Attenuation of forgetting by pharmacological stimulation of aminergic neurotransmitter systems

A narrative synthesis of research with 5-MeO-DMT

BACKGROUND

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring, short-acting psychedelic tryptamine, produced by a variety of plant and animal species. Plants containing 5-MeO-DMT have been used throughout history for ritual and spiritual purposes. The aim of this article is to review the available literature about 5-MeO-DMT and inform subsequent clinical development.

METHODS

We searched PubMed database for articles about 5-MeO-DMT. Search results were cross-checked against earlier reviews and reference lists were hand searched. Findings were synthesised using a narrative synthesis approach. This review covers the pharmacology, chemistry and metabolism of 5-MeO-DMT, as well epidemiological studies, and reported adverse and beneficial effects.

RESULTS

5-MeO-DMT is serotonergic agonist, with highest affinity for 5-HT_1A receptors. It was studied in a variety of animal models, but clinical studies with humans are lacking. Epidemiological studies indicate that, like other psychedelics, 5-MeO-DMT induces profound alterations in consciousness (including mystical experiences), with potential beneficial long-term effects on mental health and well-being.

CONCLUSION

5-MeO-DMT is a potentially useful addition to the psychedelic pharmacopoeia because of its short duration of action, relative lack of visual effects and putatively higher rates of ego-dissolution and mystical experiences. We conclude that further clinical exploration is warranted, using similar precautions as with other classic psychedelics.

The acute effect of Hypericum perforatum on short-term memory in healthy adults

RATIONALE

Over-the-counter drugs containing Hypericum perforatum (H. perforatum) have been argued to improve memory and sustained attention. So far, these claims have not been supported in human studies. However, previous studies used rather high dosages, and little is known about the acute effect of small dosages.

OBJECTIVE

We evaluated whether an acute treatment with Remotiv 500 and Remotiv 250 (500 or 250 mg of H. perforatum quantified to either 1 or 0.5 mg of hypericin) improved memory and sustained attention, as well as mood and state anxiety in healthy adults.

METHOD

A single dosage, randomized, double-blind, placebo-controlled trial was conducted with 82 student participants (33 women). Each participant received placebo in one session and one of two dosages in the other session. Order of the sessions and dosage conditions were randomized between subjects. Participants completed a battery of tasks assessing short-term memory capacity and sustained attention.

RESULTS

A significant positive effect of Remotiv 250 on digit span (mean Cohen's d = 0.58; p = .01) was observed. By contrast, Remotiv 500 had a negative effect on digit span (mean d = - 0.48, p = 0.04). A similar effect emerged when factoring across tests of short-term memory. Both dosages improved mood (d = 0.60, p = .03).

CONCLUSIONS

The results indicate that acute treatment with small (250 mg) dosages of H. perforatum has a positive effect on the capacity of short-term verbal memory, and stress the importance of maintaining small dosages in nootropic applications.

TRIAL REGISTRATION

www.clinicaltrials.gov NCT02862236.

α-Adrenergic receptors in auditory cue detection: α2 receptor blockade suppresses false alarm responding in the rat

Numerous studies have suggested a facilitatory role of the noradrenergic system in attention. Cognitive functions relating to attentive states--arousal, motivation, behavioral flexibility, and working memory--are enhanced by norepinephrine release throughout the brain. The present study addresses the role of the adrenergic system on stimulus validity and sustained attention within the auditory system. We examined the effects of adrenoceptor stimulation via systemic injection of α1 and α2-adrenoceptor antagonist and agonist drugs, prazosin (1 mg/kg), phenylephrine (0.1 mg/kg), yohimbine (1 mg/kg), and clonidine (0.0375 mg/kg), respectively. Our results indicate that α1-adrenergic stimulation is ineffective in modulating the biological assessment of auditory signal validity in the non-stressed rat, while α2-adrenoceptor antagonist and agonist drugs were effective in modulating both accuracy and response latencies in the habituated animal. Remarkably, blockade of α2-adrenoceptors significantly improved the animal's ability to correctly reject non-signal events. These findings indicate not only a state dependent noradrenergic component of auditory attentional processing, but a potential therapeutic use for drugs targeting norepinephrine release in neurological disorders ranging from Alzheimer's disease to schizophrenia.

Effects of Clozapine and Sulpiride on Morphine State-Dependent Memory in the Step-Down Passive Avoidance Test

The effects of antipsychotic drugs sulpiride and clozapine on morphine state-dependent memory of passive avoidance task were examined in mice. Post-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory retrieval on the test day which was reversed by pre-test administration of the same dose of the opioid (5 mg/kg). In animals where memory was impaired by post-training morphine, the administration of either sulpiride or clozapine before pre-test morphine reduced the improvement of memory produced by the opioid. Co-administration of sulpiride with clozapine did not potentiate their antagonistic response. In conclusion, the inhibition of improvement of memory retrieval by morphine treatment on the test day by the two dopamine receptor antagonists seems to be induced through two different receptor mechanisms.

Adrenoceptor mechanisms underlying imipramine-induced memory deficits in rats

The post-training administration of tricyclic antidepressant imipramine impairs memory consolidation in the passive avoidance task. The present study investigated the effects of intrahippocampal (i.h.) injection of adrenoceptor agents on imipramine-induced (2-8 microg/rat) amnesia. The administration of the alpha1-adrenoceptor agonist phenylephrine (0.05 microg/rat) and the alpha1-adrenceptor antagonist prazosin (0.5 microg/rat) did not alter the effect of imipramine. The lower doses of phenylephrine (0.005 and 0.015 microg/rat) impaired, while the higher dose of the drug (0.025 and 0.05 microg/rat) improved retention. The effect of phenylephrine was not altered by prazosin (0.5 and 1 microg/rat) pretreatment, although prazosin alone decreased retention latencies. The alpha2-adrenoceptor antagonist yohimbine (0.5 and 1 microg/rat) decreased the response induced by imipramine. However, the alpha2-adrenoceptor agonist clonidine (0.08 microg/rat) did not alter the effect of the drug. Clonidine (0.15 and 0.3 microg/rat) by itself decreased, while yohimbine (1 and 2 microg/rat) increased retention latencies. Yohimbine pretreatment attenuated the effect of clonidine. It is concluded that alpha2-adrenoceptor mechanism(s) may be involved in imipramine-induced impairment of memory.

Effects of alpha-adrenoceptor agonists and antagonists on histamine-induced impairment of memory retention of passive avoidance learning in rats

The effect of alpha-adrenoceptor agents on the impairment induced by histamine was measured for memory retention of passive avoidance learning in rats. Post-training intracerebroventricular (i.c.v.) injection was carried out in all the experiments. Histamine (5, 10 and 20 microg/rat) reduced, while a histamine H(1) receptor antagonist, chlorpheniramine (0.1, 1 and 10 microg/rat), increased memory retention. The histamine H(2) receptor antagonist, ranitidine (0.1, 1, 10 and 20 microg/rat), did not elicit any response in this respect. Different doses of chlorpheniramine but not ranitidine reversed the histamine-induced impairment of memory. Clonidine and prazosin decreased, but yohimbine and phenylephrine increased, memory retention. Yohimbine decreased the inhibitory response to histamine. Phenylephrine, clonidine and prazosin did not alter the histamine effect. It is concluded that a histamine-induced impairment of memory retention through histamine H(1) receptors and an alpha(2)-adrenoceptor mechanism may be involved in the histamine response.

The role of alpha-adrenoceptors in the amnestic effect of intracerebroventricular dexamethasone

Corticosteroids exert dual enhancing or impairing effects on cognitive functions. While their memory-enhancing effects have been well investigated, the mechanisms involved in their amnestic effects are not completely understood. Thus, we examined the role of alpha-adrenoceptors on dexamethasone-induced amnesia using step-through passive avoidance test in rat. Intracerebroventricular (i.c.v.) injection of dexamethasone (5 and 10 microg per rat) decreased the retention latencies. Likewise, intraperitoneal administration of alpha(2)-adrenoceptor agonist clonidine (0.1-0.3 mg kg(-1)) but not alpha(2)-adrenoceptor antagonist yohimbine (0.5-2 mg kg(-1)) decreased the retention latency. Yohimbine pre-treatment decreased the amnestic effects of dexamethasone or dexamethasone plus clonidine. On the other hand, intraperitoneal administration of alpha(1)-adrenoceptor agonist phenylephrine (0.5-2 mg kg(-1)) per se increased, while prazosin at 2 mg kg(-1) decreased the retention latency. Administration of phenylephrine before dexamethasone completely reversed the amnestic effect of the latter, while prozosin did not affect dexamethasone-induced amnesia. These data suggest that dexamethasone may induce its amnestic effect through activation of alpha(2)-adrenoceptors, leading to decreased alpha(1)-adrenergic activity.

Phenylephrine and ECS-induced retrograde amnesia

Phenylephrine is a nonselective alpha-receptor agonist. This study examined whether the administration of phenylephrine immediately before electroconvulsive shocks (ECS) attenuated ECS-induced retrograde amnesia. Adult male Wistar rats received phenylephrine (0.25 mg/kg i.p.) or saline 3 min before each of three once-daily true or sham ECS. Retention of pre-ECS learning was studied 1 day after the ECS course using a passive avoidance task. Phenylephrine increased seizure duration in ECS-treated rats, and also enhanced recall in both true and sham ECS groups. The latter finding suggests that phenylephrine nonspecifically improves cognitive functions, perhaps through adrenergic mechanisms that improve memory consolidation and storage. Since phenylephrine increases blood pressure, its cognitive effects also weaken the hypothesis that ECT-induced cognitive impairment results from the seizure-related hypertensive surge.

Hypericum perforatum as a nootropic drug: enhancement of retrieval memory of a passive avoidance conditioning paradigm in mice

Depression, among other non-cognitive symptoms, is common in patients with dementia. The effect of Hypericum perforatum (St. John's Wort) extract, with well-documented antidepressant activity, was tested on memory retrieval 24 h after training on a one-trial passive avoidance task in mice. Acute administration of Hypericum extract (4.0, 8.0, 12.0, and 25.0 mg/kg i.p.) before retrieval testing increased the step-down latency during the test session. The same doses of Hypericum extract, on the other hand, failed to reverse scopolamine-induced amnesia of a two-trial passive avoidance task. The involvement of serotonergic, adrenergic, and dopaminergic mechanisms in the facilitatory effect of Hypericum extract on retrieval memory was investigated. Pretreatment of the animals with serotonergic 5-HT1A receptor antagonist (-)-pindolol (0.3, 1.0, and 3.0 mg/kg), serotonergic 5-HT2A receptor blocker spiperone (0.01, 0.03, and 0.1 mg/kg), alpha adrenoceptor antagonist phentolamine (1, 5, and 10 mg/kg), beta receptor antagonist propranolol (5, 7.5, and 10 mg/kg), dopaminergic D1 receptor antagonist SCH 23390 (0.01, 0.05, and 0.1 mg/kg), and dopaminergic D2 receptor antagonist sulpiride (5, 7.5, and 10 mg/kg) revealed the involvement of adrenergic and serotonergic 5-HT1A receptors in the facilitatory effect of Hypericum extract on retrieval memory. It is concluded that Hypericum extract may be a better alternative for treatment of depression commonly associated with dementia than other antidepressants known to have anticholinergic side effects causing delirium, sedation and even exacerbating already existing impaired cognition. In dementias of old age, Hypericum perforatum would, therefore, serve as one medication targeting both depression and amnesia with lower potential side effects.

Behavioral and neurochemical effects induced by chronic L-DOPA administration

L-DOPA, in combination with benserazide, in the ratio 4:1 (w/w), was administered orally to rats. In the staircase maze test a low dose of L-DOPA (3 mg/kg/day) reduced the increase in errors caused by 20 days interruption of daily training, while a higher dose (30 mg/kg/day) was ineffective. A decrease in levels of dopamine in the olfactory system and DOPAC in the striatum was seen at all tested doses of L-DOPA, while an increase in 5-HT levels was seen in the hippocampus and in the striatum. 5-HIAA levels did not change. Levels of ACh in the olfactory system were reduced at all doses of L-DOPA, while in the hippocampus this effect was seen only at the dose of 90 mg/kg/day. The density of muscarinic receptors was not altered. All tested doses of L-DOPA caused norepinephrine levels to fall in the hippocampus and increase in the striatum. The density of alpha 1-adrenoceptors was reduced only at the two lower doses of L-DOPA. A comparison of the neurochemical results with the behavioral modifications seen in the staircase maze test suggests that the catecolaminergic systems are implicated in the memory process.

Effects of nicotine on memory retrieval in mice

The effect of nicotine was tested on retrieval 24 h after training on a passive avoidance task. Intraperitoneal (i.p.) injection of nicotine (0.25-1.5 mg/kg) increased the step-down latency in mice dose dependently. Pretreatment with the nicotinic receptor antagonist mecamylamine (0.5-1 mg/kg) decreased, whereas pretreatment with the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol maleate) (0.01, 0.05 and 0.1 mg/kg) and the beta-adrenoreceptor antagonist propranolol (10 mg/kg) increased the nicotine response. The dopamine receptor D2 receptor antagonist sulpiride (5-10 mg/kg), the anti-muscarinic agent atropine (2.5-10 mg/kg), the peripheral nicotinic receptor antagonist hexamethonium (0.01-0.5 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (1 and 10 mg/kg) and the peripheral dopamine D2 receptor antagonist domperidone (5 and 10 mg/kg) did not change the response induced by nicotine. Single administration of the antagonists did not cause response; however, a high dose of domperidone (10 mg/kg) and propranolol alone increased the step-down latencies. It may be concluded that a nicotinic receptor mechanism is involved in the nicotine-induced improvement of memory retrieval.

Effects of high doses of theophylline on memory acquisition

Intraperitoneal (IP) injection of high doses of theophylline decreased memory acquisition dose dependently. Low but not high doses of the dopamine receptor D2 antagonist sulpiride and the dopamine D1 receptor antagonist SCH 23390 decreased the theophylline response. Pimozide and the beta-adrenoreceptor propranolol also decreased the drug response. The anti-muscarinic agent atropine, the alpha-adrenoceptor phenoxybenzamine and the 5-HT antagonist metergoline did not affect the response induced by theophylline. It is concluded that dopamine and beta-adrenergic receptor mechanisms may be involved in the theophylline-induced attenuation of memory acquisition.

Cognitive effects of milacemide and methylphenidate in healthy young adults

Cognitive effects of the novel glycine prodrug milacemide (400 mg), the catecholaminergic agonist methylphenidate (20 mg), and placebo were evaluated in 48 healthy young adults. Throughout a 6-h drug treatment day, subjects repeatedly performed tests of target-detection vigilance, immediate and delayed verbal free recall, and Buschke Selective Reminding; total free recall and forced-choice recognition tests were administered at the end of the day. Significant improvement in both vigilance reaction time and Selective Reminding Sum Recall was observed in the methylphenidate group. Contrary to expectations, the milacemide group evidenced significant declines in both vigilance perceptual sensitivity and free-recall difference scores (delayed-immediate). Vigilance reaction times significantly decreased over repeat testing in all groups, but only the methylphenidate group differed from placebo. The reaction-time functions for milacemide and placebo were similar, suggesting arousal was not diminished under milacemide and could not account for the cognitive decrements. No significant drug effects obtained for total free recall or recognition performance. Although the glycine prodrug milacemide was ineffective as a cognitive enhancer, the involvement of the NMDA receptor in memory function reported in the literature supports continued exploration of other approaches for manipulating NMDA receptor activity.

Effects of adenosine receptor agonists and antagonists on acquisition of passive avoidance learning

A series of experiments examined the effect of the adenosine receptor agonists N(6)-cyclohexyladenosine (CHA), N(6)-phenylisopropyladenosine (R-PIA) and 5'-N-ethylcarboxamidoadenosine (NECA); the adenosine receptor antagonists theophylline, caffeine and 8-phenyltheophylline (8-PT), and the adenosine uptake inhibitor dipyridamole on acquisition of a single trial passive avoidance learning by mice. The adenosine receptor agonists CHA (0.05, 0.1, 0.2 and 0.4 mg/kg) and R-PIA (0.00625, 0.125, 0.25 and 0.5 mg/kg) administered 1 h before the training session decreased retention dose dependently while the other adenosine receptor agonist NECA (0.0025, 0.005 and 0.01 mg/kg) had no effect. The response induced by the adenosine receptor agonists CHA and R-PIA was attenuated by the pretreatment of animals with low doses of the adenosine receptor antagonists 8-PT (2.0 mg/kg) and theophylline (25 mg/kg) but not with higher doses of the antagonists. The higher doses of the antagonists decreased the passive avoidance. Dipyridamole (7.5, 15, 30, 60 and 120 mg/kg) showed no significant effect. It is concluded that adenosine A1 receptor activation decreases the acquisition of a passive avoidance response.

Effect of the calcium channel blocker amlodipine on memory in mice

Five experiments were performed to investigate the effects of amlodipine, a calcium channel antagonist of the 1,4-dihydropyridine class, on consolidation and retrieval of memory in mice. In a single-trial passive avoidance task, amlodipine was administered pretraining, posttraining, or pretesting. Results of temporal and dose-response studies showed that memory enhancement (significant increase in step-through latency) occurred when amlodipine (5, 7, 9, 15, and 30 mg/kg) was given either immediately post-training or (15 mg/kg) 15 min pretesting. Using a conditioned emotional response task, tone was paired with shock using Pavlovian conditioning procedures. Strength of conditioning was assessed by measuring suppression of drinking in the presence of a tone. Amlodipine (7 mg/kg) given immediately following both high- and low-intensity shock significantly enhanced conditioned suppression. In the third experiment thirsty mice were trained on a spatial discrimination task in a linear maze. Correct choices were reinforced with liquid reinforcement. Amlodipine (10 mg/kg) injected immediately after the training session produced a significant enhancement of discrimination performance on a 24-h retention test. In the fourth experiment mice were given 25 training trials in a two-way active avoidance task and were treated with either amlodipine (10 mg/kg) or saline after training. Amlodipine-treated mice made significantly more avoidances on the test session than control animals. The final experiment demonstrated that the deficit in approach-avoidance behavior seen in 18-month-old mice could be reversed by amlodipine treatment after the training session. These studies suggest that amlodipine can facilitate memory consolidation and retrieval.

5-HT1A agonists disrupt memory of fear conditioning in mice

A series of experiments was carried out to analyze the effects of the 5-HT1A agonists tandospirone or buspirone on the retention of fear conditioning in mice. Fear was produced by pairing tone and shock in a conditioned emotional response (CER) paradigm and strength of conditioning was assessed by measuring suppression of drinking in presence of tone. Fear conditioning was disrupted if tandospirone and buspirone were administered before the conditioning session but not before the test trial. Diazepam disrupted conditioning at both times. Tandospirone did not disrupt performance if conditioning was tested 1 hr rather than 24 hr after training, suggesting that disrupted memory rather than impaired acquisition was responsible for the deficit. The effect of tandospirone on fear conditioning could be reversed by administration of d-amphetamine prior to the retention test, which suggests that information was stored but is inaccessible to normal retrieval cues. Tandospirone and buspirone also retarded extinction, a clear indication that the disruption caused by these drugs is unrelated to their anxiolytic action.

Cholinergic and dopaminergic agents which inhibit a passive avoidance response attenuate the paradigm-specific increases in NCAM sialylation state

The influence of cholinergic and dopaminergic agents on the acquisition of a passive avoidance response in the rat is demonstrated. Trifluoperazine (0.12 mg/kg), a dopamine antagonist, inhibited task acquisition when present during training or later, during consolidation, at the 10-12 h post-training period and at no other intervening time point. Induction of amnesia was dose-dependent and was not apparent when the dose exceeded 0.12 mg/kg. This effect appears to be due to an increase in dopamine release through presynaptic receptor antagonism as similar results could be obtained by the administration of apomorphine (0.5 mg/kg), a dopamine agonist, and this effect could be antagonized by the D1 receptor selective antagonist SCH-23390. Scopolamine (0.15 mg/kg), a muscarinic antagonist, impaired acquisition of the passive avoidance response when administered during training and, separately, at the 6 h post-training period. This could not be attributed to presynaptic antagonism as oxotremorine (0.2 mg/kg), a muscarinic agonist, had no amnesic action. Administration of apomorphine or scopolamine during training and at the appropriate post-training period prevented subsequent paradigm-specific increases of neural cell adhesion molecule sialylation state in hippocampal immunoprecipitates obtained at 24 h after task acquisition and 4 h following intraventricular infusion of the labelled sialic acid precursor - N-acetyl-D-mannosamine. Oxotremorine alone did not influence neural cell adhesion molecule sialylation state. These observations provide further evidence of a regulatory role for neural cell adhesion molecule sialylation state in information storage processes.

Delayed-non-match-to-sample performance in the radial arm maze: effects of dopaminergic and gabaergic agents

Central dopaminergic transmission has been implicated in memory processes. The present experiments examined the effects of several direct acting dopaminergic agents on performance of a delayed-non-match-to-sample radial arm maze task. Preadministration of apomorphine (D1-D2 agonist; 0.25, 0.5, and 1.0 mg/kg), quinpirole (D2 agonist; 0.1 mg/kg), or SKF38393 (D1 agonist; 3 mg/kg) increased the latency of choices but did not affect any index of accuracy with a 1 h retention interval. Post-training administration of quinpirole (0.1, 0.2, 1.0, and 2.0 mg/kg), SKF38393 (0.3, 3.0, and 6.0 mg/kg), sulpiride (D2 antagonist; 3, 10, and 30 mg/kg), or SCH23390 (D1 antagonist; 0.01, 0.1, and 1.0 mg/kg) also did not affect accuracy, although quinpirole produced a dose-dependent increase in the latency of choices, assessed 10 h post-treatment. For comparison, pretraining and post-training administration of the benzodiazepine chlordiazepoxide (1, 3, 5 mg/kg) was also tested and produced dose-dependent impairments in mnemonic performance at either a 1 or 4 h retention interval. The effects of chlordiazepoxide are consistent with evidence indicating that GABAergic agents can influence memory processes. In contrast, the present findings indicate that (peripheral administration of dopaminergic agents IS) not sufficient to alter the mnemonic processes required for accurate performance of this DNMTS-RAM task.

Effect of D-1 or D-2 receptor stimulation on memory retrieval in mice

Mice were trained in one-way active avoidance procedure and retention was tested at 4, 8, 16 and 24 h after training of animals and compared with non-shocked or untrained animals. The effect of drugs was tested on retrieval 24 h after training in other groups of mice. High doses of apomorphine or bromocriptine impaired, while low doses of the drugs improved, retrieval of avoidance. High doses of sulpiride reversed the impairment induced by high doses of these dopamine agonists. Low doses of sulpiride antagonized the improvement of retrieval induced by low doses of apomorphine. SKF 38393 treatment of animals also improved the retrieval. The retrieval impairment induced by higher doses of apomorphine or the improvement induced by different doses of SKF 38393 was antagonized by SCH 23390 pre-treatment. Single administration of SCH 23390 or low doses of sulpiride also impaired retrieval. It is concluded that stimulating post-synaptic D-2 dopamine receptors impairs retrieval whilst activation of pre-synaptic D-2 or post-synaptic D-1 receptors improves memory retrieval.

Action of a chronic disulfiram administration on memory decay and on central cholinergic and adrenergic systems

The chronic administration (15 days) of disulfiram reduced the levels of noradrenaline (NA) in the olfactory system and in the subcortex at all the tested doses (50, 200 and 400 mg/kg/day). No modification of the density (Bmax) and of the dissociation constant (Kd) for alpha 1-adrenergic receptors was observed (radioligand [3H]prazosin). Only in the hippocampus the acetylcholine (ACh) levels and the Bmax for muscarinic receptors (radioligand [3H]QNB) were increased at all doses tested. Modifications of the cholinergic system were observed in the subcortex and in the cortex only at the higher doses of disulfiram. After 20 days interruption of the daily training in the staircase maze, 50, 100 and 200 mg/kg/day of disulfiram accelerated spontaneous decay of memory. It is hypothesized that the modifications of the hippocampal cholinergic system (increase of the ACh levels and of the density of the muscarinic receptors) may be the condition determining the acceleration of the decay of memory caused by disulfiram.

Behavioral and neurochemical modifications caused by chronic alpha-methylparatyrosine administration

The chronic administration of alpha-methylparatyrosine (AMT) caused a reduction of the noradrenaline levels in the hippocampus (at 150 and 300 mg/kg/day) and in the subcortex (at 30, 150 and 300 mg/kg/day). The acetylcholine levels were reduced in the hippocampus and in the olfactory brain at all the tested doses of AMT. An increase of the Bmax of muscarinic and alpha 1-adrenoceptors was observed at 30 mg/kg/day of AMT; only in the subcortex AMT caused no modification of the density of muscarinic receptors. The degree of increase of the receptors density at 30 mg/kg/day was reduced at the higher doses of AMT. AMT 30 mg/kg/day caused a reduction of the errors in the staircase maze after 20 days of interruption of the daily training. These results might suggest a correlation between the behavioral effect and the increase of density, not only of the adrenoceptors, but also of the muscarinic receptors. It is proposed that the behavioral effects caused by chronic AMT are the consequence of complex neurochemical interactions.

Milacemide enhances memory storage and alleviates spontaneous forgetting in mice

The objective of this study was to evaluate the effectiveness of milacemide as a memory-enhancing drug in mice. Experiment 1 showed that forgetting of active avoidance learning produced by a 14-day training to test delay could be alleviated by milacemide (10 mg/kg) administered before the retention test. Experiment 2 demonstrated that the same dose of milacemide could also attenuate spontaneous forgetting of passive avoidance learning, thereby ruling out nonspecific effects as an explanation for the enhancement of performance following pretesting drug administration. A third experiment showed that the facilitation of retrieval induced by milacemide could be blocked by the NMDA receptor antagonist AP-7, suggesting that the effects of milacemide on memory may be mediated by NMDA receptor activation. A final experiment demonstrated that retention was improved when milacemide was administered immediately following active avoidance training, indicating that the drug can also facilitate remembering by its actions on consolidation and storage processes.

Strength of scopolamine-induced amnesia as a function of time between training and testing

Variations in the strength of scopolamine-induced amnesia as a function of age of the habit were studied in Swiss Webster mice. Animals were trained in an active avoidance task to a criterion of 9/10 avoidances and immediately following training injected with scopolamine hydrochloride (1.0 mg/kg) or saline. Retention of the avoidance learning was evaluated by testing different groups of animals 1, 3, 7, 10, 14, and 28 days following training. The retention test consisted of five trials in which the CS but not the UCS was presented. Results indicated that saline-treated mice exhibited near-perfect retention up to 14 days post-training with forgetting beginning to be apparent at 28 days. Scopolamine treatment produced strong amnesia in animals tested 1 and 3 days post-training but normal retention in animals tested 7 and 10 days after learning. The amnesia abruptly reappeared at 14 days after which time it remained stable. The marked similarity of the scopolamine retention curve to changes in the strength of memory of discrimination learning in undertrained rats reported by Deutsch suggested that scopolamine resulted in the storage of a weak memory of the avoidance response. To explore this idea further we trained mice to a criterion (4/5) which would result in a weak avoidance response and tested different groups 1, 3, 10, 14, and 28 days following learning. Results showed that strength of the memory of avoidance learning increased up to 10 days and then decreased abruptly at 14 days thus replicating the general shape of the retention curve produced by injecting scopolamine following strong training. These data suggest that scopolamine disrupts processes essential for the formation of durable memories.

Alleviation of scopolamine amnesia by different retrieval enhancing treatments

Mice were trained in a one-way active avoidance task to a criterion of 9/10 avoidances. Immediately following training they were injected with scopolamine hydrochloride (1 mg/kg SC) or with saline. Retention was assessed 3 days after training by 5 test trials on which the UCS was not present. Thirty min prior to the test, groups were injected with different doses of arecoline, d-amphetamine sulphate or with saline. Other scopolamine-treated mice were exposed to the CS or the UCS 24 hr prior to the test. The scopolamine-induced amnesia was attenuated by both 0.5 and 1.0 mg/kg arecoline and by 2.0 mg/kg d-amphetamine. Retention was also improved by exposure to the CS and the UCS. These data show that scopolamine amnesia can be alleviated by treatments which activate retrieval processes.