Using high resolution imaging to determine trafficking of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Trafficking of G protein-coupled receptors (GPCRs) is a critical determinant of cellular sensitivity of neurons. To understand how endogenous or exogenous ligands impact cell surface expression of GPCRs, it is essential to employ approaches that achieve superior anatomical resolution at the synaptic level. In situations in which light and fluorescence microscopy techniques may provide only limited resolution, electron microscopy provides enhanced subcellular precision. Dual labeling immunohistochemistry employing visually distinct immunoperoxidase and immunogold markers has been an effective approach for elucidating complex receptor profiles at the synapse and to definitively establish the localization of individual receptors and neuromodulators to common cellular profiles. The immuno-electron microscopy approach offers the potential for determining membrane versus intracellular protein localization, as well as the association with various identifiable cellular organelles. Corticotropin-releasing factor (CRF) is an important regulator of endocrine, autonomic, immunological, behavioral and cognitive limbs of the stress response. Dysfunction of this neuropeptide system has been associated with several psychiatric disorders. This review summarizes findings from neuroanatomical studies, with superior spatial resolution, that indicate that the distribution of CRF receptors is a highly dynamic process that, in addition to being sexually dimorphic, involves complex regulation of receptor trafficking within extrasynaptic sites that have significant consequences for adaptations to stress, particularly within the locus coeruleus (LC), the major brain norepinephrine-containing nucleus.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Peptides that fine-tune the serotonin system

The dorsal raphe nucleus (DR) contains serotonin (5-HT) neurons that innervate the cortex and limbic system and through these projections is thought to regulate cognition and behavior. Clinical and pharmacological findings implicate dysfunctions in the DR-5-HT system in affective disorders, including anxiety, depression and suicide. Although the DR is often considered in light of its 5-HT neurons, recent studies underscore the complexity of this nucleus and its heterogeneous nature. Of particular interest, are peptides that are either present within neurons in the DR, innervate DR-5-HT neurons or act upon local circuitry within the DR to indirectly impact on this 5-HT system. These peptides are positioned to fine-tune the activity of selective groups of serotonergic neurons within the DR and thereby 5-HT release in different terminal fields. This review will focus on substance P and corticotropin-releasing factor as two peptides that act independently and interdependently to influence DR-5-HT function. The role of non-serotonergic components of the DR in translating the effect of each of these peptides is discussed. This synthesis refines our views on the regulation of the DR-5-HT system and importantly, gives insight into mechanisms of endogenous control of DR function, the dysregulation of which may contribute to pathophysiology.

Cellular interactions between axon terminals containing endogenous opioid peptides or corticotropin-releasing factor in the rat locus coeruleus and surrounding dorsal pontine tegmentum

Recent evidence suggests that certain stressors release both endogenous opioids and corticotropin-releasing factor (CRF) to modulate activity of the locus coeruleus (LC)-norepinephrine (NE) system. In ultrastructural studies, axon terminals containing methionine(5)-enkephalin (ENK) or CRF have been shown to target LC dendrites. These findings suggested the hypothesis that both neuropeptides may coexist in common axon terminals that are positioned to have an impact on the LC. This possibility was examined by using immunofluorescence and immunoelectron microscopic analysis of the rat LC and neighboring dorsal pontine tegmentum. Ultrastructural analysis indicated that CRF- and ENK-containing axon terminals were abundant in similar portions of the neuropil and that approximately 16% of the axon terminals containing ENK were also immunoreactive for CRF. Dually labeled terminals were more frequently encountered in the "core" of the LC vs. its extranuclear dendritic zone, which included the medial parabrachial nucleus (mPB). Triple labeling for ENK, CRF, and tyrosine hydroxylase (TH) showed convergence of opioid and CRF axon terminals with noradrenergic dendrites as well as evidence for inputs to TH-labeled dendrites from dually labeled opioid/CRF axon terminals. One potential source of ENK and CRF in the dorsal pons is the central nucleus of the amygdala (CNA). To determine the relative contribution of ENK and CRF terminals from the CNA, the CNA was electrolytically lesioned. Light-level densitometry revealed robust decreases in CRF immunoreactivity in the LC and mPB on the side ipsilateral to the lesion but little or no change in ENK immunoreactivity, confirming previous studies of the mPB. Degenerating terminals from the CNA in lesioned rats were found to be in direct contact with TH-labeled dendrites. Together, these data indicate that ENK and CRF may be colocalized to a subset of individual axon terminals in the LC "core." The finding that the CNA provides, to dendrites in the area examined, a robust CRF innervation, but little or no opioid innervation, suggests that ENK and CRF axon terminals impacting LC neurons originate from distinct sources and that terminals that colocalize ENK and CRF are not from the CNA.

Fos immunoreactivity in hypocretin-synthesizing and hypocretin-1 receptor-expressing neurons: effects of diurnal and nocturnal spontaneous waking, stress and hypocretin-1 administration

Hypocretin/orexin modulates sleep-wake state via actions across multiple terminal fields. Within waking, hypocretin may also participate in high-arousal processes, including those associated with stress. The current studies examined the extent to which alterations in neuronal activity, as measured by Fos immunoreactivity, occur within both hypocretin-synthesizing and hypocretin-1 receptor-expressing neurons across varying behavioral state/environmental conditions associated with varying levels of waking and arousal. Double-label immunohistochemistry was used to visualize Fos and either prepro-hypocretin in the lateral hypothalamus or hypocretin-1 receptors in the locus coeruleus and select basal forebrain regions involved in the regulation of behavioral state/arousal. Animals were tested under the following conditions: 1). diurnal sleeping; 2). diurnal spontaneous waking; 3). nocturnal spontaneous waking; and 4). high-arousal waking (diurnal novelty-stress). Additionally, the effects of hypocretin-1 administration (0.07 and 0.7 nmol) on levels of Fos were examined within these two neuronal populations. Time spent awake, scored for the 90-min preceding perfusion, was largely comparable in diurnal spontaneous waking, nocturnal spontaneous waking and high-arousal waking. Nocturnal spontaneous waking and high-arousal waking, but not diurnal spontaneous waking, were associated with increased levels of Fos within hypocretin-synthesizing neurons, relative to diurnal sleeping. Within hypocretin-1 receptor-expressing neurons, only high-arousal waking was associated with increased levels of Fos. Hypocretin-1 administration dose-dependently increased levels of Fos within hypocretin-1 receptor-expressing neurons to levels comparable to, or exceeding, levels observed in high-arousal waking. Combined, these observations support the hypothesis that hypocretin neuronal activity varies across the circadian cycle. Additionally, these data suggest that waking per se may not be associated with increased hypocretin neurotransmission. In contrast, high-arousal states, including stress, appear to be associated with substantially higher rates of hypocretin neurotransmission. Finally, these studies provide further evidence indicating coordinated actions of hypocretin across a variety of arousal-related basal forebrain and brainstem regions in the behavioral state modulatory actions of this peptide system.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3+/-1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8+/-0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4+/-4.4) than 5-HT-immunonegative cells (tau=33.5+/-4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT(1A) receptor. These results suggested that 5-HT(1A) receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT(1A) receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT(1A) receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT(1A) receptors on non-5-HT neurons in the dorsal raphe nucleus.

Corticotropin-releasing factor neurones of the central nucleus of the amygdala mediate locus coeruleus activation by cardiovascular stress

Hypotensive stress engages corticotropin-releasing factor (CRF) release within the rat locus coeruleus (LC), which activates LC neurones, initiating norepinephrine release in forebrain and activation of forebrain electroencephalographic activity. This study identified CRF afferents to the LC that are engaged during hypotensive stress. One of two potential CRF afferents, the central nucleus of the amygdala (CNA) or bed nucleus of the stria terminalis (BNST), was electrolytically lesioned and LC activation during hypotensive stress was quantified. Neither lesion altered LC spontaneous discharge rate or activation by intra-LC administered CRF. By contrast, LC activation by hypotensive stress was greatly attenuated in CNA-lesioned, but not BNST-lesioned, rats. Hypotensive stress-induced changes in transcriptional activation were immunohistochemically identified in CRF neurones that were retrogradely labelled from the LC region. c-fos immunoreactivity was prevalent in the paraventricular nucleus of the hypothalamus (PVN), CNA and BNST. However, only the PVN contained a substantial number of neurones that were doubly immunolabelled for CRF and c-fos, and few of these were retrogradely labelled from the LC. By contrast, immunoreactivity for the phosporylated form of cyclic AMP response-element binding protein (PCREB) was prevalent in CRF neurones in the CNA and BNST. Moreover, approximately one-third of the PCREB-expressing CRF neurones in the CNA were retrogradely labelled from the LC. These electrophysiological and anatomical findings implicate the CNA as a primary source of CRF that activates the LC during hypotensive stress. Additionally, CREB phosphorylation, rather than c-fos induction, is associated with hypotensive activation of CRF-CNA neurones that project to the LC.

Corticotropin-releasing factor is preferentially colocalized with excitatory rather than inhibitory amino acids in axon terminals in the peri-locus coeruleus region

Corticotropin-releasing factor(CRF)-immunoreactive terminals form synaptic specializations with locus coeruleus (LC) dendrites in rat brain. Within these terminals, CRF-immunoreactive dense core vesicles are colocalized with non-labeled dense core vesicles and clear vesicles, implicating other neuromodulators in the actions of CRF on LC neurons. Excitatory (glutamate) and inhibitory (GABA) amino acid afferents to the LC, have been identified which regulate noradrenergic responses to sensory stimuli. This study was designed to determine whether these amino acid neurotransmitters are colocalized with CRF in terminals within the LC/peri-LC region in the rat. Sections through the LC region that were dually labeled using immunohistochemical techniques to visualize either CRF and glutamate or CRF and GABA were examined using electron microscopy. Numerous terminals that contained immunolabeling for both CRF and glutamate (e.g. 30% of 106 CRF-immunoreactive terminals and 13% of 232 glutamate-immunolabeled terminals) were observed in the peri-LC. Additionally, single labeled CRF and glutamate terminals were often apposed to one another or found to converge on common dendritic targets. In contrast, relatively few terminals exhibited immunolabeling for both GABA and CRF (5% of 317 CRF-immunoreactive terminals). However, evidence for a postsynaptic effect of CRF on GABA-containing profiles included synapses between CRF axon terminals and GABA-labeled dendrites (10% of 317 CRF-labeled terminals), as well as appositions between CRF- and GABA-labeled terminals. These results indicate that CRF is preferentially colocalized with glutamate in the rostrolateral LC region and may impact on glutamate neurotransmission in the LC via presynaptic or postsynaptic actions. They argue against colocalization of CRF with GABA, although CRF may modulate GABA release via postsynaptic effects in the peri-LC region.

Opposing regulation of the locus coeruleus by corticotropin-releasing factor and opioids. Potential for reciprocal interactions between stress and opioid sensitivity

RATIONALE

Substantial clinical and preclinical findings support an association between stress and opiate abuse. To understand the mechanisms underlying this association, it is important to identify substrates of the stress response and endogenous opioid systems that interact and specific points at which stress circuits and endogenous opioid systems intersect.

OBJECTIVE

This review focuses on corticotropin-releasing factor (CRF), a critical substrate of the stress response, and its potential for interactions with endogenous opioid systems within the pontine nucleus, locus coeruleus (LC), a brain region that has been implicated as a target in response to stress and opiates.

RESULTS

Evidence is reviewed supporting the hypothesis that CRF and endogenous opioids interact to co-regulate the LC. Thus, CRF- and enkephalin-immunoreactive fibers innervating LC dendritic fields overlap, and some axon terminals in this region co-localize CRF and enkephalin. CRF and opioids have opposing effects on LC neuronal discharge and on intracellular signaling mechanisms within LC neurons. Finally, a history of stress or opiate use induces plasticity in CRF-LC or opiate-LC interactions, respectively. Disruptions in the CRF/opioid balance as a result of this plasticity are proposed to result in hyperactivity or hyperresponsiveness of the LC-norepinephrine (NE) system.

CONCLUSIONS

Co-regulation of the LC-NE system by CRF and opioids may be important in acute adaptation to stress. Potential clinical consequences of an imbalance in this regulation as a result of prior stress include increased risk of opiate self administration and decreased sensitivity to opiates used in clinical settings. Conversely, chronic exposure to opiates may predispose individuals to stress-related psychiatric disorders.

Role of Barrington's nucleus in the activation of rat locus coeruleus neurons by colonic distension

The locus coeruleus (LC)-noradrenergic system, which has been implicated in arousal and attention, is activated by visceral stimuli such as colon and bladder distension. Neurons of Barrington's nucleus (the pontine micturition center) have been identified which project to both the LC and preganglionic column of the lumbosacral spinal cord. Thus, Barrington's nucleus is positioned to coordinate brain noradrenergic activity with pelvic visceral functions. The aim of this study was to determine whether LC activation by colonic distension was mediated by projections from Barrington's nucleus to the LC in the rat. Lesions of Barrington's nucleus were performed unilaterally by local injection of ibotenic acid (microg/microl, 90 nl) 10 days prior to recording: (i) ipsilateral spontaneous LC discharge rate; (ii) LC responses to colonic distension; and (iii) LC responses to sciatic nerve stimulation. In some rats LC activation by hypotensive challenge was also examined. Lesions of Barrington's nucleus significantly reduced LC activation by colon distension from a magnitude of 26.6+/-6% increase in discharge rate (n=8) to 6.9+/-3% (n=6), while having no effect on basal LC discharge rate. In contrast, LC responses to sciatic nerve stimulation were not altered in rats with lesions of Barrington's nucleus and LC neurons were still activated by hypotensive challenge. These results support the hypothesis that Barrington's nucleus selectively relays input from pelvic visceral afferents to the LC. This may serve as a limb in a circuit designed to coordinate central and peripheral responses to pelvic visceral stimuli.

Evidence for regional heterogeneity in corticotropin-releasing factor interactions in the dorsal raphe nucleus

The dorsal raphe nucleus (DR) is innervated by fibers containing the stress-related neurohormone corticotropin-releasing factor (CRF), which alters DR neuronal activity and serotonin release in rats. This study examined the relative distribution of CRF-immunoreactive fibers in the rat DR by using light level densitometry. Additionally, CRF-immunoreactive processes within specific subregions of the DR were examined at the ultrastructural level by using electron microscopy. CRF-immunoreactive fibers were organized within the DR along a caudal-rostral gradient, such that proceeding rostrally, innervation shifted from dorsolateral to ventromedial. Numerous CRF-immunoreactive axon terminals containing dense-core vesicles were found in both the caudal dorsolateral region and the rostral ventromedial/interfascicular region. These formed synaptic specializations with unlabeled dendrites and frequently contacted nonlabeled axon terminals. Semiquantitative analysis revealed certain differences between the two regions with respect to the types of associations made by CRF-immunoreactive terminals. Associations with dendrites were more frequent in the dorsolateral vs. ventromedial region (65% of 171 terminals vs. 39% of 233 terminals, respectively), whereas associations with axon terminals were more frequent in the ventromedial/interfascicular vs. the dorsolateral region (72% of 233 terminals vs. 57% of 171 terminals, respectively). Additionally, synaptic specializations between CRF-immunoreactive terminals and dendrites were more frequently asymmetric in the dorsolateral region (60%) and symmetric (49%) in the ventromedial/interfascicular region. Regional differences in CRF terminal interactions in the DR could account for the reported heterogeneous effects of CRF on DR neuronal activity and forebrain serotonin release. Importantly, the present results provide anatomical substrates for regulation of the DR by endogenous CRF.

Evidence for functional release of endogenous opioids in the locus ceruleus during stress termination

Endogenous opioids target noradrenergic locus ceruleus (LC) neurons and potently inhibit LC activity. Nonetheless, it has been difficult to demonstrate functional regulation of the LC-noradrenergic system by endogenous opioids because of the lack of effect of opiate antagonists. The present findings provide evidence that endogenous opioids regulate LC neuronal activity during the termination of a stressor. LC neuronal discharge was recorded from halothane-anesthetized rats before, during, and after hypotensive stress elicited by intravenous nitroprusside infusion. In naive rats, mean arterial blood pressure was temporally correlated with LC activity such that hypotension was associated with increased LC discharge and a return to the normotensive state was associated with a decrease in LC discharge below pre-stress values. After microinfusion of an antagonist of the stress neuropeptide corticotropin-releasing factor (CRF) into the LC, the increase in LC discharge associated with hypotension was prevented, whereas LC inhibition associated with termination of the challenge occurred at an earlier time and was of a greater magnitude. In contrast, microinfusion of naloxone into the LC completely abolished LC inhibition associated with termination of the stressor. Naloxone microinfusion did not prevent LC inhibition associated with hypertension produced by intravenous vasopressin administration, suggesting that endogenous opioids may be selectively engaged during the termination of hypotensive stress. These results provide evidence for a functional release of endogenous opioids within the LC. This action of endogenous opioids may serve to counterbalance excitatory effects of CRF on the LC-norepinephrine system, thereby limiting its activation by stress.

Topographic architecture of stress-related pathways targeting the noradrenergic locus coeruleus

Peripheral sympathetic nerves and brainstem noradrenergic neurons of the locus coeruleus (LC) respond in parallel to a variety of stress-related stimuli which results in norepinephrine release both peripherally and centrally. Elucidation of central pathways subserving modulation of LC neurons point to extranuclear noradrenergic dendrites of LC somata that extend into peri-coerulear areas as a major target of afferents that participate in behavioral and physiological responses to stress. Anterograde tract tracing combined with immunoelectron microscopic detection of the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) has demonstrated that the nucleus of the solitary tract (NTS) and the ventrolateral aspect of the periaqueductal gray (PAG), regions that participate in coordinating autonomic and motor behavior in response to stress, preferentially target the rostral ventromedial aspect of the peri-LC. In contrast, limbic forebrain afferents including the central nucleus of the amygdala (CNA) and the bed nucleus of the stria terminalis (BNST), regions that coordinate emotional responses to external stressors, provide direct synaptic input to noradrenergic dendrites that extend into rostral dorsolateral peri-coerulear areas. Neurochemical identification of transmitter systems impinging on LC indicate that the CNA provides corticotropin-releasing factor (CRF), a peptide essential for integrated physiological responses to stress, to the dorsolateral LC. Endogenous opioid peptides that originate from medullary sources, however, target primarily the "core" of the LC. Our physiological data suggest that stress engages CRF and opioid afferents to the LC, which have opposing influences on this noradrenergic system. The balance between opioid and CRF influences acting in the LC may, in part, maintain the balance of active and passive coping behaviors in response to stress. Understanding the afferent and neurochemical organization of the LC may help elucidate adaptations in neural circuits associated with stress which impact on central noradrenergic function.

Transneuronal labeling from the rat distal colon: anatomic evidence for regulation of distal colon function by a pontine corticotropin-releasing factor system

Neural circuits that are positioned to regulate rat distal colon function were identified by immunohistochemical detection of pseudorabies virus (PRV) and corticotropin-releasing factor (CRF). The distribution of PRV-immunoreactive neurons was examined in spinal cord and brain at increasing times (72-118 hours) after distal colon injection. At 72-80 hours, PRV-labeling was confined to the spinal cord, in the parasympathetic preganglionic column in the lumbosacral spinal cord and in the intermediolateral column of the thoracic spinal cord. At longer survival times (88 hours), PRV-immunolabeled neurons in the lumbosacral spinal cord were also distributed in superficial layers of the dorsal horn, the dorsal commissure, and around the central canal. Trans-synaptic labeling was identified in the medullary raphe nuclei, parapyramidal region, A5, Barrington's nucleus, A7, and the dorsal cap of the paraventricular nucleus of the hypothalamus after longer survival times (88-91 hours). Substantial labeling of the locus coeruleus, periaqueductal gray and forebrain regions occurred at later survival times (> or = 96 hours). In dual-labeled sections, CRF terminal labeling surrounded PRV-labeled neurons in the parasympathetic preganglionic column of the lumbosacral spinal cord. Additionally, many neurons in Barrington's nucleus, but not other CRF-containing nuclei, were double labeled for CRF and PRV. These results, taken with previous studies, support a convergence in transneuronal labeling from different pelvic viscera that may be related to coordination of overall pelvic visceral functions. Importantly, they provide an anatomic substrate for an impact of CRF from Barrington's nucleus in normal and pathophysiological functions of the distal colon.

Effects of corticotropin-releasing factor on neuronal activity in the serotonergic dorsal raphe nucleus

The present study examined the regional localization of corticotropin-releasing factor (CRF)- and 5-hydroxytryptamine (5-HT)-immunoreactive (IR) fibers within the rat dorsal raphe nucleus (DRN) using immunohistochemistry. Additionally, the effects of CRF, administered intracerebroventricularly (0.1-3.0 micrograms) or intraraphe (0.3-30 ng), on discharge rates of putative 5-HT DRN neurons were quantified using in vivo single unit recording in halothane-anesthetized rats. CRF-IR fibers were present at all rostrocaudal levels of the DRN and exhibited a topographical distribution. CRF produced predominantly inhibitory effects on DRN discharge at lower doses and these effects diminished or became excitatory at higher doses. Inhibition of DRN discharge by CRF was attenuated by the nonselective CRF antagonist, DPheCRF12-41 and the CRF-R1-selective antagonist, antalarmin, implicating the CRF-R1 receptor subtype in these electrophysiological effects. The present findings provide anatomical and physiological evidence for an impact of CRF on the DRN-5HT system.

A.E. Bennett Research Award. Anatomic basis for differential regulation of the rostrolateral peri-locus coeruleus region by limbic afferents

BACKGROUND

Neurochemical and electrophysiological studies indicate that the locus coeruleus (LC)-norepinephrine system is activated by physiological and external stressors. This activation is mediated in part by corticotropin-releasing factor (CRF), the hypothalamic neurohormone that initiates the endocrine response to stress. We have previously shown that the central nucleus of the amygdala (CNA) provides CRF afferents to noradrenergic processes in the peri-LC area that may serve to integrate emotional and cognitive responses to stress. The bed nucleus of the stria terminalis (BNST) shares many anatomical and neurochemical characteristics with the CNA, including a high density of CRF-immunoreactive cells and fibers; however, recent studies have suggested that the CNA and the BNST may differentially regulate responses to conditioned and unconditioned fear, respectively, suggesting divergent neuroanatomical circuits underlying these processes.

METHODS

In the present study, neuroanatomical substrates subserving regulation of the LC by the BNST were examined. Anterograde tract-tracing was combined with immunoelectron microscopy to test the hypotheses that BNST efferents target noradrenergic neurons of the LC and that these efferents exhibit immunolabeling for CRF.

RESULTS

Ultrastructural analysis of sections that were dually labeled for the anterograde tracer biotinylated dextran amine (BDA) injected into the BNST and tyrosine hydroxylase (TH)-immunoreactivity demonstrated that BDA-labeled axon terminals formed synaptic specializations (primarily inhibitory) with TH-labeled dendrites and dendrites that lacked TH immunoreactivity. In contrast to CNA efferents that exhibited substantial immunolabeling for CRF, far fewer BDA-labeled terminals from the BNST in the rostrolateral peri-LC contained CRF.

CONCLUSIONS

The present results indicate that the BNST may provide distinct neurochemical regulation of the peri-LC as compared to other limbic afferents such as the CNA. These data are interesting in light of behavioral studies showing that the CNA and BNST may be differentially involved in fear versus anxiety, respectively.

Long-term regulation of locus ceruleus sensitivity to corticotropin-releasing factor by swim stress

Corticotropin-releasing factor (CRF) acts as a putative neurotransmitter in the locus ceruleus (LC) to mediate its activation by certain stressors. In this study, we quantified LC sensitivity to CRF 24 h after swim stress, at a time when behavioral depression that is sensitive to antidepressants is apparent. Rats were placed in a tank with 30 cm (swim stress) or 4 cm water and 24 h later, either behavior was monitored in a forced swim test or LC discharge was recorded. Swim stress rats were more immobile than control animals in the swim test. LC neurons of swim stress rats were sensitized to low doses of CRF (0.1-0.3 microgram i.c.v.) that were ineffective in control animals and were desensitized to higher doses. Swim stress selectively altered LC sensitivity to CRF because neither LC spontaneous discharge nor responses to other agents (e.g., carbachol, vasoactive intestinal peptide) were altered. Finally, the mechanism for sensitization was localized to the LC because neuronal activation by low doses of CRF was prevented by the intracerulear administration of a CRF antagonist. CRF dose-response curves were consistent with a two-site model with similar dissociation constants under control conditions but divergent dissociation constants after swim stress. The results suggest that swim stress (and perhaps other stressors) functionally alters CRF receptors that have an impact on LC activity. Stress-induced regulation of LC sensitivity to CRF may underlie behavioral aspects of stress-related psychiatric disorders.

Pontine regulation of pelvic viscera: pharmacological target for pelvic visceral dysfunctions

The pathophysiology and pharmacological targets of disorders of the bladder and colon have focused predominantly on the periphery. However, these viscera are regulated by the CNS, which, in turn, must integrate their functions with compatible behaviours. This review focuses on the role of the pontine micturition centre, Barrington's nucleus, as a key to this integration. Through its efferent network this pontine centre links parasympathetic preganglionic neurones with forebrain-projecting nuclei, providing an anatomical substrate for coregulation of pelvic visceral and forebrain activity. Disorders characterized by multiple pelvic visceral symptoms and comorbidity with psychiatric disorders (for example functional bowel disorders) might have their roots in dysfunctions of this circuit, which could provide a novel target for pharmacological treatment.

Glucocorticoid receptor-immunoreactivity in corticotrophin-releasing factor afferents to the locus coeruleus

The stress-related neurohormone, corticotropin-releasing factor (CRF), also acts as a neurotransmitter to activate the brain noradrenergic nucleus, locus coeruleus (LC). Previous electrophysiological findings demonstrating that tonic CRF secretion in the LC is increased in adrenalectomized rats suggest that activity of certain CRF afferents to the LC is under inhibitory regulation by endogenous corticosteroids. The present study was designed to identify putative CRF afferents to the LC that may be regulated by glucocorticoids. Retrograde tract tracing from the rat LC and pericoerulear regions was combined with immunohistochemistry to visualize CRF and glucocorticoid receptors in the same sections of rat brain. The retrograde tracer, wheat germ agglutinin conjugated to horseradish peroxidase coupled to gold (WGA-Au-HRP) was injected into either the nucleus LC or the rostrolateral pericoerulear region (peri-LC), where CRF-immunoreactive terminals have been demonstrated to synapse with LC dendrites. Sections were processed to visualize the tracer, as well as CRF- and glucocorticoid receptor-immunoreactivity. Following injections of WGA-Au-HRP into the nuclear LC, triple labeled neurons were observed primarily in Barrington's nucleus, where 74+/-4% of retrogradely labeled CRF-immunoreactive neurons colocalized glucocorticoid receptor immunoreactivity. In contrast, injections that incorporated the rostrolateral peri-LC retrogradely labeled numerous neurons that were immunoreactive for both CRF and glucocorticoid receptors in the central nucleus of the amygdala. Thus, 94+/-2% of the retrogradely labeled CRF-immunoreactive neurons in the central nucleus of the amygdala colocalized glucocorticoid receptor immunoreactivity. Additionally, triple labeled neurons were observed in the bed nucleus of the stria terminalis following WGA-Au-HRP injections that incorporated the rostrolateral peri-LC. The present results implicate Barrington's nucleus, the central nucleus of the amygdala and the bed nucleus of the stria terminalis as glucocorticoid-sensitive sources of CRF that can influence the LC-noradrenergic system. Alterations in glucocorticoid levels or glucocorticoid receptor function would be predicted to affect the impact of these specific CRF systems on LC activity.

Amygdaloid corticotropin-releasing factor targets locus coeruleus dendrites: substrate for the co-ordination of emotional and cognitive limbs of the stress response

Corticotropin-releasing factor (CRF), the neurohormone that initiates the endocrine limb of the stress response via its actions on the anterior pituitary, also acts as a neurotransmitter in the noradrenergic locus coeruleus (LC) to activate this system during stress. Because the central nucleus of the amygdala contains numerous CRF-immunoreactive neurones, the present study examined whether CRF projections from the central nucleus of the amygdala target LC dendrites, thereby providing a mechanism for limbic-CRF modulation of brain noradrenergic activity. Retrograde tracers injected into the rostrolateral pericoerulear region, where CRF-immunoreactive fibres are dense, labelled numerous CRF-immunoreactive neurones in the central nucleus of the amygdala. Consistent with this, ultrastructural analysis of the rostrolateral pericoerulear region in sections that were dually labelled for an anterograde tracer (biotinylated dextran amine, BDA) injected into the central nucleus of the amygdala and CRF immunoreactivity revealed that a substantial percentage (35%) of amygdaloid axon terminals were CRF-immunoreactive. These terminals formed synaptic specializations with unlabelled dendrites that were more often of the asymmetric (excitatory) type. Additionally, ultrastructural analysis of sections that were dually labelled to visualize CRF-and tyrosine hydroxlase-immunoreactivity demonstrated synaptic specializations between CRF-immunoreactive terminals and LC dendrites in the rostrolateral peri-LC, which were also frequently asymmetric. Taken together with previous ultrastructural findings that LC dendrites in the rostrolateral pericoerulear region are targeted by anterogradely labelled terminals from the central nucleus of the amygdala, the present results implicate this nucleus as a source of CRF that can impact on LC activity via effects on dendrites in the rostrolateral pericoerulear region. This cellular substrate for amygdaloid-CRF modulation of brain noradrenergic activity may serve as a mechanism for the integration of emotional and cognitive responses to stress.

Effects of corticotropin-releasing factor on brain serotonergic activity

The serotonergic dorsal raphe nucleus is innervated by corticotropin-releasing factor (CRF) and expresses CRF receptors, suggesting that endogenous CRF impacts on this system. The present study characterized interactions between CRF and the dorsal raphe serotonin (5-HT) system. The effects of intracerebroventricularly (i.c.v.) administered CRF on microdialysate concentrations of 5-HT in the lateral striatum of freely moving rats were determined. CRF had biphasic effects, with 0.1 and 0.3 microgram decreasing, and 3.0 micrograms increasing 5-HT dialysate concentrations. i.c.v. administration of CRF inhibited neuronal activity of the majority of dorsal raphe neurons at both low (0.3 microgram) and high (3 micrograms) doses. Likewise, intraraphe administration of CRF (0.3 and 1.0 ng) had predominantly inhibitory effects on discharge rate. Together, these results suggest that CRF is positioned to regulate the function of the dorsal raphe serotonergic system via actions within the cell body region. This regulation may play a role in stress-related psychiatric disorders in which 5-HT has been implicated.

Novel role for the pontine micturition center, Barrington's nucleus: evidence for coordination of colonic and forebrain activity

This report provides evidence for a novel role of Barrington's nucleus, considered the pontine micturition center, in regulation of colonic function. Barrington's activation elicited increases in colonic intraluminal pressure that were eliminated by scopolamine and intrathecal lidocaine, suggesting an impact of Barrington's neurons on colonic activity via projections to lumbosacral parasympathetic neurons. Consistent with this, Barrington's neurons were transsynaptically labeled from the distal colon by pseudorabies virus and several of these were also retrogradely labeled from the locus coeruleus, which projects extensively to the forebrain. Thus, Barrington's nucleus is strategically positioned to coordinate colonic and forebrain activity. Dysfunctions within this divergent system may underlie the frequent comorbidity of colonic and psychiatric symptoms.

Lack of sensitization to the effects of d-amphetamine and apomorphine on sensorimotor gating in rats

This study assessed whether repeated injections of d-amphetamine or apomorphine could induce sensitization to the disruptive effects of these psychomotor stimulants on sensorimotor gating in rats. In the first experiment, rats were given six pre-exposures to either 2.0 mg/kg d-amphetamine or saline before being tested for the effects of d-amphetamine (0.0, 0.5, 1.0, 2.0 or 4.0 mg/kg, i.p.) on prepulse inhibition of acoustic startle (PPI) and locomotor activity. The tests for PPI confirmed that sensorimotor gating could be disrupted by a high dose of d-amphetamine (4.0 mg/kg). However, comparison of the dose-response curves for the drug and saline pre-exposed groups did not reveal evidence for sensitization to this d-amphetamine effect in drug-pre-exposed rats, despite indications that sensitization had developed to the locomotor stimulant effects of d-amphetamine. A similar pattern of results was obtained in a second experiment that examined the effects of apomorphine (0.0, 0.1, 0.2, 0.4 and 0.8 mg/kg, s.c.) on PPI and locomotion in rats pre-exposed to 2.0 mg/kg of this drug or its vehicle. These findings demonstrate that treatments which induce sensitization to the behavioral activating effects of psychomotor stimulants do not necessarily produce sensitization to the disruptive effects of stimulants on sensorimotor gating. The implications of these results for hypotheses linking sensitization-like processes to the etiology of schizophrenia are discussed.

Locus coeruleus activation by colon distention: role of corticotropin-releasing factor and excitatory amino acids

The present study was designed to elucidate the neurotransmitters involved in activation of the noradrenergic nucleus, locus coeruleus, by distention of the distal colon. Locus coeruleus spontaneous discharge rate was recorded from halothane-anesthetized rats before, during and after distention of the colon produced by inflation of a balloon catheter with varying volumes of water. Locus coeruleus activation by colon distention was volume-dependent and reversible. Activation of cortical electroencephalographic activity was temporally correlated with locus coeruleus activation during colon distention and prolonged distention (greater than 2 min) resulted in tachyphalaxis to both locus coeruleus and cortical electroencephalographic activation. The corticotropin-releasing factor antagonist, DPheCRF(12-41), administered intracerebroventricularly (3 microg) or microinfused into the locus coeruleus (10 ng) significantly attenuated locus coeruleus activation produced by lower, but not higher magnitudes of colon distention, implicating corticotropin-releasing factor afferents to the locus coeruleus in this response. Consistent with this, prior exposure to 30 min of footshock stress, which desensitizes locus coeruleus neurons to corticotropin-releasing factor, produced a similar attenuation of locus coeruleus activation by low, but not high magnitudes of distention. Kynurenic acid, administered intracerebroventricularly (5 micromol), significantly antagonized locus coeruleus activation by all magnitudes of colon distention. However, this excitatory amino acid antagonist was ineffective when administered directly into the locus coeruleus (0.3 nmol). Together, these findings suggest that low magnitudes of colon distention activate the locus coeruleus-noradrenergic system via corticotropin-releasing factor release within the locus coeruleus and that excitatory amino acid neurotransmission at a site distal to the locus coeruleus is necessary for this response. Activation of the locus coeruleus-noradrenergic system during colon distention may serve as a cognitive limb of the peripheral parasympathetic response. This activation may also play a role in disorders characterized by comorbidity of colonic and psychiatric symptoms, such as irritable bowel syndrome.

Activation of the locus coeruleus noradrenergic system by intracoerulear microinfusion of corticotropin-releasing factor: effects on discharge rate, cortical norepinephrine levels and cortical electroencephalographic activity

Corticotropin-releasing factor (CRF) administered intracerebroventricularly (i.c.v.) activates noradrenergic locus coeruleus (LC) neurons of halothane-anesthetized and unanesthetized rats. This study used a technique for microinfusing CRF into the LC from calibrated micropipettes to characterize and quantify the effects of locally administered CRF on LC discharge in halothane-anesthetized rats. CRF (3-100 ng) microinfusion into the LC increased discharge rate in a dose-dependent manner from 28 +/- 8 to 105 +/- 26% above preinfusion discharge rates. The CRF dose-response curve generated by local microinfusion was parallel to, and shifted approximately 200-fold to the left, of that generated by i.c.v. administration. Intracoerulear microinfusion of the CRF antagonist, [DPhe12,Nle(21,38),CalphaMeLeu37]r/hCRF(12-41), greatly attenuated LC activation produced by a maximally effective dose of i.c.v. administered CRF, suggesting that these effects are primarily due to actions within the LC. In rats in which both LC discharge rate and norepinephrine levels in prefrontal cortex were measured by in vivo microdialysis, CRF microinfused into the LC increased both endpoints. Finally, LC activation produced by CRF (60 ng) microinfusion into the LC was associated with cortical electroencephalographic activation. Taken together with previous anatomical and electrophysiological evidence for endogenous CRF interactions in the LC, our results support the hypothesis that CRF serves as an excitatory neurotransmitter in the LC, and suggest that its actions on LC neurons are translated to enhanced norepinephrine release and an impact on cortical targets.

Regulation of a putative neurotransmitter effect of corticotropin-releasing factor: effects of adrenalectomy

This study tested the hypothesis that endogenous glucocorticoids regulate a putative neurotransmitter function of corticotropin-releasing factor (CRF) in the locus coeruleus (LC). LC spontaneous discharge and activation by intracerebroventricularly administered CRF, hypotensive challenge, sciatic nerve stimulation, and carbachol were compared in adrenalectomized and sham-operated halothane-anesthetized rats. LC spontaneous discharge was higher in adrenalectomized versus sham-operated rats. Intracoerulear microinfusion of a CRF antagonist decreased LC discharge rates of adrenalectomized rats to rates comparable with those observed in sham-operated rats but had no effect in sham-operated rats. The CRF dose-response curve was shifted in a complex manner in adrenalectomized rats, suggesting that a proportion of CRF receptors were occupied before CRF administration, and low doses of CRF were additive. Higher doses of CRF produced effects that were greater than predicted by simple additivity. Hypotensive challenge increased LC discharge rates of adrenalectomized rats by a magnitude greater than that predicted on the basis of additivity. In contrast, LC responses to carbachol and sciatic nerve stimulation were similar in both groups. The results suggest that adrenalectomy enhances tonic and stress-induced CRF release within the LC and also alters postsynaptic sensitivity of LC neurons to CRF. Because adrenalectomy also alters release of neurohormone CRF, the present study suggests that CRF actions as a neurohormone and as a neurotransmitter in the LC may be co-regulated. Such parallel regulation may underlie the coexistence of neuroendocrine and noradrenergic dysfunctions in stress-related psychiatric disorders.

Enhanced norepinephrine release in prefrontal cortex with burst stimulation of the locus coeruleus

The present study was designed to determine the relationship between the discharge of noradrenergic locus coeruleus (LC) neurons and norepinephrine release in the medial prefrontal cortex, a target of LC projections. The LC was electrically stimulated at varying frequencies and patterns for 20 min and extracellular norepinephrine levels were measured in the medial prefrontal cortex of halothane-anesthetized rats using in vivo microdialysis. Electrical stimulation of the LC at 3-10 Hz with an evenly spaced pattern of pulses (tonic stimulation) increased cortical norepinephrine levels in a frequency-dependent manner, with 5- and 10-Hz stimulation increasing norepinephrine levels by 49 +/- 3% and 66 +/- 20%, respectively. The LC was also stimulated with bursts of pulses designed to deliver physiologically relevant phasic stimulation using the same number of stimuli in a 20-min period as delivered by tonic stimulation at 3 Hz. Results revealed that norepinephrine levels were significantly higher with phasic stimulation compared to tonic stimulation. The present findings indicate that both frequency and pattern of LC discharge are determinants of norepinephrine terminal release. Additionally, bursts of LC activity, similar to those that occur in behaving animals, may be more effective in increasing terminal norepinephrine release on a per spike basis than tonic increases in activity.

Evidence for divergent projections to the brain noradrenergic system and the spinal parasympathetic system from Barrington's nucleus

The present study was designed to determine whether Barrington's nucleus, which lies ventromedial to the locus coeruleus (LC) and projects to the sacral parasympathetic nucleus, is a source of afferent projections to the LC. Restricted injections of the anterograde tracer, biocytin, into Barrington's nucleus labeled varicose fibers that extended from the injection site into the LC. Consistent with this, injections of the retrograde tracers, wheatgerm agglutinin conjugated to horseradish peroxidase coupled to gold particles (WGA-Au-HRP) or fluorescein-conjugated latex beads, into the LC labeled numerous (approximately 10%) Barrington's neurons that were also retrogradely labeled by Fluoro-Gold (FG) injections in the spinal cord. Retrograde tracing from the LC combined with corticotropin-releasing hormone (CRH) immunohistochemistry revealed that at least one third of the retrogradely labeled neurons in Barrington's nucleus were CRH-immunoreactive (CRH-IR). Finally, in triple labeling studies, CRH-Barrington's neurons were consistently observed that were retrogradely labeled from both the and spinal cord. These findings implicate Barrington's nucleus as an LC afferent and a source of CRH-IR fibers in the LC. Additionally, the results suggest that some Barrington's neurons diverge to innervate both the spinal cord and the LC. This divergent innervation may serve to coregulate the sacral parasympathetic nervous system and brain noradrenergic system, thus providing a mechanism for coordinating pelvic visceral functions with forebrain activity.

Corticotropin-releasing factor-containing axon terminals synapse onto catecholamine dendrites and may presynaptically modulate other afferents in the rostral pole of the nucleus locus coeruleus in the rat brain

Physiological and immunohistochemical studies have suggested that corticotropin-releasing factor (CRF), the hypophysiotropic peptide that initiates endocrine responses to stress, may serve as a neurotransmitter to activate noradrenergic neurons in the nucleus locus coeruleus (LC). We combined immunoperoxidase labeling for CRF and immunogold-silver localization of the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) in single sections through the rat LC to determine potential substrates for interactions between these two transmitters. Light microscopic analysis indicated that CRF processes are dense and highly varicose in the rostral LC region in the vicinity of noradrenergic dendrites. Electron microscopy of this rostral region revealed that immunoperoxidase labeling for CRF was mainly restricted to axons and axon terminals and was rarely seen in somata or dendrites. Axon terminals containing CRF immunoreactivity varied in size, content of synaptic vesicles, and formation of synaptic specializations. The postsynaptic targets of the CRF-labeled axon terminals consisted of both TH-labeled dendrites and dendrites lacking detectable TH-immunoreactivity. Of 113 CRF-immunoreactive axon terminals, approximately 70% were in direct contact with TH-labeled and unlabeled dendrites. Of the CRF-labeled axon terminals forming synapses with TH-labeled and unlabeled dendrites, they were either of the asymmetric (excitatory type; 19%) or symmetric (inhibitory type; 11%) variety or did not form identifiable contacts in the plane of section analyzed. Unlabeled axon terminals and glial processes were also commonly located adjacent to the plasma membranes of CRF-labeled axon terminals. These results provide the first direct ultrastructural evidence that axon terminals containing CRF-immunoreactivity 1) directly contact catecholamine-containing dendrites within the rostral pole of the LC, 2) may presynaptically modulate other afferents, and 3) are often enveloped by astrocytic processes.

Evidence for corticotropin-releasing hormone projections from Barrington's nucleus to the periaqueductal gray and dorsal motor nucleus of the vagus in the rat

The present study used anterograde and retrograde tract tracing and immunohistochemistry to determine the efferent projections of corticotropin-releasing hormone (CRH) neurons of Barrington's nucleus in the rat. Injections of Phaseolus vulgaris-leucoagglutinin into Barrington's nucleus resulted in anterograde labeling in the dorsal motor nucleus of the vagus, periaqueductal gray, medial thalamic nuclei, lateral hypothalamus, paraventricular nucleus of the hypothalamus, lateral preoptic area, and lateral septum. The retrograde tract tracer, fluorogold, injected into the lumbosacral spinal cord labeled many, but not all, CRH-immunoreactive neurons in Barrington's nucleus. Moreover, some Barrington's neurons that were retrogradely labeled from the spinal cord were not CRH-immunoreactive. Several CRH-immunoreactive Barrington's neurons were retrogradely labeled by fluorogold injections into the periaqueductal gray, and these were located predominantly in the dorsal part of the nucleus. Additionally, some CRH-immunoreactive Barrington's neurons were retrogradely labeled from fluorogold injections into the dorsal motor nucleus of the vagus. In contrast, fluorogold injections into the lateral hypothalamus, lateral preoptic area, or lateral septum did not result in double labeling of CRH-immunoreactive neurons in Barrington's nucleus. These results suggest that many, but not all, CRH-containing neurons of Barrington's nucleus project to the lumbosacral spinal cord. In addition to their previously documented projections to the spinal cord, these neurons may be a source of CRH in the periaqueductal gray and dorsal motor nucleus of the vagus. CRH projections of Barrington's nucleus may play a role in behavioral or autonomic aspects of stress responses, in addition to their proposed role in micturition.

Central regulation of micturition in the rat the corticotropin-releasing hormone from Barrington's nucleus

Barrington's nucleus, a pontine nucleus implicated in micturition, contains numerous corticotropin-releasing hormone (CRH) neurons that project to the spinal parasympathetic nucleus that innervates the bladder. We now report that CRH from Barrington's nucleus may serve to inhibit micturition. Selective chemical activation of Barrington's nucleus by microinjection of glutamate evoked bladder contractions that were increased in magnitude after intrathecal administration of a CRH antagonist, D-PheCRH12-41. In contrast, intrathecally administered CRH decreased the magnitude of Barrington's stimulated bladder contractions. These results suggest that activation of Barrington's nucleus releases an excitatory neurotransmitter responsible for bladder contractions, and CRH, which inhibits this neurotransmitter. The balance between these two neuromediators may regulate bladder contractility, and thereby, urinary continence.

Previous stress alters corticotropin-releasing factor neurotransmission in the locus coeruleus

Spontaneous and stress-evoked discharge of locus coeruleus neurons were characterized in rats with a history of stress. Rats exposed to one or five daily 30-min sessions of footshock were anesthetized with halothane and surgically prepared for locus coeruleus single-unit recording immediately following the last session. Locus coeruleus spontaneous discharge rate and discharge evoked by sciatic nerve stimulation were comparable between acutely and repeatedly stressed rats and controls. In contrast, locus coeruleus activation produced by intracerebroventricular administration of corticotropin-releasing factor (3 micrograms) or by hypotensive challenge (which requires endogenous corticotropin-releasing factor release in the locus coeruleus) was greatly attenuated in acutely stressed rats. The corticotropin-releasing factor dose-response curve was shifted to the right in acutely stressed rats compared with controls. In repeatedly stressed rats, the effects of 3 micrograms corticotropin-releasing factor on locus coeruleus discharge were similarly diminished. Although the maximum effect produced by corticotropin-releasing factor was decreased in these rats, the dose-response curve was shifted to the left, indicative of sensitization. Hypotensive challenge, which was ineffective in acutely stressed rats, increased locus coeruleus discharge of repeatedly stressed rats by a similar magnitude as in matched controls. The return of locus coeruleus responsiveness to hypotension in repeatedly stressed rats may be related to the sensitization to corticotropin-releasing factor. Finally, the protocol of repeated stress did not alter the affinity or density of corticotropin-releasing factor receptors in either the frontal cortex or brainstem. Taken together, the results suggest that a history of stress alters corticotropin-releasing factor neurotransmission in the locus coeruleus at the postsynaptic level. However, these effects are not reflected by corticotropin-releasing factor binding kinetics in brainstem. Stress-induced changes in corticotropin-releasing factor neurotransmitter function in the locus coeruleus may play a role in certain symptoms of stress-related psychiatric disorders.

Evidence for widespread afferents to Barrington's nucleus, a brainstem region rich in corticotropin-releasing hormone neurons

Supraspinal afferents to the pontine micturition center, Barrington's nucleus, were investigated in the rat by visualization of the retrograde tracer, cholera-toxin subunit B, in neurons following iontophoretic injection into Barrington's nucleus. Tissue sections from five rats with injections primarily localized in Barrington's nucleus revealed numerous retrogradely labeled neurons throughout all rostrocaudal levels of the periaqueductal gray (particularly its ventrolateral division), in the lateral hypothalamic area (particularly medial to the fornix), and in the medial preoptic nucleus. Retrogradely labeled neurons were also consistently found in the nucleus of the solitary tract, in the vicinity of the lateral reticular nucleus, nucleus paragigantocellularis, parabrachial nucleus, Kölliker-Fuse nucleus, cuneiform nucleus, raphe nucleus and zona incerta. In the hypothalamus, in addition to the perifornical region, retrogradely labeled neurons were found in all cases in the tuberomammillary nucleus, premammillary nucleus, dorsal hypothalamic area, ventromedial hypothalamic nucleus, and the paraventricular nucleus. At more rostral levels, in addition to the medial preoptic area, retrogradely labeled neurons were seen in the bed nucleus of the stria terminalis and in a region just lateral to the supraoptic nucleus near the medial amygdaloid nucleus. Retrogradely labeled neurons were also observed in the motor, insular, and infralimbic cortices. Injections of anterograde tracers (cholera-toxin subunit B or Phaseolus vulgaris leucoagglutinin) into the Kölliker-Fuse nucleus, the ventrolateral periaqueductal gray, lateral hypothalamic area, or medial preoptic area, resulted in fiber labeling within Barrington's nucleus, confirming the retrograde tracing studies. As previously reported, numerous neurons in Barrington's nucleus were immunoreactive for corticotropin-releasing hormone. Double-labeling studies revealed afferent fibers from the periaqueductal gray and lateral hypothalamic area overlapping the corticotropin-releasing hormone-immunoreactive neurons of Barrington's nucleus, and in some cases anterogradely labeled fibers with varcosities appeared to target these neurons. The present results suggest that Barrington's nucleus in the rat receives neuronal inputs from brainstem nuclei as well as from forebrain limbic structures including hypothalamic nuclei, the medial preoptic nucleus, and cortical areas involved in fluid balance or blood pressure regulation. In light of the role of Barrington's nucleus in micturition, the integration of these various inputs may be important for co-ordinating urinary function with fluid and cardiovascular homeostasis. Additionally, as neurons in Barrington's nucleus are immunoreactive for the stress-related neurohormone, corticotropin-releasing hormone, these diverse inputs may regulate stress-related functions of this nucleus.

Corticotropin-releasing factor neurotransmission in locus coeruleus: a possible site of antidepressant action

Hypersecretion of corticotropin-releasing factor (CRF), has been hypothesized to occur in depression. Because CRF may serve as a neurotransmitter in the locus coeruleus (LC), it was proposed that CRF hypersecretion in the LC is responsible for some characteristics of depression, and that antidepressants act by interfering with CRF neurotransmission in the LC. To test this hypothesis, the acute and chronic effects of four antidepressants and cocaine were characterized on LC spontaneous and sensory-evoked discharge, LC activation by a stressor that requires CRF release, and LC activation by exogenously administered CRF. None of the antidepressants or cocaine altered LC activation by intracerebroventricularly administered CRF (3.0 microgram) after chronic administration. However, chronic administration of desmethylimipramine and mianserin inhibited LC activation by a hypotensive stress that requires endogenous CRF release, suggesting that they decrease CRF release in the LC. Chronic administration of sertraline and phenelzine altered LC responses to repeated sciatic nerve stimulation in a manner opposite to the effect produced by CRF, suggesting that these drugs may functionally antagonize CRF actions in the LC. Cocaine did not appear to interfere with CRF actions in the LC. In conclusion, chronic administration of antidepressants may have the potential to interfere with CRF neurotransmission in the LC.

Pharmacological comparison of two corticotropin-releasing factor antagonists: in vivo and in vitro studies

The present study compared the effects of two analogs of corticotropin-releasing factor (CRF), [D-Phe12,Nle21,38, C alpha MeLeu37]CRF12-41 (D-PheCRF12-41) and alpha helical CRF9-41, as antagonists of CRF in in vivo and in vitro assays. In halothane-anesthetized rats, intracerebroventricular (i.c.v.) administration of both analogs inhibited the activation of locus coeruleus (LC) neuronal discharge produced by CRF (3.0 micrograms, i.c.v.). LC activation by hypotensive stress elicited by intravenous (i.v.) infusion of nitroprusside was antagonized by the same doses of the CRF antagonists that were effective in antagonizing CRF, suggesting that the receptors involved in LC activation by CRF and by hypotensive stress are similar. However, D-PheCRF12-41 was approximately 100 times more potent than alpha helical CRF9-41 when administered i.c.v. The IC50 values for D-PheCRF12-41 as an antagonist of CRF and of nitroprusside were 0.16 and 0.14 microgram, i.c.v., respectively. The IC50 values for alpha helical CRF9-41 as an antagonist of CRF and of nitroprusside were 18 and 27 micrograms, i.c.v., respectively. In contrast, D-PheCRF12-41 was only slightly more potent than alpha helical CRF9-41 in antagonizing CRF-stimulated cyclic AMP production in rat brain homogenates, with IC50s of 78 +/- 15 and 260 +/- 30 nM for D-PheCRF12-41 and alpha helical CRF9-41, respectively. Moreover, the antagonists had similar affinities for CRF binding sites in rat brain homogenates, with Kis of 15.5 +/- 4 nM and 10.3 +/- 6 nM for D-PheCRF12-41 and alpha helical CRF9-41, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Locus coeruleus activation by physiological challenges

Studies were designed to elucidate the neurotransmitter(s) and circuitry involved in activation of noradrenergic locus coeruleus (LC) neurons by different physiological challenges in halothane-anesthetized rats, and to understand the functional consequences of LC activation by these stimuli. LC spontaneous discharge rate was increased by a hypotensive challenge and by bladder distention. The effect produced by hypotension, but not by bladder distention, was prevented by antagonists of the stress-related neurohormone, corticotropin-releasing factor (CRF), administered ICV or directly into the LC. In contrast, ICV administration of excitatory amino acid antagonists prevented LC activation by bladder distention, but not by hypotension. These results suggest that LC activation by hypotension and bladder distention requires separate neurotransmitter systems, with CRF mediating activation by hypotension and excitatory amino acids mediating activation by bladder distention. Both physiological challenges activated forebrain electroencephalographic (EEG) activity, as indicated by a shift from low frequency, high amplitude activity to high frequency, low amplitude activity recorded from the frontal cortex. The EEG effects appeared to be temporally correlated with LC activation. Bilateral LC inactivation or microinfusion of CRF antagonists into the LC prevented both LC and EEG activation by hypotension. These results suggest that one consequence of LC activation during stress or physiological challenges may be to increase or maintain arousal.

Corticotropin-releasing factor in the locus coeruleus mediates EEG activation associated with hypotensive stress

Although corticotropin-releasing factor (CRF) is thought to act as a neurotransmitter to activate the locus coeruleus (LC) during hypotensive stress, the consequences of LC activation by CRF are unknown. In the present study a hypotensive challenge that activated rat LC neurons also produced cortical electroencephalographic (EEG) correlates of arousal. Selective, bilateral LC inactivation by local clonidine infusion prevented EEG activation associated with hypotension. Additionally, bilateral LC infusion of CRF antagonists prevented both LC and EEG activation by this challenge. These results indicate that CRF, acting as a neurotransmitter to activate LC during stress, has a powerful of modulatory influence over global forebrain electrophysiological activity.

The locus coeruleus as a site for integrating corticotropin-releasing factor and noradrenergic mediation of stress responses

Anatomic and electrophysiologic studies have provided evidence that CRF meets some of the criteria as a neurotransmitter in the noradrenergic nucleus, the locus coeruleus (LC), although some of the criteria have yet to be satisfied. Thus, immunohistochemical findings suggest that CRF innervates the LC, but this must be confirmed at the ultrastructural level. CRF alters discharge activity of LC neurons and these effects are mimicked by some stressors. Moreover, the effects of hemodynamic stress on LC activity are prevented by a CRF antagonist. However, it has not been demonstrated that stimulation of CRF neurons that project to the LC activates the LC or that the effects of such stimulation are prevented by a CRF antagonist. The role of CRF in LC activation by stressors other than hemodynamic stress needs to be determined. It could be predicted that the effects of CRF neurotransmission in the LC during stress would enhance information processing concerning the stressor or stimuli related to the stressor by LC target neurons. One consequence of this appears to be increased arousal. Although this may be adaptive in the response to an acute challenge, it could be predicted that chronic CRF release in the LC would result in persistently elevated LC discharge and norepinephrine release in targets. This could be associated with hyperarousal and loss of selective attention as occurs in certain psychiatric diseases. Manipulation of endogenous CRF systems may be a novel way in which to treat psychiatric diseases characterized by these maladaptive effects.

Effects of locus coeruleus inactivation on electroencephalographic activity in neocortex and hippocampus

The effects of inhibition of locus coeruleus neuronal discharge activity on cortical and hippocampal electroencephalographic activity were examined in halothane-anesthetized rats. A combined recording/infusion probe was used to place 35-150-nl infusions of the alpha 2-noradrenergic agonist, clonidine (1 ng/nl) which inhibits locus coeruleus neuronal discharge activity, immediately adjacent to the locus coeruleus. The recording electrode allowed verification and quantification of the electrophysiological effects of these infusions. Simultaneously, electroencephalographic activity was recorded from sites in frontal neocortex and dorsal hippocampus and subjected to power spectrum analyses. Neither cortical nor hippocampal electroencephalographic activity was substantially affected following unilateral locus coeruleus inactivation. In contrast, bilateral clonidine infusions that completely suppressed locus coeruleus neuronal discharge activity in both hemispheres altered cortical and hippocampal electroencephalographic status. The cortical response to bilateral LC inhibition was characterized by a shift from low-amplitude, high-frequency to large-amplitude, slow-wave activity. Additionally, theta-dominated activity in the hippocampus was replaced with mixed frequency activity. The onset of these changes in forebrain electroencephalographic activity was coincident with the complete bilateral inhibition of locus coeruleus neuronal discharge activity. The resumption of pre-infusion electroencephalographic patterns closely followed recovery of locus coeruleus neuronal activity or could be induced with systemic administration of the alpha 2-noradrenergic antagonist, idazoxan. Clonidine infusions placed 800-1200 microns from the locus coeruleus were less effective at inducing a complete suppression of locus coeruleus activity. These infusions either did not completely inhibit locus coeruleus discharge (35 nl infusions), or did so with a longer latency to complete locus coeruleus inhibition and a shorter duration of inhibition (150 nl infusions). Changes in forebrain electroencephalographic activity occurred only following the complete bilateral suppression of locus coeruleus neuronal discharge activity. These electroencephalographic responses closely followed or coincided with the onset of complete bilateral locus coeruleus inhibition and persisted throughout the period during which bilateral LC neuronal discharge activity was completely absent (60-240 min). Recovery of electroencephalographic patterns was coincident with the reappearance of locus coeruleus discharge activity. These results suggest that the clonidine-induced changes in forebrain electroencephalographic activity were dependent on the complete bilateral suppression of locus coeruleus discharge activity, and that under the present experimental conditions the locus coeruleus/noradrenergic system exerts a potent and tonic activating influence on forebrain electroencephalographic state. These results support the hypothesis that this system may be an important modulator of behavioral state and/or state-dependent processes.

Cocaine effects on brain noradrenergic neurons of anesthetized and unanesthetized rats

The present study characterized and quantified the effects of systemically administered cocaine on spontaneous, sensory-evoked and stress-elicited activity of noradrenergic locus coeruleus (LC) neurons of anesthetized and unanesthetized rats. Cocaine (0.1-3.0 mg/kg, i.v.) decreased LC spontaneous discharge rate and discharge evoked by repeated sciatic nerve stimulation in halothane-anesthetized rats. In unanesthetized rats cocaine (0.3-10.0 mg/kg, i.v.) also decreased LC spontaneous discharge rate and LC discharge evoked by repeated auditory stimulation. However, analysis of variance revealed a statistically significant shift to the right in the cocaine dose-response curves for effects on tonic and evoked LC discharge in unanesthetized compared to anesthetized rats. Thus, cocaine was somewhat less potent in inhibiting tonic and evoked discharge of unanesthetized rats compared to anesthetized rats. In anesthetized rats cocaine (1.0 mg/kg) did not affect LC activation by intracerebroventricularly (i.c.v.) administered corticotropin-releasing factor (3.0 micrograms in 3.0 microliters) or by hemodynamic stress elicited by i.v. nitroprusside infusion. The present findings demonstrate that cocaine has similar effects on LC neurons of anesthetized and unanesthetized rats but that it is less potent in unanesthetized rats. These effects of cocaine at noradrenergic cell bodies acting in concert with its effects at noradrenergic terminals in LC target regions may be important in the overall action of cocaine on arousal and cortical information processing.

Hemodynamic stress activates locus coeruleus neurons of unanesthetized rats

The effects of hypotensive stress elicited by nitroprusside infusion on discharge activity of noradrenergic locus coeruleus (LC) neurons of unanesthetized rats were characterized. Nitroprusside (75 micrograms/30 microliters/min, 15 min IV infusion) decreased mean arterial pressure of unanesthetized rats by 50 +/- 2 mmHg (n = 5). Simultaneous recordings of LC spontaneous discharge revealed an increase in discharge rate (197 +/- 87%) that was associated with hypotension. A lower concentration of nitroprusside (10 micrograms/30 microliters/min) that decreased blood pressure of halothane-anesthetized rats by 55 +/- 2 mmHg was much less effective in producing hypotension and did not increase LC discharge when administered to unanesthetized rats. Prior administration of the CRF antagonist, alpha helical CRF9-41 (50 micrograms, ICV) greatly attenuated LC activation by nitroprusside. These findings demonstrate that LC activation elicited by nitroprusside is dependent on the magnitude of hypotension. The present results also demonstrate that nitroprusside is a less potent hemodynamic challenge in unanesthetized rats. Finally, LC activation associated with nitroprusside administration to unanesthetized rats is mediated to a large extent by CRF, confirming findings in anesthetized rats.

Bladder distention activates noradrenergic locus coeruleus neurons by an excitatory amino acid mechanism

The present study was designed to determine the neurotransmitter(s) involved in activation of noradrenergic locus coeruleus neurons by urinary bladder distention. The spontaneous discharge rate of single locus coeruleus neurons was recorded from halothane-anesthetized rats during the physiological challenge of bladder distention. Intrabladder saline infusion (0.5 ml) increased bladder pressure by 77 +/- 9.7 mmHg (n = 19) and this was associated with an increase in locus coeruleus discharge rate of 53 +/- 4.8% (n = 29). Simultaneous recordings of cortical electroencephalographic activity demonstrated that electroencephalographic activation, characterized by a decreased amplitude and tendency to shift from low frequency activity to higher frequency activity, was also associated with bladder distention. The role of corticotropin-releasing factor and excitatory amino acid inputs to the locus coeruleus in activation by bladder distention was tested in rats pretreated with a corticotropin-releasing factor antagonist, or excitatory amino acid antagonists. Intracerebroventricular administration of the corticotropin-releasing factor antagonist did not alter locus coeruleus activation by bladder distention. In contrast, both locus coeruleus activation and electroencephalographic activation associated with bladder distention were prevented by intracerebroventricular administration of kynurenic acid. The same dose of kynurenic acid also prevented locus coeruleus activation by repeated sciatic nerve stimulation, as previously reported. Local administration of kynurenic acid into the locus coeruleus greatly attenuated, but did not completely prevent the increase in locus coeruleus discharge elicited by bladder distention.(ABSTRACT TRUNCATED AT 250 WORDS)

Common mechanisms underlying the proconflict effects of corticotropin-releasing factor, a benzodiazepine inverse agonist and electric foot-shock

The effects of corticotropin-releasing factor (CRF), a benzodiazepine inverse agonist (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; DMCM) and electric foot-shock on rat conflict behavior were characterized and compared. Rats were trained to lever press under a multiple fixed-ratio schedule (FR 20) of food reinforcement in which responses during the first component were not punished, and the first response of each FR during the second component produced electric shock of an intensity sufficient to suppress responding by 10% to 15%. Intracerebroventricular injection of CRF (0.1-5.6 micrograms) caused a dose-dependent decrease in the rate of responding in both components of the schedule. However, CRF was more potent in decreasing rates of punished responding (proconflict effect). DMCM (10-100 micrograms; i.c.v.) also decreased rates of punished and nonpunished responding and was more potent during the punishment component. The suppression of punished and nonpunished responding by CRF and DMCM was mimicked by increasing the shock intensity (delta = 0.1 to 0.6 mA) during the punishment component. To determine whether CRF, DMCM and electric shock shared common mechanisms for these effects, rats were pretreated with i.c.v. injections of either a CRF antagonist (alpha helical CRF9-41, 50 micrograms), a benzodiazepine agonist (chlordiazepoxide, 10 micrograms) or a benzodiazepine antagonist (flumazenil, 10 micrograms) before the administration of equieffective doses of CRF or DMCM or an increase in shock intensity. Chlordiazepoxide attenuated the effects of all three stimuli. Flumazenil antagonized DMCM and CRF, but not shock, implicating a pharmacologic interaction between CRF and benzodiazepine systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Corticotropin-releasing factor innervation of the locus coeruleus region: distribution of fibers and sources of input

Electrophysiologic studies support the hypothesis that corticotropin-releasing factor, the neurohormone that initiates adrenocorticotropin release during stress, also serves as a neurotransmitter in the pontine noradrenergic nucleus, the locus coeruleus. To elucidate the circuitry underlying proposed corticotropin-releasing factor neurotransmission in the locus coeruleus, the present study utilized immunohistochemical techniques to characterize corticotropin-releasing factor innervation of rat locus coeruleus and pericoerulear regions. Corticotropin-releasing factor-like immunoreactive fibers were identified in the locus coeruleus of colchicine- and non-colchicine-treated rats. However, corticotropin-releasing factor innervation of pericoerulear regions rostral and lateral to the locus coeruleus was more dense than that of the locus coeruleus proper. Double-labeling studies utilizing antisera directed against corticotropin-releasing factor and tyrosine hydroxylase indicated that corticotropin-releasing factor-like immunoreactive fibers overlap with tyrosine hydroxylase-like immunoreactive processes of locus coeruleus neurons, particularly in rostral medial and lateral regions. A group of corticotropin-releasing factor-like immunoreactive neurons was localized just lateral to the locus coeruleus and numerous corticotropin-releasing factor-like immunoreactive neurons were visualized just ventral to the rostral pole of the locus coeruleus in a region corresponding to Barrington's nucleus. None of these corticotropin-releasing factor-like immunoreactive neurons were tyrosine hydroxylase-positive. To determine the source of corticotropin-releasing factor-like immunoreactive fibers in the locus coeruleus, injections of the retrograde tracer [wheat germ agglutinin conjugated to inactivated (apo) horseradish peroxidase coupled to gold particles] were made into the locus coeruleus and sections were processed for corticotropin-releasing factor-like immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of noradrenergic locus coeruleus neurons by hemodynamic stress is due to local release of corticotropin-releasing factor

The present study was designed to determine whether activation of locus coeruleus (LC) neurons by hemodynamic stress is mediated by local release of corticotropin-releasing factor (CRF) within the LC. The ability of local LC injection of the CRF antagonist, alpha helical CRF9-41, to prevent LC activation elicited by i.v. nitroprusside infusion was investigated in halothane-anesthetized rats. Nitroprusside infusion (10 micrograms/30 microliters/min for 15 min) consistently increased LC spontaneous discharge rate with the mean maximum increase of 32 +/- 5% (n = 8) occurring between 3 and 9 min after the initiation of the infusion. Prior local LC injection of alpha helical CRF9-41 (150 ng), but not of saline (150 nl), prevented LC activation by nitroprusside. Alpha helical CRF9-41 did not alter LC spontaneous discharge rate or LC discharge evoked by repeated sciatic nerve stimulation suggesting that the CRF antagonist selectively attenuates stress-elicited LC activation. In contrast to alpha helical CRF9-41, the excitatory amino acid antagonist, kynurenic acid, did not attenuated LC activation by nitroprusside at a dose (0.5 mumol in 5 microliters, i.c.v.) that prevented LC activation by sciatic nerve stimulation. Taken together, these findings suggest that hemodynamic stress elicited by nitroprusside infusion activates LC neurons by releasing CRF within the LC region. The onset of LC activation by nitroprusside was temporally correlated with electroencephalographic (EEG) activation recorded from the frontal cortex and hippocampus. EEG activation was characterized by a change from low frequency, high amplitude activity to high frequency low amplitude activity recorded from the cortex and theta rhythm recorded from the hippocampus. LC activation usually outlasted the EEG activation. Nitroprusside infusion following local LC injection of alpha helical CRF9-41 was also associated with EEG activation in most rats. However, the duration of hippocampal theta rhythm was shorter in rats administered alpha helical CRF9-41. Thus, LC activation during cardiovascular challenge may play some role in EEG activation but is not necessary for this effect.

Pharmacology of locus coeruleus spontaneous and sensory-evoked activity

Neuroendocrine and catecholamine dysfunctions in depression may be linked by corticotropin-releasing factor (CRF) effects on locus coeruleus (LC) neurons. One consequence of CRF hypersecretion in depression would be persistent elevated levels of LC discharge and diminished responses to phasic sensory stimuli. The hypothesis that antidepressants could reverse these changes was tested by characterizing effects of pharmacologically distinct antidepressants on LC sensory-evoked discharge, LC activation by stress, and LC activation by CRF. The most consistent effect of all of the antidepressants tested was a decrease in LC sensory-evoked discharge after acute administration. However, tolerance occurs to these effects after chronic administration. With chronic administration each of the antidepressants produced effects which could potentially interfere with CRF function in the LC. Desmethylimipramine and mianserin attenuated LC activation by a stressor which requires endogenous CRF, suggesting that these antidepressants attenuate stress-elicited release of CRF and perhaps the hypersecretion that occurs in depression. The serotonin reuptake inhibitor, sertraline (SER), enhanced the signal-to-noise ratio of the LC sensory response, an effect opposite to that of CRF. Thus, SER could serve as a functional antagonist of CRF that is hypersecreted in depression. The finding that three pharmacologically distinct antidepressants share the potential to interfere with CRF function in the LC implies that this may be an important common mechanism for antidepressant activity.

Acute and chronic effects of the atypical antidepressant, mianserin on brain noradrenergic neurons

Corticotropin-releasing factor (CRF), which may serve as a neurotransmitter in the noradrenergic nucleus, locus coeruleus (LC), has been postulated to be hypersecreted in depression. The present study was designed to test the hypothesis that antidepressants interfere with CRF putative neurotransmission in the LC. The acute and chronic effects of the atypical antidepressant mianserin on LC spontaneous discharge, LC sensory-evoked discharge, LC activation by a stressor which requires endogenous CRF, and LC activation by ICV CRF were characterized in halothane-anesthetized rats. Acute IV administration of mianserin (0.0001-1.0 mg/kg) increased LC spontaneous discharge and decreased LC discharge evoked by repeated sciatic nerve stimulation in a dose-dependent manner. Additionally, mianserin (0.1 mg/kg) inhibited LC activation by hemodynamic stress (IV infusion of nitroprusside) and by ICV administration of CRF (3.0 micrograms). In rats chronically administered mianserin LC spontaneous and sensory-evoked discharge rates, and LC activation by CRF were similar to those of untreated rats or rats chronically administered saline. Moreover, acute IV administration of mianserin (0.1 mg/kg) to rats chronically treated with mianserin was less effective in altering LC spontaneous and sensory-evoked discharge. In contrast, LC activation by hemodynamic stress was still greatly attenuated in rats chronically administered mianserin. This is similar to the previously reported effect produced by chronic administration of the antidepressant, desmethylimipramine. The present results demonstrate that acute administration of low doses of mianserin attenuates LC activation by a variety of stimuli and suggest that tolerance develops with chronic administration to some of the effects of mianserin on LC discharge characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)