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)