Cholinergic function and intellectual decline in Alzheimer's disease

Biological and immunological properties of recombinant human, rat, and chicken nerve growth factors: a comparative study

Biological and immunological properties of recombinant human, rat, and chicken nerve growth factors (NGFs) were studied and compared. Recombinant NGF proteins were produced in a transient expression system using COS cells and levels of secreted NGF protein were assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of conditioned media from in vivo [35S]cysteine-labeled cell cultures. Antigenic differences among the three NGFs were studied by immunoblotting and immunoprecipitation of secreted cell products using a rabbit polyclonal antiserum against purified mouse NGF, and by a two-site enzyme immunoassay (EIA) with a monoclonal antibody against mouse NGF. Although all three NGFs were recognized equally well in the immunoblotting, only one-third of the chicken NGF protein could be detected by immunoprecipitation or by the EIA as compared to the rat and human NGFs. Thus, changes in the three-dimensional structure of the NGF molecule are most likely responsible for the antigenic differences between avian and mammalian NGFs. The three NGF proteins were also compared in their ability to displace 125I-mouse NGF from low-affinity NGF receptors on rat pheochromocytoma PC12 cells. Similar displacement curves and values were obtained for each NGF protein, indicating that structural differences among these molecules do not affect low-affinity binding to NGF receptors. Biological activities were studied by the ability of the conditioned media to promote neurite outgrowth from explants of E9 chick sympathetic ganglia and from PC12 cells. Although the rat system showed a slight preference for the homologous molecule, the morphological changes, dose-response curves, and maximal stimulation values obtained with the different NGFs were practically indistinguishable in the chicken bioassay.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic neurons of the central nervous system: morphofunctional aspects

Impairment of central cholinergic function is an early and constant finding in a number of mental disorders associated with amnesia or dementia. Although knowledge of the detailed functional implications of cholinergic mechanisms in cognition is still very incomplete, some recent results and concepts about the morphofunctional organization of the cholinergic basalo-cortical projection system are reviewed. This cholinergic system is quite different from other so-called unspecific subcortical projection systems and might have some bearing on understanding the role of cholinergic mechanisms in cognitive function and its disorders.

Central cholinergic functioning and aging

Normal aging in experimental animals and humans has been demonstrated to affect various aspects of central cholinergic functions. Although deficits at the levels of the number of cholinergic neurons, the acetylcholine synthesis, and the number of muscarinic cholinergic receptors are probably less relevant, deficits at the levels of acetylcholine release, muscarinic cholinergic receptor plasticity, as well as muscarinic cholinergic receptor function are fairly pronounced and seem to justify the assumption that the functioning of the central cholinergic system is impaired by aging. However, whether these cholinergic deficits of normal aging are the sole neurochemical basis to explain age-associated memory impairment or whether other transmitter systems also play a role is still a matter of controversy.

Anti-muscarinic drug effects in a swim-to-platform test: dose-response relations

Rats were given subcutaneous or intraperitoneal injections of scopolamine, intraperitoneal injections of atropine, or control injections, and trained on a simple swim-to-platform task. Errors were proportional to dose for both drugs over a wide range. No difference was found in the effects of intraperitoneal or subcutaneous scopolamine but scopolamine was 25 times more potent than atropine. The potency of both drugs in impairing swim-to-platform behavior was similar to their potency in abolishing the cholinergic component of neocortical low voltage fast activity. The electrocortical effect of anticholinergic drugs may be a major factor in the behavioral impairment they produce.

Disruption of classical conditioning in patients with Alzheimer's disease

One of the primary features of Alzheimer's disease (AD) is a disorder of memory. Although considerable effort has been devoted to characterizing this memory disorder, simple forms of memory such as classical (Pavlovian) conditioning have not been studied. The prevailing view has been that these simple forms of memory are not affected in AD. These forms of memory, however, may be of particular interest because they are beginning to be well understood at the neurobiological level. Because of this, when memory disorders are detected, it may be possible to specify their neurobiological substrate. We now report that classical conditioning of the eyeblink response is disrupted in AD patients compared to age-matched controls. This deficit in conditioning is not due to nonassociative factors such as changes in sensitivity to stimuli or disruption of the motor response. The results are considered in terms of using simple forms of memory to help generate hypotheses regarding the neurobiology of age-related memory disorders.

Cholinergic mechanisms in learning, memory and dementia: a review of recent evidence

The discovery in the late 1970s that cholinergic neurons in the basal forebrain degenerate in Alzheimer's disease (AD) greatly accelerated research on the role of cholinergic mechanisms in learning and memory. As is often the case in science, the early enthusiasm for the cholinergic hypothesis has been tempered by the results of subsequent research. Although there is substantial pharmacological evidence that unspecified cholinergic systems in the brain play important roles in some forms of learning and memory, recent findings in humans indicate that antimuscarinic drugs do not model the deficits seen in AD. In addition, the goal of elucidating the functions of these basal forebrain neurons in animals has proved to be difficult and is yet to be achieved. Despite substantial effort, therefore, the cognitive and behavioral consequences of cholinergic pathology in AD remain unknown. Under these circumstances, attempts to develop cholinergic pharmacotherapies for these deficits in AD are based on questionable assumptions.

Reversal of age-related increase in brain protein oxidation, decrease in enzyme activity, and loss in temporal and spatial memory by chronic administration of the spin-trapping compound N-tert-butyl-alpha-phenylnitrone

Oxygen free radicals and oxidative events have been implicated as playing a role in bringing about the changes in cellular function that occur during aging. Brain readily undergoes oxidative damage, so it is important to determine if aging-induced changes in brain may be associated with oxidative events. Previously we demonstrated that brain damage caused by an ischemia/reperfusion insult involved oxidative events. In addition, pretreatment with the spin-trapping compound N-tert-butyl-alpha-phenylnitrone (PBN) diminished the increase in oxidized protein and the loss of glutamine synthetase (GS) activity that accompanied ischemia/reperfusion injury in brain. We report here that aged gerbils had a significantly higher level of oxidized protein as assessed by carbonyl residues and decreased GS and neutral protease activities as compared to young adult gerbils. We also found that chronic treatment with the spin-trapping compound PBN caused a decrease in the level of oxidized protein and an increase in both GS and neutral protease activity in aged Mongolian gerbil brain. In contrast to aged gerbils, PBN treatment of young adult gerbils had no significant effect on brain oxidized protein content or GS activity. Male gerbils, young adults (3 months of age) and retired breeders (15-18 months of age), were treated with PBN for 14 days with twice daily dosages of 32 mg/kg. If PBN administration was ceased after 2 weeks, the significantly decreased level of oxidized protein and increased GS and neutral protease activities in old gerbils changed in a monotonic fashion back to the levels observed in aged gerbils prior to PBN administration. We also report that old gerbils make more errors than young animals and that older gerbils treated with PBN made fewer errors in a radial arm maze test for temporal and spatial memory than the untreated aged controls. These data can be interpreted to indicate that oxidation of cellular proteins may be a critical determinant of brain function. Moreover, it also implies that there is an age-related increase in vulnerability of tissue to oxidation that can be modified by free radical trapping compounds.

A postmortem study of brain nicotinic receptors in Parkinson's and Alzheimer's disease

Brain nicotinic receptors were studied in the frontal cortex, temporal cortex, hippocampus and caudate nucleus in patients with Parkinson's disease (PD), Alzheimer's disease (AD) and control. The Bmax and Kd values of (-)-[3H]nicotine binding were determined with a Scatchard analysis. The number of nicotinic receptors declined both in PD and in AD patients in all brain areas examined. The Kd values were unchanged. There was a negative correlation between the degree of dementia in PD patients and the number of nicotinic receptors in the frontal cortex. A similar correlation was seen between the muscarinic/nicotinic receptor ratio in the frontal cortex and the degree of dementia in PD patients. The present findings indicate that nicotinic receptors are affected not only in AD, but also in PD and that dysfunction of the cholinergic system in the frontal cortex is involved in the dementia process in PD.

Nerve growth factor receptor immunoreactivity within the nucleus basalis (Ch4) in Parkinson's disease: reduced cell numbers and co-localization with cholinergic neurons

In a effort to better define the role cholinergic basal forebrain neurons play in human cognitive processes, a quantitative assessment of cholinergic nucleus basalis (Ch4) neurons was carried out in 5 patients with Parkinson's disease (PD; 4 non-demented and 1 demented) and 4 age-matched controls using nerve growth factor (NGF) receptor immunohistochemistry as a direct marker for cholinergic basal forebrain neurons. Virtually all (greater than 90%) NGF receptor-containing neurons co-localize with the specific cholinergic marker choline acetyltransferase (ChAT) within the nucleus basalis in PD. NGF receptor-containing neurons were reduced on average by 68% (range 38.6-87.4%) in the non-demented PD cases and by 88.6% in the demented PD patient. Loss of these neurons was heterogeneous across the nucleus basalis subfields with only the anterolateral and posterior Ch4 subregions demonstrating significant reductions of NGF receptor-containing neurons. The reduction in NGF receptor-containing neurons was accompanied by a decrease of acetylcholinesterase (AChE) containing fibers within temporal cortex and in some cases ChAT immunoreactivity in the basolateral amygdaloid nucleus. The numerous non-cholinergic AChE-rich pyramidal cells which were observed throughout the cortex of aged controls were also virtually absent in PD. Although PD patients exhibited severe reductions in Ch4 neurons, few neuritic plaques or neurofibrillary tangles were observed within the PD cortex or Ch4 and similar numbers of these AD-type pathologies were seen within age-matched controls. This suggests that Ch4 degeneration alone is not sufficient to induce such cytoskeletal abnormalities and that the neuron loss seen within Ch4 in AD and PD may be mediated through different processes. These results, coupled with the extensive basic and clinical literature linking acetylcholine and memory function, further indicate that Ch4 degeneration without additional cortical and/or subcortical pathology is not sufficient to impair cognition in PD. Perhaps additional pathology must be superimposed upon nucleus basalis degeneration to induce dementia in humans.

Electrophysiological studies of the functions of the nucleus basalis in primates

In summary, the studies reviewed here have indicated which neural functions might be directly influenced by the nucleus basalis. Basalis neurons do not appear to be directly involved in trial-specific memory because, in memory tasks, they have non-differential responses that do not correspond to the information being remembered by the monkey. Similarly, basalis neurons do not appear to be related to movements because, in a go/no-go task, similar neuronal responses occur whether the animal moves or does not move, and, in a delayed response task, different neuronal responses occur during the same arm movement made under different conditions. Basalis neurons also respond differently to the same sensory stimuli presented under different conditions, which indicates that the nucleus basalis is not involved in basic sensory perception. The responses of basalis neurons therefore appear to be strongly influenced by the context or behavioral significance of stimuli. Many basalis neurons respond to appetitive stimuli. In trained animals, the most frequently observed responses have been to a water reward or to stimuli that consistently precede the reward. In naive, thirsty animals, a large proportion of basalis neurons respond to the delivery of water. However, a large number of neurons also respond to an aversive air puff, which indicates that the nucleus basalis cannot be exclusively related to appetitive stimuli. Although some basalis neurons apparently respond only to the appetitive stimulus and others respond only to the aversive stimulus, the majority appear to respond similarly to both stimuli. In particular, almost all of the neurons whose response magnitudes covary with the volume of the water respond similarly to the air puff. Hence, the neurons that appear most likely to be related to the appetitive component of the water are also responsive to an aversive stimulus. Basalis neurons may therefore be related to some common characteristic of aversive and appetitive stimuli, such as the arousing quality of these stimuli. The hypothesis that most basalis neurons are particularly responsive to arousing stimuli could account for the abundance of responses to rewards and stimuli associated with rewards. These phasic responses of basalis neurons are hypothesized to be related to a transient increase in the cortical activation component of arousal, just as the tonic activity of basalis neurons appears to be related to sustained cortical activation.

Graft-induced behavioral recovery from subcallosal septohippocampal damage in rats depends on maturity stage of donor tissue

Long-Evans female rats sustained electrolytic lesions of the fimbria and the dorsal fornix and, 10-14 days later, received intrahippocampal suspension grafts of septal-diagonal band tissue from either 14-day-old (Group S14, n = 8) or 16-day-old fetuses (Group S16, n = 10), or of parietal cortex from 16-day-old fetuses (Group Cx, n = 10). Sham-operated (Group S, n = 10) and lesion-only (Group Fifo, n = 21) rats served as non-grafted controls. Spontaneous alternation was assessed in a T-maze at three weeks and two months post-grafting. Home cage and open field activity as well as radial maze learning were assessed from two months post-grafting onwards. Fimbria-fornix lesions induced lasting hyperactivity in both the open field and the home cage, impaired radial maze learning and transiently reduced spontaneous alternation rates. Neither type of graft significantly affected home cage activity. Septal-diagonal band grafts improved open field habituation (within trial decline of ambulatory activity) and radial maze learning; the former was observed only in S16 rats, whereas the latter was observed only in S14 rats. Acetylcholinesterase histochemistry revealed an initial lesion-induced depletion of hippocampal acetylcholinesterase (eight days post-surgery) which was no longer observed at the end of the experiment. Acetylcholinesterase positivity was similar in S14 and S16 grafts, which also contained many choline acetyltransferase-positive neurons. Cortical grafts were found to be almost devoid of acetylcholinesterase positivity and no well-stained choline acetyltransferase-positive neurons could be identified. Septal-diagonal band grafts from 14-day-old fetuses and cortical grafts contained more parvalbumin-positive neurons than septal-diagonal band grafts provided by 16-day-old fetuses. These results suggest that grafts rich in cholinergic neurons may promote behavioral recovery from fimbria-fornix lesion-induced deficits. However, such a recovery may concern different behavioral deficits as a function of the age of the implanted tissue, suggesting that the maturity stage of the donor may critically influence the functional expression in the lesioned recipient. Also, such a recovery does not appear to be related solely to cholinergic hippocampal (re)innervation and might depend on the presence, not only of cholinergic neurons, but also of non-cholinergic neuronal populations, such as parvalbumin-positive (probably GABAergic) neurons.

Effect of recombinant human nerve growth factor on presynaptic cholinergic function in rat hippocampal slices following partial septohippocampal lesions: measures of [3H]acetylcholine synthesis, [3H]acetylcholine release and choline acetyltransferase activity

To determine whether intraventricular administration of nerve growth factor alters presynaptic cholinergic function in the intact hippocampus or following partial lesions of the fimbria, we investigated the effects of recombinant human nerve growth factor treatment on [3H]acetylcholine synthesis and release by hippocampal slices following various treatment regimens. For chronic nerve growth factor treatment, 1 microgram of recombinant human nerve growth factor was injected intraventricularly every second day. Lesions reduced [3H]acetylcholine synthesis (by 48%) and spontaneous and evoked [3H]acetylcholine release by 35 and 61%, respectively. Chronic nerve growth factor treatment over three weeks elevated [3H]acetylcholine synthesis (by 39%) and spontaneous and evoked [3H]acetylcholine release by 27 and 64%, respectively, over values in lesioned hippocampi of animals treated with a control protein (cytochrome c). The nerve growth factor-induced enhancement of presynaptic cholinergic function persisted for three weeks following the termination of nerve growth factor administration. Furthermore, chronic (nine-week) treatment with nerve growth factor increased [3H]acetylcholine by 118% over values in lesioned hippocampi of animals treated with cytochrome c. These findings indicate that chronic treatment with recombinant human nerve growth factor increases the capacity of hippocampal cholinergic neurons surviving a partial fimbrial transection to synthesize, store and release acetylcholine. Application of recombinant human nerve growth factor during the initial weeks after lesioning was necessary to product significant elevations in acetylcholine synthesis, since chronic recombinant human nerve growth factor treatment after delays of three or more weeks were ineffective. Furthermore, chronic nerve growth factor treatment failed to stimulate acetylcholine synthesis and release in intact hippocampal cholinergic systems. Single intraventricular injections of recombinant human nerve growth factor at the time of lesioning resulted in a small decrease in acetylcholine synthesis which, however, was not accompanied by a change in the rate of evoked acetylcholine release from cholinergic neurons surviving the lesion. The study indicates that chronic or repeated administration of nerve growth factor during the onset of degenerative events is necessary for the stimulation of presynaptic cholinergic function in the hippocampus of adult rats with partial fimbrial transections.

Models of memory dysfunction? A comparison of the effects of scopolamine and lorazepam on memory, psychomotor performance and mood

The effects on memory, psychomotor functions and mood of intramuscular scopolamine (0.3 mg, 0.6 mg) were compared with those of oral lorazepam (2 mg) and placebo. Thirty-six volunteers took part in a double-blind, independent groups design. Subjects completed a battery of tests 1 and 3 h after drug administration. Both doses of scopolamine produced levels of sedation comparable to that produced by lorazepam. The time course of effects of scopolamine and lorazepam differed but the pattern of psychomotor impairments and amnestic effects produced was very similar. In terms of mood, lorazepam had an anxiolytic effect whereas scopolamine increased ratings of anxiety. Levels of sedation, indexed by either subjective ratings or motor retardation (tapping speed), were related more to psychomotor performance than to performance on memory tasks. The results suggest that benzodiazepines and scopolamine have similar amnestic and sedative effects and as such may not offer distinct models of memory dysfunction.

The hippocampal formation: morphological changes induced by thyroid, gonadal and adrenal hormones

The hippocampal formation is of considerable interest due to its proposed role in a number of important functions, including learning and memory processes. Manipulations of thyroid, gonadal and adrenal hormones have been shown to influence hippocampal physiology as well as learning and memory. The cellular events which underlie these hormone-induced functional changes are largely unexplored. However, studies suggest that hormonal manipulations during development and in adulthood result in dramatic morphological changes within the hippocampal formation. Because neuronal physiology has been suggested to depend upon neuronal morphology, we have been determining the morphologic sensitivity of hippocampal neurons to thyroid and steroid hormones in an effort to elucidate possible structural mechanisms to account for differences in hippocampal function. In this review, hormone-induced structural changes in the developing and adult hippocampal formation are discussed, with particular emphasis on their functional relevance. Sex differences, as well as the developmental effects of thyroid hormone and glucocorticoids, are described. Moreover, the effects of ovarian steroids, thyroid hormone and glucocorticoids on neuronal morphology in the hippocampal formation of the adult rat are reviewed. These hormone-induced structural changes may account, at least in part, for previously reported hormone-induced changes in hippocampal function.

Emerging pharmacology of nerve growth factor

1. Partial transection of the septo-hippocampal pathway decreased measures of presynaptic cholinergic function in the rat hippocampal formation. 2. Chronic intraventricular treatment with recombinant human nerve growth factor attenuated lesioned-induced deficits in cholinergic function. Following nerve growth factor treatment measures of choline acetyltransferase activity, acetylcholine synthesis and release were significantly increased compared to cytochrome c-treated lesioned animals. 3. Single injections of nerve growth factor were ineffective in altering lesioned-induced deficits in cholinergic function. 4. Chronic nerve growth factor treatment was ineffective in increasing presynaptic cholinergic function if administered 3 or more weeks following fimbrial transections. 5. The nerve growth factor-induced increases of presynaptic cholinergic function persisted for 3 weeks following the cessation of chronic 3 week nerve growth factor treatment.

Comparison of tetrahydroaminoacridine and physostigmine on scopolamine-induced free swim behavior in the rat

The effect of acetylcholinesterase inhibitors on free swim behavior in rats pretreated with scopolamine (0.32 mg/kg, IP) was examined. Long-Evans rats received a single 5-min testing trial in a 1.5 m black swimming pool, and swim distance in three concentric annulus corridors (peripheral, middle, and inner) and the number of body-turn transitions (greater than 45 degrees) were measured. Physostigmine (1.0 mg/kg, IP) increased swim distance in the middle and inner annulus corridors, compared to tetrahydroaminoacridine (2.0 mg/kg and 10 mg/kg, IP) and scopolamine alone (control) (Ps less than 0.01), and increased body-turn transitions, compared to all the other groups (Ps less than 0.05), but had no significant effect on peripheral annulus corridor swim distance, total swim distance, or swim speed. The results suggest that physostigmine produces uniquely different free swim patterns from tetrahydroaminoacridine following cholinergic blockade. These findings have implications for investigations attempting to restore spatial learning and navigation (e.g., Morris water maze) using acetylcholinesterase inhibitors following experimentally-induced cholinergic losses.

Effects of acute infusion of the muscarinic cholinergic agonist arecoline on verbal memory and visuo-spatial function in dementia of the Alzheimer type

1. Treatment of patients with dementia of the Alzheimer type (DAT) with arecoline, a muscarinic cholinergic receptor agonist, reportedly improves performance on a picture recognition memory task, but not on other memory measures. To examine further possible performance improvements following arecoline treatment, patients with DAT were treated with a 30 min intravenous infusion of arecoline (5 mg). 2. Psychometric testing was done at five time points (two before and three following the infusion). Patients were tested on a memory task (Buschke selective reminding) and a test of visuo-spatial performance (figure copying). 3. No net change from baseline was seen in mean scores following arecoline infusion. However, the changes in performance on the two tasks were correlated (p less than 0.02) over subjects at 10 min but not at 1.5 or 5.5 hr following the infusion. 4. This result suggests that although individual patients vary in their response to a given dose of arecoline, their responses are consistent across types of tasks. Thus the lack of a mean drug effect may be due to individual differences in response rather than to a lack of response.

Cholinergic modulation of excitability in the rat olfactory bulb: effect of local application of cholinergic agents on evoked field potentials

The effect of exogenously applied cholinergic agents upon mitral-granule cell complex activity of the olfactory bulb was studied in anesthetized rats. Output neurons were activated by electrical paired-pulse stimulation (40-80 ms time interval) applied either to the olfactory nerve (orthodromic stimulation) or to the lateral olfactory tract (antidromic stimulation). Evoked field potentials were recorded in the granule cell layer. Cholinergic agents were introduced close to the mitral cell body layer through a push-pull cannula. With both orthodromic and antidromic stimulations, acetylcholine in the presence of eserine (an acetylcholinesterase blocker), did not alter the conditioning volley, while it induced a significant increase in the amplitude of the test volley. This effect could be replicated using the cholinergic agonist carbachol. This attenuation of the paired-pulse inhibition is due to a reduction of the dendrodendritic inhibitory action of granule cells upon relay cells. Muscarinic and nicotinic transmission were studied using antidromic and orthodromic stimulations, respectively. The selective effect of acetylcholine on the test volley was totally abolished by the blockade of the muscarinic transmission (by atropine). The blockade of the GABAergic transmission (by picrotoxin), could also prevent the acetylcholine-induced effect. The results lead us to propose that in deep bulbar layers, acetylcholine may activate muscarinic receptors situated on second-order GABAergic interneurons. These interneurons could in turn inhibit granule cells (first-order interneurons). The nicotinic antagonist d-tubocurarine selectively enhanced the duration of the late component and did not appear to modify early components when stimulation was applied to the olfactory nerve. This effect related to both the conditioning and the test volleys and the enhancement in the duration of depolarization of granule cell dendrites suggests that normal activation of nicotinic receptors contributes to a faster repolarization of granule cells. Since nicotinic receptors belong to the outer glomerular layer, this result points to the existence of interneurons belonging to the periglomerular region where they receive nicotinic input and project to deep layers where they modulate granule cell activity. Taken together, our results suggest the presence of a phasic muscarinic and a tonic nicotinic modulation of bulbar interneuronal activity. Since both could finally reduce the inhibitory action of granule cells, the action of cholinergic afferents would facilitate transmission of bulbar output neurons to central structures.

Thiamine-dependent enzyme changes in temporal cortex of patients with Alzheimer's disease

Activities of thiamine-dependent enzymes [pyruvate dehydrogenase (PDHC), alpha-ketoglutarate dehydrogenase (alpha KGDH), and transketolase (TK)] were measured in autopsied samples of temporal cortex from six patients with Alzheimer's disease and from eight age-matched control subjects who were free from neurological or psychiatric diseases. Times from death to freezing of dissected material at -70 degrees C were matched. Significant decreases in PDHC (decreased by 70%; P less than 0.01), alpha KGDH (decreased by 70%; p less than 0.01), and TK (decreased by 52%; P less than 0.01) were observed in brain tissue from patients with Alzheimer's disease. In contrast, activities of glutamate dehydrogenase were within normal limits. These findings suggest a possible role for alterations of brain thiamine metabolism or utilization in Alzheimer's disease.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Long-term administration of mouse nerve growth factor to adult rats with partial lesions of the cholinergic septohippocampal pathway

Nerve growth factor (NGF), a neurotrophic factor acting on cholinergic neurons of the basal forebrain, has been proposed as a treatment for Alzheimer's disease. Experimental support for its pharmacological use is derived from short-term studies showing that intraventricular administration of NGF during 2-4 weeks protects cholinergic cell bodies from lesion-induced degeneration, stimulates synthesis of choline acetyltransferase, and improves various behavioral impairments. To investigate the consequences of long-term NGF administration, we tested whether cholinergic cell bodies are protected from lesion-induced degeneration and whether cholinergic axons are stimulated to regrow into the denervated hippocampus following fimbrial transections. We found that intraventricular injections of NGF twice a week for 5 months to adult rats resulted in extended protection of cholinergic cell bodies from lesion-induced degeneration and did not produce obvious detrimental effects on the animals. NGF treatment mildly stimulated growth of cholinergic neurites within the 2-mm area directly adjacent to the fimbrial lesion but it failed to induce significant homotypic growth of cholinergic neurites into the deafferented hippocampus.

Interaction between raphe dorsalis and nucleus basalis magnocellularis in spatial learning

We compared the effects on spatial learning of an ibotenic acid lesion of the nucleus basalis magnocellularis (NBM), a 5, 7-dihydroxytryptamine lesion of the raphe dorsalis (RD) and a combined NBM and RD lesion. The RD lesion reduced serotonin levels, and the NBM lesion reduced cholineacetyltransferase (ChAT) activity in the cortex. Although RD lesions alone did not affect spatial learning in the water-maze, the lesion aggravated the spatial navigation deficit produced by NMB lesioning. The current results suggest a functional interaction between the RD and NBM in spatial navigation.

The effects of concurrent manipulations of cholinergic and noradrenergic systems on neocortical EEG and spatial learning

In the spatial learning test, young animals were divided into three groups receiving saline, scopolamine (0.15 mg/kg), or scopolamine (0.8 mg/kg). Half of the animals in each group were lesioned with DSP-4 to destroy noradrenergic fibers. DSP-4 lesions did not produce any significant impairment alone or in combination with a lower dose of scopolamine (0.15 mg/kg), but they did further augment the scopolamine (0.8 mg/kg)-induced defect. In the electroencephalography (EEG) experiment, both control rats and DSP-4-lesioned rats were recorded after receiving saline, scopolamine (0.15 mg/kg), and scopolamine (0.8 mg/kg) injections. Scopolamine induced a dose- and behavioral state-dependent EEG slowing, whereas DSP-4 lesions did not change either baseline EEG activity or EEG reactivity to scopolamine.

A comparison of the working memory performances of young and aged mice combined with parallel measures of testing and drug-induced activations of septo-hippocampal and nbm-cortical cholinergic neurones

The spatial working memory performances of young (2 months) and aged (24-26 months) mice of the C57BL/6 strain were compared using a delayed nonmatching to place (DNMTP) protocol in an automated 8-arm radial maze. The aged mice were observed to exhibit a selective and interference-related memory deficit. Parallel neurochemical analysis of the activity of septo-hippocampal and nbm-cortical cholinergic neurones in vivo was conducted using measures of sodium-dependent high-affinity choline uptake. Results showed that whereas the level of cholinergic activity in both brain regions varied less than 10% between young and aged mice in quiet conditions (basal) the activation usually observed at 30-sec posttest (+20-25%) in young mice was greatly attenuated in the frontal cortex and almost totally absent in the hippocampus of aged mice. In view of these results a complementary experiment was carried out in order to test the intrinsic ability of septo-hippocampal cholinergic neurones to activate using acute injection of scopolamine (1 mg/kg IP 20 min) to both young and aged mice in quiet conditions. The drug injection resulted in a very large (+70%) increase in hippocampal high-affinity choline uptake and with amplitudes which did not vary significantly between young and aged subjects. These observations attest to a relatively well-preserved state of central cholinergic neurones and an intact capacity to activate normally when challenged pharmacologically in aged mice. The results strongly suggest that the loss of cholinergic activation and associated memory deficit in aged mice might rather be related to a hypofunction of phasically active transsynaptic processes which normally mediate the activation of these cholinergic pathways during memory testing.

Animal models of age-related dementia: neurobehavioral dysfunctions in autoimmune mice

The development of strategies for treatment of Alzheimer's disease and other age-associated dementias is an important goal of research in the neurosciences. It is suggested that advances in understanding of the etiology of those disorders would provide the most obvious avenues to development of preventative treatments. Research findings from both clinical investigations and studies of animal models are presented which suggest a neuroimmunologic component in age-associated dementia. Clinical studies suggest an association between dementia and brain-reactive autoantibodies in subsets of patients with Alzheimer's disease. Studies of mice suggest that: 1) when compared with normal genotypes, mutant mice with accelerated autoimmunity show learning and memory impairments at earlier chronological ages; 2) the learning and memory deficits of autoimmune and normal mice are qualitatively similar; 3) the behavioral deficits of normal aged and autoimmune mice are sensitive to similar pharmacologic interventions. Overall, these findings suggest that intervention strategies targeting the immune system might be useful in the treatment or prevention of aging-associated dementia. Autoimmune mice would be useful as models for the development and testing of such immune-based interventions.

Hippocampal synaptic plasticity and NMDA receptors: a role in information storage?

There has recently been renewed interest in the idea that alterations in synaptic efficacy may be the neural basis of information storage. Particular attention has been focused upon long-term potentiation (LTP), a long-lasting, but experimentally induced synaptic change whose physiological properties point to it being a candidate memory mechanism. However, considerations of storage capacity and the possibility of concomitant activity-dependent synaptic depression make it unlikely that individual learning experiences will give rise to gross changes in field potentials similar to those that occur in LTP, even if learning and LTP utilize common neural mechanisms. One way of investigating the functional significance of LTP is to use selective antagonists of those excitatory amino acid receptors whose activation is essential for its induction. This paper discusses various design requirements for such experiments and reviews work indicating that the N-methyl-D-aspartate receptor antagonist AP5 causes a behaviourally selective learning impairment having certain common features to the behavioural profile seen after hippocampal lesions. Two new studies are described whose results show that AP5 has no effect upon the retrieval of previously established memories, and that the dose-response profile of the impairment of spatial learning occurs across a range of extracellular concentrations in hippocampus for which receptor selectivity exists. These experiments show that activation of NMDA receptors is essential for certain kinds of learning.

The nucleus basalis is involved in brain modulation of the immune system in rats

Male rats were subjected to bilateral or unilateral excitotoxic lesions of the nucleus basalis magnocellularis (NBM). Three weeks after surgery, mitogen-induced lymphoproliferation and natural killer (NK) cell activity were determined in the spleen. T-cell mitogenesis and NK cell activity were strongly enhanced after bilateral lesions but were not affected after right or left unilateral lesions. B-cell mitogenesis and blood T-cell subset distribution remained unchanged after bilateral or unilateral lesions of the NBM. These results demonstrate that NBM cells are involved in the complex interrelations existing between the central nervous system and the immune system.

Amelioration of spatial memory impairment by intrahippocampal grafts of mixed septal and raphe tissue in rats with combined cholinergic and serotonergic denervation of the forebrain

Previous studies in the rat have shown that a serotonergic depletion greatly potentiates the learning and memory impairments produced by pharmacological or lesion-induced cholinergic blockade in the forebrain. The impairment produced by combined serotonergic-cholinergic lesions is reminiscent of that seen in memory-impaired aged rats. In the present experiment, we investigated whether grafts of cholinergic septal tissue and serotonergic mesencephalic raphe tissue, placed in the hippocampus, could reverse the severe memory impairment produced by combined cholinergic-serotonergic lesions. Adult rats were given an intraventricular injection of 5,7-dihydroxytryptamine followed by a radiofrequency lesion of the septum 1-2 weeks later. Three weeks after lesion surgery, the rats were given bilateral intrahippocampal cell suspension grafts of either fetal septal or mesencephalic raphe tissue, or both. The rats were tested for spatial learning and memory in the Morris water maze task at 4 and 10 months after grafting. At 4 months, lesioned and grafted groups were all impaired compared to the normal controls in their swim time and distance swum to find the platform, and they did not show any spatially focussed search strategy in the spatial probe trial when the platform was removed from the tank. At 10 months, the rats with mixed cholinergic and serotonergic grafts were no longer impaired compared to normals in their swim time and distance to find the platform, and they were significantly improved compared to the other grafted groups. Moreover, in the spatial probe trial, the rats with mixed cholinergic and serotonergic grafts displayed a spatially focussed search behaviour over the previous platform site, which was not seen in the lesioned control rats or in the other graft groups. Morphological analysis of the hippocampus revealed that the septal grafts produced an acetylcholinesterase-positive innervation but were totally devoid of serotonin innervation. The raphe grafts produced mainly a serotonin innervation, of both acetylcholinesterase- and serotonin-positive fibres. The results suggest that a mixture of septal and raphe tissue is required when grafted to the hippocampal formation in order to ameliorate the severe spatial learning and memory impairments produced by a combined cholinergic and serotonergic denervation, and that each of these graft types separately are not sufficient to ameliorate such deficits.

Effects of p-chlorophenylalanine and scopolamine on retention of habits in rats

Rats were trained on a conventional maze test or on a swim-to-platform test. Retention of swim-to-platform performance 7 days later was severely impaired by posttraining treatment with a combination of p-chlorophenylalanine (PCPA) and scopolamine although neither drug alone had any effect. Retention of the maze habit was moderately impaired by scopolamine alone and severely impaired by a combination of scopolamine and PCPA, but was unaffected by PCPA alone. Polygraphic recordings confirmed previous reports that a combination of PCPA and scopolamine can abolish neocortical low voltage fast activity and hippocampal rhythmical slow activity. Combined blockade of central cholinergic and serotonergic neurotransmission in rats may provide a useful animal model of Alzheimer's disease.

4-Aminopyridine increases acetylcholine release without diminishing membrane phosphatidylcholine

4-Aminopyridine (10(-4)-10(-5) M) increased severalfold the release of acetylcholine from rat striatal slices superfused with an eserine-containing, choline-free medium, and caused stoichiometric decreases in the release of choline. It had no effect on tissue acetylcholine and choline levels. Electrical stimulation of the striatal slices increased acetylcholine release without affecting that of choline. Superfusion of the stimulated slices with 4-aminopyridine decreased choline release and increased the ratio of acetylcholine to choline in superfusates. As shown previously, electrical stimulation of the striatal slices decreased their contents of phospholipids, principally phosphatidylcholine; 4-aminopyridine fully protected against these membrane changes. In synaptosomal preparations, 4-aminopyridine was found to enhance the high-affinity uptake of [14C]choline and its conversion to [14C]acetylcholine. This effect on choline uptake may underlie 4-aminopyridine's ability to enhance acetylcholine release in the absence of supplemental choline while suppressing the "autocannibalism" of membrane phospholipids.

Atrophy of cholinergic neurons within the rat nucleus basalis magnocellularis following intracortical AF64A infusion

The rat nucleus basalis magnocellularis (nBM) was morphometrically analyzed following multiple intracortical AF64A infusions. At 3 weeks post-infusion, brains were histochemically double-stained for acetyl-cholinesterase and Nissl substance following diisopropylfluorophosphate pretreatment. Intracortical AF64A induced significant atrophy, but not degeneration, of nucleus basalis cholinergic cell bodies. These results suggest that retrograde cellular atrophy is associated with inhibition of presynaptic high-affinity choline transport on cortical terminals of nBM cholinergic neurons.

Chronic infusion of GABA and saline into the nucleus basalis magnocellularis of rats: II. Cognitive impairments

In order to assess sensorimotor and/or cognitive modifications following chronic inhibition of nucleus basalis magnocellularis (NBM) neurons, rats trained in two radial maze paradigms (the classical version of the test and a modified version introducing a one-hour delay between the fourth and the fifth choice) received chronic infusion of gamma-aminobutyric acid (GABA) into the NBM area. GABA (10 and 50 micrograms/microliters/h) was infused for 3 days into the NBM contralateral to their preferred turning direction in the radial maze. Simultaneously, saline (NaCl 0.9%; 1 microliter/h) was infused into the contralateral NBM. GABA and saline infusions were alternated for the subsequent 3-day period. One week later, we investigated the rats' ability to learn a multiple trial passive avoidance task. At the dose of 50 micrograms/microliters, GABA infusion produced (1) a turning bias ipsilateral to the side first infused with GABA, (2) transitory cognitive impairments in radial maze tasks and (3) a deficit in the acquisition of the passive avoidance task. At the dose of 10 micrograms/microliters, the same behavioral deficits were observed except that (1) the turning bias was reversed by the contralateral GABA infusion and (2) cognitive impairments in the radial maze were observed only when a delay was inserted between the fourth and the fifth choice. Histologically, we found a dose-dependent gliosis in the NBM area first infused with GABA. These data suggest a reactivity of the NBM to GABAergic manipulations and the intervention of this structure in both sensorimotor and cognitive processes involved in the radial maze paradigms.

Central muscarinic activities of an M1-selective agonist: preferential effect on reversal of amnesia

Effects of FKS-508 (AF102B; cis-2-methylspiro (1,3-oxathiolane-5,3')-quinuclidine), a novel M1-selective agonist, on central muscarinic responses in mice were examined in comparison with oxotremorine. FKS-508 was slightly less potent (6 times) in reversal of scopolamine-induced amnesia (passive avoidance failure), but far less potent (260 and 55 times) in producing hypothermia and tremor than oxotremorine. These results show that the selective M1 agonist FKS-508 differentiates highly between the central muscarinic effects.

Developing forebrain astrocytes are sensitive to thyroid hormone

Previous studies have shown that developing neurons of the basal forebrain and hippocampus are sensitive to thyroid hormone (Gould and Butcher: J. Neurosci., 9:3347-3358, 1989; Rami et al: Neuroscience, 19:1217-1226, 1986). In order to determine whether or not thyroid hormone influences the development of astrocytes in brain regions where neurons are affected, we performed vimentin and glial fibrillary acidic protein (GFAP) immunocytochemical and single-section Golgi-impregnation analyses on the basal forebrain and hippocampus of control and neonatally thyroid hormone treated rats. For purposes of comparison, glial cells of the pontomesencephalotegmental (PMT) region, a region where developing neurons are not morphologically affected by thyroid hormone imbalances (Gould and Butcher, op. cit.), were also examined. Neonatal thyroid hormone treatment resulted in a premature disappearance of vimentin-immunoreactive radial glia in the basal forebrain and hippocampus. In addition, a premature appearance of GFAP-immunoreactive astrocytes with mature morphological characteristics was observed in the basal forebrain and hippocampus of thyroid hormone treated animals. Quantitative analyses revealed significant increases in the density of GFAP-immunostained astrocytes and in the cross-sectional cell body area and the number of primary processes in Golgi-impregnated astrocytes of the basal forebrain and hippocampus of animals treated neonatally with thyroid hormone. In contrast, no changes in any of these parameters were observed in glial cells of the PMT region with neonatal thyroid hormone treatment.

Cholinergic modulation of aged-like retention deficits in young autoimmune mice

Separate age groups of autoimmune NZB/BINJ and non-autoimmune C57BL/BNNia mice were compared for habituation of locomotor activity and its retention over four separate testing sessions spaced at 24-hr intervals. A decline in locomotion (distance in cm) or in the time spent in the center zone as a function of sessions was taken to indicate retention for habituation to stimuli within the test apparatus. The time spent in the center zone decreased as a function of sessions in young and mature C57BL/6NNia mice but failed to show reliable between-session decreases in old (24-26-months) C57BL/6NNia mice. When compared with the old C57BL/6NNia mice, young NZB/BINJ mice showed similar impairments. Habituation of locomotion was present in all age groups of C57BL/6NNia mice, but absent in NZB/BINJ mice regardless of age. The retention impairments of 2-4 month old NZB/BINJ mice were attenuated when i.p. injections of 0.04-0.16 mg physostigmine/kg were given just following each habituation session. The effectiveness of physostigmine was substantially reduced when injections were delayed by 20 min or longer following each habituation session. The time-dependent reversal of the aged-like retention deficits by the cholinesterase inhibitor, physostigmine, suggests that cholinergic modulation of memory storage processes may be impaired in NZB/BINJ mice.

Habituation and retention of the head-shake response: lack of impairment by nucleus basalis magnocellularis lesions

Bilateral ibotenic acid lesions of the nucleus basalis magnocellularis (NBM) were examined on the habituation and retention of the head-shake response (HSR). HSR, a rapid, stereotyped rotation of the head about a front-to-rear axis, was elicited by a stream of mildly pressurized air directed at the ear in Fischer 344 rats. HSR training consisted of 40 stimulated trials followed by a 30-min retention test of 20 stimulated trials. Stimulus duration was 15 sec per trial with a 15-sec intertrial interval. Frontal cortex choline acetyltransferase was reduced by 22% in the NBM-lesioned group compared to the controls. The NBM-lesioned rats and the controls were not significantly different on either frequency or latency measures of HSR habituation or retention. The results do not support a role for cortical cholinergic mediation of the HSR.

Activating the damaged basal forebrain cholinergic system: tonic stimulation versus signal amplification

The hypothesis that the cognitive decline in senile dementia is related to the loss of cortical cholinergic afferent projections predicts that pharmacological manipulations of the remaining cholinergic neurons will have therapeutic effects. However, treatment with cholinesterase inhibitors or muscarinic agonists has been, for the most part, largely unproductive. These drugs seem to disrupt the normal patterning of cholinergic transmission and thus may block proper signal processing. An alternative pharmacological strategy which focuses on the amplification of presynaptic activity without disrupting the normal patterning of cholinergic transmission appears to be more promising. Such a strategy may make use of the normal GABAergic innervation of basal forebrain cholinergic neurons in general, and in particular of the inhibitory hyperinnervation of remaining cholinergic neurons which may develop under pathological conditions. Disinhibition of the GABAergic control of cholinergic activity is assumed to intensify presynaptic cortical cholinergic activity and to enhance cognitive processing. Although the extent to which compounds such as the benzodiazepine receptor antagonist beta-carboline ZK 93,426 act via the basal forebrain GABA-cholinergic link is not yet clear, the available data suggest that the beneficial behavioral effects of this compound established in animals and humans are based on indirect cholinomimetic mechanisms. It is proposed that an activation of residual basal forebrain cholinergic neurons can be achieved most physiologically via inhibitory modulation of afferent GABAergic transmission. This modulation may have a therapeutic value in treating behavioral syndromes associated with cortical cholinergic denervation.

Ante mortem cerebral amino acid concentrations indicate selective degeneration of glutamate-enriched neurons in Alzheimer's disease

There is little information about major constituents of the brain in Alzheimer's disease. In the case of amino acids most of the previous data are contradictory. These have been interpreted in an anatomic and neurotransmitter as well as a metabolic context. To help clarify this, the contents of 14 amino acids and ethanolamine were determined in samples of neocortex from diagnostic craniotomies of 15 demented patients (10 with Alzheimer's disease) and other neurosurgical procedures (57 patients, 18 with intractable depression). A comprehensive survey of the effects of possible complicating factors on the concentrations of amino acids showed that artefacts were few; this was in contrast to a post mortem series of brains (16 with Alzheimer's disease and 16 controls; six regions assayed). We have used the ante mortem data to provide the basis for an accurate comparison of amino acid values between Alzheimer and control samples. In Alzheimer's disease, the mean contents of many amino acids were slightly higher (sum of the increases of those significantly affected was 15 nmol/mg protein) whereas glutamate content alone was significantly reduced (by 16 nmol/mg protein). This was not a feature of depression or a group of patients with other dementias. Glutamate content of Alzheimer samples was related to pyramidal neuron density in cortical layer III. These alterations were detected relatively early during the course of Alzheimer's disease and are considered to be due to loss of corticocortical glutamatergic association pathways.

Lipid abnormalities in the brain in adult Down's syndrome and Alzheimer's disease

Quantitative analysis by HPTLC of the major lipid classes and dolichol, and of fatty acyl groups of separated phosphoglycerides by capillary GLC, has been carried out on the gray matter of frontal cerebral cortex of brains from six Down's syndrome (DS) and six Alzheimer's disease (AD) adults, and six each of two corresponding sets of age-matched controls; specimens of DS and control cerebellum and corpus callosum were also analyzed. In DS frontal cortex, but not in AD frontal cortex, compared to their respective controls there was a decrease in the fraction of phosphatidylethanolamine (PE) and an increase in the fractions of sphingomyelin (SPM) and phosphatidylserine (PS). Abnormalities were not found in the proportions of major lipid classes in DS cerebellum or corpus callosum. The concentration of dolichol was elevated for age in the frontal cortex of DS and of AD. In the phosphoglycerides of DS frontal cortex, the fatty acyl composition showed small, but statistically significant, differences from those of age-matched controls, and some slight abnormalities were also detected in DS corpus callosum. The alterations in DS frontal cortex included decreases in (n-6) and increases in (n-3) groups in choline and ethanolamine phosphoglycerides (CPG and EPG), as had previously been found in EPG and serine phosphoglyceride (SPG) of the DS fetal brain. In DS frontal cortex, the proportion of 22:4(n-6) groups was decreased in SPG, and in inositol phosphoglyceride (IPG) 18:1(n-9) was increased. There were also small but significant alterations in DS frontal cortex in the fractions of shorter chain groups in CPG. In marked contrast, most of the fatty acyl abnormalities seen in DS were absent in the AD frontal cortex. It is therefore suggested that some abnormalities in the composition of cerebral membranes present prenatally in DS may persist into adulthood, and are not directly related to AD-type pathology.