Idebenone and vinpocetine augment long-term potentiation in hippocampal slices in the guinea pig

Synthesis and pharmacological activity of vinpocetine derivatives

Vinpocetine and its derivatives were extensively employed in the treatment of ischemic stroke, serving as effective cerebrovascular vasodilators. They could also be utilized for neuroprotection, anti-inflammatory purposes, anti-aging interventions, insomnia treatment, and antidepressant effects. However, due to issues such as hepatic first-pass effect, low bioavailability, and poor patient compliance with multiple dosing, the secondary development of Vinpocetine to address these limitations became a prominent area of research. Five primary methodologies were employed for the synthesis of Vinpocetine derivatives. These included substitution on the A ring to modify the 14-ester group, alteration of the 16-ethyl group, simplification of the D and E rings, and modification of the conformation of Vinpocetine. This paper summarized the current synthesis and activity studies of Vinpocetine and its derivatives, with the aim of providing a reference for the discovery of more potent derivatives of Vinpocetine.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Reactive oxygen species in the regulation of synaptic plasticity and memory

The brain is a metabolically active organ exhibiting high oxygen consumption and robust production of reactive oxygen species (ROS). The large amounts of ROS are kept in check by an elaborate network of antioxidants, which sometimes fail and lead to neuronal oxidative stress. Thus, ROS are typically categorized as neurotoxic molecules and typically exert their detrimental effects via oxidation of essential macromolecules such as enzymes and cytoskeletal proteins. Most importantly, excessive ROS are associated with decreased performance in cognitive function. However, at physiological concentrations, ROS are involved in functional changes necessary for synaptic plasticity and hence, for normal cognitive function. The fine line of role reversal of ROS from good molecules to bad molecules is far from being fully understood. This review focuses on identifying the multiple sources of ROS in the mammalian nervous system and on presenting evidence for the critical and essential role of ROS in synaptic plasticity and memory. The review also shows that the inability to restrain either age- or pathology-related increases in ROS levels leads to opposite, detrimental effects that are involved in impairments in synaptic plasticity and memory function.

The nootropic drug vinpocetine modulates different types of potassium currents in molluscan neurons

Three types of high-threshold K+ currents were recorded in isolated neurons of the snail Helix pomatia using a two-microelectrode voltage clamp technique: transient K+ current (I(A)), delayed rectifier (I(KD)) and Ca2+-dependent K+ current (I(K(Ca))). Vinpocetine (1-100 microM) applied to the bath affected different types of K+ current in different ways: I(A) was increased (35+/-14%), I(KD) was moderately inhibited (20+/-9%) and I(K(Ca)) was strongly suppressed (45+/-15%). When I(A) and I(K(Ca)) were present in the same cell, vinpocetine exerted a dual effect on the total K+ current, depending on the amplitude of the test stimulus. In the presence of vinpocetine, the I-V curve crossed the control I-V curve. The inhibition of I(K(Ca)) by vinpocetine between 1 and 100 microM is unlikely to be a result of Ca2+ current (I(Ca)) suppression, as the latter was inhibited only at vinpocetine concentrations exceeding 300 microM. Dibutyryl cyclic GMP (dbcGMP) (but not dbcAMP) mimicked the effects of vinpocetine in the majority of cells tested (coefficient of correlation r=0.60, P<0.05, n=22). The data suggest that modulation of different types of K+ current in neuronal membrane can contribute, at least partially, to the nootropic effect of vinpocetine through the regulation of intracellular Ca2+ concentration.

NIK-247 induces long-term potentiation of synaptic transmission in the CA1 region of rat hippocampal slices through M2 muscarinic receptors

1. The purpose of this study was to examine whether NIK-247 can, by itself, induce long-lasting changes in synaptic efficacy in the hippocampus. Population spikes evoked by electrical stimulation of the stratum radiatum were recorded in the pyramidal cell layer of the CA1 region of the isolated hippocampus. 2. NIK-247 at 1 x 10(-7) - 1 x 10(-5) M dose dependently increased the amplitude of these spikes. The increase in population spikes by NIK-247 outlasted, for 2 hr, its presence. In addition, the increase in population spikes recovered to 2 hr after washout of NIK-247. Therefore, it was concluded that NIK-247 induced long-term potentiation (LTP) by itself. However, tacrine and physostigmine at 1 x 10(-7) - 1 x 10(-5) M did not increase the amplitude of population spikes and did not induce LTP by themselves. 3. The increase in amplitude of population spikes induced by NIK-247 was completely blocked sensitively by atropine (IC50 = 4.3 x 10(-8)M) but insensitively by pirenzepine (IC50 = 9.1 x 10(-7) M). Carbachol also increased the amplitude of population spikes in the presence of pirenzepine. 4. These findings indicate that the LTP induced by NIK-247 is due to its M2 muscarinic agonistic effect in the CA1 region of the rat hippocampus. It is expected that NIK-247 may be useful for the treatment of Alzheimer disease.

FK960, a novel potential anti-dementia drug, augments long-term potentiation in mossy fiber-CA3 pathway of guinea-pig hippocampal slices

Our previous studies have demonstrated that FK960 (FR59960; N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate), a novel antidementia piperazine derivative, exerts beneficial effects on memory deficits in various animal models of amnesia in rats [M. Yamazaki, N. Matsuoka, N. Maeda, Y. Ohkubo, I. Yamaguchi, FK960 N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate ameliorates the memory deficits in rats through a novel mechanism of action, J. Pharmacol. Exp. Ther., 279 (1996) 1157-1173.] and in rhesus monkeys [N. Matsuoka, T.G. Aigner, FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel potential antidementia drug, improves visual recognition memory in rhesus monkeys: comparison with physostigmine, J. Pharmacol. Exp. Ther., 280 (1997) 1201-1209]. To clarify the synaptic mechanisms of its antiamnesic action, FK960 was investigated for its effects on the development of long-term potentiation (LTP) in guinea-pig hippocampal slices. The magnitude of LTP of population spike recorded in CA3 pyramidal neurons was significantly augmented by perfusing FK960 (10-9-10-6 M) for 25 min before and during tetanic stimulation of the mossy fibers, whereas the basal amplitude of population spikes before tetanus was hardly affected by the drug. The dose-response curve was bell-shaped with a maximal augmentation at 10-7 M. Scopolamine (10-6 M) per se had little effect on the magnitude of LTP in the mossy fiber-CA3 pathway, but significantly attenuated its enhancement by FK960 (10-7 M). In hippocampal slices from animals treated with cysteamine (200 mg/kg, s.c.), which was shown to deplete the hippocampal somatostatin, FK960 (10-7 M) hardly affected the LTP. These results suggest that FK960 enhances the magnitude of LTP in the mossy fiber-CA3 pathway through an activation of the cholinergic-somatostatinergic link in the hippocampal formation. Furthermore, it can be postulated that the drug regulates the cognitive function by modulating directly synaptic plasticity in the hippocampal neuronal network.

Nootropic agent vinpocetine blocks delayed rectified potassium currents more strongly than high-threshold calcium currents

A two-microelectrode potential clamping method was used on isolated common snail neurons to measure high-threshold Ca2+ and delayed rectified K+ currents. Addition of the nootropic agent vinpocetine (VPC) to the bathing solution rapidly and reversibly inhibited both types of current. The effects of VPC were dose-dependent and were independent of the test stimulus voltage. Maximum blockade of the Ca2+ current averaged 27% at a VPC concentration of 600 microM. Maximum blockade of the K+ current averaged 76% at a VPC concentration of 30 microM. It is concluded that K+ channels are more likely targets of VPC than Ca2+ channels.

Cognitive enhancers and hippocampal long-term potentiation in vitro

We review our works on the pharmacological modulation of long-term potentiation (LTP) at guinea pig hippocampal mossy fiber-CA3 synapses in vitro. The magnitude of tetanus-induced LTP at the mossy fiber synapse was augmented by perfusion of slices with several cognitive enhancers, such as bifemelane (1 microM). The mossy fiber LTP was enhanced by somatostatin (0.32 microM) and inhibited in somatostatin-depleted slices from cysteamine-treated guinea pigs. An involvement of the 5-HT3 receptor also showed that granisetron (0.1 microM) enhanced the mossy fiber LTP. The above-mentioned enhancements by perfused agents were commonly reversed, at least in part, by muscarinic antagonists. However, the magnitude of mossy fiber LTP was bidirectionally modulated by muscarinic stimulations of slices with physostigmine or carbachol at different concentrations. The enhancing effects of high-concentration carbachol was antagonized by pirenzepine, and in contrast, the inhibition by low-concentration carbachol was antagonized in the presence of AF-DX116. When guinea pigs were preinjected with the cholinotoxin AF64A, the magnitude of LTP was decreased in the slices prepared from AF64A-treated animals. These results suggest that endogenous acetylcholine dominantly plays facilitatory roles through muscarinic M1 receptors in the induction of mossy fiber LTP. The pharmacological characterization of mossy fiber LTP may be of help to the evaluation of cognitive enhancers at a neuronal circuit level.

Behavioral LTP during learning in rat hippocampal CA3

Behavioral long-term potentiation (LTP) of the potentials with the advance of learning were investigated in hippocampal mossy fiber (MF)-CA3 synapses. In both acute and chronic experiments, control and test groups of rats were used. Daily learning tasks in the radial arm maze were given to animals in the test groups. Complete acquisition of learning was observed on day 5. In acute experiments, when MF stimulation-induced population spikes (PS) were examined on day 7 of learning task, relatively higher PS were observed in the test group than those in the control group, and tetanic stimulation (TS)-induced LTP was still observed from the increased PS before TS in the test groups. In chronic experiments, in which the MF stimulation-induced PS were recorded daily after the learning tasks with implanted electrodes, amplitude of PS of the test groups, but not the control, increased concomitantly with the progress in learning. The PS potentiation persisted steadily from day 4 to day 7 of training. The fact that LTP in the MF-CA3 system coincided well with the advance of learning suggests that the MF-CA3 system is involved in the acquisition of learning and/or memory storage.

Roles of endogenous cholinergic neurons in the induction of long-term potentiation at hippocampal mossy fiber synapses

To evaluate the functional role of endogenous acetylcholine (ACh) in the induction of long-term potentiation (LTP) at mossy fiber-CA3 synapses, the influence of cholinergic hypofunction on it was investigated. Administration of a cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A; 5 nmol, i.c.v.), to guinea pigs one week prior to preparing slices resulted in a significant decrease in the magnitude of LTP, associated with a significant decrease in cholineacetyltransferase (ChAT) activity and the number of ChAT immunoreactive cells in the hippocampal slices. Bath-application of a cholinesterase inhibitor, physostigmine at 0.1 microM and 10 microM, attenuated and augmented, respectively, the magnitude of LTP in slices prepared from vehicle-treated animals (naive slices), whereas that in slices prepared from AF64A-treated animals (lesioned slices) was not significantly affected by physostigmine at any concentration tested. The induction of LTP in naive slices was inhibited or facilitated by a muscarinic M1 antagonist pirenzepine (1 microM) and by an M2 antagonist AF-DX 116 (1 microM) alone, respectively, whereas that in lesioned slices was not significantly changed by either of them. Furthermore, bath-applied carbachol (CCh) at 0.01-10 microM augmented the magnitude of LTP in lesioned slices, whereas the induction of LTP in naive slices was inhibited and facilitated by CCh at 0.01-0.1 microM and 1-10 microM, respectively, as reported previously. Such an augmentation of LTP by CCh was reversed by pirenzepine, but not by AF-DX 116. These observations suggest that AF64A induces the defect in ACh release and the hypofunction of M2 receptors, but not of M1, at least during the induction of LTP at mossy fiber-CA3 synapses.(ABSTRACT TRUNCATED AT 250 WORDS)

LTP of mossy fiber-stimulated potentials in CA3 during learning in rats

The study was done to determine whether long-term potentiation (LTP) of the potentials occurs at mossy fiber (MF)-CA3 synapses with the advance of learning in unrestrained and unanesthetized rats. The rats were divided into two groups, the test and control groups. The test group was given daily learning tasks in the radial arm maze, whereas the control group was similarly handled without learning tasks. Complete acquisition of learning was observed in the test group on day 5, and the learning was maintained over 3 days. Under freely moving conditions, a significant increase in population spikes (PS) elicited by MF stimulation with the progress in learning was observed in the test group, and the PS potentiation remained stable after day 4. Furthermore, on day 7, when MF stimulation-induced PS in the test group were compared with that in the control group in pentobarbital-anesthetized rats, the responses were comparatively higher in the former. As the training-induced PS potentiation in CA3 occurred with the advance of learning, these findings suggest that LTP in CA3 induced by learning may be related to memory storage.

Pharmacological and electrographic properties of epileptiform activity induced by elevated K+ and lowered Ca2+ and Mg2+ concentration in rat hippocampal slices

We studied some of the physiological and pharmacological properties of an in vitro model of epileptic seizures induced by elevation of [K+]0 (to 8 mM and 10 mM) in combination with lowering of [Mg2+]0 (to 1.4 mM and 1.6 mM) and [Ca2+]0 (to 0.7 mM and 1 mM) in rat hippocampal slices. These concentrations correspond to the ionic constitution of the extracellular microenvironment during seizures in vivo. The resulting activity was rather variable in appearance. In area CA3 recurrent discharges were observed which resulted in seizure-like events with either clonic-like or tonic-clonic-like ictaform events in area CA1. With ion-sensitive electrodes, we measured the field potential and the changes in extracellular ion concentrations which accompany this activity. The recurrent discharges in area CA3 were accompanied by small fluctuations in [K+]0 and [Ca2+]0. The grouped clonic-like discharges in area CA1 were associated with moderate increases in [K+]0 and small decreases in [Ca2+]0 in the order of 2 mM and 0.2 mM, respectively. Large, negative field-potential shifts and increases in [K+]0 to 13 mM, as well as decreases in [Ca2+]0 by up to 0.4 mM, accompanied the tonic phase of ictaform events. The ictaform events were not blocked by D-2-aminophosphonovalerate (2-APV) but were sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) alone and in combination with 2-APV and ketamine. In order to determine the pharmacological characteristics of the ictaform events we bath-applied most clinically employed anticonvulsants (carbamazepine, phenytoin, valproate, phenobarbital, ethosuximide, trimethadione) and some experimental anticonvulsants (losigamone, vinpocetine, and apovincaminic acid). Carbamazepine, phenytoin, valproate, and phenobarbital were effective at clinically relevant doses. The data suggest that the high-K+ model of epileptiform activity is a good model of focal convulsant activity.

Role of somatostatin in the augmentation of hippocampal long-term potentiation by FR121196, a putative cognitive enhancer

N-(4-Acetyl-1-piperazinyl)-4-fluorobenzenesulfonamide (FR121196), a newly introduced putative cognitive enhancer of a derivative of piperazine, was investigated for its effects on long-term potentiation in guinea-pig hippocampal slices. The magnitude of long-term potentiation of population spikes recorded in CA3 pyramidal neurons was significantly augmented by perfusing FR121196 (10(-9)-10(-6) M) for 25 min before and during tetanic stimulation of the mossy fibers; the basal amplitude of population spikes before tetanus was hardly affected by the drug. The dose-response curve was bell-shaped with a maximal augmentation at 10(-7) M. Similar activity and bell-shaped dose-response curve were observed with methamphetamine (10(-8)-10(-6) M). Physostigmine (10(-8)-10(-6) M) also facilitated long-term potentiation of this pathway and the magnitude of augmentation was concentration-dependent. Scopolamine (10(-6) M) per se had little effect on the magnitude of long-term potentiation in the mossy fiber-CA3 pathway, but significantly attenuated its enhancement by FR121196 (10(-7) M) and physostigmine (10(-6) M), although it failed to influence that by methamphetamine (10(-7) M). In hippocampal slices from animals treated with cysteamine, which was shown to deplete hippocampal somatostatin, FR121196 (10(-7) M) hardly affected long-term potentiation generation, whereas physostigmine (10(-6) M) and methamphetamine (10(-7) M) augmented it significantly. These results suggest that FR121196 enhances the development of long-term potentiation in the mossy fiber-CA3 pathway through activation of somatostatinergic neurons in the hippocampal formation.

Bidirectional modulation of long-term potentiation by carbachol via M1 and M2 muscarinic receptors in guinea pig hippocampal mossy fiber-CA3 synapses

Participation of muscarinic M1 and M2 receptors in the modulation of long-term potentiation (LTP) was studied in the mossy fiber-CA3 synapse of guinea pig hippocampal slices. The magnitude of tetanus-induced LTP was attenuated in the presence of 0.01-0.1 microM carbachol, at which concentration the pre-tetanus amplitude of field excitatory postsynaptic potential (fEPSP) was not affected. The attenuation of LTP by the low concentration of carbachol was reversed by an M2 muscarinic antagonist, AF-DX 116, but not by an M1 antagonist, pirenzepine. On the contrary, a high concentration (10 microM) of carbachol decreased the pre-tetanic amplitude of fEPSP, however, the magnitude of LTP was significantly larger than that in control slices in which pre-tetanic amplitude of fEPSP was reduced to the level of carbachol-treated slices by reducing the intensity of stimulation or extracellular Ca2+ concentration. The augmentation of LTP by 10 microM carbachol was blocked by pirenzepine but not by AF-DX 116. These results suggest that the synaptic plasticity in the guinea pig hippocampal mossy fiber-CA3 synapse is inhibited and facilitated by muscarinic agonist through muscarinic M2 and M1 receptors to inhibit and facilitate the LTP, respectively.

Effect of different subtypes of cognition enhancers on long-term potentiation in the rat dentate gyrus in vivo

The effect of four drugs considered as cognition enhancers on the amplitude of the population spikes and on the long-term potentiation (LTP) evoked by perforant path stimulation was investigated in rat dentate gyrus in vivo. LTP was characterized by the absolute increase in the amplitude of the population spikes, which were expressed in mV, contrary to the widely used percentage value, because the absolute increase was independent of the pretetanus level, whereas the percentage increase was found to be negatively correlated with it. Intravenous administration of the drugs (piracetam 500 mg/kg, hydergine 2 mg/kg, vinpocetine 0.1 and 5 mg/kg and physostigmine 0.01 and 0.1 mg/kg) did not influence the amplitude of the population spikes itself. Piracetam and hydergine did not have an effect on LTP, while vinpocetine and physostigmine altered LTP in a similar manner. The higher doses of the two latter drugs, administered 5 min before tetanic stimulation, induced a significant potentiation of LTP, whereas a significant inhibition of LTP was obtained when the drugs were administered 30 min before tetanic stimulation. Based on the results obtained from guinea pig hippocampal slices, an LTP-potentiating effect of all compounds tested could have been anticipated, but this was not supported by our data. The apparent contradiction between the in vivo and in vitro results is discussed.

Effects of L-threo-DOPS, a noradrenaline precursor, on the long-term potentiation in the rat hippocampal mossy fiber-CA3 region

The effects of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS), a synthetic precursor of norepinephrine (NE), on the long-term potentiation (LTP) in the hippocampal mossy fiber-CA3 system was examined in urethane-anesthetized rats, the objective being to determine whether or not this drug acts as NE on the LTP. L-threo-DOPS may be effective for treating some type of mental disorders, including dementia. The LTP, induced in CA3 by tetanic stimulation (100 Hz for 1 s) applied to the mossy fiber persisted for more than 4 h. When L-threo-DOPS (50 and 150 micrograms) was injected into the lateral ventricle 30 min prior to the tetanic stimulation, there were no significant alterations in the LTP. However, in animals treated with reserpine (5 mg/kg i.p.) 24 h before the experiment, LTP was not induced with tetanic stimulation alone yet was obtained when tetanic stimulation was preceded by L-threo-DOPS (50 and 150 micrograms) applied to the ventricle. The LTP obtained by L-threo-DOPS in the reserpine-treated animal was inhibited by pretreatment with benserazide and was completely blocked by the simultaneous administration of sotalol. These results suggest that NE converted from L-threo-DOPS plays an important role in inducing LTP in the mossy fiber-CA3 system in the animals deficient in catecholamines.

Specific binding sites for bifemelane in the hippocampus of the guinea pig, relevant to its pharmacological actions

Bifemelane has an anti-amnesic effect, produces the translocation of protein kinase C in the hippocampal CA3 region but not in CA1 and enhances long-term potentiation in the mossy fibre-CA3 system but not in the Schaffer collateral-CA1 system. The present study examined the specific binding of [3H]bifemelane in membrane preparations of guinea pig hippocampus and regional differences in such a binding. The binding of [3H]bifemelane was reversible and greater when incubated at 4 degrees C than at 25 or 37 degrees C. The binding of [3H]bifemelane appeared to be composed of at least 2 different affinity components. Imipramine significantly suppressed the binding of [3H]bifemelane at 1 microM and, in the presence of 1 microM imipramine, the low-affinity component of the binding of [3H]bifemelane was eliminated. The density of specific binding sites for 1 nM [3H]bifemelane was significantly higher in the hippocampal CA3 region than in the CA1. The specific binding of 1 nM [3H]bifemelane was not inhibited by other nootropic drugs, such as idebenone, calcium hopantenate, vinpocetine, indeloxazine and piracetam. The present results suggest that there are specific binding sites for bifemelane in hippocampus, which are different from those for other nootropic drugs tested and that the regional differences in the pharmacological susceptibilities to bifemelane are at least, in part, attributed to those in the density of binding sites for bifemelane.

Effects of thyrotropin-releasing hormone and a related analog, CNK-602A, on long-term potentiation in the mossy fiber-CA3 pathway of guinea pig hippocampal slices

The effects of thyrotropin-releasing hormone (TRH) and a related analog, CNK-602A, that induces the release of catecholamines, on long-term potentiation (LTP) of the population spike in mossy fiber-CA3 pathways were investigated in guinea pig hippocampal slices. TRH augmented LTP of the population spike at concentrations of 10(-6)-10(-5) M. CNK-602A also augmented LTP at concentrations of 10(-6)-10(-5) M in a dose-dependent manner. LTP in slices of the hippocampus obtained from animals given 6-hydroxydopamine (6-OHDA) intraventricularly was significantly lower than that in non-treated animals. However, both TRH and CNK-602A (10(-6) M) augmented LTP in slices from 6-OHDA-treated animals. These findings suggest that TRH and CNK-602A augment LTP in the mossy fiber-CA3 pathway without activating noradrenergic and/or dopaminergic fibers.

Somatostatin augments long-term potentiation of the mossy fiber-CA3 system in guinea-pig hippocampal slices

The effect of exogenously applied somatostatin (1-14), which is one of the candidates of neuromodulators in the hippocampus, on long-term potentiation (LTP) was investigated in the CA1 and CA3 subfields of guinea-pig hippocampal slices. In the mossy fiber-CA3 pyramidal cell system, the magnitude of LTP of both population excitatory postsynaptic potential (pEPSP) and population spike was significantly augmented by somatostatin (10(-7)-10(-6) M) perfused before and during tetanic stimulation which never affected basal amplitude of population spikes before tetanus. The enhancement of LTP by somatostatin lasted for at least one hour after washout. On the other hand, somatostatin at the most effective concentration (3.2 x 10(-7) M) in the above described system failed to affect the magnitude of the LTP of population spikes in Schaffer collateral-CA1 pathway. The enhancing effect of somatostatin on LTP in the mossy fiber CA3 system was inhibited either by a muscarinic antagonist, scopolamine (10(-6) M), or a beta-adrenoceptor antagonist, timolol (10(-6) M). These results suggest that somatostatin enhances the production of LTP in the mossy fiber-CA3 pathway of the guinea-pig hippocampus through the intervention of cholinergic and noradrenergic neurons.

The use of Xenopus oocytes to evaluate drugs affecting brain Ca2+ channels: effects of bifemelane and several nootropic agents

Effects of nootropic or cerebroprotective drugs on voltage-sensitive Ca2+ channels (VSCC) in the mammalian brain were evaluated comparatively by depolarization-evoked Ca2+ channel currents in Xenopus oocytes injected with brain mRNA and by a high K(+)-stimulated 45Ca uptake into synaptosomes. It was found that several anti-amnesic agents (bifemelane, idebenone and vinpocetine) inhibited these VSCC-related responses with stronger potency than phenytoin and flurazepam. On the inhibiting potency of various compounds, there was a significant but weak correlation between the results from synaptosomes and those from the injected oocytes, since nifedipine, verapamil and diltiazem at 100 microM did not reduce 45Ca influx in synaptosomes but partly inhibited VSCCs in the oocytes. The blockade of neuronal L- and N-type VSCCs may participate in the anti-ischemic/hypoxic actions of nootropic drugs.