Nicotinic cholinergic receptors in human brain: effects of aging and Alzheimer

Effects of a memory enhancing peptide on cognitive abilities of brain-lesioned mice: additivity with huperzine A and relative potency to tacrine

Alzheimer's disease (AD) and related dementing disorders having cognitive manifestations represent an increasing threat to public health. In the present study, the effects of a memory enhancing NLPR tetra-peptide (MEP), huperzine A (Hup A), or a combination of the two on the cognitive abilities of brain-lesioned mice were evaluated and compared with tacrine in the passive avoidance and Y-water maze tests for the acquisition and retention aspects of cognitive functions. MEP at microg kg(-1) doses, and Hup A or tacrine at mg kg(-1) doses significantly reversed the cognition deficits induced by scopolamine. For acquisition ability, it was observed that mice administered with MEP (4.0 microg kg(-1)) spent less time escaping onto the platform in the water maze than those treated with tacrine (1.5 mg kg(-1)); whereas for memory retention, tacrine-administration resulted in a higher step-through latency in mice at the tested dose regime. In addition, co-administration of MEP (2.0 microg kg(-1)) and Hup A (0.1 mg kg(-1)) exhibited an additive effect resulting in considerable improvements in both acquisition and retention abilities of brain-lesioned mice. The results demonstrated that MEP was highly efficient in the rescue of cognitive abilities of brain-lesioned mice and in particular, the effective doses of MEP were about two orders of magnitude lower than that of tacrine, a therapeutic currently used in the treatment of AD. Moreover, MEP and Hup A were effective at reduced doses when the two were co-administered, providing a rationale for their combined usage in the treatment of cognitive deficits.

Ammonia and Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder. Behavioural, cognitive and memory dysfunctions are characteristic symptoms of AD. The formation of amyloid plaques is currently considered as the key event of AD. Other histological hallmarks of the disease are the formation of fibrillary tangles, astrocytosis, and loss of certain neuronal systems in cortical areas of the brain. A great number of possible aetiologic and pathogenetic factors of AD have been published in the course of the last two decades. Among the toxic factors, which have been considered to contribute to the symptoms and progression of AD, ammonia deserves special interest for the following reasons: (a) Ammonia is formed in nearly all tissues and organs of the vertebrate organism; it is the most common endogenous neurotoxic compounds. Its effects on glutamatergic and GABAergic neuronal systems, the two prevailing neuronal systems of the cortical structures, are known for many years. (b) The impairment of ammonia detoxification invariably leads to severe pathology. Several symptoms and histologic aberrations of hepatic encephalopathy (HE), of which ammonia has been recognised as a pathogenetic factor, resemble those of AD. (c) The excessive formation of ammonia in the brains of AD patients has been demonstrated, and it has been shown that some AD patients exhibit elevated blood ammonia concentrations. (d) There is evidence for the involvement of aberrant lysosomal processing of beta-amyloid precursor protein (beta-APP) in the formation of amyloid deposits. Ammonia is the most important natural modulator of lysosomal protein processing. (e) Inflammatory processes and activation of microglia are widely believed to be implicated in the pathology of AD. Ammonia is able to affect the characteristic functions of microglia, such as endocytosis, and cytokine production. Based on these facts, an ammonia hypothesis of AD has first been suggested in 1993. In the present review old and new observations are discussed, which are in support of the notion that ammonia is a factor able to produce symptoms of AD and to affect the progression of the disease.

Evaluation of radioiodinated 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine as a ligand for SPECT investigations of brain nicotinic acetylcholine receptors

5-Iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), an A-85380 analog iodinated at the 5-position of the pyridine ring, was evaluated as a radiopharmaceutical for investigating brain nicotinic acethylcholine receptors (nAChRs) by single photon emission computed tomography (SPECT). [123/125I]5IA was synthesized by the iododestannylation reaction under no-carrier-added conditions and purified by high-performance liquid chromatography (HPLC) with high radiochemical yield (50%), high radiochemical purity (> 98%), and high specific radioactivity (> 55 GBq/micromol). The binding affinity of 5IA for brain nAChRs was measured in terms of displacement of [3H]cytisine and [125I]5IA from binding sites in rat cortical membranes. The binding data revealed that the affinity of 5IA was the same as that of A-85380 and more than seven fold higher than that of (-)-nicotine, and that 5IA bound selectively to the alpha4beta2 nAChR subtype. Biodistribution studies in rats indicated that the brain uptake of [125I]51A was rapid and profound. Regional cerebral distribution studies in rats demonstrated that the accumulation of [125I]5IA was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus, moderate uptake in the cortex and lowest uptake in the cerebellum. Administration of the nAChR agonists (-)-cytisine and (-)-nicotine reduced the uptake of [125I]5IA in all regions studied with most pronounced reduction in the thalamus, and resulted in similar levels of radioactivity throughout the brain. [125I]5IA binding sites were shown to be saturable with unlabeled 5IA. Behavioral studies in mice demonstrated that 5IA did not show signs of behavioral toxicity. Furthermore, SPECT studies with [123I]5IA in the common marmoset demonstrated appropriate brain uptake and regional localization for a high-affinity nAChR imaging radiopharmaceutical. These results suggested that [123I]5IA is a promising radiopharmaceutical for SPECT studies of central nAChRs in human subjects.

Phenotypic characterization of an alpha 4 neuronal nicotinic acetylcholine receptor subunit knock-out mouse

Neuronal nicotinic acetylcholine receptors (nAChR) are present in high abundance in the nervous system (Decker et al., 1995). There are a large number of subunits expressed in the brain that combine to form multimeric functional receptors. We have generated an alpha(4) nAChR subunit knock-out line and focus on defining the behavioral role of this receptor subunit. Homozygous mutant mice (Mt) are normal in size, fertility, and home-cage behavior. Spontaneous unconditioned motor behavior revealed an ethogram characterized by significant increases in several topographies of exploratory behavior in Mt relative to wild-type mice (Wt) over the course of habituation to a novel environment. Furthermore, the behavior of Mt in the elevated plus-maze assay was consistent with increased basal levels of anxiety. In response to nicotine, Wt exhibited early reductions in a number of behavioral topographies, under both unhabituated and habituated conditions; conversely, heightened levels of behavioral topographies in Mt were reduced by nicotine in the late phase of the unhabituated condition. Ligand autoradiography confirmed the lack of high-affinity binding to radiolabeled nicotine, cytisine, and epibatidine in the thalamus, cortex, and caudate putamen, although binding to a number of discrete nuclei remained. The study confirms the pivotal role played by the alpha(4) nAChR subunit in the modulation of a number of constituents of the normal mouse ethogram and in anxiety as assessed using the plus-maze. Furthermore, the response of Mt to nicotine administration suggests that persistent nicotine binding sites in the habenulo-interpeduncular system are sufficient to modulate motor activity in actively exploring mice.

6-[18F]Fluoro-A-85380: an in vivo tracer for the nicotinic acetylcholine receptor

6-[18F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380 or 6-[18F]FA), a new tracer for positron emission tomography, was synthesized by no-carrier-added [18F] fluorination of 6-iodo-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)methoxy)pyridine followed by acidic deprotection. 6-[18F]FA followed the regional densities of brain nicotinic acetylcholine receptors (nAChRs) reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 6-FA. A preliminary toxicology study of the 6-FA showed a relatively low biological effect.

The cholinergic system in Alzheimer's disease

The past decade has witnessed an enormous increase in our knowledge of the variety and complexity of neuropathological and neurochemical changes in Alzheimer's disease. Although the disease is characterized by multiple deficits of neurotransmitters in the brain, this overview emphasizes the structural and neurochemical localization of the elements of the acetylcholine system (choline acetyltransferase, acetylcholinesterase, and muscarinic and nicotinic acetylcholine receptors) in the non-demented brain and in Alzheimer's disease brain samples. The results demonstrate a great variation in the distribution of acetylcholinesterase, choline acetyltransferase, and the nicotinic and muscarinic acetylcholine receptors in the different brain areas, nuclei and subnuclei. When stratification is present in certain brain regions (olfactory bulb, cortex, hippocampus, etc.), differences can be detected as regards the laminar distribution of the elements of the acetylcholine system. Alzheimer's disease involves a substantial loss of the elements of the cholinergic system. There is evidence that the most affected areas include the cortex, the entorhinal area, the hippocampus, the ventral striatum and the basal part of the forebrain. Other brain areas are less affected. The fact that the acetylcholine system, which plays a significant role in the memory function, is seriously impaired in Alzheimer's disease has accelerated work on the development of new drugs for treatment of the disease of the 20th century.

Increased regional cerebral glucose metabolism and semantic memory performance in Alzheimer's disease: a pilot double blind transdermal nicotine positron emission tomography study

Nicotinic receptor dysfunction and impaired semantic memory occur early in Alzheimer's disease patients (AD). Previous research implied that nicotine's ability to enhance alertness, arousal, and cognition in a number of nonclinical populations was a function of its ability to stimulate CNS nicotinic cholinergic receptors. In this study it was hypothesized that transdermal administration of nicotine would increase both regional cerebral glucose metabolism (rCMRglc) and semantic memory (as assessed by verbal fluency). Two mild AD and two elderly controls underwent positron emission tomography scanning during a double blind nicotinic agonist verbal fluency challenge procedure. rCMRglc increases occurred in both AD patients, but not controls. In the two AD patients, verbal fluency scores increased by an average of 17%. One elderly control's verbal fluency increased, and the other decreased. These findings suggest that nicotine's effect on metabolism and verbal fluency is due to its ability to stimulate the cholinergic system.