Development and plasticity of the locus coeruleus: a review of recent physiological and pharmacological experimentation

Noradrenergic neuromodulation in ageing and disease

The locus coeruleus (LC) is a small brainstem structure located in the lower pons and is the main source of noradrenaline (NA) in the brain. Via its phasic and tonic firing, it modulates cognition and autonomic functions and is involved in the brain's immune response. The extent of degeneration to the LC in healthy ageing remains unclear, however, noradrenergic dysfunction may contribute to the pathogenesis of Alzheimer's (AD) and Parkinson's disease (PD). Despite their differences in progression at later disease stages, the early involvement of the LC may lead to comparable behavioural symptoms such as preclinical sleep problems and neuropsychiatric symptoms as a result of AD and PD pathology. In this review, we draw attention to the mechanisms that underlie LC degeneration in ageing, AD and PD. We aim to motivate future research to investigate how early degeneration of the noradrenergic system may play a pivotal role in the pathogenesis of AD and PD which may also be relevant to other neurodegenerative diseases.

Dynamic developmental changes in neurotransmitters supporting infant attachment learning

Infant survival relies on rapid identification, remembering and behavioral responsiveness to caregivers' sensory cues. While neural circuits supporting infant attachment learning have largely remained elusive in children, use of invasive techniques has uncovered some of its features in rodents. During a 10-day sensitive period from birth, newborn rodents associate maternal odors with maternal pleasant or noxious thermo-tactile stimulation, which gives rise to a preference and approach behavior towards these odors, and blockade of avoidance learning. Here we review the neural circuitry supporting this neonatal odor learning, unique compared to adults, focusing specifically on the early roles of neurotransmitters such as glutamate, GABA (Gamma-AminoButyric Acid), serotonin, dopamine and norepinephrine, in the olfactory bulb, the anterior piriform cortex and amygdala. The review highlights the importance of deepening our knowledge of age-specific infant brain neurotransmitters and behavioral functioning that can be translated to improve the well-being of children during typical development and aid in treatment during atypical development in childhood clinical practice, and the care during rearing of domestic animals.

Autocrine Neuromodulation and Network Activity Patterns in the Locus Coeruleus of Newborn Rat Slices

Already in newborns, the locus coeruleus (LC) controls multiple brain functions and may have a complex organization as in adults. Our findings in newborn rat brain slices indicate that LC neurons (i) generate at ~1 Hz a ~0.3 s-lasting local field potential (LFP) comprising summated phase-locked single spike discharge, (ii) express intrinsic ‘pacemaker’ or ‘burster’ properties and (iii) receive solely excitatory or initially excitatory−secondary inhibitory inputs. μ-opioid or ɑ2 noradrenaline receptor agonists block LFP rhythm at 100−250 nM whereas slightly lower doses transform its bell-shaped pattern into slower crescendo-shaped multipeak bursts. GABAA and glycine receptors hyperpolarize LC neurons to abolish rhythm which remains though unaffected by blocking them. Rhythm persists also during ionotropic glutamate receptor (iGluR) inhibition whereas <10 mV depolarization during iGluR agonists accelerates spiking to cause subtype-specific fast (spindle-shaped) LFP oscillations. Similar modest neuronal depolarization causing a cytosolic Ca2+ rise occurs (without effect on neighboring astrocytes) during LFP acceleration by CNQX activating a TARP-AMPA-type iGluR complex. In contrast, noradrenaline lowers neuronal Ca2+ baseline via ɑ2 receptors, but evokes an ɑ1 receptor-mediated ‘concentric’ astrocytic Ca2+ wave. In summary, the neonatal LC has a complex (possibly modular) organization to enable discharge pattern transformations that might facilitate discrete actions on target circuits.

Longitudinal Analysis of Sleep-Wake States in Neonatal Rats Subjected to Hypoxia-Ischemia

OBJECTIVE

Sleep is necessary for brain maturation in infants. Perinatal hypoxic-ischemic encephalopathy (HIE) is a major cause of chronic neurological disease in infants. Although the developmental changes of electroencephalogram (EEG) in human newborns have been described, little is known about the EEG normal maturation characteristics in rodents and the changes in sleep-awake states caused by hypoxia-ischemia (HI). This study aimed to investigate the pathological response of sleep-wake states in neonatal rats with HIE.

METHODS

We constructed HIE and sham models on postnatal day (P) 3 rats and continuously monitored them using electroencephalography and electromyography for up to P12. The distribution of sleep-wake states was analyzed to estimate the effects of HIE.

RESULTS

Compared with the sham group, the HI group showed lower rapid eye movement (REM) sleep percentage, but wake percentage and frequency was higher during P4-P12. The frequency of REM and non-rapid eye movement (NREM) sleep increased and the duration of REM and NREM sleep decreased after HI induction. However, it gradually returned to the normal level with an increase in daytime.

CONCLUSION

HI damage alters the sleep-wake patterns during early neural development. The findings provide a comprehensive assessment of serial sleep-wake state recordings in neonatal rats from P4-P12.

From Brain Organoids to Networking Assembloids: Implications for Neuroendocrinology and Stress Medicine

Brain organoids are three-dimensional cultures that contain multiple types of cells and cytoarchitectures, and resemble fetal human brain structurally and functionally. These organoids are being used increasingly to model brain development and disorders, however, they only partially recapitulate such processes, because of several limitations, including inability to mimic the distinct cortical layers, lack of functional neuronal circuitry as well as non-neural cells and gyrification, and increased cellular stress. Efforts to create improved brain organoid culture systems have led to region-specific organoids, vascularized organoids, glia-containing organoids, assembloids, sliced organoids and polarized organoids. Assembloids are fused region-specific organoids, which attempt to recapitulate inter-regional and inter-cellular interactions as well as neural circuitry development by combining multiple brain regions and/or cell lineages. As a result, assembloids can be used to model subtle functional aberrations that reflect complex neurodevelopmental, neuropsychiatric and neurodegenerative disorders. Mammalian organisms possess a highly complex neuroendocrine system, the stress system, whose main task is the preservation of systemic homeostasis, when the latter is threatened by adverse forces, the stressors. The main central parts of the stress system are the paraventricular nucleus of the hypothalamus and the locus caeruleus/norepinephrine-autonomic nervous system nuclei in the brainstem; these centers innervate each other and interact reciprocally as well as with various other CNS structures. Chronic dysregulation of the stress system has been implicated in major pathologies, the so-called chronic non-communicable diseases, including neuropsychiatric, neurodegenerative, cardiometabolic and autoimmune disorders, which lead to significant population morbidity and mortality. We speculate that brain organoids and/or assembloids could be used to model the development, regulation and dysregulation of the stress system and to better understand stress-related disorders. Novel brain organoid technologies, combined with high-throughput single-cell omics and gene editing, could, thus, have major implications for precision medicine.

Sleep-wake regulation in preterm and term infants

Study Objectives

In adults, wakefulness can be markedly prolonged at the expense of sleep, e.g. to stay vigilant in the presence of a stressor. These extra-long wake bouts result in a heavy-tailed distribution (highly right-skewed) of wake but not sleep durations. In infants, the relative importance of wakefulness and sleep are reversed, as sleep is necessary for brain maturation. Here, we tested whether these developmental pressures are associated with the unique regulation of sleep–wake states.

Methods

In 175 infants of 28–40 weeks postmenstrual age (PMA), we monitored sleep–wake states using electroencephalography and behavior. We constructed survival models of sleep–wake bout durations and the effect of PMA and other factors, including stress (salivary cortisol), and examined whether sleep is resilient to nociceptive perturbations (a clinically necessary heel lance).

Results

Wake durations followed a heavy-tailed distribution as in adults and lengthened with PMA and stress. However, differently from adults, active sleep durations also had a heavy-tailed distribution, and with PMA, these shortened and became vulnerable to nociception-associated awakenings.

Conclusions

Sleep bouts are differently regulated in infants, with especially long active sleep durations that could consolidate this state’s maturational functions. Curtailment of sleep by stress and nociception may be disadvantageous, especially for preterm infants given the limited value of wakefulness at this age. This could be addressed by environmental interventions in the future.

Alzheimer's disease pathology: pathways between central norepinephrine activity, memory, and neuropsychiatric symptoms

The locus coeruleus (LC) supplies norepinephrine to the brain, is one of the first sites of tau deposition in Alzheimer's disease (AD) and modulates a variety of behaviors and cognitive functions. Transgenic mouse models showed that norepinephrine dysregulation after LC lesions exacerbates inflammatory responses, blood-brain barrier leakage (BBB), and cognitive deficits. Here, we investigated relationships between central norepinephrine metabolism, tau and beta-amyloid (Aβ), inflammation, BBB-dysfunction, neuropsychiatric problems, and memory in-vivo in a memory clinic population (total n = 111, 60 subjective cognitive decline, 36 mild cognitively impaired, and 19 AD dementia). Cerebrospinal fluid (CSF) and blood samples were collected and analyzed for 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), CSF/plasma albumin ratio (Q-alb), Aβ, phosphorylated tau, and interleukins. The verbal word learning task and the neuropsychiatric inventory assessed memory functioning and neuropsychiatric symptoms. Structural equation models tested the relationships between all fluid markers, cognition and behavior, corrected for age, education, sex, and clinical dementia rating score. Our results showed that neuropsychiatric symptoms show strong links to both MHPG and p-tau, whereas memory deficits are linked to MHPG via a combination of p-tau and inflammation-driven amyloidosis (30-35% indirect effect contribution). These results suggest that the LC-norepinephrine may be pivotal to understand links between AD pathology and behavioral and cognitive deficits in AD.

Defining Immediate Effects of Sensitive Periods on Infant Neurobehavioral Function

During a sensitive period associated with attachment, the infant brain has unique circuitry that enables the specialized adaptive behaviors required for survival in infancy. This infant brain is not an immature version of the adult brain. Within the attachment relationship, the infant remains close (proximity seeking) to the caregiver for nurturing and survival needs, but the caregiver also provides the immature infant with the physiological regulation interaction needed before self-regulation matures. Here we provide examples from the human and animal literature that illustrate some of these regulatory functions during sensitive periods, recent advances demonstrating the supporting transient neural mechanisms, and how these systems go awry in the absence of species-expected caregiving.

Norepinephrine, neurodevelopment and behavior

Neurotransmitters play critical roles in the developing nervous system. Among the neurotransmitters, norepinephrine (NE) is in particular postulated to be an important regulator of brain development. NE is expressed during early stages of development and is known to regulate both the development of noradrenergic neurons and the development of target areas. NE participates in the shaping and the wiring of the nervous system during the critical periods of development, and perturbations in this process can alter the brain's developmental trajectory, which in turn can cause long-lasting and even permanent changes in the brain function and behavior later in life. Here we will briefly review evidence for the role of noradrenergic system in neurodevelopmental processes and will discuss about the potential disruptors of noradrenergic system during development and their behavioral consequences.

Cultural Attachment: From Behavior to Computational Neuroscience

Cultural attachment (CA) refers to processes that allow culture and its symbols to provide psychological security when facing threat. Epistemologically, whereas we currently have an adequate predictivist model of CA, it is necessary to prepare for a mechanistic approach that will not only predict, but also explain CA phenomena. Toward that direction, we here first examine the concepts and mechanisms that are the building blocks of both the prototypical maternal attachment as well as CA. Based on existing robust neuroscience models we associate these concepts and mechanisms with bona fide neurobiological functions to advance an integrative neurobiological model of CA. We further discuss the unresolved relationship of CA to other similar socio-cognitive concepts such as familiarity. Overall aim of the paper is to highlight the importance of integrating CA theory to computational approaches to culture and evolution (such as predictive processing computations explaining niche construction), as this will allow a dynamic interpretation of cultural processes.

Contemplating the GANE model using an extreme case paradigm

Early experiences play a crucial role in programming brain function, affecting selective attention, learning, and memory. Infancy literature suggests an extension of the GANE (glutamate amplifies noradrenergic effects) model to conditions with minimal priority-map inputs, yet suggests qualifications by noting that its efficacy is increased when tonic levels of arousal are maintained in an optimal range, in manners that are age and exposure dependent.

Early life adversity during the infant sensitive period for attachment: Programming of behavioral neurobiology of threat processing and social behavior

Animals, including humans, require a highly coordinated and flexible system of social behavior and threat evaluation. However, trauma can disrupt this system, with the amygdala implicated as a mediator of these impairments in behavior. Recent evidence has further highlighted the context of infant trauma as a critical variable in determining its immediate and enduring consequences, with trauma experienced from an attachment figure, such as occurs in cases of caregiver-child maltreatment, as particularly detrimental. This review focuses on the unique role of caregiver presence during early-life trauma in programming deficits in social behavior and threat processing. Using data primarily from rodent models, we describe the interaction between trauma and attachment during a sensitive period in early life, which highlights the role of the caregiver's presence in engagement of attachment brain circuitry and suppressing threat processing by the amygdala. These data suggest that trauma experienced directly from an abusive caregiver and trauma experienced in the presence of caregiver cues produce similar neurobehavioral deficits, which are unique from those resulting from trauma alone. We go on to integrate this information into social experience throughout the lifespan, including consequences for complex scenarios, such as dominance hierarchy formation and maintenance.

Attachment Figure's Regulation of Infant Brain and Behavior

Altricial infants (i.e., requiring parental care for survival), such as humans and rats, form an attachment to their caregiver and receive the nurturing and protections needed for survival. Learning has a strong role in attachment, as is illustrated by strong attachment formed to non-biological caregivers of either sex. Here we summarize and integrate results from animal and human infant attachment research that highlights the important role of social buffering (social presence) of the stress response by the attachment figure and its effect on infant processing of threat and fear through modulation of the amygdala. Indeed, this work suggests the caregiver switches off amygdala function in rodents, although recent human research suggests a similar process in humans and nonhuman primates. This cross-species analysis helps provide insight and unique understanding of attachment and its role in the neurobiology of infant behavior within attachment.

Ecologically relevant neurobehavioral assessment of the development of threat learning

As altricial infants gradually transition to adults, their proximate environment changes. In three short weeks, pups transition from a small world with the caregiver and siblings to a complex milieu rich in dangers as their environment expands. Such contrasting environments require different learning abilities and lead to distinct responses throughout development. Here, we will review some of the learned fear conditioned responses to threats in rats during their ontogeny, including behavioral and physiological measures that permit the assessment of learning and its supporting neurobiology from infancy through adulthood. In adulthood, odor-shock conditioning produces robust fear learning to the odor that depends upon the amygdala and related circuitry. Paradoxically, this conditioning in young pups fails to support fear learning and supports approach learning to the odor previously paired with shock. This approach learning is mediated by the infant attachment network that does not include the amygdala. During the age range when pups transition from the infant to the adult circuit (10-15 d old), pups have access to both networks: odor-shock conditioning in maternal presence uses the attachment circuit but the adult amygdala-dependent circuit when alone. However, throughout development (as young as 5 d old) the attachment associated learning can be overridden and amygdala-dependent fear learning supported, if the mother expresses fear in the presence of the pup. This social modulation of the fear permits the expression of defense reactions in life threatening situations informed by the caregiver but prevents the learning of the caregiver itself as a threat.

Norepinephrine Modulates Pyramidal Cell Synaptic Properties in the Anterior Piriform Cortex of Mice: Age-Dependent Effects of β-adrenoceptors

Early odor preference learning in rodents occurs within a sensitive period [≤postnatal day (P)10–12], during which pups show a heightened ability to form an odor preference when a novel odor is paired with a tactile stimulation (e.g., stroking). Norepinephrine (NE) release from the locus coeruleus during stroking mediates this learning. However, in older pups, stroking loses its ability to induce learning. The cellular and circuitry mechanisms underpinning the sensitive period for odor preference learning is not well understood. We first established the sensitive period learning model in mice – odor paired with stroking induced odor preference in P8 but not P14 mice. This learning was dependent on NE-β-adrenoceptors as it was prevented by propranolol injection prior to training. We then tested whether there are developmental changes in pyramidal cell excitability and NE responsiveness in the anterior piriform cortex (aPC) in mouse pups. Although significant differences of pyramidal cell intrinsic properties were found in two age groups (P8–11 and P14+), NE at two concentrations (0.1 and 10 μM) did not alter intrinsic properties in either group. In contrast, in P8–11 pups, NE at 0.1 μM presynaptically decreased miniature IPSC and increased miniature EPSC frequencies. These effects were reversed with a higher dose of NE (10 μM), suggesting involvement of different adrenoceptor subtypes. In P14+ pups, NE at higher doses (1 and 10 μM) acted both pre- and postsynaptically to promote inhibition. These results suggest that enhanced synaptic excitation and reduced inhibition by NE in the aPC network may underlie the sensitive period.

Developmental neurobiology of the rat attachment system and its modulation by stress

Stress is a powerful modulator of brain structure and function. While stress is beneficial for survival, inappropriate stress dramatically increases the risk of physical and mental health problems, particularly when experienced during early developmental periods. Here we focus on the neurobiology of the infant rat's odor learning system that enables neonates to learn and approach the maternal odor and describe the unique role of the stress hormone corticosterone in modulating this odor approach learning across development. During the first nine postnatal days, this odor approach learning of infant rats is supported by a wide range of sensory stimuli and ensures attachment to the mother's odor, even when interactions with her are occasionally associated with pain. With maturation and the emergence of a stress- or pain-induced corticosterone response, this odor approach learning terminates and a more adult-like amygdala-dependent fear/avoidance learning emerges. Strikingly, the odor approach and attenuated fear learning of older pups can be re-established by the presence of the mother, due to her ability to suppress her pups' corticosterone release and amygdala activity. This suggests that developmental changes in stress responsiveness and the stimuli that produce a stress response might be critically involved in optimally adapting the pup's attachment system to its respective ecological niche.

Maternal regulation of infant brain state

Patterns of neural activity are critical for sculpting the immature brain, and disrupting this activity is believed to underlie neurodevelopmental disorders [1-3]. Neural circuits undergo extensive activity-dependent postnatal structural and functional changes [4-6]. The different forms of neural plasticity [7-9] underlying these changes have been linked to specific patterns of spatiotemporal activity. Since maternal behavior is the mammalian infant's major source of sensory-driven environmental stimulation and the quality of this care can dramatically affect neurobehavioral development [10], we explored, for the first time, whether infant cortical activity is influenced directly by interactions with the mother within the natural nest environment. We recorded spontaneous neocortical local field potentials in freely behaving infant rats during natural interactions with their mother on postnatal days ∼12-19. We showed that maternal absence from the nest increased cortical desynchrony. Further isolating the pup by removing littermates induced further desynchronization. The mother's return to the nest reduced this desynchrony, and nipple attachment induced a further reduction but increased slow-wave activity. However, maternal simulation of pups (e.g., grooming and milk ejection) consistently produced rapid, transient cortical desynchrony. The magnitude of these maternal effects decreased with age. Finally, systemic blockade of noradrenergic beta receptors led to reduced maternal regulation of infant cortical activity. Our results demonstrate that during early development, mother-infant interactions can immediately affect infant brain activity, in part via a noradrenergic mechanism, suggesting a powerful influence of the maternal behavior and presence on circuit development.

Brain plasticity-based therapeutics

The primary objective of this review article is to summarize how the neuroscience of brain plasticity, exploiting new findings in fundamental, integrative and cognitive neuroscience, is changing the therapeutic landscape for professional communities addressing brain-based disorders and disease. After considering the neurological bases of training-driven neuroplasticity, we shall describe how this neuroscience-guided perspective distinguishes this new approach from (a) the more-behavioral, traditional clinical strategies of professional therapy practitioners, and (b) an even more widely applied pharmaceutical treatment model for neurological and psychiatric treatment domains. With that background, we shall argue that neuroplasticity-based treatments will be an important part of future best-treatment practices in neurological and psychiatric medicine.

Brief maternal separation affects brain α1-adrenoceptors and apoptotic signaling in adult mice

Exposure to adversity during early life is a risk factor for the development of different mood and psychiatric disorders, including depressive-like behaviors. Here, neonatal mice were temporarily but repeatedly (day 1 to day 13) separated from mothers and placed in a testing environment containing a layer of odorless clean bedding (CB). We assessed in adult animals the impact of this early experience on binding sites and mRNA expression of α1-adrenergic receptor subtypes, heat shock proteins (HSPs) and proapoptotic and antiapoptotic members of the Bcl-2 family proteins in different brain regions involved in processing of olfactory information and rewarding stimuli. We found that repeated exposure to CB experience produced anhedonic-like behavior in terms of reduced saccharin intake and α1-adrenoceptor downregulation in piriform and somatosensory cortices, hippocampus, amygdala and discrete thalamic nuclei. We also found a selective decrease of α1B-adrenoceptor binding sites in the cingulate cortex and hippocampus and an increase of hippocampal α1A and α1B receptor, but not of α1D-adrenoceptor, mRNA levels. Moreover, while a significant decrease of antiapoptotic heat shock proteins Hsp72 and Hsp90 was identified in the prefrontal cortex, a parallel increase of antiapoptotic members of Bcl-2 family proteins was found at the hippocampal level. Together, these data provide evidence that the early exposure to CB experience produced enduring downregulation of α1-adrenoceptors in the prefrontal-limbic forebrain/limbic midbrain network, which plays a key role in the processing of olfactory information and reaction to rewarding stimuli. Finally, these data show that CB experience can "prime" the hippocampal circuitry and promote the expression of antiapoptotic factors that can confer potential neuroprotection to subsequent adversity.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

Mechanisms underlying early odor preference learning in rats

Early odor preference training in rat pups produces behavioral preferences that last from hours to lifetimes. Here, we discuss the molecular and circuitry changes we have observed in the olfactory bulb (OB) and in the anterior piriform cortex (aPC) following odor training. For normal preference learning, both structures are necessary, but learned behavior can be initiated by initiating local circuit change in either structure. Our evidence relates dynamic molecular and circuit changes to memory duration and storage localization. Results using this developmental model are consistent with biological memory theories implicating N-methyl-D-aspartate (NMDA) receptors and β-adrenoceptors, and their associated cascades, in memory induction and consolidation. Finally, our examination of the odor preference model reveals a primary role for increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor synaptic strength, and in network strength, in the creation and maintenance of preference memory in both olfactory structures.

Asphyxia induced by umbilical cord occlusion alters glutamatergic and GABAergic synaptic transmission in neurons of the superior colliculus in fetal rats

Using optical recordings, we studied the effects of asphyxia on intracellular Cl(-) and Ca(2+) concentrations ([Cl(-)]i; [Ca(2+)]i) in the superior colliculus of fetal rats, which were connected via the umbilical cord to the dam. Acute asphyxia was induced by umbilical cord occlusion. The number of fetal superior colliculus neurons showing GABA-mediated increases in [Cl(-)]i (leading to hyperpolarization) following local synaptic electrical stimulation had decreased by 3 h post-asphyxiation, while the number showing GABA-mediated decreases in [Cl(-)]i (leading to depolarization) increased. [Ca(2+)]i rise, which occurred after acute asphyxiation, was antagonized by both non-NMDA and NMDA receptor antagonists. The increase in [Ca(2+)]i following focal superior colliculus stimulation was markedly attenuated at 3 h post-asphyxiation. These findings suggest that asphyxia induced by umbilical occlusion induces changes in glutamatergic and GABAergic synaptic transmission in the fetal brain.

The development and neurobiology of infant attachment and fear

Survival of altricial infants depends on attachment to the caregiver - a process that requires infants to identify, learn, remember, and approach their attachment figure. Here we review the neurobiology of attachment in infant rats where learning about the caregiver is supported by a specialized attachment neural circuitry to promote the infant-caregiver relationship. Specifically, the attachment circuit relies on infants acquiring learned preferences to the maternal odor, and this behavior is supported by the hyperfunctioning locus coeruleus and generous amounts of norepinephrine to produce experience-induced changes in the olfactory bulb and anterior piriform cortex. Infants also possess a reduced ability to acquire learned aversions or fear, and this behavior is facilitated through attenuated amygdala plasticity to block fear learning. Presumably, this attachment circuitry constrains the infant animal to express only learned preferences regardless of the quality of care received. As pups mature, and begin to travel in and out of the nest, the specialized attachment learning becomes contextually confined to when pups are with the mother. Thus, when outside the nest, these older pups show learning more typical of adult learning, presumably to prepare for independent life outside the nest. The quality of attachment can alter this circuitry, with early life stress prematurely terminating the pups' access to the attachment system through premature functional activation of the amygdala. Overall, the attachment circuit appears to have a dual function: to keep pups close to the caregiver but also to shape pups' behavior to match the environment and define long-term emotion and cognition.

The neurobiology of infant maternal odor learning

Infant rats must learn to identify their mother's diet-dependent odor. Once learned, maternal odor controls pups' approach to the mother, their social behavior and nipple attachment. Here we present a review of the research from four different laboratories, which suggests that neural and behavioral responses to the natural maternal odor and neonatal learned odors are similar. Together, these data indicate that pups have a unique learning circuit relying on the olfactory bulb for neural plasticity and on the hyperfunctioning noradrenergic locus coeruleus flooding the olfactory bulb with norepinephrine to support the neural changes. Another important factor making this system unique is the inability of the amygdala to become incorporated into the infant learning circuit. Thus, infant rats appear to be primed in early life to learn odors that will evoke approach responses supporting attachment to the caregiver.

Transitions in sensitive period attachment learning in infancy: the role of corticosterone

Survival of altricial infants, including humans and rats, depends on attachment to the caregiver - a process that requires infants to recognize, learn, and remember their attachment figure. The demands of a dynamic environment combined with a maturing organism require frequent neurobehavioral reorganization. This restructuring of behavior and its supporting neural circuitry can be viewed through the unique lens of attachment learning in rats in which preference learning is enhanced and aversion learning is attenuated. Behavioral restructuring is well adapted to securing the crucial infant-caregiver relationship regardless of the quality of care. With maturation and the end of the infant-caregiver attachment learning period, the complex interplay of neural structures, hormones, and social behavior coordinates the developing rat's eventual transition to life outside of the nest. Nevertheless, early-life environmental and physiological stressors can alter the resilient nature of this system, particularly with respect to the amygdala, and these changes may provide important clues to understanding the lasting effects of early stress.

Early-life stress disrupts attachment learning: the role of amygdala corticosterone, locus ceruleus corticotropin releasing hormone, and olfactory bulb norepinephrine

Infant rats require maternal odor learning to guide pups' proximity-seeking of the mother and nursing. Maternal odor learning occurs using a simple learning circuit including robust olfactory bulb norepinephrine (NE), release from the locus ceruleus (LC), and amygdala suppression by low corticosterone (CORT). Early-life stress increases NE but also CORT, and we questioned whether early-life stress disrupted attachment learning and its neural correlates [2-deoxyglucose (2-DG) autoradiography]. Neonatal rats were normally reared or stressed-reared during the first 6 d of life by providing the mother with insufficient bedding for nest building and were odor-0.5 mA shock conditioned at 7 d old. Normally reared paired pups exhibited typical odor approach learning and associated olfactory bulb enhanced 2-DG uptake. However, stressed-reared pups showed odor avoidance learning and both olfactory bulb and amygdala 2-DG uptake enhancement. Furthermore, stressed-reared pups had elevated CORT levels, and systemic CORT antagonist injection reestablished the age-appropriate odor-preference learning, enhanced olfactory bulb, and attenuated amygdala 2-DG. We also assessed the neural mechanism for stressed-reared pups' abnormal behavior in a more controlled environment by injecting normally reared pups with CORT. This was sufficient to produce odor aversion, as well as dual amygdala and olfactory bulb enhanced 2-DG uptake. Moreover, we assessed a unique cascade of neural events for the aberrant effects of stress rearing: the amygdala-LC-olfactory bulb pathway. Intra-amygdala CORT or intra-LC corticotropin releasing hormone (CRH) infusion supported aversion learning with intra-LC CRH infusion associated with increased olfactory bulb NE (microdialysis). These results suggest that early-life stress disturbs attachment behavior via a unique cascade of events (amygdala-LC-olfactory bulb).

Nicotinic acetylcholine receptor subunit mRNA expression in adult and developing rat medullary catecholamine neurons

Nicotinic acetylcholine receptors (nAChRs) mediate numerous visceral functions via medullary catecholamine (CA) neurons found in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (DMV), and ventrolateral medulla (VLM). However, the nAChR subtypes involved are not known. We have therefore characterized expression of nine nAChR subunit mRNAs in adult and developing rat medullary CA nuclei using combined isotopic/nonisotopic in situ hybridization. Tyrosine hydroxylase (TH) mRNA, the CA-synthesizing enzyme, was used as a marker for CA neurons, because these nuclei consist of heterogeneous populations of cells. Subunit mRNA expression varied within and between nuclei, along the rostrocaudal axis, between cell types, and across development. All CA neurons expressed beta2 mRNA, whereas alpha2 mRNA was completely absent. alpha6 And beta3 mRNA expression were restricted mainly to the VLM. alpha4, alpha5, And alpha7 mRNA expression was significantly greater in the rostral than in the caudal VLM. alpha3 And beta4 mRNAs were highly expressed in the dorsal region of the NTS, whereas dense alpha7 mRNA expression was restricted to the DMV and ventral NTS. The remaining subunit mRNAs were detected to some degree in both DMV and NTS. Except for alpha4 mRNA, which peaked prenatally, expression levels of subunit transcripts in the NTS and DMV were lower during development compared with adults. In the VLM, alpha3, alpha4, and alpha5 mRNAs expression peaked perinatally, whereas alpha6 and beta3 levels increased with age. These variations in nAChR subunit mRNA expression suggest that different receptor subtypes may produce function-specific regulation of medullary CA systems.

The temporal dynamics of consolidation and reconsolidation decrease during postnatal development

The temporal dynamics of consolidation and reconsolidation of taste/odor aversion memory are evaluated during rat pup growth at postnatal days 3, 10, and 18. This is assessed through the temporal gradients of efficacy of a protein synthesis inhibitor (anisomycin) in inducing amnesia after either acquisition (consolidation) or reactivation (reconsolidation). The results show a progressive reduction with age of the delay during which the inhibitor is able to induce amnesia. Control experiments rule out a reduction of anisomycin efficacy due to blood brain barrier growth or decrease in protein synthesis inhibition. Thus, these results present the first evidence that the protein synthesis-dependent phase of memory stabilization requires less time with age. This decrease occurs in parallel for consolidation and reconsolidation. Such changes in the dynamics of memory processing could contribute to the cognitive improvement associated with development.

Norepinephrine depletion facilitates recovery of function after focal ischemia in the rat

Previous studies have suggested that increased norepinephrine plays an important role in recovery of function after brain injury; however, the majority of these studies used drugs that are known to also affect other monoamines to increase or decrease norepinephrine. The purpose of the present study was to determine if norepinephrine is required to promote recovery after ischemia. A form of enriched rehabilitation was used to rehabilitate animals after ischemia and the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine was used to selectively destroy norepinephrine projections from the locus coeruleus. Three sensorimotor tests were used to evaluate the recovery of the animals. Depletion of norepinephrine improved sensorimotor recovery in standard-housed animals and did not impede recovery in the rehabilitation groups. Dopamine beta hydroxylase staining was used to confirm N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-depleted terminal norepinephrine levels. The amount of norepinephrine terminal staining negatively correlated with recovery of function in the staircase test after ischemia. In addition, enriched rehabilitation increased, but depletion of norepinephrine had no effect on, brain-derived neurotrophic factor protein levels, which have also been linked to improved recovery of function. Together the above findings question the previously postulated role of norepinephrine in recovery of function after stroke.

Neurobiology of infant attachment

A strong attachment to the caregiver is critical for survival in altricial species, including humans. While some behavioral aspects of attachment have been characterized, its neurobiology has only recently received attention. Using a mammalian imprinting model, we are assessing the neural circuitry that enables infant rats to attach quickly to a caregiver, thus enhancing survival in the nest. Specifically, the hyper-functioning noradrenergic locus coeruleus (LC) enables pups to learn rapid, robust preference for the caregiver. Conversely, a hypo-functional amygdala appears to prevent the infant from learning aversions to the caregiver. Adult LC and amygdala functional emergence correlates with sensitive period termination. This study suggests the neonatal brain is not an immature version of the adult brain but is uniquely designed to optimize attachment to the caregiver. Although human attachment may not rely on identical circuitry, the work reviewed here suggests a new conceptual framework in which to explore human attachments, particularly attachments to abusive caregivers.

In vivooptical recordings of synaptic transmission and intracellular Ca2+and Cl-in the superior colliculus of fetal rats

Although the N-methyl-D-aspartate (NMDA) receptor is known to play a crucial role in activity-dependent remodeling of synaptic connections in the fetal superior colliculus (SC), its contribution to the electrical activity of fetal SC neurons has not been determined. Furthermore, whether gamma-aminobutyric acid (GABA)-mediated inhibition occurs either as early as prenatal periods or only after eye opening has been controversial. We therefore performed optical recordings using voltage-, Ca2+- and Cl--sensitive fluorescent dyes to analyse synaptic transmission and changes in intracellular Ca2+ and Cl- in the SC of fetal rats that were still connected with the dams by the umbilical cord. Excitatory and inhibitory responses were evoked by focal SC stimulation. The excitatory synaptic responses are composed of early and late components. The early component was mediated by both non-NMDA and NMDA receptors, whereas the late component occurred mainly via NMDA receptors. Train pulse stimulation at higher currents was required for induction of the inhibition, which was antagonized by bicuculline, and blocking of the GABA-mediated inhibition by bicuculline uncovered masked excitatory synaptic responses. Focal SC stimulation induced increases in [Cl-]i and [Ca2+]i that were mediated by GABA-A receptors and mainly by NMDA receptors, respectively. GABA antagonists augmented SC-induced increases in [Ca2+]i. These results indicate that, in the fetal SC, excitatory and inhibitory synaptic transmissions occur before birth, that the NMDA receptor is a major contributor to excitatory synaptic transmission and increased [Ca2+]i, and that the GABA-A receptor is already functioning to inhibit excitatory neurotransmission.

Enhanced nicotinic acetylcholine receptor-mediated [3H]norepinephrine release from neonatal rat hypothalamus

Nicotinic acetylcholine receptor (nAChR)-evoked release of norepinephrine (NE) has been demonstrated in a number of brain regions that receive sole noradrenergic innervation from the locus coeruleus (LC). Many of these structures display enhanced nicotine-stimulated NE release in the neonate. We have examined the hypothalamus in order to determine if this region, which receives NE projections from both the LC and medullary catecholaminergic nuclei, also demonstrates maturational changes in nAChR-mediated NE release. Quantification of radiolabeled-NE release from rat hypothalamus slices by a maximally effective dose of nicotine revealed a peak response during the first postnatal week. This was followed by a decrease at postnatal day (P) 14, and a second peak at P21. Thereafter, release was equivalent to that observed at P14. Comparison of the pharmacological properties of nAChRs mediating NE release in neonatal (P7) and mature hypothalamus suggested involvement of different nAChR subtypes at the two ages. Using the selective toxin, DSP-4, nAChR-mediated NE release in the neonatal hypothalamus was shown to be from LC terminals. Our findings demonstrate an early sensitivity of hypothalamic LC terminals to nAChR regulation that may be associated with development of systems controlling critical homeostatic functions such as stress, feeding and cardiovascular regulation.

Memory of early maltreatment: neonatal behavioral and neural correlates of maternal maltreatment within the context of classical conditioning

BACKGROUND

While children form an attachment to their abusive caregiver, they are susceptible to mental illness and brain abnormalities. To understand this important clinical issue, we have developed a rat animal model of abusive attachment where odor paired with shock paradoxically produces an odor preference. Here, we extend this model to a seminaturalistic paradigm using a stressed, "abusive" mother during an odor presentation and assess the underlying learning neural circuit.

METHODS

We used a classical conditioning paradigm pairing a novel odor with a stressed mother that predominantly abused pups to assess olfactory learning in a seminaturalistic environment. Additionally, we used Fos protein immunohistochemistry to assess brain areas involved in learning this pain-induced odor preference within a more controlled maltreatment environment (odor-shock conditioning).

RESULTS

Odor-maternal maltreatment pairings within a seminatural setting and odor-shock pairings both resulted in paradoxical odor preferences. Learning-induced gene expression was altered in the olfactory bulb and anterior piriform cortex (part of olfactory cortex) but not the amygdala.

CONCLUSIONS

Infants appear to use a unique brain circuit that optimizes learned odor preferences necessary for attachment. A fuller understanding of infant brain function may provide insight into why early maltreatment affects psychiatric well-being.

Corticosterone influences on Mammalian neonatal sensitive-period learning

Infant rats exhibit sensitive-period odor learning characterized by olfactory bulb neural changes and odor preference acquisitions critical for survival. This sensitive period is coincident with low endogenous corticosterone (CORT) levels and stress hyporesponsivity. The authors hypothesized that low corticosterone levels modulate sensitive-period learning. They assessed the effects of manipulating CORT levels by increasing and removing CORT during (Postnatal Day 8) and after (Postnatal Day 12) the sensitive period. Results show that (a) exogenous CORT prematurely ends sensitive-period odor-shock-induced preferences; (b) adrenalectomy developmentally extends the sensitive period as indicated by odor-shock-induced odor-preference learning in older pups, whereas CORT replacement can reinstate fear learning; and (c) CORT manipulation modulates olfactory bulb correlates of sensitive-period odor learning in a manner consistent with behavior.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Developing a sense of safety: the neurobiology of neonatal attachment

Clinical data suggests a strong negative impact of traumatic attachments on adult mental illness, presumably through organizing brain development. To further explore this clinical issue, a mammalian model of imprinting was developed to characterize the neural basis of attachment in both healthy and traumatic attachments. The altricial neonatal rat must learn the mother's odor for nipple attachment, huddling, and orienting to the mother, all of which are required for pup survival. While it appears maladaptive to depend upon learning for attachment, the unique learning system of neonatal pups greatly enhances odor-preference learning and attachment while pups are confined to the nest. This heightened learning is expressed behaviorally as an enhanced ability to acquire learned odor preferences and a decreased ability to acquire learned odor aversions. Specifically, both odor-milk and odor-shock (0.5 mA) conditioning result in odor-preference acquisition. It appears as though there are at least three brain structures underlying the neonatal rat's sensitive period for heightened odor learning: (1) odor learning is encoded in the olfactory bulb; (2) the hyperfunctioning noradrenergic locus coeruleus (LC) appears to support preference conditioning through release of NE; and (3) the hypofunctioning amygdala appears to underlie pups' difficulty in learning odor aversions. Overall, this suggests that the CNS of altricial infants is specialized for optimizing attachments to their caregiver.

Unique neural circuitry for neonatal olfactory learning

Imprinting ensures that the infant forms the caregiver attachment necessary for altricial species survival. In our mammalian model of imprinting, neonatal rats rapidly learn the odor-based maternal attachment. This rapid learning requires reward-evoked locus ceruleus (LC) release of copious amounts of norepinephrine (NE) into the olfactory bulb. This imprinting ends at postnatal day 10 (P10) and is associated with a dramatic reduction in reward-evoked LC NE release. Here we assess whether the functional emergence of LC alpha2 inhibitory autoreceptors and the downregulation of LC alpha1 excitatory autoreceptors underlie the dramatic reduction in NE release associated with termination of the sensitive period. Postsensitive period pups (P12) were implanted with either LC or olfactory bulb cannulas, classically conditioned with intracranial drug infusions (P14), and tested for an odor preference (P15). During conditioning, a novel odor was paired with either olfactory bulb infusion of abeta-receptor agonist (isoproterenol) to assess the target effects of NE or direct LC cholinergic stimulation combined with alpha2 antagonists and alpha1 agonists in a mixture to reinstate neonatal levels of LC autoreceptor activity to assess the source of NE. Pups learned an odor preference when the odor was paired with either olfactory bulb isoproterenol infusion or reinstatement of neonatal LC receptor activity. These results suggest that LC autoreceptor functional changes rather than olfactory bulb changes underlie sensitive period termination.

Postnatal development of the cerebellum and the CNS adrenergic system is independent of norepinephrine and epinephrine

A fundamental question in the formation of the nervous system is the extent to which a neurotransmitter contributes to the development of the neurons that synthesize and release it. A complementary question is whether neurotransmitter signaling contributes to the development of postsynaptic targets. Prior studies have suggested that adrenergic signaling may promote adrenergic neuronal proliferation or survival and may be critical for the postnatal development of the cerebellum. To test these possibilities genetically, we studied mice that are unable to synthesize norepinephrine and epinephrine (NE/E), the endogenous adrenergic receptor ligands, due to a disruption the gene for dopamine beta-hydroxylase. These mice develop postnatally in the absence of NE/E. Here we report that the adrenergic neurons of these mutant mice are present in normal numbers and locations and exhibit typical innervation patterns throughout the central nervous system (CNS), as assessed by immunostaining for tyrosine hydroxylase and the NE transporter. Furthermore, cerebellar cortical development (size, foliation, layering, cell number, and position), which proceeds to a large degree postnatally, is unaltered in the mutants. These results indicate that the fate and innervation pattern of the adrenergic neurons, as well as the development of the cerebellum, do not depend on postnatal signaling by NE/E. The results also suggest that when restoration of adrenergic signaling is performed in this mutant mouse model (by administering a synthetic precursor of NE), reversal of phenotypes is due to the synthesis and release of NE/E from adrenergic terminals that are distributed normally within the CNS.

Novel objects in a holeboard probe the role of the locus coeruleus in curiosity: support for two modes of attention in the rat

Idazoxan, an alpha 2 adrenoceptor antagonist (2 mg/kg), enhanced novel object investigation in a holeboard in rats as previously reported (V. Devauges & S. J. Sara, 1990). Two weeks of 10 min/day in 37 degrees C water increased dopamine-beta-hydroxylase staining density in the locus coeruleus but did not enhance novel object investigation. In contrast to idazoxan, however, the warm water treatment increased rearing, center entries, and activity, a pattern previously described during tonic infusion of norepinephrine into the hippocampus. Correlations among dopamine-beta-hydroxylase measures and behavior reinforced these tonic norepinephrine/behavior associations. The behavioral effects across the idazoxan and warm water experiments support G. Aston-Jones et al.'s (1999) 2 modes of attention hypothesis for locus coeruleus function: Phasic locus coeruleus activity promotes focused attention; tonic locus coeruleus activity promotes scanning attention.

Developmental regulation of nicotinic acetylcholine receptor-mediated [3H]norepinephrine release from rat cerebellum

Presynaptic modulation of synaptic transmission is the primary function of central nicotinic acetylcholine receptors (nAChRs) in developing and adult brain. nAChR activation regulates release of various neurotransmitters, including norepinephrine (NA). Given evidence that NA may serve a critical functional role in cerebellar development, we have undertaken studies to determine whether nAChRs modulate NA release in developing cerebellum. In vitro experiments using cerebellar slices examined the effects of nAChR stimulation on release of radiolabeled NA ([3H]NA). Our data indicate the presence of functional nAChRs on NA terminals in immature cerebellum and subsequent developmental regulation of receptor properties. During postnatal week one, the maximally effective dose of nicotine released 35.0 +/- 1.2% of cerebellar [3H]NA stores. There was a subsequent decline in maximal nicotine-stimulated NA release until postnatal day 30, when Emax values were statistically indistinguishable from adult. Although the efficacy of nicotine changed substantially throughout development, EC50 values did not differ significantly (EC50 = 4.4-12.0 micro m). Pharmacological analysis indicated that this developmental shift in maximum nicotine effect reflects a change in the properties of the nAChRs. These data support recent findings of a possible functional role of nAChRs in regulating cerebellar ontogeny, and provides further support for the role of NA as a neurotrophic factor during development.

Paradoxical rise in brainstem PO(2) following umbilical cord occlusion in full-term rat fetuses

Although the fetus experiences severe hypoxia and ischemia during delivery, the fetal brain is protected from hypoxic-ischemic insults by unknown mechanisms. To investigate this phenomenon, fetal asphyxia was induced in pregnant rats by occlusion of the umbilical cord. Rather than producing cerebral hypoxia, the brainstem PO(2) of at-term fetuses increased following umbilical cord occlusion, while brainstem blood flow was markedly reduced. This paradoxical increase in brainstem PO(2) during asphyxia occurred in the majority of at-term fetuses, less frequently in fetuses 1 day prior to term and did not occur in animals following birth. Because occlusion of the umbilical cord prevents maternal delivery of oxygen to the fetus, we propose that the ability to maintain PO(2) is the result of pre-existing fetal stores of oxygen or from de novo generation of oxygen in the fetal brainstem.

Circuits and systems in stress. I. Preclinical studies

This paper reviews the preclinical literature related to the effects of stress on neurobiological and neuroendocrine systems. Preclinical studies of stress provide a comprehensive model for understanding neurobiological alterations in post-traumatic stress disorder (PTSD). The pathophysiology of stress reflects long-standing changes in biological stress response systems and in systems involved in stress responsivity, learning, and memory. The neural circuitry involved includes systems mediating hypothalamic-pituitary-adrenal (HPA) axis, norepinephrine (locus coeruleus), and benzodiazepine, serotonergic, dopaminergic, neuropeptide, and central amino acid systems. These systems interact with brain structures involved in memory, including hippocampus, amygdala, and prefrontal cortex. Stress responses are of vital importance in living organisms; however excessive and/or repeated stress can lead to long-lasting alterations in these circuits and systems involved in stress responsiveness. Intensity and duration of the stressor, and timing of the stressor in life, have strong impact in this respect.

Hypocretin (orexin) enhances neuron activity and cell synchrony in developing mouse GFP-expressing locus coeruleus

The noradrenergic neurons of the locus coeruleus (LC) play an important role in modulating arousal and selective attention. A similar function has been attributed to the hypocretin neurons of the hypothalamus which maintain a strong synaptic projection to the LC. As the LC can be difficult to detect in the embryonic and neonatal mouse brain, we used a new transgenic mouse with strong GFP expression in the LC under the regulation of a mouse prion promoter. GFP colocalized with immunoreactive tyrosine hydroxylase in sections and dispersed cultures of the LC, allowing visualization and whole cell or single-unit recording from the LC in early stages of cellular development. GFP expression in the LC had no apparent effect on cellular physiology, including resting membrane potential, input resistance, spike threshold, depolarization-induced spike frequency increase, current-voltage relations, or hypocretin responses. In slices of the mature mouse and rat LC, hypocretin-1 and -2 increased spike frequency, with hypocretin-1 being an order of magnitude more potent. In the postnatal day (P) 0-2 developing mouse slice during a developmental period when spikes could be elicited in some cells, other developing LC neurons showed rhythmic, subthreshold oscillations (approximately 1 Hz) in membrane potential (2.9-7.4 mV amplitude); others were arrhythmic. Hypocretin-1 depolarized the membrane potential, resulting in the appearance of spikes in developing LC cells that showed no spikes under control conditions. In the presence of TTX and glutamate receptor antagonists, hypocretin-1-mediated inward currents were blocked by substitution of choline-Cl for NaCl, suggesting an excitatory mechanism based on an inward cation current. Hypocretin-1 initiated strong regular membrane voltage oscillations in arrhythmic immature neurons. Hypocretin increased the temporal synchrony of action potentials studied with dual-cell recording in P1-P5 mouse LC slices, consistent with the view that synchrony of LC output, associated with improved cognitive performance, may be increased by hypocretin. Together these data suggest that the hypothalamus, via hypocretin projections, may therefore be in a position to enhance arousal and modulate plasticity in higher brain centres through the developing LC.

Potential role of medullary raphe-spinal neurons in cutaneous vasoconstriction: an in vivo electrophysiological study

In rabbits, raphe magnus/pallidus neurons form a link in the CNS pathway regulating changes in cutaneous blood flow elicited by nociceptive stimulation and activation of the central nucleus of the amygdala. To characterize relevant raphe-spinal neurons, we performed extracellular recordings from the rostral medullary raphe nuclei in anesthetized, paralyzed, mechanically ventilated rabbits. All studied neurons were antidromically activated from the dorsolateral funiculus of the spinal cord (C(8)-T(2)). Of 129 studied neurons, 40% were silent. The remaining neurons discharged spontaneously at 0.3-29 Hz. Nociceptive stimulation (lip squeeze with pliers) excited 63 (49%), inhibited 9 (7%), and did not affect 57 (44%) neurons. The same stimulation also elicited falls in ear pinna blood flow. In neurons activated by the stimulation, the increase in discharge preceded the fall in flow. Electrical stimulation of the spinal trigeminal tract excited 61/63 nociception-activated neurons [onset latencies range: 6-75 ms, mean: 28 +/- 3 (SE) ms], inhibited 9/9 nociception-inhibited neurons (onset latencies range: 9-85 ms, mean: 32 +/- 10 ms), and failed to affect 55/57 neurons insensitive to nociceptive stimulation. Neurons insensitive to nociceptive/trigeminal stimulation were also insensitive to nonnociceptive tactile stimulation and to electrical stimulation of the amygdala. They were either silent (32/45) or discharged regularly at low frequencies. They possessed long-duration action potentials (1.26 +/- 0.08 ms) and slow-conducting axons (6.0 +/- 0.5 m/s). These neurons may be serotonergic raphe-spinal cells. They do not appear to be involved in nociceptive-related cutaneous vascular control. Of the 63 neurons sensitive to nociceptive and trigeminal tract stimulation, 35 also responded to tactile stimulation (wide receptive field). These neurons possessed short action potentials (0.80 +/- 0.03 ms) and fast-conducting axons (30.3 +/- 3.1 m/s). In this subpopulation, electrical stimulation of the amygdala activated nearly all neurons tested (10/12), with a mean onset latency of 34 +/- 3 ms. The remaining 28 neurons sensitive to nociceptive and trigeminal stimulation did not respond to tactile stimuli and were mainly unaffected by amygdala stimulation. It may be that fast-conducting raphe-spinal neurons, with wide multimodal receptive fields and with input from the central nucleus of the amygdala, constitute the bulbo-spinal link in the CNS pathway regulating cutaneous blood flow in response to nociceptive and alerting stimuli.

Unique Characteristics of Neonatal Classical Conditioning: The Role of the Amygdala and Locus Coeruleus

The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. During the first postnatal days, infant rats show a sensitive period for learning and are particularly susceptible to learning an attraction to their mother's odor. Classical conditioning appears to underlie this learning that is expressed behaviorally as an increased ability to acquire odor preferences and a decreased ability to acquire odor aversions. Specifically, in neonatal rats, pairing an odor with moderately painful shock (0.5mA) or milk produces a subsequent relative preference for that odor. The neural circuitry supporting the increased ability to acquire odor preferences appears to be the heightened functioning of the noradrenergic pontine nucleus locus coeruleus. Indeed, norepinephrine from the locus coeruleus appears to be both necessary and sufficient for learning during the sensitive period. On the other hand, the decreased ability to acquire odor aversions seems to be due to the lack of participation of the amygdala in at least some aversive learning situations. The site of plasticity in the pup's brain appears to be limited to the olfactory bulb. This neonatal sensitive period for learning ends around postnatal day 9-10, at which time pups make the transition from crawling to walking and classical conditioning becomes "adultlike." The neonatal behavioral and neural induced changes are retained into adulthood where it modifies sexual behavior.

Injections of an opioid antagonist into the locus coeruleus and periaqueductal gray but not the amygdala precipitates morphine withdrawal in the 7-day-old rat

Opiate withdrawal behaviors in the infant differ from those of the adult. The neural circuitry underlying opioid withdrawal in the adult rat is well defined and includes the locus coeruleus (LC) and periaqueductal gray (PAG), with a minor role of the amygdala. Because the different behaviors that constitute the infant syndrome may be mediated by different neural circuits, we tested the hypothesis that these three sites are involved in opiate withdrawal. Pups were injected with morphine from day 1-6 after birth (b.i.d.) and on the morning of the seventh day. Withdrawal was then elicited by local injection of the opioid antagonist methylnaloxonium into the LC, PAG, or amygdala. Withdrawal signs were precipitated in a dose-dependent manner following injection into the LC or PAG, but not the amygdala. The withdrawal behaviors elicited from the LC and PAG included both the same and different behaviors. The results support the hypothesis that the neural circuitry mediating opiate withdrawal behaviors is similar in infant and adult animals, but the behaviors expressed are age-specific.