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

Behavioral and Transcriptomic Changes Following Brain-Specific Loss of Noradrenergic Transmission

Noradrenaline (NE) plays an integral role in shaping behavioral outcomes including anxiety/depression, fear, learning and memory, attention and shifting behavior, sleep-wake state, pain, and addiction. However, it is unclear whether dysregulation of NE release is a cause or a consequence of maladaptive orientations of these behaviors, many of which associated with psychiatric disorders. To address this question, we used a unique genetic model in which the brain-specific vesicular monoamine transporter-2 (VMAT2) gene expression was removed in NE-positive neurons disabling NE release in the entire brain. We engineered VMAT2 gene splicing and NE depletion by crossing floxed VMAT2 mice with mice expressing the Cre-recombinase under the dopamine β-hydroxylase (DBH) gene promotor. In this study, we performed a comprehensive behavioral and transcriptomic characterization of the VMAT2DBHcre KO mice to evaluate the role of central NE in behavioral modulations. We demonstrated that NE depletion induces anxiolytic and antidepressant-like effects, improves contextual fear memory, alters shifting behavior, decreases the locomotor response to amphetamine, and induces deeper sleep during the non-rapid eye movement (NREM) phase. In contrast, NE depletion did not affect spatial learning and memory, working memory, response to cocaine, and the architecture of the sleep-wake cycle. Finally, we used this model to identify genes that could be up- or down-regulated in the absence of NE release. We found an up-regulation of the synaptic vesicle glycoprotein 2c (SV2c) gene expression in several brain regions, including the locus coeruleus (LC), and were able to validate this up-regulation as a marker of vulnerability to chronic social defeat. The NE system is a complex and challenging system involved in many behavioral orientations given it brain wide distribution. In our study, we unraveled specific role of NE neurotransmission in multiple behavior and link it to molecular underpinning, opening future direction to understand NE role in health and disease.

Noradrenergic Modulation of Fear Conditioning and Extinction

The locus coeruleus norepinephrine (LC-NE) system plays a broad role in learning and memory. Here we begin with an overview of the LC-NE system. We then consider how both direct and indirect manipulations of the LC-NE system affect cued and contextual aversive learning and memory. We propose that NE dynamically modulates Pavlovian conditioning and extinction, either promoting or impairing learning aversive processes under different levels of behavioral arousal. We suggest that under high levels of stress (e.g., during/soon after fear conditioning) the locus coeruleus (LC) promotes cued fear learning by enhancing amygdala function while simultaneously blunting prefrontal function. Under low levels of arousal, the LC promotes PFC function to promote downstream inhibition of the amygdala and foster the extinction of cued fear. Thus, LC-NE action on the medial prefrontal cortex (mPFC) might be described by an inverted-U function such that it can either enhance or hinder learning depending on arousal states. In addition, LC-NE seems to be particularly important for the acquisition, consolidation and extinction of contextual fear memories. This may be due to dense adrenoceptor expression in the hippocampus (HPC) which encodes contextual information, and the ability of NE to regulate long-term potentiation (LTP). Moreover, recent work reveals that the diversity of LC-NE functions in aversive learning and memory are mediated by functionally heterogeneous populations of LC neurons that are defined by their projection targets. Hence, LC-NE function in learning and memory is determined by projection-specific neuromodulation that accompanies various states of behavioral arousal.

On the persistence of cocaine-induced place preferences and aversions in rats

RATIONALE

Rats develop preferences for places associated with the immediate rewarding effects of cocaine and aversions for places paired with the drug's delayed negative effects. The motivation to seek cocaine should therefore depend upon the relative magnitude of these two opposing effects of the drug.

OBJECTIVE

The current study tested this notion by assessing the relative persistence of the positive and negative associations formed between environmental cues and the immediate or delayed effects of cocaine.

METHODS

Rats were administered 1.0 mg/kg intravenous cocaine and placed into a distinctive environment either immediately or 15-min after injection, alternating daily with pairings of a second environment with saline. After four drug-place and four saline-place pairings, rats were returned to their home cages for 1, 7, or 21 days after which a 15-min place preference test was conducted. In a second experiment, the effectiveness of a single reconditioning session (one drug-place and one saline-place pairing) to reactivate learned cocaine-place associations was assessed after 1 or 3 weeks of drug abstinence.

RESULTS

Places associated with the immediate effects of cocaine were preferred (CPP), while places associated with the delayed effects of cocaine were avoided (CPA). The persistence of these effects differed with CPP remaining viable at 3 weeks of withdrawal, while CPA was no longer present after 1 week. Reconditioning with an additional cocaine-place pairing failed to reinstate the CPA.

CONCLUSIONS

Cue-induced "relapse" of cocaine-seeking behavior may be fueled in part by an increased persistence of positive relative to negative associations with drug-paired stimuli.

Alpha(2)-noradrenergic antagonist administration into the central nucleus of the amygdala blocks stress-induced hypoalgesia in awake behaving rats

Stress-induced hypoalgesia (SIH) is an adaptive behavioral phenomenon mediated in part by the amygdala. Acute stress increases amygdalar noradrenaline levels and focal application of alpha(2)-adrenoceptor agonists in the central nucleus of the amygdala (CeA) is antinociceptive. We hypothesized that alpha(2)-adrenoceptor antagonist administration into the CeA may block SIH. Bilateral microinjections of drug or saline via chronically implanted CeA cannulae were followed by either a period of restraint stress or rest. The nocifensive paw-withdrawal latency (PWL) to a focused beam of light was measured. PWLs were longer in restrained rats, constituting SIH. Microinjection of the alpha(2)-adrenoceptor antagonist idazoxan into the CeA prior to restraint blocked SIH. Idazoxan administration in unrestrained rats had no effect. Microinjection of the alpha(2)-adrenoceptor agonist clonidine in unrestrained rats caused dose dependent hypoalgesia, mimicking the effects of environmental stress. alpha(2)-Adrenoceptor function in the CeA is necessary for restraint-induced SIH.

Comparison of bilaterally 6-OHDA- and MPTP-lesioned rats as models of the early phase of Parkinson's disease: Histological, neurochemical, motor and memory alterations

This study compares histological, neurochemical, behavioral, motor and cognitive alterations as well as mortality of two models of Parkinson's disease in which 100 microg 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6 microg 6-hydroxydopamine (6-OHDA) was bilaterally infused into the central region of the substantia nigra, compact part, of adult male Wistar rats. Both neurotoxins caused a significant loss of nigral tyrosine hydroxylase-immunostained cells and striatal dopamine depletion, but 6-OHDA caused more widespread and intense cell loss, more intense body weight loss and more mortality than MPTP. Both 6-OHDA- and MPTP-lesioned rats presented similar deficits in performing a working memory and a cued version of the Morris water maze task and few exploratory/motor alterations in the open field and catalepsy tests. However, rats presented a significant and transitory increase in locomotor activity after the MPTP lesion and a hypolocomotor behavior tended to be present after the 6-OHDA lesion. The picture of mild motor effects and robust impairment of habit learning and spatial working memory observed in MPTP-lesioned rats models the early phase of Parkinson's disease.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Startle reactivity and anxiety disorders: aversive conditioning, context, and neurobiology

The aim of this article is to review studies on human anxiety using the startle reflex methodology and to apply the literature on context conditioning in rats to interpret the results. A distinction is made between cued fear (as in specific phobia), a phasic response to an explicit threat cue, and anxiety, a more sustained and future-oriented response not linked to a specific discrete cue. Experimentally, contextual fear, as opposed to cued fear, may best reflect the feeling of aversive expectation about potential future dangers that characterizes anxiety. Following a brief description of the neurobiology of cued fear and context conditioning, evidence is presented showing that anxious patients are overly sensitive to threatening contexts. It is then argued that the degree to which contextual fear is prompted by threat depends on whether the danger is predictable or unpredictable. Consistent with animal data, unpredictable shocks in humans result in greater context conditioning compared to predictable shocks. Because conditioning promotes predictability, it is proposed to use conditioning procedures to study the development of appropriate and inappropriate aversive expectations. Cued fear learning is seen as an adaptive process by which undifferentiated fear becomes cue-specific. Deficits in cued fear learning lead to the development of nonadaptive aversive expectancies and an attentional bias toward generalized threat. Lacking a cue for threat, the organism cannot identify periods of danger and safety and remains in a chronic state of anxiety. Factors that may affect conditioning are discussed.

Effects of lesioning noradrenergic neurones in the locus coeruleus on conditioned and unconditioned aversive behaviour in the rat

1. The brain noradrenergic system may have a role in anxiety disorder. This study has examined the effect of bilateral 6-hydroxydopamine lesions of the noradrenergic neurones in the locus coeruleus (LC) of male Lister hooded rats on behaviour produced by unconditioned and conditioned aversive stimuli. 2. The 6-hydroxydopamine (4 microg) lesions markedly reduced the noradrenaline content of the locus coeruleus hypothalamus, frontal cortex and the periaqueductal grey area without altering the levels of either dopamine or 5-hydroxytryptamine measured 14 days after administration. 3. Exposure to ultrasound (20 kHz at 98 dB for 60 sec), an unconditioned aversive stimulus, induced a defence response in the rats characterised by an increase in activity (running and jumping) followed by a period of inactivity (freezing). 4. Lesioning of the LC significantly attenuated the duration of freezing but was without effect on the active phase of the response. A similar reduction in freezing behaviour was seen with LC lesions when rats were exposed (3 hours after the acquisition) to the contextual cue of the conditioned emotion response paradigm. 5. These findings confirm that the locus coeruleus is involved in the regulation of fear-related behaviour in the rat both in an unconditioned and a conditioned model. Furthermore the results indicate that noradrenaline modifies defence behaviour rather than being the principle activating mechanism.

Conditioned responses of monkey locus coeruleus neurons anticipate acquisition of discriminative behavior in a vigilance task

Impulse activity was recorded extracellularly from noradrenergic neurons in the nucleus locus coeruleus of three cynomolgus monkeys performing a visual discrimination (vigilance) task. For juice reward, the subjects were required to release a lever rapidly in response to an improbable target stimulus (20% of trials) that was randomly intermixed with non-target stimuli presented on a video display. All locus coeruleus neurons examined were phasically and selectively activated by target stimuli in this task. Other task events elicited no consistent response from these neurons (juice reward, lever release, fix spot stimuli, non-target stimuli). With reversal of the task contingency, locus coeruleus neurons ceased responding to the former target stimuli, and began responding instead to the new target (old non-target) stimuli. In addition, the latency of locus coeruleus response to target stimuli increased after reversal (by about 140 ms) in parallel with a similar increase in the latency of the behavioral response. These results indicate that the conditioned locus coeruleus responses reflect stimulus meaning and cognitive processing, and are not driven by physical sensors attributes. Notably, the reversal in locus coeruleus response to stimuli after task reversal occurred rapidly, hundreds of trials before reversal was expressed in behavioral responses. These findings indicate that conditioned responses of locus coeruleus neurons are plastic and easily altered by changes in stimulus meaning, and that the locus coeruleus may play an active role in learning the significance of behaviorally important stimuli.

Norepinephrine depletion impairs neuroendocrine responses to fear but not novel environmental stimuli in the rat

Emotional stimuli suppress vasopressin secretion and potentiate oxytocin and prolactin secretion by the pituitary in the rat. We studied effects of central norepinephrine depletion on these hormonal responses to novel environmental or fear stimuli. Male Wistar rats were injected intracerebroventricularly with 5-amino-2,4-dihydroxy-alpha- methylphenylethylamine, a selective neurotoxin to noradrenergic fibers. The neurotoxin treatment reduced the hypothalamic content of norepinephrine by 71% but did not significantly affect the dopamine content. Novel environmental stimuli suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected rats. The neurotoxin did not block the neuroendocrine responses. Intermittently applied electric footshocks also induced the similar neuroendocrine responses in the vehicle-injected rats. The neurotoxin significantly reduced the neuroendocrine responses. The drug, however, did not significantly alter vasopressin release after continuously applied footshocks. Environmental stimuli previously paired with footshocks (conditioned fear stimuli) suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected animals. Motor activity was suppressed during the conditioned fear stimuli. The neurotoxin impaired the neuroendocrine and behavioral responses whether the drug was injected before or after the conditioning. These data demonstrate the distinction between the neural mechanisms underlying the neuroendocrine responses to fear and to novel stimuli, suggesting that noradrenergic neurons are selectively involved in the hypothalamo-hypophysial responses to fear stimuli.

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.

Diencephalic Noradrenaline Depletion Impairs the Corticosterone Response to Footshock but does not Affect Conditioned Fear

This study tested the hypothesis that hypothalamic noradrenaline (NA) depletion induced by 6-hydroxydopamine alters neuroendocrine, but not behavioural, responses to aversive stimuli. Sham-operated and NA depleted rats were exposed to pairings of an auditory (clicker) CS and (footshock) US in a distinctive environment. Subjects were tested for preference of a 'safe' environment over the one in which they were shocked, as a measure of effective conditioning to the contextual stimuli present in the distinctive environment. Subjects were also tested, in a separate operant chamber, for the suppression of drinking in the presence of the auditory stimulus, as a measure of effective conditioning to the explicit auditory CS. Blood samples were collected immediately following each phase of the behavioural experiment and were later analysed for plasma Corticosterone concentration. Behavioural and Corticosterone responses of individual control animals to the CS were positively correlated, consistent with previous results. This correlation was not present in the NA depleted group. The lesioned rats also showed a severely attenuated Corticosterone response to the footshock US. By contrast, NA depletion had no effect on any behavioural measure of CS or contextual conditioning. Together with previous experiments, these results suggest that diencephalic NA projections are more likely to mediate neuroendocrine, and coeruleo-cortical NA projections are more likely to mediate behavioural responses to conditioned and unconditioned aversive stimuli.