Sex and Gender Differences in the Brain Cholinergic System and in the Response to Therapy of Alzheimer Disease with Cholinesterase Inhibitors.

This review has two aims. First, to examine whether or not sex and gender may influence the brain cholinergic system in animals and in humans. Second, to examine the available evidence of sexually dimorphic response to the therapeutic and toxic effects of cholinesterase inhibitors. Animal research reveals no marked difference in the general morphology of the brain cholinergic system but subtle functional gender differences have been reported. In humans, gender differences in nucleus basalis of Meynert (NBM) exist. In animals, some cholinergic neurons express estrogen alpha receptors in females and androgens in males. It is known that sex hormones exert trophic effects on the cholinergic system. Females show higher frontal cortex cholinergic activity whereas males have higher activity in the hippocampus. Gender differences in the pharmacological effects result in higher sensitivity to the toxic effects of organophosphate cholinesterase inhibitors in males. A stronger and more selective benefit of ChEI treatment in AD has been reported in men by several authors. Sex and estrogen receptor phenotype may both influence the response to donepezil and rivastigmine. Hence, aged male and female individuals might respond differently to ChEI due to either sex-specific differences in structures and function of the cholinergic system, pharmacokinetics, memory function or in the way aging or AD affects these processes.

The fate of the brain cholinergic neurons in neurodegenerative diseases.

The aims of this review are: 1) to describe which cholinergic neurons are affected in brain neurodegenerative diseases leading to dementia; 2) to discuss the possible causes of the degeneration of the cholinergic neurons, 3) to summarize the functional consequences of the cholinergic deficit. The brain cholinergic system is basically constituted by three populations of phenotypically similar neurons forming a series of basal forebrain nuclei, the midpontine nuclei and a large population of striatal interneurons. In Alzheimer's disease there is an extensive loss of forebrain cholinergic neurons accompanied by a reduction of the cholinergic fiber network of the cortical mantel and hippocampus. The midpontine cholinergic nuclei are spared. The same situation occurs in the corticobasal syndrome and dementia following alcohol abuse and traumatic brain injury. Conversely, in Parkinson's disease, the midpontine nuclei degenerate, together with the dopaminergic nuclei, reducing the cholinergic input to thalamus and forebrain whereas the forebrain cholinergic neurons are spared. In Parkinson's disease with dementia, Lewis Body Dementia and Parkinsonian syndromes both groups of forebrain and midpontine cholinergic nuclei degenerate. In Huntington's disease a dysfunction of the striatal cholinergic interneurons without cell loss takes place. The formation and accumulation of misfolded proteins such as ß-amyloid oligomers and plaques, tau protein tangles and α-synuclein clumps, and aggregated mutated huntingtin play a crucial role in the neuronal degeneration by direct cellular toxicity of the misfolded proteins and through the toxic compounds resulting from an extensive inflammatory reaction. Evidences indicate that ß-amyloid disrupts NGF metabolism causing the degeneration of the cholinergic neurons which depend on NGF for their survival, namely the forebrain cholinergic neurons, sparing the midpontine and striatal neurons which express no specific NGF receptors. It is feasible that the latter cholinergic neurons may be damaged by direct toxicity of tau, α-synuclein and inflammations products through mechanisms not fully understood. Attention and learning and memory impairment are the functional consequences of the forebrain cholinergic neuron dysfunction, whereas the loss of midpontine cholinergic neurons results primarily in motor and sleep disturbances.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory.

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and ß-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

Do cholinesterase inhibitors act primarily on attention deficit? A naturalistic study in Alzheimer's disease patients.

Attention is the first non-memory domain affected in Alzheimer's disease (AD), before deficits in language and visuo-spatial function, and it is claimed that attention deficits are responsible for the difficulties with daily living in early demented patients. The aim of this longitudinal study in a group of 121 Caucasian, community-dwelling, mild-to-moderate AD patients (Mini-Mental State Examination (MMSE) score >17) was to detect which cognitive domains were most affected by the disease and whether one year treatment with cholinesterase inhibitors was more effective in preserving attention than memory. All subjects were evaluated by a neuropsychological battery including global measurements (MMSE, Information-Memory-Concentration Test) and tasks exploring verbal long-term memory, language, attention, and executive functions. The comparison between two evaluations, made 12 months apart, shows statistically significant differences, indicating deterioration compared to baseline, in the following tests: MMSE (with no gender differences), Composite Memory Score, Short Story Delayed Recall, Trail-Making Test A, Semantic Fluency Test, and Token Test. Conversely, there were no differences in the two evaluations of the Digit Span, Corsi Tapping Test, Short Story Immediate Recall, and Phonemic Fluency Tests. It appears that the treatment specifically attenuated the decline in tests assessing attention and executive functions. A stabilization of the ability to pay attention, with the ensuing positive effects on executive functions, recent memory, and information acquisition which depend on attention, appears to be the main neuropsychological mechanism through which the activation of the cholinergic system, resulting from cholinesterase inhibition, exerts its effect on cognition.

Effect of cholinesterase inhibitors on attention.

Advantages and limits of the use of cholinesterase inhibitors (ChEI) in Alzheimer's disease (AD) are well established. Their effects result from an increase in extracellular acetylcholine (ACh) whose hydrolysis is prevented by cholinesterase inhibition. In this way, the cholinergic deficit which characterizes AD may be corrected. This overview discusses which components of the cognitive process are improved by ChEI administration. In animal experiments, the increase in ACh release, detected in brain areas during behavioral tasks designed to tax attentional processes, demonstrates that an activation of cholinergic neurons underlies arousal and attention. Since arousal and attention depend on activation of the forebrain cholinergic system, it is to be expected that the loss of cholinergic neurons occurring in AD may lead to impairment of the attentional processes. Indeed, a consensus exists that attention is the first non-memory domain to be affected in AD, before deficits in language and visuo-spatial functions. The difficulties with daily living, which occur even in mild AD, may be related to attentional deficits. ChEIs, by restoring the cholinergic activity, should improve attention. If the cognitive changes resulting from ChEI treatment in AD patients are assessed with appropriate tests or selected items of the scales, a predominant effect on attention and executive functions emerges. In a group of 121 subjects with mild to moderate AD, (MMSE score 21.88 ± 3.63) followed in the Alzheimer Unit in Florence, after a year of treatment with standard doses of ChEIs, it was observed a stabilization of the disease, characterized by no changes of the tests evaluating attention and executive functions but a worsening of those involving memory mechanisms. These findings suggest that ChEI treatment preserves attention more than memory. Finally, the electrophysiological and neurochemical mechanisms through which the activation of the cholinergic forebrain neurons enhance attention and create the condition for information acquisition are reviewed.

Cholinesterase inhibitors and memory.

A consensus exists that cholinesterase inhibitors (ChEIs) are efficacious for mild to moderate Alzheimer's Disease (AD). Unfortunately, the number of non-responders is large and the therapeutic effect is usually short-lasting. In experimental animals, ChEIs exert three main actions: inhibit cholinesterase (ChE), increase extracellular levels of brain acetylcholine (ACh), improve cognitive processes, particularly when disrupted in models of AD. In this overview we shall deal with the cognitive processes that are improved by ChEI treatment because they depend on the integrity of brain cholinergic pathways and their activation. The role of cholinergic system in cognition can be investigated using different approaches. Microdialysis experiments demonstrate the involvement of the cholinergic system in attention, working, spatial and explicit memory, information encoding, sensory-motor gating, skill learning. No involvement in long-term memory has yet been demonstrated. Conversely, memory consolidation is facilitated by low cholinergic activity. Experiments on healthy human subjects, notwithstanding caveats concerning age, dose, and different memory tests, confirm the findings of animal experiments and demonstrate that stimulation of the cholinergic system facilitates attention, stimulus detection, perceptual processing and information encoding. It is not clear whether information retrieval may be improved but memory consolidation is reduced by cholinergic activation. ChEI effects in AD patients have been extensively investigated using rating scales that assess cognitive and behavioural responses. Few attempts have been made to identify which scale items respond better to ChEIs and therefore, presumably, depend on the activity of the cholinergic system. Improvement in attention and executive functions, communication, expressive language and mood stability have been reported. Memory consolidation and retrieval may be impaired by high ACh levels. Therefore, considering that in AD the degeneration of the cholinergic system is associated with alteration of other neurotransmitter systems and a diffuse synaptic loss, a limited efficacy of ChEIs on memory processes should be expected.

Cholinesterase inhibitors and beyond.

Cholinesterase inhibitors (ChEIs) were introduced in the therapy of Alzheimer Disease (AD) in the nineteen nineties with great expectations. The hopes and large interest raised by these drugs are well demonstrated by 12,000 references listed by PubMed under 'ChEI' for 1995-2007. The list is reduced to 2500 if we confine ourselves to 'ChEIs and dementia'. Of them, about 500 were published in the last two years. Whereas an increase in brain acetylcholine and an improvement of cognitive deficits have been consistently demonstrated in animal models of AD, from aging rats to transgenic mice, the clinical effectiveness of ChEIs has been and is still a matter of contrasting opinions. These range from the negative conclusions of the AD2000 trial on donepezil, claiming that it is not cost effective, with benefits below a minimally relevant threshold, to the NICE appraisal of 2007 declaring that donepezil, rivastigmine, galantamine are efficacious for mild to moderate AD, irrespective of their different selectivity for acetyl- (AChE) and butyrylcholinesterase (BuChE). The possibility that ChEIs may exert their effects through mechanisms beyond cholinesterase inhibition has been envisaged. However, according to the information presented in this review, the "classical" ChEIs, donepezil, rivastigmine and galantamine, show no pharmacological actions beyond cholinesterase inhibition which may play an important role in their therapeutic efficacy. The diverging opinions on clinical efficacy do not discourage from developing new ChEIs, and particularly the so called multifunctional ChEIs. They represent the future of the cholinergic therapy for AD but other indications for these drugs may be considered, including vascular dementia, mild cognitive impairment, and the ethically sensitive improvement of memory and learning in healthy subjects.

Extracellular levels of brain aspartate, glutamate and GABA during an inhibitory avoidance response in the rat.

The extracellular levels of aspartate, glutamate and GABA were measured by microdialysis, coupled with an HPLC method, in rat prefrontal cortex (mPFC) and ventral hippocampus (VH) before and during the performance of a step-down inhibitory task. The basal levels of glutamate were about 50% higher than those of aspartate, and GABA levels were about 20-folds smaller than those of the excitatory amino acids. There were no significant differences in the basal levels of any of the three amino acids between the two brain regions. The extracellular levels of aspartate increased during acquisition and recall trials in both VH and mPFC, whereas those of glutamate increased in the VH during acquisition only. A significant increase in GABA levels was also detected during acquisition but only in the mPFC. The neuronal origin of the increased extracellular levels of aspartate, glutamate and GABA was demonstrated by administering tetrodotoxin directly into the mPFC or VH by reverse dialysis. These findings, together with previous evidence from our and other laboratories, indicate a differential release of aspartate and glutamate from excitatory neurons during the performance of behavioral responses, and therefore, distinct roles for the two excitatory amino acids should be envisaged.

N1phenethyl-norcymserine, a selective butyrylcholinesterase inhibitor, increases acetylcholine release in rat cerebral cortex: a comparison with donepezil and rivastigmine.

The effects of (-)-N(1)phenethyl-norcymserine (PEC, 5 mk/kg, i.p.) on acetylcholine release and cholinesterase activity in the rat cerebral cortex were compared with those of donepezil (1 mg/kg, i.p.), a selective acetylcholinesterase inhibitor, and rivastigmine (0.6 mg/kg, i.p.), an inhibitor of acetylcholinesterase and butyrylcholinesterase. Acetylcholine extracellular levels were measured by microdialysis coupled with HPLC; acetylcholinesterase and butyrylcholinesterase activity were measured with colorimetric and radiometric methods. It was found that comparable 2-3 fold increases in cortical extracellular acetylcholine level, calculated as areas under the curve, followed the administration of the three drugs at the doses used. At the peak of acetylcholine increase, a 27% acetylcholinesterase inhibition and no butyrylcholinesterase inhibition was found after donepezil (1 mg/kg, i.p) administration. At the same time point, rivastigmine (0.6 mg/kg, i.p.) inhibited acetylcholinesterase by 40% and butyrylcholinesterase by 25%. After PEC (5 mg/kg, i.p.) administration, there was a 39% butyrylcholinesterase inhibition and no effect on acetylcholinesterase. Since in the present study it was also confirmed that in the brain butyrylcholinesterase activity is only about 10% of acetylcholinesterase activity, it is surprising that its partial inhibition is sufficient to increase extracellular acetylcholine levels. The importance of butyrylcholinesterase as a "co-regulator" of synaptic acetylcholine levels should thus be reconsidered.

Acetylcholine release from fetal tissue homotopically grafted to the motoneuron-depleted lumbar spinal cord. An in vivo microdialysis study in the awake rat.

Grafts of spinal cord (SC) tissue can survive and develop into the severed SC, but no conclusive data are available concerning the functional activity of transplanted neurons. In the present study, suspensions of prelabeled embryonic ventral SC tissue were grafted to the lumbar SC of rats with motoneuron loss induced by perinatal injection of volkensin. Eight to ten months post-grafting, acetylcholine (ACh) release was measured by microdialysis in awake rats, under either basal or stimulated conditions. In normal animals, baseline ACh output averaged 1.6 pmol/30 microl, it exhibited a 4-fold increase after KCl-induced depolarization or handling, and it was completely inhibited by tetrodotoxin administration. Moreover, ACh levels did not change following acute SC transection performed under anesthesia during ongoing dialysis, suggesting an intrinsic source for spinal ACh. Treatment with volkensin produced a severe (>85%) motoneuronal loss accompanied by a similar reduction in baseline ACh release and almost completely abolished effects of depolarization or handling. In transplanted animals, many motoneuron-like labeled cells were found within and just outside the graft area, but apparently in no case were they able to extend fibers towards the denervated muscle. However, the grafts restored baseline ACh output up to near-normal levels and responded with significantly increased release to depolarization, but not to handling. The present findings indicate that spinal neuroblasts can survive and develop within the motoneuron-depleted SC and release ACh in a near-normal, but apparently non-regulated, manner. This may be of importance for future studies involving intraspinal stem cell grafts.

Cholinergic dysfunction, neuronal damage and axonal loss in TgCRND8 mice.

In 7-month-old TgCRND8 mice, the extracellular cortical acetylcholine levels in vivo, the number and morphology of cholinergic neurons in the nucleus basalis magnocellularis and the ability to acquire an inhibitory avoidance response in the step-down test were studied. The TgCRND8 mouse brain is characterized by many beta-amyloid plaques, reduced neuronal and axonal staining, white matter demyelination, glia reaction and inducible nitric oxide synthase immunoreactivity. Choline acetyltransferase immunoreactivity in the nucleus basalis magnocellularis was significantly decreased. Basal and potassium-stimulated extracellular acetylcholine levels, investigated by microdialysis, and m2 muscarinic receptor immunoreactivity were reduced in the cortex of TgCRND8 mice, and scopolamine administration increased cortical extracellular acetylcholine levels in control but not in TgCRND8 mice. A cognitive impairment was demonstrated in the step-down test. These findings demonstrate that neuronal damage and cholinergic dysfunction in vivo underlie the impairment in learning and memory functions in this mouse model of Alzheimer's disease.

Neuronal control of the cardiac responses to osmotic stress in the gastropod limpet Patella caerulea.

Mediterranean limpets Patella caerulea were exposed to different salinity conditions and treated with drugs interfering with neuronal control of heartbeat. Heart rate was monitored using a non-invasive method. Limpets were superfused with control (33 g l(-1)), hyposaline (0 and 10 g l(-1)) or hypersaline (56 and 66 g l(-1)) artificial seawater. Under osmotic stress the limpets showed an initial increase of heart rate, followed by acardia, particularly under hyposalinity. The tachycardia observed after exposure to 56 g l(-1) was abolished in the animals injected with a selective sodium channel blocker, tetrodotoxin (TTX), or with a serotoninergic antagonist, methysergide. Injection of TTX also partly prevented the acardia occurring at 0 g l(-1). The acardia was completely prevented after injection with atropine and benzoquinonium, two selective cholinergic antagonists. These findings indicate that cardiac responses of P. caerulea to variations in external salinity are regulated by an extrinsic neuronal control involving the serotoninergic and the cholinergic systems in the tachycardic and acardic responses, respectively.

Tetrodotoxin prevents copper-induced bradycardia in gastropod limpets.

Acute exposure to waterborne copper is followed by a reduction in heart rate in gastropod limpets. In order to understand the mechanism of this effect, exposure to copper (0.25 or 0.5 mg l(-1); for 3 and 6 h) was combined with an injection of tetrodotoxin (TTX, 20 microl, 0.5 or 1 microM), a natural toxin that inhibits the propagation and transmission of impulses in excitable tissues. Experiments were performed on the Mediterranean limpet Patella caerulea, using a non-invasive method for the recording of cardiac activity. TTX did not affect the bradycardic effect of the cholinergic agonist carbachol. However, this toxin significantly antagonized the bradycardia induced by 0.25 and 0.5mg l(-1) of copper exposure and prevented the acardia observed in some limpets exposed to 0.5mg l(-1) of copper for 6h. These results are consistent with the hypothesis that the inhibitory action of copper on limpet cardiac activity involves an extrinsic, cholinergic neuronal control.

Selective butyrylcholinesterase inhibition elevates brain acetylcholine, augments learning and lowers Alzheimer beta-amyloid peptide in rodent.

Like acetylcholinesterase, butyrylcholinesterase (BChE) inactivates the neurotransmitter acetylcholine (ACh) and is hence a viable therapeutic target in Alzheimer's disease, which is characterized by a cholinergic deficit. Potent, reversible, and brain-targeted BChE inhibitors (cymserine analogs) were developed based on binding domain structures to help elucidate the role of this enzyme in the central nervous system. In rats, cymserine analogs caused long-term inhibition of brain BChE and elevated extracellular ACh levels, without inhibitory effects on acetylcholinesterase. In rat brain slices, selective BChE inhibition augmented long-term potentiation. These compounds also improved the cognitive performance (maze navigation) of aged rats. In cultured human SK-N-SH neuroblastoma cells, intra- and extracellular beta-amyloid precursor protein, and secreted beta-amyloid peptide levels were reduced without affecting cell viability. Treatment of transgenic mice that overexpressed human mutant amyloid precursor protein also resulted in lower beta-amyloid peptide brain levels than controls. Selective, reversible inhibition of brain BChE may represent a treatment for Alzheimer's disease, improving cognition and modulating neuropathological markers of the disease.

A cortical GABA-5HT interaction in the mechanism of action of the antidepressant trazodone.

The aim of the study was to investigate whether the antidepressant trazodone (TRZ), a serotonin-2 receptor antagonist/reuptake inhibitor, modifies gamma-amino-butyric acid (GABA) extracellular levels in the cerebral cortex, by acting on 5-HT(2A) receptors, and through this mechanism increases 5-HT levels. For this purpose the effect of TRZ on the release of GABA was studied in adult male rats in synaptosomes, cortical slices, and "in vivo" by microdialysis. In cortical slices, the release of both GABA and 5-HT was determined. GABA and 5-HT were identified and their levels quantified by HPLC. The inhibition of 5-HT uptake by TRZ was also measured. In synaptosomes, TRZ antagonized dose-dependently, at concentrations from 10(-10) to 10(-6) M, the increase in GABA release induced by (+/-)DOI, a 5-HT(2A/2C) agonist, and the alpha receptor agonist phenylephrine, both 10(-6) M. The pIC50 values were 8.31+/-0.24, and 5.99+/-0.52, respectively. In the same preparation, [3H]5-HT accumulation was inhibited by citalopram and TRZ with pIC(50) of 7.8+/-0.44 and 5.9+/-0.09, respectively, a finding confirming the weak activity of TRZ in comparison with a SSRI. In cortical slices, TRZ exerted a biphasic effect on GABA release. At concentrations from 10(-10) to 10(-7) M it inhibited and from 10(-6) to 10(-4) M increased GABA release. 5-HT release was enhanced by TRZ throughout the entire range of concentrations tested. However, the increase was delayed after low and rapid after high concentrations. AMI-193, a 5-HT(2A) antagonist (10(-10) to 10(-5) M), reduced GABA release in a dose-response manner, while it induced an increase of 5-HT outflow. On the contrary, (+/-)DOI (10(-10) to 10(-5) M) increased GABA release and inhibited 5-HT levels. Perfusion with the GABA(A) receptor antagonist bicuculline was also followed by an increase in 5-HT release. In microdialysis experiments, TRZ 1.25 mg kg(-1) s.c. brought about a decrease in GABA extracellular levels, while an increase was found after the dose of 2.5 mg kg(-1). These findings demonstrate that TRZ, at concentrations which do not inhibit 5-HT uptake, reduces the cortical GABAergic tone by decreasing GABA extracellular levels, through the blockade of 5-HT(2A) receptors. The attenuation of GABAergic tone is responsible for an increase in 5-HT levels. A further increase also results from 5-HT uptake inhibition caused by higher doses of TRZ. The ensuing high 5-HT levels enhance GABA release, which in turn inhibits 5-HT release.

S 18986, a positive modulator of AMPA receptors with cognition-enhancing properties, increases ACh release in the hippocampus of young and aged rat.

The effect of S 18986, positive AMPA receptor modulator, on acetylcholine (ACh), gamma-aminobutyric acid (GABA) and glutamate (Glu) release from the hippocampus of freely moving young and aged rats was investigated by microdialysis coupled to HPLC. The cognition-enhancing properties were evaluated by a passive avoidance test. In 3 month-old rats, S 18986 (10 mg/kg i.p.) increased by 70% ACh release, which returned to basal level within 2 h, while 3 mg/kg had no effect. In 22 month-old rats, both 3 and 10 mg/kg i.p. induced a long lasting increase in ACh release, as large as that induced by 10 mg/kg in young rats. S 18986 did not modify GABA and glutamate release. No effect on general behavior was observed, but S 18986 at both doses prevented the disrupting effect of scopolamine (1 mg/kg i.p.) on passive avoidance acquisition.

Changes in acetylcholine extracellular levels during cognitive processes.

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

Mild cognitive impairment: animal models.

Mild cognitive impairment (MCI) is an aspect of cognitive aging that is considered to be a transitional state between normal aging and the dementia into which it may convert. Appropriate animal models are necessary in order to understand the pathogenic mechanisms of MCI and develop drugs for its treatment. In this review, we identify the features that should characterize an animal model of MCI, namely old age, subtle memory impairment, mild neuropathological changes, and changes in the cholinergic system, and the age at which these features can be detected in laboratory animals. These features should occur in aging animals with normal motor activity and feeding behavior. The animal models may be middle-aged rats and mice, rats with brain ischemia, transgenic mice overexpressing amyloid precursor protein and presenilin 1 (tested at an early stage), or aging monkeys. Memory deficits can be detected by selecting appropriately difficult behavioral tasks, and the deficits can be associated with neuropathological alterations. The reviewed literature demonstrates that, under certain conditions, these animal species can be considered to be MCI models, and that cognitive impairment in these models responds to drug treatment.

Beta-amyloid-induced inflammation and cholinergic hypofunction in the rat brain in vivo: involvement of the p38MAPK pathway.

Injection into the nucleus basalis of the rat of preaggregated Abeta(1-42) produced a congophylic deposit and microglial and astrocyte activation and infiltration and caused a strong inflammatory reaction characterized by IL-1beta production, increased inducible cyclooxygenase (COX-2), and inducible nitric oxide synthase (iNOS) expression. Many phospho-p38MAPK-positive cells were observed around the deposit at 7 days after Abeta injection. Phospho-p38MAPK colocalized with activated microglial cells, but not astrocytes. The inflammatory reaction was accompanied by cholinergic hypofunction. We investigated the protective effect of the selective COX-2 inhibitor rofecoxib in attenuating the inflammatory response and neurodegeneration evoked by Abeta(1-42). Rofecoxib (3 mg/kg/day, 7 days) reduced microglia and astrocyte activation, iNOS induction, and p38MAPK activation to control levels. Cholinergic hypofunction was also significantly attenuated by treatment with rofecoxib. We show here for the first time in vivo the pivotal role played by the p38MAPK microglial signal transduction pathway in the inflammatory response to the Abeta(1-42) deposit.

An autocrine role for pituitary GABA: activation of GABA-B receptors and regulation of growth hormone levels.

There is increasing evidence suggesting that the neurotransmitter gamma-aminobutyric acid (GABA) is a local factor involved in the regulation of endocrine organs. Examples of such functions are documented in the pancreas, but recent results suggest that GABA may act in a similar way in the pituitary, in which GABA receptors are expressed and pituitary growth hormone (GH) cells provide a source of GABA. We hypothesised that GABA secreted in somatotropes may act as an autoregulatory signaling molecule. To test this hypothesis we first examined the nature of GABA receptors expressed by GH cells. RT-PCR analysis demonstrated that GABA-B receptor subunits R1 and R2 are present in the whole rat pituitary. Laser microdissection of immunostained GH cells, followed by RT-PCR as well as immunoelectron microscopy, showed that GABA-B receptors are expressed on somatotropes. To investigate GABA-B receptor function in somatotropes, we used rat GH3 adenoma cells, which, like pituitary GH cells, express GABA-B R1 and R2 (as assessed by RT-PCR and immunoelectron microscopy) and produce GABA (checked by high performance liquid chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by baclofen, a chromatography). After inhibition of endogenous GABA synthesis, GH production was stimulated by baclofen, a GABA-B receptor agonist. By contrast, blocking GABA-B receptors by an antagonist, phaclofen, decreased GH levels. We conclude that in GH-producing cells, GABA acts as an autocrine factor via GABA-B receptors to control GH levels.