Pertussis toxin treatment increases glutamate release and dihydropyridine binding sites in cultured rat cerebellar granule neurons

Protein kinase C inhibitor chelerythrine attenuates the morphine-induced excitatory amino acid release and reduction of the antinociceptive effect of morphine in rats injected intrathecally with pertussis toxin

Neuropathic pain syndromes respond poorly to opioid treatment. In our previous studies, we found that intrathecal (i.t.) injection of pertussis toxin (PTX) produces thermal hyperalgesia, which is poorly responsive to morphine and is accompanied by an increase in cerebrospinal fluid (CSF) levels of excitatory amino acids (EAAs) and protein kinase C (PKC) activation. In the present study, rats were implanted with an i.t. catheter for drug injection and a microdialysis probe for CSF dialysate collection. On the fourth day after injection of PTX (2 microg, i.t.), there was a significant reduction in the antinociceptive effect of morphine (10 microg, i.t.) which was accompanied by an increase in levels of EAAs. Pretreatment with the PKC inhibitor, chelerythrine (25 microg, i.t.) one hour before morphine injection markedly inhibited both effects. These results suggest that, in PTX-treated rats, PKC plays an important role in inhibiting the morphine-induced spinal EAA release, which might be related to the reduced antinociceptive effect of morphine.

Selective block of the human 2-P domain potassium channel, TASK-3, and the native leak potassium current, IKSO, by zinc

Background potassium channels control the resting membrane potential of neurones and regulate their excitability. Two-pore-domain potassium (2-PK) channels have been shown to underlie a number of such neuronal background currents. Currents through human TASK-1, TASK-2 and TASK-3 channels expressed in Xenopus oocytes were inhibited by extracellular acidification. For TASK-3, mutation of histidine 98 to aspartate or alanine considerably reduced this effect of pH. Zinc was found to be a selective blocker of TASK-3 with virtually no effect on TASK-1 or TASK-2. Zinc had an IC(50) of 19.8 microM for TASK-3, at +80 mV, with little voltage dependence associated with this inhibition. TASK-3 H98A had a much reduced sensitivity to zinc suggesting this site is important for zinc block. Surprisingly, TASK-1 also has histidine in position 98 but is insensitive to zinc block. TASK-3 and TASK-1 differ at position 70 with glutamate for TASK-3 and lysine for TASK-1. TASK-3 E70K also had a much reduced sensitivity to zinc while the corresponding reverse mutation in TASK-1, K70E, induced zinc sensitivity. A TASK-3-TASK-1 concatamer channel was comparatively zinc insensitive. For TASK-3, it is concluded that positions E70 and H98 are both critical for zinc block. The native cerebellar granule neurone (CGN) leak current, IK(SO), is sensitive to block by zinc, with current reduced to 0.58 of control values in the presence of 100 microM zinc. This suggests that TASK-3 channels underlie a major component of IK(SO). It has recently been suggested that zinc is released from inhibitory synapses onto CGNs. Therefore it is possible that inhibition of IK(SO) in cerebellar granule cells by synaptically released zinc may have important physiological consequences.

Intrathecal pertussis toxin induces thermal hyperalgesia: involvement of excitatory and inhibitory amino acids

Intrathecal pertussis toxin injection has been used as a neuropathic pain model. In the present study, its effects on cerebrospinal fluid biochemistry and nociceptive behavioral expression were examined in rats. Cerebrospinal fluid dialysate samples were collected and pertussis toxin was injected using an intrathecally implanted dialysis loop catheter; samples were collected and hyperalgesia behavior was noted every 2 days for 8 days after pertussis toxin injection. Pertussis toxin injection induced thermal hyperalgesia which peaked between day 2 and 4; no cold allodynia was observed. Pertussis toxin at all doses tested (0.5, 1, or 2 microg) also induced a significant increase in cerebrospinal fluid concentrations of aspartate and glutamate between days 2 and 8, while level of the inhibitory amino acid glycine were significantly decreased by the two higher doses of pertussis toxin. Intrathecal administration of the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosponovaleric acid (10 microg) or glycine (200 microg), inhibited pertussis toxin-induced thermal hyperalgesia. Pertussis toxin injection had no effect on serine, glutamine, and taurine concentrations. These results show that intrathecal pertussis toxin injection induces thermal hyperalgesia and it is associated with an increasing of excitatory and a decreasing of inhibitory amino acids release in the spinal cord.

D-2-amino-5-phosphonopentanoic acid inhibits intrathecal pertussis toxin-induced thermal hyperalgesia and protein kinase Cgamma up-regulation

The aim of the present study was to examine the effect of intrathecal (i.t.) injection of pertussis toxin (PTX) on the nociceptive threshold and protein kinase C (PKC) expression in the rat spinal cord. The role of N-methyl-D-aspartic acid (NMDA) receptors in these changes was also examined. Male Wistar rats were implanted with two i.t. catheters, one of which was connected to a mini-osmotic pump and used to infuse saline or D-2-amino-5-phosphonopentanoic acid (D-AP5) (2 microg/h) starting on day 3 after i.t. catheter insertion. Two days later, a single injection of saline or PTX (2 microg) was given via the other catheter, followed by a flush with 10 microl of saline. On day 4 after PTX or saline injection, the thermal paw withdrawal latency was measured, then the rats were sacrificed by decapitation, and the dorsal part of the lumbosacral spinal segments was removed for PKC Western blotting assays. In PTX-treated rats, thermal hyperalgesia was observed, and the PKCgamma content of both the synaptosomal membrane and cytosolic fractions was significantly increased. The levels of alpha-, betaI-, or betaII-PKC isozymes in these fractions were unaffected by PTX treatment. Infusion of the NMDA antagonist, D-AP5, prevented both the thermal hyperalgesia and the increase in PKCgamma isoform expression in PTX-treated rats, and had no effect on these values in nai;ve rats. Intrathecal injection of the PKC inhibitor, chelerythrine (10 microg), significantly inhibited the thermal hyperalgesia observed in PTX-treated rats. These results show that i.t. injection of PTX induced thermal hyperalgesia accompanied by a selective increase in PKCgamma expression in both the synaptosomal membrane and cytosolic fractions of the dorsal horn of the rat lumbar spinal cord, and both effects were inhibited by the NMDA receptor antagonist, D-AP5.

Inhibition of the potassium current IK(SO), in cerebellar granule cells, by the inhibitors of MEK1 activation, PD 98059 and U 0126

IK(SO) is a standing-outward potassium current found in cerebellar granule neurons which is inhibited by the activation of muscarinic M(3) receptors. However the pathway between muscarinic receptor activation and current inhibition is unknown. Using two structurally distinct inhibitors of the activation of MEK1 (mitogen activated protein (MAP) kinase kinase 1), PD 98059 and U 0126, we have shown that the MAP kinase signalling cascade does not appear to underlie muscarinic inhibition of IK(SO), recorded using whole-cell patch-clamp methods. Nevertheless, both PD 98059 and U 0126 caused an inhibition of IK(SO) when applied acutely with 30 microM of each compound producing around 50% inhibition of the current. In addition, U 0125, which is structurally related to U 0126 but has a much lower potency for inhibiting MEK1 activation, was also able to inhibit IK(SO) to a similar degree. Neither the inhibition by PD 98059 nor that by U 0126 was found to be voltage dependent. This was true whether the IK(SO) current was outward or inward. Block of IK(SO) by these two compounds may compromise interpretation of studies in intact neuronal preparations when they are used as MEK1 inhibitors.

Differential effects of unaggregated and aggregated amyloid beta protein (1-40) on K(+) channel currents in primary cultures of rat cerebellar granule and cortical neurones

The effects of amyloid beta protein on voltage-gated K(+) channel currents were studied using the whole-cell patch-clamp technique. The 1-40 amino acid form of amyloid beta protein was applied to primary cultures of rat cerebellar granule and cortical neurones for 24 h. Both the unaggregated and aggregated forms of the peptide, which have differing biological activities, were used. In cerebellar granule neurones, 24-h pre-incubation with 1 microM unaggregated amyloid beta protein resulted in a 60% increase in the 'A'-type component of K(+) current. Increased delayed rectifier activity was Cd(2+)-sensitive and was presumed to be secondary to an increase in voltage-gated Ca(2+) channel current activity. Unaggregated amyloid beta protein had no effect on any component of the K(+) channel current in cortical neurones. One micromolar of aggregated amyloid beta protein had no effect on K(+) channel current in either cell type but reduced cell survival within 24 h as measured using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assays. The unaggregated form of amyloid beta protein had no neurotoxic effects when applied to either neurone type for up to 72 h. These data indicate that the unaggregated, non-pathological form of amyloid beta protein causes changes in the ion channel function of neurones, possibly reflecting a physiological role for the peptide.

Identification of galpha subtype(s) involved in gamma-aminobutyric acid(B) receptor-mediated high-affinity guanosine triphosphatase activity in rat cerebral cortical membranes

The ability of a series of specific Galpha carboxyl-terminal antisera, (i.e. anti-Gsalpha, anti-Gi1/2alpha, anti-Gi3alpha/Goalpha, anti-Goalpha/Gi3alpha, and anti-Gq/11alpha) to disrupt (+/-)-baclofen-stimulated high-affinity guanosine triphosphatase (GTPase) activity was explored in rat cerebral cortical membranes to identify the Galpha subunit(s) involved in gamma-aminobutyric acid(B) (GABA(B)) receptor-mediated signal transduction. Pretreatment of the membranes with the AS/7 (anti-Gi1/2alpha) antiserum inhibited GABA(B) receptor-mediated response without affecting the basal activity. The RM/1 (anti-Gsalpha) and QL (anti-Gq/11alpha) antisera failed to inhibit GABA(B) receptor-coupled responses. The results of the EC/2 (anti-Gi3alpha/Goalpha) and GO/1 (anti-Goalpha/Gi3alpha) antisera were difficult to interpret since the basal activities were influenced by these antisera. These results, in conjunction with the data in our previous reconstitution study, indicate that Gi2alpha is a main transducer of GABA(B) receptor-mediated signaling in rat cerebral cortex.

The role of Ca2+ stores in the muscarinic inhibition of the K+ current IK(SO) in neonatal rat cerebellar granule cells

Cerebellar granule neurons (CGNs) possess a standing outward potassium current (IK(SO)) which shares many similarities with current through the two-pore domain potassium channel TASK-1 and which is inhibited following activation of muscarinic acetylcholine receptors. The action of muscarine on IK(SO) was unaffected by the M2 receptor antagonist methoctramine (100 nM) but was blocked by the M3 antagonist zamifenacin, which, at a concentration of 100 nM, shifted the muscarine concentration-response curve to the right by around 50-fold. Surprisingly, M3 receptor activation rarely produced a detectable increase in [Ca2+]i unless preceded by depolarization of the cells with 25 mM K+. Experiments with thapsigargin and ionomycin suggested that the endoplasmic reticulum Ca2+ stores in CGNs were depleted at rest. In contrast, cerebellar glial cells in the same fields of cells possessed substantial endoplasmic reticulum Ca2+ stores at rest. Pretreatment of the cells with BAPTA AM, thapsigargin or the phospholipase C (PLC) inhibitor U-73122 all blocked the muscarine-induced Ca2+ signal but had little or no effect on muscarinic inhibition of IK(SO). Raising [Ca2+]i directly with ionomycin caused a small but significant inhibition of IK(SO). It is concluded that muscarine acts on M3 muscarinic acetylcholine receptors both to inhibit IK(SO) and to mobilize Ca2+ from intracellular stores in CGNs. While the mobilization of Ca2+ occurs through activation of PLC, this does not seem to be the primary mechanism underlying muscarinic inhibition of IK(SO).

Interleukin-1beta-dependent changes in the hippocampus following parenteral immunization with a whole cell pertussis vaccine

Neurological side effects are a major cause of concern following immunization with a number of vaccines, especially the whole cell pertussis vaccine (Pw). In this study we report that IL-1beta concentrations were significantly increased in the hippocampus following subcutaneous (s.c.) injection of Pw, and that this was accompanied by increased activity of the stress-activated kinase, c-Jun-N-terminal kinase (JNK) and a decrease in glutamate release. These effects were mimicked by s.c injection of active pertussis toxin (PT) or lipopolysaccharide (LPS). Incubation of hippocampal synaptosomes in the presence of Pw, PT or LPS also resulted in increased JNK activation and decreased glutamate release, effects which were mimicked by IL-1beta and blocked by the IL-1 receptor antagonist (IL-ra). Our observations are consistent with the hypothesis that IL-1beta induced by active bacterial toxins present in vaccine preparations, mediate the neurochemical and perhaps the neurological effects of Pw.

Inhibition of delayed rectifier K+ conductance in cultured rat cerebellar granule neurons by activation of calcium-permeable AMPA receptors

Activation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors in cerebellar granule cells during perforated-patch whole-cell recordings activated an inward current at negative voltages which was followed, after a delay, by the inhibition of an outward potassium current at voltages positive to -20 mV. The activated inward current was inwardly rectifying suggesting that the AMPA receptors were Ca2+-permeable. This was confirmed by direct measurements of intracellular calcium where Ca2+ rises were seen following AMPA receptor activation in Na+-free external solution. Ca2+ rises were equally large in the presence of 100 microM Cd2+ to block voltage-gated Ca2+ channels. Specific voltage-protocols, allowing selective activation of the delayed rectifier potassium current (KV) and the transient A current (KA), showed that kainate inhibited KV, but not to any great extent KA. The inhibition of KV was blocked by the AMPA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and was no longer observed when the KV current was abolished with high concentrations of Ba2+. The responses to kainate were not altered by pre-treating the cells with pertussis toxin, suggesting that the AMPA receptor stimulation of the G-protein Gi cannot account for the effects observed. Replacing extracellular Na+ with choline did not alter the inhibition of KV by kainate, however, removing extracellular Ca2+ reduced the kainate response. The inhibition of KV by kainate was unaffected by the presence of 100 microM Cd2+. The guanylyl cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), did not alter kainate inhibition of KV. It is concluded that ion influx (particularly Ca2+ ions) through AMPA receptor channels following receptor activation leads to an inhibition of KV currents in cerebellar granule neurons.

Entrapping of impermeant probes of different size into nonpermeabilized synaptosomes as a method to study presynaptic mechanisms

Small molecules present during brain tissue homogenization are known to be entrapped within subsequently isolated synaptosomes. We have revisited this technique in view of its systematic utilization to incorporate into nerve endings impermeant probes of large size. Rat neocortical synaptosomes were prepared in the absence or in the presence of each of the following compounds: 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), tetanus toxin (TeTx) or its light chain (TeTx-LC), pertussis toxin (PTx), anti-syntaxin, or anti-SNAP25 monoclonal antibodies. Release of endogenous GABA and glutamate was then evoked by high K+ depolarization. GABA and glutamate overflows were inhibited by entrapped BAPTA and in synaptosomes prepared by homogenization in the presence of varying concentrations of TeTx or TeTx-LC. When synaptobrevin cleavage in synaptosomes entrapped with TeTx was monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by western blotting, the extent of proteolysis was found to correspond quantitatively to that of release inhibition. GABA and glutamate overflows were increased by entrapped PTx; moreover, (-)-baclofen inhibited amino acid overflow more potently in standard than in PTx-containing synaptosomes. The overflows of GABA and glutamate were similarly decreased following incorporation of anti-syntaxin or anti-SNAP25 antibodies. Synaptosomal entrapping may be routinely used to internalize membrane-impermeant agents of different size in studies of presynaptic mechanisms.

Possible involvement of GABA(A) and GABA(B) receptors in the inhibitory action of lindane on transmitter release from cerebellar granule neurons

Cerebellar granule cells in culture express receptors for GABA belonging to the GABA(A) and GABA(B) classes. In order to characterize the ability of the insecticide lindane to interact with these receptors cells were grown in either plain culture media or media containing 150 microM THIP as this is known to influence the properties of both GABA(A) and GABA(B) receptors. It was found that lindane regardless of the culture condition inhibited evoked (40 mM K+) release of neurotransmitter ([3H]D-aspartate as label for glutamate). In naive cells both GABA(A) and GABA(B) receptor active drugs prevented the inhibitory action of lindane but in THIP treated cultures none of the GABA(A) and GABA(B) receptor active drugs had any effect on the inhibitory action of lindane. This lack of effect was not due to inability of baclofen itself to inhibit transmitter release. It is concluded that lindane dependent on the state of the GABA(A) and GABA(B) receptors is able to indirectly interfere with both GABA(A) and GABA(B) receptors. In case of the latter receptors it was shown using [3H]baclofen to label the receptors that lindane could not displace the ligand confirming that lindane is likely to exert its action at a site different from the agonist binding site.

Identification of the amino terminus of neuronal Ca2+ channel alpha1 subunits alpha1B and alpha1E as an essential determinant of G-protein modulation

We have examined the basis for G-protein modulation of the neuronal voltage-dependent calcium channels (VDCCs) alpha1E and alpha1B. A novel PCR product of alpha1E was isolated from rat brain. This contained an extended 5' DNA sequence and was subcloned onto the previously cloned isoform rbEII, giving rise to alpha1Elong whose N terminus was extended by 50 amino acids. VDCC alpha1 subunit constructs were co-expressed with the accessory alpha2-delta and beta2a subunits in Xenopus oocytes and mammalian (COS-7) cells. The alpha1Elong showed biophysical properties similar to those of rbEII; however, when G-protein modulation of expressed alpha1 subunits was induced by activation of co-expressed dopamine (D2) receptors with quinpirole (100 nM) in oocytes, or by co-transfection of Gbeta1gamma2 subunits in COS-7 cells, alpha1Elong, unlike alpha1E(rbEII), was found to be G-protein-modulated, in terms of both a slowing of activation kinetics and a reduction in current amplitude. However, alpha1Elong showed less modulation than alpha1B, and substitution of the alpha1E1-50 with the corresponding region of alpha1B1-55 produced a chimera alpha1bEEEE, with G-protein modulation intermediate between alpha1Elong and alpha1B. Furthermore, deletion of the N-terminal 1-55 sequence from alpha1B produced alpha1BDeltaN1-55, which could not be modulated, thus identifying the N-terminal domain as essential for G-protein modulation. Taken together with previous studies, these results indicate that the intracellular N terminus of alpha1E1-50 and alpha1B1-55 is likely to contribute to a multicomponent site, together with the intracellular I-II loop and/or the C-terminal tail, which are involved in Gbetagamma binding and/or in subsequent modulation of channel gating.

Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

The role of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in controlling the neuronal release of excitatory amino acids has been investigated. Stimulation of presynaptic AMPA receptors by the endogenous agonist L-glutamate, or by (R,S)-AMPA, dose-dependently enhanced the Ca(2+)-dependent, tetrodotoxin-insensitive, electrically-stimulated release of [3H]D-aspartate from rat forebrain slices. This AMPA receptor-mediated response showed marked stereoselectivity with the activity residing solely in the (S)-isomer. (R)-AMPA was inactive in this respect. AMPA-evoked responses were significantly enhanced in the presence of the AMPA receptor desensitization inhibitor, cyclothiazide (10 microM). Moreover, responses to both AMPA and glutamate were inhibited by competitive (NBQX) and non-competitive (GYKI 52466) AMPA receptor-selective antagonists in a dose-dependent manner. These results provide strong support for the existence of presynaptic AMPA receptors acting to enhance the synaptic release of excitatory amino acids in the mammalian forebrain. Such a positive feedback system may play an important functional role in physiological (e.g., long-term potentiation) and/or pathological (e.g., epileptogenesis) processes in the mammalian central nervous system. AMPA-type autoreceptors may provide new targets for drug action.

Inhibition of AMPA receptor-stimulated 57Co2+ influx by D- and L-2-amino-4-phosphonobutanoic acid (D- and L-AP4) and L-serine-O-phosphate (L-SOP) in cultured cerebellar granule cells

This study describes the inhibition of 57Co2+ influx through Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, consequent to the application of L-2-amino-4-phosphonobutanoic acid (L-AP4), D-AP4 and L-serine-O-phosphate (L-SOP) in cultured cerebellar granule cells. The forskolin-stimulated accumulation of cyclic AMP was inhibited by (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1) with an IC50 = 491 +/- 135 nM and by L-AP4 in a biphasic manner (IC50(1) = 232 +/- 61 nM and IC50(2) = >300 microM), confirming the presence of group II and group III mGlu receptors, respectively. 57Co2+ influx was stimulated by kainate (EC50 = 42.2 +/- 11.3 microM) and, in the presence of 30 microM cyclothiazide, by (S)-5-fluorowillardiine (EC50 = 0.7 +/- 0.1 microM) and (S)-AMPA (EC50 = 2.8 +/- 0.5 microM). The effects of the latter were abolished by 10 microM 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). L-AP4 (IC50 = >300 microM), D-AP4 (IC50 = >100 microM) and L-SOP (IC50 = 199 +/- 6 microM) inhibited 6 microM (S)-AMPA-stimulated 57Co2+ influx, whereas L-CCG-1 (up to 10 microM), 300 microM (RS)-3,5-dihydroxyphenylglycine, 300 microM (+/-)-baclofen and 1 mM carbachol were ineffective. Pre-incubation with either pertussis toxin (250 ng/ml, 48 hr), 1 mM dibutyryl cyclic AMP, or the potent group III mGlu receptor antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine ((RS)-CPPG), tested at 400 microM, failed to alter the inhibition of AMPA receptor activity by 300 microM L-SOP. Unlike 10 microM NBQX, neither L-AP4, D-AP4 or L-SOP (tested at 1 mM) inhibited the binding of 10 nM (S)-[3H]5-fluorowillardiine (a selective AMPA receptor ligand) to granule cell membranes. Therefore, in these neurones, high concentrations (>100 microM) of L-AP4, L-SOP and D-AP4 inhibit Ca2+-permeable AMPA receptors by a mechanism distinct from known mGlu receptor action and at a site independent from that for AMPA receptor agonists.

Signalling functions and biochemical properties of pertussis toxin-resistant G-proteins

Pertussis toxin (PTX) has been widely used as a reagent to characterize the involvement of heterotrimeric G-proteins in signalling. This toxin catalyses the ADP-ribosylation of specific G-protein alpha subunits of the Gi family, and this modification prevents the occurrence of the receptor-G-protein interaction. This review focuses on the biochemical properties and signalling of those G-proteins historically classified as 'PTX-resistant' due to the inability of the toxin to influence signalling through them. These G-proteins include members of the Gq and G12 families and one Gi family member, i.e. Gz. Signalling pathways controlled by these G-proteins are well characterized only for Gq family members, which activate specific isoforms of phospholipase C, resulting in increases in intracellular calcium and activation of protein kinase C (PKC), among other responses. While members of the G12 family have been implicated in processes that regulate cell growth, and Gz has been shown to inhibit adenylate cyclase, the specific downstream targets to these G-proteins in vivo have not been clearly established. Since two of these proteins, G12 alpha and Gz alpha, are excellent substrates for PKC, there is the potential for cross-talk between their signalling and Gq-dependent processes leading to activation of PKC. In tissues that express these G-proteins, a number of guanine-nucleotide-dependent, PTX-resistant, signalling pathways have been defined for which the G-protein involved has not been identified. This review summarizes these pathways and discusses the evidence both for the participation of specific PTX-resistant G-proteins in them and for the regulation of these processes by PKC.

Effects on K+ currents in rat cerebellar granule neurones of a membrane-permeable analogue of the calcium chelator BAPTA

1. Whole cell recordings of voltage-activated K+ currents were made with the amphotericin B perforated patch technique from cerebellar granule (CG) neurones of 6-8 days rats that had been in culture for 1 to 16 days. By use of appropriate voltage protocols, the effects of the membrane-permeant form of BAPTA, 1,2-bis-(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), on the transient A current (IKA), the delayed rectifier current (IKV) and a standing outward current (IKSO) were investigated. 2. Bath application of 25 microM BAPTA-AM inhibited both IKV and IKSO in cultured neurones, but did not seem to affect IKA. Neither 25 microM BAPTA (free acid) nor 25 microM ethylenediaminetetraacetic acid acetoxymethyl ester (EDTA-AM) had any significant effect on the magnitude of IKSO. Similarly in short-term (1-2 days) cultured CG neurones IKV, but not IKA, was inhibited by 25 microM BAPTA-AM. 3. BAPTA-AM (2.5 microM) reduced IKV in short-term culture CG neurones, with further inhibition being seen when the perfusate was changed to one containing 25 microM BAPTA-AM. 4. Tetraethylammonium ions (TEA) (10 mM) reversibly inhibited IKV in these cells with a similar rate of block of IKV to that induced by 25 microM BAPTA-AM. 5. The degree of inhibition of IKV by 25 microM BAPTA-AM was both time- and voltage-dependent, in contrast to the inhibition of this current by TEA. 6. These data indicate that BAPTA-AM reduces K+ currents in cerebellar granule neurones and that this inhibition cannot be explained in terms of intracellular Ca2+ chelation, but is a direct effect on the underlying channels.

Effects of pentoxifylline or dexamethasone in combination with amphotericin B in experimental murine cerebral cryptococcosis: evidence of neuroexcitatory pathogenic mechanisms

In a murine model of intracerebral infection by Cryptococcus neoformans the therapeutic effects of pentoxifylline or dexamethasone were studied alone and in combination with amphotericin B. Assessed parameters were mean survival time, brain histopathology index, amounts of glutamate and gamma-aminobutyric acid in the brain, and yeast CFU per brain. Survival increased significantly in mice treated with dexamethasone, amphotericin B, amphotericin B plus dexamethasone, and amphotericin B plus pentoxifylline; the latter had significantly longer survival than other treated groups. Indices of histopathological damage were similar in all treated groups. In infected untreated mice, the amounts of glutamate in the brain were decreased, presumably by depletion. In mice treated with amphotericin B plus dexamethasone, glutamate levels returned to the range of control mice. No differences in the amounts of gamma-aminobutyric acid were found between control and treatment groups. Brain fungal counts were significantly lower in mice treated with amphotericin B, amphotericin B plus dexamethasone, and amphotericin B plus pentoxifylline than in untreated animals. In this model, pentoxifylline in combination with amphotericin B improved survival, decreasing the fungal burden, and has potential as adjuvant therapy in cerebral cryptococcosis.

A non-inactivating K+ current sensitive to muscarinic receptor activation in rat cultured cerebellar granule neurons

1. Whole-cell recordings were made from cultured cerebellar granule neurons using perforated patch clamp techniques. The primary cultures were prepared using 6- to 9-day-old Sprague-Dawley rats. 2. Neurons in culture for less than 48 h possessed resting membrane potentials of -29 mV. However, neurons in culture for 7 days had much more hyperpolarized resting membrane potentials (-89 mV). Over the same period, these neurons developed an additional component of outward current. 3. This non-inactivating current was activated by depolarization, exhibited outward rectification and reversed close to the potassium equilibrium potential. The kinetics of activation and deactivation were very rapid. 4. Muscarine ((+)-muscarine chloride) reversibly inhibited the current with an EC50 of 0.17 microM. The inhibition by muscarine was unaffected by pre-incubation for 17-20 h with 120 micrograms ml-1 pertussis toxin. 5. The current and its inhibition by muscarine were unaffected by 100 microM Cd2+. In Ca(2+)-free conditions, the current was significantly larger than in 0.5 mM Ca2+, but inhibition by 10 microM muscarine was significantly reduced. 6. The standing outward current was not obviously affected by 50 microM 5-HT, 50 microM noradrenaline, 50 microM 2-chloroadenosine or 5 mM tetraethylammonium. It was reduced by 10 microM La3+, 10 microM Zn2+ and 1 mM Ba2+. 7. Muscarinic agonists increased the input resistance of neurons and shifted the zero current level in the depolarized direction when voltage clamped. This enhanced excitability was evident under current clamp, where 10 microM muscarine depolarized granule neurons such that action potentials became evident.

Facilitation of Ca2+ current in excitable cells

Voltage-dependent Ca2+ channels are one of the main routes for the entry of Ca2+ into excitable cells. These channels are unique in cell-signalling terms in that they can transduce an electrical signal (membrane depolarization) via Ca2+ entry into a chemical signal, by virtue of the diverse range of intracellular Ca(2+)-dependent enzymes and processes. In a variety of cell types, currents through voltage-dependent Ca2+ channels can be increased in amplitude by a number of means. Although the term facilitation was originally defined as an increase of Ca2+ current resulting from one or a train of prepulses to depolarizing voltages, there is a great deal of overlap between facilitation by this means and enhancement by other routes, such as phosphorylation.

Assessment of a mutation in the H5 domain of Girk2 as a candidate for the weaver mutation

A mutation in the GIRK2 inwardly rectifying K+ channel was mapped recently to the region of mouse chromosome 16 containing the wv gene and shown to occur in mutant but not in wild-type mice. We demonstrate tight linkage of the Girk2 mutation to the wv phenotype and refine the localization of the weaver (wv) gene on recombinational and physical maps. This linkage between Girk2 and wv has existed since at least 1988 in descendants of the original mutation maintained in C57BL/6 animals. Girk2 is shown to be transcribed in brain before the first recognized manifestation of the wv phenotype and in cultures of granule cells (GCs) isolated from cerebellum at postnatal day 8. Wild-type GCs grown in this culture system display an important developmental property--the ability to extend neurites. However, no inwardly rectifying K+ current is detected in GCs cultured from either wv/wv or +/+ cerebellum under a variety of conditions that activate related channels in other tissues. This suggests that if the Girk2 mutation is responsible for the wv phenotype, it does not act by altering these electrical properties of developing GCs.

The involvement of multiple calcium channel sub-types in glutamate release from cerebellar granule cells and its modulation by GABAB receptor activation

In this study, we have examined both the ability of various Ca2+ channel sub-types to support the release of [3H]glutamate from cerebellar granule neurons and the mechanism of action involved in the modulation of glutamate release by the GABAB receptor agonist, (-)-baclofen. Cerebellar granule neurons were stimulated to release newly synthesized [3H]glutamate by K(+)-evoked depolarization. Stimulated release was entirely calcium-dependent and abolished by the presence of 200 microM cadmium. Release of glutamate was not affected by either tetrodotoxin or 5-aminophosphonovaleric acid but was potentiated by dihydrokainate and inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione. Stimulated glutamate release was partially inhibited by both the L-type calcium channel blocker, nicardipine, and the N-type calcium channel blocker, omega-conotoxin GVIA; however, the P/Q-type calcium channel blocker omega-agatoxin IVA inhibited release of glutamate only after pre-incubation of cells with omega-conotoxin GVIA. K(+)-stimulated release of glutamate was observed when stimulated either in the presence of Ca2+ or of Ba2+ and similar inhibition of release by (-)-baclofen was seen under both conditions. In contrast to these results, ionomycin-evoked glutamate release was greatly reduced as compared to K(+)-evoked release and was not modulated by (-)-baclofen. In the presence of omega-conotoxin GVIA alone, inhibition of release by (-)-baclofen was attenuated but not abolished. Following block of nicardipine-sensitive channels, inhibition of release by (-)-baclofen was still present, and after prior block of omega-conotoxin GVIA-sensitive channels the presence of nicardipine restored the ability of (-)-baclofen to inhibit residual release of glutamate. Modulation of glutamate release by (-)-baclofen was unaffected by the presence of omega-agatoxin IVA alone; however, after block of both omega-conotoxin GVIA- and omega-agatoxin IVA-sensitive channels, inhibition of release by (-)-baclofen was completely abolished. These results indicate that multiple sub-types of voltage-dependent calcium channels are present on the presynaptic terminals of cerebellar granule neurons and support K(+)-stimulated release of [3H]glutamate. Modulation of release by GABAB receptor activation appears to be dependent upon interaction of this receptor with a number of voltage-sensitive calcium channels, including omega-conotoxin GVIA-sensitive and omega-agatoxin IVA-sensitive channels.

Co-existence and interaction between facilitatory and inhibitory metabotropic glutamate receptors and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals

We have investigated the interaction between facilitatory and inhibitory metabotropic glutamate receptors (mGluRs) and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals from young (3 weeks postnatal) rats. The adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) (1 microM) and the mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) (100 microM) inhibited Ca(2+)-dependent release of glutamate evoked by depolarization of synaptosomes with 30 mM KCl to 33 +/- 6 and 30 +/- 4% of control values, respectively. The CHA and L-AP4 inhibition of release was consistent with the reduction of a component of Ca2+ entry in nerve terminals which was also sensitive to omega-Aga-IVA. When the inhibitory agonists were co-applied at optimal concentrations, no additivity of the inhibitory effects on either glutamate release or [Ca2+]c was observed. The nerve terminals from young rats also exhibit the facilitatory pathway for glutamate release that is observed during 4-aminopyridine-evoked depolarization after stimulation of mGluRs with the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) in the presence of arachidonic acid (AA). The addition of ACPD or AA alone did not alter the ability of CHA and L-AP4 to reduce the release, however the co-application of AA and ACPD abolished the inhibitory effect induced by CHA and L-AP4 whether alone or in combination. These results indicate the co-existence of the three modulatory pathways of glutamate release and the dominant role of the ACPD/AA activated facilitatory pathway in its interaction with the inhibitory pathways activated by L-AP4 and CHA.

Characterization of Ca2+ channel currents in cultured rat cerebellar granule neurones

1. High-threshold voltage-gated calcium channel currents (IBa) were studied in cultured rat cerebellar granule neurones using the whole-cell patch clamp technique with 10 mM Ba2+ as the charge carrier. The putative P-type component of whole-cell current was characterized by utilizing the toxin omega-agatoxin IVA (omega-Aga IVA) in combination with other blockers. 2. omega-Aga IVA (100 nM) inhibited the high voltage-activated (HVA) IBa by 40.9 +/- 3.4% (n = 27), and the dissociation constant Kd was 2.7 nM. Maximal inhibition occurred within a 2-3 min time course, and was irreversible. The isolated omega-Aga IVA-sensitive current was non-inactivating. 3. omega-Aga IVA exhibited overlapping selectivity with both N- and L-channel blockers; omega-conotoxin GVIA (omega-CTX GVIA) (1 microM) and the dihydropyridine (-)-202-709 (1 microM), respectively. Together these toxins reduced the omega-Aga IVA-sensitive component to just 4.5 +/- 1.4% (n = 3). Thus only a small proportion of the current can be unequivocally attributed to P-type current. Inhibition of the HVA IBa by omega-Aga IA also reduced the proportion of omega-Aga IVA-sensitive current to 28.0 +/- 3.2% (n = 3). 4. Application of omega-Aga IVA and a synthetic form of funnel-web toxin, N-(7-amino-4-azaheptyl)-L-argininamide (sFTX-3.3; 10 microM), produced an additive block of the HVA IBa. Consequently these two toxins do not act on the same channel in cerebellar granule neurones. 5. omega-Aga IVA inhibition of low voltage-activated (LVA) IBa was studied in the ND7-23 neuronal cell line. omega-Aga IVA (100 nM) reduced the LVA current by 41.3 +/- 3.2% (n = 17) in a fully reversible manner with no shift in the steady-state inactivation of the channel. 6. A component of current insensitive to N-, L- and P-channel blockers remained unclassified in all our studies. This component, and also that remaining following block by omega-Aga IVA and omega-Aga IA, exhibited relatively rapid, although incomplete, inactivation compared to the other currents isolated in this study. 7. In conclusion, omega-Aga IVA inhibits a component of current in cultured cerebellar granule neurones which overlaps almost completely with that inhibited by L- and N-channel blockers. In addition, a large component of whole-cell current in these neurones still remains unclassified.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Role of GABAB receptors in intracellular Ca2+ homeostasis and possible interaction between GABAA and GABAB receptors in regulation of transmitter release in cerebellar granule neurons

The expression of GABAB receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [3H](S,R)-baclofen as well as in functional assessment of the ability of (R)-baclofen to interact with depolarization (15-40 mM KCl) coupled changes in intracellular Ca2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABAA and GABAB receptors was investigated. The binding studies showed that the granule cells express specific binding sites for (R)-baclofen. The number of binding sites could be increased by exposure of the cells to the GABAA receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABAB receptors are coupled to G-proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45Ca2+ uptake was measured or the intracellular Ca2+ concentration ([Ca2+]i) determined using the fluorescent Ca2+ chelator Fluo-3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABAB receptors as (R)-baclofen inhibited depolarization-induced increases in 45Ca2+ uptake and [Ca2+]i. (R)-Baclofen also inhibited K(+)-induced transmitter release from the neurons as monitored by the use of [3H]D-aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABAA receptor agonist isoguvacine it could be demonstrated that the GABAB receptors are functionally coupled to GABAA receptors in the neurons leading to a disinhibitory action of GABAB receptor agonists.

Modulation of the gating of the transient outward potassium current of rat isolated cerebellar granule neurons by lanthanum

The effects of the trivalent cation, lanthanum (La3+) on voltage-dependent K+ conductances were studied in rat isolated cerebellar granule neurons under whole-cell voltage-clamp conditions. La3+ at low micromolar concentrations caused a pronounced enhancement in the outward current evoked by depolarising steps from -50 mV, with the apparent recruitment of an inactivating component. The steady-state inactivation curve for the transient outward current, evoked by depolarising steps from -140 mV, was shifted by approximately 40 mV in the depolarising direction by 10 microM La3+, with a slight increase in the slope factor. The kinetics of activation and inactivation were slowed in the presence of La3+. A shift of 10 mV in the depolarising direction was seen for the activation curve of the delayed rectifier current in the presence of 10 microM La3+. These results indicate that La3+ has a potent effect on the gating characteristics of voltage-activated K+ currents. This effect cannot be explained by surface charge considerations.

Tonic inhibition of neuronal calcium channels by G proteins removed during whole-cell patch-clamp experiments

The barium current through voltage-dependent calcium channels was recorded from cultured rat cortical neurons with the whole-cell configuration of the patch-clamp technique. The maximal current evoked by depolarising pulses from -80 mV to 0 mV was divided into inactivating and non-inactivating fractions. During the first minutes of whole-cell recording, the amplitude of the inactivating fraction increased from less than 0.1 nA to an average value of 1 nA, whereas the amplitude of the non-inactivating component remained essentially the same. This increase in amplitude was prevented when the "perforated-patch technique" was used, suggesting that some intracellular factor that inhibited the barium current was lost or destroyed during conventional whole-cell experiments. When GTP[gamma-S] or GTP was added to the pipette solution, no increase or only a weak rise of the inactivating current was seen, whereas GDP[beta-S] accelerated its increase. The results suggest that some of the calcium channels expressed in cultured cortical neurons are inhibited by a G protein even in the absence of added neurotransmitter. The current increase observed during whole-cell recordings may be due to a loss of intracellular GTP and the subsequent inactivation of an inhibitory G protein.

Cycloheximide abolishes pertussis toxin-induced increase in glutamate release from cerebellar granule neurones

Release of glutamate from cerebellar granule neurones was stimulated either by adding 50 mM K+ to normal Krebs medium, or by adding 5 mM Ca2+ to neurones continuously depolarised with 50 mM K+ in the absence of Ca2+. Pre-incubation of neurones for 16 h with pertussis toxin (PTX) increased the stimulated glutamate release in both K(+)-stimulated and continuously depolarised neurones. Under both conditions, the PTX-induced increase in release was abolished by cycloheximide. In contrast, in the presence of cycloheximide, PTX still prevented the GABAB agonist (-)-baclofen from inhibiting glutamate release. These results suggest that G-protein ADP-ribosylation by PTX in cerebellar granule neurones may increase synthesis of a protein associated with the L-type calcium channel.

Ca2+ currents in cerebellar granule neurones: role of internal Mg2+ in altering characteristics and antagonist effects

Using the whole-cell patch-clamp technique, Ca2+ channel currents were measured in cultured rat cerebellar granule neurones in the presence of 10 mM Ba2+. Two different solutions were used to fill patch pipettes, one containing mainly tetraethylammonium acetate (TEA-Ac solution), and the other mainly caesium and HEPES (Cs-HEPES solution). Under these two different intracellular conditions markedly different Ca2+ channel currents were recorded. When TEA-Ac solution was used intracellularly, small, Cd(2+)-sensitive inward currents (approx. -55 pA) that were inhibited by the dihydropyridine antagonist (-)-202-791 and the GABAB agonist (-)-baclofen were observed. These currents were insensitive to the Ca2+ channel clocking toxins omega-conotoxin GVIA (omega-CgTX) and omega-agatoxin IVA and were enhanced by the dihydropyridine agonist (+)-202-791. In contrast, when the Cs-HEPES solution was used, currents were 2-3 times larger (approx. -130 pA), inhibited by (-)-202-791, omega-CgTX and omega-agatoxin IVA but were unaffected by (-)-baclofen. Furthermore, both (+)-202-791 and Bay K8644 in the presence of Cs-HEPES solution produced only a transient enhancement that was followed by an inhibition. Analysis of steady-state inactivation revealed two components of current in both cases, with similar voltage dependencies. The factor(s) giving rise to these differences were investigated in terms of current amplitude and responses to (-)-baclofen and omega-CgTX and were found to be mainly due to the concentrations of Mg2+ and ATP added to the patch pipette solutions. Furthermore, free internal Mg2+ concentrations of greater than 0.2 mM selectively inhibited omega-CgTX-sensitive Ca2+ channels. Preliminary evidence indicates that the same may be true of omega-Aga IVA-sensitive P-type current. These data suggest that the N-type Ca2+ channels in these cells are preferentially inhibited by intracellular Mg2+ and this may provide an explanation for discrepancies between the results of different groups investigating Ca2+ channel currents in similar cell types.