A combined immunocytochemical and retrograde tracing study of noradrenergic connections between the caudal medulla and bed nuclei of the stria terminalis

An ascending vagal sensory-central noradrenergic pathway modulates retrieval of passive avoidance memory

Background

Visceral feedback from the body is often subconscious, but plays an important role in guiding motivated behaviors. Vagal sensory neurons relay "gut feelings" to noradrenergic (NA) neurons in the caudal nucleus of the solitary tract (cNTS), which in turn project to the anterior ventrolateral bed nucleus of the stria terminalis (vlBNST) and other hypothalamic-limbic forebrain regions. Prior work supports a role for these circuits in modulating memory consolidation and extinction, but a potential role in retrieval of conditioned avoidance remains untested.

Results

To examine this, adult male rats underwent passive avoidance conditioning. We then lesioned gut-sensing vagal afferents by injecting cholecystokinin-conjugated saporin toxin (CSAP) into the vagal nodose ganglia (Experiment 1), or lesioned NA inputs to the vlBNST by injecting saporin toxin conjugated to an antibody against dopamine-beta hydroxylase (DSAP) into the vlBNST (Experiment 2). When avoidance behavior was later assessed, rats with vagal CSAP lesions or NA DSAP lesions displayed significantly increased conditioned passive avoidance.

Conclusions

These new findings support the view that a gut vagal afferent-to-cNTSNA-to-vlBNST circuit plays a role in modulating the expression/retrieval of learned passive avoidance. Overall, our data suggest a dynamic modulatory role of vagal sensory feedback to the limbic forebrain in integrating interoceptive signals with contextual cues that elicit conditioned avoidance behavior.

Involvement of the Dorsal Vagal Complex in Alcohol-Related Behaviors

The neurobiological mechanisms that regulate the development and maintenance of alcohol use disorder (AUD) are complex and involve a wide variety of within and between systems neuroadaptations. While classic reward, preoccupation, and withdrawal neurocircuits have been heavily studied in terms of AUD, viable treatment targets from this established literature have not proven clinically effective as of yet. Therefore, examination of additional neurocircuitries not classically studied in the context of AUD may provide novel therapeutic targets. Recent studies demonstrate that various neuropeptides systems are important modulators of alcohol reward, seeking, and intake behaviors. This includes neurocircuitry within the dorsal vagal complex (DVC), which is involved in the control of the autonomic nervous system, control of intake of natural rewards like food, and acts as a relay of interoceptive sensory information via interactions of numerous gut-brain peptides and neurotransmitter systems with DVC projections to central and peripheral targets. DVC neuron subtypes produce a variety of neuropeptides and transmitters and project to target brain regions critical for reward such as the mesolimbic dopamine system as well as other limbic areas important for the negative reinforcing and aversive properties of alcohol withdrawal such as the extended amygdala. This suggests the DVC may play a role in the modulation of various aspects of AUD. This review summarizes the current literature on neurotransmitters and neuropeptides systems in the DVC (e.g., norepinephrine, glucagon-like peptide 1, neurotensin, cholecystokinin, thyrotropin-releasing hormone), and their potential relevance to alcohol-related behaviors in humans and rodent models for AUD research. A better understanding of the role of the DVC in modulating alcohol related behaviors may lead to the elucidation of novel therapeutic targets for drug development in AUD.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Central afferents to the nucleus of the solitary tract in rats and mice

The nucleus of the solitary tract (NTS) regulates life-sustaining functions ranging from appetite and digestion to heart rate and breathing. It is also the brain's primary sensory nucleus for visceral sensations relevant to symptoms in medical and psychiatric disorders. To better understand which neurons may exert top-down control over the NTS, here we provide a brain-wide map of all neurons that project axons directly to the caudal, viscerosensory NTS, focusing on a medial subregion with aldosterone-sensitive HSD2 neurons. Injecting an axonal tracer (cholera toxin b) into the NTS produces a similar pattern of retrograde labeling in rats and mice. The paraventricular hypothalamic nucleus (PVH), lateral hypothalamic area, and central nucleus of the amygdala (CeA) contain the densest concentrations of NTS-projecting neurons. PVH afferents are glutamatergic (express Slc17a6/Vglut2) and are distinct from neuroendocrine PVH neurons. CeA afferents are GABAergic (express Slc32a1/Vgat) and are distributed largely in the medial CeA subdivision. Other retrogradely labeled neurons are located in a variety of brain regions, including the cerebral cortex (insular and infralimbic areas), bed nucleus of the stria terminalis, periaqueductal gray, Barrington's nucleus, Kölliker-Fuse nucleus, hindbrain reticular formation, and rostral NTS. Similar patterns of retrograde labeling result from tracer injections into different NTS subdivisions, with dual retrograde tracing revealing that many afferent neurons project axon collaterals to both the lateral and medial NTS subdivisions. This information provides a roadmap for studying descending axonal projections that may influence visceromotor systems and visceral "mind-body" symptoms.

Aldosterone-sensitive HSD2 neurons in mice

Sodium deficiency elevates aldosterone, which in addition to epithelial tissues acts on the brain to promote dysphoric symptoms and salt intake. Aldosterone boosts the activity of neurons that express 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), a hallmark of aldosterone-sensitive cells. To better characterize these neurons, we combine immunolabeling and in situ hybridization with fate mapping and Cre-conditional axon tracing in mice. Many cells throughout the brain have a developmental history of Hsd11b2 expression, but in the adult brain one small brainstem region with a leaky blood-brain barrier contains HSD2 neurons. These neurons express Hsd11b2, Nr3c2 (mineralocorticoid receptor), Agtr1a (angiotensin receptor), Slc17a6 (vesicular glutamate transporter 2), Phox2b, and Nxph4; many also express Cartpt or Lmx1b. No HSD2 neurons express cholinergic, monoaminergic, or several other neuropeptidergic markers. Their axons project to the parabrachial complex (PB), where they intermingle with AgRP-immunoreactive axons to form dense terminal fields overlapping FoxP2 neurons in the central lateral subnucleus (PBcL) and pre-locus coeruleus (pLC). Their axons also extend to the forebrain, intermingling with AgRP- and CGRP-immunoreactive axons to form dense terminals surrounding GABAergic neurons in the ventrolateral bed nucleus of the stria terminalis (BSTvL). Sparse axons target the periaqueductal gray, ventral tegmental area, lateral hypothalamic area, paraventricular hypothalamic nucleus, and central nucleus of the amygdala. Dual retrograde tracing revealed that largely separate HSD2 neurons project to pLC/PB or BSTvL. This projection pattern raises the possibility that a subset of HSD2 neurons promotes the dysphoric, anorexic, and anhedonic symptoms of hyperaldosteronism via AgRP-inhibited relay neurons in PB.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Cocaine self-administration and extinction alter medullary noradrenergic and limbic forebrain cFos responses to acute, noncontingent cocaine injections in adult rats

Central noradrenergic (NA) signaling contributes critically to multiple behavioral effects of cocaine administration, particularly stress- and anxiety-related effects. The present study examined the ability of acute cocaine to induce the immediate early gene product, cFos, in NA neurons and stress-related neural circuits in rats that were cocaine-naïve, or had a history of cocaine self-administration with or without extinction. Rats implanted with jugular catheters were trained to self-administer cocaine (0.5-mg/kg/infusion), with a subset subsequently trained on extinction. Cocaine-naïve controls were handled daily. After a final day of self-administration, extinction, or handling, rats received an i.p. injection of either cocaine (20-mg/kg) or saline, and 90min later were anesthetized and perfused. Tissue sections were processed for immunoperoxidase labeling of nuclear cFos with either immunoperoxidase or immunofluorescent cytoplasmic labeling of dopamine beta hydroxylase or tyrosine hydroxylase. Acute cocaine increased the number of activated NA neurons within the caudal nucleus of the solitary tract (NTS; A2 cell group) in cocaine-naïve and extinguished rats, but not in rats that only self-administered. Extinction attenuated cocaine-induced cFos activation in NA neurons of the caudal ventrolateral medulla (A1/C1 cell groups), and attenuated cFos within the paraventricular nucleus of the hypothalamus, the apex of the central neuroendocrine stress axis. Cocaine consistently increased cFos in the bed nucleus of the stria terminalis, regardless of history. NA neurons of the locus coeruleus (A6 cell group) were not activated after cocaine administration in any experimental group. Thus, the ability of cocaine to activate central stress circuitry is altered after cocaine self-administration. Our results suggest a unique role for the NTS in cocaine-induced reinstatement, as extinction training enhanced the ability of cocaine to activate NA neurons within this region. These findings suggest central NA systems originating in the caudal brainstem as potential targets for the treatment of cocaine addiction.

Amygdalar connections in the lesser hedgehog tenrec

The present study analyses the overall extrinsic connectivity of the non-olfactory amygdala (Ay) in the lesser hedgehog tenrec. The data were obtained from tracer injections into the lateral and intermediate portions of the Ay as well as several non-amygdalar brain regions. Both the solitary and the parabrachial nucleus receive descending projections from the central nucleus of the Ay, but only the parabrachial nucleus appears to project to the Ay. There is one prominent region in the ventromedial hypothalamus connected reciprocally with the medial and central Ay. Amygdalar afferents clearly arise from the dorsomedial thalamus, the subparafascicular nuclei and the medial geniculate complex (GM). Similar to other subprimate species, the latter projections originate in the dorsal and most caudal geniculate portions and terminate in the dorsolateral Ay. Unusual is the presence of amygdalo-projecting cells in the marginal geniculate zone and their virtual absence in the medial GM. As in other species, amygdalo-striatal projections mainly originate in the basolateral Ay and terminate predominantly in the ventral striatum. Given the poor differentiation of the tenrec's neocortex, there is a remarkable similarity with regard to the amygdalo-cortical connectivity between tenrec and rat, particularly as to prefrontal, limbic and somatosensorimotor areas as well as the rhinal cortex throughout its length. The tenrec's isocortex dorsomedial to the caudal rhinal cortex, on the other hand, may not be connected with the Ay. An absence of such connections is expected for primary auditory and visual fields, but it is unusual for their secondary fields.

Discrete forebrain neuronal networks supporting noradrenergic regulation of sensorimotor gating

Prepulse inhibition (PPI) refers to the reduction in the startle response when a startling stimulus is preceded by a weak prestimulus, and is an endophenotype of deficient sensorimotor gating in several neuropsychiatric disorders. Emerging evidence suggests that norepinephrine (NE) regulates PPI, however, the circuitry involved is unknown. We found recently that stimulation of the locus coeruleus (LC), the primary source of NE to the forebrain, induces a PPI deficit that is a result of downstream NE release. Hence, this study sought to identify LC-innervated forebrain regions that mediate this effect. Separate groups of male Sprague-Dawley rats received a cocktail solution of the α1-NE receptor agonist phenylephrine plus the β-receptor agonist isoproterenol (equal parts of each; 0, 3, 10, and 30 μg) into subregions of the medial prefrontal cortex (mPFC), nucleus accumbens (NAcc), extended amygdala, mediodorsal thalamus (MD-thalamus), or the dorsal hippocampus (DH) before PPI testing. NE agonist infusion into the posterior mPFC, NAcc shell, bed nucleus of the stria terminalis, basolateral amygdala, and the MD-thalamus disrupted PPI, with particularly strong effects in MD-thalamus. Sites in which NE receptor stimulation did not disrupt PPI (anterior mPFC, NAcc core, central amygdala, and DH) did support PPI disruptions with the dopamine D2 receptor agonist quinpirole (0, 10 μg). This pattern reveals new pathways in the regulation of PPI, and suggests that NE transmission within distinct thalamocortical and ventral forebrain networks may subserve the sensorimotor gating deficits that are seen in disorders such as schizophrenia, Tourette syndrome, and post-traumatic stress disorder.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Contribution of limbic norepinephrine to cannabinoid-induced aversion

RATIONALE

The cannabinoid system has risen to the forefront in the development of novel treatments for a number of pathophysiological processes. However, significant side effects have been observed in clinical trials raising concerns regarding the potential clinical utility of cannabinoid-based agents. Understanding the neural circuits and neurochemical substrates impacted by cannabinoids will provide a better means of gaging their actions within the central nervous system that may contribute to the expression of unwanted side effects.

OBJECTIVES

In the present study, we investigated whether norepinephrine (NE) in the limbic forebrain is a critical determinant of cannabinoid receptor agonist-induced aversion and anxiety in rats.

METHODS

An immunotoxin lesion approach was combined with behavioral analysis using a place conditioning paradigm and the elevated zero maze.

RESULTS

Our results show that the non-selective CB1/CB2 receptor agonist, WIN 55,212-2, produced a significant place aversion in rats. Further, NE in the nucleus accumbens was critical for WIN 55,212-2-induced aversion but did not affect anxiety-like behaviors. Depletion of NE from the bed nucleus of the stria terminalis was ineffective in altering WIN 55,212-2-induced aversion and anxiety.

CONCLUSIONS

These results indicate that limbic, specifically accumbal, NE is required for cannabinoid-induced aversion but is not essential to cannabinoid-induced anxiety.

Paraventricular hypothalamic nucleus: axonal projections to the brainstem

The paraventricular hypothalamic nucleus (PVH) contains many neurons that innervate the brainstem, but information regarding their target sites remains incomplete. Here we labeled neurons in the rat PVH with an anterograde axonal tracer, Phaseolus vulgaris leucoagglutinin (PHAL), and studied their descending projections in reference to specific neuronal subpopulations throughout the brainstem. While many of their target sites were identified previously, numerous new observations were made. Major findings include: 1) In the midbrain, the PVH projects lightly to the ventral tegmental area, Edinger-Westphal nucleus, ventrolateral periaqueductal gray matter, reticular formation, pedunculopontine tegmental nucleus, and dorsal raphe nucleus. 2) In the dorsal pons, the PVH projects heavily to the pre-locus coeruleus, yet very little to the catecholamine neurons in the locus coeruleus, and selectively targets the viscerosensory subregions of the parabrachial nucleus. 3) In the ventral medulla, the superior salivatory nucleus, retrotrapezoid nucleus, compact and external formations of the nucleus ambiguous, A1 and caudal C1 catecholamine neurons, and caudal pressor area receive dense axonal projections, generally exceeding the PVH projection to the rostral C1 region. 4) The medial nucleus of the solitary tract (including A2 noradrenergic and aldosterone-sensitive neurons) receives the most extensive projections of the PVH, substantially more than the dorsal vagal nucleus or area postrema. Our findings suggest that the PVH may modulate a range of homeostatic functions, including cerebral and ocular blood flow, corneal and nasal hydration, ingestive behavior, sodium intake, and glucose metabolism, as well as cardiovascular, gastrointestinal, and respiratory activities.

In vivo voltammetric monitoring of norepinephrine release in the rat ventral bed nucleus of the stria terminalis and anteroventral thalamic nucleus

The role and contribution of the dense noradrenergic innervation in the ventral bed nucleus of the stria terminalis (vBNST) and anteroventral thalamic nucleus (AV) to biological function and animal behaviors is poorly understood due to the small size of these nuclei. The aim of this study was to compare norepinephrine release and uptake in the vBNST with that in the AV of anesthetized rats. Measurements were made in vivo with fast-scan cyclic voltammetry following electrical stimulation of noradrenergic projection pathways, either the dorsal noradrenergic bundle (DNB) or the ventral noradrenergic bundle (VNB). The substance detected was identified as norepinephrine based upon voltammetric, anatomical, neurochemical and pharmacological evidence. Fast-scan cyclic voltammetry enables the selective monitoring of local norepinephrine overflow in the vBNST evoked by the stimulation of either the DNB or the VNB while norepinephrine in the AV was only evoked by DNB stimulation. The alpha2-adrenoceptor antagonist yohimbine and the norepinephrine uptake inhibitor desipramine increased norepinephrine overflow and slowed its disappearance in both regions. However, control of extracellular norepinephrine by both autoreceptors and uptake was greater in the AV. The greater control exerted by autoreceptors and uptake in the AV resulted in reduced extracellular concentration compared with the v BNST when large numbers of stimulation pulses were employed. The differences in noradrenergic transmission observed in the terminal fields of the v BNST and the AV may differentially regulate activity in these two regions that both contain high densities of norepinephrine terminals.

Estradiol-17beta-responsive A1 and A2 noradrenergic cells of the brain stem project to the bed nucleus of the stria terminalis in the ewe brain: a possible route for regulation of gonadotropin releasing hormone cells

We have studied brain stem cells in the ewe brain that project to the bed nucleus of the stria terminalis (BNST) and determined if these cells are activated by estradiol-17beta. This would predicate an indirect role in the estradiol-17beta regulation of gonadotropin releasing hormone (GnRH) cells, since these receive input from the BNST. Ovariectomized ewes received 50 mug estradiol-17beta benzoate (i.m.) 1 h prior to brain collection, so that activated cells could be identified by Fos immunohistochemistry. Retrograde tracer (FluoroGold; FG), was injected into the three divisions of the BNST and labeled cells were mapped to the A1 and A2 regions and the parabrachial nucleus (PBN) of the brain stem. With FG injection into the dorsal and lateral BNST, all FG-containing cells in the caudal A1 and 45% of those in A2 stained for dopamine-beta-hydroxylase (DBH), indicating noradrenergic type. No FG-labelled cells in the PBN were DBH-positive. In A1 and A2 respectively, 42% and 46% of FG-labelled cells were Fos-positive, with no double-labeling in cells of the PBN. In ewes receiving FG injections into the ventral BNST, estrogen receptor (ER)alpha-immunoreactive nuclei were found in 82% of A1-FG labeled and 38% of A2-FG labeled cells. No FG-labelled cells of the PBN were ERalpha-positive. Anterograde tracing from A1 with microruby injection identified projections to the PBN, BNST and preoptic area (POA). Thus, A1 and A2 noradrenergic neurons project to the BNST in the ewe brain, express ERalpha and are activated by estradiol-17beta. These noradrenergic, estrogen-responsive cells may provide indirect input to GnRH cells, via the BNST.

Inputs to the ventrolateral bed nucleus of the stria terminalis

The ventrolateral bed nucleus of the stria terminalis (BSTvl) receives direct input from two specific subpopulations of neurons in the nucleus tractus solitarius (NTS). It is heavily innervated by aldosterone-sensitive NTS neurons, which are selectively activated by sodium depletion, and by the A2 noradrenergic neurons, which are activated by visceral and immune- and stress-related stimuli. Here, we used a retrograde neuronal tracer to identify other brain sites that innervate the BSTvl. Five general brain regions contained retrogradely labeled neurons: cerebral cortex (infralimbic and insular regions), rostral forebrain structures (subfornical organ, organum vasculosum of the lamina terminalis, taenia tecta, nucleus accumbens, lateral septum, endopiriform nucleus, dorsal BST, substantia innominata, and, most prominently the amygdala--primarily its basomedial and central subnuclei), thalamus (central medial, intermediodorsal, reuniens, and, most prominently the paraventricular thalamic nucleus), hypothalamus (medial preoptic area, perifornical, arcuate, dorsomedial, parasubthalamic, and posterior hypothalamic nuclei), and brainstem (periaqueductal gray matter, dorsal and central superior raphe nuclei, parabrachial nucleus, pre-locus coeruleus region, NTS, and A1 noradrenergic neurons in the caudal ventrolateral medulla). In the arcuate hypothalamic nucleus, some retrogradely labeled neurons contained either agouti-related peptide or cocaine/amphetamine-regulated transcript. Of the numerous retrogradely labeled neurons in the perifornical hypothalamic area, few contained melanin-concentrating hormone or orexin. In the brainstem, many retrogradely labeled neurons were either serotoninergic or catecholaminergic. In summary, the BSTvl receives inputs from a variety of brain sites implicated in hunger, salt and water intake, stress, arousal, and reward.

Organization of immune-responsive medullary projections to the bed nucleus of the stria terminalis, central amygdala, and paraventricular nucleus of the hypothalamus: evidence for parallel viscerosensory pathways in the rat brain

Immune-responsive neurons in the brainstem, primarily in the nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM), contribute to a significant drive on forebrain nuclei responsible for brain-mediated host defense responses. The current study investigated the relative contribution of brainstem-derived ascending pathways to forebrain immune-responsive nuclei in the rat by means of retrograde tract tracing and c-Fos immunohistochemistry. Fluorogold was iontophoresed into the bed nucleus of stria terminalis (BST), central nucleus of the amygdala (CEA), paraventricular nucleus of the hypothalamus (PVN), and the pontine lateral parabrachial nucleus (PBL; an important component of ascending viscerosensensory pathways) followed 2 weeks later by intraperitoneal injection of lipopolysaccharide (LPS, 0.1 mg/kg) or saline. The NTS and VLM provide immune-responsive input to all four regions, via direct, predominantly catecholaminergic, projections to the PVN, the lateral BST, and the CEA, and mostly non-catecholaminergic projections to the PBL. The PBL provides a major LPS-activated input to the BST and CEA. The pattern of LPS-activated catecholaminergic projections from the VLM and NTS to the forebrain is characterized by a strong predominance of VLM input to the PVN, whereas the NTS provides a greater contribution to the BST. These findings indicate that direct and indirect pathways originate in the caudal brainstem that propagate immune-related information from the periphery with multiple levels of processing en route to the forebrain nuclei, which may allow for integration of brain responses to infection.

Activation in extended amygdala corresponds to altered hedonic processing during protracted morphine withdrawal

Previously we reported that during protracted morphine abstinence rats show reduced conditioned place preferences (CPP) for food-associated environments, compared to non-dependent subjects. To determine the brain regions involved in this altered reward behavior, we examined neural activation (as indexed by Fos-like proteins) induced by a preference test for a food-associated environment in 5-week morphine-abstinent versus non-dependent animals. The results indicate that elevated Fos expression in the anterior cingulate cortex (Cg) and basolateral amygdala (BLA) correlated positively with preference behavior in all groups. In contrast, Fos expression in stress-associated brain areas, including the ventral lateral bed nucleus of the stria terminalis (VL-BNST), central nucleus of the amygdala (CE), and noradrenergic (A2) neurons in the nucleus tractus solitarius (NTS) was significantly elevated only in morphine-abstinent animals. Furthermore, the number of Fos positive neurons in these areas was found to correlate negatively with food preference in abstinent animals. These results indicate that the altered hedonic processing during protracted morphine withdrawal leading to decreased preference for cues associated with natural rewards may involve heightened activity in stress-related brain areas of the extended amygdala and their medullary noradrenergic inputs.

The anxiogenic drug yohimbine activates central viscerosensory circuits in rats

Systemic administration of the alpha(2)-adrenoceptor antagonist yohimbine (YO) activates the HPA stress axis and promotes anxiety in humans and experimental animals. We propose that visceral malaise contributes to the stressful and anxiogenic effects of systemic YO and that YO recruits brainstem noradrenergic (NA) and peptidergic neurons that relay viscerosensory signals to the hypothalamus and limbic forebrain. To begin testing these hypotheses, the present study explored dose-related effects of YO on food intake, conditioned flavor avoidance (CFA), and Fos immunolabeling in rats. Systemic YO (5.0 mg/kg BW, i.p.) inhibited food intake, supported CFA, and increased Fos immunolabeling in identified NA neurons in the ventrolateral medulla, nucleus of the solitary tract, and locus coeruleus. YO also increased Fos in the majority of corticotropin releasing hormone-positive neurons in the paraventricular nucleus of the hypothalamus. YO administered at 1.0 mg/kg BW did not inhibit food intake, did not support CFA, and did not increase Fos immunolabeling. Retrograde neural tracing demonstrated that neurons activated by YO at 5.0 mg/kg BW included medullary and pontine neurons that project to the central nucleus of the amygdala and to the lateral bed nucleus of the stria terminalis, the latter region receiving comparatively greater input by Fos-positive neurons. We conclude that YO produces anorexigenic and aversive effects that correlate with activation of brainstem viscerosensory inputs to the limbic forebrain. These findings invite continued investigation of how central viscerosensory signaling pathways interact with hypothalamic and limbic regions to influence interrelated physiological and behavioral components of anxiety, stress, and visceral malaise.

Distribution of norepinephrine transporters in the non-human primate brain

Noradrenergic terminals in the central nervous system are widespread; as such this system plays a role in varying functions such as stress responses, sympathetic regulation, attention, and memory processing, and its dysregulation has been linked to several pathologies. In particular, the norepinephrine transporter is a target in the brain of many therapeutic and abused drugs. We used the selective ligand [(3)H]nisoxetine, therefore, to describe autoradiographically the normal regional distribution of the norepinephrine transporter in the non-human primate central nervous system, thereby providing a baseline to which alterations due to pathological conditions can be compared. The norepinephrine transporter in the monkey brain was distributed heterogeneously, with highest levels occurring in the locus coeruleus complex and raphe nuclei, and moderate binding density in the hypothalamus, midline thalamic nuclei, bed nucleus of the stria terminalis, central nucleus of the amygdala, and brainstem nuclei such as the dorsal motor nucleus of the vagus and nucleus of the solitary tract. Low levels of binding to the norepinephrine transporter were measured in basolateral amygdala and cortical, hippocampal, and striatal regions. The distribution of the norepinephrine transporter in the non-human primate brain was comparable overall to that described in other species, however disparities exist between the rodent and the monkey in brain regions that play a role in such critical processes as memory and learning. The differences in such areas point to the possibility of important functional differences in noradrenergic information processing across species, and suggest the use of caution in applying findings made in the rodent to the human condition.

Aldosterone-sensitive neurons in the nucleus of the solitary tract: Efferent projections

The nucleus of the solitary tract (NTS) contains a subpopulation of neurons that express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them uniquely sensitive to aldosterone. These neurons may drive sodium appetite, which is enhanced by aldosterone. Anterograde and retrograde neural tracing techniques were used to reveal the efferent projections of the HSD2 neurons in the rat. First, the anterograde tracer Phaseolus vulgaris leucoagglutinin was used to label axonal projections from the medial NTS. Then, NTS-innervated brain regions were injected with a retrograde tracer, cholera toxin beta subunit, to determine which sites are innervated by the HSD2 neurons. The HSD2 neurons project mainly to the ventrolateral bed nucleus of the stria terminalis (BSTvl), the pre-locus coeruleus (pre-LC), and the inner division of the external lateral parabrachial nucleus (PBel). They also send minor axonal projections to the midbrain ventral tegmental area, lateral and paraventricular hypothalamic nuclei, central nucleus of the amygdala, and periaqueductal gray matter. The HSD2 neurons do not innervate the ventrolateral medulla, a key brainstem autonomic site. Additionally, our tracing experiments confirmed that the BSTvl receives direct axonal projections from the neighboring A2 noradrenergic neurons in the NTS, and from the same pontine sites that receive major inputs from the HSD2 neurons (PBel and pre-LC). The efferent projections of the HSD2 neurons may provide new insights into the brain circuitry responsible for sodium appetite.

Effects of chronic cocaine self-administration on norepinephrine transporters in the nonhuman primate brain

RATIONALE

While cocaine blocks dopamine and serotonin transporters, considerably less emphasis has been placed on its effects following blockade of the norepinephrine transporter (NET). To date, no studies have made a systematic investigation of the effects of chronic cocaine on primate NET density.

OBJECTIVE

We previously reported increases in NET density in portions of the monkey bed nucleus of stria terminalis after 100 days of cocaine self-administration. In the present study we extend these findings and assess the changes in [3H]nisoxetine binding in additional brain regions of rhesus monkeys chronically self-administrating cocaine.

RESULTS

[3H]Nisoxetine binding sites in the A1 noradrenergic cell group were significantly higher after 5 days of cocaine exposure. One hundred days of self-administration also induced a higher density of NET binding within the A1 cell group; however, in addition, the effects extended to the nucleus prepositus, as well as forebrain regions such as hypothalamic nuclei, basolateral amygdala, parasubiculum, and entorhinal cortex.

CONCLUSIONS

These data demonstrate that cocaine self-administration alters the noradrenergic system of nonhuman primates. Although cocaine affected NET binding sites in the forebrain projections of both the ventral (VNAB) and dorsal (DNAB) noradrenergic bundles, the alteration in the A1 cell group at the early time-point suggests that the VNAB appears to be more sensitive than the DNAB to the effects of cocaine. Given the role of norepinephrine in arousal and attention, as well as mediating responses to stress, long-term exposure to cocaine is likely to result in significant changes in the way in which information is perceived and processed by the central nervous system of long-term cocaine users.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in the regulation of the hypothalamic-pituitary-adrenal axis

The bed nucleus of the stria terminalis (BNST) plays an important role in the regulation of the hypothalamic-pituitary-adrenal (HPA) axis during stress and it is a major extrahypothalamic relay to the paraventricular nucleus of the hypothalamus (PVN) from the amygdala and the hippocampus. In this review, we discuss the anatomical, neurochemical and behavioral evidence that substantiate a role for noradrenergic terminals of the anterior BNST in the regulation of the HPA axis. We propose the hypothesis that BNST noradrenaline (NA) participates in the regulation of the hippocampal inhibitory influence on the HPA axis activation. The observation that NA exerts a tonic inhibitory effect upon glutamatergic transmission in the anterior BNST supports this hypothesis. We also discuss the known mechanisms involved in the regulation of BNST NA extracellular levels and the possible interactions between NA and corticotropin-releasing hormone (CRH), and of CRH with glutamate (GLU) in the regulation of the HPA axis activity exerted by the BNST. The evidence discussed in the present review situates the BNST as a key extrahypothalamic center that relays and integrates limbic and autonomic information related to stress responses suggesting that dysregulation in the functioning of the BNST may underlie the pathophysiology of stress-related psychiatric disorders.

Autonomic brainstem nuclei are linked to the hippocampus

Vagal nerve stimulation has been reported to enhance memory in both rats and humans, and to be an effective treatment for epilepsy in some patients, but the underlying neuroanatomical substrate(s) responsible for these effects remains unknown. Since there is no direct anatomical projection from the nucleus tractus solitarius, the main vagal relay site of the brain, to the hippocampus, we tested whether a multisynaptic pathway exists. Pseudorabies virus, a pig herpesvirus that can be used as a retrograde transneuronal tracer, was injected into the ventral CA1 hippocampus of rats, and after 4 days, pseudorabies virus infected neurons were identified in the general visceral portion of the nucleus tractus solitarius, with the majority being localized in the A2 noradrenergic cell group. Other autonomic brainstem nuclei, including the parabrachial nucleus, locus coeruleus, A1 and A5 noradrenergic cell groups, and C1 adrenergic cell group, were labeled. In order to identify some of the potential relay sites of the nucleus tractus solitarius-->hippocampal pathway, immunotoxin lesions of the ventral CA1 region were made that selectively destroyed either the noradrenergic or cholinergic fibers. After 2 weeks' recovery, pseudorabies virus was injected in this same CA1 area, and 4 days later, the transneuronal labeling in the nucleus tractus solitarius was reduced by approximately 65%. These findings suggest that the noradrenergic neurons of the locus coeruleus and cholinergic neurons of the medial septum/diagonal band are likely to be relay sites for this pathway. Other potential linkages are discussed. In summary, this is the first anatomical report to show that the general visceral region of nucleus tractus solitarius is linked via multisynaptic relays to the hippocampus.

Effect of lesioning the suprachiasmatic nuclei on behavioral despair in rats

The suprachiasmatic nucleus (SCN) is involved in regulating many biological rhythms. Several lines of research implicate the SCN in affective behavior. The SCN is directly involved in regulating the daily rhythms of the hypothalamo-pituitary-adrenal (HPA) axis hormones involved in stress. Bilateral lesions of the SCN disrupt both the rhythms and the basal levels of the HPA axis hormones involved in coping with stress. Moreover, stress can affect the biological rhythms regulated by the SCN, and disruption of biological rhythms in turn can cause stress. The present study assessed the effect of bilateral destruction of the SCN on behavioral despair, an animal model of depression sensitive to antidepressant treatment. The results indicate that bilateral destruction of the SCN results in reduced immobility in the second forced swimming test (FST) compared to sham controls and animals with incomplete lesions. These results indicate that bilateral destruction of the SCN has a protective effect in the induction of behavioral despair which may arise out of disruption of the secretion of the HPA axis hormones and/or of the neural connections between the SCN and the limbic structures that modulate the response to swim stress.

Role of the bed nucleus of the stria terminalis versus the amygdala in fear, stress, and anxiety

The bed nucleus of the stria terminalis is a limbic forebrain structure that receives heavy projections from, among other areas, the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. Despite its strategic anatomical position, initial attempts to implicate the bed nucleus of the stria terminalis in conditioned fear were largely unsuccessful. Recent studies have shown, however, that the bed nucleus of the stria terminalis does participate in certain types of anxiety and stress responses. In this work, we review these findings and suggest from the emerging pattern of evidence that, although the bed nucleus of the stria terminalis may not be necessary for rapid-onset, short-duration behaviors which occur in response to specific threats, the bed nucleus of the stria terminalis may mediate slower-onset, longer-lasting responses that frequently accompany sustained threats, and that may persist even after threat termination.

Stress reactivity of the brain noradrenergic system in three rat strains differing in their neuroendocrine and behavioral responses to stress: implications for susceptibility to stress-related neuropsychiatric disorders

The brain noradrenergic system is activated by stress, modulating the activity of forebrain regions involved in behavioral and neuroendocrine responses to stress. In this study, we characterized brain noradrenergic reactivity to acute immobilization stress in three rat strains that differ in their neuroendocrine stress response: the inbred Lewis (Lew) and Wistar-Kyoto (WKY) rats, and outbred Sprague-Dawley (SD) rats. Noradrenergic reactivity was assessed by measuring tyrosine hydroxylase mRNA expression in locus coeruleus, and norepinephrine release in the lateral bed nucleus of the stria terminalis. Behavioral measures of arousal and acute stress responsivity included locomotion in a novel environment, fear-potentiated startle, and stress-induced reductions in social interaction and open-arm exploration on the elevated-plus maze. Neuroendocrine responses were assessed by plasma adrenocorticotropic hormone. Compared to SD, adrenocorticotropic hormone responses of Lew rats were blunted, whereas those of WKY were enhanced. The behavioral effects of stress were similar in Lew and SD rats, despite baseline differences. Lew had similar elevations of tyrosine hydroxylase mRNA, and initially greater norepinephrine release in the lateral bed nucleus of the stria terminalis during stress, although both noradrenergic responses returned toward baseline more rapidly than in SD rats. WKY rats showed depressed baseline startle and lower baseline exploratory and social behavior than SD. However, unlike the Lew or SD rats, WKY exhibited a lack both of fear potentiation of the startle response and of stress-induced reductions in exploratory and social behavior, indicating attenuated stress responsivity. Acute noradrenergic reactivity to stress, measured by either tyrosine hydroxylase mRNA levels or norepinephrine release, was also attenuated in WKY rats. Thus, reduced arousal and behavioral responsivity in WKY rats may be related to deficient brain noradrenergic reactivity. This deficit may alter their ability to cope with stress, resulting in the exaggerated neuroendocrine responses and increased susceptibility to stress-related pathology exhibited by this strain.

Stress-induced relapse to drug seeking in the rat: role of the bed nucleus of the stria terminalis and amygdala

There is growing interest in the role that the bed nucleus of the stria terminalis (BNST) and central nucleus of the amygdala (CeA), components of the extended amygdala, play in drug addiction. Within the BNST and CeA, there is an extensive system of intrinsic, primarily GABAergic, interconnections known to synthesize a variety of neuropeptides, including corticotrophin-releasing factor (CRF). The actions of CRF at extrahypothalamic sites,including the BNST and CeA, have been implicated in stress responses and in the aversive effects of withdrawal from drugs of abuse. Most recently, we have shown a critical role for extrahypothalamic CRF in stress-induced reinstatement of drug seeking in rats. In attempting to determine which brain circuitry mediates the effect of stress on relapse and, more specifically, where in the brain CRF acts to initiate the behaviours involved in relapse, we focused on the BNST and CeA. In the present paper, we summarize studies we have conducted that explore the role of these brain sites in stress-induced relapse to heroin and cocaine seeking, and then consider how our findings can be understood within the more general context of what is known about the role of the BNST and CeA in stress-related and general approach behaviours, such as drug seeking.

Basic organization of projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis in adult rat brain

The organization of axonal projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis (BST) was characterized with the Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing method in adult male rats. Within the BST, the oval nucleus (BSTov) projects very densely to the fusiform nucleus (BSTfu) and also innervates the caudal anterolateral area, anterodorsal area, rhomboid nucleus, and subcommissural zone. Outside the BST, its heaviest inputs are to the caudal substantia innominata and adjacent central amygdalar nucleus, retrorubral area, and lateral parabrachial nucleus. It generates moderate inputs to the caudal nucleus accumbens, parasubthalamic nucleus, and medial and ventrolateral divisions of the periaqueductal gray, and it sends a light input to the anterior parvicellular part of the hypothalamic paraventricular nucleus and nucleus of the solitary tract. The BSTfu displays a much more complex projection pattern. Within the BST, it densely innervates the anterodorsal area, dorsomedial nucleus, and caudal anterolateral area, and it moderately innervates the BSTov, subcommissural zone, and rhomboid nucleus. Outside the BST, the BSTfu provides dense inputs to the nucleus accumbens, caudal substantia innominata and central amygdalar nucleus, thalamic paraventricular nucleus, hypothalamic paraventricular and periventricular nuclei, hypothalamic dorsomedial nucleus, perifornical lateral hypothalamic area, and lateral tegmental nucleus. Moderately dense inputs are found in the parastrial, tuberal, dorsal raphé, and parabrachial nuclei and in the retrorubral area, ventrolateral division of the periaqueductal gray, and pontine central gray. Light projections end in the olfactory tubercle, lateral septal nucleus, posterior basolateral amygdalar nucleus, supramammillary nucleus, and nucleus of the solitary tract. These and other results suggest that the BSTov and BSTfu are basal telencephalic parts of a circuit that coordinates autonomic, neuroendocrine, and ingestive behavioral responses during stress.

Vagal immune-to-brain communication: a visceral chemosensory pathway

The immune system operates as a diffuse sensory system, detecting the presence of specific chemical constituents associated with dangerous micro-organisms, and then signalling the brain. In this way, immunosensation constitutes a chemosensory system. Several submodalities of this sensory system function as pathways conveying immune-related information, and can be classified as either primarily brain barrier associated or neural. The vagus nerve provides the major neural pathway identified to date. The initial chemosensory transduction events occur in immune cells, which respond to specific chemical components expressed by dangerous micro-organisms. These immune chemosensory cells release mediators, such as cytokines, to activate neural elements, including primary afferent neurons of the vagal sensory ganglia. Primary afferent activation initiates local reflexes (e.g. cardiovascular and gastrointestinal) that support host defense. In addition, at least three parallel pathways of ascending immune-related information activate specific components of the illness response. In this way, immunosensory systems represent highly organized and coherent pathways for activating host defense against infection.

Stress-induced relapse to heroin and cocaine seeking in rats: a review

Studies in humans suggest that exposure to stress increases the probability of relapse to drug use, but until recently there has been no animal model to study the mechanisms that mediate this effect. We have developed a reinstatement procedure that allows us to study the effect of stress on relapse to drug seeking in rats. Using this procedure, we have shown that exposure to intermittent footshock stress reliably reinstates heroin and cocaine seeking after prolonged drug-free periods. In the present paper, we summarize results from several studies on stress-induced reinstatement of heroin and cocaine seeking in rats. We first assess the degree to which the phenomenon of stress-induced relapse generalizes to other stressors, to behaviors controlled by other drugs of abuse, and to behaviors controlled by non-drug reinforcers. We then review evidence from studies concerned with the neurotransmitters, the brain sites, and the neural systems involved in stress-induced reinstatement of drug seeking. Finally, we consider the mechanisms that might underlie stress-induced relapse to drug seeking and the possible implications of the findings for the treatment of relapse to drug use in humans.

Noradrenaline in the ventral forebrain is critical for opiate withdrawal-induced aversion

Cessation of drug use in chronic opiate abusers produces a severe withdrawal syndrome that is highly aversive, and avoidance of withdrawal or associated stimuli is a major factor contributing to opiate abuse. Increased noradrenaline in the brain has long been implicated in opiate withdrawal, but it has not been clear which noradrenergic systems are involved. Here we show that microinjection of beta-noradrenergic-receptor antagonists, or of an alpha2-receptor agonist, into the bed nucleus of the stria terminalis (BNST) in rats markedly attenuates opiate-withdrawal-induced conditioned place aversion. Immunohistochemical studies revealed that numerous BNST-projecting cells in the A1 and A2 noradrenergic cell groups of the caudal medulla were activated during withdrawal. Lesion of these ascending medullary projections also greatly reduced opiate-withdrawal-induced place aversion, whereas lesion of locus coeruleus noradrenergic projections had no effect on opiate-withdrawal behaviour. We conclude that noradrenergic inputs to the BNST from the caudal medulla are critically involved in the aversiveness of opiate withdrawal.

Distribution of prolactin-releasing peptide mRNA in the rat brain

In order to identify the distribution of prolactin-releasing peptide (PrRP) mRNA in the rat brain, we independently cloned cDNA of PrRP. Brains were removed from three adult males, and brains from three females each at 0200 and 1400 h on day 7 of pregnancy were obtained. By the nonradioactive in situ hybridization method, the location of PrRP mRNA was detected in very restricted brain areas. The distribution of PrRP mRNA signals was very similar in both sexes. In the hypothalamus, only the ventral part of the caudal dorsomedial nucleus had PrRP mRNA signals. Other forebrain areas did not show any positive signals. In the medulla oblongata, two discrete areas contained PrRP mRNA signals. No positive signal was found in the rostral part of the medulla oblongata extending to the anterior part of the area postrema. The caudal part of the nucleus of the solitary tract (NTS) had neurons with very strong signals of PrRP mRNA. The reticular nucleus showed a few PrRP mRNA positive neurons. The number of PrRP mRNA positive cells in the NTS was not different between experimental groups, although plasma prolactin levels in these animals were different. This anatomical information on the location of PrRP mRNA in the brain provides the framework to understand the physiological functions of PrRP in vivo.

Electrophysiological effects of oxytocin within the bed nuclei of the stria terminalis: influence of reproductive stage and ovarian steroids

The bed nuclei of the stria terminalis (BNST) is a target site for the central actions of oxytocin (OT) in promoting behavioural and neuroendocrine responses involved in female reproduction, and binding studies suggest that OT sensitivity may be modulated over the peripartum period. Electrophysiological recordings from brain slices in vitro showed that OT sensitivity of BNST neurones is relatively low in late pregnancy, but is high during lactation. In vivo studies over the immediate peri-partum period revealed that although BNST neurones can be excited by i.c.v. OT at day 22 of pregnancy, there is a 5-10 min delay in their response which is not present in lactation. This delay can be reversed by naltrexone, or lesioning the stria terminalis, and may involve an inhibitory opioid input to the BNST from the amygdala. Examination of the role of steroids in regulating OT responses of BNST neurones showed that oestradiol pre-treatment in late pregnant ovariectomized rats increased OT excitation of BNST neurones in vitro, and a similar result was observed with in vivo recordings. Progesterone also augmented OT excitation of BNST neurones in vitro, but no such effect was observed in vivo. This difference could indicate that an additional effect of progesterone is to potentiate extraneous inhibitory inputs to the BNST, or may reflect the ability of this steroid to suppress OT sensitivity by a direct membrane action. Changes in the response of BNST neurones to OT may have functional implications for the action of central OT in facilitating the neuroendocrine milk-ejection reflex (i.e. increasing milk-ejection frequency), an effect which first appears at around day 3 of lactation. Studies involving steroid treatment of late pregnant ovariectomized rats showed that this facilitatory mechanism can be induced to appear early (i.e. on day 22 of pregnancy) by oestradiol, but not progesterone treatment. Collectively, these results support this view, that the action of OT in the BNST is regulated by the changing levels of steroids towards the end of pregnancy, thereby ensuring appropriate neuroendocrine responses necessary for motherhood.

Patterns of c-fos expression induced by fluvoxamine are different after acute vs. chronic oral administration

Fluvoxamine is a selective serotonin (5-HT) reuptake inhibitor (SSRI) with a broad spectrum of behavioral and therapeutic effects, e.g. in depressive illness. We used the expression of c-fos, after both acute and chronic oral administration of fluvoxamine in the rat, to study its immediate and long-term effects, in relation to the distribution of Galanin (GAL) and Vasoactive Intestinal Polypeptide (VIP). After acute oral administration, most consistent increases were apparent in (parts of); the nucleus of the solitary tract, medial part; the lateral parabrachial nucleus, external part; the bed nucleus of the stria terminalis, dorsolateral part; and the central nucleus of the amygdala, lateral part. After chronic administration, distribution of Fos-IR was similar to acute administration, although numbers of Fos-IR neurons were no longer significantly different from control values. It is concluded that activation of 5-HT3-receptors in the caudal brainstem or gastro-intestinal afferents of the vagal nerve may play a role in the observed pattern of Fos-IR after fluvoxamine administration. The relationship with the antidepressant effects of fluvoxamine needs further investigations.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in neuroendocrine and behavioral responses to fear-related stimuli in rats

The bed nucleus of the stria terminalis (BNST) receives dense noradrenergic projections from the brainstem and has been claimed to play a role in expression of a variety of stress responses. Fear-related stimuli suppress vasopressin and facilitate oxytocin release from the neurohypophysis and induce behavioral suppression. Here we investigated in male rats whether conditioned fear stimuli increase noradrenergic activity in the BNST and whether depletion of epinephrine content in the BNST prevents neuroendocrine and behavioral responses to fear stimuli. Environmental stimuli previously paired with electric footshocks increased the ratio of 3-methoxy-4-hydroxyphenylglycol to norepinephrine contents in the BNST, suggesting that the stimuli activated noradrenergic projections to the BNST. 5-Amino-2, 4-dihydroxy-alpha-methylphenylethylamine, a neurotoxin relatively selective for noradrenergic fibers, when injected into the BNST 7 days before measurement, decreased the content of norepinephrine by 95% and that of dopamine or serotonin by about 50%. In the rats that received the neurotoxin, the suppressive vasopressin but not the augmentative oxytocin response to intermittent footshocks was abolished. In the experiments with conditioned fear stimuli, the neurotoxin given before training partially but significantly impaired the suppressive vasopressin and behavioral responses to testing stimuli. The neurotoxin given after training, however, did not prevent the vasopressin, oxytocin or behavioral responses. The results suggest that noradrenergic fibers in the BNST mediate the suppressive vasopressin but not the augmentative oxytocin response to nonassociatively applied fear stimuli and that they modulate, in a facilitative fashion, acquisition but not retention or recall of the emotional memory associated with the vasopressin and behavioral responses to conditioned fear stimuli.