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

Neuropsychological issues in the assessment of refugees and victims of mass violence

Brain injury, stressor severity, depression, premorbid vulnerabilities, and PTSD are frequently intertwined in trauma populations. This interaction is further complicated when the neuropsychologist evaluates refugees from other cultures. In addition, the observed psychiatric symptoms reported in refugees and victims of mass violence may in fact not be the primary features of PTSD and depression but psychiatric symptoms secondary to the effects of traumatic brain injury. This paper reviews the occurrence of starvation, torture, beatings, imprisonment, and other head injury experiences in refugee and POW populations to alert treators to the presence of chronic and persistent neuropsychiatric morbidity, with implications for psychosocial adjustment. The concept of fixed neural loss may also interact with environmental and emotional stresses, and a model of neuropsychological abnormalities triggered by traumatic events and influenced by subsequent stress will also be considered. Neuropsychologists working with refugees play an important role in assessing the possibility of traumatic brain injury with tools that are relatively culture-fair.

Chronic subordination stress in male tree shrews: replacement of testosterone affects behavior and central alpha(2)-adrenoceptors

Subordination stress induced by social defeat in male animals is known to inhibit gonadal functions and it has been discussed whether the resulting deficit in testosterone might play a role in subordination behavior. One of the major transmitter systems involved in regulation of behavior is the noradrenergic system. To analyze whether a testosterone replacement can alter subordination behavior and whether this might be related to changes in the brain noradrenergic system, we quantified alpha(2)-adrenoceptors (alpha(2)-ARs) in the central nervous system of male tree shrews. Animals were submitted to chronic subordination stress and received testosterone at the same time. Behavior was monitored during all phases of the experiment: the control period of 10 days, the period of social stress lasting 10 days when subordinates were confronted daily with a dominant male, and, subsequently, the stress and treatment period of 18 days when in parallel to the stress, animals received either injections of testosterone or vehicle. Brain alpha(2)-ARs were quantified by in vitro receptor autoradiography using the antagonist ligand (3)H-RX821002. Locomotor activity decreased significantly during the stress period and was not re-normalized by testosterone. In contrast, testosterone re-normalized scent marking behavior and autogrooming, parameters that had both been reduced due to the subordination stress. Vehicle injections improved none of these behaviors. In 8 of 10 brain regions that were analyzed, numbers of alpha(2)-adrenergic binding sites were increased in stressed animals that received vehicle injections, but a difference between testosterone and vehicle injected animals was only observed in five regions. These brain regions are all known to be involved in emotional behavior (anterior hypothalamus, medial nucleus of the amygdala, cingulate cortex) or autonomic regulation, respectively (solitary tract nucleus, dorsal motor nucleus of vagus). Therefore, our data show that testosterone influences behavior of male subordinates and modulates alpha(2)-AR expression in their brains. Androgen-mediated alterations in receptors occur in brain regions that are known to be involved in emotionality, e.g., in the anterior hypothalamus which regulates aggressive behavior. One can therefore conclude that alpha(2)-ARs contribute to neuronal functions that are responsible for subordination of stress behavior, and that testosterone-induced receptor changes are related to the partial restoration of normal behavior.

A behavioral study of alpha-1b adrenergic receptor knockout mice: increased reaction to novelty and selectively reduced learning capacities

Knockout mice lacking the alpha-1b adrenergic receptor were tested in behavioral experiments. Reaction to novelty was first assessed in a simple test in which the time taken by the knockout mice and their littermate controls to enter a second compartment was compared. Then the mice were tested in an open field to which unknown objects were subsequently added. Special novelty was introduced by moving one of the familiar objects to another location in the open field. Spatial behavior and memory were further studied in a homing board test, and in the water maze. The alpha-1b knockout mice showed an enhanced reactivity to new situations. They were faster to enter the new environment, covered longer paths in the open field, and spent more time exploring the new objects. They reacted like controls to modification inducing spatial novelty. In the homing board test, both the knockout mice and the control mice seemed to use a combination of distant visual and proximal olfactory cues, showing place preference only if the two types of cues were redundant. In the water maze the alpha-1b knockout mice were unable to learn the task, which was confirmed in a probe trial without platform. They were perfectly able, however, to escape in a visible platform procedure. These results confirm previous findings showing that the noradrenergic pathway is important for the modulation of behaviors such as reaction to novelty and exploration, and suggest that this is mediated, at least partly, through the alpha-1b adrenergic receptors. The lack of alpha-1b adrenergic receptors in spatial orientation does not seem important in cue-rich tasks but may interfere with orientation in situations providing distant cues only.

Modest neuropsychological deficits caused by reduced noradrenaline metabolism in mice heterozygous for a mutated tyrosine hydroxylase gene

Tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme for the biosynthesis of catecholamines that are considered to be involved in a variety of neuropsychiatric functions. Here, we report behavioral and neuropsychological deficits in mice carrying a single mutated allele of the TH gene in which TH activity in tissues is reduced to approximately 40% of the wild-type activity. In the mice heterozygous for the TH mutation, noradrenaline accumulation in brain regions was moderately decreased to 73-80% of the wild-type value. Measurement of extracellular noradrenaline level in the frontal cortex by the microdialysis technique showed a reduction in high K(+)-evoked noradrenaline release in the mutants. The mutant mice displayed impairment in the water-finding task associated with latent learning performance. They also exhibited mild impairment in long-term memory formation in three distinct forms of associative learning, including active avoidance, cued fear conditioning, and conditioned taste aversion. These deficits were restored by the drug-induced stimulation of noradrenergic activity. In contrast, the spatial learning and hippocampal long-term potentiation were normal in the mutants. These results provide genetic evidence that the central noradrenaline system plays an important role in memory formation, particularly in the long-term memory of conditioned learning.

The effects of a specific alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and brain neurochemistry in aged Fisher 344 rats

The present experiments investigated the effects of a specific and potent alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and neurochemistry in aged rats. Aged control Fisher 344 rats, which had lower activities of choline acetyltransferase in the frontal cortex, were impaired in the acquisition of the linear arm maze task both in terms of repetition errors and their behavioural activity (the speed of arm visits), and they needed longer time to complete this task as compared to adult control rats. Atipamezole treatment (0.3 mg/kg) facilitated the acquisition of this task in the aged rats as they committed fewer errors and completed the task more quickly than saline-treated aged control rats. A separate experiment indicated that atipamezole enhanced the turnover of noradrenaline both in the adult and aged rats, but this effect was more pronounced in the aged rats. Furthermore, atipamezole enhanced significantly the turnover of serotonin and dopamine only in the aged rats when analysed in the whole brain samples. As alpha(2)-adrenoceptor antagonists are known to alleviate akinesia in the experimental models of Parkinson's disease, the present results could be especially relevant for the development of palliative treatment for demented Parkinsonian patients.

Short-term effects of postnatal manipulation on central beta-adrenoceptor transmission

Neonatal handling is known to induce long-lasting changes in behavioral and neuroendocrine responses to stress. Since the central noradrenergic system participates in the adaptive responses to stressful conditions we have analyzed the effects of postnatal handling on beta-adrenoceptor binding sites and isoprenaline- and forskolin-stimulated cyclic AMP accumulation in cerebral cortex, hippocampus and cerebellum of rats at 1 and 3 months of age. Handled animals showed reduced emotional reactivity and lower ACTH and corticosterone secretion after stress. Binding studies using [(3) H]CGP12-177 revealed increased beta-adrenoceptor binding sites in handled rats in cerebellum and cerebral cortex with no changes in hippocampus, and decreased affinity in all cerebral regions. Handling reduced basal levels of cyclic AMP in hippocampus and cerebellum but not in cerebral cortex. The concentration-response curves of cyclic AMP to isoprenaline were displaced to the right in cerebellum of handled rats without differences in Emax; however, Emax was significantly reduced in cerebral cortex and hippocampus. Direct stimulation of the catalytic subunit of adenylyl cyclase by forskolin reduced the efficiency in hippocampus and cerebellum, but not in cerebral cortex of handled animals. It is concluded that neonatal handling reduces the binding properties of beta-adrenoceptor and its primary biochemical responses in the young rat brain, which may account for the reduced responsiveness to stress attained in the handled rats, and may explain the persistence of the effect. The present study emphasizes the role of the central noradrenergic system in modulating the behavioral and neurendocrine responses to neonatal handling.

The role of norepinephrine in the pathophysiology of cognitive disorders: potential applications to the treatment of cognitive dysfunction in schizophrenia and Alzheimer's disease

The role of noradrenergic neurotransmission in normal cognitive functions has been extensively investigated, however, the involvement of noradrenergic functions in the cognitive impairment associated with schizophrenia and Alzheimer's disease has not been as intensively considered. The limited ability of atypical antipsychotics to treat the cognitive impairment of schizophrenia, and cholinomimetics to treat the cognitive impairment of Alzheimer's disease, may be related to the influence of a multiplicity of neurotransmitter abnormalities including noradrenergic dysfunction, which these treatments do not address. The evidence of noradrenergic dysfunction occurring concomitantly with dopamine dysfunction in schizophrenia and acetylcholine dysfunction in Alzheimer's disease supports therapeutic approaches using noradrenergic drugs in combination with neuroleptics and cholinesterase inhibitors, respectively, to enhance the treatment of cognitive impairment. Given the results of animal and human studies, it appears that alpha-2A agonists may be the optimal choice for this purpose.

Noradrenergic and serotonergic projections to the superior olive: potential for modulation of olivocochlear neurons

The distribution and density of noradrenergic (NA) and serotonergic (5-HT) varicosities in the superior olive (SO) and periolivary region (PO) and their relationship to olivocochlear neurons was studied. Antibodies against 5-HT and the NA precursor enzyme dopamine beta-hydroxylase were utilized to examine the density of innervation of SO and PO. To determine the relationship of these varicosities to efferent neurons projecting to the cochlea, olivocochlear neurons were retrogradely labeled with biotinylated dextranamine (BDA). NA and 5-HT varicosities were found adjacent to labeled olivocochlear neuron cell bodies and dendrites. More than 50% of labeled medial olivocochlear (MOC) neurons showed likely contact with NA varicosities and more than 90% of labeled MOC neurons with 5-HT varicosities. There was no apparent difference in the number of lateral olivocochlear (LOC) neurons in close proximity to NA and 5-HT varicosities versus MOCs in close proximity to NA and 5-HT varicosities. Our results suggest that the NA and 5-HT systems are in a position to modulate auditory brainstem processing. The specific relationship of NA and 5-HT varicosities to olivocochlear neurons suggests that one possible level of modulation is prior to signal transduction.

Locus coeruleus neurons: cessation of activity during cataplexy

Cataplexy, a symptom of narcolepsy, is a loss of muscle tone usually triggered by sudden, emotionally significant stimuli. We now report that locus coeruleus neurons cease discharge throughout cataplexy periods in canine narcoleptics. Locus coeruleus discharge rates during cataplexy were as low as or lower than those seen during rapid-eye-movement sleep. Prazosin, an alpha1 antagonist, and physostigmine, a cholinesterase inhibitor, both of which precipitate cataplexy, decreased locus coeruleus discharge rate. Our results are consistent with the hypothesis that locus coeruleus activity contributes to the maintenance of muscle tone in waking, and that reduction in locus coeruleus discharge plays a role in the loss of muscle tone in cataplexy and rapid-eye-movement sleep. Our results also show that the complete cessation of locus coeruleus activity is not sufficient to trigger rapid-eye-movement sleep in narcoleptics.

Pharmacologic strategies for augmenting cognitive performance in schizophrenia

There is recognition that the cognitive symptoms of schizophrenia have the most substantial impact on illness outcome. Domains of cognition reported to be significantly affected include serial learning, executive function, vigilance, and distractibility, to name a few. Dopamine activity at D1 receptors mediates many cognitive processes subserved by the prefrontal cortex (PFC), particularly working memory. The number of D1 receptors in the PFC is decreased in schizophrenics and is unaffected by chronic administration of typical neuroleptics. Therefore, medications that increase dopamine in the PFC, such as atypical neuroleptics, or that directly activate the D1 receptor may prove useful in the remediation of prefrontal-dependent cognitive deficits in schizophrenia. Decreased levels of cortical norepinephrine (NE) are associated with impaired learning and working memory in animal models, and can be reversed by drugs that restore NE activity. More specifically, alpha-2 adrenergic receptor agonists have been particularly effective in improving delayed response performance in young monkeys with localized 6-hydroxydopamine lesions in the PFC. Furthermore, human postmortem studies have demonstrated decreased NE in the frontal cortex of demented schizophrenic patients. Therefore, alpha-2 receptor agonists hold promise as drugs to improve cognitive performance on tasks dependent upon PFC function in schizophrenics. Finally, the finding that cortical choline acetyl transferase activity correlates with Clinical Dementia Rating scores in schizophrenic patients and that cholinomimetic drugs enhance cognition in healthy subjects suggests that cholinergic drugs may also treat cognitive symptoms in schizophrenia. Two potential types of cholinomimetics for use in schizophrenics are the acetylcholinesterase inhibitors and M1/M4 muscarinic agonists, both of which increase cortical cholinergic activity.

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.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

Effects of noradrenaline on frequency tuning of rat auditory cortex neurons

The selectivity of rat auditory cortex neurons for pure tone frequency was studied during and after ionophoretic application (5-40 nA) of noradrenaline in urethane-anaesthetized rats. The dominant effect induced by noradrenaline was a significant decrease in spontaneous (93/268 cells) and evoked activity (133/268 cells) which outlasted the application. In the whole population of cells (n = 268) the signal-to-noise ratio, computed using as the signal either the mean evoked response or the response at the best frequency, was unchanged during noradrenaline application. It was significantly increased only for cells showing significantly decreased spontaneous activity, and was significantly decreased for cells showing increased spontaneous activity. Frequency selectivity was significantly increased for the whole population during and after noradrenaline application. It was also significantly increased for cells showing significantly decreased evoked activity, and was significantly decreased for cells showing increased evoked activity. The noradrenaline-induced inhibition was not blocked by propranolol (beta antagonist); it was blocked by prazosin (alpha1 antagonist) and partly mimicked by phenylephrine (alpha1 agonist). GABA, which also inhibited spontaneous and evoked activity, slightly increased the signal-to-noise ratio and significant increased frequency selectivity. However, when noradrenaline was ejected in the presence of bicuculline at doses that were able to block GABAergic inhibition, the inhibitory effects of noradrenaline on spontaneous and evoked activity were still observed. The possible function of noradrenaline-induced inhibitions in sensory cortices is briefly discussed.

Depression as a spreading neuronal adjustment disorder

The outlines of a theory of the pathophysiology of depression are presented. The classic monoamine theory of depression as well as its more recent elaborations suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity and, concomitantly, exaggerated responses to acute increases in presynaptic firing rate and transmitter release. We propose that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus, which in turn leads to dysregulation of 5-HT-ergic and dopaminergic neurotransmission.

Alpha 2-adrenoceptor antagonists potentiate acetylcholinesterase inhibitor effects on passive avoidance learning in the rat

The cholinergic hypothesis of Alzheimer's disease (AD) has strongly influenced research on learning and memory over the last decade. However, there has been limited success treating AD dementia with cholinomimetics. Furthermore, there are indications that other neurotransmitter systems affected by this disease may be involved in cognitive processes. Animal studies have suggested that norepinephrine and acetylcholine may interact in learning and memory. The current experiments investigate this interaction in a step-down passive avoidance paradigm after coadministration of acetylcholinesterase inhibitors and alpha 2-adrenoceptor antagonists. Administration of acetylcholinesterase inhibitors heptylphysostigmine (0.625-5.0 mg/kg, IP), tacrine (2.5-10.0 mg/kg, PO), velnacrine (0.312-2.5 mg/kg, SC), and galanthamine (0.312-2.5 mg/kg IP) each enhanced retention of a passive avoidance response at selected moderate doses administered 30-60 min prior to training. The alpha 2-adrenoceptor antagonists idazoxan (0.312-2.5 mg/kg, IP), yohimbine (0.078-0.312 mg/kg, IP) and P86 7480 (0.156-0.625 mg/kg, IP) alone failed to enhance learning in this paradigm. Coadministration of a subthreshold dose of heptylphysostigmine (0.625 mg/kg, IP) with doses of idazoxan, yohimbine or P86 7480 enhanced passive avoidance learning. This synergistic interaction may represent effects of antagonism of presynaptic alpha 2-adrenoceptor since coadministration of heptylphysostigmine and the selective postsynaptic alpha 2-adrenoceptor antagonist SKF 104856 did not result in enhanced learning. Taken together these data suggest noradrenergic activation through pre-synaptic alpha 2-adrenoceptor blockade may potentiate cholinergic activity in the formation of a long-term memory trace. These observations may have implications for the treatment of AD with cholinergic and adrenergic agents.

Modulation of the reaction of hippocampal neurons to sensory stimuli by cholinergic substances

The influences of increasing endogenous acetylcholine (eserine) and its blockade (scopolamine) on the effects of sensory stimuli were analyzed through the extracellular recording of the activity of individual hippocampal neurons of awake rabbits. An increase in the level of acetylcholine, accompanied by the appearance of stable theta rhythm, leads to a substantial decrease in the reactivity of neurons, the suppression, attenuation, and inversion of the majority of inhibitory reactions and of a substantial proportion of activational reactions including on-responses of a specific type. At the same time, a limited group of activational reactions is intensified and extended against the background of eserine. Scopolamine, which blocks theta rhythm, does not change or intensifies inhibitory and some activational reactions, including on-responses. Tonic reactions are shortened; however, their gradual extinction disappears. The effects described are preserved in the hippocampus in the presence of basal undercutting of the septum which eliminates ascending brainstem pathways. These data make it possible to draw the conclusion that, under normal conditions, a new (significant) sensory stimulus elicits in the hippocampus an initial stoppage (reset) of activity with the coordinated triggering of theta rhythm and the passage against this background of signals along the cortical input in a specific phase relationship to it. The period of theta modulation switched on by the signal fosters its recording and the limitation of the passage of subsequent, interfering signals. The septohippocampal influences may thus support the mechanism of selective attention, as a necessary precondition for memory.

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.

The alpha 2-adrenergic antagonist idazoxan enhances the frequency selectivity and increases the threshold of auditory cortex neurons

Idazoxan (IDA), an alpha 2 antagonist of adrenoceptors, has been shown to increase cortical release of norepinephrine (NE) by an action mediated primarily by the alpha 2 autoreceptors located on the NE terminals. In the present experiment, IDA application was used to increase the cortial concentration of NE. Single unit activity (n = 107) was recorded in the rat auditory cortex, and the neurons' frequency receptive fields (FRF) were determined before and after systemic (intraperitoneal or intravenous) or local application of IDA. In the whole population (n = 107) there was a decrease in spontaneous activity and/or evoked activity for 84% of the recordings (90/107 cells). Decreased tone-evoked responses were obtained after systemic injections (n = 39), as well as after local applications (n = 68) of IDA. These effects were not observed after either systemic injections (n = 13) or local applications (n = 9) of saline. The signal-to-noise ratio (the mean evoked responses divided by the spontaneous activity) was slightly decreased after systemic injections and slightly increased after local applications. However, after both systemic and local injections the frequency selectivity of the neuronal responses was increased. For a group of neurons (n = 27), testing the FRF at three intensities indicated that this increased selectivity can be expressed at high or middle range intensity but not at low intensity. For 37 cells, the intensity function was tested at the best frequency before and after IDA application, and the threshold for excitatory responses was determined in 28 cases. An increased threshold was observed in 16 of 28 cases after IDA application. Thus, using a pharmacological procedure to increase the extracellular concentration of NE, the dominant inhibitory effect on the auditory cortex neurons led to an enhancement of the frequency selectivity, but also an increase in the threshold of these neurons.

Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

These experiments were designed to investigate the role of the noradrenergic system in promoting investigation of novelty in rats. Behavior was monitored in a hole board equipped with photoelectric cells strategically placed so that locomotor activity, rearing and investigation of each of the holes could be quantified independently. Specially designed computer software permitted recording of the sequence and cumulative duration of the visits to specific holes throughout the session. Dose-response curves of the sedative effect of the alpha 2 adrenergic receptor agonist clonidine were established, a sedative effect being defined as a decrease in overall horizontal displacements, rearings and hole visits. After a one week interval, the rats were rerun in the holeboard, with novel objects placed in four of the nine holes. Previous experiments had shown that rats spend significantly more time investigating holes containing objects than empty holes in this apparatus and this was replicated here. Doses of clonidine which were below threshold for inducing any sedative effect (10 micrograms/kg) totally eliminated preference for holes with objects while having no effect on total time investigating the holes. A subsequent experiment showed that the beta receptor antagonist propranolol (10 mg/kg) produced a similar effect. These results suggest that the noradrenergic system is implicated in stimulus seeking behavior and the post-synaptic beta receptors are involved in mediating the behavior.

Ultrastructural organization of the noradrenergic innervation of the superficial gray layer of the hamster's superior colliculus

Immunocytochemistry with an antibody-directed tyrosine hydroxylase (TH) was combined with electron microscopy and serial-section analysis to examine the synaptic organization of the catecholaminergic projection to the stratum griseum superficiale (SGS) of the hamster's superior colliculus (SC). A total of 250 TH-immunoreactive profiles within SGS were examined. Of these, 114 (45.6%) made synaptic contacts; 81 (71.1%) were axodendritic, and the remainder (33, 28.9%) were axo-axonic. Serial-section analysis was employed to evaluate the presence or absence of synaptic contacts for 26 profiles. Overall, 19 (73.1%) of the profiles followed through serial sections exhibited synaptic contacts. Double staining of single sections with antibodies directed against TH and dopamine-beta-hydroxylase (D beta H) and examination in the light microscope indicated that virtually all TH-positive fibers also contained D beta H. This indicated that the fibers examined at the electron microscopic level were noradrenergic rather than dopaminergic. These results suggest that norepinephrine may have both pre- and postsynaptic actions in the hamster's SC and that at least some of these effects are mediated by conventional synapses.

Effects of intraseptally injected noradrenergic drugs on hippocampal sodium-dependent-high-affinity-choline-uptake in 'resting' and 'trained' mice

It has been shown in numerous studies that memory testing can alter presynaptic cholinergic activity within the hippocampus. In the present experiments, the role of the noradrenergic input to the septal cholinergic neurons in the immediate increase in cholinergic activity induced by the first training session of a spatial reference memory task in an 8-arm radial maze was investigated. The effects of bilateral intraseptal injections of noradrenergic drugs on hippocampal sodium-dependent-high-affinity-choline-uptake (SDHACU) were studied in 'resting' animals (basal level) or in 'trained' animals injected 20 min before training and sacrificed immediately after the test. The results showed that: (1) the injection of maprotiline, a noradrenaline reuptake inhibitor (0.06 ng/site), induced an increase in hippocampal SDHACU in 'resting' animals, whereas the alpha 2-adrenoceptor agonist UK 14304 (1.5 ng) significantly reduced the basal level of SDHACU; (2) none of the alpha-adrenoceptor antagonists used (phenoxybenzamine, 10 and 100 ng; BE 2254, 100 and 500 ng; yohimbine, 0.5 and 50 ng) significantly affected the basal level of hippocampal SDHACU, and only the alpha 1-adrenoceptor antagonist BE 2254 (500 ng) significantly reduced the testing-induced activation of SDHACU. Taken together, these findings suggest that noradrenaline may exert a bimodal regulatory influence on the activity of septo-hippocampal cholinergic neurons. The behavior-induced activation of hippocampal SDHACU could be partly mediated by the stimulation of alpha 1-adrenoceptors, whereas postsynaptic alpha 2-adrenoceptors may be important for the maintenance of a tonic inhibition of the steady-state cholinergic activity in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

The Maudsley rat strains as a probe to investigate noradrenergic-cholinergic interaction in cognitive function

Central noradrenergic function in relation to cognitive performance was studied in the Maudsley rat strains. Neurochemical studies revealed a higher response to acute stress in the locus coeruleus (LC) in the Maudsley reactives (MR) than in the Maudsley non-reactives (MNRA). Autoradiographic studies showed that MNRAs had greater 125I clonidine binding to alpha 2 receptors in LC, which was accompanied by a higher behavioral sensitivity to clonidine. MRs had a deficit in working memory, but were superior to MNRAs in two reference memory tasks. MRs displayed a stronger preference for novel objects, with no strain differences in general exploratory activity. The behavioral profile of the MRs is similar to rats treated with drugs which enhance noradrenergic function. Furthermore, MNRA rats had greater availability of muscarinic receptors, which correlated with behavioral performance in the spatial working memory task. The differences in noradrenergic and cholinergic systems and their relationship to the behavioral profile make the Maudsley strains a useful tool to probe the interaction between two neurotransmitter systems in cognitive function.

Locus coeruleus-evoked responses in behaving rats: a clue to the role of noradrenaline in memory

Neuromodulatory properties of noradrenaline (NA) suggest that the coreruleo-cortical NA projection should play an important role in attention and memory processes. Our research is aimed at providing some behavioral evidence. Single units of the locus coeruleus (LC) are recorded during controlled behavioral situations, in order to relate LC activation to specific behavioral contexts. LC cells respond in burst to imposed novel sensory stimuli or to novel objects encountered during free exploration. When there is no predictive value of the stimulus or no behavioral response required, there is rapid habituation of the LC response. When a stimulus is then associated with reinforcement, there is a renewed response, which is transient. During extinction, LC neuronal responses reappear. Thus, LC cells respond to novelty or change in incoming information, but do not have a sustained response to stimuli, even when they have a high level of biological significance. The gating and tuning action of NA released in target sensory systems would promote selective attention to relevant stimuli at the critical moment of change. The adaptive behavioral outcome would result from the integration of retrieved memory with the sensory information selected from the environment.

Maudsley rat strains, selected for differences in emotional responses, differ in behavioral response to clonidine and in [125I]clonidine binding in the locus coeruleus

Maudsley rats, selectively inbred for emotionality for over sixty generations, differ in reactivity to stress, both at the peripheral level and within the central noradrenergic system. The present experiments examine to what extent these central differences might be due to differences in the inhibitory processes mediated by alpha 2 autoreceptors within the locus coeruleus. Maudsley reactive rats (MRs), the strain which showed a much higher central noradrenergic response to immobilisation stress, required higher doses of the alpha 2 receptor agonist, clonidine, to induce behavioral sedation than the Maudsley non-reactive rats (MNRA). Autoradiographic studies showed a significantly higher level of binding of 125iodeclonidine in the locus coeruleus of the MNRAs compared to the MRs, indicating that the former had more alpha 2 receptors and/or these receptors had a greater affinity for the agonist. Thus autoinhibitory processes within the locus coeruleus are different in the two strains, which could account for the differences in reactivity to stress seen in the biochemical and behavioral studies.

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.

Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--IV. Sensory stimulation

Modifications of responses of hippocampal neurons to sensory stimuli at the background of increased endogenous acetylcholine level (injection of physostigmine) and during blocking by scopolamine were analysed in the chronic alert rabbit. A significant decrease of reactivity (about 40%) of hippocampal neurons to sensory stimuli occurred after physostigmine injection, inducing stable theta modulation. Suppression and decrease of inhibitory responses (including initial reset phase) and of some excitatory reactions (including on-effects) were observed. However, a limited group of excitatory responses was augmented and prolonged under physostigmine action. Scopolamine, which blocked electroencephalogram theta-rhythm, did not change the responsiveness of hippocampal neurons. Some of the inhibitory and excitatory effects of sensory stimuli, especially on-responses, were strongly facilitated. Tonic responses were shorter, but they were stably reproduced without typical gradual habituation. All these effects were also present in the hippocampus after basal undercutting of the septum, which eliminates ascending brainstem input. It is suggested that under normal conditions a new or significant sensory stimulus evokes, in the hippocampus, an initial inhibitory reset of neuronal activity with subsequent coordinated triggering of rhythmic theta modulation by the septal input and arrival of the cortical input signal phase-locked to it. During the period of theta triggered by the stimulus, its processing and fixation in memory occurs, while the other, interfering stimuli, which are not phase-locked to the ongoing theta activity, are actively filtered out. Thus, septohippocampal interactions may participate in the organization of selective attention as a necessary condition for memory trace formation.

Effects of locus coeruleus lesions on vigilance and attentive behaviour in cat

Previous data have suggested that in the cat, expectancy behaviour (waiting for a target to appear) and associated electrocortical, focal, synchronized activity ('mu' rhythms) are modulated by a noradrenergic system possibly originating from the locus coeruleus (LC). To test the latter hypothesis, we have examined the behavioural and ECoG changes induced after bilateral LC lesions. Our results demonstrated that destruction of the anterior 3/4th of the LC (A6 noradrenergic cell group) resulted in a considerable increase of mu rhythms and expectancy behaviour, without episodes of drowsiness that normally occur. Destruction of the posterior fourth of LC (A4 noradrenergic group) only increased the duration of slow sleep. Extending the A6 lesion to include the dorsal ascending noradrenergic bundle also increased the expectancy behaviour and mu rhythms. Finally, when the nucleus subcoeruleus was also involved, the duration of slow sleep and the frequency of paradoxical sleep episodes increased. These findings indicate that the LC exerts an inhibitory effect on structures involved in the induction and persistence of expectancy behaviour with accompanying mu rhythms.

Limbic forebrain toxin trimethyltin reduces behavioral suppression by clonidine

Trimethyltin (TMT) at moderate doses selectively damages hippocampus and related olfactory cortex and produces learning and memory impairments. TMT also increases forebrain beta-adrenergic ligand binding; this could be ancillary to reduced noradrenergic neurotransmission, which in turn could be involved in the cognitive deficit caused by TMT. If this hypothesis is correct, then the alpha 1-adrenergic agonist clonidine, which inhibits noradrenergic neurotransmission in normal subjects, should be less behaviourally effective after TMT poisoning. Thus, rats treated with water vehicle or TMT (6 mg/kg, PO) were given saline or clonidine IP (5, 10, or 20 micrograms/kg) 30 min before placement in a hole-board apparatus. Exploratory activity was reduced in controls by 10 or 20 micrograms/kg. Clonidine at 10 micrograms/kg was ineffective in rats given TMT. At 20 micrograms/kg, an apparent reduction in exploratory activity was not significant because variability of responding was higher after TMT treatment. The results suggest an impairment in noradrenergic neurotransmission following TMT poisoning.

Enhancement of excitability and inhibitory processes in hippocampal dentate gyrus by noradrenaline: a pharmacological study in awake, freely moving rats

Idazoxan (IDA), an alpha 2 receptor antagonist which increases firing rate of noradrenergic neurons in the locus coeruleus (LC) and release of noradrenaline (NA) in target structures, was used to study the neuromodulatory effects of NA in the hippocampus in awake rats. After IDA the population spike in the dentate gyrus (DG), evoked by a single pulse to the perforant path, was greatly enhanced with no effect of the drug on excitatory postsynaptic potentials (EPSPs). Paired pulses with short interpulse intervals (25-30 ms) produced inhibition of the response to the second pulse which was increased by IDA. This drug effect was independent of its effect on the amplitude of the first spike, since the increase in inhibition was seen at stimulation intensities which did not increase the response amplitude to the initial pulse. Thus both excitability and inhibitory processes can be enhanced in the same population of neurons by an alpha 2 adrenoceptor antagonist.

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.