Trends in searching for new cognition enhancing drugs

The ATP binding cassette transporter, VmTPT2/VmABCG1, is involved in export of the monoterpenoid indole alkaloid, vincamine in Vinca minor leaves

Vinca minor is a herbaceous plant from the Apocynaceae family known to produce over 50 monoterpene indole alkaloids (MIAs). These include several biologically active MIAs that have a range of pharmaceutical activities. The present study shows that the MIAs, vincamine, akuammicine, minovincinine, lochnericine and vincadifformine tend to be secreted on V. minor leaf surfaces. A secretion mechanism of MIAs, previously described for Catharanthus roseus, appears to be mediated by a member (CrTPT2) of the pleiotropic drug resistance ABC transporter subfamily. The molecular cloning of an MIA transporter (VmTPT2/VmABCG1) that is predominantly expressed in V. minor leaves was functionally characterized in yeast and established it as an MIA efflux transporter. The similar function of VmTPT2/VmABCG1 to CrTPT2 increases the likelihood that this MIA transporter family may have co-evolved within members of Apocynaceae family to secrete selected MIAs and to regulate leaf MIA surface chemistry.

Memory retrieval improvement by Heteropterys aphrodisiaca in aging rats

Few data exists about the pharmacological properties of Heteropterys aphrodisiaca O. Mach. (Malpighiaceae), which is native to the scrubland regions of Brazil. The present study investigated the effects of oral treatment with H. aphrodisiaca extract (BST0298) on the learning and memory of young (3-6 months) and aged (21-23 months) rats, and compared the in vitro antioxidant activity of three lots collected in different years. An improvement in the number of sessions to learn the task was observed in the left/right discrimination test in aged rats treated for 45 days with 25 mg/kg (7.0 ± 0.5; p=0.005) or 50 mg/kg (7.6 ± 0.6; p=0.012) compared with control old rats (11.0 ± 1.6). On the other hand, pre-treatment did not improve the performance of scopolamine-treated mice in the passive avoidance test. The in vitro malondialdehyde test showed that all three different extracts presented similar antioxidant activity. The flavonoids astilbin, isoastilbin and neoastilbin were isolated from the extract and may contribute to the biological activity. These results suggest that repeated treatment with H. aphrodisiaca improves learning and memory, probably by a non-muscarinic mechanism.

Heteropterys aphrodisiaca (extract BST0298): a Brazilian plant that improves memory in aged rats

Literature report is lacking on pharmacological studies of the plant Heteropterys aphrodisiaca, endemic to the scrublands of Brazil. The present study was carried out to investigate the effects of oral dosing with extract BST0298 from this plant, on learning and on memory, in young (3-6-month-old) and aged (20-28-month-old) rats. The aged animals presented significant memory deficits in both the passive avoidance and T-maze left/right discrimination tests. Treatment for 7 days (50 mg/kg) or 26 days (100 mg/kg) with extract BST0298 restored the memory deficits in the passive avoidance test. However, no improvement in memory was observed after acute administration of extract BST0298 (100 mg/kg) in aged rats. An improvement in learning was also observed in the left/right discrimination test in aged rats treated for 109 days with BST0298 at a dose of 50 mg/kg. These results suggest that treatment for 7 days or more with H. aphrodisiaca improves learning and memory deficits in aged rats.

Role of pregnenolone, dehydroepiandrosterone and their sulfate esters on learning and memory in cognitive aging

Aging is a general process of functional decline which involves in particular a decline of cognitive abilities. However, the severity of this decline differs from one subject to another and inter-individual differences have been reported in humans and animals. These differences are of great interest especially as concerns investigation of the neurobiological factors involved in cognitive aging. Intensive pharmacological studies suggest that neurosteroids, which are steroids synthesized in the brain in an independent manner from peripheral steroid sources, could be involved in learning and memory processes. This review summarizes data in animals and humans in favor of a role of neurosteroids in cognitive aging. Studies in animals demonstrated that the neurosteroids pregnenolone (PREG) and dehydroepiandrosterone (DHEA), as sulfate derivatives (PREGS and DHEAS, respectively), display memory-enhancing properties in aged rodents. Moreover, it was recently shown that memory performance was correlated with PREGS levels in the hippocampus of 24-month-old rats. Human studies, however, have reported contradictory results. First, improvement of learning and memory dysfunction was found after DHEA administration to individuals with low DHEAS levels, but other studies failed to detect significant cognitive effects after DHEA administration. Second, cognitive dysfunctions have been associated with low DHEAS levels, high DHEAS levels, or high DHEA levels; while in other studies, no relationship was found. As future research perspectives, we propose the use of new methods of quantification of neurosteroids as a useful tool for understanding their respective role in improving learning and memory impairments associated with normal aging and/or with pathological aging, such as Alzheimer's disease.

Effect of alaptide, its analogues and oxiracetam on memory for an elevated plus-maze in mice

In the present study, the elevated plus-maze was used to evaluate memory in female mice. In Experiment 1, the mice retested on day 1, 4 or 7 after the initial session escaped from the open arm into the enclosed arm in a significantly shorter time than those retested on day 10 or 14. Thus, a 10-day inter-session interval was chosen for testing drugs which were expected to enhance memory. In Experiment 2, in the retest performed on day 10, both alaptide (cyclo(L-alanyl-1-amino-1- cyclopentanecarbonyl)) and oxiracetam, given immediately after the 1st session, reduced the transfer latency from the open arm into the enclosed arm as compared with that of the controls. In Experiment 3, a similar effect, i.e., the retention of spatial information, was facilitated by post-session injections of 5 out of 21 alaptide analogues. The new compounds represent the 2,5-piperazinedione derivatives which contain 1-amino-1-cyclo-alkanecarboxylic acid (C3 to C7 ring). The cyclopentane- and cyclohexane-ring was substituted by an alkyl group. In the series with the cycloalkane ring, the importance of the structure of alaptide was confirmed again, which underlines the importance of the cyclopentane ring; the active structures had L-alanine instead of glycine as the second amino acid. Isomers of the cyclohexane series which contained methyl or tert-butyl were most active when the substitution was at position 3. Our results demonstrate that the model of long-term memory can be used to discriminate between closely related chemical structures.

Piracetam and other structurally related nootropics

Nearly three decades have now passed since the discovery of the piracetam-like nootropics, compounds which exhibit cognition-enhancing properties, but for which no commonly accepted mechanism of action has been established. This review covers clinical, pharmacokinetic, biochemical and behavioural results presented in the literature from 1965 through 1992 (407 references) of piracetam, oxiracetam, pramiracetam, etiracetam, nefiracetam, aniracetam and rolziracetam and their structural analogues. The piracetam-like nootropics are capable of achieving reversal of amnesia induced by, e.g., scopolamine, electroconvulsive shock and hypoxia. Protection against barbiturate intoxication is observed and some benefit in clinical studies with patients suffering from mild to moderate degrees of dementia has been demonstrated. No affinity for the alpha 1-, alpha 2-, beta-, muscarinic, 5-hydroxytryptamine-, dopamine, adenosine-A1-, mu-opiate, gamma-aminobutyric acid (GABA) (except for nefiracetam (GABAA)), benzodiazepine and glutamate receptors has been found. The racetams possess a very low toxicity and lack serious side effects. Increased turnover of different neurotransmitters has been observed as well as other biochemical findings, e.g., inhibition of enzymes such as prolylendopeptidase. So far, no generally accepted mechanism of action has, however, emerged. We believe that the effect of the racetams is due to a potentiation of already present neurotransmission and that much evidence points in the direction of a modulated ion flux by, e.g., potentiated calcium influx through non-L-type voltage-dependent calcium channels, potentiated sodium influx through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor gated channels or voltage-dependent channels or decreases in potassium efflux. Effects on carrier mediated ion transport are also possible.

Benzodiazepine inverse agonists augment long-term potentiation in CA1 and CA3 of guinea pig hippocampal slices

The effects of benzodiazepine inverse agonists on the long-term potentiation of synaptic transmission in hippocampal slices of the guinea pig were examined using an extracellular recording technique. Benzodiazepine inverse agonists, beta-carboline-3-carboxylate (beta-CCE), 2-phenylpyrazolo [4,3-c]quinolin-3(5H)-one (CGS-8216) and 2-[5-methylthien-3-yl]-2,5-dihydro-3H-pyrazolo [4,3-c]quinolin-3-one (S-135), augmented the magnitude of long-term potentiation induced by tetanic stimulation of input fibers in both the CA1 and the CA3 regions. beta-Carboline-3-carboxylate was more effective in augmenting long-term potentiation in CA1 than in CA3. Augmentation of long-term potentiation produced by beta-CCE was antagonized by concomitant application of flumazenil, a benzodiazepine receptor antagonist. Therefore, the enhancing action of benzodiazepine inverse agonists on long-term potentiation, which is suggested to be a specific action, mediated by the GABA/benzodiazepine receptor complex, might help to explain the mechanism of the memory-enhancing effects of benzodiazepine inverse agonists, observed in some in vivo behavioral paradigms.

Effect of scopolamine and nootropic drugs on rewarded alternation in a T-maze

The effects of different doses of scopolamine, and of the nootropic drugs oxiracetam and aniracetam, were investigated on the performance of male Wistar rats in a T-maze requiring a spatial discrimination in the stem (reference memory) and an alternate discrimination in the arms (working memory). Criterion (90% correct responses) was reached within 3 days of daily training for stem and 9 days for arm discrimination. Scopolamine (0.1, 0.2, 0.6, and 1.0 mg/kg, SC, 60 min before session) significantly impaired working memory, as shown by a decrease in the number of correct alternations, without affecting reference memory. Both nootropic drugs (25-50 and 100 mg/kg PO) 30 min before scopolamine) attenuated the working memory impairment induced by scopolamine.

Effects of combined acetylcholinesterase inhibition and serotonergic receptor blockade on age-associated memory impairments in rats

We recently reported that post-training administration of serotonergic receptor antagonists attenuated the inhibitory-avoidance memory deficits normally exhibited by aged rats. In the present study, we determined whether a subeffective dose of the serotonergic type-2 receptor antagonist, ketanserin, would augment the facilitative effects produced by the acetylcholinesterase inhibitor, physostigmine, on memory in aged rats using the same task. The drugs were injected intraperitoneally alone, or in combination, immediately following training. Retention testing occurred 24 hours following training. A dose-dependent enhancement of memory was demonstrated as a result of the two treatment conditions (physostigmine 0.01-10.0 micrograms/kg, ketanserin 1.0 mg/kg + physostigmine 0.001-0.01 micrograms/kg). The facilitation of memory produced by the combined treatment was observed at doses well below those required to produce a similar effect when each drug was administered alone. The results provide additional evidence for an interaction between the cholinergic and serotonergic neurotransmitter systems in learning and memory, and may have important implications in the treatment of age-related memory impairments.