Withdrawal from repeated amphetamine administration leads to disruption of prepulse inhibition but not to disruption of latent inhibition

Summary. The present study represents a continuous effort to develop an animal model of schizophrenia based on the “endogenous dopamine sensitization” hypothesis. To achieve this goal, withdrawal from an escalating amphetamine (AMPH) regime administration [three injections per day over a period of 4 days and increasing doses from 1 to 10 mg/kg of AMPH or an equivalent volume of saline (SAL)] was employed. Animals exposed to this treatment were evaluated on their performance in attentional (Latent inhibition, LI) and sensorimotor gating (Prepulse inhibition, PPI) tasks in a drug free state and tested for locomotor sensitization following a low dose of AMPH challenge administration. LI using active avoidance, tested on withdrawal day 4, was unaffected. PPI of the acoustic startle response, measured on withdrawal days 6 and 70, was disrupted. On the 76th day of withdrawal, a low challenge dose of AMPH (1 mg/kg) led to a clear locomotor sensitization effect. Present address: Brain Research Institute, University of Zurich/ETH Zurich, Switzerland     

Microsphere embolism-induced cortical cholinergic deafferentation and impairments in attentional performance

Ischemic events have been hypothesized to play a critical role on the pathogenesis of dementia and the acceleration of cognitive impairments. This experiment was designed to determine the consequences of microvascular ischemia on the cortical cholinergic input system and associated attention capacities. Injections of microspheres ( approximately 50 microm diameter; approximately 5000 microspheres/100 microL) into the right common carotid artery of rats served as a model of microvascular ischemia and resulted in decreases in the density of cholinergic fibers in the ipsilateral medial prefrontal cortex and frontoparietal areas. Furthermore, dense astrogliosis, indicated by glial fibrillary acidic protein (GFAP) immunohistochemistry, was observed in the globus pallidus, including the areas of origin of cholinergic projections to the cortex. Fluoro-Jade B staining indicated that loss of neurons in the cortex was restricted to areas of microsphere-induced infarcts. Attentional performance was assessed using an operant sustained attention task; performance in this task was previously demonstrated to reflect the integrity and activity of the cortical cholinergic input system. Embolized animals' performance was characterized by a decrease in the animals' ability to detect signals. Their performance in non-signal trials remained unaffected. The residual density of cholinergic axons in prefrontal and frontoparietal areas correlated with the animals' performance. The present data support the hypothesis that microvascular ischemia results in loss of cortical cholinergic inputs and impairs associated attentional performance. Microsphere embolism represents a useful animal model for studying the role of interactions between microvascular disorder and impaired forebrain cholinergic neurotransmission in the manifestation of cognitive impairments.     

Behavioural consequences of withdrawal from three different administration schedules of amphetamine

Different administration schedules of amphetamine (AMPH) lead to different behavioural consequences, neurochemical changes and gene expression patterns in a variety of brain areas during drug withdrawal. However, direct comparisons of these different effects within the same experiment are rare in the literature. Accordingly, in this study, rats were tested in relevant behavioural paradigms during withdrawal from three different administration schedules of AMPH. The intermittent schedule (INT) consisted of one daily injection of 1.5 mg/kg AMPH for 6 days. The escalating administration schedule consisted of three daily injections for 6 days with increasing dosages from 1 to 5 mg/kg AMPH during the first five injections and 5 mg/kg for the following 13 injections (ESC-5). A second more severe escalating administration schedule (three injections per day for 3 days escalating from 1 to 9 mg/kg AMPH and a final 10 mg/kg AMPH injection on day 4, ESC-10) was also investigated. Control animals received saline injections according to the three administration schedules. Rats pretreated with AMPH according to the ESC-10 administration schedule exhibited a transient reduction of locomotor activity on day 1, but not day 5, of withdrawal, as well as a permanent disruption of prepulse inhibition (PPI) on days 4, 20, and 40 of withdrawal. There was no effect on anxiety measured by the elevated plus-maze on withdrawal day 2, and all the AMPH pretreated animals exhibited a similar magnitude of behavioural sensitization following an AMPH challenge irrespective of administration schedule on withdrawal day 42. These data suggest that, based on their persistent disruption of PPI, animals withdrawn from AMPH ESC-10 might model specific symptoms of schizophrenic patients.     

A sensitizing regimen of amphetamine that disrupts attentional set-shifting does not disrupt working or long-term memory

Exposure to an intermittent, escalating dose of amphetamine induces a sensitized state that, both behaviourally and neurochemically, mirrors several features linked to the positive symptoms of schizophrenia. Increasingly it is being realized that cognitive deficits are a core component of schizophrenia; therefore we sought to assess the effects of inducing an amphetamine-sensitized state on memory (working and long-term) and cognitive flexibility, two cognitive domains impaired in schizophrenia. Rats were exposed to a sensitizing regimen of amphetamine (1-5 mg/kg; three times per week for 5 weeks; escalating at 1mg/kg per week) or saline. In experiment 1, animals were tested on an operant delayed non-match to position task (working memory). Experiment 2 used a standard fixed-platform location water maze task (long-term memory), while experiment 3 used a variable-platform location water maze task (long-term memory and working memory). Amphetamine-sensitized animals were not impaired on any of these tasks. In experiment 4, animals were assessed on a strategy selection task in which they were first required to learn to locate a food reward using a particular learning strategy (place or response) then to learn to shift to an alternate learning strategy (response or place). Amphetamine-sensitized animals were not impaired on this task. In the final experiment animals were found to be impaired in performance of the extra-dimensional shift component of an attentional set-shifting task. These results suggest that while amphetamine sensitization does not produce memory impairments similar to those seen in schizophrenia, it does produce strong impairments in set-shifting, suggesting changes in prefrontal function similar to those seen in schizophrenia.     

Brain-derived neurotrophic factor administration protects basal forebrain cholinergic but not nigral dopaminergic neurons from degenerative changes after axotomy in the adult rat brain.

Cell culture studies with dissociated primary cultures from embryonic rat brain revealed that brain-derived neurotrophic factor (BDNF) promotes the developmental differentiation of both basal forebrain cholinergic and mesencephalic dopaminergic neurons. These studies suggested that, in the adult brain, BDNF may be able to protect cholinergic and dopaminergic neurons from degenerative changes induced by axotomy, similar to the known protective action of NGF in cholinergic neurons. Testing this hypothesis, we found that intraventricular administration of recombinant human BDNF (rhBDNF) to adult rats with transections of the fimbria significantly reduces axotomy-induced degenerative changes of the cholinergic cells in the basal forebrain. No such effect was seen on the dopaminergic neurons of the ventral mesencephalon after transection of their axons ascending in the medial forebrain bundle. Injected in equal amounts, rhBDNF and recombinant human NGF had quantitatively different effects on the cholinergic neurons. BDNF sustained only part of the population of cholinergic neurons affected by the lesion, whereas the entire population was protected by NGF treatment.     

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.     

Lesions of the basal forebrain impair reversal learning but not shifting of attentional set in rats

The cholinergic neurons of the basal forebrain, which project to cortex, the thalamic reticular nucleus and the amygdala, are implicated in many aspects of attentional function, while the intrinsic neurons of the basal forebrain are implicated in learning and memory. This study compared the effects of lesions of the basal forebrain made with either the immunotoxin 192-IgG-saporin (which selectively destroys cholinergic neurons), or the non-selective excitotoxin, ibotenic acid (which destroys both cholinergic and non-cholinergic neurons) on a task which measure the acquisition and shifting of attentional set as well as the ability to learn reversals of specific stimulus-reward pairings. Rats learned to obtain food reward by digging in small bowls containing distinctive digging media that were differentially scented with distinct odours. They performed a series of two-choice discriminations, with the bait associated with either the odour or the digging medium. Rats with 192-IgG-saporin lesions of the basal forebrain were not impaired relative to control rats at any stage of the task. Rats with ibotenic acid lesions of the basal forebrain were impaired the first time stimulus-reward contingencies were reversed. They were not impaired in acquisition of new discriminations, even when an attentional-shift was required. These data are consistent with data from marmosets and so highlight the functional similarity of monkey and rodent basal forebrain. They also confirm the likely involvement of non-cholinergic neurons of the basal forebrain in reversal learning.     

Diminished trkA receptor signaling reveals cholinergic‐attentional vulnerability of aging

The cellular mechanisms underlying the exceptional vulnerability of the basal forebrain (BF) cholinergic neurons during pathological aging have remained elusive. Here we employed an adeno‐associated viral vector‐based RNA interference (AAV‐RNAi) strategy to suppress the expression of tropomyosin‐related kinase A (trkA) receptors by cholinergic neurons in the nucleus basalis of Meynert/substantia innominata (nMB/SI) of adult and aged rats. Suppression of trkA receptor expression impaired attentional performance selectively in aged rats. Performance correlated with trkA levels in the nMB/SI. trkA knockdown neither affected nMB/SI cholinergic cell counts nor the decrease in cholinergic cell size observed in aged rats. However, trkA suppression augmented an age‐related decrease in the density of cortical cholinergic processes and attenuated the capacity of cholinergic neurons to release acetylcholine (ACh). The capacity of cortical synapses to release ACh in vivo was also lower in aged/trkA‐AAV‐infused rats than in aged or young controls, and it correlated with their attentional performance. Furthermore, age‐related increases in cortical proNGF and p75 receptor levels interacted with the vector‐induced loss of trkA receptors to shift NGF signaling toward p75‐mediated suppression of the cholinergic phenotype, thereby attenuating cholinergic function and impairing attentional performance. These effects model the abnormal trophic regulation of cholinergic neurons and cognitive impairments in patients with early Alzheimer's disease. This rat model is useful for identifying the mechanisms rendering aging cholinergic neurons vulnerable as well as for studying the neuropathological mechanisms that are triggered by disrupted trophic signaling.     

Dementia: animal models of the cognitive impairments following damage to the basal forebrain cholinergic system

The finding that patients with Alzheimer's disease (AD) have significant degeneration of neurons in the basal forebrain cholinergic system (BFCS) stimulated a great deal of research to determine the cognitive impairments resulting from selective damage to this area. The experiments reviewed here indicate that lesions of the nucleus basalis magnocellularis (NBM) and of the medial septal area (MSA) reproduce the behavioral symptoms following lesions of their respective target sites, the frontal cortex (FC) and the hippocampus (HIP). Impairments of recent memory are one of the most striking symptoms in AD patients at the beginning of their disease, and lesions of the BFCS induce similar impairments. Comparisons of the effects of the lesions produced by different neurotoxins, ibotenic (IBO) acid and quisqualic (QUIS) acid, have raised questions about the role of cholinergic and noncholinergic neurotransmitter systems in the basal forebrain. The implications of these data for the cholinergic hypothesis of mnemonic functions are discussed.     

Selective Lesioning of the Basal Forebrain Cholinergic System by Intraventricular 192 IgG–saporin: Behavioural, Biochemical and Stereological Studies in the Rat

The elucidation of the functional role of the basal forebrain cholinergic system will require access to a highly specific and efficient cholinergic neurotoxin. Recently, selective depletion of the nerve growth factor (NGF) receptor-bearing cholinergic neurons in the rat basal forebrain and a dramatic loss of cholinergic innervation in the related cortical regions have been obtained following intraventricular injection of a newly introduced immunotoxin, 192 IgG-saporin. Here we extend these initial findings and report that administration of increasing doses (1.25, 2.5, 5.0 or 10 μg) of the 192 IgG-saporin conjugate into the lateral ventricles of adult rats induced dose-dependent impairments in the water maze task and passive avoidance retention, but only weak and inconsistent effects on locomotor activity. These behavioural changes were paralleled by a reduction in choline acetyltransferase activity in hippocampus and several cortical areas (up to 97%) and selective depletions of NGF receptor-positive cholinergic neurons in the septal-diagonal band area and nucleus basalis magnocellularis (up to 99%). By contrast, the non-cholinergic parvalbumin-containing neurons in the septum were completely spared, and other cholinergic projection systems (such as in the striatum, thalamus, brainstem and spinal cord) were unaffected even at the highest dose. The observed changes in the water maze and passive avoidance tasks, as well as the cholinergic cell loss, were maintained up to at least 8 months following the intraventricular injection of a single dose (5 μg) of the immunotoxin. The results confirm the usefulness of the 192 IgG-saporin toxin for selective and profound lesions of the basal forebrain cholinergic neurons and provide further support for a role of the basal forebrain cholinergic system in cognitive functions.     

Attentional demands for demonstrating deficits following intrabasalis infusions of 192 IgG-saporin

Previous research has shown that basal forebrain cholinergic inputs to the cerebral cortex are necessary for attentional processing. However, the key components of attention-demanding tasks for demonstrating deficits following loss of basal forebrain corticopetal cholinergic neurons are unclear. In the present experiment, rats were trained in a visual cued discrimination task with limited explicit attentional demands and then received intrabasalis infusions of the immunotoxin, 192 IgG-saporin, or saline. Postsurgically, attentional demands were increased by decreasing the signal duration or the intertrial interval or by increasing the variability of these parameters. Subsequently, rats were trained in a task that required discrimination of successively presented signals and "blank" trials with no signal presentation. Again, attentional demands were increased by manipulating signal duration or the intertrial interval. Finally, all rats were trained in a task with both the signal duration and the intertrial interval designed to increase attentional demands. Compared to sham-lesioned animals, lesioned animals exhibited deficits in signal detection only during the successive discrimination task with both the signal duration and intertrial interval shorter and variable. The present data suggest that attentional deficits following loss of cortical cholinergic inputs result from overall attentional task demands rather than being dependent on any single task parameter.     

Galanin microinjected into the medial septum inhibits scopolamine-induced acetylcholine overflow in the rat ventral hippocampus

Galanin-like immunoreactive terminals hyperinnervate the basal forebrain cholinergic neurons in Alzheimer's disease. To investigate the hypothesis that galanin acts directly on basal forebrain cell bodies, in vivo microdialysis studies were conducted in awake rats which analyzed the actions of galanin on acetylcholine release. Microinjection of galanin into the cholinergic cell body region of the medial septum-diagonal band (MS-DBB) inhibited acetylcholine release in the ventral hippocampus. These results are consistent with an interpretation that galanin terminals synapsing on cholinergic cell bodies of the basal forebrain may serve to inhibit the release of acetylcholine in the terminal fields of the cholinergic neurons.     

Age-related shrinkage of cortically projecting cholinergic neurons: A selective effect

The number and size of basal forebrain neurons that provide the cholinergic innervation for the cerebral cortex, amygdala, and hippocampus were studied in young and aged mice. The results showed that these neurons became substantially smaller with increasing age. This effect was relatively selective, since the immediately adjacent cholinergic neurons in the striatum did not show a change of similar magnitude. The shrinkage of these basal forebrain neurons may account for the decline of cholinergic innervation that occurs with age. In the material that we examined, aging did not influence the number of cholinergic neurons in the basal forebrain, only their size. It seems, therefore, that the age-related changes in cholinergic function (and their putative behavioral consequences) are not associated with a substantial component of irreversible cell death.     

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.     

Selective deficits in attentional performance on the 5-choice serial reaction time task following pedunculopontine tegmental nucleus lesions

Sustained attention requires the integrity of basal forebrain cholinergic systems. The pedunculopontine tegmental nucleus (PPTg) has direct and indirect connections (via the thalamus) with the basal forebrain, suggesting that the PPTg may also play an important role in attentional processes. We examined this hypothesis by testing the effects of PPTg lesions in rats on performance in the 5-choice serial reaction time test. Bilateral lesions reduced accuracy, increased errors of omission, and increased the latency to correct responses. The deficits were more severe when neuronal damage was bilateral and concentrated in the posterior PPTg. Attentional demands of the task were increased by decreasing the stimulus duration, the stimulus brightness, or the inter-trial interval, and by introducing random bursts of white noise. These challenges impaired performance of all animals, but the magnitude of deficit was increased in the lesioned group. Conversely, lesion-induced deficits were partially alleviated when the attentional demands of the task were reduced. This pattern of results suggests that PPTg lesions produce a global deficit in attention, rather than a specific impairment in one process. The PPTg may control attentional processes through its direct projections to the forebrain cholinergic system or, indirectly, through activation of thalamocortical projections.     

Cholinergic basal forebrain is critical for social transmission of food preferences

Studies using selective lesions of basal forebrain cholinergic neurons suggest that these neurons play a role in attentional processing, but not learning and memory. However, the tests of learning and memory used thus far have been restricted largely to spatial tasks. In the present study, we examined whether the cholinergic basal forebrain plays a role in a form of nonspatial associative memory, the social transmission of food preferences. Sham-operated control rats were compared to rats with 192 IgG-saporin lesions of the medial septum/diagonal band cholinergic projections to hippocampus or nucleus basalis magnocellularis/substantia innominata cholinergic projections to neocortex. Both lesions impaired 24-h retention of a learned social food preference relative to controls, despite performance on an immediate retention trial that was indistinguishable from controls. Moreover, 24-h retention of the socially learned food preference correlated strongly with cholinergic enzymatic activity in the neocortex, but not in the hippocampus. Immunohistochemical data confirmed significant and selective lesion-induced cholinergic depletions in the intended brain regions. These data provide evidence that the cholinergic basal forebrain, particularly the cholinergic projection to neocortex, is involved in the formation and/or retrieval of social memories related to food preference, and suggest a role for cortical acetylcholine in consolidation of associative memory processes.     

Persistent innervation of the rat neocortex by basal forebrain cholinergic neurons despite the massive reduction of cortical target neurons. I. Morphometric analysis.

In Alzheimer's disease, a characteristic neurochemical abnormality is the loss of cholinergic enzymes in the neocortex, reflecting the degeneration of basal forebrain neurons responsible for cholinergic innervation of the neocortex. It is hypothesized that basal forebrain neuronal degeneration results from a reduction in the level of trophic factors synthesized by neurons in the target regions of cholinergic projections. Data from a large number of animal lesioning studies tend to support this theory; however, most of these lesions also induce widespread, nonspecific injury responses in the CNS. To directly test the dependence of basal forebrain cholinergic neurons on target-derived neurotrophic support, pregnant Sprague-Dawley rats were injected with moderate doses of methylazoxymethanol acetate (MAM) on Gestational Days 14 and 15. Extensive morphometric analysis of offspring reveals that the prenatal administration of MAM during this period of neurogenesis results in the ablation of 40-70% of cortical neurons, without significant effects on the hippocampus or the genesis of basal forebrain cholinergic neurons. Examination of MAM-treated animals at several ages reveals no significant differences in neuronal density of cholinergic neurons as compared to controls. Extensive analysis of animal brains at several time points has failed to reveal any evidence of classical injury responses which might be responsible for preservation of basal forebrain neurons. These results contradict the theory that mature basal forebrain cholinergic neurons are critically dependent on the availability of target-derived neurotrophic factors and are therefore unlikely to be the major etiological factor in basal forebrain neuronal degeneration characteristic of Alzheimer's disease.     

Neurons with galanin innervate cholinergic cells in the human basal forebrain and galanin and acetylcholine coexist

Immunocytochemistry with antibodies against cholinacetyltransferase (ChAT) and the novel peptide galanin (GA) were conducted as a single label or as double label experiments on the basal forebrain nuclei of brains from eleven human subjects without prior history of neurological disease. ChAT positive or cholinergic neurons form the major population of cells in the basal nucleus of Meynert. A minor portion of these ChAT positive neurons demonstrate a coexistence with GA positive immunoreactivity suggesting that they are cholinergic/GA neurons. Small fusiform neurons with long dendrites and complex local axonal networks are GA immunoreactive and are local circuit interneurons. The cholinergic cells in the basal nucleus are innervated by a fine network of GA immunoreactive axons and terminals which enwrap their perikarya and dendrites. It is suggested that GA in local circuit interneurons may provide a significant control or modulation of the cholinergic neurons and of cholinergic functions within the basal nucleus. In human diseases which feature a destruction of the basal forebrain cholinergic neurons, surviving GA neurons may inhibit most remaining cholinergic neurons and their function, with severe consequences.     

Immunohistochemical detection of a monoclonal antibody directed against the NGF receptor in basal forebrain neurons following intraventricular injection.

It has been shown by autoradiography that, following intraventricular administration, a monoclonal antibody directed against the rat nerve growth factor (NGF) receptor is specifically accumulated bilaterally by numerous cholinergic neurons of the basal forebrain. This is consistent with the evidence that cholinergic basal forebrain neurons have NGF receptors and respond to NGF under a variety of experimental conditions. The present study demonstrates that the immunohistochemical detection of unmodified monoclonal antibody in cholinergic forebrain neurons following transport from CSF is feasible, although injection of larger amounts of the antibody is required to obtain an image equivalent to the one obtained with the autoradiographic method. The location of the immunohistochemical product clearly indicates that the antibody has been internalized, probably in an endosomal compartment.     

Selective induction of Fos and FRA immunoreactivity within the mesolimbic and mesostriatal dopamine terminal fields

The influence of the mesencephalic dopaminergic projections on the neurons within the basal forebrain and prefrontal cortex is not well understood although it has been intensely investigated. The purpose of this study was to evaluate the expression of Fos-like and FRA-like (Fos Related Antigens) immunoreactivity (IR) as a qualitative and quantitative marker of neuronal activity within the mesolimbic and mesostriatal dopamine terminal fields. Following the administration of apomorphine (5.0 mg/kg S.C.), a rapid increase in FRA-IR, accompanied by Fos-IR, was observed within the striatum in a patchy distribution. Apomorphine also induced the expression of FRA-IR within the nucleus accumbens, cortex, septum, and the islands of Calleja complex. This broad pattern of activation contrasts with the limited expression of Fos-IR and FRA-IR within the dorsolateral striatum, dorsomedial shell of the nucleus accumbens, and cingulate cortex following haloperidol administration (2.0 mg/kg, S.C.). Finally, it was observed that nuclei expressed Fos-IR rapidly and transiently within the striatum following haloperidol, whereas the number of FRA-IR nuclei remained elevated but changed in distribution and intensity over time. In conclusion, different regions within the dopamine terminal fields express varying concentrations of Fos-IR and FRA-IR after stimulation or blockade of dopamine receptors. These data indicate that Fos, as well as selective FRAs, can be used to delineate populations of neurons with altered metabolic activity resulting from the administration of dopaminergic agents. Furthermore, the data support the concept of segregated mesostriatal and mesolimbic projections, in particular the divison of the nucleus accumbens into the shell and core compartments. © 1993 Wiley-Liss, Inc.