Mammalian Central Neurons Research Articles

Enhancement by Propofol of the γ-Aminobuiyric AcidA Response in Dissociated Hippocampal Pyramidal Neurons of the Rat

Activation of the gamma-aminobutyric acidA (GABAA) receptor-ionophore complex has been reported as a possible molecular mechanism of the anesthetic action of propofol. Augmentation of GABA-induced inhibitory transmission has also been suggested as a mechanism. Because data describing this latter mechanism in mammalian neurons are few, we have examined the effects of propofol on the GABA response in central neurons of the rat. Hippocampal pyramidal neurons were dissociated after enzyme treatment of the rat brain slices. The neurons were voltage-clamped with the whole cell configuration of the patch clamp technique. Neurotransmitters and drugs were applied using the "Y-tube" method, which exchanges the extracellular solutions around the neuron within 10-20 ms and makes it possible to obtain the peak response before desensitization develops. In pyramidal neurons voltage-clamped at -60 mV, GABA induced an inward current. Propofol (10(-6) M) augmented the current and shifted the concentration-response curve for GABA to the left without affecting the maximum response. A low concentration of the anesthetic (10(-6) M) reduced the dissociation constant for GABA from 8.2 x 10(-6) to 4.2 x 10(-6) M without a significant effect on the Hill coefficient. Coapplication of propofol at a higher concentration (5 x 10(-6) M) also shifted the GABA dose-response curve to the left, reducing the dissociation constant to 2.8 x 10(-6) M. Potentiation by propofol was not associated with a change in the reversal potential for the GABA response and was not voltage-dependent. The inhibitory glycine response was not affected by propofol (10(-6) M or 5 x 10(-6) M). Propofol at clinically relevant concentrations enhances the inhibitory GABAA receptor-mediated response in mammalian central neurons. The enhancement may result in reduced excitability of the neuronal network and may, consequently, contribute to the anesthetic action of the agent.

Developmental autoregulation of calcium currents in mammalian central neurones.

The influence of calcium currents expressed at early stages on the subsequent development of calcium currents was studied in embryonic rat hypothalamic neurones in culture. Voltage-activated calcium currents and spontaneous fluctuations of intracellular free calcium concentration ([Ca2+]i) were monitored. Acute application of nickel chloride (0.1 mM) to 6- to 7-day-old cultures strongly reduced calcium currents and [Ca2+]i fluctuations. When cultures were maintained for 6-7 days in the presence of NiCl2 (0.05-0.1 mM), expression of the low voltage-activated current was strongly inhibited; this treatment did not affect high voltage-activated currents. Our results suggest that spontaneous activation of calcium currents early during development promotes calcium influx that regulates expression of calcium current in mature neurones.

Modifications of A‐current kinetics in mammalian central neurones induced by extracellular zinc.

1. Whole-cell voltage clamp recordings were used to study the action of the transition ion zinc on the A-current kinetics in granule cells from rat cerebellar slices. 2. The effects of zinc have been tested in the concentration range from 1 microM to 1 mM, and fully characterized on all kinetic parameters at 100 and 300 microM. All the effects observed were rapid, concentration dependent and fully reversible. 3. Steady-state inactivation curves are strongly shifted towards depolarized potentials, with activation curves much less so. These shifts lead to an increase of the peak current amplitude around physiological resting membrane potentials and to a decrease at hyperpolarized potentials. 4. The forward 'on' rate constants are slowed by Zn2+ at a concentration of 100-300 microM by a factor from 1.5 to 4. The backward 'off' rate constants are unaffected by Zn2+. 5. The development of IA inactivation, as measured from the current decay, is not affected by Zn2+ up to 1 mM. Removal of inactivation is, on the contrary, significantly slowed. 6. The results are neither compatible with the theory of the surface charge screening effect nor with a mechanism involving channel block. It seems more likely that Zn2+ interferes with the channel gating by binding to a specific domain of the channel protein. 7. After treatment with Hg2+, which is irreversible, Zn2+ still maintains its effects, which suggest that the two divalents act at different sites. 8. In view of the widespread distribution of zinc throughout the brain, its actions on the A-current could play an important role in physiological function.

Expression and function of the low density lipoprotein receptor-related protein (LRP) in mammalian central neurons

The low density lipoprotein receptor-related protein (LRP) is a multifunctional cell surface receptor, expressed in liver, that binds with high affinity and endocytoses several structurally and functionally distinct ligands, including apolipoprotein E-activated beta-migrating very low density lipoprotein, tissue-type plasminogen activator, and alpha 2-macroglobulin. Here using in situ hybridization and quantitative RNase protection assays, we show that LRP is also expressed throughout the brain. LRP message is particularly high in the cerebellum, cortex, hippocampus, and brain stem. In addition, we demonstrate that a 39-kDa protein which copurifies with LRP and regulates the binding of other ligands to LRP is also expressed throughout the brain. Interestingly, expression of the 39-kDa message is approximately 100-fold of that found in liver, suggesting that the activity of LRP is more tightly regulated in brain tissue than in liver. Using primary cultures of isolated postnatal cortical neurons, [35S]methionine biosynthetic labeling and immunoprecipitation, we also demonstrate the de novo biosynthesis of LRP, the 39-kDa protein, as well as tissue-type plasminogen activator. Finally, using radioligand binding as well as fluorescent ligand binding and uptake studies, we show that LRP is functional in cortical neurons. These results taken together thus demonstrate expression of functional LRP in neuronal cells and suggest a potential role for LRP in brain protein and lipoprotein metabolism, development, and regeneration.

Perturbation of intracellular calcium and hydrogen ion regulation in cultured mouse hippocampal neurons by reduction of the sodium ion concentration gradient

Na(+)-Ca2+ exchange has been identified as a mechanism for regulation of intracellular Ca ion concentration ([Ca2+]i) in neurons of invertebrates and vertebrates, but for mammalian central neurons its role in restoration of resting [Ca2+]i after transient increases induced by stimulation has been less clear. We have examined the recovery of [Ca2+]i following K+ depolarization and glutamate receptor activation of cultured mouse hippocampal neurons using the Ca(2+)-sensitive dye Fura-2. Reduction of the transmembrane Na+ gradient by removal of external Na+ slowed the recovery of neurons from imposed Ca2+ loads. We observed that [Ca2+]i regulation was disrupted more severely when N-methyl-D-glucamine (N-MG), Tris, or choline rather than Li+ replaced external Na+. Additional disruption of intracellular pH regulation by substitutes other than Li+ may account for this difference. Measurement of [Ca2+]i and [H+]i (using the H(+)-sensitive dye BCECF) during glutamate receptor activation indicated that Ca2+ influx resulted in production of intracellular H+, and that Li+ but not N-MG could prevent cytoplasmic acidification on removal of external Na+. We also observed that intracellular acidification alone was sufficient to slow recovery from Ca2+ load. We conclude, therefore, that Na(+)-Ca2+ exchange contributes to recovery of [Ca2+]i after stimulation leading to Ca2+ entry into hippocampal neurons, and that Na(+)-H+ exchange limits the acidification (and secondary increase in [Ca2+]i) that accompanies Ca2+ influx. We suggest that because both Na(+)-Ca2+ and Na(+)-H+ exchangers will be compromised during ischemia and hypoglycemia, increased intracellular H+ may synergize with cytoplasmic Ca2+ to potentiate excitotoxic neuronal death.

Regulation of NMDA receptors by tyrosine kinases and phosphatases.

Protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs) are key enzymes in signal-transduction pathways for a wide range of cellular processes. PTKs and PTPs are highly expressed in the central nervous system, which is consistent with the importance of tyrosine phosphorylation in neuronal function. Protein phosphorylation is known to be involved in the regulation of neurotransmitter receptors, but the effects of tyrosine phosphorylation on neurotransmitter receptor function in the central nervous system are unknown. Here we present evidence that in mammalian central neurons tyrosine phosphorylation regulates the function of the NMDA (N-methyl-D-aspartate) receptor, a subtype of excitatory amino-acid receptor. NMDA-receptor-mediated whole-cell currents and intracellular Ca2+ responses are depressed by inhibition of PTKs. Conversely, NMDA currents are potentiated by intracellular application of the well characterized PTK pp60c-src. NMDA currents are also potentiated by intracellular administration of an inhibitor of PTPs. Protein-tyrosine phosphorylation is a new mechanism for regulating NMDA receptors and may be important in neuronal development, plasticity and toxicity.

Single-channel currents trigger action potentials in small cultured hippocampal neurons.

Spontaneous neuronal impulse activity appears to play a key role in some neural processes, such as the normal establishment of interneuronal connections during development. In addition, spontaneous impulses may be essential for the functional operation of neuronal networks. Mechanisms of spontaneous non-pacemaker impulse generation are, however, not well known. In this work, spontaneous electrical activity in small cultured hippocampal neurons from rat was studied with tight-seal recording techniques. The results demonstrate that spontaneous individual openings of single ion channels can trigger impulse generation in these high-resistance cells. First, impulses recorded in the whole-cell mode were apparently induced by spontaneous plateau-potential events showing the characteristics expected from individual openings and closures of ion channels. Second, patch-clamp recordings in the cell-attached configuration showed that openings of single ion channels in the patch membrane could trigger cellular impulses, detected as biphasic current deflections. These findings suggest that the random gating of ion channel molecules can be used as a mechanism for stochastic triggering of spontaneous impulses in mammalian central neurons.

Insulin gene expression and insulin synthesis in mammalian neuronal cells.

To demonstrate the presence of de novo synthesis in central mammalian neurons, we cloned and sequenced a rabbit insulin cDNA from pancreas and used it to define sequences encoding insulin mRNA from postnatal rabbit brain. We observed transcription/elongation of nascent insulin transcripts, characterized the size of these transcripts, and localized them to specific neurons in certain catecholaminergic-rich areas of the central nervous system. RNase protection assays using a rabbit probe spanning a region from 14 bases 5' to the translation start site through all but 18 bases of the sequence encoding the A-chain of insulin showed two bands in rabbit brain RNA and only one band in pancreas. The larger band in brain was the same size as that in pancreatic RNA; the other was approximately 10 bases shorter. Because the sequence of a reverse transcription-polymerase chain reaction product from brain RNA was identical to pancreatic RNA sequence in the region corresponding to the 3' region of the probe, the smaller band in brain is most consistent with a sequence mismatch in some brain mRNA in the region corresponding to the 5'-end of the probe. In situ hybridization localized insulin mRNA to anatomical regions involved with olfaction and higher association of the limbic system. High performance liquid chromatography, radioimmunoassay, and [35S]cysteine metabolic labeling of cultured neuronal and glial cells indicated extracellular secretion of immunoprecipitable insulin by neurons only. Presence of insulin transcripts within specific neurons with extracellular secretion of the peptide suggests a specialized biological role.

Visualization of Quantal Synaptic Transmission by Dendritic Calcium Imaging

As changes in synaptic strength are thought to be critical for learning and memory, it would be useful to monitor the activity of individual identified synapses on mammalian central neurons. Calcium imaging of cortical neurons grown in primary culture was used to visualize the activation of individual postsynaptic elements by miniature excitatory synaptic currents elicited by spontaneous quantal release. This approach revealed that the probability of spontaneous activity differed among synapses on the same dendrite. Furthermore, synapses that undergo changes in activity induced by glutamate or phorbol ester treatment were identified.

Different properties of caffeine-sensitive Ca2+ stores in peripheral and central mammalian neurones

Using indo-1 based microfluorometry for measuring the cytoplasmic free calcium concentration ([Ca2+]i), the properties of caffeine-induced Ca2+ release from internal stores were studied in rat cultured central and peripheral neurones, including dorsal root ganglia (DRG) neurones, neurones from nucleus cuneatus, CA1 and CA3 hippocampal region and pyramidal neocortical neurones. Under resting conditions the Ca2+ content of internal stores in DRG neurones was high enough to produce caffeine-triggered [Ca2+]i transients. Caffeine-induced Ca2+ release depleted internal stores in DRG neurones, but they refilled themselves spontaneously up to 81.4 +/- 5.67% within 10 minutes. In contrast, in all types of central neurones the resting Ca2+ content of internal stores was low, but the stores could be charged by transmembrane Ca2+ influx through voltage-operated calcium channels. After charging, the stores in central neurones spontaneously lost releasable calcium content and within 10 minutes they emptied again. We suggest that in sensory neurones calcium stores are continuously filled by releasable calcium and after discharge they can refill themselves spontaneously, while in central neurones internal calcium stores can be charged by releasable calcium only transiently.

Effects of target depletion on adult mammalian central neurons: Functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60–85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25–30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Effects of target depletion on adult mammalian central neurons: Morphological correlates

The morphological sequelae induced by target removal were studied on adult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target—the medial rectus motoneurons of the oculomotor nucleus—was selectively destroyed by the injection of toxic ricin into the medial rectus muscle. Retrograde labeling with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after target removal. However, soma size was reduced by about 20% three months postlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was observed at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covered instead by glial processes. The detachment of synaptic endings was selective on abducens internuclear neurons since nearby motoneurons always showed a normal synaptic coverage. By one month, abducens internuclear neurons recovered a normal density of receiving axosomatic synapses. Anterogradely biocytin-labeled axon terminals of abducens internuclear neurons remained in place after the lesion of medial rectus motoneurons, although with a progressive decrease in density. Ultrastructural examination of the oculomotor nucleus 10 days after the lesion revealed numerous empty spaces left by the dead motoneurons. Targetless boutons were observed surrounded by large extracellular gaps, still apposed to remnanls of the postsynaptic membrane or, finally, ensheathed by glial processes. At longer intervals (> one month), the ultrastructure of the oculomotor nucleus was re-established and labeled boutons were observed contacting either unidentified dendrites within the neuropil or the soma and proximal dendrites of the oculomotor internuclear neurons that project to the abducens nucleus. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in control tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens internuclear neurons. Specifically, the oculomotor internuclear neurons, which likely suffer a partial deafferentation after medial rectus motoneuron loss, constitute a potential new target for the abducens internuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear neurons after medial rectus motoneuron lesion, which occurred with a similar time course, as described in the accompanying paper [de la Cruz R. R. et al. (1994) Neuroscience 58, 81–97.]

Activation of Cl? channels by avermectin in rat cultured hippocampal neurons

The actions of the insecticide avermectin (AVM) were studied in rat cultured hippocampal neurons with patch-clamp techniques. Application of micromolar concentrations of AVM to voltage-clamped cells gave rise to whole-cell currents, which showed a slow time-course of activation in the order of 10 s, and wash-out periods of typically 20 min. Dose-response curves revealed a half-maximally activating AVM concentration (EC50) of 2.0 +/- 0.6 microM and a Hill coefficient of 1.5 +/- 0.9. The current activated by AVM was carried predominantly by Cl- ions, as demonstrated by ion-substitution experiments. The Cl- channel blocker picrotoxinin (100 microM) substantially but transiently reduced the AVM response. Outside-out patch recording showed that AVM opened Cl- channels with a conductance of 40 +/- 12 pS. The open-time distribution was characterized by two time constants of 11 ms and 259 ms. It is suggested that AVM directly activates Cl- channels in mammalian central neurons, which resemble the channels activated by the physiological transmitters GABA and glycine.

Propofol Activates GABAA Receptor-Chloride Ionophore Complex in Dissociated Hippocampal Pyramidal Neurons of the Rat

The molecular mechanism of propofol anesthesia has been related to facilitation of the inhibitory neurotransmission mediated by gamma-aminobutyric acid (GABA). In the current study, the authors examined the direct actions of propofol on the acutely dissociated mammalian central neurons. Hippocampal pyramidal neurons were dissociated after enzymatic treatment of the brain slices of the rat. Single neurons were voltage-clamped using the whole cell configuration of the patch clamp technique, and drugs were applied with a rapid drug-application system. In the pyramidal neurons voltage-clamped at -60 mV, propofol evoked a transmembrane inward current, which desensitized at high concentrations of the anesthetic. The peak amplitude of the current increased sigmoidally with increasing doses of propofol applied. A least-squares fitting gave a dissociation constant of 1.2 x 10(-5) M and a Hill coefficient of 1.8, thereby indicating that clinical concentrations of propofol evoke the current, and that the anesthetic cooperatively activates the channel. The threshold concentration was less than 10(-6) M. The reversal potential for the current shifted according to the chloride equilibrium potential predicted by the Nernst equation, indicating that the current was carried by chloride ions. Bicuculline and strychnine suppressed the current in a concentration-dependent manner, in which the former was almost 40-fold more potent than the latter. The propofol-induced current cross-desensitized with the GABA-induced current, but no such interaction was observed with the glycine-induced current. Ro15-1788 (10(-6) M), an allosteric benzodiazepine antagonist, had no effect on the response. Diazepam (10(-6) M) enhanced the propofol-induced current, but pentobarbital (10(-6) M and 3 x 10(-5) M) did not affect the current. Propofol at clinically relevant concentrations directly activates the GABAA receptor-chloride ionophore complex in the mammalian central neurons and, hence, increases the chloride conductance, which may contribute to anesthesia produced by the agent. The desensitization of the GABAA receptor in the presence of high concentrations of propofol may result in a suppression of the GABAA inhibitory system.

Mechanosensitive channels transduce osmosensitivity in supraoptic neurons.

Vasopressin is a peptide hormone synthesized by neurons of the supraoptic and paraventricular nuclei, which project axon terminals to the neurohypophysis. Consistent with its antidiuretic properties, vasopressin release rises as a function of plasma osmolality, a response that results from accelerated action potential discharge. Previous studies have shown that increases in fluid osmolality depolarize supraoptic neurons in the absence of synaptic transmission, suggesting that these cells behave as intrinsic osmoreceptors. The mechanism by which changes in osmolality are transduced into an electrical signal is unknown, however. Here we report that changes in cell volume accompany physiological variations in fluid osmolality and that these modulate the activity of mechanosensitive cation channels in a way that is consistent with the macroscopic regulation of membrane voltage and action potential discharge. These findings define a function for stretch-inactivated channels in mammalian central neurons.

Expression of Kv2.1 delayed rectifier K + channel isoforms in the developing rat brain

Mammalian central neurons regulate a diverse set of functions by changes in spatial and temporal expression patterns of a large family of K + channel gene products. Here the temporal patterns of expression of Kv2.1 (drkl) K + channel mRNAs and polypeptides have been studied in rat brain. Northern blots using Kv2.1-specific probes show that three size classes (4.4, 9.0, 11.5 kb) of Kv2.1 K + channel transcripts are present in rat brain. These mRNAs show differential patterns of expression over embryonic and postnatal developmental, with the 4.4 kb transcript being the predominant embryonic Kv2.1 mRNA, and the 11.5 kb transcript being the predominant adult mRNA. Immunoblots using Kv2.1-specific site-directed antibodies show the existence of multiple Kv2.1 polypeptides, which differ in their mobility on SDS polyacrylamide gels, in their immunoreactivity to a carboxyl terminal-directed anti-Kv2.1 antibody, and in their developmental expression. Thus Kv2.1 polypeptide isoforms exhibit discrete temporal patterning during neuronal development, implying distinct roles for these channel proteins in vivo.

A simple method for internal perfusion of mammalian central nervous system neurones in brain slices with multiple solution changes

A simple system for internal perfusion of whole-cell patch-clamped neurones in brain slices allowing multiple, fast solution exchanges is described. With this system, perfusing solutions can be loaded immediately before use during the recording. At perfusion rates of 5-10 microliters/min, complete replacement of solutions in the patch pipette tip, as determined by changes in the pipette resistance and liquid junction potential, was achieved in less than 20 s. This occurred after a 1-min latency that was due to solution flow through the infusion tube. The effectiveness of the system was tested on rat hippocampal CA1 neurones in the slice preparation. The effects of replacement of internal 150 mM K+ by 150 mM Cs+ ions on voltage-activated K+ currents and of changing internal [Cl-] between 20 mM and 150 mM on evoked GABAA-mediated inhibitory postsynaptic currents (IPSCs) were studied. The blockade of K+ currents by Cs+ ions and the changes of IPSCs by altered internal [Cl-] ion concentration were achieved within 3.2 and 1.5 min, respectively, including the 'flow latency' of about 1 min, and recovery following solution change occurred within 5.2 and 1.5 min, respectively. More than 10 effective internal solution replacements could be performed within 1 h in a single neurone without affecting the recording stability.

Functional heterogeneity of calcium release by inositol trisphosphate in single Purkinje neurones, cultured cerebellar astrocytes, and peripheral tissues.

Purkinje neurones of the cerebellar cortex are rich in receptors for the Ca-mobilizing second messenger inositol trisphosphate (InsP3) in association with intracellular Ca stores. Cytosolic Ca ions are important in regulating neuronal excitability but it has proved difficult to demonstrate InsP3-evoked release of Ca in mammalian central neurones directly. Intracellular release of InsP3 by flash photolysis of caged InsP3, combined with whole-cell patch clamp and microspectrofluorimetry of Ca indicators, allows comparison of InsP3-evoked Ca release in single Purkinje cells in cerebellar slices with the same process in cultured astrocytes and peripheral tissues. In astrocytes, hepatocytes, exocrine cells, and vascular endothelium, minimal Ca release from stores requires photorelease of InsP3 at concentrations of 0.2-0.5 microM, and maximal efflux as judged by the rate of increase of Ca concentration is seen with 5-10 microM InsP3. In contrast in Purkinje cells, InsP3 concentrations of > or = 9 microM were required to produce minimal Ca release from stores under the same conditions, and Ca efflux increased with InsP3 concentrations up to 70-80 microM. Furthermore, the rate of increase and size of the Ca concentration in Purkinje cells are 10- to 30-fold greater than in astrocytes and peripheral tissues. The InsP3 sensitivity was not affected by changing exogenous cytosolic Ca buffering, suggesting that endogenous Ca binding cannot account for the difference. The results show a functional difference in InsP3-evoked Ca release between Purkinje cells and peripheral tissues.

Functional diversity of K + currents in hippocampal pyramidal neurons

Potassium channels contribute strongly to the rich variety of response properties among nerve cells. Hippocampal pyramidal cells may serve to illustrate how multiple K+ currents co-exist and function in a class of mammalian central neurons. In this brief review, six major voltage- and calcium-activated hippocampal K+ currents and their proposed functional roles are compared. Four of these K+ currents are activated by depolarization: IA, ID, IK and IM, whereas the two others are activated by intracellular calcium ions: IC and IAHP. There is evidence that IA, ID, IK, IC contribute to action potential repolarization, while IA, ID, IM, IC and IAHP control the excitability and discharge frequency in different ways. Several neurotransmitters act through modulation of these K+ channels. Recent evidence also suggests that activity-dependent expression of K+ channel genes may contribute to the diversity of response properties among neurons.

211 Direct visualization and physiological characterization of presynaptic terminals in cultured mammalian central neurons

211 Direct visualization and physiological characterization of presynaptic terminals in cultured mammalian central neurons