Locus Coeruleus Spontaneous Discharge Research Articles

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.

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.

Influence of corticotropin-releasing hormone on electrophysiological activity of locus coeruleus neurons

These experiments examined the effects of corticotropin-releasing hormone (CRH) on single-unit electrophysiological activity of locus coeruleus (LC) neurons. As has been reported previously, infusion of CRH into the ventricular system of the brain (i.c.v.) of halothane-anesthetized adult male rats increased spontaneous discharge rate of LC neurons while producing no increase, and possibly a decrease, in sensory-evoked activity. However, when i.c.v. CRH was given to female rats or immature male rats, which had not been studied previously, LC activity was not altered. To attempt to understand this sex and age difference, potential mechanisms by which i.c.v. CRH elevates LC spontaneous activity in adult male rats were examined; in that i.c.v. CRH activates the pituitary-adrenal axis and autonomic nervous system, these response systems were manipulated. Adrenalectomy (with or without corticosterone replacement by pellet) did not affect the ability of i.c.v. CRH to increase LC spontaneous activity in adult male animals, but blockade of sympathetically-mediated autonomic responses, either by chlorisondamine or the beta adrenergic receptor blocker timolol, blocked this increase, indicating that afferent feedback from peripheral autonomic responses was critical for activating LC neurons following i.c.v. CRH. To determine whether CRH neurotransmission might play a role in this feedback pathway, the CRH antagonist alpha-helical CRH (α-hCRH) was microinjected into several brain regions including LC prior to i.c.v. CRH. α-hCRH microinjected into LC reduced the increase in LC activity caused by i.c.v. CRH; however, blockade of this increase was total when α-hCRH was microinjected into the lateral parabrachial nucleus ipsilateral to the LC recording site, suggesting that increased LC activity following i.c.v. CRH is mediated by CRH acting in the parabrachial region. During these studies, it was also observed that microinjection of α-hCRH into LC increased LC spontaneous discharge rate; consequently, CRH was microinjected into LC, and produced a dose-dependent decrease in LC spontaneous activity in both male and female rats, which could be blocked by microinjection of α-hCRH — these data indicated that the direct influence of CRH on LC neurons is to decrease their spontaneous activity. To reconcile this with the original report that CRH applied to LC neurons increases their activity, one possibility suggested is that the CRH microinjection procedure used in the present study stimulated inhibitory receptors on LC dendrites whereas the original study stimulated excitatory receptors on LC cell bodies. It is concluded that an inhibitory influence of CRH on LC activity is consistent with recent data indicating that decreased LC activity increases anxiety and stress-related responses, but that direct influences of CRH appear rather minor in determining LC neuronal activity in comparison to other inputs to LC such as are seen after i.c.v. CRH infusion.

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 μg) 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.

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.

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 μg in 3.0 μl) 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 μg/30 μl/ min, 15 min IV infusion) decreased mean arterial pressure of unanesthetized rats by 50 ± 2mmHg ( 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 μg/30 μl/min) that decreased blood pressure of halothane-anesthetized rats by 55 ± 2mmHg was much less effective in producing hypotension and did not increase LC discharge when administered to unanesthetized rats. Prior administration of the CRF antagonist, α helical CRF 9–41 (50 μg, 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.

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)

Serotonin selectively attenuates glutamate-evoked activation of noradrenergic locus coeruleus neurons

The effect of 5-HT on activity of noradrenergic locus coeruleus (LC) neurons was studied using microiontophoretic and micropressure drug application in anesthetized rats. 5-HT had no consistent effect on LC spontaneous discharge, eliciting a modest decrease overall. However, 5-HT reliably attenuated responses of LC neurons to excitatory amino acids (EAAs), one of the major classes of transmitters in afferents to these neurons. This effect was specific for EAA responses because it occurred for glutamate and kainate but not for ACh. In contrast, iontophoretic norepinephrine (NE) selectively attenuated spontaneous activity but not responses evoked by either glutamate or ACh. The responsiveness of LC neurons to EAAs as quantified by a response-contrast measure (evoked excitation/basal activity) was markedly reduced by 5-HT, but was increased by NE. For ACh, such responsiveness of LC cells was not changed by 5-HT, but was increased by NE. The effects of 5-HT were prevented and reversed by iontophoretically applied antagonists of 5-HT receptors, methysergide and methiothepin. Thus, 5-HT appears to selectively interact with EAA responses of LC neurons, acting as a filter to attenuate LC activity linked to its major EAA inputs while allowing other channels afferent to the LC (e.g., those utilizing ACh) to be expressed.

Corticotropin-releasing hormone increases tonic but not sensory-evoked activity of noradrenergic locus coeruleus neurons in unanesthetized rats

These studies were designed to further test the hypothesis that corticotropin-releasing hormone (CRH) is released from axons innervating the noradrenergic neurons of the locus coeruleus (LC) and serves to activate these neurons during stress responses. Specifically, the effects of exogenous CRH on the electrophysiological activity of LC neurons in unanesthetized rats were characterized. Intracerebroventricular (I.C.V.) injection of CRH (0.3-3.0 micrograms) caused a dose-dependent increase in LC spontaneous discharge rates that became statistically significant 6-9 min after injection and was still evident 30-40 min later. A 1.0 and 3.0 micrograms amount of CRH caused peak increases of 86 +/- 32% and 184 +/- 29% (SEM), respectively. In contrast, neither the lowest dose of CRH (0.3 microgram) nor a high dose of Ala 14CRH (3.0 micrograms), an inactive analog of CRH, altered LC spontaneous activity. The effects of CRH administration on sensory-evoked activity of LC neurons were also determined. As previously reported (Foote et al., 1980; Aston-Jones and Bloom, 1981b), the repeated presentation of auditory tone stimuli resulted in a brief enhancement of LC discharge, which was usually followed by a period of relatively decreased activity. Administration of 1.0 or 3.0 micrograms CRH enhanced basal discharge during sensory testing, but discharge rates during the excitatory component of the sensory response were not altered. Quantitative analyses revealed that these doses of CRH produced a statistically significant decrease in the ratio of sensory-evoked to basal discharge rates. Additional analyses of the temporal distribution of discharge activity for individual recording sites during sensory testing demonstrated that 1.0 and 3.0 micrograms CRH altered relative response magnitudes to a statistically significant extent in 7 of 10 and 5 of 7 cases, respectively, while 0.3 microgram CRH and 3.0 micrograms Ala 14CRH were without effect. The present results are consistent with previous studies of CRH effects on LC activity in anesthetized rats (Valentino et al., 1983; Valentino and Foote, 1987). However, in unanesthetized animals, CRH is more potent in increasing tonic activity and does not decrease the absolute magnitude of sensory-evoked activity. The present results support the hypothesis that CRH released from fibers innervating the LC may affect the tonic activity of these cells and the relationship between tonic discharge and phasic, sensory-evoked activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbachol-induced increases in locus coeruleus spontaneous activity are associated with an altered pattern of response to sensory stimuli

In order to evaluate the relationship between locus coeruleus (LC) spontaneous activity and LC activity in response to sensory stimuli, the effects of carbachol on both types of LC neuronal activity were investigated in anesthetized rats. Carbachol increased LC spontaneous discharge rate and altered the pattern of LC discharge in response to sciatic nerve stimulation, while saline had no effect. Moreover, the magnitudes of increases in LC spontaneous discharge rate produced by carbachol were highly correlated ( r=0.79) with a measure of simultaneous effects on LC sensory responses. These results suggest that drugs which increase LC spontaneous activity will also disrupt the pattern of LC discharge to sensory stimuli.

Corticotropin-releasing factor disrupts sensory responses of brain noradrenergic neurons.

In order to elucidate the possible role of noradrenergic neurons of the nucleus locus coeruleus (LC) in stress responses, the effects of corticotropin-releasing factor (CRF) on LC neuronal activity were characterized. In haloth-aneanesthetized rats, intracerebroventricular administration of CRF was found to have two distinct actions: A dose-dependent increase in spontaneous discharge activity was observed 3 min after peptide injection, with 1.0 and 3.0 micrograms CRF increasing activity by 7 +/- 2 and 47 +/- 12%, respectively; Ala14CRF (3.0 micrograms), an inactive analogue of CRF, had no effect on LC spontaneous discharge rates. These results confirm and extend previous studies of CRF activation of LC basal discharge activity; additionally, CRF (1.0 and 3.0 micrograms) disrupted sensory responses of LC cells to sciatic nerve stimulation. As previously reported, responses of LC neurons to electrical stimulation of the sciatic nerve usually consisted of a brief activation beginning 20-40 ms after stimulus followed by a period of relatively suppressed activity lasting 100-200 ms. CRF attenuated both components. Responses plotted as normalized, cumulative histograms became more linear in the presence of CRF (1.0 and 3.0 micrograms), suggesting that discharge rates during phasic responses to sciatic stimulation were similar to spontaneous rates. Statistical comparison using the Kolmogorov-Smirnoff test or correlation coefficients demonstrated that both 1.0 and 3.0 micrograms CRF reduced response components, while 0.3 micrograms and Ala14CRF (3.0 micrograms) had no effect. The degree of attenuation of LC sensory responses by CRF was not linearly related to the magnitude of CRF-induced increases in spontaneous discharge rate, suggesting that these are distinct effects of CRF.(ABSTRACT TRUNCATED AT 250 WORDS)