Effects of beta-adrenoceptor antagonists on the firing rate of noradrenergic neurones in the locus coeruleus of the rat

Compromised trigemino-coerulean coupling in migraine sensitization can be prevented by blocking beta-receptors in the locus coeruleus

BACKGROUND

Migraine is a disabling neurological disorder, characterized by recurrent headaches. During migraine attacks, individuals often experience sensory symptoms such as cutaneous allodynia which indicates the presence of central sensitization. This sensitization is prevented by oral administration of propranolol, a common first-line medication for migraine prophylaxis, that also normalized the activation of the locus coeruleus (LC), considered as the main origin of descending noradrenergic pain controls. We hypothesized that the basal modulation of trigeminal sensory processing by the locus coeruleus is shifted towards more facilitation in migraineurs and that prophylactic action of propranolol may be attributed to a direct action in LC through beta-adrenergic receptors.

METHODS

We used simultaneous in vivo extracellular recordings from the trigeminocervical complex (TCC) and LC of male Sprague-Dawley rats to characterize the relationship between these two areas following repeated meningeal inflammatory soup infusions. Von Frey Hairs and air-puff were used to test periorbital mechanical allodynia. RNAscope and patch-clamp recordings allowed us to examine the action mechanism of propranolol.

RESULTS

We found a strong synchronization between TCC and LC spontaneous activities, with a precession of the LC, suggesting the LC drives TCC excitability. Following repeated dural-evoked trigeminal activations, we observed a disruption in coupling of activity within LC and TCC. This suggested an involvement of the two regions' interactions in the development of sensitization. Furthermore, we showed the co-expression of alpha-2A and beta-2 adrenergic receptors within LC neurons. Finally propranolol microinjections into the LC prevented trigeminal sensitization by desynchronizing and decreasing LC neuronal activity.

CONCLUSIONS

Altogether these results suggest that trigemino-coerulean coupling plays a pivotal role in migraine progression, and that propranolol's prophylactic effects involve, to some extent, the modulation of LC activity through beta-2 adrenergic receptors. This insight reveals new mechanistic aspects of LC control over sensory processing.

Association of Intrinsic Functional Connectivity between the Locus Coeruleus and Salience Network with Attentional Ability

The locus coeruleus (LC) is a brainstem region associated with broad neural arousal because of norepinephrine production, but it has increasingly been associated with specific cognitive processes. These include sustained attention, with deficits associated with various neuropsychological disorders. Neural models of attention deficits have focused on interrupted dynamics between the salience network (SAL) with the frontoparietal network, which has been associated with task-switching and processing of external stimuli, respectively. Conflicting findings for these regions suggest the possibility of upstream signaling leading to attention dysfunction, and recent research suggests LC involvement. In this study, resting-state functional connectivity and behavioral performance on an attention task was examined within 584 individuals. Analysis revealed significant clusters connected to LC activity in the SAL. Given previous findings that attention deficits may be caused by SAL network switching dysfunctions, findings here further suggest that dysfunction in LC-SAL connectivity may impair attention.

Altered norepinephrine transmission after spatial learning impairs sleep-mediated memory consolidation in rats

The therapeutic use of noradrenergic drugs makes the evaluation of their effects on cognition of high priority. Norepinephrine (NE) is an important neuromodulator for a variety of cognitive processes and may importantly contribute to sleep-mediated memory consolidation. The NE transmission fluctuates with the behavioral and/or brain state and influences associated neural activity. Here, we assessed the effects of altered NE transmission after learning of a hippocampal-dependent task on neural activity and spatial memory in adult male rats. We administered clonidine (0.05 mg/kg, i.p.; n = 12 rats) or propranolol (10 mg/kg, i.p.; n = 11) after each of seven daily learning sessions on an 8-arm radial maze. Compared to the saline group (n = 9), the drug-treated rats showed lower learning rates. To assess the effects of drugs on cortical and hippocampal activity, we recorded prefrontal EEG and local field potentials from the CA1 subfield of the dorsal hippocampus for 2 h after each learning session or drug administration. Both drugs significantly reduced the number of hippocampal ripples for at least 2 h. An EEG-based sleep scoring revealed that clonidine made the sleep onset faster while prolonging quiet wakefulness. Propranolol increased active wakefulness at the expense of non-rapid eye movement (NREM) sleep. Clonidine reduced the occurrence of slow oscillations (SO) and sleep spindles during NREM sleep and altered the temporal coupling between SO and sleep spindles. Thus, pharmacological alteration of NE transmission produced a suboptimal brain state for memory consolidation. Our results suggest that the post-learning NE contributes to the efficiency of hippocampal-cortical communication underlying memory consolidation.

Drug Safety in Episodic Migraine Management in Adults. Part 2: Preventive Treatments

PURPOSE OF REVIEW

The aim of this review is to aid in decision-making when choosing safe and effective options for preventive migraine medications.

RECENT FINDINGS

In Part 2, we have compiled clinically relevant safety considerations for commonly used migraine prophylactic treatments. Preventive treatment of episodic migraine includes nonspecific and migraine-specific drugs. While medications from several pharmacological classes-such as anticonvulsants, beta-blockers, and antidepressants-have an established efficacy in migraine prevention, they are associated with a number of side effects. The safety of migraine-specific treatments such as anti-CGRP monoclonal antibodies and gepants are also discussed. This review highlights safety concerns of commonly used migraine prophylactic agents and offers suggestions on how to mitigate those risks.

Recurrent administration of the nitric oxide donor, isosorbide dinitrate, induces a persistent cephalic cutaneous hypersensitivity: A model for migraine progression

Background

A subgroup of migraineurs experience an increase in attack frequency leading to chronic migraine.

Methods

We assessed in rats the roles of dose and repeat administration of systemic isosorbide dinitrate (ISDN), a nitric oxide donor, on the occurrence and development of cephalic/face and extracephalic/hindpaw mechanical allodynia as a surrogate of migraine pain, and the effect of acute systemic sumatriptan and olcegepant and chronic systemic propranolol on these behavioral changes.

Results

A single high (H-ISDN) but not low (L-ISDN) dose of ISDN induces a reversible cephalic and extracephalic mechanical allodynia. However, with repeat administration, L-ISDN produces reversible cephalic but never extracephalic allodynia, whereas H-ISDN induces cephalic and extracephalic allodynia that are both potentiated. H-ISDN-induced cephalic allodynia thus gains persistency. Sumatriptan and olcegepant block single H-ISDN-induced behavioral changes, but only olcegepant reduces these acute changes when potentiated by repeat administration. Neither sumatriptan nor olcegepant prevent chronic cephalic hypersensitivity. Conversely, propranolol blocks repeat H-ISDN-induced chronic, but not acute, behavioral changes.

Conclusions

Repeated ISDN administration appears to be a naturalistic rat model for migraine progression, suitable for screening acute and preventive migraine therapies. It suggests frequent and severe migraine attacks associated with allodynia may be a risk factor for disease progression.

Increased baroreflex sensitivity and heart rate variability in migraine patients

OBJECTIVES

We investigated whether spontaneous baroreflex sensitivity and heart rate variability (HRV) are different in migraine patients compared to healthy controls.

MATERIAL AND METHODS

Sixteen female migraine patients without aura aged 18-30 years and 14 age-matched healthy female controls were included. Continuous finger blood pressure and ECG were measured supine during paced breathing in the laboratory. Continuous finger blood pressure was measured the following 24-h period. Spontaneous baroreflex sensitivity (time-domain cross correlation baroreflex sensitivity) as well as HRV parameters were calculated.

RESULTS

Spontaneous baroreflex sensitivity measured in the 24-h period was increased in patients (20.6 ms/mmHg) compared to controls (15.7 ms/mmHg, P = 0.031). HRV parameters were increased during paced breathing in patients (P < 0.045).

CONCLUSIONS

The results suggest that central hypersensitivity in migraine also includes cardiovascular reactivity and may be important for the understanding of the mechanisms for the effect of antihypertensive drugs for migraine prophylaxis.

Purinergic modulation of cardiovascular function in the rat locus coeruleus

1 The purpose of the present study was to determine whether purines exerted a physiological role in central cardiovascular modulation at the level of the locus coeruleus (LC). 2 In pentobarbitone-anaesthetised Wistar-Kyoto rats, unilateral microinjection of ATP or alpha,beta-methyleneATP into the LC elicited dose-related decreases in blood pressure and heart rate. Unilateral microinjection of the P2 purinoceptor antagonists suramin and PPADS, caused pressor and tachycardic responses. Administration of the selective P2X(1) receptor antagonist NF-279 had no effect. While both ATP and L-glutamate (L-GLU) resulted in depressor responses after intra-LC microinjection, following intra-LC microinjection of P2 purinoceptor antagonists into the LC, the effects of subsequent administration of either ATP or L-GLU were functionally reversed, such that a pressor response ensued. 3 Microinjection of noradrenaline into the LC caused an increase in blood pressure and heart rate; however, the alpha(2)-adrenoceptor antagonist idazoxan had no cardiovascular effects, but did prevent the pressor response to PPADS or suramin. In addition, coinjection of idazoxan with either suramin or PPADS abolished the ATP and L-GLU mediated pressor responses observed following either suramin or PPADS administration. 4 The present data suggest that firstly, purines are capable of acting within the LC to ultimately modulate the cardiovascular system and secondly, that there is apparently a functional interaction between tonically active purinergic and noradrenergic systems within the LC of the rat.

Interactions between adrenergic systems, anaesthetic and TRH analogue induced analeptic effects on VBT transmission

Previous behavioural and electrophysiological studies have indicated an antinarcotic action of thyrotropin-releasing hormone (TRH) and its analogues in antagonizing the action of CNS depressant drugs, including baclofen and a variety of anesthetics. While beta-adrenergic receptors are implicated in the level of anaesthesia/arousal, whether the analeptic action of TRH involves adrenergic systems for its expression is uncertain. The object of the present experiments, therefore, was to examine interactions between adrenergic systems and the anti-anaesthetic effects of TRH analogue CG3703. It was found that pretreatment with the beta-block (+/-)propranolol did not abolish or reduce the ability of CG3703 to antagonize urethane-induced depression of VBT transmission. These results suggest therefore, that beta-adrenergic systems are unlikely to be involved in the anti-anaesthetic effect of the tripeptide.

Side effects of beta-blocker treatments as related to the central nervous system

During the last decade beta-adrenoceptor antagonists have become one of the first-line treatments for hypertension. Generally, they have been shown to be safe with a low frequency of serious side effects. However, minor subjective symptoms, usually considered to be CNS-related, have been reported for all beta-blockers used. Thus, all beta-blockers on the market seem to have a high benefit:risk ratio; independent of their physicochemical properties and pharmacodynamic profile, however, they seem to cause CNS-related side effects to about the same extent. These minor side effects, the mechanisms of which are unclear, consist of subtle effects on general well being, decreased initiative, a depressed frame of mind, and disturbed sleep. Generally, however, beta-blockers in therapeutic dosages do not affect the qualitative functions of the brain. The results so far available have been obtained primarily by using objective methods. Further comparison has now been initiated using documented subjective methods to investigate whether the objectively documented differences are of any clinical relevance to the patient's quality of life. Although it cannot be claimed with certainty, nonselective beta-blockers seem to cause CNS-related side effects to a greater extent than beta 1-selective blockers. Differences in the degree of hydrophilicity of the beta-blocker are apparently of no clinical relevance in this respect. Rather, the plasma concentration of the beta-blocking drug (degree of beta-blockade) seems to be the major determinant of whether or not CNS-related symptoms appear in susceptible patients.

Chronic antidepressant treatment reduces central beta-adrenoceptor sensitivity in a behavioural test

The ability of beta-adrenoceptor agonists to potentiate the head-twitch response to 5-hydroxytryptophan (5-HTP) in mice was used to assess the in vivo sensitivity of beta-adrenoceptors 48 h after cessation of acute or chronic administration of desmethylimipramine (DMI) or iprindole (IPD). Neither acute nor chronic antidepressant administration significantly altered the head-twitch response to 5-HTP alone. Forty-eight hours after withdrawal from chronic but not acute pretreatment with DMI or IPD the potentiating effects of dobutamine, prenalterol and salbutamol were significantly decreased. This is consistent with a reduction in beta-adrenoceptor density and suggests that the 'spare' beta-adrenoceptor pool is small enough for this to result in functional subsensitivity. Implications of these findings are discussed.

Catecholamine metabolism in the rat locus coeruleus as studied by in vivo differential pulse voltammetry. III. Evidence for the existence of an alpha 2-adrenergic tonic inhibition in behaving rats

One of the various regulations controlling the noradrenergic (NA) locus coeruleus (LC) activity has been proved to be alpha 2 adrenergic specific, on the basis of electrophysiological data obtained in anesthetized preparations. To assess, under rigorously chronic conditions, the existence of such an inhibition, recordings of LC catechol metabolic activity were performed with in vivo differential pulse voltammetry. A guiding cannula and appropriate wires were implanted under anesthesia. After 48 h of recovery a carbon fiber electrode was threaded to the LC through this cannula to monitor the LC catechol oxidation current. Piperoxane 60 mg/kg i.p. and yohimbine 10 mg/kg i.p. induced an increase in catechol oxidation current to approximately 300% of baseline (100%) values. Graded doses of piperoxane (1-100 mg/kg i.p.) induced a dose dependent increase in LC catechol metabolic activity (ED50 = 29.7 mg/kg). These changes in catechol oxidation current were confirmed either by combined electrophysiological and electrochemical recordings in the LC of an anesthetized preparation, or by postmortem HPLC catechol determinations on LC microdissections. By contrast, guanfacine 1 mg/kg and clonidine (10-200 micrograms/kg i.p.) induced a dose dependent decrease in catechol peak height. Clonidine 50 micrograms/kg reversed the effect of piperoxane 30 mg/kg i.p. On the other hand, a highly selective alpha 1 antagonist, such as prazosin (1 mg/kg i.p.), evoked only a small increase in catechol peak (11% above saline effect). This data is consistent with previously reported electrophysiological, biochemical and autoradiographic data. They confirm the presence of a tonic alpha 2 adrenergic inhibition on NA-LC cell activity, in behaving rats.

Migraine and beta-blockade: modulation of sympathetic neurotransmission

The pathophysiology of both classical and common migraine is still not understood, and there is controversy as to whether the origin is vascular, neuronal, or both. Although the mechanism for the prophylactic effect of beta-blockers in migraine has not been elucidated yet, the therapeutic action of beta 1-blockers and non-selective beta-blockers devoid of intrinsic activity is well established by many controlled trials. Mainly on the basis of data from animal experiments, the possible role of central beta-adrenoceptor-mediated mechanisms involved in the control of the activity of noradrenergic neurons will be discussed with reference to migraine.

Locus coeruleus norepinephrine-containing neurons: effects produced by acute and subchronic treatment with antipsychotic drugs and amphetamine

Extracellular single-unit recording techniques were used to determine whether chronic treatment with either a typical antipsychotic drug (APD), haloperidol (HAL) or an atypical APD clozapine (CLOZ) causes a time-dependent reduction of spontaneously active norepinephrine (NE) neurons in the locus coeruleus (LC). Neither HAL nor CLOZ, after prolonged treatment, reduced NE activity. In addition, subchronic amphetamine (AMP) treatment did not increase NE activity. If these results can be extended to humans, they suggest that NE hyperactivity is not the cause for schizophrenic symptoms. Interestingly, chronic CLOZ markedy increased NE activity which may contribute to its low potential for causing extrapyramidal side-effects.

The role of beta- and alpha-adrenoceptors in the regulation of the stages of the sleep-waking cycle in the cat

The effects of beta-adrenergic drugs alone and in combination with alpha-adrenergic drugs on the stages of the sleep-waking cycle were studied in adult cats. Polygraphic sleep recordings of 16 h showed that prenalterol (20 and 40 mg/kg i.p.), a beta 1-adrenoceptor-stimulating drug increased paradoxical sleep (PS) in a dose-related manner during 4-12 h. Salbutamol (40 mg/kg), a beta 2-adrenoceptor-stimulating drug, decreased PS during the first 4 h. Metoprolol (10 and 50 mg/kg), a relatively selective beta 1-adrenoceptor blocking drug, increased drowsy waking during the first 4 h. The larger dose also tended to decrease PS. Already at the lower dose metoprolol partially antagonized the PS increase produced by prazosin, an alpha 1-adrenoceptor blocking drug. Propranolol (5 mg/kg), a beta 1- and beta 2-adrenoceptor blocking drug, which alone decreases PS, antagonized the PS increase induced by phentolamine, an alpha 1- and alpha 2-adrenoceptor blocking drug. Atenolol (5 mg/kg), a poorly lipid-soluble beta-adrenoceptor blocking drug, failed to counteract phentolamine in increasing PS. Metoprolol (10 and 50 mg/kg) and propranolol (5 mg/kg) clearly potentiated the increase in drowsy waking and decrease in deep slow wave sleep and PS induced by clonidine (0.01 mg/kg), an alpha 2-adrenoceptor-stimulating drug. The results support the involvement of beta-adrenoceptors in the regulation of the sleep-waking cycle. A high level of beta-adrenergic activity may facilitate the production of PS. A low level of beta-adrenergic activity, especially in combination with a high level of alpha 2-adrenergic activity, may facilitate the production of drowsy waking. Central alpha 1- and beta 1-adrenoceptors may mediate opposite functions in the regulation of PS.

Brain noradrenaline and anaesthesia: further characterization of the beta-receptor

The sleeping time induced by thiopentone in rats was markedly prolonged by the (-)-isomer of propranolol while the (+)-isomer was virtually without effect. Since the two isomers are equipotent in their membrane-stabilizing effects but the (-)-isomer is about seven to ten times more potent than the (+)-isomer in beta-blockade this suggests that the potentiation of barbiturate sleeping time is due to blockade of beta-adrenergic receptors. The centrally active beta-agonist, clenbuterol, shortened thiopentone-induced sleeping time in a dose-dependent fashion while the beta-agonist, salbutamol, which fails to cross the blood-brain barrier, was without effect. This suggests a central locus of action. Destruction of the noradrenaline system in the locus coeruleus with 6-hydroxydopamine prevented the effect of a racemic mixture of propranolol in elevating thiopentone-induced sleeping time, thus confirming a noradrenergic mechanism and indicating that the coerulear, rather than the medullary, noradrenaline fibres were involved. Thiopentone-induced sleeping time was potentiated by the selective beta 2 blocker ICI 118551 but not by the selective beta 1 blocker, metoprolol, thus characterizing the relevant beta-receptor type as beta.

Anaesthesia: the role of adrenergic mechanisms

The beta-blocker propranolol administered intraperitoneally to rats prior to the barbiturate anaesthetic thiopentone caused a dose-dependent increase in anaesthesia duration. Sotalol, which only poorly crosses the blood-brain barrier, had no such effect, implying a central site of action. The selective beta 1-blockers, metoprolol and atenolol, did not alter thiopentone anaesthesia duration; implying that the effect of propranolol was mediated by a beta 2-receptor. The selective alpha 1-blocker prazocin increased thiopentone anaesthesia duration, while the alpha 1-agonist ST 587 decreased it. Since the alpha 1-agonist methoxamine, which only poorly crosses the blood-brain barrier, was ineffective, a central site of action is indicated. The alpha 2-agonist clonidine markedly increased thiopentone-anaesthesia duration, while the alpha 2-blocker yohimbine, shortened the duration. These effects were shown to be noradrenergic since they were blocked by prior depletion of brain noradrenaline using 6-hydroxydopamine. A model is proposed in which drug-induced alterations in the firing of locus coeruleus cells, or drug-induced changes in the postsynaptic effect of released noradrenaline, may be responsible for modulation of cortical arousal, wakefulness and the processing of sensory stimuli; thus affecting the duration of barbiturate anaesthesia.

Emerging aspects of the adrenergic nervous systems

The present paper reviews some recent experiments with relevance to the functional significance of brain noradrenergic systems with bearing on three clinically relevant topics: 1. The withdrawal and abstinence reactions after the antihypertensive agent clonidine as well as after opiates. 2. The mental reactions associated with alterations in blood volume and acid-base balance. 3. The central actions and side-effects of beta-adrenoceptor blocking agents. The available evidence indicates that central NA neurons may serve a function within the CNS analogous to that of the peripheral sympathetic nerves, i.e. to alert and alarm the individual to significant events in the external and internal environment. Thus, the largest brain NA system, which emanates from locus coeruleus and innervates vast regions of the neuroaxis, is largely activated in the same situations and by the same mechanisms (e.g. blood-volume and chemoreceptors) as the sympathetic system, and may provide part of the central machinery for the anxiety reaction associated with hypercapnia as well as the withdrawal reactions after clonidine or morphine. Some, but not all centrally active beta-adrenoreceptor blocking agents were found to affect the activity of brain NA systems. This result suggests that there may be significant differences with respect to the clinical, central side effects of these drugs.