Noradrenergic modulation of arousal

Abstract

Through a highly divergent efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. State-dependent neuronal discharge activity of locus coeruleus neurons has long-suggested a role of this system in the induction of an alert waking state. More recent work supports this hypothesis, demonstrating robust wake-promoting actions of the locus coeruleus-noradrenergic system. Norepinephrine enhances arousal, in part, via actions of beta- and alpha1-receptors located within multiple subcortical structures, including the general regions of the medial septal area and the medial preoptic areas. Recent anatomical studies suggest that arousal-enhancing actions of norepinephrine are not limited to the locus coeruleus system and likely include the A1 and A2 noradrenergic cell groups. Thus, noradrenergic modulation of arousal state involves multiple noradrenergic systems acting within multiple subcortical regions. Pharmacological studies indicate that the combined actions of these systems are necessary for the sustained maintenance of arousal levels associated with spontaneous waking. Enhanced arousal state is a prominent aspect of both stress and psychostimulant drug action and evidence indicates that noradrenergic systems likely play an important role in both stress-related and psychostimulant-induced arousal. These and other observations suggest that the dysregulation of noradrenergic neurotransmission could well contribute to the dysregulation of arousal associated with a variety of behavioral disorders including insomnia and stress-related disorders.

Figure 1

Effects of LC activation on cortical EEG (ECoG) activity state. In this experiment, the LC was first located electrophysiologically using a combined recording-infusion probe in the halothane-anesthetized rat. Once located, a small infusion of the cholinergic agonist, bethanechol was made adjacent to the LC. The effect of this infusion-induced activation of the LC on ECoG was monitored. A: Schematic of the infusion/recording probe used to activate LC. Bethanechol was infused 300–400 μm lateral or medial to the recording electrode that was used to locate the main body of the LC. B: Photomicrograph of a peri-LC infusion site. N indicates position within the track created by the infusion needle where the infusate exited the needle. E indicates position of the recording electrode within the LC. C: Effects of bethanechol infusion on LC activity and ECoG activity (recorded from the prefrontal cortex). ECoG activity is displayed in the top trace and the raw trigger output from LC activity in the bottom trace. Bethanechol infusion (indicated by horizontal bar) increased LC discharge rate approximately two-thirds of the way through the 60-second infusion. Several seconds following LC activation, an abrupt onset of EEG desynchronization is observed (see [19]).

Figure 2

Schematic diagram depicting bethanechol infusion sites that were effective or ineffective for activating forebrain EEG. Solid circles indicate sites at which bethanechol infusion activated the EEG. Shaded boxes indicate sites at which bethanechol infusion had no obvious EEG effects. The center of the infusion site is indicated by these symbols. There is a radius of approximately 500 μm around LC within which infusions, placed either medially or laterally, activated forebrain EEG. Infusions placed immediately anterior to the LC were also ineffective (not shown). Abbreviations: Me5, mesencephalic nucleus of the trigeminal nerve; Mo5, motor nucleus of the trigeminal nerve; Pr5, principle sensory nucleus of the trigeminal nerve; 4V, fourth ventricle (see [19]).

Figure 3

Effects of bilateral suppression of LC discharge in the lightly-anesthetized rat. Shown are the effects of bilateral clonidine infusions (100 ng in 100 nl) that resulted in the complete bilateral suppression of LC discharge activity. Shown are 25-sec raw EEG traces for cortical EEG (ECoG; top row) and hippocampal EEG (HEEG; bottom row) along with the results of power spectral analysis (PSA; graphs below the raw traces) calculated on an 8-min epoch from which the 25-sec EEG trace was taken. Data from pre-infusion (left column) and post-infusion (right column) periods are displayed. The most striking post-infusion changes in the ECoG are the increase in the slowest frequencies, and in the HEEG, the dramatic reduction in theta activity and the appearance of mixed-frequency activity. Shading in the PSA plots indicates the theta frequency band (2.3–6.9 Hz) in the HEEG power spectra (see [23]).

Figure 4

Boundaries defining the general regions of the MSA (top row) and MPOA (middle row) within which NE acts to promote waking. Previous intratissue infusion mapping studies with NE and various direct and indirect NE agonists indicate that NE acts within a nearly continuous portion of the medial basal forebrain the spans the anterior-posterior extent of the MSA and MPOA, indicated by the dotted line. The region termed the MSA encompasses the medial septum, the vertical limb of the diagonal band of Broca, the posterior portions of the shell region of the nucleus accumbens. The region termed the MPOA encompasses preoptic area of the hypothalamus and portions of the bed nucleus of the stria terminalis (BST). Previous mapping studies suggest that the shell accumbens or BST are not prominently involved in NE-induced waking. Infusions outside the MSA and MPOA are generally ineffective at increasing waking. Panels are arranged anterior-posterior with the anterior-most panel shown in the upper left and the posterior-most panel shown in the bottom right position. Photomicrographs (bottom row) are of infusion sites from experiments involving NE agonist infusions into the MSA (left image) and MPOA (right image). In these photomicrographs, the arrows indicate the most ventral extent of the infusion needle. AC, anterior commissure; CC, corpus callosum; CP, caudate-putamen; GP, globus pallidus; I, internal capsule; LS, lateral septum, LC, lateral ventricle; MS, medial septum; NA nucleus accumbens; SI, substantia innominata.

Figure 5

Wake-promoting effects of NE infusion into the MPOA. Shown are the effects of NE (1.2 μg (4 nmol)/150nl) infusions into the MPOA on ECoG and electromyographic activity (EMG) from a typical experiment. Shown are 10-min traces of ECoG and EMG recorded immediately prior to (top traces, PRE NE) and 10-min following (bottom traces, POST NE) NE infusion into MPOA. Prior to the infusion, the animal spent the majority of time in slow-wave sleep (resting with large amplitude, slow-wave activity present in ECoG and low-amplitude activity present in EMG). The most striking post-infusion changes are the decrease in large-amplitude, slow-wave ECoG activity and the increase in EMG amplitude, indicative of alert, active waking (see [22]).

Figure 6

Wake-promoting effects of intra-MPOA infusion of NE, α₁-agonist and β-agonist. Top Panel displays the effects of vehicle, 4 nmol NE, or 16 nmol NE, infused into either the MPOA or SI on total time spent awake. Bottom Panel displays the effects of the α₁-agonist, phenylephrine (PHEN; 40 nmol), and the β-agonist, isoproterenol (ISO; 15 nmol), infused into the MPOA or SI on total time spent awake. Symbols represent mean (±SEM) of time (secs) spent awake in 30-minutes epochs. PRE1 and PRE2 represent 30-min pre-infusion epochs occurring immediately prior to the infusion. POST1-POST3 represent 30-min post-infusions epochs, beginning immediately following the infusions. NE, phenylephrine and isoproterenol increased waking when infused to the MPOA. In contrast, these treatments had little impact on time awake when infused into the SI. The only exception to this being observed with the high dose of NE. In this case the latency to waking was longer and the magnitude of waking smaller than that observed with infusions into the MPOA. Lack of visible error bars indicates the magnitude of the SEM fell within the range corresponding to the dimensions of the symbol. There were no significant differences between any of the groups during the pre-infusion epochs. *P<0.05, **P<0.01 compared to vehicle-treated controls (see [22]).

Figure 7

Additive wake-promoting effects of α₁- and β-agonist receptor stimulation within the MSA and MPOA. Top Panel depicts dose-dependent wake-promoting actions of infusion of the α₁-agonist, phenylephrine (PHEN; 10 nmol, 50 nmol in 150 nl), into the MSA. Intra-MSA infusion produced dose-dependent increases in waking. Middle and bottom panels depict the effects of infusion of vehicle, 10 nmol phenylephrine (Phen), 4 nmol of the β-agonist, isoproterenol (Iso; 4 nmol) and combined phenylephrine and isoproterenol (Combined). For both regions, when administered separately at these doses each drug had a mild wake-promoting action. In the combined treatment group, the wake-promoting effects of isoproterenol and phenylephrine appeared additive and not supra-additive. Symbols represent means (± SEM) of time (secs) spent awake per 30-min testing epoch. PRE1 and PRE2 represent pre-infusion portions of the experiment. POST1-POST3 represent post-infusions epochs. *P<0.05, **P<0.01 compared to PRE1; ⁺P<0.05 compared to Combined (see [20]).

Figure 8

Synergistic sedative effects of α₁- and β-receptor blockade. Shown are the effects of the β-antagonist, timolol (ICV), the α₁-antagonist, prazosin (IP) and combined antagonist treatment on cortical EEG in animals exposed to an arousing brightly-lit novel environment. Animals were treated 30-min prior to testing with: 1) ICV vehicle + IP saline (VEH/VEH); 2) 150 μg ICV timolol + IP saline (TIM/VEH); 3) ICV vehicle + 500 μg/kg prazosin (VEH/PRAZ,), and; 4) combined timolol + prazosin (TIM/PRAZ). In this figure, EEG traces are from the second 5-min epoch of exposure to the novel environment. Vehicle-treated controls displayed behavioral and EEG indices of alert waking throughout most of the recording session. This is reflected in sustained EEG desynchronization (low-amplitude, high-frequency). β-receptor blockade alone (TIM/VEH) had no effects on EEG activity. α₁-receptor blockade alone (VEH/PRAZ) increased the frequency and duration of sleep spindles (high-voltage spindles). In contrast to that observed with β-receptor blockade alone, in the presence of α₁-receptor blockade, β-receptor blockade produced substantial increases in large-amplitude, slow-wave activity. Power spectral analyses provide quantification of these qualitative observations (data not shown, see [17]).