Trans-synaptic stimulation of cortical acetylcholine release after partial 192 IgG-saporin-induced loss of cortical cholinergic afferents

Age-related changes in basal forebrain afferent activation in response to food paired stimuli

The basal forebrain contains a phenotypically-diverse assembly of neurons, including those using acetylcholine as their neurotransmitter. This basal forebrain cholinergic system projects to the entire neocortical mantle as well as subcortical limbic structures that include the hippocampus and amygdala. Basal forebrain pathology, including cholinergic dysfunction, is thought to underlie the cognitive impairments associated with several age-related neurodegenerative conditions, including Alzheimer's disease. Basal forebrain dysfunction may stem, in part, from a failure of normal afferent regulation of cholinergic and other neurons in this area. However, little is understood regarding how aging, alone, affects the functional regulation of basal forebrain afferents in the context of motivated behavior. Here, we used neuronal tract-tracing combined with motivationally salient stimuli in an aged rodent model to examine how aging alters activity in basal forebrain inputs arising from several cortical, limbic and brainstem structures. Young rats showed greater stimulus-associated activation of basal forebrain inputs arising from prelimbic cortex, nucleus accumbens and the ventral tegmental area compared with aged rats. Aged rats also showed increased latency to respond to palatable food presentation compared to young animals. Changes in activation of intrinsic basal forebrain cell populations or afferents were also observed as a function of age or experimental condition. These data further demonstrate that age-related changes in basal forebrain activation and related behavioral and cognitive functions reflect a failure of afferent regulation of this important brain region.

Rescuing the attentional performance of rats with cholinergic losses by the M1 positive allosteric modulator TAK-071

RATIONALE

Loss of basal forebrain cholinergic neurons contributes to the severity of the cognitive decline in age-related dementia and, in patients with Parkinson's disease (PD), to impairments in gait and balance and the resulting risks for falls. Contrasting with the extensive evidence indicating an essential role of cholinergic activity in mediating cognitive, specifically attentional abilities, treatment with conventional acetylcholinesterase inhibitors (AChEIs) has not fulfilled the promise of efficacy of pro-cholinergic treatments.

OBJECTIVES

Here, we investigated the potential usefulness of a muscarinic M1 positive allosteric modulator (PAM) in an animal model of cholinergic loss-induced impairments in attentional performance. Given evidence indicating that fast, transient cholinergic signaling mediates the detection of cues in attentional contexts, we hypothesized that a M1 PAM amplifies such transient signaling and thereby rescues attentional performance.

RESULTS

Rats performed an operant sustained attention task (SAT), including in the presence of a distractor (dSAT) and during a post-distractor (post-dSAT) period. The post-dSAT period served to assess the capacity for recovering performance following a disruptive event. Basal forebrain infusions of the cholino-specific immunotoxin 192 IgG-saporin impaired SAT performance, and greater cholinergic losses predicted lower post-dSAT performance. Administration of TAK-071 (0.1, 0.3 mg/kg, p.o., administered over 6-day blocks) improved the performance of all rats during the post-dSAT period (main effect of dose). Drug-induced improvement of post-dSAT performance was relatively greater in lesioned rats, irrespective of sex, but also manifested in female control rats. TAK-071 primarily improved perceptual sensitivity (d') in lesioned rats and facilitated the adoption of a more liberal response bias (B˝_D) in all female rats.

CONCLUSIONS

These findings suggest that TAK-071 may benefit the attentional performance of patients with partial cholinergic losses and specifically in situations that tax top-down, or goal-driven, attentional control.

Upregulation of orexin/hypocretin expression in aged rats: Effects on feeding latency and neurotransmission in the insular cortex

Aging is associated with changes in numerous homeostatic functions, such as food intake, that are thought to be mediated by the hypothalamus. Orexin/hypocretin neurons of the hypothalamus regulate several physiological functions, including feeding, sleep and wakefulness. Evidence from both clinical and animal studies supports the notion that aging is associated with loss or dysregulation of the orexin system. Here, we used virus-mediated gene transfer to manipulate expression of orexin peptides in young and aged rats and examined behavioral and neurochemical correlates of food intake in these animals. Aged rats showed slower feeding latencies when presented with palatable food compared to young control rats, and these deficits were ameliorated by upregulation of orexin expression. Similarly, young animals treated with a virus designed to decrease preproorexin expression showed longer feeding latencies reminiscent of aged control rats. Feeding was also associated with increased acetylcholine, glutamate and GABA efflux in insular cortex of young control animals. Orexin upregulation did not restore deficits in feeding-elicited release of these neurotransmitters in aged rats, but did enhance basal neurotransmitter levels which may have contributed to the behavioral correlates of these genetic manipulations. These studies demonstrate that age-related deficits in behavioral and neurochemical measures of feeding are likely to be mediated, in part, by the orexin system. Because these same neurotransmitter systems have been shown to underlie orexin effects on cognition, treatments which increase orexin function may have potential for improving both physiological and cognitive manifestations of certain age-related disorders.

Basal Forebrain Cholinergic System and Memory

Basal forebrain cholinergic neurons constitute a way station for many ascending and descending pathways. These cholinergic neurons have a role in eliciting cortical activation and arousal. It is well established that they are mainly involved in cognitive processes requiring increased levels of arousal, attentive states and/or cortical activation with desynchronized activity in the EEG. These cholinergic neurons are modulated by several afferents of different neurotransmitter systems. Of particular importance within the cortical targets of basal forebrain neurons is the hippocampal cortex. The septohippocampal pathway is a bidirectional pathway constituting the main septal efferent system, which is widely known to be implicated in every memory process investigated. The present work aims to review the main neurotransmitter systems involved in modulating cognitive processes related to learning and memory through modulation of basal forebrain neurons.

Role of orexin/hypocretin in conditioned sucrose-seeking in female rats

The orexin/hypocretin system has recently been implicated in reward-seeking, especially for highly salient food and drug rewards. Given that eating disorders affect women more than men, we reasoned that the orexin system may be strongly engaged in female rats, and during periods of food restriction as we recently reported in male rats. Therefore, the present study examined the involvement of the orexin system in operant responding for sucrose, and in cue-induced reinstatement of extinguished sucrose-seeking, in ad libitum fed vs. food-restricted female subjects. Female Sprague Dawley rats were trained to self-administer sucrose pellets, and we determined the effects of pretreatment with the OxR1 receptor antagonist SB 334867 (SB; 10-30 mg/kg) on fixed ratio (FR) sucrose self-administration, and on cue-induced reinstatement of extinguished sucrose-seeking. SB decreased sucrose self-administration in food-restricted but not in ad libitum-fed females. SB did not alter active lever responding during cue-induced reinstatement of sucrose-seeking in either feeding group. These results confirm our previous results in male rats that signaling at the OxR1 receptor is involved in the sucrose reinforcement and self-administration in food-restricted subjects. However, the finding that SB is ineffective at attenuating cue-induced reinstatement in females, but was effective in food-restricted males, leads us to conclude that food seeking induced by conditioned stimuli engages the orexin system differentially in males and females.

Attenuation of saccharin-seeking in rats by orexin/hypocretin receptor 1 antagonist

RATIONALE

The orexin (Orx)/hypocretin system has been implicated in reward-seeking, especially for highly salient food and drug rewards. We recently demonstrated that signaling at the OxR1 receptor is involved in sucrose reinforcement and reinstatement of sucrose-seeking elicited by sucrose-paired cues in food-restricted rats. Because sucrose reinforcement has both a hedonic and caloric component, it remains unknown what aspect of this reward drives its reinforcing value.

OBJECTIVES

The present study examined the involvement of the Orx system in operant responding for saccharin, a noncaloric, hedonic (sweet) reward, and in cue-induced reinstatement of extinguished saccharin-seeking in ad libitum-fed vs food-restricted male subjects.

METHODS

Male Sprague Dawley rats were fed ad libitum or food-restricted and trained to self-administer saccharin. We determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) saccharin self-administration and on cue-induced reinstatement of extinguished saccharin-seeking.

RESULTS

SB decreased responding and number of reinforcers earned during FR responding for saccharin and decreased cue-induced reinstatement of extinguished saccharin-seeking. All of these effects were obtained similarly in food-restricted and ad libitum-fed rats.

CONCLUSIONS

These results indicate that signaling at the OxR1 receptor is involved in saccharin reinforcement and reinstatement of saccharin-seeking elicited by saccharin-paired cues regardless of food restriction. These findings lead us to conclude that the Orx system contributes to the motivational effects of hedonic food rewards, independently of caloric value and homeostatic needs.

Role of orexin/hypocretin in conditioned sucrose-seeking in rats

RATIONALE

The orexin/hypocretin system has recently been implicated in reward-seeking, especially for highly salient food and drug rewards. We reasoned that this system may be strongly engaged during periods of reward restriction, including food restriction.

OBJECTIVES

This study examined the involvement of the orexin (Orx) system in responding for sucrose, and in cue-induced reinstatement of extinguished sucrose-seeking, in ad libitum fed versus food-restricted male subjects.

METHODS

Sprague-Dawley rats (n = 108) were trained to self-administer sucrose, and we determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) or progressive ratio (PR) sucrose self-administration, as well as on cue-induced reinstatement of sucrose-seeking. Finally, expression of the immediate early gene c-fos in Orx neurons was examined after self-administration, late extinction or cue-induced reinstatement of sucrose seeking.

RESULTS

SB decreased lever responding (by about 1/3) and the number of reinforcers earned during FR, and less so during PR, schedules and decreased cue-induced reinstatement to sucrose-seeking to extinction levels, predominately in food-restricted rats. Additionally, Fos expression in Orx neurons in perifornical and dorsomedial hypothalamus was increased during extinction.

CONCLUSIONS

These results indicate that signaling at the OxR1 receptor is involved in pronounced sucrose reinforcement, and reinstatement of sucrose-seeking elicited by sucrose-paired cues, in food-restricted subjects. These findings lead us to conclude that conditioned activation of Orx neurons increases motivation for food reward during food restriction.

Role of orexin/hypocretin in reward-seeking and addiction: implications for obesity

Orexins (also named hypocretins) are recently discovered neuropeptides made exclusively in the hypothalamus. Recent studies have shown that orexin cells located specifically in lateral hypothalamus (LH) are involved in motivated behavior for drugs of abuse as well as natural rewards. Administration of orexin has been shown to stimulate food consumption, and orexin signaling in VTA has been implicated in intake of high-fat food. In self-administration studies, the orexin 1 receptor antagonist SB-334867 (SB) attenuated operant responding for high-fat pellets, sucrose pellets and ethanol, but not cocaine, demonstrating that signaling at orexin receptors is necessary for reinforcement of specific rewards. The orexin system is also implicated in associations between rewards and relevant stimuli. For example, Fos expression in LH orexin neurons varied in proportion to conditioned place preference (CPP) for food, morphine, or cocaine. This Fos expression was altered accordingly for CPP administered during protracted abstinence from morphine or cocaine, when preference for natural rewards was decreased and drug preference was increased. Additionally, orexin has been shown to be involved in reward-stimulus associations in the self-administration paradigm, where SB attenuated cue-induced reinstatement of extinguished sucrose- or cocaine-seeking. Although the specific circuitry mediating the effects of orexin on food reward remains unknown, VTA seems likely to be a critical target for at least some of these orexin actions. Thus, recent studies have established a role for orexin in reward-based feeding, and further investigation is warranted for determining whether function/dysfunction of the orexin system may contribute to the overeating associated with obesity. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

Food-elicited increases in cortical acetylcholine release require orexin transmission

The corticopetal basal forebrain cholinergic system (BFCS) is crucial for normal attentional function and cortical acetylcholine release is increased by stimuli with high motivational salience. Projections from the lateral hypothalamus to the basal forebrain have been previously described and have been hypothesized to relay interoceptive information to this area but little is known about the phenotypic and functional nature of hypothalamic modulation of the BFCS. We have previously shown that orexin (hypocretin) fibers from the hypothalamus distribute densely among basal forebrain choline acetyltransferase-positive neurons and that intrabasalis administration of orexin A increases cortical acetylcholine release. Here, we used in vivo microdialysis to test the hypothesis that the orexin system is necessary for activation of the BFCS in response to a food-related stimulus in food-restricted rats. Elimination of the majority of orexin neurons with the toxin orexin B-saporin significantly blunted the cholinergic response to presentation of palatable food in these animals. Similar effects were seen in animals acutely pretreated with the orexin 1 receptor antagonist, SB-334867, which also increased feeding latency. Collectively, these data suggest that orexin interactions with the BFCS may be a critical component of the neurobiological substrates by which interoceptive cues bias the allocation of attentional resources toward exteroceptive stimuli related to homeostatic challenges.

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.

Evaluation of cholinergic markers in Alzheimer's disease and in a model of cholinergic deficit

Cognitive deficits in neuropsychiatric disorders, such as Alzheimer's disease (AD), have been closely related to cholinergic deficits. We have compared different markers of cholinergic function to assess the best biomarker of cognitive deficits associated to cholinergic hypoactivity. In post-mortem frontal cortex from AD patients, acetylcholine (ACh) levels, cholinacetyltransferase (ChAT) and acetylcholinesterase (AChE) activity were all reduced compared to controls. Both ChAT and AChE activity showed a significant correlation with cognitive deficits. In the frontal cortex of rats with a selective cholinergic lesion, all cholinergic parameters measured (ACh levels, ChAT and AChE activities, "in vitro" and "in vivo" basal ACh release) were significantly reduced. AChE activity was associated to ChAT activity, and even more, to "in vivo" and "in vitro" basal ACh release. Quantification of AChE activity is performed by an easy and cheap method and therefore, these results suggest that determination of AChE activity may be used as an effective first step method to evaluate cholinergic deficits.

Facilitation of cholinergic transmission by combined treatment of ondansetron with flumazenil after cortical cholinergic deafferentation

We have studied the effects of concomitant blockade of 5-HT(3) and GABA(A) receptors on acetylcholine (ACh) release in the frontal cortex of rats with a selective cholinergic lesion. Lesions were performed by microinjection of the cholinergic toxin 192 IgG-saporin into the nucleus basalis magnocellularis. Single treatment with either the 5-HT(3) receptor antagonist ondansetron, 0.1 microg/kg, or the GABA(A) receptor benzodiazepine site antagonist flumazenil, 10 mg/kg, did not affect ACh release. However, the combined ondansetron + flumazenil administration significantly increased ACh release to a similar extent as a depolarising stimulus with K(+), 100 mM, at both 7 and 30 days post-lesion. Cortical perfusion with the combined ondansetron + flumazenil treatment also increased [(3)H]ACh efflux "in vitro" 30 days after lesion, suggesting that local events within the frontal cortex may participate in the interaction of ondansetron with GABAergic neurons, modulating ACh release in situations of cholinergic hypoactivity. No differences in the expression of 5-HT(3) and GABA(A) receptors in the frontal cortex were found after the cholinergic lesion. These results suggest that a combined ondansetron + flumazenil treatment would contribute to restoring a diminished cholinergic function and may provide a basis for using this treatment in the therapy of cognitive disorders associated with degeneration of the cholinergic system.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Selective behavioral and neurochemical effects of cholinergic lesions produced by intrabasalis infusions of 192 IgG-saporin on attentional performance in a five-choice serial reaction time task

The effects of the cholinergic immunotoxin 192 IgG-saporin (SAP) (0.0, 0.15, or 0.45 microg/microl; 0.5 microl/hemisphere) infused into the area of the nucleus basalis magnocellularis (NBM) of rats were tested in a five-choice serial reaction time task (5CSRTT) designed to assess visual attention. The effects of this manipulation on acetylcholine efflux in the medial frontal cortex were determined using in vivo microdialysis during the 5CSRTT. Rats with extensive lesions of the NBM (SAP HIGH) showed an array of behavioral deficits in the 5CSRTT hypothesized to represent deficits in central executive function that were associated with severe deficits in accuracy. Lengthening the stimulus duration ameliorated these deficits. Rats with restricted lesions of the NBM (SAP LOW) showed impairments over time on task when tested under standard conditions that were exacerbated by increases in the event rate. The number of choline acetyltransferase-immunoreactive cells in the area of the NBM but not the vertical limb of the diagonal band correlated significantly with accuracy in the task. SAP HIGH rats had significantly lower levels of cortical acetylcholine (ACh) efflux relative to SHAM both before and during the 5CSRTT. SAP LOW rats showed significantly higher levels of cortical ACh efflux before but not during the 5CSRTT. Cortical ACh efflux increased in all rats with the onset of the attentional task. These data provide the first direct evidence for a relationship between selective damage in the basal forebrain with decreased cortical ACh efflux and impaired attentional function.

Basal forebrain glutamatergic modulation of cortical acetylcholine release

The mediation of cortical ACh release by basal forebrain glutamate receptors was studied in awake rats fitted with microdialysis probes in medial prefrontal cortex and ipsilateral basal forebrain. Repeated presentation of a stimulus consisting of exposure to darkness with the opportunity to consume a sweetened cereal resulted in a transient increase in cortical ACh efflux. This stimulated release was dependent on basal forebrain glutamate receptor activity as intrabasalis perfusion with the ionotropic glutamate receptor antagonist kynurenate (1.0 mM) markedly attenuated darkness/cereal-induced ACh release. Activation of AMPA/kainate receptors by intrabasalis perfusion of kainate (100 microM) was sufficient to increase cortical ACh efflux even under basal (nonstimulated) conditions. This effect of kainate was blocked by coperfusion with the antagonist DNQX (0.1-5.0 mM). Stimulation of NMDA receptors with intrabasalis perfusion of NMDA (50 or 200 microM) did not increase basal cortical ACh efflux. However, perfusion of NMDA in rats following exposure to the darkness/cereal stimulus resulted in a potentiation of both the magnitude and duration of stimulated cortical ACh efflux. Moreover, intrabasalis perfusion of the higher dose of NMDA resulted in a rapid increase in cortical ACh efflux even before presentation of the darkness/cereal stimulus, suggesting an anticipatory change in the excitability of basal forebrain cholinergic neurons. These data demonstrate that basal forebrain glutamate receptors contribute to the stimulation of cortical ACh efflux in response to behavioral stimuli. The specific roles of basal forebrain glutamate receptor subtypes in mediating cortical ACh release differ and depend on the level of activity of basal forebrain cholinergic neurons.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Systemic and intra-accumbens administration of amphetamine differentially affects cortical acetylcholine release

The present experiments tested the hypothesis that the amphetamine-induced increase in dopamine release in the nucleus accumbens represents a necessary and sufficient component of the ability of systemically administered amphetamine to stimulate cortical acetylcholine release. The effects of systemic or intra-accumbens administration of amphetamine on accumbens dopamine release and cortical acetylcholine release were assessed simultaneously in awake animals equipped with dialysis probes inserted into the shell of the nucleus accumbens and the medial prefrontal cortex. Additionally, the ability of intra-accumbens administration of dopamine D(1) and D(2) receptor antagonists to attenuate the effects of systemic amphetamine on cortical acetylcholine was tested. The effects of all treatments were assessed in interaction with a stimulus-induced activation of cortical acetylcholine release to account for the possibility that the demonstration of the trans-synaptic effects of accumbens dopamine requires pre-activation of basal forebrain circuits. Systemic amphetamine resulted in increases in basal cortical acetylcholine and accumbens dopamine efflux. Intra-accumbens administration of amphetamine substantially increased accumbens dopamine efflux, but did not significantly affect cortical acetylcholine efflux. Furthermore, intra-accumbens administration of sulpiride or SCH 23390 did not attenuate the systemic amphetamine-induced increase in cortical acetylcholine efflux. Collectively, the present data suggest that increases in accumbens dopamine release are neither sufficient nor necessary for the effects of systemically administered amphetamine on cortical acetylcholine release. The systemic amphetamine-induced increase in cortical acetylcholine may be mediated via multiple, parallel pathways and may not be attributable to a single afferent pathway of the basal forebrain.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

Role of accumbens and cortical dopamine receptors in the regulation of cortical acetylcholine release

Cortical acetylcholine, under resting and stimulated conditions, was measured in frontoparietal and prefrontal cortex using in vivo microdialysis in freely-moving rats. Cortical acetylcholine efflux was stimulated by systemic administration of the benzodiazepine receptor partial inverse agonist FG 7142. Administration of FG 7142 (8.0 mg/kg; i.p.) significantly elevated acetylcholine efflux in both cortical regions (150-250% relative to baseline) for 30 min after drug administration. The ability of endogenous dopamine to regulate cortical acetylcholine efflux under resting or stimulated conditions and the relative contributions of D1- and D2-like dopamine receptor activation was also assessed. In a first series of experiments, systemic administration of the antipsychotic drug haloperidol (0.15, 0.9 mg/kg, i.p.) blocked FG 7142-stimulated acetylcholine efflux in frontoparietal, cortex while the D1-like antagonist, SCH 23390 (0.1, 0.3 mg/kg), was less effective in attenuating stimulated acetylcholine efflux. In a second series of experiments, the effects of infusions of these antagonists and of the D2-like antagonist sulpiride (10, 100 microM) into the nucleus accumbens were assessed. Infusions of haloperidol and sulpiride significantly blocked FG 7142-stimulated acetylcholine efflux while SCH 23390 did not. By contrast, a third series of experiments demonstrated that perfusion of these antagonists (100 microM) locally into the cortex (through the probe) did not affect FG 7142-stimulated acetylcholine efflux. Moreover, none of these dopamine receptor antagonists, whether administered systemically or perfused into the nucleus accumbens or cortex, affected basal cortical acetylcholine efflux. These results reveal similarities in stimulated cortical acetylcholine release across frontal cortical regions and suggest a prominent role for D2-mediated accumbens dopamine transmission in the regulation of cortical acetylcholine release. The findings provide evidence in support of a neural substrate that links dysregulation of mesolimbic dopaminergic transmission to changes in cortical cholinergic transmission. Dysregulation within this circuit is hypothesized to contribute to the etiology of disorders such as schizophrenia, dementia and drug abuse.

Age-related attenuation of stimulated cortical acetylcholine release in basal forebrain-lesioned rats

In vivo microdialysis was used to measure the effects of partial deafferentation of cortical cholinergic inputs on acetylcholine efflux in young (four to seven months) and aged (24-28 months) male F344/BNNIA rats. Partial deafferentation was produced by bilateral infusions of the immunotoxin 192 immunoglobulin G-saporin (0.56 microg/1.0 microl) or its vehicle solution into the ventral pallidum/substantia innominata region of the basal forebrain. The lesion produced comparable (65%) decreases in basal cortical acetylcholine efflux in young and aged rats. Presentation of a complex environmental stimulus (exposure to darkness/palatable food), in conjunction with the systemic administration of the benzodiazepine receptor weak inverse agonist ZK 93 426, increased cortical acetylcholine efflux in young shams, aged shams and young lesioned rats, but not in aged lesioned rats. Administration of the benzodiazepine receptor partial inverse agonist FG 7142, in the absence of the environmental stimulus, comparably stimulated cortical acetylcholine efflux in young and aged sham rats. FG 7142-induced increases in acetylcholine efflux were attenuated by approximately 50% following partial deafferentation in both young and aged rats. These results suggests that, under certain conditions, ageing potently interacts with the integrity of the cortical cholinergic afferent system. The effects of ageing on cortical cholinergic function may be most potently revealed by experiments assessing age-related limitations in the responsiveness of a partially deafferented cholinergic system to certain behavioral and/or pharmacological stimuli.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

Effects of local cholinesterase inhibition on acetylcholine release assessed simultaneously in prefrontal and frontoparietal cortex

To investigate whether acetylcholine is released in a similar fashion in different regions of the cortex, in vivo microdialysis was used to measure acetylcholine efflux simultaneously in the medial prefrontal and the frontoparietal cortex, under both basal conditions and following tactile stimulation. Additionally, the effects of including two different concentrations (0.05 microM and 0.5 microM) of a cholinesterase inhibitor (neostigmine) in the perfusion fluid were assessed. Basal levels of acetylcholine (i.e. during non-stimulated sessions) were similar in medial prefrontal and frontoparietal areas. Tactile stimulation reliably increased acetylcholine efflux in a similar fashion (up to 140% increase above baseline) in both cortical areas studied. Predictably, the higher concentration of neostigmine (0.5 microM) increased basal acetylcholine efflux by about 150% from levels observed with the lower neostigmine concentration (0.05 microM), but the concentration of local neostigmine had no effect on either the magnitude or the duration of the increased acetylcholine efflux following tactile stimulation. These results suggest that the pattern of acetylcholine release may be comparable in different areas of the cortex, supporting the idea that cholinergic projections from the basal forebrain to the cortex represent a globally regulated system. Furthermore, while the inclusion of neostigmine in perfusion fluid must be taken into account when interpreting acetylcholine efflux data, it appears that concentrations of up to 0.5 microM do not interfere fundamentally with the lability of cortical acetylcholine efflux in response to behavioural stimulation.

Removal of cholinergic input to rat posterior parietal cortex disrupts incremental processing of conditioned stimuli

Recent research suggests that the basal forebrain cholinergic neurons innervating the cortex play a role in attentional functions in both primates and rodents. Among the cortical targets of these projections in primates is the posterior parietal cortex (PPC), a region shown to be critically involved in the regulation of attention. Recent anatomical studies have defined a cortical region in the rat that may be homologous to the PPC of primates. In the present study, cholinergic innervation of the PPC was depleted by intracortical infusion of the immunotoxin 192 IgG-saporin. Control and lesioned rats were then tested in two associative learning paradigms designed to increase attentional processing of conditioned stimuli (CSs). In one experiment, attention was manipulated by shifting a predictive relation between a light CS and another CS to a less predictive relation. Unlike control rats, lesioned rats failed to increase attention when the predictive relation was modified. In a second experiment, attentional processing of a tone CS was increased when its introduction during training coincided with a change in the value of the unconditioned stimulus, a phenomenon referred to as unblocking. Unlike control rats, lesioned rats failed to exhibit unblocking. In both paradigms, lesioned rats conditioned normally when the training procedures did not encourage increased attentional processing. These findings, across different behavioral paradigms and stimulus modalities, provide converging evidence that intact cholinergic innervation of the PPC is important for changes in attention that can increase the processing of certain cues.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

Operant performance and cortical acetylcholine release: role of response rate, reward density, and non-contingent stimuli

The relationship between acetylcholine (ACh) efflux in medial prefrontal cortex (mPFC) and performance in a visual discrimination task and a variable interval (VI) schedule of reinforcement was studied in rats. Animals were pretrained in one of the two tasks and then unilaterally implanted with microdialysis guide cannula into the mPFC. Animals were then dialyzed, during 12 min collection intervals, in the operant chambers prior to task onset and during and after task performance. Each animal was dialyzed for a total of four sessions: two standard task sessions, one session in which a houselight was flashed at 0.5 Hz during the third 12 min block, and an extinction session (always the last session) in which reinforcement was withheld during the final three blocks. Response accuracy in the discrimination task was very high (> 95% correct) and stable across the four blocks with a progressive increase in omissions. The flashing houselight did not affect performance whereas the loss of reinforcement led to an increase in omissions. VI performance was associated with a high number of lever presses and a high reward rate that declined over the four blocks. Again, the flashing houselight did not affect VI performance whereas lever pressing declined markedly during the extinction session. ACh efflux did not change, relative to baseline, during performance in either task, or with the presentation of the flashing houselight or the loss of reinforcement. These data contrast with the changes in cortical ACh efflux observed in situations characterized by the presentation of novel stimuli or changing demands on attentional processing and, therefore, assist in the specification of hypotheses on the cognitive functions of cortical ACh.