Chronic desipramine treatment increases activity of noradrenergic postsynaptic cells

Chronic administration of tricyclic antidepressant drugs has been shown to exert multiple influences on various mechanisms of noradrenergic nervous systems. To determine the overall effect of these influences, this study examined the effect of long-term desipramine administration on the firing rate of noradrenergic postsynaptic neurons, specifically, those in the rat hippocampus that were inhibited by the nucleus locus coeruleus. Daily injections for 3 weeks of 5 or 10 mg/kg desipramine resulted in a 32% or 49% increase, respectively, of hippocampal cell activity, suggesting that long-term desipramine treatment is antagonistic to noradrenergic functions.     

Fluorescence histochemical study on the noradrenergic control to the anterior column of the spinal lumbosacral segments of the rat and dog,with special reference to motoneurons innervating the perineal striated muscles (Onuf's nucleus)

Summary The organization of noradrenergic fibers in the lumbosacral anterior column of rats and dogs was examined in detail using a modification of a highly sensitive glyoxylic acid fluorescence histochemical method. In both rat and dog, there were greater concentrations of fluorescing noradrenergic fibers around the motoneurons innervating the perineal striated muscles (Onuf's nucleus) than around other motoneuronal groups. The preferential accumulation of noradrenergic fibers in Onuf's nucleus may indicate that the noradrenergic neuron system in the spinal cord of rodents and carnivores is closely related to the functional peculiarities of the perineal striated muscles, including the external anal and urethral sphincter muscles.     

Fluorescence histochemical study on the noradrenergic control to the anterior column of the spinal lumbosacral segments of the rat and dog, with special reference to motoneurons innervating the perineal striated muscles (Onuf's nucleus)

The organization of noradrenergic fibers in the lumbosacral anterior column of rats and dogs was examined in detail using a modification of a highly sensitive glyoxylic acid fluorescence histochemical method. In both rat and dog, there were greater concentrations of fluorescing noradrenergic fibers around the motoneurons innervating the perineal striated muscles (Onuf's nucleus) than around other motoneuronal groups. The preferential accumulation of noradrenergic fibers in Onuf's nucleus may indicate that the noradrenergic neuron system in the spinal cord of rodents and carnivores is closely related to the functional peculiarities of the perineal striated muscles, including the external anal and urethral sphincter muscles.     

Central noradrenergic activity in intact and sinoaortic denervated Dahl rats

Lesions in forebrain areas richly innervated by noradrenergic terminals and involved in cardiovascular function reduce or prevent hypertension in the Dahl salt-sensitive (S) rats fed a high (H) salt diet. This led us to examine two questions. (1) Is the noradrenergic activity altered in discrete forebrain and brainstem areas of SH rats? (2) Are these changes in noradrenergic activity eliminated by sinoaortic denervation (SAD)? Studies were done in 10-week-old female SH and Dahl salt-resistant (RH) rats. Half of the rats in each group had SAD surgery 1 week prior to study. An index of norepinephrine (NE) turnover was determined by measuring the decline in tissue NE concentration 8 h after administering alpha-methyl-p-tyrosine, a NE synthesis blocker, to animals from each of four groups: sham-RH, SAD-RH, sham-SH, and SAD-SH (n = 18-20 per group). Various discrete brain areas were obtained using the "punch technique." In SH rats the index of NE turnover was increased in the median preoptic nucleus and decreased in the paraventricular nucleus compared with RH rats regardless of SAD. In contrast, in SH rats the index of NE turnover was increased in the supraoptic nucleus and locus ceruleus compared with RH rats; however, SAD-RH had greater turnover of NE at these sites than SAD-SH. In summary, changes in noradrenergic activity in the median preoptic nucleus and the paraventricular nucleus may be related to genetic predisposition to hypertension in SH rats. In contrast, changes in the locus ceruleus and the supraoptic nucleus of SH rats may be related to impaired baroreflexes and thereby contribute to hypertension.     

Modulation of the hippocampal alpha-adrenoceptor population by lesion of the serotonergic raphe-hippocampal pathway in rats

Electrolytic lesion of the ascending serotonergic fibers in the median raphe nucleus or in both the median raphe nucleus and dorsal raphe nucleus caused after 18 days more than 80% depletion of serotonin in the hippocampus and frontal cortex, respectively, without affecting norepinephrine and acetylcholine contents. alpha 1-Adrenoceptor binding of (3H) WB-4104 was increased in the hippocampus but not in the frontal cortex. Scatchard analysis revealed that the increase in (3H) WB-4101 binding in the lesioned hippocampus was the result of an elevated density of alpha 1-adrenergic receptors of about 65%. This phenomenon began 8 days postlesion and persisted for at least 90 days postlesion. Similar qualitative and quantitative results were obtained following chemical lesion of the serotonergic cells of origin in the median raphe nucleus with 5,7-dihydroxytryptamine. Selectivity of the phenomenon was further demonstrated as or beta-adrenoceptor binding with (3H) dihydroalprenolol and cholinergic muscarinic receptor binding with (3H) dexetimide were not significantly affected in the hippocampus. By comparison, when norepinephrine in the hippocampus was depleted by more than 90% by bilateral lesion of the ascending noradrenergic fibers with 6-hydroxydopamine (18 days), the alpha 1-adrenoceptor number was significantly increased by only about 20% while the beta-adrenoceptor number was enhance by 40%. The area-selective increase in alpha 1-adrenoceptor number in the hippocampus in the presence of unchanged norepinephrine content and in the absence of serotonin probably signifies that serotonin actively participates in the modulation of the noradrenergic receptor population.     

Neuroadaptive responses in brainstem noradrenergic nucleic following chronic morphine exposure

Opiate dependence and withdrawal involve neuroadaptive responses in the central nervous system. A host of studies have previously implicated the A6 noradrenergic neurons of the pontine nucleus locus coeruleus (LC) as an important mediator of somatic signs observed upon withdrawal from opiates. Recent studies, however, are showing that noradrenergic neurons of the LC may not be solely involved in mediating somatic signs of withdrawal. The A2 noradrenergic neurons of the nucleus of the solitary tract (nucleus tractus solitarius [NTS]) in the caudal brainstem may be another possible site. Neurons in the nucleus paragigantocellularis lateralis (PGi), located in the rostral ventral medulla, which are known to send collateral projections to both the LC and the NTS, may co-modulate both noradrenergic nuclei in a parallel fashion, which may represent an anatomical substrate underlying the behavioral expression of opiate withdrawal. The PGi provides glutamatergic and opioid innervation to LC neurons. Hyperactivity of LC during opiate withdrawal arises, in part, from increased glutamate transmission in this pathway. The authors have recently shown that the excitatory transmitter, glutamate, co-exists with the endogenous opioid peptide, enkephalin, in a subset of axon terminals in the LC. Decreases in endogenous opioids in afferents to LC and NTS, following chronic opiate administration, may be equally important in modulating noradrenergic neurons following chronic opiate exposure, by removing a neurochemical system that would inhibit noradrenergic neurons. A persistent decrease in opioid peptide release from afferents during withdrawal would result in glutamate acting on postsynaptic targets, in an unopposed fashion. A parallel effect in opioid projections from PGi to the NTS would potentially support similar actions in this noradrenergic nucleus. The authors' recent data show that opioid-containing neurons in the PGi project to the NTS, and that enkephalin levels are decreased in opioid afferents to the NTS. This review summarizes data that the authors have collected regarding opioid expression changes in brainstem circuits (PGi-LC and PGi-NTS), following chronic morphine treatment, which may represent a model for understanding of adaptations in endogenous opioid circuits during drug dependence and withdrawal.     

Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

In obstructive sleep apnea patients, elevated activity of the lingual muscles during wakefulness protects the upper airway against occlusions. A possibly related form of respiratory neuroplasticity is present in rats exposed to acute and chronic intermittent hypoxia (CIH). Since rats exposed to CIH have increased density of noradrenergic terminals and increased α(1)-adrenoceptor immunoreactivity in the hypoglossal (XII) nucleus, we investigated whether these anatomic indexes of increased noradrenergic innervation translate to increased sensitivity of XII motoneurons to noradrenergic activation. Adult male Sprague-Dawley rats were subjected to CIH for 35 days, with O(2) level varying between 24% and 7% with 180-s period for 10 h/day. They were then anesthetized, vagotomized, paralyzed, and artificially ventilated. The dorsal medulla was exposed, and phenylephrine (2 mM, 10 nl) and then the α(1)-adrenoceptor antagonist prazosin (0.2 mM, 3 × 40 nl) were microinjected into the XII nucleus while XII nerve activity (XIIa) was recorded. The area under integrated XIIa was measured before and at different times after microinjections. The excitatory effect of phenylephrine on XII motoneurons was similar in sham- and CIH-treated rats. In contrast, spontaneous XIIa was more profoundly reduced following prazosin injections in CIH- than sham-treated rats [to 21 ± 7% (SE) vs. 40 ± 8% of baseline, P < 0.05] without significant changes in central respiratory rate, arterial blood pressure, or heart rate. Thus, consistent with increased neuroanatomic measures of noradrenergic innervation of XII motoneurons following exposure to CIH, prazosin injections revealed a stronger endogenous noradrenergic excitatory drive to XII motoneurons in CIH- than sham-treated anesthetized rats.     

Studies on the up regulation of alpha-adrenoceptors on rat hippocampal perikarya by chemical lesion of the median raphe nucleus

Neurotoxin-induced lesion of the serotonergic raphe-hippocampal pathway produced about a 50% increase in the density of a nM affinity alpha-adrenergic binding site for (3H)WB-4101 in rat hippocampus 18 days postlesion without altering the specific binding of (3H)5-HT to serotonergic receptors. The chronic i.c.v. infusion of serotonin by minipump started at the appropriate time averted or reverted the effect. The dynamics of noradrenergic neurotransmission in the hippocampus was not impaired by lesion of the median raphe nucleus as determined by the uptake and turnover of noradrenaline as well as its release - as reflected by the normetanephrine concentration. In addition, neurotoxin-induced lesion of the dorsal noradrenergic bundle failed to alter either the Bmax or the Kd of (3H)WB-4101 binding to the nM site. Kainic acid-induced destruction of perikarya depressed the nM (3H)WB-4101 binding sites by 60% and completely prevented the up regulation caused by lesion of the median raphe nucleus. Thus, the supersensitivity-like response of the adrenoceptors to the lack of serotonin appears to be localized on kainate-sensitive cells within the hippocampus.     

Noradrenergic modulation of hypoglossal motoneuron excitability: developmental and putative state-dependent mechanisms

Hypoglossal (XII) motoneurons (MNs) contribute to diverse behaviors. Their innervation of the genioglossus muscle, a tongue protruder, plays a critical role in maintaining upper airway patency during breathing. Indeed, reduced activity in these motoneurons is implicated in sleep related disorders of breathing such as obstructive sleep apnea (OSA). The excitability of these MNs is modulated by multiple neurotransmitter systems. The focus of this review is on the modulation of XII MN excitability by norepinephrine (NE), which increases MN excitability through a variety of mechanisms. The level of noradrenergic drive, however, is very dynamic, varying on developmental, sleep-wake and even millisecond timescales relevant to transitions between behaviours. Here we review and provide new data on the maturation of the noradrenergic modulatory system, focusing on those elements specifically relevant to XII MN excitability including the: i) ontogeny of the noradrenergic cell group that provides the majority of the noradrenergic innervation to the XII nucleus, the Locus subcoeruleus (LsC); ii) time course over which the XII nucleus is innervated by noradrenergic nerve fibres, and; iii) ontogeny of XII MN sensitivity to NE. In the context of state-dependent changes in noradrenergic cell activity, we review mechanisms of NE action most relevant to its role in the muscle atonia of REM sleep. We conclude with a discussion of the hypothesis that the dynamics of MN modulation by NE extend to the spatial domain and recent data suggesting that noradrenergic modulation of the dendritic tree is not uniform but compartmentalized. Implications for information processing are discussed.     

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.     

Interactions of norepinephrine and galanin in the central amygdala and lateral bed nucleus of the stria terminalis modulate the behavioral response to acute stress

Many aspects of drug abuse and addiction share neurobiological substrates with the modulatory processes underlying the response and adaptation to acute stress. In particular, the ascending noradrenergic system has been implicated in facilitating the response to stress, and in stress-induced reinstatement of drug seeking behavior. Thus, to better understand the link between stress and addictive behaviors, it would be informative to understand better the modulatory function of the ascending noradrenergic system, and its interaction with other neurotransmitters with which it is closely associated or co-localized, such as the neuropeptide galanin. In this paper, we review a series of studies investigating the functional interactions of norepinephrine and galanin in modulating the behavioral response to acute stress in two components of the extended amygdala, the central nucleus of the amygdala and the lateral bed nucleus of the stria terminalis. We showed that norepinephrine facilitates behavioral reactivity to stress on the elevated plus-maze and social interaction tests. However, when stress-induced activation of the noradrenergic system was enhanced by blocking inhibitory adrenergic autoreceptors, galanin release was recruited in the central amygdala, acting to attenuate the behavioral response to stress. By contrast, stress-induced galanin release in the lateral bed nucleus appeared to be independent of enhanced noradrenergic activation, and unlike the central amygdala, both galanin and norepinephrine facilitated behavioral stress reactivity in the bed nucleus. The different modes of interaction and differential region- and response-specificity of galanin and norepinephrine suggest that a complex neural circuit interconnecting these two regions is involved in the modulatory effects of norepinephrine and galanin on the behavioral response to stress. Such complexity may allow for flexibility and plasticity in stress adaptation, and may also contribute to behavioral changes induced by chronic drug administration. Thus, the interaction of galanin and norepinephrine may be a viable target for the future development of novel therapeutic strategies for treating behavioral disorders related to stress or drug abuse.     

Characterization of the autonomic innervation of mammary gland in lactating rats studied by retrograde transynaptic virus labeling and immunohistochemistry

The aim of experiments was to characterize the neurons of the autonomic chain that innervates the nipple and the mammary gland of lactating rats using retrograde transynaptic virus labeling and neurotransmitter and neuropeptide immunohistochemistry. Two days after injection of green fluorescence protein labeled virus in two nipples and underlying mammary glands, labeling was observed in the ipsilateral paravertebral sympathetic trunk and the lateral horn. Three days after inoculation the labeling appeared in the brain stem and the hypothalamic paraventricular nucleus. Above the spinal cord the labeling was bilateral. A subpopulation of virus labeled cells in the paraventricular nuclei synthesized oxytocin. Labeled neurons in the lateral horn showed cholinergic immunoreactivity. These cholinergic neurons innervated the paravertebral ganglia where the virus labeled neurons were partially noradrenergic. The noradrenergic fibers in the mammary gland innervate the smooth muscle wall of vessels, but not the mammary gland in rats. The neurons in the lateral horn receive afferents from the brain stem, and paraventricular nucleus and these afferents are noradrenergic and oxytocinergic. New findings in our work: Some oxytocinergic fibers may descend to the neurons of the lateral horn which innervate noradrenergic neurons in the paravertebral sympathetic trunk, and in turn these noradrenergic neurons reach the vessels of the mammary gland.     

Benzodiazepines attenuate single unit activity in the locus coeruleus

Several classes of anxiolytic compounds have the common effect of decreasing the firing of noradrenergic neurons or attenuating the post- synaptic effects of noradrenergic activity. In order to determine whether the benzodiazepines, the most widely used anxiolytics, also decrease noradrenergic activity, the effect of acute intravenous injections of diazepam (0.1–2.0 mg/kg) and chlordiazepoxide (0.5–4.0 mg/kg) were administered to anesthetized rats while spontaneous activity of single neurons in the principal noradrenergic nucleus, the locus coeruleus, was recorded. Diazepam and chlordiazepoxide decreased spontaneous single unit activity in the locus coeruleus at relatively low doses. This net effect on noradrenergic systems is consistent with the actions of several classes of nonbenzodiazepine anxiolytics, and with the involvement of noradrenergic systems in the neural mechanisms of anxiety.     

The Inhibition of the Dorsal Paragigantocellular Reticular Nucleus Induces Waking and the Activation of All Adrenergic and Noradrenergic Neurons: A Combined Pharmacological and Functional Neuroanatomical Study

GABAergic neurons specifically active during paradoxical sleep (PS) localized in the dorsal paragigantocellular reticular nucleus (DPGi) are known to be responsible for the cessation of activity of the noradrenergic neurons of the locus coeruleus during PS. In the present study, we therefore sought to determine the role of the DPGi in PS onset and maintenance and in the inhibition of the LC noradrenergic neurons during this state. The effect of the inactivation of DPGi neurons on the sleep-waking cycle was examined in rats by microinjection of muscimol, a GABA_A agonist, or clonidine, an alpha-2 adrenergic receptor agonist. Combining immunostaining of the different populations of wake-inducing neurons with that of c-FOS, we then determined whether muscimol inhibition of the DPGi specifically induces the activation of the noradrenergic neurons of the LC. Slow wave sleep and PS were abolished during 3 and 5 h after muscimol injection in the DPGi, respectively. The application of clonidine in the DPGi specifically induced a significant decrease in PS quantities and delayed PS appearance compared to NaCl. We further surprisingly found out that more than 75% of the noradrenergic and adrenergic neurons of all adrenergic and noradrenergic cell groups are activated after muscimol treatment in contrast to the other wake active systems significantly less activated. These results suggest that, in addition to its already know inhibition of LC noradrenergic neurons during PS, the DPGi might inhibit the activity of noradrenergic and adrenergic neurons from all groups during PS, but also to a minor extent during SWS and waking.     

Net effect of acute administration of desipramine on the locus coeruleus - hippocampal system.

The tricyclic antidepressant drug desipramine (DMI) produces multiple effects on noradrenergic nervous systems. This study attempted to determine the net outcome of these effects by evaluating the firing rate of noradrenergic postsynaptic neurons. Hippocampal pyramidal cells inhibited by stimulation of the nucleus locus coeruleus were used as noradrenergic postsynaptic neurons. An intraperitoneal injection of DMI (5 or 10 mg/kg) inhibited 14 of 23 cells studied and an intravenous injection (0.3 or 0.6 mg/kg) supressed 16 of 16 cells studied. The inhibition was pronounced and lasted 18 min (i.p.) or 8 min (i.v.). It was blocked by either locus coeruleus lesions or pretreatment with reserpine and α-methyl-p-tyrosine, which suggests that the inhibition was mediated by norepinephrine. These results indicate that the net effect of DMI on noradrenergic systems is facilitation.     

Participation of Noradrenergic Neurotransmission in the Enhancement of Baroreceptor Reflex Response by Substance P at the Nucleus Tractus Solitarii of the Rat: A Reverse Microdialysis Study

Abstract: We applied reverse microdialysis and HPLC analysis to evaluate the participation of noradrenergic neurotransmission in modulation of the baroreceptor reflex response by substance P at the nucleus tractus solitarii in Sprague-Dawley rats anesthetized with pentobarbital sodium (50 mg/kg, i.p., with 20 mg/kg/h.i.v. supplement). Continuous infusion of substance P (600 µ M ) at 1 µl/min into the nucleus tractus solitarii through a stereo-taxically positioned microdialysis probe (active exchange length, 180–200 µm; diameter, 220 µm) for 1 h elicited an enhancement of the baroreceptor reflex response. This facilitatory effect correlated positively, during the 60-min infusion period, with the time course of increase in the extracellular concentration of substance P and noradrenaline in the nucleus tractus solitarii. Experimentally elevating the concentration of noradrenaline at this medullary nucleus also augmented the baroreceptor reflex sensitivity. On the other hand, depletion of the noradrenergic fibers and nerve terminals at the nucleus tractus solitarii with DSP4 diminished the enhancement of baroreceptor reflex response and the corresponding elevation in extracellular concentration of noradrenaline by substance P. Microinfusion of noradrenaline into the nucleus tractus solitarii in DSP4-treated animals, however, potentiated the baroreceptor reflex response. These results suggest that the enhancement of baroreceptor reflex response by substance P may involve an increase in the concentration of noradrenaline at the nucleus tractus solitarii via a presynaptic mechanism.     

Peripheral ghrelin transmits orexigenic signals through the noradrenergic pathway from the hindbrain to the hypothalamus

Ghrelin, a gastrointestinal peptide, stimulates feeding when administered peripherally. Blockade of the vagal afferent pathway abolishes ghrelin-induced feeding, indicating that the vagal afferent pathway may be a route conveying orexigenic ghrelin signals to the brain. Here, we demonstrate that peripheral ghrelin signaling, which travels to the nucleus tractus solitarius (NTS) at least in part via the vagus nerve, increases noradrenaline (NA) in the arcuate nucleus of the hypothalamus, thereby stimulating feeding at least partially through alpha-1 and beta-2 noradrenergic receptors. In addition, bilateral midbrain transections rostral to the NTS, or toxin-induced loss of neurons in the hindbrain that express dopamine beta hydroxylase (an NA synthetic enzyme), abolished ghrelin-induced feeding. These findings provide new evidence that the noradrenergic system is necessary in the central control of feeding behavior by peripherally administered ghrelin.     

Monoamine Metabolism in the Locus Coeruleus Measured Concurrently with Behavior During Opiate Withdrawal: An In Vivo Microdialysis Study in Freely Moving Rats

Abstract: Using microdialysis, changes in monoamine metabolism were monitored in the locus coeruleus of freely moving rats during opiate withdrawal concomitantly with behavioral symptoms. Rats were infused with morphine (2 mg/kg/h, s.c.) or saline for 5 days and challenged with naltrexone (100 mg/kg, s.c.) on day 6. Following naltrexone challenge, the classic behavioral symptoms of morphine withdrawal were observed in rats treated with morphine but not in saline-infused rats. In morphine-dependent rats, naltrexone induced a marked increase (280%) in dialysate concentrations of 3,4-dihydroxyphenylacetic acid, an index of the functional activity of the noradrenergic neurons in the locus coeruleus. The local concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid were also increased (70%) during morphine withdrawal. Taken together, these results (a) confirm in unanesthetized rats the hypothesis of an activation by opiate withdrawal of noradrenergic neurons in the locus coeruleus and (b) suggest an increase in serotonergic transmission in the same nucleus during morphine withdrawal.     

In Vivo Microdialysis Study of the Extracellular 3,4-Dihydroxyphenylacetic Acid in the Rat Locus Ceruleus: Topographical and Pharmacological Aspects

Abstract: In vivo microdialysis coupled with HPLC and electrochemical detection was used to monitor extracellular levels of 3, 4-dihydroxyphenylacetic acid (DOPAC) of the locus ceruleus (LC) in halothane-anesthetized rats. The identity of DOPAC was confirmed by experiments showing that the chromatographic peak was totally suppressed after inhibition of monoamine oxidase by pargyline. Histological examinations allowed to relate the quantity of DOPAC measured in the dialysates with the localization of the probe implantation site. We found that the DOPAC concentration was inversely proportional to the distance between the probe and the lateral border of the LC. Regardless of the caudorostral level of the nucleus, concentrations were maximal when the axis of the probe was 100 μ M from the lateral border of the LC and decreased by 53% when this distance reached 300 μ M . Activation of LC noradrenergic neurons by systemic administration of the α₂-antagonist piperoxane increased by 100% DOPAC concentrations in LC dialysates. These results suggest that the DOPAC measured by microdialysis could be considered an indicator of the functional state of LC noradrenergic neurons.