Choline uptake and permanent memory storage

Memantine Improves Memory and Neurochemical Damage in a Model of Maple Syrup Urine Disease

Maple Syrup Urine Disease (MSUD) is a metabolic disease characterized by the accumulation of branched-chain amino acids (BCAA) in different tissues due to a deficit in the branched-chain alpha-ketoacid dehydrogenase complex. The most common symptoms are poor feeding, psychomotor delay, and neurological damage. However, dietary therapy is not effective. Studies have demonstrated that memantine improves neurological damage in neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. Therefore, we hypothesize that memantine, an NMDA receptor antagonist can ameliorate the effects elicited by BCAA in an MSUD animal model. For this, we organized the rats into four groups: control group (1), MSUD group (2), memantine group (3), and MSUD + memantine group (4). Animals were exposed to the MSUD model by the administration of BCAA (15.8 µL/g) (groups 2 and 4) or saline solution (0.9%) (groups 1 and 3) and treated with water or memantine (5 mg/kg) (groups 3 and 4). Our results showed that BCAA administration induced memory alterations, and changes in the levels of acetylcholine in the cerebral cortex. Furthermore, induction of oxidative damage and alterations in antioxidant enzyme activities along with an increase in pro-inflammatory cytokines were verified in the cerebral cortex. Thus, memantine treatment prevented the alterations in memory, acetylcholinesterase activity, 2',7'-Dichlorofluorescein oxidation, thiobarbituric acid reactive substances levels, sulfhydryl content, and inflammation. These findings suggest that memantine can improve the pathomechanisms observed in the MSUD model, and may improve oxidative stress, inflammation, and behavior alterations.

Atropine, an anticholinergic drug, impairs memory retrieval of a high consolidated avoidance response in mice

Immediate post-training intraperitoneal administration of the centrally acting anticholinesterase physostigmine (70.0, or 150.0 microg/kg) enhanced retention of male CF-1 mice tested 48 h after training in a one-trial step-through inhibitory avoidance task (0.8 mA, 50 Hz, 1 s footshock). The effect was observed in mice that received saline 30 min before the retention test; on the contrary, the pre-test administration of the centrally active muscarinic cholinergic antagonist, atropine (1.0 mg/kg, i.p.), but not methylatropine (1.0 mg/kg, i.p.), instead of saline, prevents the enhancement of retention induced by both doses of the anticholinesterase when given immediately after training. The high retention performance caused by post-training physostigmine was recovered following a second administration of the same doses of the drug, 10 min after the pre-test injections of atropine. Since, physostigmine do not influence memory retrieval when given prior to the retention test, and its post-training effects are not due to the induction of state-dependency, the recover of the high retention performance was probably due to a classical interaction between a muscarinic competitive antagonist and an indirect cholinergic agonist. Further, atropine probably does not modify the memory trace by erasing it, but by producing a poor retrieval.

Involvement of central cholinergic mechanisms in the effects of oxytocin and an oxytocin receptor antagonist on retention performance in mice

Oxytocin (OT, 0.10 microg/kg, sc) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. In contrast, the immediate post-training administration of the putative oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2), Thr(4), Thy-NH(9)(2)] OVT (AOT, 0.30 microg/kg, sc) significantly enhanced retention performance. Neither OT nor AOT affected response latencies in mice not given footshock on the training trial, and neither the impairing effects of OT nor the enhancing effects of AOT were seen when the training-treatment interval was 180 min, suggesting that both treatments influenced memory storage. The effects of OT (0.10 microg/kg, sc) on retention were prevented by AOT (0.03 microg/kg, sc) given immediately after training, but 10 min prior to OT treatment. The central acting anticholinesterase physostigmine (35, 70, or 150 microg/kg, i.p.), but not its quaternary analogue neostigmine (150 microg/kg, i.p.), reversed the impairment of retention performance induced by OT, whereas low subeffective doses of the centrally active muscarinic cholinergic antagonist atropine (0.5 mg/kg, i.p.) or the central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg, i.p.), but not methylatropine (0.5 mg/kg, i.p.) or hexamethonium (5 mg/kg, i.p.) prevented the enhancement of retention performance caused by AOT. We suggest that oxytocin negatively modulates the activity of central cholinergic mechanisms during the posttraining period that follows an aversively motivated learning experience, leading to an impairment of retention performance of the inhibitory avoidance response.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

AF-DX 116, a presynaptic muscarinic receptor antagonist, potentiates the effects of glucose and reverses the effects of insulin on memory

Male Swiss mice were tested 24 h after training in a one-trial step-through inhibitory avoidance task. Low subeffective doses of d-(+)-glucose (10 mg/kg, ip), but not its stereoisomer l-(-)-glucose (30 mg/kg,ip), administered immediately after training, and AF-DX 116 (0.3 mg/kg,ip), a presynaptic muscarinic receptor antagonist, given 10 min after training, interact to improve retention. Insulin (8 IU/kg, ip) impaired retention when injected immediately after training, and the effects were reversed, in a dose-related manner, by AF-DX 116 (0.3, 1.0, or 3.0 mg/kg, ip) administered 10 min following insulin. Since AF-DX 116 possibly blocks autoreceptors mediating the inhibition of acetylcholine release from cholinergic nerve terminals, the present data support the view that changes in the central nervous system glucose availability, subsequent to modification of circulating glucose levels, influence the activity of central cholinergic mechanisms involved in memory storage of an inhibitory avoidance response in mice.

Hippocampal acetylcholine and habituation learning

Acetylcholine neurotransmission is considered to play a critical role in processes underlying behavioural activity, arousal, attention, learning, and memory. These functional attributions have largely been based on pharmacological findings. or data from brain damaged animals, and humans with neurodegenerative diseases, such as Alzheimer's disease. With the introduction of the in vivo microdialysis method it has recently become possible to monitor acetylcholine in the brain of the behaving animal, which allows to investigate its activity in specific behavioural tasks. With respect to learning and memory, one of the most elementary experimental paradigms is that of behavioural habituation, where the decrease of exploratory activity as a function of repeated exposure to the same environment is taken as an index of memory. We have used this paradigm to monitor hippocampal acetylcholine levels by means of in vivo microdialysis in rats, which were exposed to a novel open field and which were re-exposed to it on the following day (10 min each). The results show that exposure of rats to the novel environment led to increased extracellular levels of hippocampal acetylcholine which were positively correlated with exploratory behaviour. These cholinergic activations were larger than those of control animals which were handled like the experimental animals but which were not exposed to the open field. When re-exposing the experimental animals to the same environment, exploratory behaviour, but not cholinergic activation, was decreased. indicating habituation. In the subsequent 10 min, that is, when the animals where back in their home cages, cholinergic activity was still increased. The magnitude of increase was larger after re-exposure than after exposure to the novel open field. Finally, we differentiated the animals into "superior" vs "inferior" learners and found that the "superior" learners showed higher behavioural activation in the novel environment and stronger neurochemical responses, both. in the novel and familiar environment. Our data show that extracellular levels of hippocampal acetylcholine are not only elevated in relation to novelty and behavioural activation. but also during behavioural habituation. Furthermore, an inter-individual variability of cholinergic activation seems to exist which is related to individual differences in behavioural responsiveness to novelty. Such differences in cholinergic activity may be related to other known differences in hippocampal structure and function and may be important for previously reported inter-individual variabilities in sensation-seeking and related mnestic functions.

Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice

We have previously reported that spatial reference memory (RM) training-induced alterations in hippocampal cholinergic activity as measured by sodium-dependent high-affinity choline uptake (SDHACU). Each training session was found to induce an immediate (30 s) increase in SDHACU followed (30 s to 15 min posttest) by a deactivation and long-lasting inhibition (15 min to 24 h) of this cholinergic marker. The present experiments were designed to assess the role of septal glutamatergic receptors in this posttraining cholinergic deactivation. In the first experiment, the effects of intraseptal injections of different doses of glutamic acid and glutamatergic antagonists (kynurenic acid, KYN, and AP5) on hippocampal SDHACU were studied in awake but otherwise resting (i.e., naive) mice. The results showed that glutamic acid at the lowest dose used (5 ng) produced a decrease in SDHACU, whereas both glutamatergic antagonists produced a dose-related increase in this cholinergic marker. It was concluded that septal glutamatergic receptors mediate a tonic inhibitory input on the cholinergic cells. Hence, in a second experiment the effect of intraseptal injections of KYN (5 ng) on the training-induced changes in hippocampal cholinergic activity were assessed following variable amounts of radial maze RM training. Trained mice were injected 20 min before the first or the ninth training session and killed 30 s or 15 min posttraining for determination of SDHACU. KYN slowed the posttesting cholinergic deactivation (disinhibition), this effect being more marked in good learners than in bad learners. The present findings suggest that septal glutamatergic receptors mediate an inhibitory input on the cholinergic cells, and that this input could play a role in memory consolidation.

Memory-improving actions of glucose: involvement of a central cholinergic muscarinic mechanism

Post-training intraperitoneal administration of alpha-D[+]-glucose (10-300 mg/kg) facilitated 24-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted U. Glucose did not increase the retention latencies of mice that had not received a footshock during training. The effect of glucose (30 mg/kg, ip) on retention was time-dependent, which suggests that the drug facilitated memory storage. The memory facilitation induced by glucose (30 mg/kg, ip) was prevented by atropine (0.5 mg/kg, ip) administered after training, but 10 min prior to glucose treatment. In contrast, neither methylatropine (0.5 mg/kg, ip), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, ip) or hexamethonium (5 mg/kg, ip), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training glucose on retention. Low subeffective doses of the central acting anticholinesterase physostigmine (35 micrograms/kg, ip), administered immediately after training, and glucose (10 mg/kg, ip), given 10 min after training, acted synergistically to improve retention. The effects of glucose (10 mg/kg, ip) were not influenced by the peripherally acting anticholinesterase neostigmine (35 micrograms/kg, ip). Considered together, these findings suggest that the memory facilitation induced by post-training administration of glucose could result from an enhancement of brain acetylcholine synthesis and/or its release that, in turn, might modulate the activity of muscarinic cholinergic mechanisms that are critically involved in memory storage.

Blood glucose and human memory

As it has been suggested that blood glucose might play a role in the action of some cognitive enhancing drugs, the influence of glucose containing drinks on human memory was examined. In a double-blind study the influence was examined of a drink containing 50 g glucose, or a placebo, on the ability to recall a word list. There was a significant correlation between blood glucose values and the number of words recalled. Those whose blood glucose levels were increasing remembered significantly more words than those whose blood glucose levels were falling. No relationship was found between blood glucose and performance on a test of spatial memory. In a second study blood glucose levels were raised for 2 h by taking a series of glucose-containing drinks. The number of words recalled from a word list correlated significantly with blood glucose levels but not with recall of a Wechsler story. The glucose-induced improvement in memory did not occur only in those whose blood glucose levels were initially low; rather it occurred irrespective of initial blood glucose level.

Amygdala kindling-induced seizures selectively impair spatial memory. 2. Effects on hippocampal neuronal and glial muscarinic acetylcholine receptor

The muscarinic acetylcholine receptor is linked via hydrolysis of phosphoinositides to the protein kinase C pathway. In a preceding paper (Beldhuis, H. J. A., H. G. J. Everts, E. A. Vander Zee, P. G. M. Luiten, and B. Bohus (1992) Amygdala kindling-induced seizures selectively impair spatial memory. 1. Behavioral characteristics and effects on hippocampal neuronal protein kinase C isoforms. Hippocampus 2:397-410), the role of different isoforms of protein kinase C in neurobiological processes associated with plasticity was studied using both a spatial learning paradigm and amygdala kindling in the rat. This study extended the findings on protein kinase C activity to the level of the muscarinic acetylcholine receptor. Rats were trained in a spatial learning paradigm and kindled simultaneously in the amygdala to develop generalized motor convulsions. Control rats were trained only in the spatial learning paradigm to acquire stable working and reference memory performance. Alteration in the expression of the muscarinic acetylcholine receptor was investigated using a monoclonal antibody to muscarinic acetylcholine receptor proteins. Trained control rats that were exposed repeatedly to the spatial learning paradigm showed an increase in immunoreactivity for the muscarinic acetylcholine receptor located in the same hippocampal regions in which the protein kinase C activity was increased. In fully kindled rats, however, this increase located in principal neurons was absent, whereas expression of muscarinic acetylcholine receptor proteins was increased in hippocampal astrocytes. Moreover, fully kindled rats showed an impairment in reference memory performance as compared to trained control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Septo-hippocampal and nBM-cortical cholinergic neurones exhibit differential time-courses of activation as a function of both type and duration of spatial memory testing in mice

We previously showed that the initial acquisition session of a spatial discrimination (mixed reference/working memory) test in an 8-arm radial maze induced differential activations in the ascending cholinergic septo-hippocampal and nBM-cortical pathways in mice. This data showed that the duration of post-test cholinergic activation was longer in the nBM-cortical pathway than in the septo-hippocampal projection. Moreover, the post-test durations but not the immediate post-test amplitudes of activation in each pathway decreased progressively as a function of repeated daily acquisition sessions. In the present study we have thus tested the hypotheses that the time-courses of post-test cholinergic activation in the septo-hippocampal and nBM-cortical pathways may vary both as a function of the type of memory used (working vs. reference) and according to the duration of repeated daily testing. Cholinergic activity in vivo in the hippocampus or frontal cortex of mice was quantified using measures of sodium-dependent high-affinity choline uptake at two different times (30 s and 15 min) following specific spatial working or reference memory testing in an 8-arm radial maze. The memory tests were administered daily over a 13-day period to attain high levels of performance in each type of task. In comparison to control groups both types of memory testing induced significant post-test cholinergic activations in each brain region on Day 15. However, cholinergic activity remained elevated in frontal cortex at 15 min post-test following reference memory testing, whereas significantly shorter durations of cortical and hippocampal cholinergic activation were observed following working memory testing using short (1 min) retention intervals. The possible significance of these differential modifications to the time-course of the post-test activations in these cholinergic pathways in working and reference memory processes and the putative transsynaptic mechanisms involved are discussed.

MDL 26,479: a potential cognition enhancer with benzodiazepine inverse agonist-like properties

1. The present study investigated biochemical, electrophysiological and behavioural properties of the novel cognition enhancer, MDL 26,479 (5-(3-fluorophenyl)-2,4-dimethyl-3H-1,2,4-triazole-3-thione). 2. The 5-aryl-1,2,4-triazole, MDL 26,479, potently (0.22 +/- 0.05 mg kg-1) inhibited [3H]-flumazenil (Ro15-1788) binding in mouse cortex but was ineffective in vitro at displacing radioligand binding to the GABAA receptor complex. 3. Parenteral administration of MDL 26,479 (1 mg kg-1) or the benzodiazepine (BZD) inverse agonist methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (0.3 mg kg-1) increased cortical ex vivo binding of [3H]-hemicholinium-3 ([3H]-HC-3), a marker for cholinergic activation. This effect of MDL 26,479 was blocked by pretreatment with the antagonist flumazenil (1 mg kg-1). 4. MDL 26,479 (20 microM) and DMCM (1 microM) increased excitation in the hippocampal long-term potentiation (LTP) slice preparation; however, unlike DMCM, the effect of MDL 26,479 was not blocked by flumazenil. 5. In behavioural studies, MDL 26,479 did not exhibit adverse properties characteristic of drugs associated with the GABAA receptor complex. It lacked convulsant, anxiogenic, anxiolytic, or depressant effects. Since MDL 26,479 lacks activity with the BZD receptor in vitro we suggest that it acts via the GABAA receptor complex at another site on this receptor or in an as yet undefined manner or an active metabolite is formed in vivo. 6. Previous work showed that MDL 26,479 enhances learning acquisition in animal models.The present study suggests that at least some of the cognition enhancing properties are due to the enhancement of cortical and hippocampal cholinergic function and LTP.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Memory-improving action of glucose: indirect evidence for a facilitation of hippocampal acetylcholine synthesis

The effect of a 3 g/kg glucose injection on the velocity of the sodium-dependent high-affinity choline uptake mechanism in the hippocampus was both measured in quiet control mice and in mice immediately after training in an operant bar pressing task. Glucose did not significantly change high-affinity choline uptake in resting animals. High-affinity choline uptake in the hippocampus was increased by training in the operant bar pressing task. Glucose significantly reduced the amplitude of the increase in high-affinity choline uptake observed in the trained animals. Similarly, a 3 g/kg glucose injection also attenuated the increase in high-affinity choline uptake observed in animals injected with 1 mg/kg scopolamine. Finally, a 3 g/kg glucose injection significantly attenuated the amnesia produced by a post-training 1 mg/kg scopolamine injection in mice trained for an operant bar pressing task. These results provide additional evidence for an action of glucose on hippocampal cholinergic activity under conditions of high acetylcholine demand. This action may be mediated via an increase in acetyl coenzyme A availability, one of the precursors of acetylcholine. This facilitative effect of glucose on hippocampal acetylcholine synthesis may constitute the physiological basis for its facilitative action on memory and its attenuation of scopolamine amnesia.

Septal alpha-noradrenergic antagonism in vivo blocks the testing-induced activation of septo-hippocampal cholinergic neurones and produces a concomitant deficit in working memory performance of mice

In order to test the hypothesis that alpha-noradrenergic receptors in the septum 1) play an important functional role in the mediation of trans-synaptic control of the neurones of the cholinergic septo-hippocampal pathway and 2) produce resultant modulation of working memory performance, we have investigated the effects in vivo of the acute intraseptal injection of an alpha-antagonist, phenoxybenzamine, in mice. Neurochemical analysis was performed using measures of the kinetics of sodium-dependent high-affinity choline uptake in samples of hippocampus from injected mice and their relevant controls in both quiet conditions and immediately following selective working memory testing in an 8-arm radial maze. Results show that whereas the injection of phenoxybenzamine produces no significant alteration of the activity of the cholinergic septo-hippocampal neurones in quiet conditions, the pretrial (20 min) administration of this drug almost totally abolished the usually observed increase in hippocampal cholinergic activity induced by testing. This inhibition of cholinergic activation was associated with a parallel working memory deficit. The results provide further direct support for the hypothesis that septal noradrenergic afferents via alpha-receptors mediate a phasic and net excitatory trans-synaptic influence on the cholinergic septo-hippocampal pathway during working memory testing and thereby significantly contribute to the modulation of the level of working memory performance.

Differential hippocampal and cortical cholinergic activation during the acquisition, retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze by mice

Possible differentiation of the intervention of cholinergic septohippocampal and magnocellular forebrain (NBM) projections to cortex during learning and memory processes has been investigated directly using mice. High-affinity choline uptake velocities in the hippocampus and cortex were analyzed, in parallel, at various periods during the acquisition, over 8 days, as were the subsequent retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze. Initial acquisition induced an immediate (30 s) and long-lasting (approx. 3 h) increase in mean hippocampal (+33%) and cortical (+23%) cholinergic activities. The time course of this activation was structure-dependent and correlations of hippocampal-cortical cholinergic activities showed large and consistent alterations as a function of time after training. Cholinergic activation in both brain regions was observed immediately following each daily training session with amplitudes which did not vary significantly in spite of a progressive daily increment in performance. Following acquisition mice were tested for retention, reversal and extinction: 30 s following the retention session, cholinergic activation was observed in both cortex and hippocampus, with magnitudes similar to those observed at the end of acquisition. However, in the reversal and extinction groups, a treatment-dependent attenuation of cholinergic activation was observed which was accompanied by a significant loss of correlation of cholinergic activity between these two brain regions. The results are discussed in relation to the concepts of reference and working memory and also to novelty, stress, arousal and frustrative non-reward. The data constitute direct experimental evidence for a differential involvement of cholinergic septohippocampal and NBM-cortical projections in learning and memory processes.

Low-level microwave irradiations affect central cholinergic activity in the rat

Sodium-dependent high-affinity choline uptake was measured in various regions of the brains of rats irradiated for 45 min with either pulsed or continuous-wave low-level microwaves (2,450 MHz; power density, 1 mW/cm2; average whole-body specific absorption rate, 0.6 W/kg). Pulsed microwave irradiation (2-microseconds pulses, 500 pulses/s) decreased choline uptake in the hippocampus and frontal cortex but had no significant effect on the hypothalamus, striatum, and inferior colliculus. Pretreatment with a narcotic antagonist (naloxone or naltrexone; 1 mg/kg i.p.) blocked the effect of pulsed microwaves on hippocampal choline uptake but did not significantly alter the effect on the frontal cortex. Irradiation with continuous-wave microwaves did not significantly affect choline uptake in the hippocampus, striatum, and hypothalamus but decreased the uptake in the frontal cortex. The effect on the frontal cortex was not altered by pretreatment with narcotic antagonist. These data suggest that exposure to low-level pulsed or continuous-wave microwaves leads to changes in cholinergic functions in the brain.

Chronic administration of sulbutiamine improves long term memory formation in mice: possible cholinergic mediation

Thiamine deficiency in both man and animals is known to produce memory dysfunction and cognitive disorders which have been related to an impairment of cholinergic activity. The present experiment was aimed at testing whether, inversely, chronic administration of large doses of sulbutiamine would have a facilitative effect on memory and would induce changes in central cholinergic activity. Accordingly mice received 300 mg/kg of sulbutiamine daily for 10 days. They were then submitted to an appetitive operant level press conditioning test. When compared to control subjects, sulbutiamine treated mice learned the task at the same rate in a single session but showed greatly improved performance when tested 24 hr after partial acquisition of the same task. Parallel neurochemical investigations showed that the treatment induced a slight (+ 10%) but significant increase in hippocampal sodium-dependent high affinity choline uptake. The present findings and previous results suggest that sulbutiamine improves memory formation and that this behavioral effect could be mediated by an increase in hippocampal cholinergic activity.

Physostigmine reverses memory deficits produced by pretraining electrical stimulation of the dorsal hippocampus in mice

The aim of the present experiments was to test the validity of the hypothesis that presynaptic cholinergic activity has a functional significance for memory formation. The results show that electrical stimulation of the dorsal hippocampus delivered before learning in BALB/c mice which induces a decrease of about 40% in hippocampal choline acetyltransferase (ChAT) activity at the time of learning results in deficits in retention scores in two appetitive learning tasks (operant conditioning in the Skinner box or a spatial memory task using a 4-hole board). In both behavioral tasks intraventricular injection of 1 microgram of physostigmine 20 min before the acquisition session reverses the disruptive effect of pretraining hippocampal stimulation. Our results seem to indicate that the memory deficits produced by pretraining electrical stimulation of the hippocampus result from both a decrease in ChAT activity and a corresponding reduction of acetylcholine availability in the hippocampal formation.

Behavioural plasticity and the cholinergic system

Evidence for the involvement of the cholinergic system in behavioural plasticity is reviewed by considering three forms of behavioural plasticity: habituation, learning and memory, and tolerance development. Although the cholinergic system may modulate the response tendencies of an animal, it does not appear to be involved in the process of habituation. A number of studies have indicated that the cholinergic system may be involved in learning and memory processes in infrahuman animals. In general, cholinergic antagonists tend to disrupt memory while agonists may, under the appropriate conditions, facilitate memory. Recent studies have pointed to a relation between dysfunctions of the cholinergic system and dysfunctions of memory in aged animals. Studies of tolerance development suggest that the cholinergic system may undergo plastic changes which may underlie the development of tolerance to some drugs, with receptor alterations being the most reproducible finding. However, more work is necessary to establish the degree of plasticity. The cholinergic system also appears to be involved in learning and memory processes in humans. However, attempts to correct the memory deficits in aged humans by manipulating the cholinergic system have met with limited success. The reasons for this lack of success are briefly considered.

Facilitation of memory consolidation by post-training electrical stimulation of the medial septal nucleus: is it mediated by changes in rhythmic slow activity?

Sinusoidal (100 Hz) electrical stimulation was applied at a weak intensity (7.5 muA peak to peak) through bipolar electrodes located in the medial septal nucleus after partial acquisition of an appetitive operant conditioning task in a Skinner box. Analysis of performance in a retention test 24 hr later showed that (i) the presence of stimulation electrodes by itself impaired retention-test performance, and (ii) electrical stimulation applied 30 sec after the end of the acquisition session improves retention; this facilitatory effect disappeared when the treatment was delayed 15 min. Both impairment and facilitation were found to vary (considerably) among subjects. Electrodes located in the center of the medial septal nucleus led to both a greater impairment in unstimulated subjects and a greater facilitation in stimulated subjects than more anterior placements in the vicinity of the diagonal band. Finally, spectral analysis of hippocampal EEG showed that stimulation had no effect on rhythmic slow activity (RSA). These results are discussed in relation to studies showing that RSA is associated with memory-storage processes and our own hypothesis which underlines the importance of activation of septo-hippocampal cholinergic neurons in the early stages of these mnemonic processes.

Radial maze performance in three strains of mice: role of the fimbria/fornix

Three strains of mice were tested on an 8-arm radial maze, an index of hippocampus-dependent spatial memory. Levels of performance differed between strains with C57Br/cj greater than Balb/cj greater than C57Bl/6j. Lesions of the fimbria/fornix disrupted performance in the C57Br and Balb strains: the C57Bl mice never performed better than chance before or after surgery. Choline acetyltransferase activity in hippocampus was not correlated with radial maze performance. These findings suggest a possible genetic contribution towards radial maze behavior.

Motion sickness: a modulatory role for the central cholinergic nervous system

The present review has extended the general theory of motion sickness proposed by Wood and Graybiel [135, 136] by identifying specific neurophysiological mechanisms that are involved in motion sickness and by interpreting the actions of both scopolamine and amphetamine as effective anti-motion sickness drugs within this neurophysiological context. The neurochemical and neurophysiological effects of scopolamine have been reviewed in relationship to central cholinergic pathways. Cholinergic pathways have been associated with both the perception and expression of normal and excessive levels of motion stimuli. New approaches to the problem of the prevention of motion sickness have been considered. Efferent nicotinic innervation at the primary sensory hair cells and the medial vestibular nucleus were identified as sites where modulation by cholinergic drugs might exert a beneficial influence. However, it was generally conceded that the complexity of the cholinergic system and the interaction of scopolamine with that system left open the possibility that pharmacological doses of drugs specific to the cholinergic system might exert significant modulatory influences at alternative sites, as well.