Modulation of a slowly inactivating potassium current, I(D), by metabotropic glutamate receptor activation in cultured hippocampal pyramidal neurons

Dopamine D2 Receptor-Mediated Modulation of Rat Retinal Ganglion Cell Excitability

Ganglion cells (RGCs) are the sole output neurons of the retinal circuity. Here, we investigated whether and how dopamine D2 receptors modulate the excitability of dissociated rat RGCs. Application of the selective D2 receptor agonist quinpirole inhibited outward K⁺ currents, which were mainly mediated by glybenclamide- and 4-aminopyridine-sensitive channels, but not the tetraethylammonium-sensitive channel. In addition, quinpirole selectively enhanced Nav1.6 voltage-gated Na⁺ currents. The intracellular cAMP/protein kinase A, Ca²⁺/calmodulin-dependent protein kinase II, and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling pathways were responsible for the effects of quinpirole on K⁺ and Na⁺ currents, while phospholipase C/protein kinase C signaling was not involved. Under current-clamp conditions, the number of action potentials evoked by positive current injection was increased by quinpirole. Our results suggest that D2 receptor activation increases RGC excitability by suppressing outward K⁺ currents and enhancing Nav1.6 currents, which may affect retinal visual information processing.

Ca2+ Channels Involvement in Low-Frequency Stimulation-Mediated Suppression of Intrinsic Excitability of Hippocampal CA1 Pyramidal Cells in a Rat Amygdala Kindling Model

Low-frequency stimulation has demonstrated promising seizure suppression in animal models of epilepsy, while the mechanism of the effect is still debated. Changes in intrinsic properties have been recognized as a prominent pathophysiologically relevant feature of numerous neurological disorders including epilepsy. Here, it was evaluated whether LFS can preserve the intrinsic neuronal electrophysiological properties in a rat model of epilepsy, focusing on the possible involvement of voltage-gated Ca²⁺ channels. The amygdala kindling model was induced by 3 s monophasic square wave pulses (50 Hz, 1 ms duration, 12times/day at 5 min intervals). Both LFS alone and kindled plus LFS (KLFS) groups received four packages of LFS (each consisting of 200 monophasic square pulses, 0.1 ms pulse duration at 1 Hz with the after discharge threshold intensity), which in KLFS rats was applied immediately after kindling induction. Whole-cell patch-clamp recordings were made in the presence of fast synaptic blockers 24 h after the last kindling stimulations or following kindling stimulations plus LFS application. In the KLFS group, both the rebound excitation and kindling-induced intrinsic hyperexcitability were decreased, associated with a regular intrinsic firing as indicated by a lower coefficient of variation. The amplitude of afterdepolarization (ADP) and its area under the curve were both decreased in the KLFS group compared to the kindled group. LFS prevented the increasing effect of kindling on Ca²⁺ currents in the KLFS group. Findings provided evidence for a novel form of epileptiform activity suppression by LFS in the presence of synaptic blockade possibly by decreasing Ca²⁺ currents.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

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.

Paeonol promotes hippocampal synaptic transmission: The role of the Kv2.1 potassium channel

Paeonol is a major constituent of the Chinese herb Moutan cortex radices. Recent studies report that paeonol has neuroprotective effects and improves impaired learning and memory. However, its underlying mechanisms by which paeonol contributes to synaptic transmission remain unclear. In this study, we found that paeonol increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and spontaneous excitatory postsynaptic currents (sEPSCs), but had no effect on the amplitude in rat hippocampal CA1 neurons. Similarly, the acetylcholinesterase (AChE) inhibitor rivastigmine increased the frequency of mEPSCs, but had no effect upon amplitude in rat hippocampal neurons. Rivastigmine also inhibited the delayed outward K⁺ currents in rat hippocampal CA1 neurons, but had no effect in nucleus ambiguus (NA) neurons. The Kv2 blocker guangxitoxin-1E increased the frequency of both mEPSCs and sEPSCs of rat hippocampal CA1 neurons, without affecting their amplitude. Our results suggest that paeonol and rivastigmine enhance spontaneous presynaptic transmitter release, which may be associated with the inhibition of the hippocampal Kv2 current and with therapeutic potential in neurotransmitter deficits found in Alzheimer's disease (AD). Moreover, our data also show that paeonol protects against Aβ_25-35-induced impairment of long-term potentiation (LTP) in mouse hippocampal neurons. However, guangxitoxin-1E failed to potentiate the evoked field excitatory postsynaptic potentials (fEPSPs), LTP and Aβ_25-35-induced impairment of LTP. These results indicate that paeonol may has the potential to improve learning and memory in AD. Interestingly, this effect is not involved in the inhibition of the hippocampal Kv2 current.

A comparison of the delayed outward potassium current between the nucleus ambiguus and hippocampus: sensitivity to paeonol

Whole-cell patch-clamp recordings investigated the electrophysiological effects of 2'-hydroxy-4'-methoxyacetophenone (paeonol), one of the major components of Moutan Cortex, in hippocampal CA1 neurons and nucleus ambiguus (NA) neurons from neonatal rats as well as in lung epithelial H1355 cells expressing Kv2.1 or Kv1.2. Extracellular application of paeonol at 100μM did not significantly affect the spontaneous action potential frequency, whereas paeonol at 300μM increased the frequency of spontaneous action potentials in hippocampal CA1 neurons. Paeonol (300μM) significantly decreased the tetraethylammonium-sensitive outward current in hippocampal CA1 neurons, but had no effect upon the fast-inactivating potassium current (IA). Extracellular application of paeonol at 300μM did not affect action potentials or the delayed outward currents in NA neurons. Paeonol (100μM) reduced the Kv2.1 current in H1355 cells, but not the Kv1.2 current. The inhibitor of Kv2, guangxitoxin-1E, reduced the delayed outward potassium currents in hippocampal neurons, but had only minimal effects in NA neurons. We demonstrated that paeonol decreased the delayed outward current and increased excitability in hippocampal CA1 neurons, whereas these effects were not observed in NA neurons. These effects may be associated with the inhibitory effects on Kv2.1 currents.

Suppression of outward K(+) currents by activating dopamine D1 receptors in rat retinal ganglion cells through PKA and CaMKII signaling pathways

Dopamine plays an important role in regulating neuronal functions in the central nervous system by activating the specific G-protein coupled receptors. Both D1 and D2 dopamine receptors are extensively distributed in the retinal neurons. In the present study, we investigated the effects of D1 receptor signaling on outward K(+) currents in acutely isolated rat retinal ganglion cells (RGCs) by patch-clamp techniques. Extracellular application of SKF81297 (10 μM), a specific D1 receptor agonist, significantly and reversibly suppressed outward K(+) currents of the cells, which was reversed by SCH23390 (10 μM), a selective D1 receptor antagonist. We further showed that SKF81297 mainly suppressed the glybenclamide (Gb)- and 4-aminopyridine (4-AP)-sensitive K(+) current components, but did not show effect on the tetraethylammonium (TEA)-sensitive one. Both protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways were likely involved in the SKF81297-induced suppression of the K(+) currents since either Rp-cAMP (10 μM), a cAMP/PKA signaling inhibitor, or KN-93 (10 μM), a specific CaMKII inhibitor, eliminated the SKF81297 effect. In contrast, neither protein kinase C (PKC) nor mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathway seemed likely to be involved because both the PKC inhibitor bisindolylmaleimide IV (Bis IV) (10 μM) and the MAPK/ERK1/2 inhibitor U0126 (10 μM) did not block the SKF81297-induced suppression of the K(+) currents. These results suggest that activation of D1 receptors suppresses the Gb- and 4-AP-sensitive K(+) current components in rat RGCs through the intracellular PKA and CaMKII signaling pathways, thus modulating the RGC excitability.

NMDA receptors and L-type voltage-gated Ca²⁺ channels mediate the expression of bidirectional homeostatic intrinsic plasticity in cultured hippocampal neurons

Homeostatic plasticity is engaged when neurons need to stabilize their synaptic strength and excitability in response to acute or prolonged destabilizing changes in global activity. Compared to the extensive studies investigating the molecular mechanisms for homeostatic synaptic plasticity, the mechanism underlying homeostatic intrinsic plasticity is largely unknown. Through whole-cell patch-clamp recording in low-density cultures of dissociated hippocampal neurons, we demonstrate here that prolonged activity blockade induced by the sodium channel blocker tetrodotoxin (TTX) leads to increased action potential firing rates. Conversely, prolonged activity enhancement induced by the A-type gamma-aminobutyric acid receptor antagonist bicuculline (BC) results in decreased firing rates. Prolonged activity enhancement also enhanced potassium (K(+)) current through Kv1 channels, suggesting that changes in K(+) current, in part, mediate stabilization of hippocampal neuronal excitability upon prolonged activity elevation. In contrast to the previous reports showing that L-type voltage-gated calcium (Ca(2+)) channels solely mediate homeostatic regulation of excitatory synaptic strength (Ibata et al., 2008; Goold and Nicoll, 2010), inhibition of N-Methyl-d-aspartate (NMDA) receptors alone mimics the elevation in firing frequency driven by prolonged TTX application, while the decrease in firing rates induced by prolonged BC treatment involves the activity of NMDA receptors and L-type voltage-gated Ca(2+) channels. These results collectively provide strong evidence that alterations in Ca(2+) influx through NMDA receptors and L-type voltage-gated Ca(2+) channels mediate homeostatic intrinsic plasticity in hippocampal neurons in response to prolonged activity changes.

Gastrodin inhibits allodynia and hyperalgesia in painful diabetic neuropathy rats by decreasing excitability of nociceptive primary sensory neurons

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus and adversely affects the patients' quality of life. Evidence has accumulated that PDN is associated with hyperexcitability of peripheral nociceptive primary sensory neurons. However, the precise cellular mechanism underlying PDN remains elusive. This may result in the lacking of effective therapies for the treatment of PDN. The phenolic glucoside, gastrodin, which is a main constituent of the Chinese herbal medicine Gastrodia elata Blume, has been widely used as an anticonvulsant, sedative, and analgesic since ancient times. However, the cellular mechanisms underlying its analgesic actions are not well understood. By utilizing a combination of behavioral surveys and electrophysiological recordings, the present study investigated the role of gastrodin in an experimental rat model of STZ-induced PDN and to further explore the underlying cellular mechanisms. Intraperitoneal administration of gastrodin effectively attenuated both the mechanical allodynia and thermal hyperalgesia induced by STZ injection. Whole-cell patch clamp recordings were obtained from nociceptive, capsaicin-sensitive small diameter neurons of the intact dorsal root ganglion (DRG). Recordings from diabetic rats revealed that the abnormal hyperexcitability of neurons was greatly abolished by application of GAS. To determine which currents were involved in the antinociceptive action of gastrodin, we examined the effects of gastrodin on transient sodium currents (I(NaT)) and potassium currents in diabetic small DRG neurons. Diabetes caused a prominent enhancement of I(NaT) and a decrease of potassium currents, especially slowly inactivating potassium currents (I(AS)); these effects were completely reversed by GAS in a dose-dependent manner. Furthermore, changes in activation and inactivation kinetics of I(NaT) and total potassium current as well as I(AS) currents induced by STZ were normalized by GAS. This study provides a clear cellular basis for the peripheral analgesic action of gastrodin for the treatment of chronic pain, including PDN.

The CatSper channel: a polymodal chemosensor in human sperm

The sperm-specific CatSper channel controls the intracellular Ca(2+) concentration ([Ca(2+)](i)) and, thereby, the swimming behaviour of sperm. In humans, CatSper is directly activated by progesterone and prostaglandins-female factors that stimulate Ca(2+) influx. Other factors including neurotransmitters, chemokines, and odorants also affect sperm function by changing [Ca(2+)](i). Several ligands, notably odorants, have been proposed to control Ca(2+) entry and motility via G protein-coupled receptors (GPCRs) and cAMP-signalling pathways. Here, we show that odorants directly activate CatSper without involving GPCRs and cAMP. Moreover, membrane-permeable analogues of cyclic nucleotides that have been frequently used to study cAMP-mediated Ca(2+) signalling also activate CatSper directly via an extracellular site. Thus, CatSper or associated protein(s) harbour promiscuous binding sites that can host various ligands. These results contest current concepts of Ca(2+) signalling by GPCR and cAMP in mammalian sperm: ligands thought to activate metabotropic pathways, in fact, act via a common ionotropic mechanism. We propose that the CatSper channel complex serves as a polymodal sensor for multiple chemical cues that assist sperm during their voyage across the female genital tract.

A post-burst after depolarization is mediated by group i metabotropic glutamate receptor-dependent upregulation of Ca(v)2.3 R-type calcium channels in CA1 pyramidal neurons

The excitability of hippocampal pyramidal neurons is regulated by activation of metabotropic glutamate receptors, an effect that is mediated by modulation of R-type calcium channels.

Activation of group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) regulates neural activity in a variety of ways. In CA1 pyramidal neurons, activation of group I mGluRs eliminates the post-burst afterhyperpolarization (AHP) and produces an afterdepolarization (ADP) in its place. Here we show that upregulation of Ca_v2.3 R-type calcium channels is responsible for a component of the ADP lasting several hundred milliseconds. This medium-duration ADP is rapidly and reversibly induced by activation of mGluR5 and requires activation of phospholipase C (PLC) and release of calcium from internal stores. Effects of mGluR activation on subthreshold membrane potential changes are negligible but are large following action potential firing. Furthermore, the medium ADP exhibits a biphasic activity dependence consisting of short-term facilitation and longer-term inhibition. These findings suggest that mGluRs may dramatically alter the firing of CA1 pyramidal neurons via a complex, activity-dependent modulation of Ca_v2.3 R-type channels that are activated during spiking at physiologically relevant rates and patterns.

The hippocampus is an essential structure in the brain for the formation of new declarative memories. Understanding the cellular basis of memory formation, storage, and recall in the hippocampus requires a knowledge of the properties of the relevant neurons and how they are modulated by activity in the neural circuit. For many years, we have known that various chemical neurotransmitters can modulate the electrical excitability of neurons in the hippocampus. Here, we report new experiments to reveal how the chemical neurotransmitter glutamate increases neuronal excitability. The effect we study is the conversion of the afterhyperpolarization (a cellular consequence of firing an action potential) to an afterdepolarization. We identified the metabotropic glutamate receptors involved in this conversion (receptors called mGluR1 and mGluR5) as well as the final target of modulation (R-type calcium channels composed of Ca_v2.3 subunits), which cause the neurons to exhibit altered excitability in the presence of glutamate. We also determined some of the intermediate steps between activation of the glutamate receptors and modulation of the calcium channels responsible for the change in excitability, offering further mechanistic insight into how synaptic transmission can regulate cellular and network activity.

Excitatory roles of protein kinase C in striatal cholinergic interneurons

Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.

The action potential in mammalian central neurons

The action potential of the squid giant axon is formed by just two voltage-dependent conductances in the cell membrane, yet mammalian central neurons typically express more than a dozen different types of voltage-dependent ion channels. This rich repertoire of channels allows neurons to encode information by generating action potentials with a wide range of shapes, frequencies and patterns. Recent work offers an increasingly detailed understanding of how the expression of particular channel types underlies the remarkably diverse firing behaviour of various types of neurons.

Dendritic D-type potassium currents inhibit the spike afterdepolarization in rat hippocampal CA1 pyramidal neurons

In CA1 pyramidal neurons, burst firing is correlated with hippocampally dependent behaviours and modulation of synaptic strength. One of the mechanisms underlying burst firing in these cells is the afterdepolarization (ADP) that follows each action potential. Previous work has shown that the ADP results from the interaction of several depolarizing and hyperpolarizing conductances located in the soma and the dendrites. By using patch-clamp recordings from acute rat hippocampal slices we show that D-type potassium current modulates the size of the ADP and the bursting of CA1 pyramidal neurons. Sensitivity to alpha-dendrotoxin suggests that Kv1-containing potassium channels mediate this current. Dual somato-dendritic recording, outside-out dendritic recordings, and focal application of dendrotoxin together indicate that the channels mediating this current are located in the apical dendrites. Thus, our data present evidence for a dendritic segregation of Kv1-like channels in CA1 pyramidal neurons and identify a novel action for these channels, showing that they inhibit action potential bursting by restricting the size of the ADP.

Mechanisms underlying type I mGluR-induced activation of lobster gastric mill neurons

In addition to ionotropic effects, glutamate and acetylcholine have metabotropic modulatory effects on many neurons. Here we show that in the stomatogastric ganglion of the lobster, glutamate, one of the main ionotropic neurotransmitters, modulates the excitability of gastric mill neurons. The neurons in this well-studied system produce rhythmic output to a subset of lobster foregut muscles. Recently, metabotropic glutamate receptor (mGluR) agonists were suggested as modulators of the rhythmic output, in addition to the previously described muscarinic modulation by acetylcholine. However, the cellular mechanisms responsible for these effects on the pattern are not known. Using intracellular recording methods and calcium imaging, we show that glutamate has an excitatory effect on specific neurons in the stomatogastric ganglion, which is mediated by mGluRs. Responses to the application of mGluR type I agonists are transient oscillations in the system, probably arising from network interactions. We show that the excitatory effect is sensitive to phospholipase-C and IP(3) and is G-protein dependent. The G-protein dependency was demonstrated by GDPbetaS and GTPgammaS injection into identified neurons. The depolarizations and oscillations were accompanied by an increase of intracellular Ca(2+) levels and correlated Ca(2+) oscillations. By using cyclopiazonic acid, an endoreticular Ca(2+) uptake inhibitor, we show that some internal calcium release may augment the response, but is not crucial for its production. Interestingly, although Ca(2+) concentration increase is typically associated with the phosphoinositide pathway, in the lobster, the Ca(2+) concentration increase-either voltage dependent or independent-cannot account for the observed depolarization.

Pharmacology of a slowly inactivating outward current in hippocampal CA3 pyramidal neurons

The pharmacology of a slowly inactivating outward current was examined using whole cell patch-clamp recordings in CA3 pyramidal cells of guinea pig hippocampal slices. The current had a low activation threshold (about -60 mV) and inactivated slowly (time constant of 3.4 +/- 0.5 s at -50 mV) and completely at membrane voltages depolarized to -50 mV. The slowly inactivating outward current was mainly mediated by K+ with a reversal potential close to the equilibrium potential for K+. The slowly inactivating outward current had distinct pharmacological properties: its time course was not affected by extracellular Cs+ (1 mM) or 4-AP (1-5 mM)-broad spectrum inhibitors of K+ currents and of inactivating K+ currents, respectively. The presence of extracellular Mn2+ (0.5-1 mM), which suppresses several Ca2+ -dependent K+ currents, also did not affect the slowly inactivating outward current. The current was partially suppressed by TEA (50 mM) and was blocked by intracellular Cs+ (134 mM). In addition, intracellular QX-314 (5 mM), a local anesthetic derivative, inhibited this current. The slowly inactivating outward current with its low activation threshold should be operational at the resting potential. Our results suggest that the transient outward current activated at subthreshold membrane potentials in hippocampal pyramidal cells consists of at least three components. In addition to the well-described A- and D-currents, the slowest decaying component reflects the time course of a distinct current, suppressible by QX-314.

Fluoxetine inhibits A-type potassium currents in primary cultured rat hippocampal neurons

The effects of fluoxetine (Prozac) on the transient A-currents (IA) in primary cultured hippocampal neurons were examined using the whole-cell patch clamp technique. Fluoxetine did not significantly decrease the peak amplitude of whole-cell K+ currents, but it accelerated the decay rate of inactivation, and thus decreased the current amplitude at the end of the pulse. For further analysis, IA and delayed rectifier K+ currents (IDR) were isolated from total K+ currents. Fluoxetine decreased IA (the integral of the outward current) in a concentration-dependent manner with an IC50 of 5.54 microM. Norfluoxetine, the major active metabolite of fluoxetine, was a more potent inhibitor of IA than was fluoxetine, with an IC50 of 0.90 microM. Fluoxetine (3 microM) inhibited IA in a voltage-dependent manner over the whole range of membrane potentials tested. Analysis of the time dependence of inhibition gave estimates of 34.72 microM(-1) s(-1) and 116.39 s(-1) for the rate constants of association and dissociation, respectively. The resulting apparent Kd was 3.35 microM, similar to the IC50 value obtained from the concentration-response curve. In current clamp configuration, fluoxetine (3 microM) induced depolarization of resting membrane potential and reduced the rate of action potential. Our results indicate that fluoxetine produces a concentration- and voltage-dependent inhibition of IA, and that this effect could affect the excitability of hippocampal neurons.

Presynaptic group I metabotropic glutamate receptors modulate synaptic transmission in the rat superior colliculus via 4-AP sensitive K(+) channels

1. Group I metabotropic glutamate receptors (mGluRs) are thought to be important modulators of neuronal function in the superior colliculus (SC). Here, we investigated the pharmacology and signalling mechanisms underlying group I mGluR-mediated inhibition of neuronal excitability and synaptic transmission in the rat SC slice. 2. The group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) potently depressed synaptically evoked excitatory postsynaptic potentials (EPSPs), currents (EPSCs), and action potentials in a dose-dependent manner (IC50: 6.3 microm). This was strongly reduced by the broad-spectrum antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mm, approximately 95% reduction), by the mGluR1 antagonist LY367385 (100 microm, approximately 80% reduction) but not by the mGluR5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP, 1-100 microm). 3. The putative mGluR5-specific agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 microm) also inhibited EPSPs. Interestingly, CHPG's actions were not blocked by MPEP, but LY367385 (100 microm) reduced the effect of CHPG by 50%. 4. Inhibition induced by DHPG was independent of phospholipase C (PLC)/protein kinase C pathways, and did not require intact intracellular Ca2+ stores. It was not abolished but enhanced by the GABAA antagonist bicuculline (5 microm), suggesting that DHPG's action was not due to facilitated inhibition or changes in neuronal network activity. 5. The K+ channel antagonist 4-aminopyridine (4-AP, 50-100 microm) converted the inhibitory effect of DHPG into facilitation. Paired-pulse depression was strongly reduced by DHPG, an effect that was also prevented by 4-AP. 6. Our data indicate that group I agonists regulate transmitter release, presumably via an autoreceptor in the SC. This receptor may be involved in adaptation to repetitive stimulation via a non-PLC mediated pathway.

Potassium currents during the action potential of hippocampal CA3 neurons

Central neurons have multiple types of voltage-dependent potassium channels, whose activation during action potentials shapes spike width and whose activation and inactivation at subthreshold voltages modulate firing frequency. We characterized the voltage-dependent potassium currents flowing during the action potentials of hippocampal CA3 pyramidal neurons and examined the susceptibility of the underlying channel types to inactivation at subthreshold voltages. Using acutely dissociated neurons that permitted rapid voltage clamp, action potentials recorded previously were used as the command voltage waveform, and individual components of potassium current were identified by pharmacological sensitivity. The overall voltage-dependent potassium current in the neurons could be split into three major components based on pharmacology and kinetics during step voltage pulses: I(D) (fast activating, slowly inactivating, and sensitive to 4-aminopyridine at 30 microm), I(A) (fast activating, fast inactivating, and sensitive to 4-aminopyridine at 3 mm), and I(K) (slowly activating, noninactivating, and sensitive to external TEA at 3-25 mm). The potassium current during the action potential was composed of approximately equal contributions of I(D) and I(A), with a negligible contribution of I(K). I(D) and I(A) had nearly identical trajectories of activation and deactivation during the action potential. Both I(A) and I(D) showed steady-state inactivation at subthreshold voltages, but maximal inactivation at such voltages was incomplete for both currents. Because of the major contribution of both I(D) and I(A) to spike repolarization, it is likely that modulation or partial inactivation at subthreshold voltages of either current can influence spike timing with minimal effect on spike width.

Three classes of pyramidal neurons in layer V of rat perirhinal cortex

Whole-cell recordings from 140 pyramidal neurons in layer V of rat perirhinal cortex (PR) revealed three distinct firing patterns: regular spiking (RS, 76%), burst spiking (BS, 9%), and late spiking (LS, 14%). LS neurons have not previously been reported in layer V of any cortical region. LS cells in layer V of PR exhibited delays of up to 12 s from onset of a depolarizing current step to spike threshold, followed by sustained firing. In contrast, pyramidal cells in layer V of other cortical regions contain only RS and BS cells. Within PR, the percentage of LS neurons in layer V differs markedly from what we previously observed in layers II/III (50% LS) and VI (90% LS). Morphologically, BS neurons in layer V of PR had thick primary apical dendrites that terminated in a tuft within layer I, whereas RS and LS cells had relatively thin primary apicals that terminated either diffusely or in a layer I tuft. At holding potentials near rest, PR neurons exhibited small (approximately 15 pA), inward, spontaneous postsynaptic currents (PSCs) that were indistinguishable among the three cell types. Currents evoked by minimal stimulation of layer I were about 2.8 times larger than the spontaneous PSCs. Evoked currents had unusually long onset latencies with little variation in latency, consistent with monosynaptic responses evoked by stimulation of unmyelinated fibers. The prevalence of LS cells in combination with the long-latency monosynaptically evoked PSCs suggested that PR is not a region of rapid throughput. This is consistent with anatomical data suggesting that PR is a higher-level association cortex. These data further advance an emerging picture of PR as a cortical region with a unique distribution of cell types different from other cortical regions.

Group I mGluRs increase excitability of hippocampal CA1 pyramidal neurons by a PLC-independent mechanism

Previous studies have implicated phospholipase C (PLC)-linked Group I metabotropic glutamate receptors (mGluRs) in regulating the excitability of hippocampal CA1 pyramidal neurons. We used intracellular recordings from rat hippocampal slices and specific antagonists to examine in more detail the mGluR receptor subtypes and signal transduction mechanisms underlying this effect. Application of the Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) suppressed slow- and medium-duration afterhyperpolarizations (s- and mAHP) and caused a consequent increase in cell excitability as well as a depolarization of the membrane and an increase in input resistance. Interestingly, with the exception of the suppression of the mAHP, these effects were persistent, and in the case of the sAHP lasting for more than 1 h of drug washout. Preincubation with the specific mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), reduced but did not completely prevent the effects of DHPG. However, preincubation with both MPEP and the mGluR1 antagonist LY367385 completely prevented the DHPG-induced changes. These results demonstrate that the DHPG-induced changes are mediated partly by mGluR5 and partly by mGluR1. Because Group I mGluRs are linked to PLC via G-protein activation, we also investigated pathways downstream of PLC activation, using chelerythrine and cyclopiazonic acid to block protein kinase C (PKC) and inositol 1,4,5-trisphosphate-(IP(3))-activated Ca(2+) stores, respectively. Neither inhibitor affected the DHPG-induced suppression of the sAHP or the increase in excitability nor did an inhibitor of PLC itself, U-73122. Taken together, these results argue that in CA1 pyramidal cells in the adult rat, DHPG activates mGluRs of both the mGluR5 and mGluR1 subtypes, causing a long-lasting suppression of the sAHP and a consequent persistent increase in excitability via a PLC-, PKC-, and IP(3)-independent transduction pathway.

Activation of group I mGluRs elicits different responses in murine CA1 and CA3 pyramidal cells

The group I metabotropic glutamate receptor agonist DHPG has been shown to produce two major effects on CA3 pyramidal cells at rest: a reduction in the background conductance and an activation of a voltage-gated inward current (I(mGluR(V))). Both effects contribute to depolarising CA3 pyramidal cells and the latter has been implicated in eliciting prolonged epileptiform population bursts. We observed that DHPG-induced depolarisation was smaller in CA1 pyramidal cells than in CA3 cells. Voltage clamp studies revealed that while DHPG elicited I(mGluR(V)) in CA3 pyramidal cells, such a response was absent in CA1 pyramidal cells. Both mGluR1 and mGluR5 have been localised in CA3 pyramidal cells, whereas only mGluR5 has been detected in CA1 pyramidal cells. Using mGluR1 knockout mice, we evaluated whether the absence of an I(mGluR(V)) response can be correlated with the absence of mGluR1. In these experiments, DHPG failed to elicit I(mGluR(V)) in CA3 pyramidal cells. This suggests that the smaller depolarising effects of DHPG on wild-type CA1 pyramidal cells is caused, at least in part, by the absence of I(mGluR(V)) in these cells and that the difference in the responses of CA1 and CA3 cells may be attributable to the lack of mGluR1 in CA1 pyramidal cells.

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.

Modulation of excitability by alpha-dendrotoxin-sensitive potassium channels in neocortical pyramidal neurons

Many neurons transduce synaptic inputs into action potentials (APs) according to rules that reflect their intrinsic membrane properties. Voltage-gated potassium channels, being numerous and diverse constituents of neuronal membrane, are important participants in neuronal excitability and thus in synaptic integration. Here we address the role of dendrotoxin-sensitive "D-type" potassium channels in the excitability of large pyramidal neurons in layer 5 of the rat neocortex. Low concentrations of 4-aminopyridine or alpha-dendrotoxin (alpha-DTX) dramatically increased excitability: the firing threshold for action potentials was hyperpolarized by 4-8 mV, and the firing frequency during a 1-sec-long 500 pA somatic current step was doubled. In nucleated outside-out patches pulled from the soma, alpha-DTX reversibly blocked a slowly inactivating potassium current that comprised approximately 6% of the total. This current first turned on at voltages just hyperpolarized to the threshold for spiking and activated steeply with depolarization. By assaying alpha-DTX-sensitive current in outside-out patches pulled from the axon and primary apical dendrite, it was found that this current was concentrated near the soma. We conclude that alpha-DTX-sensitive channels are present on large layer 5 pyramidal neurons at relatively low density, but their strategic location close to the site of action potential initiation in the axon may ensure that they have a disproportionate effect on neuronal excitability. Modulation of this class of channel would generate a powerful upregulation or downregulation of neuronal output after the integration of synaptic inputs.

Group I metabotropic glutamate receptors elicit epileptiform discharges in the hippocampus through PLCbeta1 signaling

Activation of metabotropic glutamate receptors (mGluRs) produces multiple effects in cortical neurons, resulting in the emergence of network activities including epileptiform discharges. The cellular mechanisms underlying such network responses are largely unknown. We examined the properties of group I mGluR-mediated cellular responses in CA3 neurons and attempted to determine their role in the generation of the network activities. Group I mGluR stimulation causes depolarization of hippocampal neurons. This depolarization is primarily mediated by two sets of conductance change: the opening of a voltage-dependent cationic conductance (mediating I(mGluR(V))) and the closing of a voltage-independent (background) K(+) conductance. I(mGluR(V)) was no longer elicited by group I mGluR agonists in the presence of U73122, a phospholipase C (PLC) blocker. Also, the current could not be activated in hippocampal CA3 neurons from PLCbeta1 knock-out mice. In contrast, suppression of PLC signaling did not affect the group I mGluR-mediated suppression of background K(+) conductance. Thus, the suppression of the background K(+) conductance occurred upstream to PLC activation, whereas the generation of I(mGluR(V)) occurred downstream to PLC activation. Group I mGluR agonists normally elicited rhythmic single cell and population burst responses in the CA3 neurons. In the absence of an I(mGluR(V)) response, CA3 neurons in slices prepared from PLCbeta1-/- mutant mice could no longer generate these responses. The results suggest that I(mGluR(V)) expression in CA3 hippocampal neuron is PLCbeta1-dependent and that I(mGluR(V)) plays a necessary role in the generation of rhythmic single cell bursts and synchronized epileptiform discharges in the CA3 region of the hippocampus.

Group I mGluR activation turns on a voltage-gated inward current in hippocampal pyramidal cells

A unique property of the group I metabotropic glutamate receptor (mGluR)-induced depolarization in hippocampal cells is that the amplitude of the depolarization is larger when the response is elicited at more depolarized membrane potentials. Our understanding of the conductance mechanism underlying this voltage-dependent response is incomplete. Through the use of current-clamp and single-electrode voltage-clamp recordings in guinea pig hippocampal slices, we examined the group I mGluR-induced depolarization in CA3 pyramidal cells. The group I mGluR agonists (S)-3-hydroxyphenylglycine and (S)-3,5-dihydroxyphenylglycine turned on a voltage-gated inward current (I(mGluR(V))), which was pharmacologically distinct from the voltage-gated sodium and calcium currents intrinsic to the cells. I(mGluR(V)) was a slowly activating, noninactivating current with a threshold at about -75 mV. In addition to the activation of I(mGluR(V)), group I mGluR stimulation also produced a voltage-independent decrease in the K(+) conductance. Our results suggest that the depolarization induced by group I mGluR activation is generated by two ionic mechanisms-a heretofore unrecognized voltage-gated inward current (I(mGluR(V))) that is turned on by depolarization and a voltage-insensitive inward current that results from a turn-off of the K(+) conductance. The low-threshold and noninactivating properties of I(mGluR(V)) allow the current to play a significant role in setting the resting potential and firing pattern of CA3 pyramidal cells.