Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Expression of Calretinin in the Cecal Muscularis Interna: Observation and Hypothetical Relevance to Appendicitis

BACKGROUND

The unexpected observation of calretinin immunoreactivity in smooth muscle cells in the muscularis propria of the cecum led to a more detailed examination of calretinin expression and its possible relationship to propulsive contractile activity around the vermiform appendix.

METHODS

Immunohistochemistry and RNA in situ hybridization were performed to analyze calretinin expression in intestinal samples from 33 patients at ages ranging from mid-gestation fetuses to adults, as well as in some potentially relevant animal models. Dual immunolabeling was done to compare calretinin localization with markers of smooth muscle and interstitial cells of Cajal.

RESULTS

Calretinin expression was observed consistently in the innermost smooth muscle layers of the muscularis interna in the human cecum, appendiceal base, and proximal ascending colon, but not elsewhere in the intestinal tract. Calretinin-positive smooth muscle cells did not co-express markers located in adjacent interstitial cells of Cajal. Muscular calretinin immunoreactivity was not detected in the ceca of mice or macaques, species which lack appendices, nor in the rabbit cecum or appendix.

CONCLUSIONS

Localized expression of calretinin in cecal smooth muscle cells may reduce the likelihood of retrograde, calcium-mediated propulsive contractions from the proximal colon and suppress pro-inflammatory fecal stasis in the appendix.

Dual effects of L-DOPA on nigral dopaminergic neurons

L-DOPA (Levodopa) remains the gold standard for the treatment of motor symptoms of Parkinson's disease (PD), despite indications that the drug may have detrimental effects in cell culture. Classically, l-DOPA increases the production of dopamine (DA) in nigral dopaminergic neurons, while paradoxically inhibiting the firing of these neurons due to activation of D2 autoreceptors by extracellularly released DA. Using a combination of electrophysiology and calcium microfluorometry in brain slices, we have identified a novel effect of L-DOPA on dopaminergic neurons when D2 receptors were blocked. Under these conditions, L-DOPA (0.03-3 mM) evoked an excitatory effect consisting of two components. The 'early' component observed during and immediately after application of the drug, was associated with increased firing, membrane depolarization and inward current. This excitatory response was strongly attenuated by CNQX (10 μM), pointing to the involvement of TOPA quinone, an auto-oxidation product of L-DOPA and a potent activator of AMPA/kainate receptors. The 'late' phase of excitation persisted >30 min after brief L-DOPA application and was not mediated by ionotropic glutamate receptors, nor by D1, α1-adrenergic, mGluR1 or GABAB receptors. It was eliminated by carbidopa, demonstrating its dependence on conversion of L-DOPA to DA. Exogenous DA (50 μM) also evoked a glutamate-receptor independent increase in firing and an inward current when D2 receptors were blocked. In voltage-clamped neurons, both L-DOPA and DA produced a long-lasting increase in [Ca(2+)]i which was unaffected by block of ionotropic glutamate receptors. These results demonstrate that L-DOPA has dual, inhibitory and excitatory, effects on nigral dopaminergic neurons, and suggest that the excitation and calcium rise may have long-lasting consequences for the activity and survival of these neurons when the expression or function of D2 receptors is impaired.

Effects of calretinin on Ca2+ signals in cerebellar granule cells: implications of cooperative Ca2+ binding

Calretinin is thought to be the main endogenous calcium buffer in cerebellar granule cells (GrCs). However, little is known about the impact of cooperative Ca(2+) binding to calretinin on highly localized and more global (regional) Ca(2+) signals in these cells. Using numerical simulations, we show that an essential property of calretinin is a delayed equilibration with Ca(2+). Therefore, the amount of Ca(2+), which calretinin can accumulate with respect to equilibrium levels, depends on stimulus conditions. Based on our simulations of buffered Ca(2+) diffusion near a single Ca(2+) channel or a large cluster of Ca(2+) channels and previous experimental findings that 150 μM 1,2-bis(o-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid (BAPTA) and endogenous calretinin have similar effects on GrC excitability, we estimated the concentration of mobile calretinin in GrCs in the range of 0.7-1.2 mM. Our results suggest that this estimate can provide a starting point for further analysis. We find that calretinin prominently reduces the action potential associated increase in cytosolic free Ca(2+) concentration ([Ca(2+)]( i )) even at a distance of 30 nm from a single Ca(2+) channel. In spite of a buildup of residual Ca(2+), it maintains almost constant maximal [Ca(2+)]( i ) levels during repetitive channel openings with a frequency less than 80 Hz. This occurs because of accelerated Ca(2+) binding as calretinin binds more Ca(2+). Unlike the buffering of high Ca(2+) levels within Ca(2+) nano/microdomains sensed by large conductance Ca(2+)-activated K(+) channels, the buffering of regional Ca(2+) signals by calretinin can never be mimicked by certain concentration of BAPTA under all different experimental conditions.

Pharmacological evidence of functional inhibitory metabotrophic glutamate receptors on mouse arousal-related cholinergic laterodorsal tegmental neurons

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) are importantly involved in neurobiological mechanisms governing states of arousal such as sleep and wakefulness as well as other appetitive behaviors, such as drug-seeking. Accordingly, mechanisms controlling their excitability are important to elucidate if we are to understand how these LDT neurons generate arousal states. Glutamate mediates the vast majority of excitatory synaptic transmission in the vertebrate CNS and while presence of glutamate input in the LDT has been shown and ionotropic responses to glutamate have been reported in the LDT, characterization of metabotropic responses is lacking. Therefore, electrophysiological responses and changes in levels of intracellular Ca(2+) in mouse cholinergic LDT neurons following application of specific mGluR agonists and antagonists were examined. Unexpectedly, both the mGluR(5)specific agonist, CHPG, and the group II mGluR (mGlu(2/3)) agonist, LY379268 (LY), induced a TTX-insensitive outward current/hyperpolarization. Both outward currents were significantly reduced by the mGluR antagonist MCPG and the CHPG-induced current was blocked by the specific mGluR(5) antagonist MTEP. Concurrent Ca(2+)imaging revealed that while CHPG actions did include release of Ca(2+) from CPA/thapsigargin-sensitive intracellular stores, actions of LY did not. Both CHPG- and LY-induced outward currents were mediated by a TEA-sensitive potassium conductance. The large-conductance, Ca(2+)-dependent potassium (BK) channel blocker, iberiotoxin, attenuated CHPG actions. Consistent with actions on the BK conductance, CHPG enhanced the amplitude of the fast component of the after hyperpolarizing potential, concurrent with a reduction in the firing rate. We conclude that stimulation of mGluR(5) and group II (mGluR(2/3)) elicits postsynaptically-mediated outward currents/hyperpolarizations in cholinergic LDT neurons. Effects of glutamatergic input would be, thus, expected not only to be excitation via stimulation of ionotropic glutamate receptors and mGluR(1), but also inhibition via actions at mGluR(5) and mGluR(2/3) on these neurons. As these two processes counteract each other, these surprising findings necessitate revision of predictions regarding the net level of excitation generated by glutamate input to cholinergic LDT cells and, by extension, the functional outcome of glutamate transmission on processes which these neurons regulate. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Diabetes as a risk factor for Alzheimer's disease: insulin signalling impairment in the brain as an alternative model of Alzheimer's disease

Surprisingly little is known about the mechanisms that trigger the onset of AD (Alzheimer's disease) in sporadic forms. A number of risk factors have been identified that may shed light on the mechanisms that may trigger or facilitate the development of AD. Recently, T2DM (Type 2 diabetes mellitus) has been identified as a risk factor for AD. A common observation for both conditions is the desensitization of insulin receptors in the brain. Insulin acts as a growth factor in the brain and is neuroprotective, activates dendritic sprouting, regeneration and stem cell proliferation. The impairment of this important growth factor signal may facilitate the development of AD. Insulin as well as other growth factors have shown neuroprotective properties in preclinical and clinical trials. Several drugs have been developed to treat T2DM, which re-sensitize insulin receptors and may be of use to prevent neurodegenerative processes in the brain. In particular, the incretins GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insolinotropic polypeptide) are hormones that re-sensitize insulin signalling. Incretins also have similar growth-factor-like properties as insulin and are neuroprotective. In mouse models of AD, GLP-1 receptor agonists reduce amyloid plaque formation, reduce the inflammation response in the brain, protect neurons from oxidative stress, induce neurite outgrowth, and protect synaptic plasticity and memory formation from the detrimental effects caused by β-amyloid production and inflammation. Other growth factors such as BDNF (brain-derived neurotrophic factor), NGF (nerve growth factor) or IGF-1 (insulin-like growth factor 1) also have shown a range of neuroprotective properties in preclinical studies. These results show that these growth factors activate similar cell signalling mechanisms that are protective and regenerative, and suggest that the initial process that may trigger the cascade of neurodegenerative events in AD could be the impairment of growth factor signalling such as early insulin receptor desensitization.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

The role of GLP-1 in neuronal activity and neurodegeneration

Type 2 diabetes has been identified as a risk factor for Alzheimer's disease (AD). The underlying mechanism behind this unexpected link is most likely linked to the observed desensitization of insulin receptors in the brain. Insulin acts as a growth factor in the brain and supports neuronal repair, dendritic sprouting, and differentiation. Several drugs have been developed to treat type 2 diabetes which re-synthesize insulin receptors and may be of use to prevent neurodegenerative developments in AD. The incretin glucagon-like peptide-1 (GLP-1) is a hormone that facilitates insulin release under high blood sugar conditions. Interestingly, GLP-1 also has very similar growth factor like properties as insulin, and has been shown to protect neurons from toxic effects. In preclinical studies, GLP-1 and longer lasting analogues reduce apoptosis, protect neurons from oxidative stress, induce neurite outgrowth, protect synaptic plasticity and memory formation from the detrimental effects of β-amyloid, and reduce plaque formation and the inflammation response in the brains of mouse models of AD. An advantage of GLP-1 is that it does not affect blood sugar levels in nondiabetic people. Furthermore, recent research has shown that some GLP-1 analogues can cross the blood-brain barrier, including two that are on the market as a treatment for type 2 diabetes. Therefore, GLP-1 analogues show great promise as a novel treatment for AD or other neurodegenerative conditions.

New roles for insulin-like hormones in neuronal signalling and protection: new hopes for novel treatments of Alzheimer's disease?

Type 2 diabetes has been identified as a risk factor for Alzheimer's disease (AD). This is most likely due to the desensitisation of insulin receptors in the brain. Insulin acts as a growth factor and supports neuronal repair, dendritic sprouting, and differentiation. This review discusses the potential role that insulin-like hormones could play in ameliorating the reduced growth factor signalling in the brains of people with AD. The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) have very similar properties in protecting neurons from toxic effects, and are capable of reversing the detrimental effects that beta-amyloid fragments have on synaptic plasticity. Therefore, incretins show great promise as a novel treatment for reducing degenerative processes in AD.

mGluR1 agonists elicit a Ca 2+ signal and membrane hyperpolarization mediated by apamin-sensitive potassium channels in immature rat purkinje neurons

The type 1 metabotropic glutamate receptor (mGluR1) plays an import role in the synaptic physiology and development of cerebellar Purkinje neurons. mGluR1 expression occurs early in the developmental program of Purkinje neurons, at an age that precedes expression of the dendritic structure. Few studies have investigated the physiological response produced by mGluR1 activation in early-developing Purkinje neurons. To address this question, simultaneous recording of membrane potential and intracellular Ca(2+) was performed in immature cultured Purkinje neurons coupled with exogenous application of mGluR1 agonists. Membrane potential was measured using the perforated patch method of whole-cell recording, and intracellular Ca(2+) was measured using fura-2-based Ca(2+) imaging. Brief, 1-sec micropressure application of the group I mGluR-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked a prominent Ca(2+) signal and coincident fast hyperpolarization in the immature neurons. The mGluR1-selective antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester blocked the Ca(2+) signal and fast hyperpolarization, confirming the involvement of mGluR1s. Amplitude of the fast hyperpolarization varied as a function of membrane potential and intracellular Ca(2+) and was blocked by apamin, an antagonist of the small-conductance Ca(2+)-activated K(+) channel (SK), identifying this K(+) channel as an underlying mechanism. In similar experiments with mature cultured Purkinje neurons, DHPG elicited a Ca(2+) signal, but fast membrane hyperpolarization was not evident. These results suggest that mGluR1 activation and the resulting release of Ca(2+) from intracellular stores and activation of SK channels may be a mechanism through which mGluR1 can modulate neuronal excitability of Purkinje neurons during early development.

Delayed rectifier outward K+ current mediates the migration of rat cerebellar granule cells stimulated by melatonin

Melatonin (MT) may work as a neuromodulator through the associated MT receptors in the central nervous system. Previously, our studies have shown that MT increased the I(K) current via a G protein-related pathway. In the present study, patch-clamp whole-cell recording, transwell migration assays and organotypic cerebellar slice cultures were used to examine the effect of MT on granule cell migration. MT increased the I(K) current amplitude and migration of granule cells. Meanwhile, TEA, the I(K) channel blocker, decreased the I(K) current and slowed the migration of granule cells. Furthermore, the effects of MT on the I(K) current and cell migration were not abolished by pre-incubation with P7791, a specific antagonist of MT(3)R, but were eliminated by the application of the MT(2)R antagonists K185 and 4-P-PDOT. I(K) current and cell migration were decreased by the application of dibutyryl cyclic AMP (dbcAMP), which was in contrast to the MT effect on the I(K) current and cell migration. Incubation with dbcAMP essentially blocked the MT-induced increasing effect. Moreover, incubation of isolated cell cultures in the MT-containing medium also decreased the cAMP immunoreactivity in the granule cells. It is concluded, therefore, that I(K) current, downstream of a cAMP transduction pathway, mediates the migration of rat cerebellar granule cells stimulated by MT.

Exchange protein activated by cAMP (Epac) mediates cAMP activation of p38 MAPK and modulation of Ca2+-dependent K+ channels in cerebellar neurons

The exchange factor directly activated by cAMP (Epac) is a newly discovered direct target for cAMP and a guanine-nucleotide exchange factor for the small GTPase Rap. Little is known about the neuronal functions of Epac. Here we show that activation of Epac by specific cAMP analogs or by the pituitary adenylate cyclase-activating polypeptide induces a potent activation of the Ca2+-sensitive big K+ channel, slight membrane hyperpolarization, and increased after-hyperpolarization in cultured cerebellar granule cells. These effects involve activation of Rap and p38 MAPK, which mobilizes intracellular Ca2+ stores. These findings reveal a cAMP Epac-dependent and protein kinase A-independent signaling cascade that controls neuronal excitability.

Shank expression is sufficient to induce functional dendritic spine synapses in aspiny neurons

Shank proteins assemble glutamate receptors with their intracellular signaling apparatus and cytoskeleton at the postsynaptic density. Whether Shank plays a role in spinogenesis and synaptogenesis remained unclear. Here, we report that knock-down of Shank3/prolinerich synapse-associated protein-2 by RNA interference reduces spine density in hippocampal neurons. Moreover, transgene expression of Shank 3 is sufficient to induce functional dendritic spines in aspiny cerebellar neurons. Transfected Shank protein recruits functional glutamate receptors, increases the number and size of synaptic contacts, and increases amplitude, frequency, and the AMPA component of miniature EPSCs, similar to what is observed during synapse developmental maturation. Mutation/deletion approaches indicate that these effects require interactions of Shank3 with the glutamate receptor complex. Consistent with this observation, chronic treatment with glutamate receptor antagonists alters maturation of the Shank3-induced spines. These results strongly suggest that Shank proteins and the associated glutamate receptors participate in a concerted manner to form spines and functional synapses.

Elevation of intracellular Ca2+ modulates A-currents in rat cerebellar granule neurons

In the brain, the transient-inactivating voltage-gated potassium channel currents (called I(K(A)) or A-currents) are activated at subthreshold membrane potentials to control the excitability of neurons. In the current study, the effect of intracellular calcium on the A-current and the action mechanism of intracellular calcium was investigated by using the whole-cell voltage-clamp technique. Elevation of intracellular calcium by addition of 2 mM CaCl2 in the pipette solution significantly modulated both the peak amplitude and the kinetics of the A-current in rat granule neurons. The peak amplitudes of the A-current were 1,060 +/- 87 pA and 1,972 +/- 16 pA under conditions of no Ca2+ and elevated intracellular Ca2+, respectively. The time to peak, the time course of fast inactivation, and the steady-state inactivation property of the A-current were all significantly altered by elevating the intracellular Ca2+. Replacement of the Ca2+ in the pipette solution with the same concentration of Co2+ did not mimic the effects of intracellular Ca2+ on the A-current amplitude and kinetics. These effects are similar to the behavior of the reconstituted Kv4/KChIP (K(V) channel-interacting proteins) current induced by expression of KChIP and Kv4 together in a cell expression system. Application of 10 microM arachidonic acid, which can bind to the Kv4/KChIP complex, inhibited the A-current and eliminated the effects of intracellular Ca2+ on the A-current, suggesting that KChIP may be involved in the effects of Ca2+ on the A-current. Collectively, our results indicate that elevated intracellular Ca2+ modulates the amplitude, fast activation, and steady-state inactivation characteristics of the A-current in rat cerebellar granule neurons, and this may occur via KChIP.

Diclofenac, a nonsteroidal anti-inflammatory drug, activates the transient outward K+ current in rat cerebellar granule cells

Diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), has been widely investigated in terms of its pharmacological action, but less is known about its direct effect on ion channels. Here, the effect of diclofenac on voltage-dependent transient outward K+ currents (I(A)) in cultured rat cerebellar granule cells was investigated using the whole-cell voltage-clamp technique. At concentrations of 10(-5)-10(-3) M, diclofenac reversibly increased the I(A) amplitude in a dose-dependent manner and significantly modulated the steady-state inactivation properties of the I(A) channels, but did not alter the steady-state activation properties. Furthermore, diclofenac treatment resulted in a slightly accelerated recovery from I(A) channel inactivation. Intracellular application of diclofenac could mimic the effects induced by extracellular application, although once the intracellular response reached a plateau, extracellular application of diclofenac could induce further increases in the current. These observations indicate that diclofenac might exert its effects on the channel protein at both the inner and outer sides of the cell membrane. Our data provide the first evidence that diclofenac is able to activate transient outward potassium channels in neurons. Although further work will be necessary to define the exact mechanism of diclofenac-induced I(A) channel activation, this study provides evidence that the nonsteroidal anti-inflammatory drug, diclofenac, may play a novel neuronal role that is worthy of future study.

A biophysical model of synaptic delay learning and temporal pattern recognition in a cerebellar Purkinje cell

It has been suggested that information in the brain is encoded in temporal spike patterns which are decoded by a combination of time delays and coincidence detection. Here, we show how a multi-compartmental model of a cerebellar Purkinje cell can learn to recognise temporal parallel fibre activity patterns by adapting latencies of calcium responses after activation of metabotropic glutamate receptors (mGluRs). In each compartment of our model, the mGluR signalling cascade is represented by a set of differential equations that reflect the underlying biochemistry. Phosphorylation of the mGluRs changes the concentration of receptors which are available for activation by glutamate and thereby adjusts the time delay between mGluR stimulation and voltage response. The adaptation of a synaptic delay as opposed to a weight represents a novel non-Hebbian learning mechanism that can also implement the adaptive timing of the classically conditioned eye-blink response.

Coassembly of big conductance Ca2+-activated K+ channels and L-type voltage-gated Ca2+ channels in rat brain

Based on electrophysiological studies, Ca(2+)-activated K(+) channels and voltage-gated Ca(2+) channels appear to be located in close proximity in neurons. Such colocalization would ensure selective and rapid activation of K(+) channels by local increases in the cytosolic calcium concentration. The nature of the apparent coupling is not known. In the present study we report a direct coassembly of big conductance Ca(2+)-activated K(+) channels (BK) and L-type voltage-gated Ca(2+) channels in rat brain. Saturation immunoprecipitation studies were performed on membranes labeled for BK channels and precipitated with antibodies against alpha(1C) and alpha(1D) L-type Ca(2+) channels. To confirm the specificity of the interaction, precipitation experiments were carried out also in reverse order. Also, additive precipitation was performed because alpha(1C) and alpha(1D) L-type Ca(2+) channels always refer to separate ion channel complexes. Finally, immunochemical studies showed a distinct but overlapping expression pattern of the two types of ion channels investigated. BK and L-type Ca(2+) channels were colocalized in various compartments throughout the rat brain. Taken together, these results demonstrate a direct coassembly of BK channels and L-type Ca(2+) channels in certain areas of the brain.

Modulation of afterpotentials and firing pattern in guinea pig CA3 neurones by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGluRs) alters the firing patterns of individual CA3 pyramidal cells in guinea pig hippocampal slices. Following addition of the selective group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) to the bathing solution, pyramidal cells initially firing regular, single action potentials switched to firing in brief bursts. This change in firing pattern resulted from modulation by mGluRs of three afterpotentials. The medium and slow afterhyperpolarizations (m and sAHPs) were blocked by mGluR activation. In addition, a voltage-dependent after depolarization (ADP) was induced. Recordings from mutant mice lacking phospholipase C(beta1) (PLC(beta1)) showed that mGluR block of the mAHP, as well as induction of the ADP, depended on the phosphoinositide hydrolysis pathway. Block of the sAHP, however, was partly spared in the absence of PLC(beta1). Optical recordings of post spike intracellular Ca(2+) rises showed that mGluR block of the AHP was not mediated by alterations of action potential-associated Ca(2+) increases (Ca(2+) transients). The mGluR induction of an ADP was also independent of any changes in the Ca(2+) transient. The mGluR-induced change in the firing pattern of hippocampal pyramidal cells is thus the result of multiple mechanisms, including suppression of both m and sAHPs and activation of an ADP, that act together to produce a specific excitatory effect, namely an increased likelihood that a single action potential will lead immediately to one or more following action potentials.

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.

Luzindole, a melatonin receptor antagonist, inhibits the transient outward K+ current in rat cerebellar granule cells

The inhibitory effect of the melatonin receptor antagonist luzindole on voltage-activated transient outward K(+) current (I(K(A))) was investigated in cultured rat cerebellar granule cells using the whole cell voltage-clamp technique. At the concentration of 1 microM to 1 mM, luzindole reversibly inhibited I(K(A)) in a concentration-dependent manner. In addition to reducing the current amplitude of I(K(A)),luzindole accelerated the fast inactivation of I(K(A)) channels and shifted the curves of voltage-dependent steady-state activation and inactivation of I(K(A)) by +6.6 mV and -7.0 mV, respectively. The inhibitory effect of luzindole was neither use-dependent nor voltage-dependent, suggesting that the binding affinity of luzindole to I(K(A)) channels is state-dependent. Including luzindole in the pipette solution, or extracellular application of 4 P-PDOT, an antagonist of melatonin receptors, did not change the luzindole-induced inhibitory effect on the I(K(A)) current, indicating that luzindole exerts its channel blocking inhibitory action at the extracellular mouth of the channel, and that the effect is not due to action of the melatonin receptors. Our data are the first demonstration that luzindole is able to block transient outward K(+) channels in rat cerebellar granule cells in a state-dependent manner, likely associated with extracellular interaction of the drug with the I(K(A)) inactivation gate.

Distribution of rSlo Ca2+-activated K+ channels in rat astrocyte perivascular endfeet

Evidence that Ca(2+)-activated K(+) (K(Ca)) channels play a role in cell volume changes and K(+) homeostasis led to a prediction that astrocytes would have K(Ca) channels near blood vessels in order to maintain K(+) homeostasis. Consistent with this thinking the present study demonstrates that rSlo K(Ca) channels are in glial cells of the adult rat central nervous system (CNS) and highly localized to specializations of astrocytes associated with the brain vasculature. Using confocal and thin-section electron microscopic immunolabeling methods the distribution of rSlo was examined in adult rat brain. Strong rSlo immunolabeling was present around the vasculature of most brain regions. Examination of dye-filled hippocampal astrocytes revealed rSlo immunolabeling polarized in astrocytic endfeet. Ultrastructural analysis confirmed that the rSlo staining was concentrated in astrocytic endfeet ensheathing capillaries as well as abutting the pia mater. Immunostaining within the endfeet was predominantly distributed at the plasma membrane directly adjacent to either the vascular basal lamina or the pial surface. The distribution of the aquaporin-4 (AQP-4) water channel was also examined using dye-filled hippocampal astrocytes. In confirmation of earlier reports, intense AQP-4 immunolabeling was generally observed at the perimeter of blood vessels, and coincided with perivascular endfeet and rSlo labeling. We propose that rSlo K(Ca) channels, with their sensitivity to membrane depolarization and intracellular calcium, play a role in the K(+) modulation of cerebral blood flow. Additional knowledge of the molecular and cellular machinery present at perivascular endfeet may provide insight into the structural and functional molecular elements responsible for the neuronal activity-dependent regulation of cerebral blood flow.

Homer-dependent cell surface expression of metabotropic glutamate receptor type 5 in neurons

The metabotropic glutamate (mGlu) receptors are a family of receptors involved in the tuning of fast excitatory synaptic transmission in the brain. Experiments performed in heterologous expression systems suggest that cell surface expression of group I mGlu receptors is controlled by their auxiliary protein, Homer. However, whether or not this also applies to neurons remains controversial. Here we show that in cultured cerebellar granule cells, the group I mGlu receptor subtype, mGlu5, transfected alone is functionally expressed at the surface of these neurons. Transfected Homer1b caused intracellular retention and clustering of this receptor at synaptic sites. Recombinant Homer1a alone did not affect cell surface expression of the receptor, but in neurons transfected with Homer1b, excitation-induced expression of native Homer1a reversed the intracellular retention of mGlu5 receptors, resulting in the receptor trafficking to synaptic membranes. Transfected Homer1a also increased the latency and amplitude of the mGlu5 receptor Ca2+ response. These results indicate that Homer1 proteins regulate synaptic cycling and Ca2+ signaling of mGlu5 receptors, in response to neuronal activity.

Characterization of the mGluR(1)-mediated electrical and calcium signaling in Purkinje cells of mouse cerebellar slices

The metabotropic glutamate receptor 1 (mGluR(1)) plays a fundamental role in postnatal development and plasticity of ionotropic glutamate receptor-mediated synaptic excitation of cerebellar Purkinje cells. Synaptic activation of mGluR(1) by brief tetanic stimulation of parallel fibers evokes a slow excitatory postsynaptic current and an elevation of intracellular calcium concentration ([Ca2+](i)) in Purkinje cells. The mechanism underlying these responses has not been identified yet. Here we investigated the responses to synaptic and direct activation of mGluR(1) using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca2+](i) in mouse Purkinje cells. Following pharmacological block of ionotropic glutamate receptors, two to six stimuli applied to parallel fibers at 100 Hz evoked a slow inward current that was associated with an elevation of [Ca2+](i). Both the inward current and the rise in [Ca2+](i) increased in size with increasing number of pulses albeit with no clear difference between the minimal number of pulses required to evoke these responses. Application of the mGluR(1) agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) by means of short-lasting (5-100 ms) pressure pulses delivered through an agonist-containing pipette positioned over the Purkinje cell dendrite, evoked responses resembling the synaptically induced inward current and elevation of [Ca2+](i). No increase in [Ca2+](i) was observed with inward currents of comparable amplitudes induced by the ionotropic glutamate receptor agonist AMPA. The 3,5-DHPG-induced inward current but not the associated increase in [Ca2+](i) was depressed when extracellular Na+ was replaced by choline, but, surprisingly, both responses were also depressed when bathing the tissue in a low calcium (0.125 mM) or calcium-free/EGTA solution. Thapsigargin (10 microM) and cyclopiazonic acid (30 microM), inhibitors of sarco-endoplasmic reticulum Ca2+-ATPase, had little effect on either the inward current or the elevation in [Ca2+](i) induced by 3,5-DHPG. Furthermore, the inward current induced by 3,5-DHPG was neither blocked by 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy] ethyl-1H-imidazole, an inhibitor of store operated calcium influx, nor by nimodipine or omega-agatoxin, blockers of voltage-gated calcium channels. These electrophysiological and Ca2+-imaging experiments suggest that the mGluR(1)-mediated inward current, although mainly carried by Na+, involves influx of Ca2+ from the extracellular space.

Agonist-independent activation of metabotropic glutamate receptors by the intracellular protein Homer

G-protein-coupled receptors (GPCRs) transduce signals from extracellular transmitters to the inside of the cell by activating G proteins. Mutation and overexpression of these receptors have revealed that they can reach their active state even in the absence of agonist, as a result of a natural shift in the equilibrium between their inactive and active conformations. Such agonist-independent (constitutive) activity has been observed for the glutamate GPCRs (the metabotropic glutamate receptors mGluR1a and mGluR5) when they are overexpressed in heterologous cells. Here we show that in neurons, the constitutive activity of these receptors is controlled by Homer proteins, which bind directly to the receptors' carboxy-terminal intracellular domains. Disruption of this interaction by mutagenesis or antisense strategies, or expression of endogenous Homer1a (H1a), induces constitutive activity in mGluR1a or mGluR5. Our results show that these glutamate GPCRs can be directly activated by intracellular proteins as well as by agonists.

Theta-frequency bursting and resonance in cerebellar granule cells: experimental evidence and modeling of a slow k+-dependent mechanism

Neurons process information in a highly nonlinear manner, generating oscillations, bursting, and resonance, enhancing responsiveness at preferential frequencies. It has been proposed that slow repolarizing currents could be responsible for both oscillation/burst termination and for high-pass filtering that causes resonance (Hutcheon and Yarom, 2000). However, different mechanisms, including electrotonic effects (Mainen and Sejinowski, 1996), the expression of resurgent currents (Raman and Bean, 1997), and network feedback, may also be important. In this study we report theta-frequency (3-12 Hz) bursting and resonance in rat cerebellar granule cells and show that these neurons express a previously unidentified slow repolarizing K(+) current (I(K-slow)). Our experimental and modeling results indicate that I(K-slow) was necessary for both bursting and resonance. A persistent (and potentially a resurgent) Na(+) current exerted complex amplifying actions on bursting and resonance, whereas electrotonic effects were excluded by the compact structure of the granule cell. Theta-frequency bursting and resonance in granule cells may play an important role in determining synchronization, rhythmicity, and learning in the cerebellum.

Choline blocks AMPA-induced dark cell degeneration of Purkinje neurons: potential role of the α7 nicotinic receptor

The objective of the present study was to assess the contribution of sodium influx to development of dark cell degeneration (DCD) in Purkinje neurons (PNs) following AMPA (DL-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid) receptor activation. During the course of these experiments, we observed inconsistent protection against DCD by Na(+) ion substitutes leading us to consider other potential mechanisms. A 30 min application of AMPA (30 microM, induction or trigger phase) followed by a 90-120 min AMPA-free expression period produced DCD in the majority of PNs. Substitution of NaCl with choline chloride (120 mM) produced a marked suppression of AMPA-induced toxicity. Suppression of DCD by choline was concentration dependent. Concentrations of choline as low as 10 mM effectively attenuated DCD when substituted on an equimolar basis for NaCl in the artificial cerebrospinal fluid (ACSF). Unlike choline, substitution of NMDG for NaCl failed to suppress AMPA-induced DCD. Lidocaine and TTX (tetrodotoxin), two agents that inhibit Na(+) influx failed to significantly alter DCD. Because choline is a prototypical alpha7 nicotinic receptor selective agonist, methyllycaconitine (MLA), an alpha7 receptor antagonist was tested and significantly attenuated the protective effects of choline in a concentration-dependent manner. Nicotine (100 microM) added to normal ACSF was effective in attenuating AMPA-induced toxicity. These findings suggest that DCD is not heavily dependent on Na(+)-mediated phenomena and that nicotinic alpha7 receptor activation may be neuroprotective against some types of excitotoxicity that are mediated by active cellular programs.

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

4-aminopyridine, a specific blocker of K(+) channels, inhibited inward Na(+) current in rat cerebellar granule cells

The effects of 4-aminopyridine (4-AP), a specific blocker of outward K(+) current, on voltage-activated transient outward K(+) current (I(K(A))) and inward Na(+) current (I(Na)) were investigated on cultured rat cerebellar granule cells using the whole cell voltage-clamp technique. At the concentration of 1-5 mM, 4-AP inhibited both I(K(A)) and I(Na). It reduced the amplitude of peak Na(+) current without significant alteration of the steady-state activation and inactivation properties. The inhibitory effect was not enhanced by repeated depolarizing pulses (0.5 or 0.1 Hz), suggesting that the binding affinity of 4-AP on Na(+) channels is state-independent. In contrast, the effect of 4-AP on Na(+) channels appeared to be voltage-dependent, the weaker inhibition occurred at more depolarization. Moreover, 4-AP slowed both the activation and inactivation kinetics of Na(+) current. These effects were similar to those induced by alpha-scorpion toxin and sea anemone toxins on Na(+) channels in other cell model. Our data demonstrate for the first time that 4-AP is able to block not only A-type K(+) channels, but also Na(+) channels in rat cerebellar granule cells. It is concluded that the inhibition exerted by 4-AP on Na(+) current likely differs from that provoked by local anesthetics. The possibility that the binding site of neurotoxin receptor 3 may be involved is discussed.

Inhibition of the glutamate-induced K(+) current in identified Onchidium neurons by nitric oxide donors

Nitric oxide (NO) acts as a neurotransmitter and neuromodulator in the nervous system of many vertebrates and invertebrates. The effects of extracellularly applied sodium nitroprusside (SNP) and diethylamine NO (C(2)H(5))(2)N[N(O)NO]-Na(+) (DEA/NO), NO donors, on a glutamate (Glu)-induced K(+) current in identified Onchidium neurons were investigated using voltage clamp and pressure ejection techniques. Bath-applied SNP (10 microM) and DEA/NO (5-10 microM) reduced the Glu-induced K(+) current without affecting the resting membrane conductance and holding current. The Glu-induced K(+) current also was inhibited by the focal application of SNP to the neuron somata. The suppressing effects of NO donors were concentration-dependent and completely reversible. Pretreatment with hemoglobin (50 microM), a nitric oxide scavenger, and 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 microM), a specific inhibitor of NO-stimulated guanylate cyclase, decreased the SNP-induced inhibition of the Glu-induced current. Bath-applied 50 microM 3-isobutyl-1-methylxanthine (IBMX), a nonspecific phosphodiesterase inhibitor, or intracellular injection of 1 mM guanosine 3',5'-cyclic monophosphate (cGMP) inhibited the Glu-induced current, mimicking the effect of NO donors. These results demonstrate that SNP and DEA/NO inhibit the Glu-induced K(+) current and that the mechanism of NO inhibition of the Glu-induced current involves cGMP-dependent protein kinase.

Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons

Metabotropic glutamate receptors (mGluRs) can increase intracellular Ca2+ concentration via Ins(1,4,5)P3- and ryanodine-sensitive Ca2+ stores in neurons. Both types of store are coupled functionally to Ca2+-permeable channels found in the plasma membrane. The mGluR-mediated increase in intracellular Ca2+ concentration can activate Ca2+-sensitive K+ channels and Ca2+-dependent nonselective cationic channels. These mGluR-mediated effects often result from mobilization of Ca2+ from ryanodine-sensitive, rather than Ins(1,4, 5)P3-sensitive, Ca2+ stores, suggesting that close functional interactions exist between mGluRs, intracellular Ca2+ stores and Ca2+-sensitive ion channels in the membrane.

Group I metabotropic glutamate receptors mediate an inward current in rat substantia nigra dopamine neurons that is independent from calcium mobilization

Metabotropic glutamate receptors modulate neuronal excitability via a multitude of mechanisms, and they have been implicated in the pathogenesis of neurodegenerative processes. Here we investigated the responses mediated by group I metabotropic glutamate receptors (mGluRs) in dopamine neurons of the rat substantia nigra pars compacta, using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca(2+)](i) and [Na(+)](i). The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) was bath-applied (20 microM, 30 s to 2 min) or applied locally by means of short-lasting (2-4 s) pressure pulses, delivered through an agonist-containing pipette positioned close to the cell body of the neuron. 3,5-DHPG evoked an inward current characterized by a transient and a sustained component, the latter of which was uncovered only with long-lasting agonist applications. The fast component coincided with a transient elevation of [Ca(2+)](i), whereas the total current was associated with a rise in [Na(+)](i). These responses were not affected either by the superfusion of ionotropic excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphono-pentanoic acid (D-APV), nor by the sodium channel blocker tetrodotoxin (TTX). (S)-alpha-methyl-4-carboxyphenylglycine (S-MCPG) and the more selective mGluR1 antagonist 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) depressed both 3,5-DHPG-induced inward current components and, although less effectively, the associated [Ca(2+)](i) elevations. On repeated agonist applications the inward current and the calcium transients both desensitized. The time constant of recovery from desensitization differed significantly between these two responses, being 67.4+/-4.4 s for the inward current and 28.6+/-2.7 s for the calcium response. Bathing the tissue in a calcium-free/EGTA medium or adding thapsigargin (1 microM) to the extracellular medium prevented the generation of the [Ca(2+)](i) transient, but did not prevent the activation of the inward current. These electrophysiological and fluorometric results show that the 3, 5-DHPG-induced inward current and the [Ca(2+)](i) elevations are mediated by independent pathways downstream the activation of mGluR1.

New perspectives for the development of selective metabotropic glutamate receptor ligands

The metabotropic glutamate receptors are GTP-binding-protein (G-protein) coupled receptors that play important roles in regulating the activity of many synapses in the central nervous system. As such, these receptors are involved in a wide number of physiological and pathological processes. Within the last few years, new potent and selective agonists and antagonists as well as radioligands acting on these receptors have been developed. Molecular modeling studies revealed the structural features of the glutamate binding site, and will be useful for the design of more selective and potent ligands. More interestingly, recent data revealed new regulatory sites on the receptor protein, able either to decrease or potentiate the action of the endogenous ligand. No doubt that in the near future a multitude of new tools to modulate the activity of these receptors will be discovered, enabling the identification of the possible therapeutic applications for these new neuroactive molecules.

A simple method to transfer plasmid DNA into neuronal primary cultures: functional expression of the mGlu5 receptor in cerebellar granule cells

We describe a method to transfer cDNA into neuronal primary cultures with a commercialised cationic lipid, Transfast. Cultures were transfected at a rate of about 5% with green fluorescent protein (GFP) cDNA. Comparing Transfast to other transfection reagents, we found this compound to be the most efficient. GFP-transfected mouse cerebellar granule cells displayed normal whole-cell voltage-sensitive and unitary big K+ channel currents. We also used this transfection method with success to transfer GFP cDNA into primary cultures of striatum and colliculus. Transfast was then used to cotransfect cultured cerebellar cells with GFP cDNA, in conjunction with cDNA coding for the metabotropic glutamate receptor type 5 (mGlu5 receptor). Ninety percent of the cells expressing GFP also expressed mGlu5 receptor. Though neurones were best transfected one day after plating, they still expressed both GFP and mGlu5 receptor proteins 2 weeks after plating, i.e. after full differentiation. A functional test of the expressed mGlu5 receptor was thus performed in GFP-transfected neurones. Stimulation of mGlu5 receptor induced single big K+ channel activity, as it was the case for the native mGlu1 receptor. This indicated that the transfected mGlu5 receptor plasmid was functionally expressed and that both mGlu1 and mGlu5 receptors may share common coupling mechanisms to big K+ channels in neurones.

Pharmacological characterization of an unusual mGluR-evoked neuronal hyperpolarization mediated by activation of GIRK channels

The metabotropic glutamate receptor (mGluR) agonist ACPD exerts an unusual inhibitory effect on a population of neurons of the song-control nucleus HVc of the zebra finch via activation of the GIRK channel. We report in the present study the pharmacology of this response. ACPD directly hyperpolarized the neurons by a mechanism independent of GABA(B) receptors. The group I mGluR agonist DHPG had no effect on membrane properties and the group I mGluR antagonist 4-CPG did not affect the ACPD-induced hyperpolarization. In contrast, the ACPD response was mimicked by the group II mGluR agonist LY314593 and the group II and III agonist L-CCG-I. The group II mGluR antagonist LY307452 fully antagonized the ACPD response and reduced the response induced by L-CCG-I. The group III mGluR agonist L-AP4 induced a small hyperpolarization, which was antagonized by the group III mGluR antagonist MAP-4. These data indicate that group II and group III mGluRs are present and functional in the postsynaptic membrane of these HVc neurons, and mediate the hyperpolarizing action of mGluR agonists. In contrast, group I mGluRs are absent from these neurons, nonfunctional, or coupled to different effector systems that do not influence membrane potential or input resistance.

Reconstitution of a voltage and calcium dependent potassium channel from rat cerebellum

Membrane vesicles from rat cerebellum were reconstituted into lipid bilayers. The activity of two different potassium channels was recorded: (1) a small conducting voltage dependent potassium channel insensitive to [Ca2+]i, (2) a calcium and voltage dependent potassium channel (KCa). KCa channels had a conductance of (302+/-15) pS (n=5) and were activated by [Ca2+]i and membrane depolarizations. They were blocked by tetraethylamonium (TEA) and charybdotoxin (CTX) but insensitive to noxiustoxin (NTX). Finally, we showed the blocking effect of Androctonus australis Hector (AaH) scorpion venom on KCa channels from rat cerebellum.