Actions of arginine polyamine on voltage and ligand-activated whole cell currents recorded from cultured neurones

KINETIC MODELS ON ACETYLCHOLINESTERASE MODULATION BY SELF-SUBSTRATE AND POLYAMINES: ESTIMATION OF INTERACTION PARAMETERS AND RATE CONSTANTS FOR FREE AND ACETYLATED STATES OF THE ENZYME

Polyamines act as dual modulators on electric eel acetylcholinesterase, modifying both the apparent Km and Ki, depending on substrate levels. A kinetic model was developed to explain the results, based on two-step catalysis, a peripheral site for substrate inhibition apart from the catalytic site, and one binding site for polyamine. This model presented the best fittings to data, when compared with a simpler one considering one catalytic step. A fitting equation built up with sixteen independent parameters let us calculate the kinetic constants. In this way, we were able to solve the parameter identifiability problem arising from model uncertainty when only substrate was used in acetylcholinesterase kinetics. Besides, fitting parameters directly provide information about the binding constants of the different complexes, the modulatory strength of substrate and polyamines, and the effect on the standard activation free energy for acetylcholinesterase.

Substrate inhibition operates mainly on the first catalytic step with an affinity constant of 5.2 mM-1, which is reduced to one third for the acetylated enzyme. The interaction factor between substrate binding at both sites is about 12. The modulatory strength of polyamines is spermine > spermidine > putrescine. This order is directly related to the number of amino groups in the molecule, and to the calculated free interaction energy. The effect of the number of amino groups on the binding energy is significantly increased in acetylated acetylcholinesterase. It is also inferred that the formation of a quaternary complex enzyme-substrate-substrate-polyamine would not be possible. Some relations between polyamine structure and acetylcholinesterase activity are suggested from estimated constants. Due to the distal amino group distances, it is possible for spermine and spermidine to span along the catalytic gorge of acetylcholinesterase, binding to the catalytic and peripheral sites in a way similar to bisquaternary ammonium inhibitors.

Targeting ionotropic receptors with polyamine-containing toxins☆

This review summarises current knowledge of polyamine-containing spider toxins and their interactions with ionotropic receptors of invertebrate and vertebrate excitable cells. Their diverse actions on ionotropic glutamate and acetylcholine receptors, which include potentiation, closed channel block and open channel block, are discussed in the context of toxin and target structures. Factors that complicate attempts to identify and pharmacologically characterise the binding sites for these toxins include their ability to permeate channels of some ionotropic receptors and their apparent accumulation in a cellular compartment, possibly the membrane bilayer.

Characterisation of a novel class of polyamine-based neuroprotective compounds

Prolonged cerebral ischaemia initiates complex intra- and inter-cellular signalling cascades ultimately resulting in neuronal death. Well-characterised mediators of ischaemic cell death are glutamate, free radicals and nitric oxide. Many drugs that block these mechanisms are neuroprotective in vitro, but have unfavourable side-effect profiles in man. We have recently demonstrated that the compound L-arginyl-3,4-spermidine (L-Arg3,4) is neuroprotective in vitro through an interaction with several of these mechanisms, and prevents ischaemic neurodegeneration in vivo with no gross side effects. In this study, we have used solid-phase combinatorial chemistry, to synthesise a number of analogues of L-Arg3,4, and investigate the structure-activity relationship using an in vitro, organotypic hippocampal slice culture model of cerebral ischaemia. A number of molecular features were identified which were essential for the neuroprotective activity including the requirement for a positive charge and an amino acid in the L-configuration. Relatively minor alterations to both the terminal arginine and polyamine moieties significantly attenuated neuroprotective efficacy. Our data implies that these compounds are neuroprotective through a currently undefined mechanism rather than non-specific ionic interactions described previously for other polyamine-containing compounds.

T-channel-like pharmacological properties of high voltage-activated, nifedipine-insensitive Ca2+ currents in the rat terminal mesenteric artery

1. Pharmacological properties of nifedipine-insensitive, high voltage-activated Ca(2+) channels in rat mesenteric terminal arteries (NICCs) were investigated and compared with those of alpha1E and alpha1G heterologously expressed in BHK and HEK293 cells respectively, using the patch clamp technique. 2. With 10 mM Ba(2+) as the charge carrier, rat NICCs (unitary conductance: 11.5 pS with 110 mM Ba(2+)) are almost identical to those previously identified in a similar region of guinea-pig, such as in current-voltage relationship, voltage dependence of activation and inactivation, and divalent cation permeability. However, these properties are considerably different when compared with alpha1E and alpha1G. 3. SNX-482(200 nM and sFTX3.3 (1 micro M), in addition to omega-conotoxin GVIA (1 micro M) and omega-agatoxin IVA (100 nM), were totally ineffective for rat NICC currents, but significantly suppressed alpha1E (by 82% at 200 nM; IC(50)=11.1 nM) and alpha1G (by 20% at 1 micro M) channel currents, respectively. A non-specific T-type Ca(2+) channel blocker nimodipine (10 micro M) differentially suppressed these three currents (by 40, 3 and 85% for rat NICC, alpha1E and alpha1G currents, respectively). 4. Mibefradil, the widely used T-type channel blocker, almost equally inhibited rat NICC and alpha1G currents in a voltage-dependent fashion with similar IC(50) values (3.5 and 0.3 micro M and 2.4 and 0.14 micro M at -100 and -60 mV, respectively). Furthermore, other organic T-type channel blockers such as phenytoin, ethosuximide, an arylpiperidine derivative SUN N5030 (IC(50)=0.32 micro M at -60 mV for alpha1G) also exhibited comparable inhibitory efficacies for NICC currents (inhibited by 22% at 100 micro M; IC(50)=27.8 mM; IC(50)=0.53 micro M, respectively). 5. These results suggest that despite distinctive biophysical properties, the rat NICCs have indistinguishable pharmacological sensitivities to many organic blockers compared with T-type Ca(2+) channels.

Gastrin and the neuropeptide PACAP evoke secretion from rat stomach histamine-containing (ECL) cells by stimulating influx of Ca2+ through different Ca2+ channels

1. Gastrin and PACAP stimulate secretion of histamine and pancreastatin from isolated rat stomach ECL cells. We have examined whether or not secretion depends on the free cytosolic Ca2+ concentration ([Ca2+]i) and the pathways by which gastrin and PACAP elevate [Ca2+]i. Secretion was monitored by radioimmunoassay of pancreastatin and changes in [Ca2+]i by video imaging. The patch clamp technique was used to record whole-cell currents and membrane capacitance (reflecting exocytosis). 2. In the presence of 2 mM extracellular Ca2+, gastrin and PACAP induced secretion and raised [Ca2+]i. Without extracellular Ca2+ (or in the presence of La3+) no secretion occurred. The extracellular Ca2+ concentration required to stimulate secretion was 10 times higher for gastrin than for PACAP. Depletion of intracellular Ca2+ pools by thapsigargin had no effect on the capacity of gastrin and PACAP to stimulate secretion. 3. Gastrin-evoked secretion was inhibited 60-80 % by L-type channel blockers and 40 % by the N-type channel blocker omega-conotoxin GVIA. Combining L-type and N-type channel blockers did not result in greater inhibition than L-type channel blockers alone. Whole-cell patch clamp measurements confirmed that the ECL cells are equipped with voltage-dependent inward Ca2+ currents. A 500 ms depolarising pulse from -60 mV to +10 mV which maximally opened these channels resulted in an increase in membrane capacitance of 100 fF reflecting exocytosis of secretory vesicles. 4. PACAP-evoked secretion was reduced 40 % by L-type channel blockers but was not influenced by inhibition of N-type channels. SKF 96365, a blocker of both L-type and receptor-operated Ca2+ channels, inhibited PACAP-evoked secretion by 85 %. Combining L-type channel blockade with SKF 96365 abolished PACAP-evoked secretion. 5. The results indicate that gastrin- and PACAP-evoked secretion depends on Ca2+ entry and not on mobilisation of intracellular Ca2+. While gastrin stimulates secretion via voltage-dependent L-type and N-type Ca2+ channels, PACAP acts via L-type and receptor-operated Ca2+ channels.

Venom from Anemesia species of spider modulates high voltage-activated Ca(2+) currents from rat cultured sensory neurones and excitatory post synaptic currents from rat hippocampal slices

The actions of crude venom from Anemesia species of spider were investigated in cultured dorsal root ganglion neurones from neonatal rats and hippocampal slices. Using mass spectrometry (MALDI-TOF MS), 10-12 distinct peptides with masses between about 3 and 10kDa were identified in the crude spider venom. At a concentration of 5 microg/ml crude Anemesia venom transiently enhanced the mean peak whole cell voltage-activated Ca(2+) current in a voltage-dependent manner and potentiated transient increases in intracellular Ca(2+) triggered by 30mM KCI as measured using Fura-2 fluorescence imaging. Additionally, 5-8 microg/ml Anemesia venom increased the amplitude of glutamatergic excitatory postsynaptic currents evoked in hippocampal slices. Omega-Conotoxin GVIA (1 microM) prevented the increase in voltage-activated Ca(2+) currents produced by Anemesia venom. This attenuation occurred when the cone shell toxin was applied before or after the spider venom. Anemesia venom (5 microg/ml) created no significant change in evoked action potentials but produced modest but significant inhibition of voltage-activated K(+) currents. At a concentration of 50 microg/ml Anemesia venom only produced reversible inhibitory effects, decreasing voltage-activated Ca(2+) currents. However, no significant effects on Ca(2+) currents were observed with a concentration of 0.5 microg/ml. The toxin(s) in the venom that enhanced Ca(2+) influx into sensory neurones was heat-sensitive and was made inactive by boiling or repetitive freeze-thawing. Boiled venom (5 microg/ml) produced significant inhibition of voltage-activated Ca(2+) currents and freeze-thawed venom inhibited Ca(2+) transients measured using Fura-2 fluorescence. Our data suggest that crude Anemesia venom contains components, which increased neuronal excitability and neurotransmission, at least in part this was mediated by enhancing Ca(2+) influx through N-type voltage-activated Ca(2+) channels.

Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura-2-loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR beta2-subunit. In control cells, superfusion with nicotine (10-100 microM) caused a long-lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in beta2-/- mutant mice, leaving a small residual response to a high concentration (100 microM) of nicotine which was inhibited by the alpha7-subunit-selective antagonist, methyllycaconitine. Conversely, the alpha7-subunit-selective agonist choline (10 mM) caused a methyllycaconitine-sensitive increase in intracellular Ca2+ level both in wild-type and beta2-/- mutant mice. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas the choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of beta2-subunit-containing nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and/or activation of alpha7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long-term effects of nicotine on the dopaminergic system.

High sensitivity of immature GABAergic neurons to blockers of voltage-gated calcium channels

The involvement of voltage-gated calcium channels in the survival of immature CNS neurons was studied in aggregating brain cell cultures by examining cell type-specific effects of various channel blockers. Nifedipine (10 microM), a specific blocker of L-type calcium channels, caused a pronounced and irreversible decrease of glutamic acid decarboxylase activity, whereas the activity of choline acetyltransferase was significantly less affected. Flunarizine (1-10 microM, a relatively unspecific ion channel blocker) elicited similar effects, that were attenuated by NMDA. The glia-specific marker enzymes, glutamine synthetase and 2',3'-cyclic nucleotide 3'-phosphohydrolase, were affected only after treatment with high concentrations of nifedipine (50 microM) or NiCl2 (100 microM, shown to block T-type calcium channels). Nifedipine (50 microM), NiCl2 (100 microM), and flunarizine (5 microM) also caused a significant increase in the soluble nucleosome concentration, indicating increased apoptotic cell death. This effect was prevented by cycloheximide (1 microM). Furthermore, the combined treatment with calcicludine (10 nM, blocking L-type calcium channels) and funnel-web spider toxin-3.3 (100 nM, blocking T-type channels) also caused a significant increase in free nucleosomes as well as a decrease in glutamic acid decarboxylase activity. In contrast, cell viability was not affected by peptide blockers specific for N-, P-, and/or Q-type calcium channels. Highly differentiated cultures showed diminished susceptibility to nifedipine and flunarizine. The present data suggest that the survival of immature neurons, and particularly that of immature GABAergic neurons, requires the sustained entry of Ca2+ through voltage-gated calcium channels.

Electrophysiological characterisation of the human N-type Ca2+ channel III: pH-dependent inhibition by a synthetic macrocyclic polyamine

The effects of a novel synthetic macrocyclic polyamine (LY310315) were investigated on recombinant human N-type Ca2+ channels stabley expressed in HEK293 cells. LY310315 proved to be a potent and reversible N-type Ca2+ channel antagonist. Inhibition by this compound was dose-dependent with an IC50 of approximately 0.4 microM at pH 7.35. LY310315 blocked very rapidly at all concentrations tested. Upon washout, recovery of the Ca2+ current developed with a time constant of approximately 30 s. Use-dependence in the development of block indicated that voltage-dependent transitions in the channel protein were required to permit significant inhibition. Application of > 100 times the IC50 dose of LY310315 to the interior of the cell produced no detectable Ca2+ current inhibition. LY310315 had no effects on the kinetics of channel activation or deactivation but did slightly slow the rate of macroscopic inactivation observed during a 300 ms test depolarisation. In the presence of LY310315 the activation curve was significantly shallower. This resulted in a shift in the activation midpoint voltage to a more depolarised levels. LY310315-induced inhibition of human N-type channels was strongly dependent on the extracellular pH, with increased potency seen upon extracellular acidification. Although most effective against N-type Ca2+ channels, LY310315 was also found to inhibit both P-type and L-type Ca2+ channels. LY310315 proved to be a weak blocker of Na+ currents, but produced approximately 50% of the K+ currents of AtT20 cells at a concentration of 0.5 microM.

Extracellular or intracellular application of argiotoxin-636 has inhibitory actions on membrane excitability and voltage-activated currents in cultured rat sensory neurones

The whole cell variant of the patch clamp technique was used to investigate the actions of the polyamine amide spider toxin, argiotoxin-636, on the excitability of cultured dorsal root ganglion neurones. Synthesized argiotoxin-636 (0.1-100 microM) reduced neuronal excitability when applied to the extracellular environment by low pressure ejection or to the intracellular environment via the patch pipette solution. The toxin prolonged the duration of evoked action potentials and reduced the peak amplitude of action potentials. Intracellular and extracellular application of argiotoxin-636 also decreased the number of action potentials evoked in response to 800-ms depolarizing current commands. This action of the toxin was mimicked by 100 microM tetraethylammonium. Extracellular application of argiotoxin-636 inhibited voltage-activated K currents in a dose-dependent manner over the complete voltage range. This inhibition occurred without any significant changes in the voltage dependence of activation or inactivation. Intracellular application of argiotoxin-636, during 5-10 min of whole cell recording, also inhibited voltage-activated K+ currents without changing the voltage dependence of activation or steady-state inactivation. Extracellular or intracellular spermidine (250 microM) reversibly attenuated the inhibitory actions of extracellular argiotoxin-636. Argiotoxin-636 also inhibited voltage-activated Na + currents; this effect was dependent on repeated activation of the currents and the period during which the neurones were in culture. We conclude that application of argiotoxin-636 to either the extracellular or intracellular environment reduced excitability of cultured sensory neurones from neonatal rats and that this involved inhibition of both voltage-activated K+ and Na+ currents. The data suggest that the toxin was more effective at attenuating action potentials when neurones were repeatedly excited, and that access to inhibitory sites of action on the voltage-activated ion channels can be achieved from the inside of the neurone.

Inhibitory actions of synthesised polyamine spider toxins and their analogues on Ca2+-activated Cl- currents recorded from cultured DRG neurones from neonatal rats

The whole cell variant of the patch clamp technique was used to investigate the actions of polyamine spider toxins and their analogues on high voltage-activated Ca2+ currents and Ca2+-activated Cl- currents (I(Cl(Ca))). The actions of synthesised FTX (putative natural toxin from the American funnel web spider), sFTX-3.3, Orn-FTX-3.3, Lys-FTX-3.3, and argiotoxin-636 on cultured dorsal root ganglion neurones from neonatal rats were investigated. Synthesised FTX (1 microM) inhibited I(Cl(Ca)) but did not inhibit high voltage-activated Ca2+ currents. In contrast, sFTX-3.3 (10 microM) inhibited both high voltage-activated Ca2+ currents and the associated I(Cl(Ca)) in near equal proportions. Argiotoxin-636 (1-10 microM) inhibited I(Cl(Ca)) evoked by Ca2+ entry through voltage-activated channels and by intracellular photorelease of Ca2+ from a caged precursor DM-nitrophen. This data indicates that synthesised FTX and argiotoxin-636 directly inhibit Ca2+-activated Cl- channels. In conclusion, the potency of polyamines as non-selective inhibitors of Ca2+ channels and Ca2+-activated Cl- channels is in part determined by the presence of a terminal arginine and this may involve an interaction between terminal guanidino groups and Ca2+ binding sites.

Identifying neuronal non-L Ca2+ channels--more than stamp collecting?

The pharmacology of the majority of Ca2+ channels in the nervous system is very limited. Although attempts have been made to constrain native Ca2+ channels into the framework provided by the six pore-forming molecules cloned to date, refined biophysical analysis of Ca2+ currents, expression techniques and the use of selective toxins have helped to identify unambiguously only a limited number of Ca2+ channels. In fact, many native Ca2+ channel activities remain as 'orphans', waiting for their molecular counterparts to be defined. In this article, Janet Nooney, Régis Lambert and Anne Feltz systematically delineate the well characterized non-L Ca2+ channel activities and the missing elements in our knowledge of the Ca2+ channel family.

Toxins affecting calcium channels in neurons

Calcium enters the cytoplasm mainly via voltage-activated calcium channels (VACC), and this represents a key step in the regulation of a variety of cellular processes. Advances in the fields of molecular biology, pharmacology and electrophysiology have led to the identification of several types of VACC (referred to as T-, N-, L-, P/Q- and R-types). In addition to possessing distinctive structural and functional characteristics, many of these types of calcium channels exhibit differential sensitivities to pharmacological agents. In recent years a large number of toxins, mainly small peptides, have been purified from the venom of predatory marine cone snails and spiders. Many of these toxins have specific actions on ion channels and neurotransmitter receptors, and the toxins have been used as powerful tools in neuroscience research. Some of them (omega-conotoxins, omega-agatoxins) specifically recognize and block certain types of VACC. They have common structural backbones and some been synthesized with identical potency as the natural ones. Natural, synthetic and labeled calcium channel toxins have contributed to the understanding of the diversity of the neuronal calcium channels and their function. In particular, the toxins have been useful in the study of the role of different types of calcium channels on the process of neurotransmitter release. Neuronal calcium channel toxins may develop into powerful tools for diagnosis and treatment of neurological diseases.

Interactions of polyamines with ion channels

Endogenous polyamines, in particular spermine, have been found to cause block and modulation of a number of types of ion channel. Intracellular spermine is responsible for intrinsic gating and rectification of strong inward rectifier K+ channels by directly plugging the ion channel pore. These K+ channels control the resting membrane potential in both excitable and non-excitable cells, and control the excitability threshold in neurons and muscle cells. Intracellular spermine causes inward rectification at some subtypes of Ca2+-permeable glutamate receptors in the central nervous system, again by plugging the receptor channel pore, and spermine can even permeate the ion channel of these receptors. Extracellular spermine has multiple effects at the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor, including stimulation that increases the size of NMDA receptor currents, and voltage-dependent block. A number of polyamine-conjugated arthropod toxins and synthetic polyamine analogues are potent antagonists of glutamate receptors, and represent new tools with which to study these receptors. Interactions of polyamines with other types of cation channels have been reported. This area of research represents a new biology and a new pharmacology of polyamines.

Polyamine FTX-3.3 and polyamine amide sFTX-3.3 inhibit presynaptic calcium currents and acetylcholine release at mouse motor nerve terminals

FTX-3.3 is the proposed structure of a calcium-channel blocking toxin that has been isolated from the funnel web spider (Agelenopsis aperta). The effects of FTX-3.3 and one of its analogues, sFTX-3.3, on acetylcholine release, on presynaptic currents at mouse motor nerve terminals and on whole-cell sodium currents in SK.N.SH cells (a human neuroblastoma cell line) have been studied. FTX-3.3 (10-30 microM) and sFTX-3.3 (100-300 microM) reversibly reduced release of acetylcholine by approximately 70-90% and 40-60%, respectively. FTX-3.3 (10 microM) blocked the fast component of presynaptic calcium currents by approximately 60%. sFTX-3.3 (100 microM) reduced the duration of the slow component of presynaptic calcium currents by about 50% of the control and also reduced presynaptic sodium current by approximately 20% of the control. sFTX-3.3 (100 microM) reduced whole-cell sodium current recorded from SK.N.SH cells by approximately 15%, whereas FTX-3.3, even at 200 microM, did not affect this current. Since the only difference in chemical structures of these toxins is that sFTX-3.3 has an amide function which is absent in FTX-3.3, the amide function may be responsible for the reduced potency and selectivity of sFTX-3.3. This study also provides further support for the existence of P-type calcium channels at mouse motor nerve terminals.

Modulation and block of ion channels: a new biology of polyamines

The endogenous polyamines, spermine, spermidine, and putrescine have effects on several types of cation channels. Intracellular polyamines, in particular spermine, contribute to intrinsic gating and rectification of strong inward rectifier K+ channels. Intracellular spermine is also responsible for inward rectification of some types of Ca(2+)-permeable AMPA and kainate receptors. Spermine has a number of effects on the activity of the NMDA subtype of glutamate receptor, involving two or more extracellular polyamine binding sites on the NMDA receptor. In K+ channels and glutamate receptors, some of the amino acids in the receptor/channel structure that influence to polyamines have been identified, leading to a partial understanding of the effects of polyamines at a molecular level. Block of K+ channels by intracellular polyamines is likely to be an important receptors by intracellular spermine and modulation by extracellular spermine may affect excitability and the influx of Ca2+ in neurons and glial cells of the nervous system.

Polyamine amides are neuroprotective in cerebellar granule cell cultures challenged with excitatory amino acids

Primary cultures of rat cerebellar granule cells have been used to assess the potential neuroprotective effects of philanthotoxins and argiotoxin-636 (ArgTX-636). These polyamine amides are potent antagonists of ionotropic L-glutamate (L-Glu) receptors. In granule cells loaded with fluo-3, ArgTX-636 and philanthotoxin-343 (PhTX-343) antagonised increases of intracellular free calcium concentration ([Ca2+]i) that were stimulated by N-methyl-D-aspartate (NMDA). The antagonism was use-dependent. Antagonism by PhTX-343 was fully reversible, but recovery following antagonism by ArgTX-636 was slow and only partial during the time-course of an experiment. Neither compound inhibited K(+)-induced increases in [Ca2+]i. In excitotoxicity studies with cerebellar granule cells, the release of lactate dehydrogenase (LDH) and morphological observations were used to assess cell death. A 20-30 min exposure to 500 microM NMDA, 100 microM L-Glu or 500 microM kainate was sufficient to kill > 90% of the cells after 18-20 h. When added 5 min prior to, and during agonist exposure, PhTX-343 and ArgTX-636 provided total neuroprotection. ArgTX-636 was about 20-30 fold more potent than PhTX-343 against NMDA, but was approximately equipotent with PhTX-343 against a kainate challenge. Neither of the toxins showed any inherent toxicity even at 400 microM and 100 microM respectively. Some analogues of PhTX-343 are more potent, both in terms of antagonism of NMDA-stimulated increases of [Ca2+]i and neuroprotection, than PhTX-343 and ArgTX-636.

Selective inhibition of high voltage-activated L-type and Q-type Ca2+ currents by serotonin in rat melanotrophs

1. Whole-cell Ca2+ currents (ICa) from cultured rat melanotrophs were identified by their sensitivity to Ca2+ channel blockers, and their modulation by serotonin (5-HT) was studied. All cells displayed high voltage-activated (HVA; > -30 mV) Ca2+ currents. A low voltage-activated (LVA; > -60 mV) Ca2+ current was detected in 92% of the cells. 2. The whole-cell ICa was insensitive to omega-conotoxin GVIA (0.5-1 microM) indicating the absence of N-type Ca2+ channels. 3. At a holding potential (Vh) of -70 mV, the L-type channel blocker nifedipine reduced ICa in a dose-dependent manner with a half-maximal effective concentration (IC50) of 28 nM. The L-type current represented 39% of the total ICa. 4. omega-Agatoxin IVA (omega-Aga IVA) produced a biphasic dose-dependent inhibition of ICa, with IC50 values of 0.4 and 91 nM, indicating the presence of P-type and Q-type Ca2+ channels, which accounted respectively for 16 and 45% of the total ICa. The P-type current was also blocked by synthetic funnel-web spider toxin (sFTX 3.3; 1-10 microM) and was present only in a subpopulation (60-70%) of cells. 5. All cells possessed a Ca2+ current which was resistant to nifedipine (10 microM) and omega-Aga IVA (50 nM). This current was not affected by Ni2+ (40 microM) but was abolished by a low concentration of Cd2+ (10 microM) and by omega-conotoxin MVIIC (1 microM) indicating that it was a Q-type Ca2+ current. 6. 5-HT (10 microM) inhibited the whole-cell ICa in 70% of the cells tested (n = 120) by activating 5-HT1A and 5-HT2C receptors. 5-HT produced either a kinetic slowing of the activation phase (37% of the cells) or a scaling down (14% of the cells) of ICa. In the majority of cells (49%) both types of inhibition were found to coexist. 7. The effects of 5-HT were voltage dependent, rendered irreversible when GTP-gamma-S (30 microM) was present in the pipette solution and abolished by pretreatment of the cells with pertussis toxin (PTX; 150 ng ml-1, 18 h). 8. Low concentrations of omega-Aga IVA (20 nM), which blocked mainly P-type channels, did not reduce the effect of 5-HT on ICa. The scaling down effect of 5-HT on ICa was eliminated in the presence of nifedipine (10 microM) and the kinetic slowing effect of 5-HT persisted after blockade of L- and P-type channels but was abolished by omega-conotoxin MVIIC (1 microM). 9. We conclude that rat melanotrophs possess functional L-, P- and Q-type Ca2+ channels and that 5-HT inhibits selectively L-type and Q-type Ca2+ currents with different modalities. These effects are voltage dependent and mediated by a PTX-sensitive G-protein.

Differential inhibition of Ca2+ channels in mature rat cerebellar Purkinje cells by sFTX-3.3 and FTX-3.3

Synthetic funnel web spider toxin (sFTX-3.3) is a polyamine amide analogue of FTX, a toxin fraction isolated from the venom of the funnel web spider, Agelenopsis aperta, that blocks P-type Ca2+ channels. The structures of these polyamine containing compounds are not identical: sFTX-3.3 contains an amide carbonyl oxygen that is absent from the predicted structure of native FTX. Recently, a compound called FTX-3.3 was synthesized with the structure predicted for native FTX. We have compared the effects of polyamine amide sFTX-3.3 and polyamine FTX-3.3, on Ca2+ channel currents in the soma of mature rat cerebellar Purkinje neurons, in which the predominant Ca2+ channels are defined as P-type. Differential inhibition by sFTX-3.3 and FTX-3.3 revealed three populations of Ca2+ channels. One group, mediating approximately 66% of the current, was blocked by sFTX-3.3 with an IC50 (concentration producing half maximal inhibition) of 33 nM or by FTX-3.3 with an IC50 of 55 pM. A second population (5-25% of the total current) was inhibited by sFTX-3.3 with an IC50 of 33 nM, but was insensitive to FTX-3.3, while a third (10-30%) was blocked by FTX-3.3 with an IC50 of 125 nM and was resistant to sFTX-3.3. These channels also showed distinctive current-voltage relationships. Our results suggest that P-type Ca2+ channels in mature rat cerebellar Purkinje cells may be subdivided according to pharmacological and biophysical properties.

Inhibition of acetylcholine release from presynaptic terminals of skate electric organ by calcium channel antagonists: a detailed pharmacological study

Release of acetylcholine (ACh) from the presynaptic terminals in skate electric organ was tested for its sensitivity to calcium channel antagonists. A pharmacological profile was established by measuring inhibition of K(+)-stimulated release of [3H]ACh from prelabelled tissue slices. Peptide antagonists of N-type (omega-conotoxins GVIA and MVIIA) and P-type (omega-agatoxin-IVA) channels had no effect, whereas both omega-conotoxins MVIIC and SVIB produced concentration-dependent inhibition and could completely block ACh release. omega-Conotoxin GVIA and omega-agatoxin IVA did not attenuate the block by omega-conotoxin MVIIC. The inorganic ions, Cd2+ and Ni2+, also produced a full inhibition of release (Cd2+ > > Ni2+) and Gd3+ a partial one. Drugs targeting L-type channels (diltiazem, nifedipine and verapamil) at low microM concentrations and a synthetic analogue of the polyamine toxin from funnel web spider venom (sFTX) at 1 mM were all non-inhibitory. Inhibition by omega-conotoxins MVIIC (IC50 25 nM) and SVIB (IC50 500 nM) was reversible and modulated by external concentrations of Ca2+. Inhibitory potency was increased by lowering and decreased by elevating external Ca2+. This "antagonistic" effect of Ca2+ was also seen with Cd2+ inhibition. The inhibitory potency of omega-conotoxin MVIIC was unaffected by predepolarisation. End plate potentials generated by release of endogenous ACh in electrically-stimulated slices were also reversibly blocked by Cd2+ and omega-conotoxins MVIIC and SVIB but were unaffected by omega-conotoxin GVIA and omega-agatoxin IVA. It is concluded that ACh release in skate electric organ depends on presynaptic calcium channels which have different pharmacological properties from established sub-types.

Transgenic animals as models in the study of the neurobiological role of polyamines

Natural polyamines, putrescine, spermidine and spermine, exhibit a number of neurophysiological and metabolic effects in brain preparations. In the in vitro studies, several specific sites of action have been identified such as ion channels, transmitter release and Ca2+ homeostasis. Polyamines have been linked to the development of neuronal degeneration caused by, for instance, epileptic seizures and stroke. The role of endogenous polyamines in the functioning brain is not clear, however. We review the work carried out using state-of-the-art transgenic animal models for polyamine research. A number of transgenic mouse lines carrying human ornithine decarboxylase, spermidine synthase and S-adenosylmethionine decarboxylase gene have been generated. Of these animals those with ornithine decarboxylase transgene show an extensive and constitutive expression of the enzyme in the brain with an exceedingly high putrescine concentration, a phenotype that is not encountered under physiological conditions. In this article we review the neurometabolic, behavioural and histological data that has been obtained from these transgenic mice.

Characterization of the effects of polyamines on the modulation of the N-methyl-D-aspartate receptor by glycine

We have investigated the effects of polyamine agonists and antagonists on the modulation of the N-methyl-D- aspartate receptor by glycine using a [3H]dizocilpine ([3H]MK801) binding assay. We monitored the non-equilibrium binding of [3H]dizocilipine in the presence of 5,7-dichlorokynurenate to preclude occupation of the glycine site by endogenous glycine. Using this assay, spermine and spermidine increased both the affinity and the maximum effect of glycine. Similar effects are produced by other polyamine agonists including 1,5-diethylaminopiperidine, neomycin and Ca2+. These actions are reversed by the polyamine glycine produced by polyamine agonists appears to be due to an increase in the equilibrium affinity of [3H]dizocilpine, and cannot therefore be attributed solely to modulation of glycine binding. Interestingly, 1,5-diethylaminopiperidine increases the maximum effect of glycine to a greater extent than it alters glycine affinity, suggesting that the two effects may be mediated by different sites or mechanisms. These studies will help to define the role of the glycine site in the modulation of the N-methyl-D-aspartate receptor by polyamines.

Affinity purification of rat cortical and chicken forebrain synaptosomes using a biotinylated derivative of omega-CgTx GVIA

We describe a magnetophoretic method for the affinity purification of synaptosomes expressing omega-CgTx GVIA-sensitive, N-type voltage-sensitive calcium channels (VSCCs). The method utilizes a biotinylated derivative of omega-CgTx GVIA which retains its ability to displace [125I] omega-CgTx GVIA from its binding sites on rat synaptic membranes. When coupled to streptavidin coated magnetizable beads, the hexanoyl spacer between omega-CgTx GVIA and the biotin:streptavidin bead complex is sufficiently long to allow flexibility of the toxin to bind to its receptor on synaptosomes. We have used this ligand successfully to isolate deaggregated synaptosomes from the parent fractions of chicken forebrain and rat cortex. In the chicken synaptosome parent fraction, omega-CgTx GVIA (1 nM-1 microM) produced a concentration-dependent block of the KCl-induced intracellular free Ca2+, [Ca2+]i, elevation with an IC50 of 28 nM. After affinity magnetophoresis no increase in [Ca2+]i elevation was observed in either the bound or unbound fractions. In the rat synaptosome parent fraction, the KCl-induced increase in free intracellular Ca2+ ([Ca2+]i) elevation was partially blocked by omega-CgTx GVIA (17 +/- 2% / 1 microM) and to a greater extent by omega-Aga IVA (55 +/- / 1 microM): a combination of the two toxins was additive (72 +/- 4% / 1 microM). The block obtained by omega-CgTx GVIA (1 microM) in the unbound fraction was reduced to 3 +/- 2%, whereas that by omega-Aga IVA (1 microM) increased to 82 +/- 3%. The block obtained by a combination of both toxins (83 +/- 2%) was the same as that with omega-Aga IVA alone (82 +/- 3%). No increase in free [Ca2+]i elevation was observed in the bound fraction although single synaptosome-like structures, displaying synaptophysin immunoreactivity, were detected on the beads. We conclude that omega-CgTx GVIA-sensitive N-type calcium channels are present on all chicken forebrain synaptosomes but only a subset of rat cortical synaptosomes.

Aspects of calcium-activated chloride currents: a neuronal perspective

Ca(2+)-activated Cl- channels are expressed in a variety of cell types, including central and peripheral neurones. These channels are activated by a rise in intracellular Ca2+ close to the cell membrane. This can be evoked by cellular events such as Ca2+ entry through voltage- and ligandgated channels or release of Ca2+ from intracellular stores. Additionally, these Ca(2+)-activated Cl currents (ICl(Ca)) can be activated by raising intracellular Ca2+ through artificial experimental procedures such as intracellular photorelease of Ca2+ from "caged" photolabile compounds (e.g. DM-nitrophen) or by treating cells with Ca2+ ionophores. The potential changes that result from activation of Ca(2+)-activated Cl- channels are dependent on resting membrane potential and the equilibrium potential for Cl-. Ca2+ entry during a single action potential is sufficient to produce substantial after potentials, suggesting that the activity of these Cl- channels can have profound effects on cell excitability. The whole cell ICl(Ca) can be identified by sensitivity to increased Ca2+ buffering capacity of the cell, anion substitution studies and reversal potential measurements, as well as by the actions of Cl- channel blockers. In cultured sensory neurones, there is evidence that the ICl(Ca) deactivates as Ca2+ is buffered or removed from the intracellular environment. To date, there is no evidence in mammalian neurones to suggest these Ca(2+)-sensitive Cl- channels undergo a process of inactivation. Therefore, ICl(Ca) can be used as a physiological index of intracellular Ca2+ close to the cell membrane. The ICl(Ca) has been shown to be activated or prolonged as a result of metabolic stress, as well as by drugs that disturb intracellular Ca2+ homeostatic mechanisms or release Ca2+ from intracellular stores. In addition to sensitivity to classic Cl- channel blockers such as niflumic acid, derivatives of stilbene (4,4'diisothiocyanostilbene-2,2'-disulphonic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) and benzoic acid (5-nitro 2-(3-phenylpropylamino) benzoic acid), ICl(Ca) are also sensitive to polyamine spider toxins and some of their analogues, particularly those containing the amino acid residue arginine. The physiological role of Ca(2+)-activated Cl- channels in neurones remains to be fully determined. The wide distribution of these channels in the nervous system, and their capacity to underlie a variety of events such as sustained or transient depolarization or hyperpolarizations in response to changes in intracellular Ca2+ and variations in intracellular Cl- concentration, suggest the roles may be subtle, but important.

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.

Spermine depresses NMDA, K/AMPA and GABAA-mediated synaptic transmission in the rat hippocampal slice preparation

The effects of spermine, an endogenous polyamine, were examined in area CA1 of the rat hippocampal slice preparation. Spermine, at low millimolar concentrations, rapidly and potently depressed NMDA and K/AMPA-mediated population EPSPs, and GABA-mediated monosynaptic population IPSPs. These effects contrast with its well-known potentiation of NMDA currents at lower concentrations. Our results raise the possibility that the large intracellular stores of spermine that are released after various neural insults could act as an endogenous neuroprotective mechanism by limiting excessive calcium entry.

Pharmacological identification of a novel Ca2+ channel in chicken brain synaptosomes

Ca2+ influx was measured in rat and chicken brain synaptosomes in the presence of a number of pharmacological tools which have recently been used to define voltage-sensitive Ca(2+)-channel (VSCC) types. In chicken brain synaptosomes. VSCCs which, because of their sensitivity to inhibition by omega-conotoxin (omega-CgTx), are thought to be exclusively N-type, the P-type VSCC polyamine inhibitor FTX (from Agelenopsis aperta venom; 1 microliters/ml), its synthetic analogue, sFTX (1-5 mM) and the polypeptides AgaIVA (IC50 0.29 microM) and omega-CgTx MVIIC (IC50 0.0022 microM) inhibited 70-100% of the measurable K+ stimulated Ca2+ influx. The prototypical N-channel VSCC inhibitor omega-CgTx GVIA (IC50 0.014 microM), Cd2+ (50 microM) and diluted venom from Hololena curta (1:2,500) also caused complete or almost complete, inhibition of Ca2+ influx. In comparable studies using rat brain synaptosomes, sFTX (1-10 mM) caused a dose-dependent reduction of Ca2+ influx, while FTX (1 microliters/ml) and AgaIVA (IC50 0.02 microM) completely inhibited Ca2+ influx. Similar to the findings in chicken synaptosomes, Cd2+ (50 microM) and H. curta (1:2,500 dilution) both inhibited K+ stimulated influx by > 80% whereas omega-CgTx (1 microM) only caused a maximum 25% inhibition. Both sFTX and its congener spermine, inhibited [125I]omega-CgTx binding to rat and chicken synaptosomal membranes. These results strongly implicate P-type channels as the major VSCC in rat brain. The results also clearly demonstrate a heretofore unrecognized, novel, FTX/AgaIVA/omega-CgTx GVIA/omega-CgTx MVIIC-sensitive VSCC in chicken brain.

P-type calcium channels in rat neocortical neurones

1. The high threshold, voltage-activated (HVA) calcium current was recorded from acutely isolated rat neocortical pyramidal neurones using the whole-cell patch technique to examine the effect of agents that block P-type calcium channels and to compare their effects to those of omega-conotoxin GVIA (omega-CgTX) and nifedipine. 2. When applied at a saturating concentration (100 nM) the peptide toxins omega-Aga-IVA and synthetic omega-Aga-IVA blocked 31.5 and 33.0% of the HVA current respectively. 3. A saturating concentration of nifedipine (10 microM) inhibited 48.2% of the omega-Aga-IVA-sensitive current, whereas saturating concentrations of both omega-Aga-IVA (100 nM) and omega-CgTX (10 microM) blocked separate specific components of the HVA current. 4. Partially purified funnel web spider toxin (FTX) at a dilution of 1:1000 blocked 81.4% of the HVA current and occluded the inhibitory effect of omega-Aga-IVA. Synthetic FTX 3.3 arginine polyamine (sFTX) at a concentration of 1 mM blocked 61.2% of the HVA current rapidly and reversibly. The effects of sFTX were partially occluded by pre-application of omega-Aga-IVA. We conclude that neither FTX nor sFTX blocked a specific component of the HVA current in these cells. 5. In view of the specificity of omega-Aga-IVA for P-type calcium channels in other preparations and for a specific component of the HVA current in dissociated neocortical neurones we conclude that about 30% of the HVA current in these neurones flow through P-channels.

Low- and high-voltage-activated calcium channel currents and their modulation in the dorsal root ganglion cell line ND7-23

The dorsal root ganglion-neuroblastoma cell line ND7-23 expresses low-voltage-activated calcium channel currents, and also expresses high-voltage-activated currents in about 50% of differentiated cells. Calcium channel currents were recorded with Ba2+ as the charge carrier. Low-voltage-activated currents were maximally activated at -30 mV and completely inactivated at holding potentials of -60 to -50 mV. omega-Conotoxin GVIA produced a reversible inhibition of low-voltage-activated currents, whereas the inhibition of high-voltage-activated current was largely irreversible. Dihydropyridine antagonists did not inhibit low-voltage-activated currents, whereas they inhibited a sustained, high-voltage-activated current. Low-voltage-activated currents were inhibited to a greater extent than high-voltage-activated currents by Ni2+ (100 microM) and by phenytoin (10 microM). Bradykinin (0.1 microM), baclofen (2 microM) and internal guanosine-5'-O-3-thiotriphosphate (100 microM) inhibited low-voltage-activated currents without affecting their kinetics of activation. Two classes of low-voltage-activated current were distinguished by their kinetics of inactivation. In the majority of cells, currents were slowly inactivating with a time-constant of inactivation of about 50 ms. They also exhibited a sustained component to the current, representing about 20% of the peak current. This component could be distinguished pharmacologically from high-voltage-activated current. The remainder of cells expressed a rapidly and completely inactivating current, with a time-constant of inactivation of about 20 ms. Two distinct single channel currents were observed in these cells, from cell-attached patch measurements, one had a single channel conductance of 7.9 pS, and the ensemble average current showed some inactivation. It is likely that this channel subtype underlies the low-voltage-activated current. The other showed long openings in the presence of a dihydropyridine agonist, had a conductance of 23.1 pS, and was non-inactivating.

omega-Agatoxin-IVA-sensitive calcium channels in bovine chromaffin cells

A large component of the whole-cell currents through Ca2+ channels in bovine adrenomedullary chromaffin cells has been shown to be insensitive to both L-type and N-type Ca2+ channel blockers, suggesting the existence of a third type of Ca2+ channel. In the present paper, omega-agatoxin-IVA (AgTx), a selective blocker of P-type Ca2+ channels in mammalian neurons, has been used to investigate the presence of this subtype of Ca2+ channel in bovine chromaffin cells. Barium currents (IBa) through Ca2+ channels were recorded in whole-cell patch-clamped bovine chromaffin cells. IBa was blocked by AgTx in a dose-dependent and irreversible manner. At the maximal concentration used (1 microM), AgTx inhibited IBa by 49.5 +/- 3%. Such a blockade was also present when bovine chromaffin cells were pretreated with 10 microM furnidipine, a novel 1,4-dihydropyridine L-type channel blocker, and after treatment with 1 microM of the N-type channel blocker, omega-conotoxin GVIA (CgTx). A combination of these three types of Ca2+ channel blockers suppressed the macroscopic Ba2+ currents by 88%. We conclude that bovine chromaffin cells, in addition to N- and L-type Ca2+ channels, possess a P-like component in their whole-cell currents through the Ca2+ channels.

The calcium channel coupled to the exocytosis of L-glutamate from cerebellar granule cells is inhibited by the spider toxin, Aga-GI

The increase in cytosolic calcium, [Ca2+]c, evoked with 50 mM KCl in cerebellar granule cells consists of four components; (1) a rapidly inactivating transient or spike; (2) a nifedipine-sensitive non-inactivating plateau; (3) an Aga-GI (spider toxin) sensitive non-inactivating plateau; (4) a residual non-inactivating plateau insensitive to nifedipine and Aga-GI. None of these components is blocked by synthetic arginine polyamine toxin, spermine, (+)-MK-801 hydrogen maleate, D(-)-2-amino-5-phosphonopentanoic acid or omega-conotoxin-GVIA. The proposed P-type channel antagonist, omega-agatoxin-IVA, has a limited but non-significant effect on the elevated plateau [CA2+]c.L-type Ca2+ channels are located primarily on the soma whereas the component of the plateau which is blocked specifically by Aga-GI is localized primarily on the cell neurites. The latter component is coupled to the exocytosis of endogenous glutamate evoked with 50 mM KCl.

Interactions of polyamines with neuronal ion channels

Polyamines, a group of aliphatic amines, exert selective and complex actions on a variety of ion channels. Polyamines are found endogenously, as normal metabolic intermediates, and also in the venoms of several invertebrates, where they act as potent neurotoxins. In addition, evidence suggests that polyamines may mediate or potentiate excitotoxic mechanisms responsible for neuronal damage during ischaemia. Now that the structures and functions of various polyamines are beginning to be deduced, and synthetic analogues become available, these compounds are of importance, not only as pharmacological tools to study specific receptor/ion channel complexes, but also as templates on which to base drugs designed for neuroprotective effects in a number of neurodegenerative disorders.

Pharmacological characterization of voltage-sensitive Ca2+ channels in autonomic nerves

Hololena curta venom a potent inhibitor of voltage sensitive Ca2+ channels and neurotransmitter release in mammalian brain, and synthetic funnel web spider toxin an inhibitor of P channels, were examined for their activity on autonomic nerves. Hololena curta (0.5 to 5.0 micrograms venom protein/ml) potently inhibited motor responses of the cholinergic guinea pig ileum myenteric plexus and the adrenergic rat anococcygeus muscle. Synthetic funnel web spider toxin was inactive at concentrations up to 100 microM. Hololena curta inhibited K+, and electrically evoked release of tritium from labeled superfused tissues. Furthermore, K(+)-contracted rat aorta was not relaxed by Hololena curta thereby precluding effects of Hololena curta on postjunctional L type smooth muscle Ca2+ channels. The pattern of effects of Hololena curta on peripheral autonomic nerves was similar to the N channel inhibitor omega-conotoxin GVIA. These results suggest that Hololena curta venom constituents block Ca2+ channels in peripheral autonomic nerves. The study failed to establish the presence of functional P type Ca2+ channels on these peripheral autonomic nerves and further suggests that N type channels may be exclusively responsible for supplying the Ca2+ necessary for neurotransmitter release in these nerves.

Palmitoyl-DL-carnitine has calcium-dependent effects on cultured neurones from rat dorsal root ganglia

1. The effects of palmitoyl-DL-carnitine (0.01 to 1 mM) on whole cell voltage-activated calcium channel currents carried by calcium or barium and Ca(2+)-activated chloride currents were studied in cultured neurones from rat dorsal root ganglia. 2. Palmitoyl-DL-carnitine applied to the extracellular environment or intracellularly via the patch solution reduced Ca2+ currents activated over a wide voltage range from a holding potential of -90 mV. Inhibition of high voltage activated Ca2+ channel currents was dependent on intracellular Ca2+ buffering and was reduced by increasing the EGTA concentration from 2 to 10 mM in the patch solution. Barium currents were significantly less sensitive to palmitoyl-DL-carnitine than Ca2+ currents. 3. The amplitude of Ca(2+)-activated Cl- tail currents was reduced by palmitoyl-DL-carnitine. However, the duration of these Cl- currents was greatly prolonged by palmitoyl-DL-carnitine, suggesting slower removal of free Ca2+ from the cytoplasm following Ca2+ entry through voltage-activated channels. 4. Palmitoyl-DL-carnitine evoked Ca(2+)-dependent inward currents which could be promoted by activation of the residual voltage-activated Ca2+ currents and attenuated by intracellular application of EGTA. 5. We conclude that palmitoyl-DL-carnitine reduced the efficiency of intracellular Ca2+ handling in cultured dorsal root ganglion neurones and resulted in enhancement of Ca(2+)-dependent events including inactivation of voltage-activated Ca2+ currents. The activation of inward currents by palmitolyl-DL-carnitine may involve Ca(2+)-induced Ca2+ release from intracellular stores, or direct interaction of palmitoyl-DL-carnitine with Ca2+ stores.