Cognitive function. Acetylcholine revisited

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Anticholinergicity and cognitive processing in chronic schizophrenia

Patients with chronic schizophrenia suffer from alterations in cholinergic functioning due to several factors, including the disease diathesis and pharmacologic treatments. Acetylcholine-cognition relationships are well explored in normals but are unclear in schizophrenia. Prior work indicated serum anticholinergicity does not cause global cognitive impairment in this group (Tracy et al., 1998a), raising the possibility that anticholinergicity normalizes an abnormal hyperactive cholinergic state. Serum anticholinergic levels were determined in 38 chronic schizophrenia patients using an established radioreceptor assay method. Six cognitive functions associated with cholinergic tone in normals were tested. The potential role of autonomic arousal and cigarette smoking were also assessed as both have been linked to cholinergic functioning. Regression analyses showed measures of inhibitory executive control and effortful memory accounted for a greater proportion of the variance in the anticholinergicity measure compared to the other variables. The data demonstrate a relationship between high anticholinergicity and worse performance on two types of attention-resource demanding cognitive processes and do not support the notion that reduced cholinergic tone normalizes a hyperactive cortical acetylcholine substrate. Relevant neuroanatomic structures and implications for models of cognitive deficits in schizophrenia are discussed.

Characterization of microsphere embolism-induced impairment of learning and memory function and the cholinergic system

The impairments of learning and memory function and of the cholinergic system were examined in rats with microsphere embolism. Microsphere embolism was induced by injection of 900 microspheres with a diameter of 48 microm into the right internal carotid artery. The retention latency of a passive avoidance test was shortened and the escape latency of a water maze test was prolonged, when the animals were tested on the 5th to 10th day after the embolism, suggesting learning and memory dysfunction. Cholinergic parameters of the striatum and hippocampus, such as acetylcholine (ACh) content (67 and 60% decrease, respectively), choline acetyltransferase (ChAT) activity (45 and 56% decrease, respectively), and Bmax of muscarinic acetylcholine M1-receptor (43 and 37% decrease, respectively), were reduced on the 11th day after the embolism, suggesting attenuation of ACh synthesis and a decrease in the number of muscarinic acetylcholine M1-receptors mainly in the striatum and hippocampus. Areas not stained with triphenyltetrazolium chloride, an indication of infarction, were detected mainly in the striatum and hippocampus and partly in the frontal cortex on the 11th day after the embolism. The results suggest that an animal with microsphere embolism may be a good ischemic model with relatively sustained impairments of learning and memory function and of the striatal and hippocampal cholinergic system.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

The cholinergic 'pitfall': acetylcholine, a universal cell molecule in biological systems, including humans

1. Acetylcholine (ACh) represents one of the most exemplary neurotransmitters. In addition to its presence in neuronal tissue, there is increasing experimental evidence that ACh is widely expressed in pro- and eukaryotic non-neuronal cells. Thus, ACh has been detected in bacteria, algae, protozoa, tubellariae and primitive plants, suggesting an extremely early appearance of ACh in the evolutionary process. 2. In humans, ACh and/or the synthesizing enzyme, choline acetyltransferase, has been demonstrated in epithelial cells (airways, alimentary tract, urogenital tract, epidermis), mesothelial (pleura, pericardium) and endothelial and muscle cells. In addition, immune cells express the non-neuronal cholinergic system (i.e. the synthesis of ACh can be detected in human leucocytes (granulocytes, lymphocytes and macrophages)), as well as in rat microglia in vitro. 3. The widespread expression of non-neuronal ACh is accompanied by the ubiquitous expression of cholinesterase activity, which prevents ACh from acting as a classical hormone. 4. Non-neuronal ACh mediates its cellular actions in an auto- and paracrine manner via the activation of the widely expressed nicotinic and muscarinic acetylcholine receptors, which can interfere with virtually all cellular signalling pathways (ion channels and key enzymes). 5. Non-neuronal ACh appears to be involved in the regulation of basic cell functions, such as mitosis, cell differentiation, organization of the cytoskeleton, cell-cell contact, secretion and absorption. Non-neuronal ACh also plays a role in the regulation of immune functions. All these qualities together may mediate the so-called 'trophic property' of ACh. 6. Future experiments should be designed to analyse the cellular effects of ACh in greater detail. The involvement of the non-neuronal cholinergic system in the pathogenesis of chronic inflammatory diseases should be investigated to open up new therapeutic strategies.

Presynaptic markers of cholinergic function in the rat brain: relationship with age and cognitive status

The nature of age-related changes in cholinergic function and their relationship to age-related behavioral decline were examined in the present study. Male Fischer-344 rats of four ages (four, 11, 17 and 23 months) were tested in a battery of cognitive tasks. Discrete microdissections of brain areas involved in cognitive function were performed, and activity of choline acetyltransferase and levels of hemicholinium-3 binding were determined to assess the integrity of cholinergic innervation. Age-related changes in cholinergic markers occurred predominantly in the medial septal area and its target areas (hippocampus and cingulate cortex), and were also present in the posterior caudate. However, most of the age-related changes in cholinergic markers were already present at ages at which behavioral impairment was not yet maximal. There were some consistent correlations between behavioral and neurochemical measures, independent of age, but these accounted for relatively small proportions of variance in behavioral performance. For most of these correlations, lower levels of presynaptic cholinergic markers were related to better behavioral performance. In brain areas in which correlations changed with age, lower levels of presynaptic cholinergic markers were associated with better performance in young rats, whereas higher levels were associated with better performance in aged rats. Recent lesion studies using a toxin selective for basal forebrain cholinergic neurons have suggested that these neurons do not play as central a role in learning and memory in young and aged animals as was previously thought. When considered in this context, the present results suggest that preserved cholinergic function in old age might act indirectly to sustain cognitive ability. Changes in cholinergic function may represent one of a number of age-related neurobiological events that underlie behavioral impairments, or may be a permissive factor for other age-related processes that are more directly responsible for cognitive impairments.

Effects of extracellular acetylcholine on muscarinic receptor binding assessed by [125I]dexetimide and a simple probe

New pharmacologic approaches to enhance brain cholinergic function focus on increasing intrasynaptic acetylcholine. We examined the usefulness of a simple probe and [125I]dexetimide to evaluate in vivo the effects of extracellular acetylcholine on muscarinic receptor binding in the mouse brain. After radiotracer injection continuous time/activity curves were generated over 330 min. [125I]Dexetimide reached a plateau at 90 min post-injection. To increase extracellular acetylcholine, the anticholinesterase physostigmine was administered at 120 min, producing a reversible decrease in [125I]dexetimide specific binding (23%) for 30 min. These findings demonstrate that dynamic changes in extracellular acetylcholine can be evaluated by displacement of [125I]dexetimide binding in vivo using a simple probe system.

Non-neuronal acetylcholine, a locally acting molecule, widely distributed in biological systems: expression and function in humans

Acetylcholine acts as a neurotransmitter in the central and peripheral nervous systems in humans. However, recent experiments demonstrate a widespread expression of the cholinergic system in non-neuronal cells in humans. The synthesizing enzyme choline acetyltransferase, the signalling molecule acetylcholine, and the respective receptors (nicotinic or muscarinic) are expressed in epithelial cells (human airways, alimentary tract, epidermis). Acetylcholine is also found in mesothelial, endothelial, glial, and circulating blood cells (platelets, mononuclear cells), as well as in alveolar macrophages. The existence of non-neuronal acetylcholine explains the widespread expression of muscarinic and nicotinic receptors in cells not innervated by cholinergic neurons. Non-neuronal acetylcholine appears to be involved in the regulation of important cell functions, such as mitosis, trophic functions, automaticity, locomotion, ciliary activity, cell-cell contact, cytoskeleton, as well as barrier and immune functions. The most important tasks for the future will be to clarify the multiple biological roles of non-neuronal acetylcholine in detail and to identify pathological conditions in which this system is up- or down-regulated. This could provide the basis for the development of new therapeutic strategies to target the non-neuronal cholinergic system.

The use of nonneuronal cells for gene delivery

The implantation of genetically engineered nonneuronal cells can provide an effective method for achieving localized delivery of discrete molecules to the CNS or for providing substrates for regrowth of neural structures. Most primary nonneuronal cells have the advantage of being easily obtainable from the prospective host for ex vivo retrovirus-mediated genetic manipulation (most will be mitotic in culture) and reimplantation as an autologous graft (circumventing the problem of immune rejection). As primary cells, they are unlikely to be tumorigenic. The most vexing problem for such systems remains the apparent loss of transgene expression from viral promoters after prolonged periods of engraftment. Much effort is currently being directed at optimizing sustained transgene expression by varying the promoters, by varying the cell types to be engineered, or by regulating expression by enhancing promoter function or substrate availability. While nonneuronal cells are excellent vehicles for achieving passive delivery of substances to the CNS, they lack the ability to incorporate into the host cytoarchitecture in a functional manner (e.g., make synaptic contacts). For this reason, not only may certain essential circuits not be re-formed, but the regulated release of certain substances through feedback loops may be missing. While apparently unimportant for some substances (e.g., ACh), for others (e.g., NGF), their unregulated, inappropriate, excessive, or ectopic release may actually be inimical to the host. Furthermore, the loss of foreign gene expression (the bane of gene therapy) may leave engineered nonneural cells incapacitated, whereas donor tissue originating from brain may intrinsically produce various CNS factors allowing correction to proceed despite inactivation of the introduced gene. In fact, CNS-derived tissue may provide as-yet-unrecognized endogenous neuralspecific substances which are equally as beneficial to the host as the gene in question. Thus, future developments in gene delivery to the brain for some conditions may emphasize using neurons or neural progenitors for ex vivo genetic manipulation (Fisher, 1997) and refining techniques for the direct injection of therapeutic genes into neurons in vivo (see Snyder and Fisher, 1996). For a wide variety of conditions, however, using nonneuronal cellular vehicles or even nonbiologic synthetic vehicles may be efficient, effective, and safe strategies for the passive delivery of therapeutic molecules to discrete regions of the CNS. In fact, this approach may come closer than any other to immediate human applications.

Evidence for direct and indirect mechanisms in the potent modulatory action of interleukin-2 on the release of acetylcholine in rat hippocampal slices

1. The biphasic nature of the potent modulatory action of interleukin-2 (IL-2) on hippocampal acetylcholine (ACh) release was investigated by use of brain slice superfusion. 2. Both the potentiating (10(-13) M) and inhibitory (10(-9) M) effects of IL-2 on hippocampal ACh release were stimulation-dependent and were blocked by a neutralizing IL-2 receptor antibody, suggesting the activation of typical IL-2 receptors in both cases. 3. Tetrodotoxin (TTX: 10 microM) failed to block the potentiation of ACh release induced by a very low concentration of IL-2 (10(-13) M) suggesting a direct effect on cholinergic nerve terminals. 4. In contrast, the inhibitory effect seen at a higher concentration (10(-9) M) was TTX-sensitive, and hence indicative of an indirect action. 5. To establish the nature of this intermediate mediator, blockers of nitric oxide synthesis, and of opioid and gamma-aminobutyric acid (GABA) receptors were used. Only GABAA and GABAB receptor antagonists altered the inhibitory action of IL-2, suggesting the participation of GABA as mediator. 6. Taken together, these results provide further evidence for the potent role of IL-2 in the modulation of cholinergic function in the rat hippocampus.

Ionotropic glutamate receptor subtypes in the aged memory-impaired and unimpaired Long-Evans rat

The comparative quantitative autoradiographic distribution of ionotropic glutamate receptor subtypes were investigated in young adults (six months) and aged (24-25 months) cognitively impaired and unimpaired male Long-Evans rats. Aged rats were behaviorally characterized as either cognitively impaired or unimpaired based upon their performances in the Morris water maze task compared to the young adult controls. The status of the N-methyl-D-aspartate, [125I]dizocilpine maleate, [3H]kainate and amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA, [3H]AMPA) receptor binding sites were then established in these three subgroups of animals as a function of their cognitive performance in the Morris water maze task. The apparent densities of both N-methyl-D-aspartate/[125I]dizocilpine maleate and kainate binding sites were significantly decreased in various regions of the aged rat brain. Marked losses in [125I]dizocilpine maleate binding sites were observed in outer laminae of the frontal, parietal and temporal cortices, and the stratum radiatum of the CA3 subfield of the hippocampus. Interestingly, losses in [125I]dizocilpine maleate binding sites were generally most evident in the cognitively unimpaired aged subgroup, suggesting a possible inverse relationship between losses of this receptor subtype and cognitive performances in the Morris water maze task. The levels of [3H]kainate binding were most significantly diminished in various cortical and hippocampal areas as well as the striatum and septal nuclei of both groups of aged rats. In contrast, the apparent density of [3H]AMPA binding was increased in most hippocampal subfields and the superficial laminae of the occipital cortex of the cognitively impaired vs young adult rats. Changes in [3H]AMPA labeling failed to reach significance in the unimpaired cohort. Taken together, these results show that while losses in [3H]kainate binding were similar in both subgroups of aged rats, differences were seen with respect to cognitive status for both [125I]dizocilpine maleate/N-methyl-D-aspartate and [3H]AMPA binding sites. Decreases in [125I]dizocilpine maleate binding sites were mostly restricted to cortical areas of cognitively unimpaired rats, while increases in the AMPA binding subtype were seen in the memory-impaired subgroup. It would thus appear that changes in N-methyl-D-aspartate and AMPA receptor subtypes may be more critical than alterations in kainate binding sites for the emergence of the functional deficits seen in the aged cognitively impaired rat.

The basal forebrain cholinergic system and object memory in the rat

Rats with near complete destruction of basal forebrain cholinergic neurons from intracerebroventricular injections of 192 IgG-saporin were trained on object discrimination problems and then retrained two weeks later to measure retention. Despite dramatic reductions of acetylcholinesterase-positive fibers in hippocampus and neocortex, these animals did not differ from controls on an analysis of savings scores. Thus, the basal forebrain cholinergic system may serve functions that support non-spatial memory but are not specifically mnemonic in nature.