Mechanisms of action of noradrenaline in the brain

Noradrenergic Profile of Hippocampal Formation Theta Rhythm in Anaesthetized Rats

The present study was undertaken to identify the noradrenergic receptors underlying the production of hippocampal formation (HPC) type 2 theta rhythm. The experiments were performed on urethanized rats wherein type 2 theta is the only rhythm present. In three independent stages of experiments, the effects of noradrenaline (NE) and selective noradrenergic α and β agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and β1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC β2 and β3 noradrenergic receptors do not seem to be directly involved in the pharmacological mechanism responsible for the suppression of theta rhythm in anaesthetized rats. Obtained results provide evidence for the suppressant effect of exogenous NE on HPC type 2 theta rhythm and show the crucial role of α1, α2 and β1 noradrenergic receptors in the modulation of HPC mechanisms of oscillations and synchrony. This finding is in contrast to the effects of endogenous NE produced by electrical stimulation of the locus coeruleus (LC) and procaine injection into the LC (Broncel et al., 2020).

Afferent subcortical connections into the motor cortical larynx area in the rhesus monkey

In three rhesus monkeys (Macaca mulatta), the inferior motor cortex was explored by electrical stimulation for sites yielding vocal fold adduction. The retrograde tracer wheat germ-agglutinin-conjugated horseradish peroxidase was injected into the effective sites. Within the forebrain, retrogradely labeled cells were found in the claustrum, basal nucleus of Meynert, substantia innominata, extended amygdala, lateral and posterior hypothalamic area, field H of Forel, and a number of thalamic nuclei with the strongest labeling in the nuclei ventralis lateralis, ventralis posteromedialis, including its parvocellular part, medialis dorsalis and centrum medianum, and weaker labeling in the nuclei ventralis anterior, ventralis posterolateralis, intermediodorsalis, paracentralis, parafascicularis and pulvinaris anterior. In the midbrain, labeling was found in the deep mesencephalic nucleus, ventral tegmental area, and substantia nigra. In the lower brainstem, labeled cells were found in the pontine reticular formation, median and dorsal raphe nuclei, medial parabrachial nucleus, and locus coeruleus. The findings are discussed in terms of the possible role of these structures in voluntary vocal control.

Effects of methylphenidate on the membrane potential and current in neurons of the rat locus coeruleus

Effects of methylphenidate (MPH), a therapeutic agent used in children presenting the attention deficit hyperactivity disorder (ADHD), on the membrane potential and current in neurons of the rat locus coeruleus (LC) were examined using intracellular and whole cell patch-clamp recording techniques. Application of MPH (30 microM) to artificial cerebrospinal fluid (ACSF) produced a hyperpolarizing response with amplitude of 12 +/- 1 mV (n = 29). Spontaneous firing of LC neurons was blocked during the MPH-induced hyperpolarization. Superfusion of LC neurons with ACSF containing 0 mM Ca(2+) and 11 mM Mg(2+) (Ca(2+)-free ACSF) produced a depolarizing response associated with an increase in spontaneous firing of the action potential. The MPH-induced hyperpolarization was blocked in Ca(2+)-free ACSF. Yohimbine (1 microM) and prazosin (10 microM), antagonists for alpha(2) and alpha(2B/2C) receptors, respectively, blocked the MPH-induced hyperpolarization in LC neurons. Tetrodotoxin (TTX, 1 microM) produced a partial depression of the MPH-induced hyperpolarization in LC neurons. Under the whole cell patch-clamp condition, MPH (30-300 microM) produced an outward current (I(MPH)) with amplitude of 110 +/- 6 pA (n = 17) in LC neurons. The I(MPH) was blocked by Co(2+) (1 mM). During prolonged application of MPH (300 microM for 45 min), the hyperpolarization gradually decreased in the amplitude and eventually disappeared, possibly because of depression of norepinephrine (NE) release from noradrenergic nerve terminals. At a low concentration (1 microM), MPH produced no outward current but consistently enhanced the outward current induced by NE. These results suggest that the MPH-induced response is mediated by NE via alpha(2B/2C)-adrenoceptors in LC neurons. I(MPH) was associated with an increase in the membrane conductance of LC neurons. The I(MPH) reversed its polarity at -102 +/- 6 mV (n = 8) in the ACSF. The reversal potential of I(MPH) was changed by 54 mV per decade change in the external K(+) concentration. Current-voltage relationship showed that the I(MPH) exhibited inward rectification. Ba(2+) (100 microM) suppressed the amplitude and the inward rectification of the I(MPH.) These results suggest that the I(MPH) is produced by activation of inward rectifier K(+) channels in LC neurons.

Outlining a brain model of mental imaging abilities

This paper outlines a brain model of mental imaging abilities by reviewing neuropsychological evidence of the association and dissociation between mental imaging processes and known perceptual mechanisms; differentiating between visual-mode and auditory-mode thought systems; pointing to brain arousal systems involved in waking and dream imaging; and exploring pre-frontal involvement in deliberate imaging and cognitive state monitoring. In the process, the paper develops an emerging distinction between objective-imagery and subjective-imagery abilities. It ends by suggesting possible brain models for in-born and clinical loss of subjective imaging abilities.

Noradrenalin enhances the activity of cochlear nucleus neurons in the rat

The cochlear nucleus of rats is heavily innervated by noradrenergic fibres from the locus coeruleus. The physiological meaning of this innervation is poorly understood. Therefore, iontophoretically applied noradrenalin was tested on single neurons of the cochlear nucleus in urethane-anaesthetized rats. Iontophoresis of noradrenalin had a dual effect. During application noradrenalin led to moderate inhibition of tone-evoked activity in 37% of the tested neurons. In contrast, approximately 20-30 s after the onset of iontophoresis a long-lasting increase in discharge activity was found in most neurons. Data from iontophoresis of the alpha1-receptor agonist phenylephrine and the alpha2-receptor agonist clonidine suggest that the fast moderate inhibition is mediated by alpha2-receptors while the pronounced long-lasting elevated neuronal firing is mediated by alpha1-receptors. However, these data do not exclude the possibility that part of the response to noradrenalin is also mediated by beta-receptors. Electrical stimulation of the locus coeruleus resulted in an increase in discharge activity comparable with iontophoresis of noradrenalin or phenylephrine. Thus, activation of the locus coeruleus predominantly increases spontaneous and tone-evoked neuronal firing in the cochlear nucleus of the rat. This alpha-receptor-mediated enhanced discharge activity may serve to increase the sensitivity of acoustic processing mechanisms or to lower the threshold for short-latency acoustic reflexes.

Idazoxan-induced reductions in cortical glucose use are accompanied by an increase in noradrenaline release: complementary [14C]2-deoxyglucose and microdialysis studies

The autoradiographic [14C]2-deoxyglucose procedure was used to map function-related alterations in local cerebral glucose use following acute administration of the alpha 2-adrenoceptor antagonist, idazoxan (0.3-3 mg kg-1 s.c.). The most prominent feature of the results obtained was the significant reduction in glucose use in certain locus coeruleus projection areas. Thus, in various cortical, hippocampal and thalamic regions, as well as structures involved in auditory and visual function, idazoxan administration was associated with a 13-20% decrease in glucose use. In a complementary microdialysis study, the effect of idazoxan on extracellular noradrenaline levels in the frontal cortex of rats, manipulated in the same fashion as during the [14C]2-deoxyglucose procedure (i.e. following the application of surgery and partial restraint), was examined. Both surgery and restraint were associated with a modest but significant increase in basal noradrenaline release (+31% and +26%, respectively). Subsequent administration of idazoxan (3 mg kg-1 s.c.) evoked a further increase in noradrenaline release, the magnitude of which was the same as that observed following its administration to freely-moving rats (+113%). These combined data suggest that idazoxan-induced reductions in cerebral glucose use, at least in the frontal cortex, may occur as a consequence of the increase in noradrenaline release. In addition, it appears that surgery and partial restraint do not alter alpha 2-adrenoceptor tone in the frontal cortex.

Nanomolar concentrations of ouabain block ethanol-inducible Na+,K(+)-ATPase activity in brain

The effect of low concentrations of ethanol on Na+,K(+)-ATPase activity, defined as ouabain-inhibitable 86Rb+ (K+) uptake, was investigated in a crude synaptosome preparation which was subject to minimal subcellular fractionation procedures. Moderate (20-30%) but potent (EC50 = 3.8 mM) stimulation of total ouabain (1 mM)-inhibitable K+ uptake by ethanol was observed following incubation periods of up to 20 min. The activity of the ethanol-induced component of K+ uptake was antagonized by nanomolar concentrations of ouabain. Thus, the moderate stimulation of total ouabain-inhibitable K+ uptake by ethanol was attributable to the activation of a component of K+ uptake which was very sensitive (VS; IC50 = 2.8 x 10(-10) M) to inhibition by ouabain. Slightly higher concentrations of ouabain (10(-9) - 10(-6.6) M) stimulated K+ uptake above control (no ethanol or ouabain) in both the absence and presence of ethanol. The selectivity of the VS-ethanol interaction was demonstrated by the lack of any ethanol effect on two other components of ouabain-inhibitable K+ uptake which accounted for inhibition of K+ uptake by concentrations of ouabain above 10(-6.6) M and were defined as sensitive (S; IC50 = 10(-6) M) and insensitive (I; IC50 = 10(-4) M) to ouabain. These results define the ethanol-inducible component of ouabain-inhibitable Na+,K(+)-ATPase activity and promote the view that changes in Na+,K(+)-ATPase-dependent ion translocation may contribute to ethanol intoxication in vivo.

The human locus coeruleus and anxiogenesis

Electrical stimulation of locus coeruleus (LC), via permanently implanted electrodes with confirmed localization and effectiveness, did not elicit any subjective or behavioral manifestations of anxiety. This is evidence against the hypothesis that LC is a mediator of anxiogenesis in man.

A network model of catecholamine effects: gain, signal-to-noise ratio, and behavior

At the level of individual neurons, catecholamine release increases the responsivity of cells to excitatory and inhibitory inputs. A model of catecholamine effects in a network of neural-like elements is presented, which shows that (i) changes in the responsivity of individual elements do not affect their ability to detect a signal and ignore noise but (ii) the same changes in cell responsivity in a network of such elements do improve the signal detection performance of the network as a whole. The second result is used in a computer simulation based on principles of parallel distributed processing to account for the effect of central nervous system stimulants on the signal detection performance of human subjects.

Subacute noradrenergic agonist infusions in vivo increase Na+, K+-ATPase and ouabain binding in rat cerebral cortex

In order to investigate the specificity of noradrenergic effects on Na+, K+-ATPase, we infused noradrenergic agonists into the cerebral ventricles of rats, with or without depletion of forebrain norepinephrine. Infusion of norepinephrine, isoproterenol, or phenylephrine increased ouabain binding in intact rats, whereas clonidine infusion decreased binding. Depletion of forebrain norepinephrine by destruction of the dorsal noradrenergic bundle reduced ouabain binding. Norepinephrine infusion reversed the effect of dorsal bundle lesion; isoproterenol and phenylephrine increased ouabain binding in lesioned rats, but did not restore the effect of the lesions. Clonidine had no effect in lesioned rats. Effects on Na+, K+-ATPase activity were similar, but smaller. These results suggest that stimulation of both alpha 1- and beta-noradrenergic receptors may be necessary for optimal Na+, K+-ATPase, and that clonidine reduces Na+, K+-ATPase indirectly through decreased norepinephrine release.

Dexamethasone and adrenalectomy alter brain (Na+,K+)-ATPase responses to noradrenergic stimulation or depletion

We investigated the effects of adrenalectomy or dexamethasone treatment on the regulation of brain (Na+,K+)-ATPase by noradrenaline. Noradrenergic stimulation was produced by repeated injections of yohimbine, and noradrenaline depletion by an injection of the selective toxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). Adrenalectomy had no effect on the number of ouabain binding sites in cerebral cortex, but increased the number of sites synergistically with noradrenergic stimulation. Dexamethasone prevented the decrease in ouabain binding in rats treated with DSP4, but did not itself alter ouabain binding. Neither dexamethasone nor adrenalectomy altered the changes in beta-receptor binding associated with the noradrenergic manipulations. Changes in exposure to corticosteroids may alter the coupling between adrenoceptor binding and second messenger activation in a way that affects (Na+,K+)-ATPase regulation.

Electrophysiological effects of methylphenidate on the coeruleo-cortical noradrenergic system in the rat

The effect of methylphenidate on noradrenergic neurotransmission was investigated in urethane-anesthetized rats. The spontaneous activity of locus coeruleus noradrenergic neurons was the same in rats treated for 7 days with methylphenidate as in the controls. In control rats, i.v. methylphenidate induced a reduction of locus coeruleus neuronal firing whereas in rats treated for 7 days with methylphenidate, the same dose of methylphenidate failed to induce any change in locus coeruleus activity. At this time, clonidine induced a lesser reduction of locus coeruleus neuronal firing than in the controls, indicating that their autoreceptors had become desensitized. Following electrical stimulation of the locus coeruleus, most of the spontaneously firing cortical neurons were inhibited but the percentage of such neurons was reduced and the neurons showed a decreased responsiveness after methylphenidate treatment. The responsiveness of cortical neurons to microiontophoretic applications of NA as assessed by the I.T50 method was reduced after 7 days of treatment with methylphenidate. These findings suggest that the efficacy of cortical NA neurotransmission is markedly reduced following methylphenidate treatment.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. III. Glucoregulatory challenge

Insulin-induced hypoglycemia and the subsequent administration of glucose were examined for their effects on single unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats. LC-NE neuronal activity showed an inverse relationship to blood glucose levels. The activity of most cells increased during sustained hypoglycemia, and then decreased following glucose administration. Some neurons were unaffected by hypoglycemia, but were inhibited following glucose. The activation of LC-NE neurons in response to insulin administration generally paralleled the increase in plasma epinephrine, although the adrenal response was more sensitive. These data, together with those reported in the preceding papers, suggest the following general conclusions: (1) physiological stimuli can influence the activity of LC-NE neurons in unanesthetized subjects (although they do so less strongly than environmental stimuli); (2) these effects of physiological stimuli upon LC-NE neurons can be exerted independent of changes in behavioral state; (3) LC-NE neurons do not appear to play a specific role in the regulation of any of the systems examined, but may instead play a more global role in the response to physiological challenges in general; (4) LC-NE neurons are generally co-activated with both the neural and hormonal components of the sympatho-adrenal system, although sympathetic activation can occur in the absence of increased LC-NE activity. A previously hypothesized role for LC-NE neurons in facilitating the behavioral response to environmental stressors may thus be extended to include the response to physiological challenges, and perhaps facilitation of the physiological as well as the behavioral components of the stress response.

Long-term effects of yohimbine on behavioral sensitivity to a stressor

Two experiments examined the long-term effects of repeated administration of yohimbine, a suspected anxiogenic drug, on behavioral sensitivity to a conditioned cue for shock. In Experiment 1, rats were trained to bar press following injection of yohimbine (either 1 or 2 mg/kg) or saline. At the end of this training, injections were suspended and the rats were given Pavlovian fear conditioning to establish a light as a shock signal. Rats were then returned to the bar press situation (about 2 weeks after their last injection) and the capacity of the light to disrupt responding was tested. Rats previously treated with yohimbine were less disrupted by the light than were controls. In contrast, Experiment 2 found that previously experiencing the yohimbine-induced state only in the home cage increased subsequent disruption of bar pressing by the light. Yohimbine pretreatment had no effect on acquisition of freezing behavior to the light, nor on bar pressing during testing in the absence of the light, in either experiment. The results show that yohimbine can have long-term effects on behavior in the presence of a stressor, and that the nature of these effects are dependent upon environmental and/or behavioral context in which the yohimbine-induced state was experienced. These findings appear compatible with an internal stimulus view of stress and stress inoculation.

No increase in reaction-time after lesion of the dorsal noradrenergic bundle

In this study we examined whether lesions of rats' dorsal noradrenergic bundles affect their reaction times to temporally unpredictable stimuli. Injection of 6-hydroxydopamine into rats' dorsal bundles drastically reduced the noradrenaline content of their cerebral cortex. Nevertheless, 6OHDA treated rats could still react as quickly as controls. Moreover, the treatment did not affect the efficiency with which rats performed the task, even when they were forced to respond very quickly.