Serotonin-dependent maintenance of spatial performance and electroencephalography activation after cholinergic blockade: effects of serotonergic receptor antagonists

Cholinergic modulation of motor sequence learning

The cholinergic system plays a key role in motor function, but whether pharmacological modulation of cholinergic activity affects motor sequence learning is unknown. The acetylcholine receptor antagonist biperiden, an established treatment in movement disorders, reduces attentional modulation, but whether it influences motor sequence learning is not clear. Using a randomized, double-blind placebo-controlled crossover design, we tested 30 healthy young participants and showed that biperiden impairs the ability to learn sequential finger movements, accompanied by widespread oscillatory broadband power changes (4-25 Hz) in the motor sequence learning network after receiving biperiden, with greater power in the theta, alpha and beta bands over ipsilateral motor and bilateral parietal-occipital areas. The reduced early theta power during a repeated compared with random sequence, likely reflecting disengagement of top-down attention to sensory processes, was disrupted by biperiden. Alpha synchronization during repeated sequences reflects sensory gating and lower visuospatial attention requirements compared with visuomotor responses to random sequences. After biperiden, alpha synchronization was greater, potentially reflecting excessive visuospatial attention reduction, affecting visuomotor responding required to enable sequence learning. Beta oscillations facilitate sequence learning by integrating visual and somatosensory inputs, stabilizing repeated sequences and promoting prediction of the next stimulus. The beta synchronization after biperiden fits with a disruption of the selective visuospatial attention enhancement associated with initial sequence learning. These findings highlight the role of cholinergic processes in motor sequence learning.

The involvement of monoaminergic neurotransmission in the antidepressant-like action of scopolamine in the tail suspension test

Some clinical studies indicate that scopolamine may induce a rapid antidepressant effect. Although scopolamine is a muscarinic antagonist, it seems that not only cholinergic but also glutamatergic and GABAergic systems might be involved in the mechanism of its antidepressant activity in animal models of depression. Here, we present a set of behavioral data aimed at investigating the role of monoaminergic system activity in the mechanism of the antidepressant-like action of scopolamine in an animal model based on behavioral despair, namely, the tail suspension test (TST). It was found that AMPT induced a partial reduction in the antidepressant-like effect of scopolamine (0.3mg/kg) in the TST in C57BL/6 mice and that the effect of scopolamine was comparable to the effect of reboxetine (10mg/kg), which was used in this study as a reference drug. The attenuated antidepressant-like effect of scopolamine in AMPT-treated mice was observed in both its immediate (30min after administration) and prolonged (24h after administration) action in the TST. On the other hand, serotonin depletion by PCPA-pretreatment had no effect on the antidepressant effect of scopolamine (0.3mg/kg) either 30min or 24h after administration. Furthermore, a dose-dependent decrease in the immobility time of mice treated with a non-active dose of reboxetine (2mg/kg) together with non-active doses of scopolamine (0.03 and 0.1mg/kg) was found, suggesting a synergistic interaction between reboxetine and scopolamine in the TST. In contrast, a subeffective dose of the SSRI citalopram co-administered with subeffective doses of scopolamine did not induce significant changes in the behavior of mice in this test. Altogether, these data suggest that activation of the noradrenergic system might be involved in the antidepressant-like effect of scopolamine in the TST.

Antidepressant-like effects of scopolamine in mice are enhanced by the group II mGlu receptor antagonist LY341495

Clinical studies have shown that the muscarinic receptor antagonist scopolamine induces a potent and rapid antidepressant effect relative to conventional antidepressants. However, potential undesirable effects, including memory impairment, partially limit the use of scopolamine in psychiatry. In the present study, we propose to overcome these limitations and enhance the therapeutic effects of scopolamine via administration in combination with the group II metabotropic glutamate (mGlu) receptor antagonist, LY341495. Joint administration of sub-effective doses of scopolamine (0.03 or 0.1 mg/kg, i.p.) with a sub-effective dose of LY341495 (0.1 mg/kg, i.p.) induced a profound antidepressant effect in the tail suspension test (TST) and in the forced swim test (FST) in mice. This drug combination did not impair memory, as measured using the Morris water maze (MWM), and did not influence the locomotor activity of mice. Furthermore, we found that an AMPA receptor antagonist, NBQX (10 mg/kg), completely reversed the antidepressant-like activity of a mixture of scopolamine and LY341495 in the TST. However, this effect was not influenced by para-chlorophenylalanine (PCPA) pre-treatment, indicating a lack of involvement of serotonergic system activation in the antidepressant-like effects of jointly given scopolamine and LY341495. Therefore, the combined administration of low doses of the antimuscarinic drug scopolamine and the group II mGlu receptor antagonist LY341495 might be a new, effective and safe strategy in the therapy of depression.

α-Synuclein induced toxicity in brain stem serotonin neurons mediated by an AAV vector driven by the tryptophan hydroxylase promoter

We studied the impact of α-synuclein overexpression in brainstem serotonin neurons using a novel vector construct where the expression of human wildtype α-synuclein is driven by the tryptophan hydroxylase promoter, allowing expression of α-synuclein at elevated levels, and with high selectivity, in serotonergic neurons. α-Synuclein induced degenerative changes in axons and dendrites, displaying a distorted appearance, suggesting accumulation and aggregation of α-synuclein as a result of impaired axonal transport, accompanied by a 40% loss of terminals, as assessed in the hippocampus. Tissue levels of serotonin and its major metabolite 5-HIAA remained largely unaltered, and the performance of the α-synuclein overexpressing rats in tests of spatial learning (water maze), anxiety related behavior (elevated plus maze) and depressive-like behavior (forced swim test) was not different from control, suggesting that the impact of the developing axonal pathology on serotonin neurotransmission was relatively mild. Overexpression of α-synuclein in the raphe nuclei, combined with overexpression in basal forebrain cholinergic neurons, resulted in more pronounced axonal pathology and significant impairment in the elevated plus maze. We conclude that α-synuclein pathology in serotonergic or cholinergic neurons alone is not sufficient to impair non-motor behaviors, but that it is their simultaneous involvement that determines severity of such symptoms.

A hippocampal nicotinic acetylcholine alpha 7-containing receptor complex is linked to memory retrieval in the multiple-T-maze in C57BL/6j mice

The link between the cholinergic and serotonergic system in cognitive function is well-documented. There is, however, limited information on spatial memory and this formed the rationale to carry out a study with the aim to show a specific link between nicotinic and serotonergic receptor complexes rather than the corresponding subunits, to spatial memory retrieval in a land maze. A total of 46 mice were used and divided into two groups, trained and untrained (yoked) in the multiple-T-Maze (MTM) and following training during the first four days, probe trials for memory retrieval were performed on days 8, 16 and 30. Six hours following scarification, hippocampi were taken for the analysis of native receptor complex levels using blue-native gels followed by immunoblotting with specific antibodies. 5-HT1A-, 5-HT7-, nAChα4- and nACh-α7-containing receptor complexes were observed and were paralleling memory retrievals and receptor complex levels were shown to be significantly different between trained and yoked animals. Only levels of a nicotinic acetylcholine α7 receptor-containing complex at an apparent molecular weight of approximately 480kDa were shown to be linked to memory retrieval on day 8 but not to retrievals on days 16 and 30 when memory extinction has taken place. Correlation between nAChα4-, 5-HT1A- and 5-HT7-containing receptors and latencies on day 16 may point to a probable link in extinction mechanisms. A series of the abovementioned receptor complexes were correlating among each other probably indicating a serotonergic/cholinergic network paralleling spatial memory formation.

Role of cholinergic-muscarinic receptors in visual discrimination performance of rats: importance of stimulus load

Central cholinergic transmission has long been implicated in various cognitive processes, including memory acquisition, consolidation, and attentional processes. Here, we examined the role of muscarinic receptors in visual discrimination performance under conditions of altered visual information availability. Adult rats were trained to discriminate two visual cues (indicating the presence and absence of a hidden escape platform, respectively) in a water maze-based, trapezoidal-shaped apparatus. Following task acquisition, testing continued with two types of trials: regular trials (RTs; both visual cues present, identical to training conditions) and probe trials (PTs; only one of the two cues present). In Experiment 1, removal of one visual cue on PTs impaired discrimination performance. Moreover, scopolamine administration (0.125-1.0 mg/kg, i.p.) tended to further suppress performance in a dose-dependent manner on PTs, while discriminations on RTs were left intact. In Experiment 2, these results were confirmed and extended by showing that PT (one visual cue) performance could improve with training in undrugged, but not in scopolamine-treated rats. Together, these experiments reveal that visual discrimination performance of rats benefits from the concurrent availability of two visual cues that provide complimentary and consistent information. Furthermore, muscarinic receptors are particularly important under conditions of reduced visual information availability, as well as in the adoption of new behavioral strategies, such as switching from two-cue to single-cue guided navigation.

Effects of early protein malnutrition and scopolamine on learning and memory in the Morris water maze

The present study investigated the effects of early protein malnutrition on the spatial learning and memory processes. The consequences of malnutrition for the cholinergic system were evaluated by comparing the performance of malnourished and control animals in the Morris water maze after treatment with scopolamine. The learning test consisted of placing the animal in the maze to escape to a submerged platform with 12 trials per day for two consecutive days. After 24 trials, the platform was removed, the rats were placed in the maze and the time spent by them in each quadrant was recorded. After 28 days the animals were tested in a single trial to verify the retention of the spatial information. In the first Experiment, scopolamine (0.0, 0.2, 0.4 and 0.6 mg/kg per ml. i.p.) was administered 20 min before the experimental sessions. In the second experiment, a dose of 0.6 mg/kg was administered after the sessions, during the period in which learning consolidation occurs. In the first experiment, there was a significant effect of the drug, with scopolamine impairing, learning in both nutritional conditions. In the saline condition, control animals presented a better performance when compared with malnourished animals. However, 28 days later, both groups increased their latencies. With 0.2 and 0.4 mg/kg of scopolamine, the performance of both nutritional groups was similar and with 0.6 mg/kg malnourished animals performed better than controls. In the second experiment, malnourished animals were also less reactive to the effects of scopolamine, resulting in lower impairments as compared to control animals. These data suggest long-term changes in learning and memory as the result of changes produced by protein malnutrition in the cholinergic neurotransmitter system.

Serotonergic modulation of plasticity of the auditory cortex elicited by fear conditioning

In the awake big brown bat, 30 min auditory fear conditioning elicits conditioned heart rate decrease and long-term best frequency (BF) shifts of cortical auditory neurons toward the frequency of the conditioned tone; 15 min conditioning elicits subthreshold cortical BF shifts that can be augmented by acetylcholine. The fear conditioning causes stress and an increase in the cortical serotonin (5-HT) level. Serotonergic neurons in the raphe nuclei associated with stress and fear project to the cerebral cortex and cholinergic basal forebrain. Recently, it has been shown that 5-HT(2A) receptors are mostly expressed on pyramidal neurons and their activation improves learning and memory. We applied 5-HT, an agonist (alpha-methyl-5-HT), or an antagonist (ritanserin) of 5-HT(2A) receptors to the primary auditory cortex and discovered the following drug effects: (1) 5-HT had no effect on the conditioned heart rate change, although it reduced the auditory responses; (2) 4 mm 5-HT augmented the subthreshold BF shifts, whereas 20 mm 5-HT did not; (3) 20 mm 5-HT reduced the long-term BF shifts and changed them into short-term; (4) alpha-methyl-5-HT increased the auditory responses and augmented the subthreshold BF shifts as well as the long-term BF shifts; (5) in contrast, ritanserin reduced the auditory responses and reversed the direction of the BF shifts. Our data indicate that the BF shift can be modulated by serotonergic neurons that augment or reduce the BF shift or even reverse the direction of the BF shift. Therefore, not only the cholinergic system, but also the serotonergic system, plays an important role in cortical plasticity according to behavioral demands.

D1 and D2 receptor antagonist injections in the prefrontal cortex selectively impair spatial learning in mice

The prefrontal cortex (PFC) is a cortical area involved in selecting and retaining information to produce complex behaviors. Within the PFC, the dopaminergic system plays an important role in information processing. Thus, the objective of this study was to test whether bilateral administration of the D1 and D2 receptor antagonists in the prelimbic region of the PFC influenced the performance of mice in a non-associative spatial learning task. CD1 mice were bilaterally microinjected in the PFC with either the D1 receptor antagonist, SCH23390 (SCH 6.25; 12.5; 50 ng), or the D2 receptor antagonist, sulpiride (SULP 12.5; 50; 100 ng) and placed into an open field containing five different objects. After three sessions of habituation two objects were repositioned (spatial change) and in the subsequent session one of the objects was substituted (non-spatial change). No significant alteration was observed in the habituation pattern of the animals after D1 or D2 receptor blockade. When two of the objects were displaced, control mice explored the displaced objects far more than the non-displaced ones, while mice treated with SCH or SULP spent a comparable amount of time re-exploring the two object categories. Conversely, DA antagonists had no effects on the discrimination of the new object. Thus, the administration of both SCH and SULP selectively impaired the ability of mice to discriminate a spatial change, without affecting any other behavioral parameter. These findings could provide a model to study the role of the PFC dopaminergic system in spatial learning and to study the neural mechanisms underlying cognitive and attention deficits often observed in psychiatric disorders.

Neurotransmitter deficits in behavioural and psychological symptoms of Alzheimer's disease

Behavioural and psychological symptoms of dementia (BPSD) occur in 50-90% of Alzheimer's disease (AD) patients. Imbalance of different neurotransmitters (acetylcholine, dopamine, noradrenaline and serotonin), involvement of specific brain regions responsible for emotional activities (parahippocampal gyrus, dorsal raphe and locus coeruleus) and cortical hypometabolism have been proposed as neurobiological substrate of BPSD. Compared to with respect to the neurochemical component, the cholinergic dysfunction seems to play a major role in contributing to BPSD occurrence. This view is also supported by the findings of recent trials with cholinesterase inhibitors, showing that these drugs are effective in controlling and/or improving BPSD, independent on effects on cognitive dysfunction. On the site of psychotropic drugs, atypical or novel antipsychotics represent the reference drugs for treating BPSD, whereas classic antipsychotic drugs for their profile and the potential side effects should be avoided.

Cholinergic-serotonergic imbalance contributes to cognitive and behavioral symptoms in Alzheimer's disease

Neuropsychiatric symptoms seen in Alzheimer's disease (AD) are not simply a consequence of neurodegeneration, but probably result from differential neurotransmitter alterations, which some patients are more at risk of than others. Therefore, the hypothesis of this study is that an imbalance between the cholinergic and serotonergic systems is related to cognitive symptoms and psychological syndromes of dementia (BPSD) in patients with AD. Cholinergic and serotonergic functions were assessed in post-mortem frontal and temporal cortex from 22 AD patients who had been prospectively assessed with the Mini-Mental State examination (MMSE) for cognitive impairment and with the Present Behavioral Examination (PBE) for BPSD including aggressive behavior, overactivity, depression and psychosis. Not only cholinergic deficits, but also the cholinacetyltransferase/serotonin ratio significantly correlated with final MMSE score both in frontal and temporal cortex. In addition, decreases in cholinergic function correlated with the aggressive behavior factor, supporting a dual role for the cholinergic system in cognitive and non-cognitive disturbances associated to AD. The serotonergic system showed a significant correlation with overactivity and psychosis. The ratio of serotonin to acetylcholinesterase levels was also correlated with the psychotic factor at least in women. It is concluded that an imbalance between cholinergic-serotonergic systems may be responsible for the cognitive impairment associated to AD. Moreover, the major findings of this study are the relationships between neurochemical markers of both cholinergic and serotonergic systems and non-cognitive behavioral disturbances in patients with dementia.

The 5-HT1A receptor and recognition memory

Functional activation of the 5-HT1A receptor inhibits cognition, although discrepant findings have also been reported. The present study was designed to investigate the role of the 5-HT1A receptor on recognition memory in the rat. For this purpose, the effects induced by the 5-HT1A agonist R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and the 5-HT1A antagonist WAY 100635 on memory were evaluated by using the object recognition task. In addition, the possible involvement of the nitrergic system on 5-HT1A receptor's effects was also assessed by using the same behavioral procedure. In the first dose-response study, post-training administration of 8-OH-DPAT (0.1 and 0.3 mg/kg, subcutaneously (s.c.)) dose-dependently impaired animals' performance in this test. WAY 100635 (0.3 and 1 mg/kg, intraperitoneally (i.p.)) successfully antagonized these 8-OH-DPAT-induced performance deficits. The NO donor molsidomine (2 and 4 mg/kg, i.p.) counteracted cognition deficits produced by the highest dose of 8-OH-DPAT (0.3 mg/kg). Our findings indicate (a) that the 5-HT1A receptor is involved in recognition memory, and (b) that a NO component modulates the effects of the 5-HT1A receptor on learning and memory.

Acute amygdalar activation induces an upregulation of multiple monoamine G protein coupled pathways in rat hippocampus

A "partial" rodent model for schizophrenia has been used to characterize the regulation of hippocampal genes in response to amygdalar activation. At 96 h after the administration of picrotoxin into the basolateral nucleus, we have observed an increase in the expression of genes associated with 18 different monoamine (ie adrenergic alpha 1, alpha 2 and beta 2, serotonergic 5HT5b and 5HT6, dopamine D4 and muscarinic m1, m2 and m3) and peptide (CCK A and B, angiotensin 1A, mu and kappa opiate, FSH, TSH, LH, GNRH, and neuropeptide Y) G-protein coupled receptors (GPCRs). These latter receptors are associated with three different G protein signaling pathways (Gq, Gs, and Gi) in which significant changes in gene expression were also noted for adenylate cyclase (AC4), phosphodiesterase (PDE4D), protein kinase A (PKA), and protein kinase C (PKC). Quantitative RT-PCR was used to validate the results and demonstrated that there were predictable increases of three GPCRs selected for this analysis, including the dopamine D4, alpha 1b, and CCK-B receptors. Eight out of the nine monoamine receptors showing these changes have moderate to high affinity for the atypical antipsychotic, clozapine. Taken together, these results suggest that amygdalar activation may play a role in the pathophysiology and treatment of psychosis by regulating the activity of multiple GPCR and metabolic pathways in hippocampal cells.

The 5-HT 1A receptor antagonist WAY 100635 improves rats performance in different models of amnesia evaluated by the object recognition task

The effects of the 5-HT(1A) receptor antagonist WAY 100635 on recognition memory were investigated in two different amnestic models in the rat by using the object recognition task. WAY 100635 at 1 mg/kg, but not at 0.3 mg/kg, counteracted scopolamine-induced performance deficits in the acquisition version of this behavioral paradigm. At the same dose, WAY 100635 antagonized extinction of recognition memory in the normal rat, suggesting that it affected acquisition, storage and retrieval of information. These results support and extend prior findings that interactions between the serotonergic and cholinergic systems are relevant to cognition and indicate that WAY 100635 modulates different aspects of recognition memory.

Cholinergic and serotonergic neocortical projection lesions given singly or in combination cause only mild impairments on tests of skilled movement in rats: evaluation of a model of dementia

The cholinergic (ACh) projections of the nucleus basalis and the serotonergic (5-HT) projections of the raphe nuclei to the neocortex are required for the normal function of the neocortex. Nevertheless, damage to either system alone has little effect on the behavior of rats, but conjoint damage to both systems is reported to produce dementia to the point that animals are described as being unable to engage in intelligent behavior. Because rats with bilateral damage to both systems are so severely impaired, they are not useful for chronic studies. The objective of the present research was to determine whether unilateral depletions produce a functional impairment. Rats received unilateral neurotoxic lesions to either the nucleus basalis (quisqualic acid), or the medial forebrain bundle (5,7-dihydroxytryptamine), or both, which reduced neocortical levels of ACh (55%) and 5-HT (63%). The rats then received a battery of tests sensitive to unilateral neocortical injury. The 5-HT lesion produced no quantitative or qualitative deficits on reaching for food, walking across a horizontal ladder, forelimb placement in a cylinder, sensory detection of adhesive paper applied to the wrists, or forelimb inhibition during swimming. The ACh lesion produced mild qualitative deficits in reaching. Combined lesions produced mild deficits in skilled reaching, ladder walking, and sensory detection. In contrast to the mild impairments produced by the lesions, pharmacological blockade of either ACh with atropine or 5-HT with methiothepin mesylate systemically blocked skilled motor behavior as assessed by skilled reaching. The results are discussed in relation to the problems associated with the development of a unilateral model of dementia.

Postmortem serotoninergic correlates of cognitive decline in Alzheimer's disease

Serotonin1A receptor density and serotonin concentration were measured in the postmortem neocortex of 17 AD patients who had been prospectively assessed every four months with the Mini-Mental State Examination (MMSE) for a mean of 2.6 years till death. In the frontal cortex, serotonin levels correlated negatively with the annual rate of MMSE decline, while serotonin1A receptor density was positively correlated with the rate of MMSE decline. Our study suggests that reduced serotonin levels and increased serotonin1A receptor density are markers for accelerated cognitive decline in AD, and provides support for the use of serotonin1A antagonists in the treatment of AD.

Effects of excitotoxic median raphe lesions on scopolamine-induced working memory deficits in inhibitory avoidance

The aim of the present study was to investigate the effects of excitotoxic damage of the serotonergic cell bodies in the median raphe nucleus (MRN) on the scopolamine-induced working memory deficits in a single-trial light/dark inhibitory avoidance task. Rats were given 1 mg/kg of scopolamine hydrobromide (intraperitonal, i.p.) or saline before the inhibitory avoidance training, in which initial preference to the dark compartment (escape latency) was used to measure nonmnemonic behaviors, and response latency to enter the dark compartment immediately after the shock was used to measure working memory. It was found that scopolamine significantly reduced escape latencies in sham-lesioned rats, whereas it had no effect in the rats with MRN lesions. Although MRN lesion per se did not alter response latency, it prevented scopolamine-induced decrease in this parameter. These results suggest that the antagonistic interactive processes between serotonergic projections of the MRN and the muscarinic cholinergic system modulate nonmnemonic attentional component of working memory formation in the inhibitory avoidance.

Excitotoxic median raphe lesions aggravate working memory storage performance deficits caused by scopolamine infusion into the dentate gyrus of the hippocampus in the inhibitory avoidance task in rats

The interactions between the median raphe nucleus (MRN) serotonergic system and the septohippocampal muscarinic cholinergic system in the modulation of immediate working memory storage performance were investigated. Rats with sham or ibotenic acid lesions of the MRN were bilaterally implanted with cannulae in the dentate gyrus of the hippocampus and tested in a light/dark step-through inhibitory avoidance task in which response latency to enter the dark compartment immediately after the shock served as a measure of immediate working memory storage. MRN lesion per se did not alter response latency. Post-training intrahippocampal scopolamine infusion (2 and 4 microg/side) produced a more marked reduction in response latencies in the lesioned animals compared to the sham-lesioned rats. Results suggest that the immediate working memory storage performance is modulated by synergistic interactions between serotonergic projections of the MRN and the muscarinic cholinergic system of the hippocampus.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.

Electroencephalographic activation by fluoxetine in rats: role of 5-HT(1A) receptors and enhancement of concurrent acetylcholinesterase inhibitor treatment

Considerable evidence indicates that both cholinergic (ACh) and serotonergic (5-HT) inputs to the neocortex play a direct role in maintaining the activated state of the electroencephalogram (EEG). Here, we investigated frequency-specific EEG effects of the 5-HT re-uptake inhibitor fluoxetine (10 and 20 mg/kg) to restore EEG activation abolished by combined treatment with the monoamine depletor reserpine (10 mg/kg) and the muscarinic antagonist scopolamine (1 mg/kg). Fluoxetine alone was ineffective in reversing EEG slowing produced by reserpine-scopolamine administration. However, fluoxetine suppressed EEG synchronization in the alpha (8-12 Hz) band when administered concurrently with the 5-HT(1A) receptor antagonist WAY 100635 (0.5 mg/kg). Further, fluoxetine, with and without WAY 100635, markedly potentiated the effectiveness of the acetylcholinesterase (AChE) inhibitor tacrine (5 mg/kg) to restore EEG activation between 4-30 Hz. These data indicate that 5-HT uptake inhibition and concurrent 5-HT(1A) receptor blockade produce a limited normalization of the cortical EEG after monoaminergic-cholinergic blockade. However, fluoxetine strongly potentiates the effectiveness of tacrine to restore EEG activation. Inhibitors of AChE are used to delay cognitive decline and EEG slowing in patients with Alzheimer's disease. We suggest that doses of AChE inhibitors required for these effects can be reduced by concurrent 5-HT stimulation.

Effects of phencyclidine (PCP) and MK 801 on the EEGq in the prefrontal cortex of conscious rats; antagonism by clozapine, and antagonists of AMPA-, alpha(1)- and 5-HT(2A)-receptors

1. The electroencephalographic (EEG) effects of the propsychotic agent phencyclidine (PCP), were studied in conscious rats using power spectra (0 - 30 Hz), from the prefrontal cortex or sensorimotor cortex. PCP (0.1 - 3 mg kg(-1) s.c.) caused a marked dose-dependent increase in EEG power in the frontal cortex at 1 - 3 Hz with decreases in power at higher frequencies (9 - 30 Hz). At high doses (3 mg kg(-1) s.c.) the entire spectrum shifted to more positive values, indicating an increase in cortical synchronization. MK 801 (0.05 - 0.1 mg kg(-1) i.p.) caused similar effects but with lesser changes in power. 2. In contrast, the non-competitive AMPA antagonists GYKI 52466 and GYKI 53655 increased EEG power over the whole power spectrum (1 - 10 mg kg(-1) i.p.). The atypical antipsychotic clozapine (0.2 mg kg(-1) s.c.) synchronized the EEG (peak 8 Hz). The 5-HT(2A)-antagonist, M100907, specifically increased EEG power at 2 - 3 Hz at low doses (10 and 50 microg kg(-1) s.c.), whereas at higher doses (0.1 mg kg(-1) s.c.) the profile resembled that of clozapine. 3. Clozapine (0.2 mg kg(-1) s.c. ), GYKI 53655 (5 mg kg(-1) i.p.), prazosin (0.05 and 0.1 mg kg(-1) i.p.), and M100907 (0.01 and 0.05 mg kg(-1) s.c.) antagonized the decrease in power between 5 and 30 Hz caused by PCP (1 mg kg(-1) s.c.), but not the increase in power at 1 - 3 Hz in prefrontal cortex.

Beta-amyloid(1-42)-induced cholinergic lesions in rat nucleus basalis bidirectionally modulate serotonergic innervation of the basal forebrain and cerebral cortex

Ample experimental evidence suggests that beta-amyloid (A beta), when injected into the rat magnocellular nucleus basalis (MBN), impels excitotoxic injury of cholinergic projection neurons. Whereas learning and memory dysfunction is a hallmark of A beta-induced cholinergic deficits, anxiety, or hypoactivity under novel conditions cannot be attributed to the loss of cholinergic MBN neurons. As mood-related behavioral parameters are primarily influenced by the central serotonergic system, in the present study we investigated whether A beta(1-42) toxicity in the rat MBN leads to an altered serotonergic innervation pattern in the rat basal forebrain and cerebral cortex 7 days postsurgery. A beta infusion into the MBN elicited significant anxiety in the elevated plus maze. A beta toxicity on cholinergic MBN neurons, expressed as the loss of acetylcholinesterase-positive cortical projections, was accompanied by sprouting of serotonergic projection fibers in the MBN. In contrast, the loss of serotonin-positive fiber projections, decreased concentrations of both serotonin and 5-hydroxyindoleacetic acid, and decline of cortical 5-HT(1A) receptor binding sites indicated reduced serotonergic activity in the somatosensory cortex. In conclusion, the A beta-induced primary cholinergic deficit in the MBN and subsequent cortical cholinergic denervation bidirectionally modulate serotonergic parameters in the rat basal forebrain and cerebral cortex. We assume that enhanced serotonin immunoreactivity in the damaged MBN indicates intrinsic processes facilitating neuronal recovery and cellular repair mechanisms, while diminished cortical serotonergic activity correlates with the loss of the subcortical cholinergic input, thereby maintaining the balance of neurotransmitter concentrations in the cerebral cortex.

Increased effectiveness of tacrine by deprenyl co-treatment in rats: EEG and behavioral evidence

The acetylcholinesterase inhibitor tacrine and the monoamine oxidase inhibitor deprenyl are considered useful pharmacotherapies for Alzheimer's disease (AD). We assessed whether co-administration of these two compounds increases their effectiveness against two measures of cholinergic-monoaminergic hypofunction in rats, cortical EEG slowing and impaired spatial performance. EEG slowing induced by cholinergic-monoaminergic blockade was reversed by both deprenyl (10 - 50 mg/kg) and tacrine (1 - 20 mg/kg), but co-treatment with a subthreshold dose of deprenyl plus tacrine was markedly more effective. Neither tacrine (5 mg/kg) nor deprenyl (10 mg/kg) alone reduced water maze deficits due to cholinergic-monoaminergic hypofunction, but co-treatment (using these doses) improved performance. Cholinergic-monoaminergic co-treatment may constitute a useful pharmacotherapy to correct physiological and behavioral dysfunction due to neurotransmitter deficiencies in AD.

Alzheimer's disease: more than a 'cholinergic disorder' - evidence that cholinergic-monoaminergic interactions contribute to EEG slowing and dementia

The loss of cognitive (particularly mnemonic) abilities constitutes a prominent symptom of Alzheimer's disease (AD). These cognitive symptoms occur in close relation to the slowing of the electroencephalogram (EEG), and it is likely that the inability of cortical circuits to maintain an activated state contributes to the behavioral disorganization in AD. The 'cholinergic hypothesis' of AD suggests that many of the cognitive and EEG symptoms are related to the atrophy of basal forebrain cholinergic neurons, which innervate the neocortex and hippocampus, among others. However, data from behavioral and electrophysiological studies in rats suggest that selective reductions in cholinergic transmission result in relatively small mnemonic impairments, and only a partial reduction in EEG activation. Thus, cholinergic atrophy alone may not be sufficient to cause the marked changes in cognition and cortical activity typical of AD. Cholinergic deficits do, however, make neural circuits susceptible to additional neurodegenerative processes. In rats, lowered serotonergic or noradrenergic activity alone often produces only minor impairments in learning/memory tasks and does not block EEG activation. The same monoaminergic deficits, however, result in severe behavioral impairments, and reduce or abolish EEG activation when they occur in a brain already affected by lowered cholinergic activity. There is an abundance of evidence that monoamines are reduced in AD. These degenerative processes, when occurring in a neural environment compromised by cholinergic atrophy, may then contribute to the disturbances in cortical processing and cognition/behavior in AD. A prediction derived from this theory is that an enhancement of monoaminergic functions may have beneficial effects on behavior and cortical activity. Preliminary experiments support this idea: combined cholinergic-monoaminergic stimulation can be more effective in reversing behavioral (Morris water maze) impairments and EEG slowing in rats with multiple neurotransmitter deficiencies than cholinergic enhancement alone. Thus, a stimulation of monoaminergic activity, in conjunction with cholinergic therapies, may provide an effective treatment option for AD.