The effects of 4-aminopyridine and tetrodotoxin on the release of acetylcholine from rat striatal slices

Comparative effects of chlorpyrifos in wild type and cannabinoid Cb1 receptor knockout mice

Endocannabinoids (eCBs) modulate neurotransmission by inhibiting the release of a variety of neurotransmitters. The cannabinoid receptor agonist WIN 55.212-2 (WIN) can modulate organophosphorus (OP) anticholinesterase toxicity in rats, presumably by inhibiting acetylcholine (ACh) release. Some OP anticholinesterases also inhibit eCB-degrading enzymes. We studied the effects of the OP insecticide chlorpyrifos (CPF) on cholinergic signs of toxicity, cholinesterase activity and ACh release in tissues from wild type (+/+) and cannabinoid CB1 receptor knockout (-/-) mice. Mice of both genotypes (n=5-6/treatment group) were challenged with CPF (300 mg/kg, 2 ml/kg in peanut oil, sc) and evaluated for functional and neurochemical changes. Both genotypes exhibited similar cholinergic signs and cholinesterase inhibition (82-95% at 48h after dosing) in cortex, cerebellum and heart. WIN reduced depolarization-induced ACh release in vitro in hippocampal slices from wild type mice, but had no effect in hippocampal slices from knockouts or in striatal slices from either genotype. Chlorpyrifos oxon (CPO, 100 μM) reduced release in hippocampal slices from both genotypes in vitro, but with a greater reduction in tissues from wild types (21% vs 12%). CPO had no significant in vitro effect on ACh release in striatum. CPF reduced ACh release in hippocampus from both genotypes ex vivo, but reduction was again significantly greater in tissues from wild types (52% vs 36%). In striatum, CPF led to a similar reduction (20-23%) in tissues from both genotypes. Thus, while CB1 deletion in mice had little influence on the expression of acute toxicity following CPF, CPF- or CPO-induced changes in ACh release appeared sensitive to modulation by CB1-mediated eCB signaling in a brain-regional manner.

Epileptiform activity in rat spinal dorsal horn in vitro has common features with neuropathic pain

Neuropathic pain and epileptic seizures bear several similarities, among them is the response to anticonvulsant drugs. It has therefore been hypothesized that epileptiform activity of nociceptive spinal dorsal horn neurons may contribute to paroxysmal forms of neuropathic pain. We used patch-clamp and field potential recordings from young rat spinal cord slices to test if nociceptive dorsal horn structures are indeed able to sustain epileptiform activity. Application of the convulsant 4-aminopyridine (100 microM) evoked epileptiform activity that was most pronounced in superficial dorsal horn and involved nociceptive lamina I neurons with a projection to the brain. The epileptiform activity was dependent on fast excitatory and inhibitory synaptic transmission through ionotropic glutamate receptors and GABA(A) receptors. During epileptiform activity, previously silent polysynaptic pathways from primary afferent C-fibers to superficial dorsal horn neurons were opened. Stimulation of primary afferents at Adelta- and C-fiber intensity interfered with the epileptiform rhythm, suggesting that both affect the same dorsal horn structures. Similar to neuropathic pain, spinal dorsal horn epileptiform activity was much less reduced by classical analgesics than by anticonvulsant agents.

Action of 4-aminopyridine on extracellular amino acids in hippocampus and entorhinal cortex: a dual microdialysis and electroencehalographic study in awake rats

In order to study the role of amino acids in the hippocampus and the entorhinal cortex during the convulsive process induced by 4-aminopyridine (4-AP), we have used a device allowing the simultaneous microdialysis and the recording of their electrical activity of both regions in freely moving rats. We found that infusion of 4-AP into the entorhinal cortex resulted in a large increase in extracellular glutamate and glutamine and small increases in glycine and taurine levels. Likewise, infusion of 4-AP into the hippocampus resulted in a major increase in glutamate, as well as slight increases in taurine and glycine. In both infused regions the peak concentration of extracellular glutamate was observed 15 min after 4-AP administration. No significant changes were found in the non-infused hippocampus or entorhinal cortex of the same rats. Simultaneous electroencephalographic recordings showed intense epileptiform activity starting during 4-AP infusion and lasting for the rest of the experiment (1 h) in both the entorhinal cortex and the hippocampus. The discharges were characterized by poly-spikes and spike-wave complexes that propagated almost immediately to the other region studied. These findings suggest that increased glutamatergic synaptic function in the circuit that connects both regions is involved in the epileptic seizures induced by 4-AP.

The effects of brucine and alcuronium on the inhibition of [3H]acetylcholine release from rat striatum by muscarinic receptor agonists

1. Radioligand binding experiments indicate that the affinity of muscarinic receptors for their agonists may be enhanced by allosteric modulators. We have now investigated if brucine can enhance the inhibitory effects of muscarinic receptor agonists on the electrically evoked release of [3H]acetylcholine ([3H]ACh) from superfused slices of rat striatum. 2. The evoked release of [3H]ACh was inhibited by all agonists tested (i.e., furmethide, oxotremorine-M, bethanechol and oxotremorine). 3. Brucine enhanced the inhibitory effects of furmethide, oxotremorine-M and bethanechol on the evoked [3H]ACh release without altering the inhibitory effect of oxotremorine. 4. Alcuronium was applied for comparison and found to diminish the inhibitory effect of furmethide on the evoked [3H]ACh release. 5. The results demonstrate that it is possible both to enhance and diminish the functional effects of muscarinic receptor agonists by allosteric modulators. 6. The direction of the observed effects of brucine and alcuronium on [3H]ACh release fully agrees with the effects of these modulators on the affinities of human M4 receptors for furmethide, oxotremorine-M, bethanechol and oxotremorine, as described by Jakubik et al. (1997). This supports the view that the presynaptic muscarinic receptors responsible for the autoinhibition of ACh release in rat striatum belong to the M4 muscarinic receptor subtype.

Attenuation of the locomotor activating effects of D-amphetamine, cocaine, and scopolamine by potassium channel modulators

1. Locomotor activating effects of D-amphetamine, cocaine, and scopolamine were determined alone and after pretreatment with K-channel modulators in mice. 2. When administered alone, D-amphetamine (1.0- 30 mg/kg) and cocaine (3.0- 56 mg/kg) produced inverted U-shaped dose-effect curves characteristic of psychomotor stimulant drugs. 3. When administered alone, scopolamine (3.0-56 mg/kg) also produced dose-dependent increases in locomotor activity but these effects plateaued with similar increases in locomotor activity induced by 10-56 mg/kg of scopolamine. 4. Pretreatment with the K-channel blockers 4-aminopyridine (0.3-1.7 mg/kg), quinine (30-100 mg/kg) or apamin (0.3-1.0 mg/kg) attenuated the locomotor increases induced by d-amphetamine, cocaine, and scopolamine. 5. Like the K-channel blockers, pretreatment with the K-channel openers cromakalim (1.0-3.0 mg/kg) and pinacidil (3.0-10 mg/kg) also attenuated the locomotor increases induced by D-amphetamine and scopolamine but did not modify the locomotor activating effects of cocaine. 6. These results demonstrate that K-channel modulation modifies the effects of D-amphetamine, cocaine, and scopolamine. 7. The results also demonstrate that K-channel openers can differentially alter the behavioral effects of cocaine and D-amphetamine.

Properties of 3,4-diaminopyridine-evoked dopamine and acetylcholine release in rabbit caudate nucleus slices: involvement of facilitatory adenosine A2 receptors or nitric oxide?

The 3H-overflow from slices of the rabbit caudate nucleus preincubated with tritiated dopamine (DA), or choline, and then superfused and stimulated twice with 3,4-diaminopyridine (3,4-DAP; 25 microM, 1 min), was explored as an in vitro model for evoked release of DA, or acetylcholine (ACh), respectively. In both cases the 3,4-DAP-evoked 3H-overflow was tetrodotoxin-sensitive and Ca(2+)-dependent and hence most probably represents action potential-induced exocytotic release of DA or ACh, respectively. Using pairs of preferential agonists/antagonists it was shown, that evoked DA release was inhibited via presynaptic D2 autoreceptors (quinpirole/domperidone) and kappa-opioid receptors (U-50488H/norbinaltorphimine). No evidence was found for the presence of presynaptic adenosine A1 or A2 receptors on dopaminergic terminals. Moreover, 3,4-DAP-evoked DA release was unaffected by increased intracellular cyclic AMP levels or by drugs affecting the NO/guanylate cyclase pathway. In a similar manner it was shown that 3,4-DAP-evoked ACh release was inhibited via presynaptic muscarine autoreceptors (oxotremorine/atropine) and dopamine D2 heteroreceptors (quinpirole/domperidone). Again, no evidence for the involvement of the NO/guanylate cyclase system in the modulation of ACh release was found, whereas the presence of inhibitory adenosine A1 receptors, but not of facilitatory A2 receptors, could be clearly established. It is concluded, that 3,4-DAP-evoked 3H-overflow from rabbit caudate nucleus slices preincubated with [3H]DA or [3H]choline, represents a simple and useful in vitro model for action potential-induced DA or ACh release, respectively. Moreover, at least in this model or rabbit brain region, facilitatory adenosine A2 receptors and the NO/guanylate cyclase system seem not to be involved in the release of these transmitters.

Protection by NMDA receptor antagonists against seizures induced by intracerebral administration of 4-aminopyridine

The effects of NMDA receptor antagonists on the convulsant action of the administration of 4-aminopyridine in the rat lateral cerebral ventricle (i.c.v. injection) and motor cerebral cortex (i.cx. injection) were studied. 4-Aminopyridine administration in both regions induced various preconvulsive symptoms, such as salivation, tremors, chewing and rearing, followed by continuous clonic convulsions and, only after i.c.v. injection, running fits and generalized tonic convulsions. This behavioral pattern appeared 5-9 min after administration of 4-aminopyridine and persisted for 100-150 min. 4-Aminopyridine also generated epileptiform electroencephalographic (EEG) discharges characterized by isolated spikes, poly-spikes and spike-wave complexes, which began some seconds after administration of the drug and were present for more than 2 h. The NMDA receptor antagonists (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP), (+/-)-2-amino-7-phosphono-heptanoic acid (AP7) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) clearly protected against some of the behavioral alterations induced by i.c.v. 4-aminopyridine, particularly the tonic convulsions, but were less effective against those produced by i.cx. 4-aminopyridine. These antagonists also delayed the appearance of EEG epileptiform discharges, reduced its amplitude, frequency and duration, and blocked their propagation to other cortical regions after i.cx. 4-aminopyridine. These results, together with previous data showing that 4-aminopyridine stimulates the release of glutamate in vivo, suggest that an excessive glutamatergic neurotransmission involving NMDA receptors is implicated in 4-amino-pyridine-induced seizures.

Presynaptic receptors and the control of glutamate exocytosis

When a typical glutamate-containing neurone fires, an action potential is propagated down the branching axon through more than a thousand varicosities. At each of these release sites the probability that a synaptic vesicle will be exocytosed into the synaptic cleft is individually controlled by means of presynaptic receptors: autoreceptors responding by positive or negative feedback to previously released transmitter, or heteroreceptors under the influence of other neurotransmitters or modulators. The simplest system in which to investigate presynaptic modulation is the isolated nerve terminal or synaptosome; studies with this preparation have revealed a complex interplay of signal-transduction pathways.

Preferential stimulation of glutamate release by 4-aminopyridine in rat striatum in vivo

The potassium channel blocker 4-aminopyridine (4-AP) is a potent convulsant drug which, in vitro, stimulates the release of neurotransmitter amino acids. We have studied the effect of 4-AP in vivo on the extracellular concentration of amino acids in rat striatum, by means of microdialysis and HPLC. Perfusion with 4-AP in the awake animal produced intense motor alterations, including barrel turning and running fits. Therefore, most microdialysis experiments were carried out in anesthetized rats. Perfusion with 20-75 mM 4-AP for 12.5 min resulted in a massive increase in extracellular glutamate (up to 20-fold), smaller increases in aspartate and taurine (up to 10-fold) and slight increments in glutamine, alanine, glycine and GABA. In contrast, perfusion with 100 mM K+ produced, mainly, an increment in taurine (7-fold) and modest increases in glutamate and aspartate (100-300%), as well as a notable decrease in glutamine. Tetraethylammonium (TEA, 120 mM) perfusion induced taurine and glutamate elevations similar to those after high K+, but glutamine was not affected. In unanesthetized rats, perfusion with 40 mM 4-AP induced changes in extracellular amino acids similar to those observed under anesthesia. In these animals neither high K+ nor TEA affected significantly the motor behavior. The results suggest that an enhancement of glutamatergic synaptic transmission, rather than a general depolarizing action, is an important factor in the neuronal hyperexcitability induced by 4-AP, which is consistent with the previously demonstrated inhibition of its convulsant effect by glutamate receptor antagonists.

3,4-Diaminopyridine and choline increase in vivo acetylcholine release in rat striatum

We investigated the effects of choline, 3,4-diaminopyridine and their combination on acetylcholine release from the corpus striatum of freely moving rats which were treated or not with atropine. Intraperitoneal administration of choline or intrastriatal administration of 3,4-diaminopyridine increased acetylcholine levels in striatal dialysates in a dose-dependent manner. When 3,4-diaminopyridine treatment was combined with choline, the observed effect was considerably greater than the sum of the increases produced by choline or 3,4-diaminopyridine alone. Administration of atropine (1 microM) in the dialysing medium was also found to be effective to stimulate striatal acetylcholine levels. 3,4-Diaminopyridine did not affect acetylcholine levels under these conditions. Whereas the choline-induced increase in acetylcholine release was significantly potentiated by atropine, co-administration of 3,4-diaminopyridine with choline failed to produce a further significant increase in the presence of atropine. These results suggest that a highly effective means for increasing acetylcholine release involves two concurrent treatments that increase neuronal choline levels and inhibition of the negative feedback modulation of acetylcholine release.

4-Aminopyridine differentially affects the spontaneous release of radiolabelled transmitters from rat brain slices in vitro

4-Aminopyridine increased the release of [3H]noradrenaline from dorsal hippocampus slices in vitro in a concentration-dependent manner. When the slices were exposed to 4-aminopyridine for 5 min, the overflow of radioactivity returned to pre-exposure values within 20-25 min. When the exposure of the slices was continued, a sustained enhancement of the release of [3H]noradrenaline was observed for the duration of the exposure. 4-Aminopyridine, 10(-4) M, had an effect of similar magnitude, or an even more pronounced effect, on the release of [3H]catecholamine from cortex, septum, periaqueductal gray and striatum slices. The effects of the compound on the release of [3H]5-hydroxytryptamine and [14C]acetylcholine were less pronounced. At this concentration 4-aminopyridine had no effect on the release of [3H]D-aspartate from hippocampus or septum slices, whereas the effect on the release of this transmitter in striatal slices was marginal. The effect of 4-aminopyridine on the release of [3H]noradrenaline in hippocampus slices was largely dependent on the presence of Ca2+ in the superfusion medium. This was also the case for the effect on the release of [3H]noradrenaline from preloaded dorsal hippocampus synaptosomes. In the presence of nitrendipine the effect of 4-aminopyridine was dose-dependently reduced, but the maximal reduction, at a nitrendipine concentration of 10(-4) M, was only 40%. Cd2+ completely abolished the effect of 4-aminopyridine on the release of [3H]noradrenaline. These results confirm that the enhancing effect of 4-aminopyridine on the release of [3H]noradrenaline depends on the entry of extracellular Ca2+ into the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine-induced increase in basal and stimulation-evoked [3H]-NA release in slices from rat hippocampus: Ca2+ sensitivity and presynaptic control

1. We have examined the mechanisms by which the K(+)-channel blocker 4-aminopyridine (4-AP) can dose-dependently increase both basal [3H]-noradrenaline ([3H]-NA) release and the [3H]-NA release evoked by electrical stimulation, but not the release of [3H]-acetylcholine ([3H]-ACh), from slices of rat hippocampus. 2. Both the electrically evoked and the 4-AP-induced release were blocked by tetrodotoxin (TTX) (3 microM). The Ca(2+)-dependence of the 4-AP-induced release (EC50 0.15 mM) was, however, different from that of the electrically evoked [3H]-NA release (EC50 0.76 mM). 3. The 4-AP-induced release could be inhibited by CdCl2(10 microM) and omega-conotoxin (30 nM), but not by nifedipine (1 microM). 4. Transmitter release evoked by 100 microM 4-AP could be blocked by the alpha 2-adrenoceptor agonist, UK 14,304 (0.1 microM) and by the A1-receptor agonist R-N6-phenylisopropyl adenosine (R-PIA, 1 microM) and increased by the alpha 2-adrenoceptor antagonist, yohimbine (1 microM), both in 0.25 and 1.3 mM Ca(2+)-containing medium. By contrast, the effect of alpha 2-adrenoceptor agonist and antagonists on transmitter release evoked by electrical stimulation was markedly reduced in the presence of 4-AP (100 microM). 5. The results suggest that 4-AP can depolarize some nerve endings in the central nervous system, leading to transmitter release that is dependent on nerve impulses and Ca2+. Furthermore, the fact that alpha 2-receptors and adenosine A1 receptor agonists can influence the release of NA evoked by 4-AP suggests that these drugs may have actions that are independent of blockade of aminopyridine-sensitive K(+)-channels.

Role of NMDA, AMPA and kainate receptors in mediating glutamate- and 4-AP-induced dopamine and acetylcholine release from rat striatal slices

Striatal slices, preincubated with [3H]dopamine and [14C]choline, were superfused continuously and subjected to electrical field stimulation (3 Hz) and perfused with amino acid analogues or 4-amino pyridine (4-AP). The released radioactivity was used to monitor release of the neurotransmitters dopamine (DA) and acetylcholine (ACh). Glutamate, NMDA (in the absence of Mg2+), AMPA, kainic acid, domoate and 4-AP all induced DA and ACh release in a concentration-dependent manner. The DA and ACh release induced by NMDA (15 microM) and glutamate (1 mM) was essentially abolished by Mg2+ (1.15 mM), whereas release induced by AMPA (100 microM), kainic acid (100 microM) or 4-AP (30 microM) was not reduced. Tetrodotoxin (1 microM) essentially abolished the effects of NMDA, markedly reduced the effects of glutamate, AMPA and 4-AP, whereas the effect of kainic acid was only modestly affected. MK-801 (30 nM) reduced NMDA-induced DA release by some 70% and ACh release by 30%. MK-801 reduced 4-AP-induced DA release by 40% but not ACh release. CNQX in a concentration (10 microM) that scarcely affected NMDA-induced ACh release, but blocked that induced by AMPA, kainic acid or domoate, reduced the ACh release induced by 4-AP. In summary, DA and ACh release from rat striatum can be stimulated by activation of NMDA and non-NMDA glutamate receptors, and this mechanism is activated by the potassium channel blocker 4-AP.

Tacrine increases stimulation-evoked acetylcholine release from rat hippocampal slices

We examined the effects of tacrine (9-amino-1,2,3,4-tetrahydroacridine) on endogenous acetylcholine (ACh) release from rat hippocampal slices. Tacrine (more than 1 microM) increased the measurable amount of basal ACh release. On the other hand, in the presence of physostigmine (50 microM; under this condition, cholinesterase activity was inhibited), tacrine did not enhance the basal ACh release. Tacrine at more than 100 microM increased the submaximal electrical stimulation-evoked release of ACh in both the absence and presence of physostigmine (50 microM). This effect of tacrine was abolished by a combination of atropine (100 mM) and physostigmine. These results indicate that a high-dose of tacrine increases cholinergic neurotransmission not only by inhibition of cholinesterase but also by increasing ACh release through an atropine-like effect, perhaps by blockade of part of the process of muscarinic autoinhibition.

Neurotensin modulates differently potassium, veratridine and 4-aminopyridine-evoked release of dopamine in rat striatal slices

We have studied the effects of neurotensin (NT) on the release of [3H]dopamine ([3H]DA) evoked by terminal depolarization with either K+, veratridine or 4-aminopyridine (4-AP). NT (1-1000 nM) induced a net potentiation (up to 170%) of the K+ (25 mM)-evoked release of [3H]DA. The capacity of NT to potentiate the effect of K+ ions decreased as the K+ concentration rose from 25 to 50 mM and totally disappeared at this high K+ concentration. NT (100 nM; 1,000 nM) had no significant effect on the veratridine (1.5; 5 microM) or 4-AP (20 microM) -evoked release of [3H]DA. The relevance of these experimental models of DA release to physiological transmitter release remains to be established. Those data highlight the complexity of the modulation of evoked neurotransmitter release by pharmacological agents.

Investigation of the mechanism of the effect of tacrine (tetrahydroaminoacridine) on the metabolism of acetylcholine and choline in brain cortical prisms

The mechanism by which tacrine increases the content and synthesis of acetylcholine (ACh) in cerebrocortical prisms exposed to an irreversible inhibitor of cholinesterases and incubated under resting conditions (Dolezal and Tucek, 1991) is not known. As found in the present experiments, this effect of tacrine is only apparent if its application had been preceded by a period of preincubation, but the preincubation is ineffective if it occurs in the presence of hemicholinium-3. Apparently, choline or a choline-containing compound accumulates in the slices during the preincubation and is then utilized for the enhanced synthesis of ACh in the presence of tacrine. Tacrine did not induce a decrease in the amount of radiolabel that had been incorporated from choline into acid-insoluble compounds, which suggests that the choline which is used for the synthesis of additional ACh does not originate from choline lipids. However, tacrine was found to diminish the efflux of choline from prisms which had been preincubated with an increased concentration of choline in the medium, and from prisms incubated in the presence of hemicholinium-3. It also diminished the efflux of radioactive choline that had accumulated in the prisms during preincubation with a very low concentration of tacrine, when the prisms were subsequently incubated with 4-aminopyridine. It is proposed that the potency of tacrine to increase the content and synthesis of ACh in cerebrocortical prisms whose cholinesterases had been inhibited is due to its ability to diminish the efflux of endogenous choline from the nerve terminals.

NMDA receptor antagonists protect against seizures and wet-dog shakes induced by 4-aminopyridine

The effect of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on the generalized tonic-clonic convulsions and wet-dog shakes induced by the intraperitoneal (i.p.) or the intrahippocampal (i.h., stereotaxic microinjection into the CA1 region) administration of 4-aminopyridine (4-AP) was studied in rats. Pretreatment with NMDA competitive and non-competitive antagonists resulted in potent protection against the motor effects of both the i.p. and the i.h. administration of 4-AP. MK-801 (0.25 mg/kg i.p.) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP, 0.8 nmol intracerebroventricular, i.c.v.) showed the most powerful anticonvulsive effect, since they prevented the occurrence of generalized tonic convulsions and the death of the animals in convulsions after i.p. 4-AP. The i.c.v. injection (10 nmol) of the NMDA competitive antagonists 2-amino-5-phosphonopentanoate (AP-5) and 2-amino-5-phosphonoheptanoate (AP-7) also showed a clear though less potent protective effect. Similarly, the frequency of wet-dog shakes induced by i.h. 4-AP was markedly decreased by pretreating the animals with i.p. MK-801 or with i.c.v. CPP or AP-7. However, the co-injection of CPP with 4-AP failed to protect against the occurrence of wet-dog shakes. The i.c.v. pretreatment with the unselective antagonist, kynurenate (up to 68 nmol) or with the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (0.5 nmol), did not significantly modify the effects of 4-AP when administered either i.p. or i.h. We conclude that NMDA receptors are involved in the mechanism of the convulsive activity induced by 4-AP, probably because this drug induces the release of glutamate.

Differential effects of potassium channel blockers on dopamine release from rat striatal slices

The effects of different potassium channel blockers on tritiated dopamine [( 3H]DA) release were investigated in rat striatal slices in the presence of pargyline and nomifensine (10 microM each). 4-Aminopyridine (4-AP; 10 and 30 microM) and 3,4-diaminopyridine (3,4-DAP; 30 microM) markedly increased the basal tritium outflow, whereas tetraethylammonium (TEA; 100-1000 microM) was without effect. The facilitating effect of 4-AP (10 microM) on spontaneous release was Ca(2+)- and K(+)-dependent. Moreover, the 4-AP-induced increase in spontaneous release was abolished in the presence of tetrodotoxin, indicating that voltage-dependent Na+ channels were involved in the release mechanism. 4-AP (10 and 30 microM) induced a dose-dependent decrease in K(+)-evoked [3H]DA release. This effect was confirmed with 3,4-DAP (30 microM). When striatal slices were depolarized with veratridine (5 microM), these two aminopyridines increased the evoked release of [3H]DA. TEA increased both K(+)- and veratridine-evoked [3H]DA release. These biochemical results are consistent with electrophysiological differences between the mechanism of action of aminopyridines and that of TEA.

Presynaptic mechanism of action of 4-aminopyridine: changes in intracellular free Ca2+ concentration and its relationship to B-50 (GAP-43) phosphorylation

Recently we have shown that 4-aminopyridine (4-AP), a drug known to enhance transmitter release, stimulates the phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat brain synaptosomes and that this effect is dependent on the presence of extracellular Ca2+. Hence, we were interested in the relationship between changes induced by 4-AP in the intracellular free Ca2+ concentration ([Ca2+]i) and B-50 phosphorylation in synaptosomes. 4-AP (100 microM) elevates the [Ca2+]i (as determined with fura-2) to approximately the same extent as depolarization with 30 mM K+ (from an initial resting level of 240 nM to approximately 480 nM after treatment). However, the underlying mechanisms appear to be different: In the presence of 4-AP, depolarization with K+ still evoked an increase in [Ca2+]i, which was additive to the elevation caused by 4-AP. Several Ca2+ channel antagonists (CdCl2, LaCl3, and diphenylhydantoin) inhibited the increase in B-50 phosphorylation by 4-AP. It is interesting that the increase in [Ca2+]i and the increase in B-50 phosphorylation by 4-AP were attenuated by tetrodotoxin, a finding pointing to a possible involvement of Na+ channels in this action. These results suggest that 4-AP (indirectly) stimulates both Ca2+ influx and B-50 phosphorylation through voltage-dependent channels by a mechanism dependent on Na+ channel activity.

Negative effects of tacrine (tetrahydroaminoacridine) and methoxytacrine on the metabolism of acetylcholine in brain slices incubated under conditions stimulating neurotransmitter release

The effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) and 7-methoxytacrine on the metabolism of brain acetylcholine were investigated in experiments in which acetylcholine turnover was stimulated by tissue depolarization or by 4-aminopyridine. Rat cerebrocortical prisms were preincubated under "resting" conditions (Krebs-Ringer buffer with 3 mmol/L K+ and with paraoxon to inhibit cholinesterases) and then incubated in the presence of tacrine or methoxytacrine and of 50 mmol/L K+. Both drugs diminished the amount of acetylcholine released by depolarization and the amount of acetylcholine synthesized during incubation; in experiments in which [14C]choline was present in the incubation medium simultaneously with tacrine or methoxytacrine, the drugs diminished the uptake of [14C]choline by the tissue and the amount of [14C]-acetylcholine synthesized and released into the medium. In these experiments, it was not possible to distinguish whether the inhibitory effects of tacrine and methoxytacrine were primarily on the process of acetylcholine synthesis (particularly on the uptake of choline), or whether the drugs also acted directly on the process of neurotransmitter release. In subsequent experiments the prisms were preincubated with [14C]choline and only then subjected to a short depolarization in the presence of hemicholinium-3 and tacrine or methoxytacrine. Both drugs severely inhibited the release of preformed [14C]acetylcholine and prevented the diminution of tissue [14C]acetylcholine stores. Methoxytacrine was also found to diminish the release of acetylcholine induced by 4-aminopyridine while increasing the content of acetylcholine in the tissue. Tacrine and methoxytacrine had no effect on the activity of choline acetyltransferase (EC 2.3.1.6).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of 3,4-diaminopyridine and choline in stimulating acetylcholine release and protecting membrane phospholipids

We investigated the effects of 3,4-DAP on ACh release from rat striatal slices superfused with or without choline, at rest and during electrical stimulation. In a choline-free medium, 3,4-DAP increased basal and stimulated ACh release while lowering the net efflux of choline; thus while the sum of ACh plus choline released remained constant, the ratio of released ACh to that of choline was increased. The drug failed to affect tissue ACh, choline or membrane phospholipid levels (including those of phosphatidylcholine). In a choline-containing medium, 3,4-DAP potentiated the enhancement by choline of both basal and electrically stimulated ACh release. Electrical stimulation alone increased ACh release from the slices without altering choline efflux or depleting tissue choline or ACh stores; however, this treatment did deplete membranes of phosphatidylcholine and of other major phospholipids. Superfusion of the slices with 3,4-DAP protected the slices from stimulation-induced phospholipid depletion. Calcium-dependent activation of high-affinity choline uptake may underlie the observed effects of 3,4-DAP.

Mechanism of aminopyridine-induced release of [3H]dopamine from rat brain synaptosomes

1. Aminopyridines (APs) induced the release of [3H]dopamine (3H-DA) from rat synaptosomal preparations. 2. 4-AP and 3,4-DAP were of equal efficacy in inducing release of 3H-DA; 3-AP, 2-AP and 2,6-AP were less active; pyridine and pyridine-4-carboxylamide were inactive. 3. Cd2+ was more effective in inhibiting 4-AP-induced release of 3H-DA (IC50 approximately 4 microM) than Co2+ and Ni2+ (IC50s approximately 500 microM). 4. While 4-AP increased the 45Ca2+ content of whole synaptosomal preparations, no effect of 4-AP on 45Ca2+ content was observed in lysed synaptosomal preparations. 5. 4-AP-induced 45Ca2+ uptake was inhibited by Cd2+, Ni2+ and Co2+ in concentration ranges similar to those inhibiting 3H-DA release.

4-Aminopyridine increases acetylcholine release without diminishing membrane phosphatidylcholine

4-Aminopyridine (10(-4)-10(-5) M) increased severalfold the release of acetylcholine from rat striatal slices superfused with an eserine-containing, choline-free medium, and caused stoichiometric decreases in the release of choline. It had no effect on tissue acetylcholine and choline levels. Electrical stimulation of the striatal slices increased acetylcholine release without affecting that of choline. Superfusion of the stimulated slices with 4-aminopyridine decreased choline release and increased the ratio of acetylcholine to choline in superfusates. As shown previously, electrical stimulation of the striatal slices decreased their contents of phospholipids, principally phosphatidylcholine; 4-aminopyridine fully protected against these membrane changes. In synaptosomal preparations, 4-aminopyridine was found to enhance the high-affinity uptake of [14C]choline and its conversion to [14C]acetylcholine. This effect on choline uptake may underlie 4-aminopyridine's ability to enhance acetylcholine release in the absence of supplemental choline while suppressing the "autocannibalism" of membrane phospholipids.

Seizures and wet-dog shakes induced by 4-aminopyridine, and their potentiation by nifedipine

The behavioral and electrographic effects of 4-aminopyridine (4-AP) administered i.p. or microinjected into the hippocampal CA1 region (i.h.) were studied in rats. The modification of such effects by the systemic administration of the Ca2+ antagonist dihydropyridine, nifedipine, was also studied. 4-AP i.p. (5 mg/kg) induced generalized tonic convulsions in 74% of the animals and death in 13%. Convulsions were characterized by electrical discharges of relatively short duration in all structures studied (frontal cortex, amygdala, dorsal hippocampus and dorsal raphe). Limbic seizures and frequent wet-dog shakes were observed when 4-AP was administered i.h. (2-4 nmol) and this behavior was correlated with hippocampal discharges, which rapidly propagated to the other structures. Pretreatment with nifedipine (7.5-50 mg/kg s.c.) markedly potentiated the effects of 4-AP. The percentage of rats that died during generalized convulsion after i.p. 4-AP increased to 56-87% and the frequency of wet-dog shakes increased after i.h. microinjection of 4-AP. Moreover, nifedipine-treated rats showed long-lasting (greater than 60 min) continuous discharges in all structures studied (status epilepticus). These results are discussed in the light of the possible participation of Ca2+ channels in the convulsant effect of 4-AP and its potentiation by nifedipine.

4-Aminopyridine stimulates B-50 (GAP43) phosphorylation and [3H]noradrenaline release in rat hippocampal slices

In situ phosphorylation of the presynaptic protein kinase C substrate B-50 was investigated in rat hippocampal slices incubated with the convulsant drug 4-aminopyridine (4-AP). Phosphorylation of B-50 was significantly enhanced 1 min after the addition of 4-AP (100 microM). This increase by 4-AP was concentration dependent (estimated EC50 30-50 microM). Concomitant with the changes in B-50 phosphorylation, 4-AP also dose-dependently stimulated [3H]noradrenaline [( 3H]NA) release from the slices. 4-AP stimulated [3H]NA release within 5 min to seven times the control level. The B-50 phosphorylation induced by 4-AP remained elevated after removal of the convulsant, this is contrast to B-50 phosphorylation induced by depolarization with K+. A similar persistent increase was observed for [3H]NA release after a 5-min incubation period with 4-AP. These results give more insight into the molecular mechanisms underlying 4-AP-induced epileptogenesis and provide further evidence for the correlation between B-50 phosphorylation and neurotransmitter release in the hippocampal slice.

4-Aminopyridine stimulates B-50 (GAP-43) phosphorylation in rat synaptosomes

Recently, we have shown that stimulation of [3H]-noradrenaline release from hippocampal slices by 4-aminopyridine (4-AP) is accompanied by an enhancement of the phosphorylation of B-50, a major presynaptic substrate of protein kinase C (PKC). PKC has been implicated in the regulation of transmitter release. In this study, we investigated the effects of 4-AP on B-50 phosphorylation in synaptosomes from rat brain and compared the effects of 4-AP with those of depolarization with K+, in order to gain more insight into the mechanism of action of 4-AP. B-50 phosphorylation was stimulated by incubation with 4-AP for 2 minutes at concentrations ranging from 10 microM to 5 mM. 4-AP (100 microM) stimulated B-50 phosphorylation already within 15 seconds; longer incubations revealed a sustained increase in the presence of 4-AP. B-50 phosphorylation was also stimulated by depolarization with 30 mM K+ for 15 seconds. The effects of both 4-AP or K+ depolarization on B-50 phosphorylation were abolished at low extracellular Ca2+ concentrations. The increase in B-50 phosphorylation induced by 4-AP seemed to be dependent on the state of depolarization, since the effect of 4-AP was largest under nondepolarizing conditions. Comparing the effects of 4-AP and K+ depolarization on B-50 phosphorylation suggests that a different mechanism of action is involved. These results indicate that the stimulation of B-50 phosphorylation by 4-AP in hippocampal slices can be attributed to a direct action of 4-AP on presynaptic terminals. In addition, our results support the hypothesis that B-50 phosphorylation by PKC is involved in Ca2(+)-dependent transmitter release evoked by 4-AP.

Effects of atropine on the release of newly synthesized acetylcholine from rat striatal slices at various concentrations of calcium ions

The release of acetylcholine (ACh) from brain tissue is known to be inhibited by muscarinic autoreceptors on cholinergic nerve terminals but the mechanism of the inhibition is not understood. Atropine brings about an increase of ACh release by removing the inhibitory action of autoreceptors. We investigated whether the effect of atropine on the release of [14C]ACh newly synthesized during incubations from [U-14C] glucose depends on the concentration of Ca2+ in the medium. In rat striatal slices incubated in the presence of an inhibitor of cholinesterases and of 30 mmol/l K+, significant increases in the release of [14C]ACh elicited by atropine were only observed during incubations with very low concentrations of Ca2+. This finding supports the view that the activation of presynaptic muscarinic autoreceptors in the brain affects the release of ACh by reducing the availability of Ca2+ that is required for transmitter liberation.

Activation of protein kinase C by 4-aminopyridine dependent on Na+ channel activity in rat hippocampal slices

The convulsant drug 4-aminopyridine (4-AP) stimulates the phosphorylation of the neuron-specific presynaptic protein B-50 in hippocampal slices. This effect could be attenuated by the protein kinase C (PKC) inhibitor staurosporine. Moreover, the endogenous phosphorylation of B-50 was found to be restricted to the 15 kDa Staphylococcus aureus protease fragment of B-50, known to contain the PKC acceptor site. The effect of 4-AP on B-50 phosphorylation was sensitive to the Na+ channel blocker tetrodotoxin. These results indicate that 4-AP stimulates PKC activity in hippocampal slices by a mechanism dependent on Na+ channel activity.

The mechanism of tetrahydroaminoacridine-evoked release of endogenous 5-hydroxytryptamine and dopamine from rat brain tissue prisms

1 Tetrahydroaminoacridine (THA) is an acetylcholinesterase (AChE) inhibitor which may have a greater therapeutic effect in Alzheimer-type dementia (ATD) than other cholinergic agents. This suggests possible non-cholinergic properties. We have therefore studied the effects of THA on the release of endogenous 5-hydroxytryptamine (5-HT) from rat cortical prisms and dopamine from striatal prisms. 2 In the presence of K+ (1 mM), THA stimulated release of both 5-HT and dopamine. THA (100 microM)-evoked monoamine release was comparable, but not additive with the release produced by K+ (35 mM). The effect was not maximal at 1 mM THA. THA-evoked release of 5-HT was independent of the presence of Ca2+ in the external medium. 3 Drugs acting on the cholinergic system, nicotine, mecamylamine, atropine, oxotremorine, physostigmine and neostigmine (all 10 microM) had no effect on 5-HT and dopamine-release. 4-Aminopyridine (4-AP), a potent acetylcholine-releasing agent, had no effect on 5-HT release and was approximately 100 fold less active than THA on dopamine release. 4 Both THA and reserpine enhanced the release of 5-HT in the presence of the monoamine oxidase inhibitor, pargyline. Reserpine- but not THA-evoked release was abolished in the absence of pargyline. Reserpine (5 mg kg-1, i.p.) markedly depleted brain monoamine concentrations 3 h after injection, while THA (15 mg kg-1, i.p.) had no effect. 5 Chloroamphetamine and fenfluramine both released 5-HT in a Ca2(+)-independent manner and with a similar potency to THA, while (+)-amphetamine released dopamine with a similar potency to THA. The effects of the amphetamines were not maximal at 1 mM. However, unlike THA, chloroamphetamine-evoked release of 5-HT was additive with release evoked by K+ (35 mM). 6 Clomipramine (IC50 = 0.036 microM) and THA (IC50 = 19.9 microM) all inhibited the uptake of [3H]-5-HT into a P2 membrane preparation. However, none of these compounds inhibited [3H]-5-HT uptake into tissue prisms during the release experiments in which the reuptake inhibitor fluoxetine (5 microM) was present. 7 We conclude that THA does not release endogenous 5-HT through a cholinergic, reserpine- or amphetamine-like mechanism or through inhibition of reuptake. The possibility exists that the release may occur via blockade of 4-AP-insensitive K+ channels.

3,4-Diaminopyridine-induced noradrenaline release from CNS tissue as a model for action potential-evoked transmitter release: effects of phorbol ester

We used rabbit hippocampus slices preincubated with [3H]noradrenaline (NA) and applied short pulses of 3,4-diaminopyridine (3,4-DAP) during superfusion to investigate the mechanism underlying the 3H overflow evoked by 3,4-DAP and the effects of the protein kinase C (PKC) activator, 4 beta-phorbol 12,13-dibutyrate (PDB), in this model. The 3H overflow evoked by 200 microM 3,4-DAP (about 4-5% of tissue-tritium) was largely Ca2+-dependent, tetrodotoxin-sensitive and markedly reduced by clonidine, but it was enhanced by yohimbine. We also demonstrated that the response could be inhibited via presynaptic adenosine (A1-) and opioid (kappa-) receptors. PDB (1 microM) markedly increased the 3,4-DAP-evoked 3H overflow, its effect being almost unchanged following activation of presynaptic alpha 2-, A1- or kappa-receptors. Inhibitors of PKC (polymyxin B, staurosporine) almost abolished the 3,4-DAP-evoked 3H overflow and antagonized the effects of PDB. It is concluded that application of 3,4-DAP (200 microM for 2 min) to brain slices leads to depolarization of the neuronal membrane, Na+ current-carried action potentials, Ca2+ influx and the exocytotic release of NA, which in many aspects resembles the release evoked by electrical field stimulation. The findings with phorbol ester further support the involvement of PKC in transmitter release. Activation of PKC apparently does not directly interfere with signal transduction mechanisms of presynaptic inhibitory receptors on noradrenergic nerve terminals.

Binding sites for [3H]AF-DX 116 and effect of AF-DX 116 on endogenous acetylcholine release from rat brain slices

The present study shows that the putative M2 ligand, [3H]AF-DX 116, binds to two classes of muscarinic sites in homogenates of rat hippocampus, striatum and cerebral cortex: one with a high affinity (Kd less than 5 nM)/low capacity (Bmax = 30-63 fmol/mg protein), and a second of lower affinity (Kd greater than 65 nM) and higher capacity (Bmax greater than 190 fmol/mg protein). In experiments which tested the effects of the muscarinic antagonists on acetylcholine (ACh) release from brain slices, the non-selective antagonist (-)-quinuclidinyl benzylate and atropine significantly enhanced the potassium (25 mM)-evoked release of ACh. This effect was mimicked by the M2 ligand AF-DX 116, but neither the M1-selective antagonist pirenzepine, nor the putative M3-muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), altered ACh release. Also, the muscarinic agonist, oxotremorine, significantly depressed evoked ACh release from brain slices, an effect that was completely antagonized by atropine or by AF-DX 116, but not by pirenzepine or 4-DAMP. Thus, it appears that presynaptic muscarinic autoreceptors in the rat hippocampus, striatum and cerebral cortex belong to the M2 subtype of muscarinic receptors.

Differential effects of 4-aminopyridine on acetylcholine release triggered by K+ depolarization, veratridine, or A23187 in rat cerebral cortical synaptosomes

The effect of 4-aminopyridine on [3H]acetylcholine release was studied in rat cerebral cortical synaptosomes in the presence of a several secretagogues that have different mechanisms of action. As found previously, 4-aminopyridine increased [3H]acetylcholine release in a concentration-dependent manner (5-10 mM); a high concentration (10 mM) also elevated [3H]choline efflux. However, the 35 mM K+ induced release of [3H]acetylcholine was attenuated by 4-aminopyridine at concentrations (less than 5 mM) that had no effect on transmitter release. At no concentration of 4-aminopyridine was the release of transmitter additive with 35 mM K+ induced release. Veratridine-induced release was neither attenuated nor additive with low concentrations of 4-aminopyridine, even when a sub-maximal concentration of the sodium ionophore was used (10 microM). In contrast, A23187-induced release was additive with that caused by 4-aminopyridine. These results suggest that: 1) 4-aminopyridine blocks potassium channels involved in regulating membrane potential in isolated cholinergic terminals; and 2) changes in the activity of these 4-aminopyridine sensitive K+ channels are not important in the nerve terminal's response to depolarization caused by sodium influx.

Physostigmine stimulates phosphoinositide breakdown in the rat neostriatum

The reversible acetylcholinesterase inhibitor, physostigmine, stimulated in a dose-dependent manner the accumulation of [3H]inositol monophosphate ([3H]IP1) in lithium-treated neostriatal slices. The muscarinic agonists, carbachol and oxotremorine, also stimulated [3H]IP1 accumulation. Atropine completely blocked the physostigmine-induced accumulation but had no effect on the basal accumulation. Tetrodotoxin partially inhibited the physostigmine-induced [3H]IP1 accumulation but had no effect on the carbachol-induced accumulation. 4-Aminopyridine stimulated the basal [3H]IP1 accumulation and potentiated the physostigmine-induced accumulation. This potentiation was blocked by tetrodotoxin. The physostigmine dose-response curve for the stimulation of [3H]IP1 accumulation was similar to its dose-response curve to inhibit acetylcholinesterase activity in the neostriatum. The results suggest that, under our experimental conditions, the acetylcholine released spontaneously from intrinsic cholinergic neurons does not activate the striatal muscarinic receptors coupled to phosphoinositide breakdown unless the intrinsic acetylcholinesterases are inhibited.

Blockade of a cardiac K+ channel by tacrine: interactions with muscarinic and adenosine receptors

The centrally acting anticholinesterase drug tacrine has been shown to block K+ channels in guinea pig left atrium. It competitively blocks the negative inotropic effects of adenosine, 2-chloroadenosine and carbachol. Ka values obtained from dose ratio plots were 2.5, 3.5 and 2.9 microM respectively. It was also able to antagonize the shortening of the action potential due to these compounds. Doses of tacrine ranging from 1 to 4 microM restored the AP configuration close to control values. Tacrine also antagonized the binding of 1-quinuclidinyl[phenyl-4-3H]benzilate ([3H]QNB) to membranes derived from the atrium and cerebral cortex. Ki values of 1.8 +/- 0.33 and 1.3 +/- 0.47 microM were obtained respectively. Tacrine was a weak competitor of [3H]phenylisopropyladenosine ([3H]L-PIA) binding in brain membranes. Its diverse pharmacological effects may be relevant to its use in Alzheimer's disease.

The influence of aminopyridines on Ca2+-dependent evoked release of acetylcholine from rat cortex slices

The release of acetylcholine (ACh) elicited by electrical stimulation was investigated in rat brain cortical slices preloaded with 3H-choline. Decreasing the [Ca2+]o from 2.5 to 0.3 mM caused a progressive reduction of the evoked release of ACh. 4-Aminopyridine (4AP) or LF14 [(1,1-dimethyl-3-(4-amino-3-pyridyl)], 4 x 10(-5) M doubled the evoked release of ACh when the [Ca2+]o was 2.5 mM and quadrupled it when it was 0.3 mM, to levels higher than those obtained with 2.5 mM [Ca2+]o alone. This indicates that both 4AP and LF14 decrease the Ca2+ requirements for the evoked release of ACh. The findings of this study indicate that LF14 may be suitable for the symptomatic treatment of senile dementia of Alzheimer's type, presumably caused by dysfunction of cholinergic transmission in the brain.

Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.

Differential effects of 4-aminopyridine and 2,4-diaminopyridine on the in vivo release of acetylcholine and dopamine in freely moving rats measured by intrastriatal dialysis

The central effects of 4-aminopyridine (4-AP), a blocking agent of voltage-dependent potassium channels, and its more polar analogue, 2,4-diaminopyridine (2,4-DAP), were studied after i.p. injection and direct intrastriatal administration in rats. The effects of the drugs on the release of acetylcholine (ACh) and dopamine (DA) were quantified by means of an in vivo microdialysis sampling technique. Both neurotransmitters were determined by on-line HPLC analysis. Both aminopyridines increased the release of ACh dose dependently when administered intrastriatally. After peripheral administration, however, 4-AP but not 2,4-DAP induced an increase in the release of ACh. These results are interpreted as being due to the greater lipid solubility of 4-AP compared to 2,4-DAP and hence its better penetration through the blood-brain barrier. Intrastriatal administration of 4-AP induced a much lower increase in the release of DA compared to ACh, whereas there was no change in the release of DA after peripheral administration. These results indicate that the sensitivity of excitable membranes to the releasing effects of 4-AP is not the same for DA- and ACh-containing neurotransmitter systems.

9-Amino-1,2,3,4-tetrahydroacridine (THA), an alleged drug for the treatment of Alzheimer's disease, inhibits acetylcholinesterase activity and slow outward K+ current

The in vitro release of acetylcholine in rat brain tissue was inhibited by 9-amino-1,2,3,4-tetrahydroacridine (THA). Atropine antagonized this effect of THA. As THA does not display an affinity for muscarinic receptors, we conclude that THA inhibits acetylcholinesterase activity. In electrophysiological studies with neurons of Lymnaea stagnalis, THA inhibited the slow outward K+ current and consequently increased the duration of the action potentials. It is discussed that both effects of THA possibly contribute to its reported effect in the treatment of patients with Alzheimer's disease.

Neurotoxicity of quinolinate in the rat nucleus basalis magnocellularis

Injections of quinolinic acid (QUIN) alone or combinations of QUIN with dipicolinic acid, Mg2+ or 3,4-diaminopyridine were made into the nucleus basalis magnocellularis of the rat. Choline acetyltransferase activity in the cortex decreased following injections of QUIN and this was antagonized by co-injections of QUIN with dipicolinic acid or Mg2+. Co-injections of QUIN with 3,4-diaminopyridine increased the neurotoxicity of quinolinic acid, while diaminopyridine alone was not toxic.

Failure of the calcium channel activator, Bay K 8644, to increase the release of acetylcholine from nerve terminals in brain and diaphragm

The calcium channel activator Bay K 8644 did not increase the release of acetylcholine from rat brain cortex prisms incubated in the presence of 3 mmol l-1 or 25 mmol l-1 K+ nor from rat diaphragms incubated in the presence of 5 mmol l-1 or 25 mmol l-1 K+. It also did not influence the release of acetylcholine from cortex prisms incubated in the presence of 25 mmol l-1 K+ and of lowered concentrations of Ca2+ ions. It is concluded that the voltage-dependent Ca2+ channels in the nerve terminals, responsible for the depolarization-induced influx of Ca2+ ions into the nerve terminals and thus for the depolarization-evoked release of acetylcholine from the nerve terminals, are different from the voltage-dependent Ca2+ channels in the heart and smooth muscle cells.

Mechanism of the calcium-dependent stimulation of transmitter release by 4-aminopyridine in synaptosomes

The mechanism of the Ca2+-dependent stimulation of neurotransmitter release by 4-aminopyridine in synaptosomes was studied. The stimulation of gamma-[3H]aminobutyric acid and [3H]acetylcholine release by 4-aminopyridine was not significantly affected either by tetrodotoxin or by the absence of Na+ in the medium, whereas the toxin notably inhibited the release of both transmitters induced by veratridine. On the other hand, the release of labeled gamma-aminobutyric acid induced by 4-aminopyridine was inhibited by both La3+ and ruthenium red, two blockers of Ca2+ transport in synaptosomes. In other experiments, 4-aminopyridine had only a slight stimulatory effect, if any, on the influx of 45Ca2+ into synaptosomes, under both resting and K+-depolarizing conditions. Ruthenium red inhibited the stimulation by K+ of the 45Ca2+ uptake, and 4-aminopyridine did not antagonize this inhibition. We conclude that the transmitter-releasing action of 4-aminopyridine in synaptosomes does not involve an excitatory effect on the membrane which may result in the opening of voltage-sensitive Na+ channels. 4-Aminopyridine does not seem to act either by enhancing Ca2+ entry into the synaptosomes. It is proposed that 4-aminopyridine facilitates the coupling between Ca2+ binding and transmitter secretion at the presynaptic membrane.

Modulation of the endogenous acetylcholine release from rat striatal slices

The dopaminergic modulation of the acetylcholine release from rat striatal slices has been investigated using a chemiluminescent method. Dopamine, more efficiently than apomorphine, decreased the potassium-evoked release of acetylcholine. The effect of dopamine antagonists, haloperidol and sulpiride, has been studied, and haloperidol was a better antagonist than sulpiride to the dopamine effect. Haloperidol elicited an acetylcholine release from striatal slices at 0.1 nM, probably by removing endogenous dopamine from dopaminergic receptors.

The effect of nicotine and cytisine on 3H-acetylcholine release from cortical slices of guinea-pig brain

Nicotine 1.8 X 10(-5)-1.8 X 10(-4) mol/l enhanced the spontaneous 3H-efflux from guinea-pig cortical slices preloaded with 3H-choline and perfused in the presence of hemicholinium (HC-3). The facilitation of tritium outflow was prevented by tetrodotoxin 5 X 10(-7) mol/l and by D-tubocurarine 4.5 X 10(-6) mol/l. Nicotine 1.8 X 10(-6)-1.8 X 10(-4) mol/l, and the agonist cytisine 5 X 10(-7)-5 X 10(-5) mol/l increased, in a concentration-dependent way, 3H-efflux from electrically-stimulated slices (0.2 Hz). The concentration-response curves of both drugs were parallelly shifted to the right by D-tubocurarine 4.5 X 10(-6) mol/l. The EC50 values (i.e. the concentrations required to cause a 50% increase in the S2/S1 ratio) changed for nicotine from 5.58 X 10(-5) to 4.34 X 10(-4) and for cytisine from 6.3 X 10(-6) to 2.75 X 10(-4) mol/l in the absence and in the presence of the antagonist, respectively. In the range of 0.2-2 Hz the magnitude of the effect of nicotine was inversely related to the rate of stimulation. The response to nicotine was subject to rapidly developing tachyphylaxis; it was resistant to atropine. It is concluded that nicotine and cytisine facilitate 3H-efflux from the cholinergic nerve endings of guinea-pig cerebral cortex. This effect involves sodium-dependent mechanisms and is due to an interaction of the drugs with receptors showing affinity for D-tubocurarine.