Intact selective attention in rats with lesions of the dorsal noradrenergic bundle

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Systemic idazoxan impairs performance in a non-reversal shift test: implications for the role of the central noradrenergic systems in selective attention

Two experiments examined the effect of pharmacological stimulation of the locus coeruleus in a non-reversal shift paradigm to test predictions concerning the role of the ceruleo-cortical noradrenergic system in processes of selective attention. Food-deprived rats were trained to make either visual (experiment 1) or spatial (experiment 2) discriminations in a parallel alley with both sets of cues being present at all times. Two groups of rats received treatments of either 2 mg/kg i.p. of the selective α(2) adrenoceptor antagonist idazoxan or saline control injections before each daily block of trials. Following attainment of criterion, the reinforcement contingencies were altered according to a non-reversal shift design, so that the alternative (i.e. either spatial or visual) set of cues now predicted reward. Rats treated with idazoxan were not impaired in the acquisition of either the spatial or visual discrimination task, but they were impaired in both forms of non-reversal shift. These deficits are interpreted as resulting from narrowed attention in idazoxan-treated rats, thus supporting a selective attention hypothesis of locus coeruleus function.

Plasticity of sensory responses of locus coeruleus neurons in the behaving rat: implications for cognition

The gating and tuning actions of noradrenaline (NA) at post-synaptic sites have been highly suggestive of an important role for the locus coeruleus (LC) in attention, learning and memory. By recording the activity of single units in the LC in behaving rats in a strictly controlled conditioning paradigm, direct evidence was provided that this nucleus is engaged during specific aspects of learning. The neuronal response to a discrete sensory stimulus was monitored as a function of the changing significance of the stimulus i.e., when it was novel, during habituation, associative learning, reversal and extinction. Both appetitive and aversive paradigms were used. We consistently observed differential conditioned responding with food reinforcement, while when footshock reinforcement was used, there was an increase in response to both CS+ and CS-. In both paradigms, the LC response disappeared when the conditioning was expressed at a behavioral level, to reappear vigorously as soon as the stimulus reinforcement contingencies were changed, i.e., during reversal or extinction. These results suggest that the LC does not mediate specific sensory or associative information necessary for ongoing performance but shows remarkable plasticity of sensory responding as a function of changing cognitive significance of the stimulus.

Disruption of noradrenergic, but not serotonergic or opiate, functioning blocks both cardiac and behavioral components of the orienting response in preweanling rats

Previous work in this laboratory established that selective attention, as measured by the behavioral and autonomic expressions of the orienting response (OR), is not disrupted by either dopaminergic or cholinergic receptor blockade. The present experiments extended this pharmacological analysis of the OR. In Experiment 1, preweanling rats were injected with methysergide maleate, a serotonin receptor blocker. Neither the behavioral nor the heart rate (HR) component of the OR was attenuated. In Experiment 2, the opiate receptor blocker naltrexone also failed to inhibit the HR and behavioral expressions of the OR. alpha-1 adrenergic receptor blockade with WB-4101 in Experiment 3 abolished both the HR and behavioral ORs to the pulsating tone. In Experiment 4, clonidine, which inhibits release of norepinephrine by stimulating alpha-2 autoreceptors, attenuated both behavioral and HR ORs to the pulsating tone in a dose-dependent manner. These data, in combination with the prior findings, suggest that norepinephrine is critically involved in the central process underlying the OR in the rat. Dopaminergic, cholinergic, serotonergic, and opiate receptor blockades do not impair selective attention as indexed by HR and behavioral ORs to an auditory stimulus. In contrast, disruption of noradrenergic functioning via either alpha-1 receptor blockade or alpha-2 receptor stimulation disrupts both the HR and behavioral components of the OR. These results indicate that integrity of central noradrenergic functioning is essential for expression of the OR and for stimulus-directed attention.

Activation of the noradrenergic system facilitates an attentional shift in the rat

The noradrenergic system was pharmacologically activated with the alpha 2 receptor antagonist, idazoxan (2 mg/kg i.p.), during the acquisition of a complex appetitive task requiring a shift in attention to stimulus dimension and in response strategy. Rats first learned a fixed path of 6 successive choices in a linear maze. The task was then changed to a visual discrimination task in which the spatial configuration of the correct path was indicated by visual cues and changed on each daily trial. During this part of the task, the rats were injected before each trial with idazoxan, a drug which increases the firing rate of neurons in the locus coeruleus and the release of noradrenaline in the cortex and hippocampus. Two control experiments showed that the drug treatment had no effect on the acquisition of either component of the task - the successive place learning or the visual discrimination. The drug was found to be effective only during the shift phase of the experiment, the idazoxan-treated rats taking fewer trials to reach criterion than the saline. A second experiment showed that idazoxan increased the amount of time spent investigating novel and unexpected objects in a familiar hole board. These results implicate the noradrenergic system in problem-solving which requires an attentional shift or a shift in responding from familiar to novel stimuli.

Forebrain norepinephrine involvement in selective attention and neophobia

It has been reported that depletion of forebrain norepinephrine via 6-hydroxydopamine infusion into the dorsal bundle decreases the rat's ability to selectively attend to relevant stimuli and thus increases the rat's responsiveness to novelty. In this study we measured 6-hydroxydopamine lesion effects on 1) selective attention via the nonreversal shift task and extinction of continuous reinforcement bar pressing and on 2) neophobia via consumption of a novel solution in a familiar environment; exploratory behaviors and consumption of a familiar food in a novel environment; and consumption of familiar and novel foods in a novel environment. Our data do not support a role for the dorsal bundle in selective attention. Our data do support a role for forebrain norepinephrine in neophobia and suggest that the lesion effects on neophobia result from an interaction between novelty of environment and novelty of food.

Lesions of the dorsal noradrenergic bundle simultaneously enhance and reduce responsivity to novelty in a food preference test

In conclusion, DNAB lesions have been shown to have two, apparently contradictory effects in a food preference test: to increase neophobia to a novel environment, and to increase the tendency to eat novel food in a novel environment. It has been suggested that these two effects occur because, although NA has a common action on neuronal firing in terminal fields, the dissociable consequences reflect the different functions of areas in receipt of these noradrenergic afferents. In addition, it has been shown that DNAB lesions, VNAB lesions, and a benzodiazepine, chlordiazepoxide, all have somewhat different behavioral effects in the food preference test. Taken together with the lack of correlation between the various behavioral measures used in these experiments, this suggests that neophobia does not reflect a single behavioral process, such as anxiety, or reactivity to novelty.

On the role of the dorsal noradrenergic bundle in learning and habituation to novelty

In Experiment 1, the performance of vehicle control rats and rats with 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle (DB) was examined in acquisition and extinction of bar pressing and in spontaneous and food-reinforced alternation in a T-shape maze. Plasma corticosterone levels in basal conditions, after chronic food restriction, after transportation to a novel environment, and after sessions of either rewarded or nonrewarded bar pressing were assayed. DB lesions produced a significant decrease of spontaneous alternation and a significant but small resistance to extinction, without reliably altering either corticosterone responses or instrumental spatial alternation. In Experiment 2, bar-press extinction and instrumental alternation were reexamined in new groups of control rats and rats with DB lesions without any blood collection procedures. The DB lesions did not reliably alter either behaviors on any measures. Taken together, these data indicate no consistent effects of forebrain noradrenaline depletion on either extinction or spatial memory or pituitary-adrenocortical function. However, the impairment of spontaneous alternation found in a previous study was confirmed. These findings are discussed in terms of the proposed roles of the dorsal noradrenergic bundle in learning and habituation to novelty.

A strong static-magnetic field alters operant responding by rats

Forty male rats of the Wistar ST strain were trained and observed for Sidman avoidance (SA) for 7 weeks or for discriminative avoidance (DA) for 14 weeks to determine the effects of exposure to a strong static-magnetic field. Before avoidance conditioning was completed, rats in the SA group were exposed to the static field at 0.6 T, 16 h/day for 4 days during the fifth week, and those in the DA group were exposed for 6 h/day for 4 days during the seventh week. In the SA conditioning, frequency of lever-pressing by exposed rats gradually decreased during 1 week of exposure and stayed low for at least 2 weeks after exposure. Frequencies of electric shocks received by the rats increased dramatically during the second day of exposure and consistently stayed higher than those of control rats. In the DA condition, exposed rats responded at lower rates than did control rats throughout the observation period. They received more shocks during the 2 weeks following exposure. The data indicate that performance of avoidance responses was inhibited by a comparatively long exposure to a strong magnetic field.

Amphetamine impairs the discriminative performance of rats with dorsal noradrenergic bundle lesions on a 5-choice serial reaction time task: new evidence for central dopaminergic-noradrenergic interactions

A series of experiments examined the effects of lesions of the dorsal noradrenergic bundle (DNAB), induced by 6-hydroxydopamine (6-OHDA), on the behavioural response to systemic and intra-accumbens amphetamine, using a rat analogue of Leonard's 5-choice serial reaction time task for humans. Although the 6-OHDA DNAB lesion produced a profound depletion of cortical noradrenaline (NA) (to around 5% of control levels) it did not impair any aspect of performance on this task. Both systemic and intra-accumbens amphetamine increased behavioural measures of impulsivity of responding, but neither impaired discriminative accuracy in the sham-operated control rats. However, the DNAB lesioned rats did show a discriminative impairment following both low doses of systemic amphetamine, and intra-accumbens amphetamine. The latter effect was antagonised by systemic administration of the specific dopaminergic (DA) antagonist alpha-flupenthixol. The DNAB lesion did not alter the effect of amphetamine on any other behavioural measure, including speed and impulsivity of responding. These results suggest that although DA and NA participate in qualitatively different behavioural processes, the effects of DNAB lesions on attentional processes depend on the level of DA activity within the nucleus accumbens.

No increase in reaction-time after lesion of the dorsal noradrenergic bundle

In this study we examined whether lesions of rats' dorsal noradrenergic bundles affect their reaction times to temporally unpredictable stimuli. Injection of 6-hydroxydopamine into rats' dorsal bundles drastically reduced the noradrenaline content of their cerebral cortex. Nevertheless, 6OHDA treated rats could still react as quickly as controls. Moreover, the treatment did not affect the efficiency with which rats performed the task, even when they were forced to respond very quickly.

6-Hydroxydopamine lesion of the ventral noradrenergic bundle blocks the effect of amphetamine on hippocampal acetylcholine

Rats treated with amphetamine exhibited an increase in hippocampal acetylcholine turnover, as determined by a mass fragmentographic technique. However, administration of amphetamine to rats which had received stereotaxically placed bilateral injections of 6-hydroxydopamine in the ventral noradrenergic bundle 10 days previously did not increase hippocampal acetylcholine turnover. Because the ventral noradrenergic bundle projects to the septal area, it is suggested that amphetamine increases acetylcholine turnover in the hippocampus by an action on noradrenergic neurons in this pathway.

Behavioral effects of xylamine-induced depletions of brain norepinephrine: interaction with LSD

Male rats were treated with a combination of systemic fluoxetine and intraventricular xylamine (under ether anesthesia) to deplete brain norepinephrine (NE) in the projection areas of the locus coeruleus. Four days later, control and lesioned rats were tested following injections of either saline or 80 micrograms/kg LSD in a Behavioral Pattern Monitor which recorded the sequential patterns of their locomotor and investigatory (holepokes) responses. Liquid chromatographic measures of brain monoamines confirmed that xylamine reduced hippocampal NE by 80.8% and hypothalamic NE by 26% without affecting levels of serotonin or dopamine. Relative to controls, NE-depleted rats exhibited repetitive spatial patterns of locomotion with no alteration in the amount or rate of habituation of locomotor activity. Lesioned animals made fewer rearings and holepokes, particularly early in the hour test session. When given 80 micrograms/kg LSD, sham-lesioned rats exhibited the expected decreases in entries into and time spent in the center of the chamber, an increase in time spent in the corners, and fewer holepokes and rearings early in the session. With the exception of the effect on rearings and holepokes, the effects of LSD were diminished in rats depleted of brain NE. These results indicate that this profile of behavioral effects of LSD, which has been interpreted as a potentiation of neophobia, may be dependent upon the noradrenergic projections of the locus coeruleus.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Manipulation of the amygdala noradrenergic system impairs extinction of passive avoidance

In a consolidation paradigm, noradrenergic manipulation of the rat amygdala results in disruption of extinction for passive avoidance. These results are discussed in light of a recent controversy concerning the role of central norepinephrine systems in general extinction behavior. The discussion is cast primarily within the framework of rodent natural life history, the presence of innate behavioral strategies, and the organization of information processing as an integration of experiential attributes.

Brain norepinephrine depleting lesions selectively enhance behavioral responsiveness to novelty

The noradrenergic innervation of the forebrain by cells from the locus coeruleus (LC) was interrupted by either electrolytic lesions of the LC or 6-hydroxydopamine (6-OHDA) lesions of the dorsal noradrenergic bundle (DB). Animals so treated were then tested in a modified open field test designed to measure responsiveness to environmental novelty and also tested for food consumption in their home cages. In addition, DB lesioned animals were tested in photocell activity cages for both their initial locomotor response to the novel cages as well as their activity level after habituation. The DB lesioned animals were also tested for rates of acquisition and extinction of an operant response. The DB lesion produced no deficits in either the acquisition or the extinction of a food rewarded operant response. The DB lesion did reduce the initial amount of locomotor activity in response to introduction to the activity cages but did not alter the rate of habituation of the locomotor response nor the "basal" level of activity at the end of two hr of exposure. Neither lesion affected the amount of food eaten by 24 hr-fasted animals in their home cages during the first 15 min. When fasted prior to being given access to food in a novel open field, however, both lesions resulted in decreased food consumption and decreased amounts eaten per approach to the food pedestal. The DB lesion, but not the LC lesion, resulted in decreased rearing and grooming in this setting.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitated reversal learning of a spatial-memory task by medial septal injections of 6-hydroxydopamine

To assess the role of hippocampal norepinephrine in learning and memory, rats were treated with medial septal injections of 6-hydroxydopamine either prior to or after acquisition of a spatial-memory task. No effect on acquisition learning or retention was observed. However, reversal learning was significantly enhanced in all treated animals regardless of whether treatment was prior to or after acquisition. Our results do not support a role of hippocampal norepinephrine in selective attention, but rather indicate a direct involvement in memory processes.

The effects of lesions to ascending noradrenergic neurons on discrimination learning and performance in the rat

Three experiments examined the effect of central noradrenergic depletion on the acquisition and performance of a temporal discrimination in auditory or visual modalities. In Experiment 1, 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle significantly retarded acquisition compared to a similarly lesioned ventral noradrenergic bundle-group, two sham-operated and one unoperated control group. In Experiment 2, the acquisition impairment produced in the dorsal noradrenergic bundle group was replicated, for both auditory and visual modalities, by lesions depleting hippocampal and neocortical noradrenaline by over 80%. In rats subsequently switched to discriminations involving the unfamiliar modality, the dorsal-bundle lesion also impaired acquisition several weeks after surgery. Experiment 3 showed significantly impaired performance in rats with dorsal bundle lesions when training prior to surgery had not resulted in better than chance performance. In rats previously trained to criterion, the dorsal bundle lesion transiently, but non-significantly, impaired performance. In rats performing better than chance, but not having reached criterion, there were no significant effects of the dorsal bundle lesion. Subsequent manipulations of deprivation and difficulty of discrimination in general failed to distinguish between the dorsal bundle lesion and sham-groups, suggesting that the acquisition impairment did not result from simple sensory or motivational effects. Reducing the interstimulus interval did impair the dorsal bundle group more than controls. However, there was no evidence of altered "distractibility" in the lesioned group when the alternative modality was introduced as a distractor. The results are discussed in terms of other acquisition deficits shown by rats with central noradrenaline depletion and their significance for determining the functions of the projections from the locus caeruleus via the dorsal noradrenergic bundle to the neocortex and hippocampus.

Evidence against a role of the rat's dorsal noradrenergic bundle in selective attention and place memory

In Experiment 1, 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle (DNB) in rats did not impair either acquisition of non-delayed alternation, retention of non-delayed alternation, or performance of alternation with delays in a T-maze, whether or not the goal arms of the maze were visually distinctive. These results were in contrast with those of a previous report indicating that DNB lesions impair learning of spatial alternation. In Experiment 2, the lack of a reliable effect of DNB lesions on learning performance of spatial alternation was confirmed. However, the rats with DNB lesions showed an impairment of spontaneous alternation. The negative results of the present study do not support either the hypothesis that depletion of forebrain noradrenaline impairs selective attention or the hypothesis that such depletion induces an amnesia for past places. On the other hand, the finding of impaired spontaneous alternation is consistent with previous observations suggesting that depletion of forebrain noradrenaline impairs habituation of fear reactions.