a slow, 4-AP-resistant outward current and a Na(+)-dependent inward current, were occasionally observed in the cultured sensory neurons. Responses consistent with all three currents were observed in sensory neurons in situ in intact pleural ganglia.

Ion channel diversity in the feline smooth muscle esophagus

We have characterized ion-channel identity and density differences along the feline smooth muscle esophagus using patch-clamp recording. Current clamp recording revealed that the resting membrane potential (RMP) of esophageal smooth muscle cells (SMC) from the circular layer at 4 cm above the lower esophageal sphincter (EBC4; LES) were more depolarized than at 2 cm above LES. Higher distal Na(+) permeability (but not Cl(-) permeability) contributes to this RMP difference. K(+) channels but not large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels contribute to RMP at both levels, because nonspecific K(+)-channel blockers depolarize all SMC. Depolarization of SMC under voltage clamp revealed that the density of voltage-dependent K(+) channels (K(V)) was greatest at EBC4 due to increased BK(Ca.) Delayed rectifier K(+) channels (K(DR)), compatible with subtype K(V)1.2, were present at both levels. Differences in K(Ca)-to-K(DR) channel ratios were also manifest by predictable shifts in voltage-dependent inactivation at EBC4 when BK(Ca) channels were blocked. We provide the first evidence for regional electrophysiological differences along the esophageal body resulting from SMC ion channel diversity, which could allow for differential muscular responses to innervation and varied muscular contribution to peristaltic contractions along the esophagus.

Involvement of calmodulin-dependent protein kinase II in carbachol-induced rhythmic activity in the hippocampus of the rat

The role of calcium and protein kinases in rhythmic activity induced by muscarinic receptor activation in the CA1 area in rat hippocampal slices was investigated. Extracellular recording showed that carbachol (20 microM) induced synchronized field potential activity with a dominant frequency of 7.39+/-0.68 Hz. Pretreatment with the membrane permeable Ca(2+) chelator BAPTA-AM (50 microM) or with thapsigargin (1 microM), a compound which depletes intracellular calcium stores, reduced the dominant power of carbachol-induced theta-like activity by 83% and 78%, respectively. Inhibition of calmodulin-dependent protein kinase II (CaMKII) by the cell permeable inhibitor KN-93 (10 microM) reduced the power of carbachol-induced theta-like activity by 80%. In contrast the protein kinase C (PKC) inhibitor calphostin C did not significantly (P>0.05) affect the effect of carbachol. Whole-cell recording indicated that KN-93 also blocked carbachol-induced suppression of slow I(AHP) and strongly inhibited the carbachol-induced plateau potential. Our data suggest that activation of CaMKII by carbachol is crucial for local theta-like activity in the CA1 area of the rat hippocampus in vitro. Furthermore, involvement of CaMKII in carbachol-induced suppression of the slow I(AHP) and the induction of plateau potentials could play a role in the induction of theta-like rhythmic activity by carbachol.

Properties of voltage-gated potassium currents in nucleated patches from large layer 5 cortical pyramidal neurons of the rat

Voltage-gated potassium currents were studied in nucleated outside-out patches obtained from large layer 5 pyramidal neurons in acute slices of sensorimotor cortex from 13- to 15-day-old Wistar rats (22-25 C). Two main types of current were found, an A-current (IA) and a delayed rectifier current (IK), which were blocked by 4-aminopyridine (5 mM) and tetraethylammonium (30 mM), respectively. Recovery from inactivation was mono-exponential (for IA) or bi-exponential (for IK) and strongly voltage dependent. Both IA and IK could be almost fully inactivated by depolarising prepulses of sufficient duration. Steady-state inactivation curves were well fitted by the Boltzmann equation with half-maximal voltage (V ) and slope factor (k) values of -81.6 mV and -6.7 mV for IA, and -66.6 mV and -9.2 mV for IK. Peak activation curves were described by the Boltzmann equation with V and k values of -18.8 mV and 16.6 mV for IA, and -9.6 mV and 13.2 mV for IK. IA inactivated mono-exponentially during a depolarising test pulse, with a time constant ( approximately 7 ms) that was weakly dependent on membrane potential. IK inactivated bi-exponentially with time constants ( approximately 460 ms, approximately 4.2 s) that were also weakly voltage dependent. The time to peak of both IA and IK depended strongly on membrane potential. The kinetics of IA and IK were described by a Hodgkin-Huxley-style equation of the form mNh, where N was 3 for IA and 1 for IK. These results provide a basis for understanding the role of voltage-gated potassium currents in the firing properties of large layer 5 pyramidal neurons of the rat neocortex.

Ca2+ channels that activate Ca2+-dependent K+ currents in neostriatal neurons

It is demonstrated that not all voltage-gated calcium channel types expressed in neostriatal projection neurons (L, N, P, Q and R) contribute equally to the activation of calcium-dependent potassium currents. Previous work made clear that different calcium channel types contribute with a similar amount of current to whole-cell calcium current in neostriatal neurons. It has also been shown that spiny neurons possess both "big" and "small" types of calcium-dependent potassium currents and that activation of such currents relies on calcium entry through voltage-gated calcium channels. In the present work it was investigated whether all calcium channel types equally activate calcium-dependent potassium currents. Thus, the action of organic calcium channel antagonists was investigated on the calcium-activated outward current. Transient potassium currents were reduced by 4-aminopyridine and sodium currents were blocked by tetrodotoxin. It was found that neither 30 nM omega-Agatoxin-TK, a blocker of P-type channels, nor 200 nM calciseptine or 5 microM nitrendipine, blockers of L-type channels, were able to significantly reduce the outward current. In contrast, 400 nM omega-Agatoxin-TK, which at this concentration is able to block Q-type channels, and 1 microM omega-Conotoxin GVIA, a blocker of N-type channels, both reduced outward current by about 50%. These antagonists given together, or 500 nM omega-Conotoxin MVIIC, a blocker of N- and P/Q-type channels, reduced outward current by 70%. In addition, the N- and P/Q-type channel blockers preferentially reduce the afterhyperpolarization recorded intracellularly. The results show that calcium-dependent potassium channels in neostriatal neurons are preferentially activated by calcium entry through N- and Q-type channels in these conditions.

Apamin-sensitive conductance mediates the K(+) current response during chemical ischemia in CA3 pyramidal cells

Pyramidal cells typically respond to ischemia with initial transient hyperpolarization, which may represent a neuroprotective response. To identify the conductance underlying this hyperpolarization in CA3 pyramidal neurons of rat hippocampal organotypic slice cultures, recordings were obtained using the single-electrode voltage-clamp technique. Brief chemical ischemia (2 mM 2-deoxyglucose and 3 mM NaN(3), for 4 min) induced a response mediated by an increase in K(+) conductance. This current was blocked by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), reduced with low external [Ca(2+)], and inhibited by a selective L-type Ca(2+) channel inhibitor, isradipine, consistent with the activation of a Ca(2+)-dependent K(+) conductance. Experiments with charybdotoxin (10 nM) and tetraethylammonium (TEA; 1 mM), or with the protein kinase C activator, phorbol 12,13-diacetate (PDAc; 3 microM), ruled out an involvement of a large conductance-type or an apamin-insensitive small conductance, respectively. In the presence of apamin (1 microM), however, the outward current was significantly reduced. These results demonstrate that in rat hippocampal CA3 pyramidal neurons an apamin-sensitive Ca(2+)-dependent K(+) conductance is activated in response to brief ischemia generating a pronounced outward current.

2-Methyl-6-(phenylethynyl)-pyridine (MPEP), a potent, selective and systemically active mGlu5 receptor antagonist

In the present paper we describe 2-methyl-6-(phenylethynyl)-pyridine (MPEP) as a potent, selective and systemically active antagonist for the metabotropic glutamate receptor subtype 5 (mGlu5). At the human mGlu5a receptor expressed in recombinant cells, MPEP completely inhibited quisqualate-stimulated phosphoinositide (PI) hydrolysis with an IC50 value of 36 nM while having no agonist or antagonist activities at cells expressing the human mGlu1b receptor at concentrations up to 30 microM. When tested at group II and III receptors, MPEP did not show agonist or antagonist activity at 100 microM on human mGlu2, -3, -4a, -7b, and -8a receptors nor at 10 microM on the human mGlu6 receptor. Electrophysiological recordings in Xenopus laevis oocytes demonstrated no significant effect at 100 microM on human NMDA (NMDA1A/2A), rat AMPA (Glu3-(flop)) and human kainate (Glu6-(IYQ)) receptor subtypes nor at 10 microM on the human NMDA1A/2B receptor. In rat neonatal brain slices, MPEP inhibited DHPG-stimulated PI hydrolysis with a potency and selectivity similar to that observed on human mGlu receptors. Furthermore, in extracellular recordings in the CA1 area of the hippocampus in anesthetized rats, the microiontophoretic application of DHPG induced neuronal firing that was blocked when MPEP was administered by iontophoretic or intravenous routes. Excitations induced by microiontophoretic application of AMPA were not affected.

The mechanism gated by external potassium and sodium controls the resting conductance in hippocampal and cortical neurons

The excitation of densely packed mammalian central neurons is followed by a substantial transitory elevation of external K+ concentration. This phenomenon may have a different functional significance depending on how the resting membrane conductance mechanisms react to the changes in the gradient of these ions. We have found that in the hippocampal and cortex neurons of rat a large fraction of the membrane conductance in the vicinity of the resting potential is provided by the K+ permeability mechanism which is gated by external K+ and Na+. The responses of acutely isolated pyramidal neurons to rapidly altered external [K+] were investigated using the whole-cell patch clamp and concentration clamp techniques. Elevation of [K+]out induced a biphasic inward current at membrane potentials more negative than the reversal potential for K+ ions. This current consisted of an "instantaneously" increased leakage component and a slowly activated current (tau = 48 ms at 21 degrees C) designated below as I(deltaK). The latter demonstrated a first order activation kinetics with a remarkably high Q10 = 7.31. I(deltaK) was absent in the peripheral sensory neurons as well as in the Purkinje neurons. Slow activation of I(deltaK) was critically dependent on [Na+]out: substitution of the extracellular Na+ with choline chloride or Li+ led to the "instantaneous" reaction of the membrane to the changes in [K+]out. By slowing down potassium influx, I(deltaK) may be of importance in preserving densely packed pyramidal neurons from immediate excitation following rapid increases in [K+]out.

Enhancement of glutamate release uncovers spillover-mediated transmission by N-methyl-D-aspartate receptors in the rat hippocampus

Properties of excitatory postsynaptic currents during increased glutamate release were investigated by means of a whole-cell voltage-clamp in CA1 pyramidal neurons of rat hippocampal slices. Enhancement of transmitter release by 50 microM 4-aminopyridine or by elevated extracellular Ca2+ (up to 5 mM) resulted in a substantial increase in the peak excitatory postsynaptic current amplitude and in the significant stimulus-dependent prolongation of the excitatory postsynaptic current decay. The stronger the stimulus, the slower the excitatory postsynaptic current decay became. The pharmacologically isolated N-methyl-D-aspartate, but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid component of the excitatory postsynaptic current exhibited this phenomenon. The possible connection of such behaviour of the N-methyl-D-aspartate component to the loss of voltage control was tested in the following way: the peak of the N-methyl-D-aspartate component was enhanced under 50 microM 4-aminopyridine and then returned back to the control level by a low dose of D-2-amino-5-phosphonopentanoic acid. However, the decay of the decreased N-methyl-D-aspartate component remained slow suggesting another origin of the stimulus-dependent kinetics. Dihydrokainate, a non-competitive inhibitor of glutamate uptake, did not influence the kinetics of the N-methyl-D-aspartate component in control but induced its dramatic stimulus-dependent prolongation when applied on the background of a low dose of 4-aminopyridine (10 microM) which did not affect the decay by itself. We propose that the delayed stimulus-dependent kinetics of the N-methyl-D-aspartate component is due to the saturation of uptake mechanisms and subsequent activation of extrasynaptic N-methyl-D-aspartate receptors. Our present observations therefore support the hypothesis that N-methyl-D-aspartate receptors may play a role in the cross-talk between synapses by means of the transmitter spillover.

Relationships among seizures, extracellular amino acid changes, and neurodegeneration induced by 4-aminopyridine in rat hippocampus: a microdialysis and electroencephalographic study

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.

Ca2+-activated outward currents in neostriatal neurons

Whole-cell voltage-clamp recordings of outward currents were obtained from acutely dissociated neurons of the rat neostriatum in conditions in which inward Ca2+ current was not blocked and intracellular Ca2+ concentration was lightly buffered. Na+ currents were blocked with tetrodotoxin. In this situation, about 53 +/- 4% (mean +/- S.E.M.; n = 18) of the outward current evoked by a depolarization to 0 mV was sensitive to 400 microM Cd2+. A similar percentage was sensitive to high concentrations of intracellular chelators or to extracellular Ca2+ reduction (<500 microM); 35+/-4% (n=25) of the outward current was sensitive to 3.0 mM 4-aminopyridine. Most of the remaining current was blocked by 10 mM tetraethylammonium. The results suggest that about half of the outward current is activated by Ca2+ entry in the present conditions. The peptidic toxins charybdotoxin, iberotoxin and apamin confirmed these results, since 34 +/- 5% (n = 14), 29 5% (n= 14) and 28 +/- 6% (n=9) of the outward current was blocked by these peptides, respectively. The effects of charybdotoxin and iberotoxin added to that of apamin, but their effects largely occluded each other. There was additional Cd2+ block after the effect of any combination of toxins. Therefore, it is concluded that Ca2+-activated outward currents in neostriatal neurons comprise several components, including small and large conductance types. In addition, the present experiments demonstrate that Ca2+-activated K+ currents are a very important component of the outward current activated by depolarization in neostriatal neurons.

Serotonin modulates spike backpropagation and associated [Ca2+]i changes in the apical dendrites of hippocampal CA1 pyramidal neurons

The effect of serotonin (5-HT) on somatic and dendritic properties was analyzed in pyramidal neurons from the CA1 region in slices from the rat hippocampus. Bath-applied 5-HT (10 microM) hyperpolarized the soma and apical dendrites and caused a conductance increase at both locations. In the dendrites (200-300 microm from the soma) trains of antidromically activated, backpropagating action potentials had lower peak potentials in 5-HT than in normal artificial cerebrospinal fluid. Spike amplitudes were about the same in the two solutions. Similar results were found when the action potentials were evoked synaptically with stimulation in the stratum oriens. In the soma, spike amplitudes increased in 5-HT, with only a small decrease in the peak potential. Calcium concentration measurements, made with bis-fura-2 injected through patch electrodes, showed that the amplitude of the [Ca2+]i changes was reduced at all locations in 5-HT. The reduction of the [Ca2+]i change in the soma was confirmed in slices where cells were loaded with fura-2-AM. The reduction at the soma in 5-HT, where the spike amplitude increased, suggests that the reduction is due primarily to direct modulation of Ca2+ channels. In the dendrites, the reduction is due to a combination of this channel modulation and the lowering of the peak potential of the action potentials.

Functional and ionic properties of a slow afterhyperpolarization in ferret perigeniculate neurons in vitro

Intracellular recordings from spontaneously spindling GABAergic neurons of the ferret perigeniculate nucleus in vitro revealed a fast afterhyperpolarization after each action potential, a medium-duration afterhyperpolarization after each low-threshold Ca2+ spike, and a slow afterhyperpolarization after the cessation of spindle waves. The slow afterhyperpolarization was associated with an increase in membrane conductance, and the reversal potential was sensitive to extracellular [K+]o, indicating that it is mediated at least in part by the activation of a K+ conductance. However, the block of Ca2+ channels did not block the slow afterhyperpolarization, whereas the block of Na+ channels did block this event, even after the generation of repetitive Ca2+ spikes, indicating that it is mediated by a Na+-activated K+ current. Application of apamin reduced the afterhyperpolarization and enhanced a plateau potential after each low-threshold Ca2+ spike. This plateau potential could result in a prolonged depolarization of perigeniculate neurons, even before the application of apamin, resulting in the generation of tonic discharge. The plateau potential was blocked by the local application of tetrodotoxin, indicating that it is mediated by a persistent Na+ current. The activation and interaction of these slowly developing and persistent currents contributes significantly to low-frequency components of spindle wave generation. In particular, we suggest that the activation of the slow afterhyperpolarization may contribute to the generation of the spindle wave refractory period in vitro.

The voltage-dependent conductances of rat neocortical layer I neurons

Whole cell patch-clamp techniques were used to study voltage-dependent sodium (Na+), calcium (Ca2+), and potassium (K+) conductances in acutely isolated neurons from cortical layer I of adult rats. Layer I cells were identified by means of gamma-aminobutyric acid (GABA) immunocytochemistry. Positive stainings for the Ca2+-binding protein calretinin in a subset of cells, indicated the presence of Cajal-Retzius (C-R) cells. All investigated cells displayed a rather homogeneous profile of voltage-dependent membrane currents. A fast Na+ current activated at about -45 mV, was half-maximal steady-state inactivated at -66.6 mV, and recovery from inactivation followed a two-exponential process (tau1 = 8.4 ms and tau2 = 858.8 ms). Na+ currents declined rapidly with two voltage-dependent time constants, reaching baseline current after some tens of milliseconds. In a subset of cells (< 50%) a constant current level of < 65 pA remained at the end of a 90 ms step. A transient outward current (Ifast) activated approximately -40 mV, declined rapidly with a voltage-insensitive time constant (tau approximately 350 ms) and was relatively insensitive to tetraethylammonium (TEA, 20 mM). Ifast was separated into two components based on their sensitivity to 4-aminopyridine (4-AP): one was blocked by low concentrations (40 microM) and a second by high concentrations (6 mM). After elimination of Ifast by a conditioning prepulse (50 ms to -50 mV), a slow K+ current (I(KV)) could be studied in isolation. I(KV) was only moderately affected by 4-AP (6 mM), while TEA (20 mM) blocked most (> 80%) of the current. I(KV) activated at about -40 mV, declined monoexponentially in a voltage-dependent manner (tau approximately 850 ms at -30 mV), and revealed an incomplete steady-state inactivation. In addition to Ifast and I(KV), indications of a Ca2+-dependent outward current component were found. When Na+ currents, Ifast, and I(KV) were blocked by tetrodotoxin (TTX, 1 microM), 4-AP (6 mM) and TEA (20 mM) an inward current carried by Ca2+ was found. Ca2+ currents activated at depolarized potentials at about -30 mV, were completely blocked by 50 microM cadmium (Cd2+), were sensitive to verapamil (approximately 40% block by 10 microM), and were not affected by nickel (50 microM). During current clamp recordings, isolated layer I neurons displayed fast spiking behaviour with short action potentials (approximately 2 ms, measured at half maximal amplitude) of relative small amplitude (approximately 83 mV, measured from the action potential threshold).

Ca2+-inhibited non-inactivating K+ channels in cultured rat hippocampal pyramidal neurones

1. Whole-cell perforated-patch recording from cultured CA1-CA3 pyramidal neurones from neonatal rat hippocampus (20-22 C; [K+]o = 2.5 mM) revealed two previously recorded non-inactivating (sustained) K+ outward currents: a voltage-independent 'leak' current (Ileak) operating at all negative potentials, and, at potentials >= -60 mV, a time- and voltage-dependent 'M-current' (IK(M)). Both were inhibited by 1 mM Ba2+ or 10 microM oxotremorine-M (Oxo-M). In ruptured-patch recording using Ca2+-free pipette solution, Ileak was strongly enhanced, and was inhibited by 1 mM Ba2+ but unaffected by 0.5 mM 4-aminopyridine (4-AP), 1 mM tetraethylammonium (TEA) or 1-10 nM margatoxin. 2. Single channels underlying these currents were sought in cell-attached patch recordings. A single class of channels of conductance approximately 7 pS showing sustained activity at resting potential and above was identified. These normally had a very low open probability (Po < 0. 1), which, however, showed a dramatic and reversible increase (to about 0.9 at approximately 0 mV) following the removal of Ca2+ from the bath. Under these (Ca2+-free) conditions, single-channel Po showed both voltage-dependent and voltage-independent components on patch depolarization from resting potential. The mean activation curve was fitted by a modified Boltzmann equation. When tested, all channels were reversibly inhibited by addition of 10 microM Oxo-M to the bath solution. 3. The channels maintained their high Po in patches excised in inside-out mode into a Ca2+-free internal solution and were strongly inhibited by application of Ca2+ to the inner face of the membrane (IC50 = 122 nM); this inhibition was observed in the absence of MgATP, and therefore was direct and unrelated to channel phosphorylation/dephosphorylation. 4. Channels in patches excised in outside-out mode were blocked by 1 mM Ba2+ but were unaffected by 4-AP or TEA. 5. Channels in cell-attached patches were inhibited after single spikes, yielding inward ensemble currents lasting several hundred milliseconds. This was prevented in Ca2+-free solution, implying that it was due to Ca2+ entry. 6. The properties of these channels (block by internal Ca2+ and external Oxo-M and Ba2+, and the presence of both voltage-dependent and voltage-independent components in their Po/V relationship) show points of resemblance to those expected for channels associated with both Ileak and IK(M) components of the sustained macroscopic currents. For this reason we designate them Ksust ('sustained current') channels. Inhibition of these channels by Ca2+ entry during an action potential may account for some forms of Ca2+-dependent after-depolarization. Their high sensitivity to internal Ca2+ may provide a new, positive feedback mechanism for cell excitation operating at low (near-resting) [Ca2+]i.

Metabotropic glutamate receptors limit adenylyl cyclase-mediated effects in rat hippocampus via protein kinase C

Glutamate receptors of the metabotropic type (mGluRs) activate protein kinase C in hippocampus, but few physiological functions of this pathway are known. The present data show that mGluRs utilize protein kinase C to inhibit another second messenger system, the adenylyl cyclase pathway, in neurons of the CA1 area of hippocampus. Activation of mGluRs prevented beta-adrenergic receptors, which couple to adenylyl cyclase, from blocking the slow Ca2+-dependent afterhyperpolarization (AHP). Since the afterhyperpolarization modulates neuronal responsiveness, crosstalk between protein kinase C and the adenylyl cyclase pathway is likely to have physiological consequences. Moreover, mGluRs themselves block the afterhyperpolarization, so the observed interference with the beta-adrenergic response constitutes a hierarchical relationship in which mGluRs are dominant over beta-adrenergic receptors.

Ionic mechanisms of intrinsic oscillations in neurons of the basolateral amygdaloid complex

Ionic mechanisms underlying low-threshold (LTO) and high-threshold (HTO) oscillations occurring in a class of spiny neurons within the basolateral amygdaloid complex (see companion paper) were investigated in slice preparations of the guinea pig amygdala in vitro. LTOs were abolished through local application of tetrodotoxin (TTX, 10-20 microM) or a decrease in the extracellular sodium concentration ([Na+]o) from 153 to 26 mM, whereas HTOs were more readily elicited under these conditions. The effects of TTX and low [Na+]o were accompanied by a hyperpolarizing shift of the membrane potential by 3 +/- 1 mV and a decrease in apparent input resistance by 14 +/- 11 MOmega. LTOs were not observed during intracellular recording with QX 314 (50 microM) or Cs-acetate (2 M) containing micropipettes. At membrane potentials associated with LTO generation, voltage responses to sinusoidal current input with changing frequency between 0 and 10 Hz were characterized by a peak in the response (resonance) at 2.4 +/- 1 Hz, largely corresponding to the frequency range of the LTOs. Resonance behavior was evident as a peak in the impedance amplitude plot (ZA-plot) and a maximum in the fast Fourier transformation (FFT). Resonance and LTOs were concomitantly reduced by TTX and barium (Ba2+;2-10 mM) and were preserved during action of extracellular cesium (Cs+; 10-30 mM) or tetraethylammonium chloride (TEA; 20-50 mM), although the peak in the frequency domain tended to shift to lower values in TEA. Application of carbachol (50-200 microM) significantly reduced or blocked LTOs, whereas 4-aminopyridine (4-AP; 10 mM), iberiotoxin (Ibtx, 10 microM), and apamin (20 microM) had no effect. Slow depolarizing/repolarizing current ramps (12.5-125 pA/s) evoked HTOs as rhythmic deflections in membrane potential at either phase of the current ramp. Substitution of extracellular calcium (Ca2+) by magnesium and addition of cobalt chloride (2-4 mM) blocked HTOs but had no measurable effect on the propensity of the cells to produce LTOs. HTOs were abolished within approximately 10 min after impalement of the cells with a bis-(2-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA; 200 mM)-containing micropipette. Intracellular Cs+, extracellular Ba2+ (2-10 mM), or extracellular TEA (20-50 mM) induced an increase in amplitude of the rhythmic discharges and an increasingly slowed time course of repolarization at successive oscillatory events, until a steady depolarization was reached at -20 to -10 mV. Application of Ibtx (10 microM) reversibly abolished rhythmic activity during the repolarizing phase of the current ramp, whereas charybdotoxin (2-10 microM) and apamin (20 microM) had no effect. Changes in the chloride (Cl-) equilibrium potential by approximately +30 mV through intracellular recording with a KNO3 (3 M)-containing micropipette or lowering [Cl-]o from 128 to 4 mM, or blockade of Cl- conductances through niflumic acid (100 microM), did not significantly effect LTOs or HTOs. The generation of repetitive spike patterns on membrane depolarization was substantially influenced through removal of extracellular Ca2+ and associated blockade of HTOs, in that the initial high frequent discharge was abolished, frequency adaptation toward slow-rhythmic firing was delayed, and firing occurred at a more irregular pattern during strong depolarizing stimuli. It is concluded that a TTX-sensitive Na+ conductance and the M current contribute to generation of the LTOs, although their exact role in rhythmogenesisremains to be determined. HTOs seem to largely depend on a functional coupling between high-voltage-activated Ca2+ conductances, a Ca2+-activated K+ current presumably carried through BKCa channels, and additional voltage-dependent K+ conductances. In functional terms, the HTOs are important in determining spike frequency adaptation toward a slow-rhythmic firing pattern during maintained depolarizing influence.

Two types of intrinsic oscillations in neurons of the lateral and basolateral nuclei of the amygdala

Intracellular recordings in the guinea pig and cat basolateral amygdaloid (BL) complex maintained as slices in vitro revealed that a subpopulation of neurons (79%) in the lateral (AL) and basolateral (ABl) nuclei generated two types of slow oscillations of the membrane potential upon steady depolarization from resting potential. The cells were of a stellate or pyramidal-like shape and possessed spiny dendrites and an axon leaving the local synaptic environment, and thus presumably represented projection neurons. Similar oscillatory activity was observed in projection neurons of the cat AL nucleus recorded in vivo. Oscillatory activity with a low threshold of activation (low-threshold oscillation, LTO) appeared as rhythmic deflections (amplitudes, 2-6 mV) of the membrane potential positive to -60 mV. Fast Fourier transformation (FFT) demonstrated a range of frequencies of LTOs between 0.5 and 9 Hz, with >80% occurring at 1-3.5 Hz and an average at 2.3 +/- 1.1 Hz. LTOs were more regular after pharmacological blockade of synaptic transmission and were blocked by tetrodotoxin (TTX). Blockade of LTOs and Na+ spikes revealed a second type of oscillatory activity (high-threshold oscillation, HTO) at depolarizations beyond -40 mV, which was capable of triggering high-threshold spikes. HTOs ranged between 1 and 7.5 Hz, with >80% occurring at 2-6 Hz and an average at 5.8 +/- 1.1 Hz. HTOs vanished at a steady membrane polarization positive to -20 mV. Current versus voltage relations obtained under voltage-clamp conditions revealed two regions of negative slope conductance at -55 to -40 mV and at around -30 mV, which largely overlapped with the voltage ranges of LTOs and HTOs. TTX abolished the first region of negative slope conductance (-55 to -40 mV) and did not significantly influence the second region of negative slope conductance. Neuronal responses to maintained depolarizing current pulses consisted of an initial high-frequency discharge (up to 100 Hz), the frequency of which depended on the amplitude of the depolarizing current pulse, followed by a progressive decline ("adaptation") toward a slow-rhythmic firing pattern. The decay in firing frequency followed a double-exponential function, with time constants averaging 57 +/- 28 ms and 3.29 +/- 1.85 s, and approached steady-state frequencies at 6.3 +/- 2.9 Hz (n = 17). Slow-rhythmic firing remained at this frequency over a wide range of membrane polarization between approximately -50 and -20 mV, although individual electrogenic events changed from Na+ spikes and underlying LTOs to high-threshold spikes and underlying HTOs. Rhythmic regular firing was only interrupted at an intermediate range of membrane polarization by the occurrence of spike doublets. In conclusion, the integrative behavior of a class of neurons in the BL complex appears to be largely shaped by the slow-oscillatory properties of the membrane. While LTOs are likely to synchronize synaptic signals near firing threshold, HTOs are a major determinant for the slow steady-state firing patterns during maintained depolarizing influence. These intrinsic oscillatory mechanisms, in turn, can be assumed to promote population activity at this particular frequency, which ranges well within that of the limbic theta (Theta) rhythm and the delta (delta) waves in the electroencephalogram during slow-wave sleep.

The oxytocin-induced inward current in vagal neurons of the rat is mediated by G protein activation but not by an increase in the intracellular calcium concentration

The neuropeptide oxytocin can depolarize parasympathetic preganglionic neurons in the dorsal motor nucleus of the vagus nerve of the rat by generating a sustained inward current, which is sodium-dependent and tetrodotoxin-insensitive. The second messenger activated by oxytocin receptor binding is, however, not yet known. In the present study, we attempted to characterize it by using the whole-cell recording technique and brainstem slices. When loaded with GTP-gamma-S, a non-hydrolysable analogue of GTP, vagal neurons generated a persistent inward current in the absence of agonist and the oxytocin effect was suppressed, suggesting that the peptide-evoked current was mediated by G-protein activation. Loading vagal neurons with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid (BAPTA) suppressed a calcium-dependent, slowly decaying potassium aftercurrent but did not affect the oxytocin response, suggesting that the latter was not mediated by an agonist-induced increase in the intracellular calcium concentration. Protein kinase C (PKC) activation was probably not involved, since the peptide-evoked current was not modified by loading neurons with the PKC inhibitor H7. Thus, the oxytocin-evoked current in vagal neurons was probably not mediated by phospholipase C-beta (PLC-beta) activation. Loading neurons with 8-Br-cAMP or with an adenylyl cyclase activator (forskolin) reduced the oxytocin-evoked current by about half. SQ 22536, an adenylyl cyclase inhibitor, reduced this current by a similar amount. However, the peptide-evoked current was unaffected by Rp-cAMPS and Sp-cAMPS, an inhibitor and an activator, respectively, of cAMP-dependent protein kinase (PKA). We suggest that oxytocin activates two distinct signalling pathways in vagal neurons: one which is cAMP-dependent, but PKA-independent, and one, unidentified, which is PLC-beta-and cAMP-independent. Each pathway accounts for about half of the peptide effect and both appear to involve G-protein activation.

Properties of a Ca2+-activated K+ conductance in acutely isolated pyramidal-like neurons from the rat basolateral amygdaloid complex

A calcium (Ca2+)-activated potassium (K+) conductance was studied in large pyramidal-like neurons acutely dissociated from the rat basolateral amygdaloid complex. Neurons were immunoreactive to anti-alpha(913-926), a sequence-directed antibody directed against the pore-forming alpha-subunit of the BK(Ca) channel, also termed slo. Whole cell current-voltage (I-V) relationships obtained on application of slow (46.7 mV/s) voltage ramps from -110 to +100 mV were N shaped positive to -30 mV. Maximal current activation occurred at +9.8 +/- 2.7 (SE) mV, with a mean current density of 404.8 +/- 25.0 pA/pF. Substitution of extracellular Ca2+ with manganese (Mn2+), or with magnesium (Mg2+) and addition of 5 mM ethyleneglycol-bis (beta-aminoethylether)-N,N,N',N'-tetraacetic acid, abolished the N-shaped I-V relationship with a reduction in maximal outward current to 15.3 +/- 2.3% of the control value. The Ca2+-sensitive K+ current component, as revealed by voltage step protocols, activated at depolarizations positive to -30 mV with a slow time course (time constant 430.7 +/- 78.6 ms). The current was reduced by 80.4 +/- 4.6% through 1 mM tetraethyammonium chloride and by 66.8 +/- 3.4% through 100 nM iberiotoxin, whereas apamin up to 1 microM had no effect. It is concluded that pyramidal-like neurons of the basolateral amygdaloid complex possess BK(Ca) channels and the corresponding macroscopic Ca2+-sensitive K+ conductance, activation of which will substantially contribute to the Ca2+-dependent regulation of electrogenic behavior in these neurons.

Activity-dependent [Ca2+]i changes in guinea pig vagal motoneurons: relationship to the slow afterhyperpolarization

Vagal motoneurons in slices from the guinea-pig brain stem were injected with the fluorescent [Ca2+]i indicators fura-2, furaptra, or Calcium Green-1. Spike-induced fluorescence changes were measured in the soma and dendrites and simultaneously the long-lasting afterhyperpolarization was recorded with a sharp microelectrode in the soma. Na+ spikes or Ca2+ spikes increased [Ca2+]i (measured as a change in indicator fluorescence) in all locations in the soma and dendrites. Each spike in a train of action potentials caused a step increase in fluorescence of about equal amplitude when nonsaturating indicators were used. Peak changes at all locations occurred at the time of the last action potential. Transients measured with low concentrations of Calcium Green-1 or furaptra had a recovery time constant of approximately 500-1,500 ms in the cell body. The recovery time course was faster in the dendrites than in the soma. The norepinephrine-sensitive, slow afterhyperpolarization (sAHP) had a time to peak of approximately 800 ms and a recovery time constant of 2-5 s, much longer than the recovery time course of the fluorescence changes. Some of these experiments were repeated on pyramidal neurons from the CA1 region of the rat hippocampus with similar results. In both cell types, the data suggest that the time course of neither the rising phase nor the falling phase of the sAHP, nor the underlying conductance, directly reflects the time course of the [Ca2+]i change. The mechanism connecting the parameters remains unclear. One possibility is that an additional second messenger system is involved.

1S, 3R-ACPD induces a region of negative slope conductance in the steady-state current-voltage relationship of hippocampal pyramidal cells

Synaptic responses mediated by metabotropic glutamate receptors (mGluRs) display a marked voltage-dependent increase in amplitude when neurons are moderately depolarized beyond membrane potential. We have investigated the basis for this apparent nonlinear behavior by activating mGluRs with 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (1S, 3R-ACPD; 10 microM) in CA3 pyramidal cells from rat hippocampal slice cultures with the use of the single-electrode voltage-clamp technique. Under control conditions, cells depolarized from resting potential by 10-20 mV responded with delayed outwardly rectifying currents due to activation of voltage- and Ca(2+)-dependent K+ conductances. In contrast, in the continuous presence of 1S, 3R-ACPD, small depolarizations (10-20 mV) induced a delayed inward current. The steady-state current-voltage relationship for this response displayed a region of negative slope conductance at potentials between -55 and -40 mV. The reversal potential of the corresponding 1S,3R-ACPD-sensitive tail currents (-93.0 +/- 2.2 mV, mean +/- SE) was close to the potassium reversal potential, consistent with an mGluR-mediated suppression of K+ current. When external K+ concentration was increased to 8 mM, there was a positive shift in reversal potential to -76.9 +/- 5.1 mV. The depolarization-induced inward current in the presence of 1S,3R-ACPD was blocked by Ba2+ (1 mM). The response was not dependent on changes in intracellular Ca2+ concentration and was insensitive to bath-applied Cs+ (1 mM), ruling out a contribution of Ca(2+)-dependent currents or the inward rectifier lQ. Furthermore, the effect of 1S,3R-ACPD was not mimicked by inhibiting afterhyperpolarizing current and M current with low-Ca2+ saline (0.5 mM Ca2+, 10 mM Mg2+) containing 10 mM tetraethylammonium chloride. A comparison of the responses induced by 1S,3R-ACPD and N-methyl-D-aspartate showed that both induce an inward current with small depolarizations from resting potential but with different kinetics and Mg2+ sensitivity. These results indicate that the suppression of K+ currents in response to activation of mGluRs is markedly voltage dependent, increasing at depolarized potentials and decreasing at hyperpolarized potentials. The negative slope conductance at membrane voltages positive to resting potential may underlie the amplification of mGluR-mediated responses when the membrane potential approaches action potential threshold.

Chaos and synchrony in a model of a hypercolumn in visual cortex

Neurons in cortical slices emit spikes or bursts of spikes regularly in response to a suprathreshold current injection. This behavior is in marked contrast to the behavior of cortical neurons in vivo, whose response to electrical or sensory input displays a strong degree of irregularity. Correlation measurements show a significant degree of synchrony in the temporal fluctuations of neuronal activities in cortex. We explore the hypothesis that these phenomena are the result of the synchronized chaos generated by the deterministic dynamics of local cortical networks. A model of a "hypercolumn" in the visual cortex is studied. It consists of two populations of neurons, one inhibitory and one excitatory. The dynamics of the neurons is based on a Hodgkin-Huxley type model of excitable voltage-clamped cells with several cellular and synaptic conductances. A slow potassium current is included in the dynamics of the excitatory population to reproduce the observed adaptation of the spike trains emitted by these neurons. The pattern of connectivity has a spatial structure which is correlated with the internal organization of hypercolumns in orientation columns. Numerical simulations of the model show that in an appropriate parameter range, the network settles in a synchronous chaotic state, characterized by a strong temporal variability of the neural activity which is correlated across the hypercolumn. Strong inhibitory feedback is essential for the stabilization of this state. These results show that the cooperative dynamics of large neuronal networks are capable of generating variability and synchrony similar to those observed in cortex. Auto-correlation and cross-correlation functions of neuronal spike trains are computed, and their temporal and spatial features are analyzed. In other parameter regimes, the network exhibits two additional states: synchronized oscillations and an asynchronous state. We use our model to study cortical mechanisms for orientation selectivity. It is shown that in a suitable parameter regime, when the input is not oriented, the network has a continuum of states, each representing an inhomogeneous population activity which is peaked at one of the orientation columns. As a result, when a weakly oriented input stimulates the network, it yields a sharp orientation tuning. The properties of the network in this regime, including the appearance of virtual rotations and broad stimulus-dependent cross-correlations, are investigated. The results agree with the predictions of the mean field theory which was previously derived for a simplified model of stochastic, two-state neurons. The relation between the results of the model and experiments in visual cortex are discussed.

Cholinergic modulation of the action potential in rat hippocampal neurons

The cholinergic input to the hippocampus from the medial septum is important for modulating hippocampal activity and functions, including theta rhythm and spatial learning. Neuromodulation by transmitters in central nervous system neurons usually affects cell excitability by modifying the membrane potential, discharge pattern and spike frequency. Here we describe another type of neuromodulation: changing the action potential waveform. During intracellular recordings from CA1 pyramidal cells in hippocampal slices from rats, the cholinergic agonist carbachol caused several reversible changes in the action potential: low doses (2 microM) caused an increase in spike duration; high doses (10-40 microM) or long-lasting applications also reduced the spike amplitude and rate of rise, and raised the spike threshold. These effects are similar to those of metabotropic glutamate receptor agonists or phorbol esters, both of which activate protein kinase C. The effects were blocked by the muscarinic antagonist atropine, and were prevented by Ca(2+)-free medium and by Ca(2+)-channel blockers. However, the cholinergic spike modulation was not occluded or mimicked by blocking the Ca(2+)-dependent K+ currents IC or IAHP, suggesting that these K+ currents are not involved in the modulation. We conclude that muscarinic receptor activation modulates the action potential in CA1 pyramidal cells via a Ca(2+)-dependent mechanism, possibly involving protein kinase C. This modulation and the similar effects mediated by metabotropic glutamate receptors to our knowledge provide the only examples of neuromodulation of the action potential in the vertebrate central nervous system-a form of modulation known to regulate Ca2+ influx and transmitter release, and to mediate synaptic plasticity and learning in invertebrates.

Modulation of N-methyl-D-aspartate receptor activity by oxidative stress conditions in chick retinal cells

The effect of oxidative stress, induced by ascorbate (1.5 mM)/Fe2+ (7.5 microM), on the cellular responses to N-methyl-D-aspartate (NMDA) receptor activation was evaluated by measuring the release of [3H]GABA induced by NMDA from cultured retina cells. In retina cells submitted to oxidative stress the [3H]GABA release evoked by NMDA, in a medium containing physiological concentrations of Mg2+ (1.6 mM) and K+ (4 mM), was significantly higher than in control cells. The [3H]GABA release evoked by NMDA was potentiated by glycine and was abolished by MK-801, suggesting that the [3H]GABA release was due to NMDA receptor activation. The increased effect of NMDA in peroxidized cells was significantly reduced by TTX, suggesting that the higher cellular responses to the activation of NMDA receptors are due to a hyperexcitability of retina cells submitted to oxidative stress. No significant differences were found between the average resting membrane potential of control and peroxidized cells. However, membrane potential is more tightly regulated by K(+)-channels sensitive to 4-aminopyridine (100 microM), alpha-dendrotoxin (100 nM) and gamma-dendrotoxin (100 nM) under oxidative stress.

Hyperexcitability in CA1 of the rat hippocampal slice following hypoxia or adenosine

Participation of adenosine receptors in the depression of synaptic transmission during hypoxia, and the production of multiple populations spikes in the pyramidal neurons following hypoxia, has been investigated in the CA1 area of the rat hippocampal slice. A method is presented for analysing such hyperexcitability, using input/output curves of the second population spike. This method provides evidence that rebound hyperexcitability following hypoxia or prolonged adenosine-mediated inhibition results from an increase in excitability of the CA1 pyramidal neurons rather than from an increase in excitatory neurotransmitter release. Hypoxia-induced depression of the synaptic components of evoked field potentials was blocked in a concentration dependent manner by the selective A1 receptor antagonist 8-cyclopentyltheophylline (8-CPT), demonstrating extracellular accumulation of adenosine during hypoxia. Upon reoxygenation of slices following 30 min hypoxia, multiple population spikes were evoked by a single orthodromic stimulus in slices that exhibited only a single population spike prior to hypoxia. Such post-hypoxic hyperexcitability was not prevented by superfusion of slices with 8-CPT during hypoxia. Depression of synaptic transmission by 30 min superfusion of slices with 50 microM adenosine was also followed, upon washout, by the appearance of multiple population spikes. However, such hyperexcitability could not be produced by superfusion with adenosine analogues selective for A1 receptors, cyclopentyladenosine, selective for A2a receptors, 2-p-(2-carboxyethyl)phenetheylamino-5'-ethylcarboxamidoadenosine (CGS 21680), or active at A2a and A2b receptors, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine, suggesting that adenosine receptors other than the A1, A2a or A2b subtypes are involved in its generation.