The locus coeruleus and noradrenergic modulation of cognition

Genetic dissection of dopaminergic and noradrenergic contributions to catecholaminergic tracts in early larval zebrafish

The catecholamines dopamine and noradrenaline provide some of the major neuromodulatory systems with far-ranging projections in the brain and spinal cord of vertebrates. However, development of these complex systems is only partially understood. Zebrafish provide an excellent model for genetic analysis of neuronal specification and axonal projections in vertebrates. Here, we analyze the ontogeny of the catecholaminergic projections in zebrafish embryos and larvae up to the fifth day of development and establish the basic scaffold of catecholaminergic connectivity. The earliest dopaminergic diencephalospinal projections do not navigate along the zebrafish primary neuron axonal scaffold but establish their own tracts at defined ventrolateral positions. By using genetic tools, we study quantitative and qualitative contributions of noradrenergic and defined dopaminergic groups to the catecholaminergic scaffold. Suppression of Tfap2a activity allows us to eliminate noradrenergic contributions, and depletion of Otp activity deletes mammalian A11-like Otp-dependent ventral diencephalic dopaminergic groups. This analysis reveals a predominant contribution of Otp-dependent dopaminergic neurons to diencephalospinal as well as hypothalamic catecholaminergic tracts. In contrast, noradrenergic projections make only a minor contribution to hindbrain and spinal catecholaminergic tracts. Furthermore, we can demonstrate that, in zebrafish larvae, ascending catecholaminergic projections to the telencephalon are generated exclusively by Otp-dependent diencephalic dopaminergic neurons as well as by hindbrain noradrenergic groups. Our data reveal the Otp-dependent A11-type dopaminergic neurons as the by far most prominent dopaminergic system in larval zebrafish. These findings are consistent with a hypothesis that Otp-dependent dopaminergic neurons establish the major modulatory system for somatomotor and somatosensory circuits in larval fish.

Noradrenaline activation of neurotrophic pathways protects against neuronal amyloid toxicity

Degeneration of locus coeruleus (LC) noradrenergic forebrain projection neurons is an early feature of Alzheimer's disease. The physiological consequences of this phenomenon are unclear, but observations correlating LC neuron loss with increased Alzheimer's disease pathology in LC projection sites suggest that noradrenaline (NA) is neuroprotective. To investigate this hypothesis, we determined that NA protected both hNT human neuronal cultures and rat primary hippocampal neurons from amyloid-beta (Abeta(1-42) and Abeta(25-35)) toxicity. The noradrenergic co-transmitter galanin was also effective at preventing Abeta-induced cell death. NA inhibited Abeta(25-35)-mediated increases in intracellular reactive oxygen species, mitochondrial membrane depolarization, and caspase activation in hNT neurons. NA exerted its neuroprotective effects in these cells by stimulating canonical beta(1) and beta(2) adrenergic receptor signaling pathways involving the activation of cAMP response element binding protein and the induction of endogenous nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Treatment with functional blocking antibodies for either NGF or BDNF blocked NA's protective actions against Abeta(1-42) and Abeta(25-35) toxicity in primary hippocampal and hNT neurons, respectively. Taken together, these data suggest that the neuroprotective effects of noradrenergic LC afferents result from stimulating neurotrophic NGF and BDNF autocrine or paracrine loops via beta adrenoceptor activation of the cAMP response element binding protein pathway.

Combined administration of alpha1-adrenoceptor antagonist prazosin and beta-blocker propranolol impairs spatial avoidance learning on a dry arena

Spatial learning is a widely studied type of animal behavior often considered as a model of higher human cognitive functions. Noradrenergic receptors play a modulatory role in many nerve functions, including vigilance, attention, reward, learning and memory. The present study aimed at studying the effects of separate or combined systemic administration of the alpha1-adrenergic antagonist prazosin (1 and 2 mg/kg) and beta-blocker propranolol (5 and 20 mg/kg) on the hippocampus-dependent learning in the active allothetic place avoidance (AAPA) task. Both centrally active drugs impaired spatial learning when administered together, exerting no effect in separate applications. Locomotion was impaired only in a combined application of higher doses of both drugs (2 mg/kg prazosin and 20 mg/kg propranolol). These results suggest an in vivo interaction between these two types of receptors in spatial navigation regulation.

Noradrenergic modulation of extinction learning and exposure therapy

Memory consolidation is enhanced by emotional arousal, an effect mediated by noradrenergic beta-receptor signaling. Norepinephrine strengthens consolidation of both appetitive and aversive learning, and is implicated in extinction of conditioned responses. In this review, we summarize work on the noradrenergic mechanisms of extinction learning and implications for extinction-based exposure therapy. The evidence suggests that norepinephrine release evoked by conditioned stimuli during extinction strengthens extinction memory via beta-receptor signaling. The modulatory effect of norepinephrine during extinction depends on predictable presentation of conditioned stimuli and optimal levels of norepinephrine release. Mechanistically, norepinephrine acts to increase cellular excitability and enhance synaptic plasticity within extinction-related neural circuitry. Currently, drugs that modulate norepinephrine are being used to treat symptoms of anxiety disorders, and are now being tested as pharmacotherapeutic prophalactics in the prevention of chronic posttraumatic stress reactions and as adjuncts to extinction-based exposure therapy. Studies of these new applications of noradrenergic drugs show a converging pattern of results with basic science suggesting ways in which basic laboratory findings can be translated into procedures to enhance clinical outcomes.

Post-retrieval disruption of a cocaine conditioned place preference by systemic and intrabasolateral amygdala beta2- and alpha1-adrenergic antagonists

Previous work has demonstrated post-retrieval impairment in associative learning paradigms, including those mediated by drugs of abuse, using nonspecific beta-adrenergic receptor (beta-AR) antagonists. Remarkably little is known about the role of the specific beta-AR subtypes, or other adrenergic receptors, in these effects. The current study examined the effects of beta(1) and beta(2), as well as alpha(1)-adrenergic receptor antagonism following retrieval of a cocaine conditioned place preference (CPP). We found that rats administered the beta(2) antagonist ICI 118,551 (8 mg/kg intraperitoneal [IP]) or the alpha(1) antagonist prazosin (1 mg/kg IP) following a drug-free test for CPP showed attenuated preference during a subsequent test, while the beta(1) antagonist betaxolol (5 or 10 mg/kg IP) and a lower dose of prazosin (0.3 mg/kg IP) had no effect. Furthermore, post-test microinfusion of ICI 118,551 (6 nmol/side) or prazosin (0.5 nmol/side) into the basolateral amygdala (BLA) also impaired a subsequent preference. Systemic or intra-BLA ICI 118,551 or prazosin administered to rats in their home cages, in the absence of a preference test, had no effect on CPP 24 h later. ICI 118,551 also attenuated the FOS response in the BLA induced by the CPP test. These results are the first to demonstrate a role for alpha(1)- and beta(2)-specific adrenergic mechanisms in post-retrieval memory processes. These systemic and site-specific injections, as well as the FOS immunohistochemical analyses, implicate the importance of specific noradrenergic signaling mechanisms within the BLA in post-retrieval plasticity.

Excitation of locus coeruleus noradrenergic neurons by thyrotropin-releasing hormone

Locus coeruleus (LC) noradrenergic neurons are implicated in a variety of functions including the regulation of vigilance and the modulation of sensory processing. Thyrotropin-releasing hormone (TRH) is an endogenous neuropeptide that induces a variety of behavioural changes including arousal and antinociception. In the present study, we explored whether the activity of LC noradrenergic neurons is modulated by TRH. Using current-clamp recording from isolated rat LC neurons, we found that TRH increased the firing rate of spontaneous action potentials. The TRH action was mimicked by TRH analogues including taltirelin and TRH-gly. In voltage-clamp recording at a holding potential of 50 mV, TRH produced an inward current associated with a decrease in the membrane K+ conductance. This current was inhibited by the TRH receptor antagonist chlordiazepoxide. Following inhibition of the pH-sensitive K+ conductance by extracellular acidification, the TRH response was fully inhibited. The TRH-induced current was also inhibited by the phospholipase C (PLC) inhibitor U-73122, but not by the protein kinase C inhibitor chelerythrine nor by chelation of intracellular Ca2+ by BAPTA. The recovery from the facilitatory action of TRH on the spike frequency was markedly inhibited by a high concentration of wortmannin. These results suggest that TRH activates LC noradrenergic neurons by decreasing an acid-sensitive K+ conductance via PLC-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. The present findings demonstrate that TRH activates LC neurons and characterize the underlying signalling mechanisms. The action of TRH on LC neurons may influence a variety of CNS functions related to the noradrenergic system which include arousal and analgesia.

Genetic enhancement of memory and long-term potentiation but not CA1 long-term depression in NR2B transgenic rats

One major theory in learning and memory posits that the NR2B gene is a universal genetic factor that acts as rate-limiting molecule in controlling the optimal NMDA receptor's coincidence-detection property and subsequent learning and memory function across multiple animal species. If so, can memory function be enhanced via transgenic overexpression of NR2B in another species other than the previously reported mouse species? To examine these crucial issues, we generated transgenic rats in which NR2B is overexpressed in the cortex and hippocampus and investigated the role of NR2B gene in NMDA receptor-mediated synaptic plasticity and memory functions by combining electrophysiological technique with behavioral measurements. We found that overexpression of the NR2B subunit had no effect on CA1-LTD, but rather resulted in enhanced CA1-LTP and improved memory performances in novel object recognition test, spatial water maze, and delayed-to-nonmatch working memory test. Our slices recordings using NR2A- and NR2B-selective antagonists further demonstrate that the larger LTP in transgenic hippocampal slices was due to contribution from the increased NR2B-containing NMDARs. Therefore, our genetic experiments suggest that NR2B at CA1 synapses is not designated as a rate-limiting factor for the induction of long-term synaptic depression, but rather plays a crucial role in initiating the synaptic potentiation. Moreover, our studies provide strong evidence that the NR2B subunit represents a universal rate-limiting molecule for gating NMDA receptor's optimal coincidence-detection property and for enhancing memory function in adulthood across multiple mammalian species.

State-dependent performance of optic-flow processing interneurons

Active locomotive states are metabolically expensive and require efficient sensory processing both to avoid wasteful movements and to cope with an extended bandwidth of sensory stimuli. This is particularly true for flying animals because flight, as opposed to walking or resting, imposes a steplike increase in metabolism for the precise execution and control of movements. Sensory processing itself carries a significant metabolic cost, but the principles governing the adjustment of sensory processing to different locomotor states are not well understood. We use the blowfly as a model system to study the impact on visual processing of a neuromodulator, octopamine, which is known to be involved in the regulation of flight physiology. We applied an octopamine agonist and recorded the directional motion responses of identified visual interneurons known to process self-motion-induced optic flow to directional motion stimuli. The neural response range of these neurons is increased and the response latency is reduced. We also found that, due to an elevated spontaneous spike rate, the cells' negative signaling range is increased. Meanwhile, the preferred self-motion parameters the cells encode were state independent. Our results indicate that in the blowfly energetically expensive sensory coding strategies, such as rapid, large responses, and high spontaneous spike activity could be adjusted by the neuromodulator octopamine, likely to save energy during quiet locomotor states.

Stressed memories: how acute stress affects memory formation in humans

Stressful, aversive events are extremely well remembered. Such a declarative memory enhancement is evidently beneficial for survival, but the same mechanism may become maladaptive and culminate in mental diseases such as posttraumatic stress disorder (PTSD). Stress hormones are known to enhance postlearning consolidation of aversive memories but are also thought to have immediate effects on attentional, sensory, and mnemonic processes at memory formation. Despite their significance for our understanding of the etiology of stress-related mental disorders, effects of acute stress at memory formation, and their brain correlates at the system scale, remain elusive. Using an integrated experimental approach, we probed the neural correlates of memory formation while participants underwent a controlled stress induction procedure in a crossover design. Physiological (cortisol level, heart rate, and pupil dilation) and subjective measures confirmed acute stress. Remarkably, reduced hippocampal activation during encoding predicted stress-enhanced memory performance, both within and between participants. Stress, moreover, amplified early visual and inferior temporal responses, suggesting that hypervigilant processing goes along with enhanced inferior temporal information reduction to relay a higher proportion of task-relevant information to the hippocampus. Thus, acute stress affects neural correlates of memory formation in an unexpected manner, the understanding of which may elucidate mechanisms underlying psychological trauma etiology.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

Relation of locus coeruleus neurons in monkeys to Pavlovian and operant behaviors

Noradrenaline is released throughout the forebrain from locus coeruleus (LC) projections in close temporal proximity to emotional and goal-directed events. To examine interactive influences of these processes on LC neuronal activity, we used a task where Pavlovian and operant processes vary and can be easily identified. We recorded 69 single LC neurons from two monkeys performing a task where cues indicate the progression through schedules of one, two, or three operant trials. Pavlovian responses and phasic LC activations occur following the appearance of conditioned visual cues (54/69 neurons), especially those at the beginning of new schedules, whether the current trial will be rewarded (single trial schedule) or not (2 or 3 trial schedules), and after visual imperative signals eliciting the operant response (64/69 neurons), whether the current trial will be rewarded or not. The modulation of LC responses seems to be relatively independent of attention or motivation, because the responses do not covary with operant performance in the task. The magnitude of LC responses across the schedules varied in close relation to the intensity of Pavlovian behavior but these responses were also modulated by operant processes. Our conclusion is that LC activation occurs when task-relevant stimuli evoke a conditioned instinctive (Pavlovian) response, with the strength of the activation also being modulated by goal-directed processes. Thus locus coeruleus neurons broadcast information about stimulus-elicited primitive and goal-directed behaviors to forebrain structures important for executive functions and emotions.