Functional hypotheses of the coeruleocortical noradrenergic projection: A review of recent experimentation and theory

Neuroendocrine, behavioral and macrophage activity changes induced by picrotoxin effects in mice

The relevance and property of studies related to stress effects on immune function are undisputable. All studies conducted on stress-immune relationships, however, provide from physical and/or psychological stressors. Indeed, as far as it is of our knowledge brain-innate immune responses were not analyzed after anxiogenic-like drugs use. The present experiment was then undertaken to analyze the effects of picrotoxin (0.3, 0.6 and 1.0mg/kg doses) on behavior, macrophage activity, serum corticosterone and noradrenaline (NE) levels and turnover in the brain of adult mice. Results showed that picrotoxin treatment in mice: (1) decreased motor and rearing activities in an open-field; (2) decreased the number of entries into the plus-maze open-arms and decreased the time spent in the exploration of the plus-maze open-arms; (3) decreased both motor activity and the level of holes exploration in the hole-board; (4) increased the levels of serum corticosterone in dose-dependent way; (5) increased noradrenaline (NE) and MHPG levels and NE turnover in the hypothalamus; and (6) increased Staphylococcus aureus and PMA-induced macrophage oxidative burst. However, and contrary to that reported after physical or psychological stress, this drug induced no effects on macrophage phagocytosis and NE levels and turnover in the frontal cortex. The present results are thus showing that picrotoxin induces some but not all neuro-innate immunity changes previously reported for inescapable foot-shock and psychological stressors in mice. These facts suggest that this chemical stressor triggers CNS pathways that might be somehow different from those fired by inescapable foot-shock and psychological stressors, leading to different neuro-innate immune responses.

Cocaine-induced inhibition of process outgrowth in locus coeruleus neurons: role of gestational exposure period and offspring sex

Cocaine use during pregnancy is associated with neurobehavioral problems in school-aged children that implicate alterations in attentional processes, potentially due to impairments in the noradrenergic system. We analyzed locus coeruleus (LC) neurite outgrowth characteristics following the administration of a physiologically relevant dose of cocaine (3.0 mg/kg) issued during critical phases of gestation (gestational day (GD)8-14, GD15-21, GD8-21). Results showed that cocaine inhibits LC neurite outgrowth and development, as evidenced by a decrease in total neurite length, a decrease in neurite length per cell, and a decrease in the percentage of cells with neurites. Morphological differences between cultures treated with and without cocaine were also evident. Further, the specific gestational exposure period effects were also dependent upon sex of the fetus. Finally, a discriminant function analysis suggested that the pattern and magnitude of alterations that defined the GD8-14 exposure were significantly different from that of the GD15-21 or GD8-21 exposures. Collectively, these data demonstrate a direct, disruptive effect of cocaine on noradrenergic neurons and may provide a neurobiological basis for changes in attentional function seen in offspring exposed to cocaine in utero.

Prenatal cocaine exposure alters sensitivity to the effects of idazoxan in a distraction task

The present study was designed to test whether prenatal cocaine (COC) exposure alters sensitivity to the attentional effects of idazoxan (IDZ), an alpha-2 adrenergic antagonist that increases coeruleocortical NE activity. The task assessed subjects' ability to selectively attend to an unpredictable light cue and disregard olfactory distractors. IDZ increased commission errors specifically under conditions of distraction, an effect that was similar in the COC and control groups. In contrast, COC animals were significantly more sensitive than controls to the effects of IDZ on omission errors and nontrials. The pattern of effects suggests that the differential treatment response to IDZ on these latter measures resulted from an alteration in norepinephrine (NE)-modulated dopamine release in the COC animals, reflecting lasting changes in dopaminergic and/or noradrenergic systems as a result of the early cocaine exposure. Based on the behavioral measures that showed a differential response to IDZ in the COC animals, it seems likely that these changes may contribute to the alterations in sustained attention and arousal regulation that have been reported in both animals and humans exposed to cocaine in utero.

Depletion of brain norepinephrine does not reduce spontaneous ambulatory activity of rats in the home cage

6-Hydroxydopamine (6-OHDA) lesions of brain noradrenergic neurons and terminals were made in rats to assess the importance of forebrain norepinephrine (NE) for mediating circadian patterns of spontaneous ambulatory activity that rats show in the home cage. 6-OHDA was injected intracranially into the fibers of the ascending noradrenergic dorsal and ventral bundle pathways or infused into the lateral ventricle or both. Rats living in a 12/12 h light/dark cycle exhibit a marked increase in ambulatory activity during the dark period in comparison to the light period and a 'W-shaped' pattern of activity during the 12 h of the dark phase. Results showed that near-total depletion of brain NE did not impair the capacity to generate normal patterns of spontaneous ambulatory activity that occur in the home cage. In the animals that sustained the most complete NE lesions, the amounts of activity generated at times of peak activity were exaggerated in comparison to the control animals, which is consistent with the possibility that NE in the brain exerts a moderating influence on behavior.

Enduring effects of early lead exposure: evidence for a specific deficit in associative ability

Long-Evans dams were exposed to Pb acetate in the drinking water during both gestation and lactation, or lactation only. This report presents the results of an automated, olfactory, serial reversal task administered to the adult offspring. Although overall learning rate was not significantly affected by Pb exposure, analyses of specific phases of the learning process revealed that all three exposed groups required significantly more trials than controls to reach criterion from the point at which perseverative responding to the previously correct cue ended. These in-depth analyses revealed that the reversal learning impairment of the Pb-exposed animals was not due to a deficit in inhibiting responses to the previously correct cue, the mechanism commonly assumed to underlie impaired reversal learning. Instead, the analyses revealed that two other independent Pb effects were responsible for the prolonged postperseverative learning period: a response bias and an impaired ability to associate cues and/or actions with affective consequences. The contribution of these two factors varied as a function of the timing and intensity of the Pb exposure. It is hypothesized that the Pb-induced associative deficit may reflect lasting damage to the amygdala and/or nucleus accumbens, which comprise a system thought to modulate the process by which environmental cues acquire affective significance.

Effects of noradrenaline on frequency tuning of auditory cortex neurons during wakefulness and slow-wave sleep

This study shows the effects of noradrenaline (NA) on receptive fields of auditory cortex neurons in awake animals; it is the first one to describe the effects of NA on neurons in sensory cortex, in different natural states of vigilance. The frequency receptive field of 250 auditory cortex neurons was determined before, during and after ionophoretic application of NA while recording the state of vigilance of unanaesthetized guinea-pigs. When NA significantly changed the spontaneous activity (85 out of 250 cells), the dominant effect was a decrease (61 out of 85 cells, 72%). When NA significantly changed the evoked activity (107 out of 250 cells), the dominant effect was also a decrease (84 out of 107 cells, 78%). During and after NA application, the signal-to-noise ratio (S/N, i.e. evoked/spontaneous activity) was unchanged, but the selectivity for pure-tone frequencies was enhanced. When the effects occurring in wakefulness and in slow-wave sleep (SWS) were compared, it appeared that the predominantly inhibitory effect of NA on spontaneous and evoked activity was present in both states. The S/N ratio was unchanged and the selectivity was increased in both states. However, during SWS, the percentage of cells inhibited by NA was lower, and the effects on the frequency selectivity were smaller than in wakefulness. In contrast, GABA produced similar inhibitory effects on spontaneous and on evoked activity during wakefulness and SWS. Comparisons with previous data obtained using the same protocol in urethane anaesthetized animals (Manunta & Edeline 1997) indicate that the effects of NA were qualitatively the same. Based on these results, we suggest that any hypothesis concerning the role of NA in cortical plasticity should take into account the fact that the predominantly inhibitory effects of NA lead to decrease the size of the receptive field.

Facilitation of cognitive functions by a specific alpha2-adrenoceptor antagonist, atipamezole

The present experiments investigated the effects of a specific and potent alpha2-adrenoceptor antagonist, atipamezole (as a stimulator of the noradrenergic system) on cognitive performance in rats. Atipamezole enhanced the acquisition of a linear-arm maze test and also improved the choice accuracy of poorly performing rats in a delayed (20 min) three-choice maze test. Furthermore, atipamezole improved the achievement of a one-trial appetite-maze when injected immediately after teaching, thus having an effect on consolidation. Atipamezole clearly impaired the acquisition of the active avoidance test. The present results indicate that stimulation of noradrenergic system by atipamezole improves the performance of animals in tasks assessing relational learning and memory, possibly affecting attention, short-term memory and the speed of information processing. It has also an effect on a consolidation process unrelated to attentional or motivational mechanisms. In a stressful test. stimulation of noradrenaline release leads to impairment of performance.

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.

Noradrenergic mechanisms in stress and anxiety: I. Preclinical studies

There is considerable preclinical evidence for a relationship between noradrenergic brain systems and behaviors associated with stress and anxiety. The majority of noradrenergic neurons are located in the locus coeruleus (pons), with projections throughout the cerebral cortex and multiple subcortical areas, including hippocampus, amygdala, thalamus, and hypothalamus. This neuroanatomical formation of the noradrenergic system makes it well suited to rapidly and globally modulate brain function in response to changes in the environment, as occurs during the presentation of stressors. Stress exposure is associated with an increase in firing of the locus coeruleus and with associated increased release and turnover of norepinephrine in brain regions which receive noradrenergic innervation. Increased firing of the locus coeruleus is also associated with behavioral manifestations of fear, such as arched back and piloerection in the cat. Exposure to chronic stress results in long-term alterations in locus coeruleus firing and norepinephrine release in target brain regions of the locus coeruleus. Norepinephrine is also involved in neural mechanisms such as sensitization and fear conditioning, which are associated with stress. These findings are relevant to an understanding of psychiatric disorders, such as panic disorder and post-traumatic stress disorder (PTSD), the symptoms of which have been hypothesized to be related to alterations in noradrenergic function.

Retarded acquisition of a temporal discrimination following destruction of noradrenergic neurones by systemic treatment with DSP4

This experiment examined the effect of destroying central noradrenergic neurones, using the selective neurotoxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) on the acquisition and performance of discrimination between two time intervals. Rats that had received systemic treatment with DSP4 and vehicle-treated control rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus and lever B following an 8-s presentation of the same stimulus. Both groups acquired the discrimination (> 90% correct choices) within 15 sessions; however, the DSP4-treated group showed significantly slower acquisition than the control group. When stable performance had been attained, 'probe' trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. Neither the bisection point (duration corresponding to 50% choice of lever B) nor the Weber fraction differed significantly between the DSP4-treated and control groups. The levels of noradrenaline were markedly reduced in the neocortex and hippocampus of the DSP4-treated group, but the levels of dopamine and 5-hydroxytryptamine were not altered. The results indicate that noradrenaline depletion induced by DSP4 retarded the acquisition of temporal discrimination, but did not impair steady-state discriminative precision.

Behavioral impairment in radial-arm maze learning and acetylcholine content of the hippocampus and cerebral cortex in aged mice

Age-related changes in spatial learning performance were studied in relation to acetylcholine (ACh) content of brain regions in male aged (28-month-old) and young (5-month-old) mice of BDF1 strain. As there were large individual differences in the spatial performance of aged mice, the aged mice were divided into two subgroups, old A and old B. The old A group included the six best performers out of the 12 aged mice and the old B group included the remaining 6 worst performers. In a radial-arm maze task with 8 baited arms, aged mice in the old B group showed a marked deficit in acquisition performance and habituation to the apparatus. In the more difficult maze task with only 4 baited arms, the aged mice in the old B group exhibited marked impairment both in working memory and reference memory throughout training, whereas the aged mice in the old A group showed deficits in reference memory during the first 20 days of training and working memory during the last 20 days relative to young mice. Neurochemical analysis revealed significant decreases in the ACh content of the hippocampus and striatum in both aged groups, and in the frontal cortex and posterior cortex of the old B group as compared to the young group. Correlational analysis showed significant correlations between learning performance in the spatial task and ACh levels in the hippocampus, frontal cortex, and posterior cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose- and parameter-dependent effects of atipamezole, an alpha 2-antagonist, on the performance of rats in a five-choice serial reaction time task

The present study investigated whether atipamezole (ATI), a potent alpha 2-adrenoceptor antagonist that increases the release of noradrenaline in brain, improves attention in rats. Thus, the effects of ATI on the performance of adult male rats in the five-choice serial reaction time task were studied. Food-deprived rats were trained to detect and respond to brief flashes of light presented randomly in one of five spatially diverse locations. The effects of single-dose administration of ATI (0.03-3.0 mg/kg) on the performance of rats under different parametric manipulations of the task were tested: 1) the visual stimuli were presented at unpredictable intertrial intervals (ITIs) or b) the intensity (brightness) of visual stimuli was reduced, thus placing an additional load on attentional processing for animals. Presenting the stimuli earlier than normally or reducing its intensity markedly impaired the choice accuracy of rats. At doses of 0.03, 0.3, and 1.0 mg/kg, ATI improved the choice accuracy of rats when tested using reduced stimulus intensity. ATI 3.0 mg/kg did not affect accuracy performance when tested using reduced stimulus intensity but impaired it when tested using unpredictable ITIs. The other doses of ATI (0.03, 0.3, and 1.0 mg/kg) did not markedly affect choice accuracy of rats tested using unpredictable ITI. Our results could be explained by the assumption that an acute, systemic administration of ATI affects arousal mechanisms and facilitates the processing of visual stimuli related to reward.

Olfactory recognition of infants in laboratory mice: role of noradrenergic mechanisms

Noradrenaline depletion of the olfactory bulbs induces cannibalism at parturition in primiparous mice, without producing anosmia or impairment of maternal behaviour. Similar lesions made in multiparous experienced females do not result in cannibalism. The present studies investigated 1) whether a 30-min exposure to pups or to distal cues from pups given to virgin females before noradrenaline depletion of the olfactory bulbs overcame the impairment in recognition at first parturition, and 2) whether noradrenaline-depleted females allowed to care for their pups for the 24 h following parturition showed a failure in recognition on a second parturition. Experiment 1 showed that exposure to distal cues from pups enabled the females to successfully recognize pups at parturition in comparison to naive females. However, neither the exposure to pup cues nor the fully interactive experience with pups overcame the disruptive effect on recognition at birth of the noradrenaline depletion. In Experiment 2, we found that olfactory recognition was impaired in noradrenaline-depleted females on second parturition, in spite of the mothering experience with their own pups.

Noradrenergic receptor mechanisms in neophobia

We have previously demonstrated that depletion of forebrain norepinephrine (NE) led to an attenuation of neophobia in a novel environment, as defined by a greater preference for novel food over familiar food. To study further the role of forebrain NE in neophobia we chronically infused noradrenergic receptor ligands or forskolin into the lateral ventricles of sham and 6-hydroxydopamine dorsal bundle lesioned rats. Chronic NE infusions into lesioned animals reversed the lesion-induced shift in relative food preference. The beta receptor agonist isoproterenol had moderate effects similar to those of NE in lesioned and sham animals. Phenylephrine, an alpha-1 agonist, was without effect. Forskolin, an adenylate cyclase activator, mimicked the effects of NE infusions. These data suggest a role for noradrenergic stimulation of adenylate cyclase in neophobia.

Telencephalic but not diencephalic noradrenaline depletion enhances behavioural but not endocrine measures of fear conditioning to contextual stimuli

Three experiments investigated the effects of primarily cortical or hypothalamic noradrenaline depletion on aversive conditioning of explicit and contextual stimuli in rats. In Expt. 1, two groups of rats were trained to respond under a variable interval schedule for food reward. One group of rats subsequently received injections of 6-hydroxydopamine into the dorsal noradrenergic bundle, resulting in profound depletion of cortical noradrenaline; the second group received vehicle injections. All rats were exposed to 5 pairings of an auditory stimulus (CS) and footshock (UCS) in a distinctive environment (the dark chamber of a place preference apparatus). During testing in a separate, neutral environment, DNAB-lesioned rats suppressed responding for food reward, in the presence of the aversive CS, to a greater degree than controls. Lesioned rats also showed a greater aversion to the distinctive environment in which they were shocked. In contrast, plasma corticosterone concentrations, measured immediately following each of these behavioural tests, revealed no differences between DNAB-lesioned and control rats. Expt. 2 showed that the DNAB lesion did not affect habituation to the light chamber of the place preference apparatus used in Expt. 1. Expt. 3 showed that 6-OHDA injection into the ventral noradrenergic bundle component of the central tegmental tract, which damages primarily the noradrenergic innervation of hypothalamus, had no effect on either behavioural or endocrine responses to conditioned aversive, explicit or contextual cues. The results are discussed in relation to other reports of the effects of DNAB lesions on simple associative learning in an aversive context.

Memory following cholinergic (NBM) and noradrenergic (DNAB) lesions made singly or in combination: potentiation of disruption by scopolamine

Groups of rats were trained on either delayed matching or nonmatching to position tasks, then divided into four subgroups and given the following bilateral lesions: (a) SHAM [vehicle injection into the nucleus basalis magnocellularis (NBM) and dorsal noradrenergic bundle (DNAB)], (b) DNAB (6-hydroxydopamine lesion of the DNAB, vehicle into the NBM), (c) NBM (quisqualic acid lesion of the NBM, vehicle into the DNAB) and (d) DUAL (neurotoxin lesions of both DNAB and NBM). Following postoperative recovery, the DUAL lesion subjects were slightly impaired, but by the seventh day of testing all groups were performing at similar levels. This strongly suggests that quisqualate lesions of the NBM are not sufficient to produce severe and lasting mnemonic disorders resembling those seen in Alzheimer's disease (AD). These data also indicate that the noradrenergic system may not be of critical importance with respect to cognition. It was reasoned that an additional anticholinergic treatment might exacerbate an underlying deficiency. All groups were injected, peripherally, with the cholinergic antagonist scopolamine (0-0.5 mg/kg). This drug dose-dependently disrupted performance in all groups. Moreover, the highest dose had a marked effect in the DUAL group, impairing performance even when no mnemonic burden was present (at zero delay). The results suggest that cholinergic NBM and noradrenergic DNAB lesions produce only transient mnemonic deficiencies. A combination of the two can be disruptive, but longer term task (or reference) memory is the primary process affected, and only under certain conditions. The implication of these findings to research concerning animal models relating to Alzheimer's disease is discussed.

Monoamines and abnormal behaviour. A multi-aminergic perspective

Classical nosology has been the cornerstone of biological psychiatric research; finding biological markers and eventually causes of disease entities has been the major goal. Another approach, designated as 'functional', is advanced here, attempting to correlate biological variables with psychological dysfunctions, the latter being considered to be the basic units of classification in psychopathology. Signs of diminished DA, 5-HT and NA metabolism, as have been found in psychiatric disorders, are not disorder-specific, but rather are related to psychopathological dimensions (hypoactivity/inertia, increased aggression/anxiety, and anhedonia) independent of the nosological framework in which these dysfunctions occur. Implications of the functional approach for psychiatry include a shift from nosological to functional application of psychotropic drugs. Functional psychopharmacology will be dysfunction-orientated and therefore geared towards utilising drug combinations. This prospect is hailed as progress, both practically and scientifically.

The effects of dorsal noradrenergic bundle lesions on spatial learning, locomotor activity, and reaction to novelty

The dorsal noradrenergic bundle (DNB) of male Wistar rats was lesioned bilaterally using intracerebral injections of 6-hydroxydopamine neurotoxin. Some of the rats were trained in a water maze using an "alternation" strategy, where the two positions of the hidden platform in the pool were changed between successive trials in the daily tests, and other rats in a water maze, where the temperature of the water was lowered to about 11 degrees C. The rats trained in the cold-water maze were also tested in open-field and saccharin neophobia tests. No differences were found between the two groups in learning of water maze tasks or in locomotor activity in the open-field tests. However, the saccharin neophobia test revealed an increased neophobia in the DNB-lesioned rats.

Damage to ceruleo-cortical noradrenergic projections impairs locally cued but enhances spatially cued water maze acquisition

A series of 4 experiments tested the effects of central catecholamine depletion on acquisition of an escape response in a spatial water maze. In Expt. 1, local infusions of 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle (DNAB) enhanced efficient acquisition of the spatial water maze in a stressful condition (cold water), but had no effect in warm water. In Expt. 2, lesions of the ventral noradrenergic bundle did not affect acquisition of the maze, indicating that the changes observed in Expt. 1 were unlikely to have been the result of incidental damage to the noradrenergic innervation of the hypothalamus. Measures of core body temperature and plasma corticosterone were taken in parallel with the behavioral experiments and revealed that central noradrenaline (NA) depletion did not alter these responses to cold or warm water swims. Expt. 3 revealed a contrasting pattern of effects following dopamine (DA) depletion from the caudate-putamen: swimming speed was reduced in warm, but not cold water and maze acquisition was impaired, to an equal extent in warm and cold water. Finally, in Expt. 4, rats with 6-OHDA lesions of the DNAB were impaired in discriminating local cues in a simultaneous visual discrimination water maze. These results support the hypothesis that ceruleo-cortical NA depletion broadens the span of attention, particularly under stressful circumstances. In contrast, the results also indicate that striatal DA depletion mainly affects vigour of responding, as measured by swim speed, and that this effect can be reversed by the stressful effects of cold water.

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.

The nucleus locus coeruleus modulates local cerebral glucose utilization during noise stress in rats

Local cerebral glucose utilization (LCGU), estimated by the quantitative autoradiographic 2-deoxyglucose technique, was studied in rats with bilateral 6-hydroxydopamine lesions of the locus coeruleus (LC) and in vehicle-injected controls. Unanesthetized animals were studied during exposure to stressful levels of white noise (95 dB) or in relative silence (50 dB). Results indicated that noise caused greater and more widespread increases in LCGU in animals with LC lesions than in vehicle-injected controls. Lesions alone had little or no effect in animals not subjected to noise. Analyses of variance revealed significant treatment interaction effects (intact/lesion x silence/noise) for 37 of 109 regions measured. The pattern of results suggests that the LC acts during stress to limit unnecessary cerebral activity that might interfere with efficient sensory processing and/or the organization of appropriate behavioral responses. In this respect LC function may be similar to those actions of the peripheral sympathetic nervous system that suppress vegetative functions during stress to allow for the performance of coping responses.

Neurophysiology and neurochemistry of drug dependence: a review

To understand the neurophysiological and neurochemical mechanisms of drug dependence, the functional significance of dopamine, noradrenaline and endogenous opioid peptides in the mediation of natural, self-stimulation and pharmacological reinforcement are discussed. Data on search of system(s), mediator(s) and neurons of reinforcement as well as my own notions on reinforcement as a critical element in organization and regulation of the organism's adaptive activity in variable environments are presented. The role of chronic drug-induced stable modification of central neurochemical systems' functioning as a basis for the alteration of endogenous reinforcement processes and raising drug dependence are examined in detail for main addictive drugs, opiates and psychomotor stimulants.

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.

Changes in biogenic amines in rat hippocampus during development and aging

The effects of postnatal development and aging on the concentration of dopamine, noradrenaline, serotonin and their principle metabolites have been studied in the hippocampus of the rat. During development the concentration of dopamine increases 1.5 fold during the first 90 days. 3-methoxytyramine was found in low concentrations. The homovanillic acid and DOPAC concentrations showed no changes apart from a decrease at day 15 and an increase at day 8, respectively. From birth up to 30 months, the noradrenaline concentration increased by a factor of about 10. Their metabolites each showed a different profile. The concentration of tryptophan was always the highest among the compounds studied. It decreased from birth to day 15, while the concentration of serotonin and 5-hydroxyindolacetic acid increased 3 and 5 fold respectively during this time. However, 5-hydroxytryptophan and 5-hydroxytryptophol concentrations were very low and unchanged at all stages. These findings led to the conclusion that the neurotransmitters: noradrenaline and serotonin, are developed in the hippocampus during the first three months. During aging, the serotonin concentration is increased without significant change in the other compounds studied.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. I. Thermoregulatory challenge

This and the following two papers examine the activity of locus coeruleus noradrenergic (LC-NE) neurons in response to a variety of physiological manipulations in unanesthetized, unrestrained cats. Unit responses were studied during a constant behavioral state in order to avoid the potentially confounding effects of state changes upon LC-NE unit discharge. In the present study, LC-NE unit activity was recorded during two thermoregulatory challenges: ambient heating and pyrogen-induced fever. These two conditions are particularly interesting since the direction of body temperature change that they produce is the same, but the thermoregulatory responses elicited are opposite, i.e. heat loss and heat gain. LC-NE neurons were activated by both manipulations. In response to ambient heating, neuronal activity increased only with the occurrence of panting, and not during earlier portions of the heating session. Following pyrogen administration, LC-NE neurons responded only during the peak increase in body temperature. Both of these effects on unit activity occurred independent of changes in behavioral state. Since both manipulations similarly increased LC-NE unit activity, despite eliciting opposite thermoregulatory responses, it appears that these neurons do not play a specific role in thermoregulation, but may participate in the response to physiological challenges in general.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. III. Glucoregulatory challenge

Insulin-induced hypoglycemia and the subsequent administration of glucose were examined for their effects on single unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats. LC-NE neuronal activity showed an inverse relationship to blood glucose levels. The activity of most cells increased during sustained hypoglycemia, and then decreased following glucose administration. Some neurons were unaffected by hypoglycemia, but were inhibited following glucose. The activation of LC-NE neurons in response to insulin administration generally paralleled the increase in plasma epinephrine, although the adrenal response was more sensitive. These data, together with those reported in the preceding papers, suggest the following general conclusions: (1) physiological stimuli can influence the activity of LC-NE neurons in unanesthetized subjects (although they do so less strongly than environmental stimuli); (2) these effects of physiological stimuli upon LC-NE neurons can be exerted independent of changes in behavioral state; (3) LC-NE neurons do not appear to play a specific role in the regulation of any of the systems examined, but may instead play a more global role in the response to physiological challenges in general; (4) LC-NE neurons are generally co-activated with both the neural and hormonal components of the sympatho-adrenal system, although sympathetic activation can occur in the absence of increased LC-NE activity. A previously hypothesized role for LC-NE neurons in facilitating the behavioral response to environmental stressors may thus be extended to include the response to physiological challenges, and perhaps facilitation of the physiological as well as the behavioral components of the stress response.

Noradrenaline- and time-dependent changes in neocortical alpha 2- and beta 1-adrenoceptor binding in dorsal noradrenergic bundle-lesioned rats

A 6-hydroxydopamine-induced lesion of the dorsal noradrenergic bundle (DNB) in rats markedly decreased neocortical noradrenaline concentration (NNC) by 72-100% as measured 1, 3 and 13 months after the lesioning procedure. The concomitant assessment of neocortical alpha 2- and beta 1-adrenoceptor binding (NAAB and NBAB, respectively) usually indicated significant increases of 25-72% for these two variables. There were, however, cases of unchanged NAAB and NBAB which presumably reflected an incomplete DNB lesion and a consequent time-related, partial recovery of NNC. The results emphasize the potential for long-term sequelae of the DNB lesion, and the existence of a critical NNC threshold (approximately 10-30% of control NNC values) which modulates postsynaptic alpha 2- and beta 1-adrenoceptor density.

The effects of excitotoxic lesions of the substantia innominata, ventral and dorsal globus pallidus on the acquisition and retention of a conditional visual discrimination: implications for cholinergic hypotheses of learning and memory

The effects of ibotenic acid-induced lesions of the ventral pallidum/substantia innominata region, the dorsal pallidum or both on the acquisition and retention of a conditional visual discrimination have been studied in the rat. Lesions of the ventral pallidum and large lesions of the dorsal and ventral pallidum severely impaired both the acquisition and retention of the conditional discrimination. Dorsal pallidal lesions had similar, but less marked effects. The same lesions also impaired the retention of a passive avoidance task, but had no effect on a conditioned taste aversion. Neurobiological investigations revealed that the lesions destroyed cholinergic neurons in the magnocellular nucleus basalis and caused reductions in cortical choline acetyltransferase activity of about 30-40%. Tract-tracing experiments indicated that the lesions destroyed, in particular, cholinergic neurons projecting to the frontal dorsolateral cortex and also those projecting to more posterior cortex, but not the occipital lobes. Contingency analysis of the behavioural, neurochemical and neuroanatomical data indicated that those animals with the largest decreases in choline acetyltransferase activity, or the largest areas of neuronal loss in the ventral and dorsal globus pallidus, were most impaired in the retention of the conditional discrimination. The results do not, therefore, indicate a simple relationship between cholinergic neuronal loss and the retention of response rules essential for performance of the task ("reference memory"). The relevance of the results to cholinergic hypotheses of learning and memory is discussed.

Catecholaminergic neurones assessed ante-mortem in Alzheimer's disease

Indices of dopaminergic and noradrenergic varicosities were assayed in neocortical tissue obtained at diagnostic craniotomy from patients with Alzheimer's disease in the presenium. Dopaminergic markers (concentrations of dopamine, dihydroxyphenylacetic acid and homovanillic acid) were not significantly different from controls in either frontal or temporal cortex. In the frontal cortex, the release of endogenous dopamine and noradrenaline (in the presence of both resting and stimulating concentrations of potassium) was also unaffected whereas release of endogenous serotonin was significantly reduced. In the temporal cortex, noradrenergic markers (concentration of noradrenaline and uptake of radiolabelled noradrenaline) were significantly reduced, to at least 47% of mean control values. These deficits are interpreted as reflecting denervation and were present in patients examined only some two years after developing symptoms of dementia. The ratio of 3-methoxy-4-hydroxyphenylglycol to noradrenaline (a putative index of noradrenaline turnover) was elevated in the temporal cortex, suggesting increased activity of the remaining noradrenergic varicosities. Noradrenergic markers did not correlate with either clinical or histological indices of the severity of the disease which contrasts with presynaptic cholinergic and serotonergic markers.

The emerging pharmacology of ethanol

Historically, alcohol (ethanol) has been viewed as a non-specific CNS depressant, presumed to act equally on all bioelectric membranes. In contrast to this view, cellular electrophysiological studies, supported by anatomic and neurochemical evidence, support the emergence of a more specific pharmacological profile. Four regions of the rodent brain (cerebellum, hippocampus, locus coeruleus and inferior olive) have so far been examined. The effects of acute parenteral ethanol on specific identifiable neurons within these four regions are highly consistent, dose-related, and spontaneously reversible. Nevertheless, different patterns of effects are seen in each responsive region, ranging from general increased firing in inferior olive to generally depressed synaptic transmission in hippocampus, and with more subtle effects within the cerebellum and within the locus ceruleus. This survey of consistent but differing patterns of responsiveness to ethanol at specific time points after acute exposure, suggests that the global effects of ethanol on movement, on arousal and on emotions and memory must be composed of several distinct effects both within and across many cellular systems.