Dissociation between the attentional effects of infusions of a benzodiazepine receptor agonist and an inverse agonist into the basal forebrain

Intra-accumbens infusions of antisense oligodeoxynucleotides to one isoform of glutamic acid decarboxylase mRNA, GAD65, but not to GAD67 mRNA, impairs sustained attention performance in the rat

The effects of bilateral infusions of antisense oligodeoxynucleotides (ODNs) for the two isoforms of glutamic acid decarboxylase (GAD65; GAD67) into the nucleus accumbens on the performance of intact rats in a task designed to assess sustained attention were tested. The task required the animals to discriminate between signal and non-signal events. Signals and non-signals were presented randomly and unpredictably. The task generated all four response types of a sustained attention task, i.e., hits, misses, correct rejections, false alarms. Infusions of the scrambled sequence ODNs did not affect performance. Likewise, infusions of the GAD67 ODNs failed to produce any effect. However, infusions of the GAD65 ODNs into the nucleus accumbens resulted in a robust and reliable decrease in the relative number of hits. Similarly, the combined infusion of GAD65+67 ODNs impaired the hit rate but did not affect the animals' ability to reject non-signals. Following each treatment series, performance rapidly returned to baseline, further indicating the specificity and reversibility of the effects of the infusions of the ODNs. While these data suggest that translation arrest of specifically the GAD65 isoform of the enzyme in the nucleus accumbens impairs attentional performance, the neuronal mechanisms mediating these effects remain unsettled.

Anxiety and cardiovascular reactivity: the basal forebrain cholinergic link

The relations between anxiety states and autonomic functions are considered from the vantage of a model of the neural systems underlying anxiety and autonomic control. An important component of this model is the involvement of the basal forebrain cortical cholinergic system that is seen to play a crucial role in the cognitive aspects of anxiety, and the links between anxiety and autonomic regulation. An additional aspect of the model is the detailing of the routes by which autonomic reactivity and associated visceral afference can modulate more rostral components of the system. The proposed model offers a more comprehensive framework for research on the neurobiology of anxiety and autonomic control.

Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

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.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Cognitive functions of cortical acetylcholine: toward a unifying hypothesis

Previous efforts aimed at attributing discrete behavioral functions to cortical cholinergic afferents have not resulted in a generally accepted hypothesis about the behavioral functions mediated by this system. Moreover, attempts to develop such a unifying hypothesis have been presumed to be unproductive considering the widespread innervation of the cortex by basal forebrain cholinergic neurons. In contrast to previous descriptions of the role of cortical acetylcholine (ACh) in specific behavioral phenomena (e.g., mediation of the behavioral effects of reward loss) or mnemonic entities (e.g., working or reference memory), cortical ACh is hypothesized to modulate the general efficacy of the cortical processing of sensory or associational information. Specifically, cortical cholinergic inputs mediate the subjects' abilities to detect and select stimuli and associations for extended processing and to allocate the appropriate processing resources to these functions. In addition to evidence from electrophysiological and behavioral studies on the role of cortical ACh in sensory information processing and attention, this hypothesis is consistent with proposed functions of the limbic and paralimbic networks in regulating the activity of the basal forebrain cholinergic neurons. Finally, while the proposed hypothesis implies that changes in activity in cortical ACh simultaneously occur throughout the cortex, the selectivity and precision of the functions of cholinergic function is due to its coordinated interactions with the activity of converging sensory or associational inputs. Finally, the dynamic, escalating consequences of alterations in the activity of cortical ACh (hypo- and hyperactivity) on cognitive functions are evaluated.

Trans-synaptic stimulation of cortical acetylcholine and enhancement of attentional functions: a rational approach for the development of cognition enhancers

Activation and restoration of cholinergic function remain major foci in the development of pharmacological approaches toward the treatment of cognitive dysfunctions associated with aging and dementia. Our research has been guided by the hypothesis that (re)activation of cortical cholinergic inputs is achieved as a result of trans-synaptic disinhibition of basal forebrain cholinergic neurons. This approach depends on the ability of benzodiazepine receptor (BZR) inverse agonists to reduce the potency of GABA to block neuronal excitation. BZR inverse agonists were found to augment cortical ACh efflux through interaction with cognition-associated activation of this system. Cortical cholinergic inputs have been implicated in the processing of behaviorally significant stimuli, i.e., attentional functions. Using a recently developed and validated task for the measurement of sustained attention, or vigilance, administration of BZR inverse agonists were found to selectively increase the number of false alarms in intact animals. However, in animals with a 50-70%, but not > 90%, loss of the cortical cholinergic inputs, treatment with BZR inverse agonists alleviated the lesion-induced impairment in sustained attention and enhanced activated cortical ACh efflux. A rational development of cognitive enhancers will benefit from experiments in which cognitive and neuropharmacological variables are assessed simultaneously, thus allowing the analysis of interactions between cognition-associated neuronal activity and the neuronal and cognitive effects of putative cognition enhancers.

Effects of ibotenic acid-induced loss of neurons in the medial prefrontal cortex of rats on behavioral vigilance: evidence for executive dysfunction

Rats were trained in a previously validated task for the assessment of sustained attention, or vigilance. This task required the animals to discriminate between signals of variable lengths and non-signal events by making an appropriate lever press. The performance of sham-lesioned animals in this task was characterized by a signal-length dependent number of hits. Also, approximately 70 percent of the non-signals were correctly rejected. Ibotenic acid-induced lesions of the medial prefrontal cortex decreased the relative number of hits and correct rejections and, in essence, resulted in random lever selection. The lesion did not affect the number of omissions or side bias. Furthermore, the performance of lesioned animals was insensitive to the detrimental effects of distractors. The effects of the lesions do not support an interpretation in terms of sustained attention. Rather, the pattern of the lesioned animals' performance is speculated to reveal a fundamental disruption of decisional processes, reminiscent of the executive dysfunctions observed in patients with damage to ventromedial prefrontal areas or with schizophrenia.

Central cholinergic systems and cognition

The organization and possible functions of basal forebrain and pontine cholinergic systems are reviewed. Whereas the basal forebrain cholinergic neuronal projections likely subserve a common electrophysiological function, e.g. to boost signal-to-noise ratios in cortical target areas, this function has different effects on psychological processes dependent upon the neural network operations within these various cortical domains. Evidence is presented that (a) the nucleus basalis-neocortical cholinergic system contributes greatly to visual attentional function, but not to mnemonic processes per se; (b) the septohippocampal projection is involved in the modulation of short-term spatial (working) memory processes, perhaps by prolonging the neural representation of external stimuli within the hippocampus; and (c) the diagonal band-cingulate cortex cholinergic projection impacts on the ability to utilize response rules through conditional discrimination. We also suggest that nucleus basalis-amygdala cholinergic projections have a role in the retention of affective conditioning while brainstem cholinergic projections to the thalamus and midbrain dopamine neurons affect basic arousal processes (e.g. sleep-wake cycle) and behavioral activation, respectively. The possibilities and limitations of therapeutic interventions with procholinergic drugs in patients with Alzheimer's disease and other neurodegenerative disorders in which basal forebrain cholinergic neurons degenerate are also discussed.

Behavioural vigilance in schizophrenia. Evidence for hyperattentional processing

BACKGROUND

Despite 30 years of research, some surprisingly fundamental gaps remain in our understanding of schizophrenic input dysfunctions.

METHOD

In a provisional test of a 'hyperattention' hypothesis, schizophrenic patients and control subjects performed a behavioural test that was adapted from a paradigm originally developed for characterising vigilance or sustained attention in animals. On this computerised operant testing procedure, subjects discriminated between signals of various salience and non-signal presentations. Hits and correct rejections resulted in monetary rewards while misses and false alarms entailed monetary costs.

RESULTS

Data from in-patients with schizophrenia and age, education and gender-matched controls support hypotheses not only about hyperattentional dysfunctions in schizophrenia with respect to overall signal detectability but also in terms of resistance to the vigilance decrement that normally occurs over trials.

CONCLUSIONS

The theoretical importance of impairments of this sort are discussed with respect to the cognitive and perceptual consequences of hypervigilance and 'input dysfunction'.

Neuronal mechanisms mediating drug-induced cognition enhancement: cognitive activity as a necessary intervening variable

The conceptual foundations of a research aimed at the determination of potential neuronal, neuropharmacological, and behavioral/cognitive mechanisms mediating drug-induced cognition enhancement are discussed. The available evidence justifies a focus on attentional processes as a target for drug-induced cognition enhancement. Neuropharmacological mechanisms that may mediate drug-induced enhancement of attentional functions are proposed to interact necessarily with attention-associated neuronal activity. The elements of a transsynaptic approach to increase the excitability of basal forebrain cholinergic neurons and hence, attentional functions are discussed. Experimental tests of this hypothesis require the demonstration of interactions between cognition-induced increases in the activity of cortical cholinergic afferents and the effects of putative cognition enhancers. The available data illustrate that the effects of benzodiazepine receptor (BZR) agonists and inverse agonists on cortical acetylcholine (ACh) efflux interact with the state of activity in this system. The feasibility, potential heuristic power, and the experimental and conceptual problems of studies attempting to simultaneously assess drug effects on behavioral/cognitive abilities, ACh efflux, and neuronal activity have been revealed by an experiment intended to correlate performance in a task measuring sustained attention with medial prefrontal ACh efflux and medial prefrontal single-unit activity. The rational development of a psychopharmacology of cognition enhancers requires a union among behavioral/cognitive pharmacology, neuropharmacological and electrophysiological approaches.

A central cholinergic link in the cardiovascular effects of the benzodiazepine receptor partial inverse agonist FG 7142

Previous work demonstrated that systemic administration of the benzodiazepine receptor (BZR) partial inverse agonist beta-carboline FG 7142 (FG) augments the cardiovascular response to non-signal stimuli, similar to the effects of an aversive context. Analysis of the parasympathetic and sympathetic contributions to the effects of FG prompted the hypothesis that increases in central cholinergic activity mediates the potentiation of the cardioacceleratory response by FG. Consistent with this hypothesis, the present experiments demonstrate: (a) intracerebroventricular (ICV) infusion of the cholinergic receptor agonist carbachol mimics the response-potentiating effects of FG; (b) this effect of carbachol was blocked by ICV co-administration of the muscarinic antagonist atropine; (c) ICV infusions of atropine blocked the potentiation of the cardioacceleratory response by systemically administered FG, but did not alter the basal response to the stimulus; and (d) 192 IgG-saporin-induced lesions of basal forebrain cholinergic neurons prevented the FG-induced potentiation of the cardioacceleratory response, again without altering the basal cardiac response. These data strongly support the hypothesis that the effects of FG on cardiac reactivity are mediated via an activation of central muscarinic cholinergic mechanisms.

Benzodiazepine receptor agonists and inverse agonists yield concordant rather than opposing effects on startle responses

Benzodiazepine receptor agonists and inverse agonists exert generally opposite actions at both the cellular and behavioural levels. The present study, however, reveals that both the benzodiazepine receptor agonist, chlordiazepoxide and the partial inverse agonist, FG7142, yield a dose-dependent (2-16 mg/kg, i.p) reduction in the amplitude of the acoustic startle response in the rat. The similarity in drug effects on startle was not attributable to congruent effects on basal somatic activity, as chlordiazepoxide resulted in a dose-dependent decrease in activity whereas FG7142 was associated with a small but non-significant increase in activity. As these results contrast with the bidirectional actions of benzodiazepine receptor agonists and inverse agonists in behavioural tests of fear or anxiety, the neuronal mechanisms mediating the effects of benzodiazepine receptor ligands on the acoustic startle response may be distinct from those that underlie the specific fear- attenuating and potentiating actions, respectively, of benzodiazepine receptor agonists and inverse agonists.

Bidirectional modulation of cortical acetylcholine efflux by infusion of benzodiazepine receptor ligands into the basal forebrain

In a previous in vivo microdialysis study in rats, it was found that cortical acetylcholine (ACh) efflux was reliably increased by a multimodal appetitive stimulus (onset of darkness with presentation of palatable food). Furthermore, this stimulated ACh efflux was significantly enhanced by systemic administration of a benzodiazepine receptor (BZR) weak inverse agonist and significantly reduced by a BZR full agonist. These effects contrasted with the minimal effects of BZR ligands on basal cortical ACh efflux in resting animals. The aim of the present study was to determine whether this modulation of stimulated cortical ACh efflux by BZR ligands was mediated within the basal forebrain. ACh efflux, measured with in vivo microdialysis, was stimulated by onset of darkness, an event which predicted delivery of palatable food. The BZR full inverse agonist, beta -CCM (3.0 micrograms/hemisphere) or the full agonist chlordiazepoxide (40.0 micrograms/hemisphere) was infused into the basal forebrain just prior to the darkness/food stimulus. Similar to previous results with systemic administration, the BZR full inverse agonist enhanced, while the full agonist reduced, stimulated cortical ACh efflux. These results demonstrate that the action of BZR ligands in the basal forebrain is sufficient for their modulation of cortical ACh release.