Cholinergic regulation of the central nucleus of the amygdala in rats: Effects of local microinjections of cholinomimetics and cholinergic antagonists on arousal and sleep

Dopaminergic- and cholinergic-inputs from substantia nigra and pedunculo-pontine tegmentum, respectively, converge in amygdala to modulate rapid eye movement sleep in rats

Dreams appear intermittently during phasic rapid eye movement sleep (REMS). Although reasonable progress has been made about neuro-physio-pharmacological mechanism of appearance of REMS, appearance of dreams is a mystery. Isolated studies have reported that substantia nigra (SN) withdraws inhibition from pedunculo-pontine tegmentum (PPT) acetylcholine (ACh)-ergic REM-ON neurons to trigger REMS; some REM-ON neurons become phasically active during REMS; amygdala (Amyg), a limbic structure associated with emotions, may be related with dreaming like state; Amyg receives projections from both SN-Dopamine (DA)-ergic and PPT-ACh-ergic neurons. Collating these isolated findings, we proposed that on the background of REMS, SN-DA-ergic and PPT-ACh-ergic inputs phasically activate Amyg-neurons to manifest dreams. In the absence of better criteria, we recorded electrophysiological characteristics of REMS as the closest objective read-out for dreams in surgically prepared, chronic, freely moving rats. Microinjection of either DA-ergic or ACh-ergic agonist [Quinpirole (Qnp) or Carbachol (Carb)] bilaterally into Amyg increased, while antagonists [Haloperidol (Hal) or Scopolamine (Scop)] reduced REMS. Electrical stimulation of either bilateral SN or PPT increased REMS, which however, was prevented when stimulated in presence of Hal or Scop, respectively into the Amyg. These findings confirm and support our contention that SN-DA-ergic and PPT-ACh-ergic inputs integrate in Amyg for REMS regulation. Further, subject to confirmation in humans, we propose that on the background of REMS, some phasic PPT-ACh-ergic-REM-ON neurons intermittently trigger some neurons in Amyg, the area known to be associated with memory and emotions, causing intermittent appearance of REMS-associated dreams and in REMS behavior disorder.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

Deletion of Metabotropic Glutamate Receptors 2 and 3 (mGlu2 & mGlu3) in Mice Disrupts Sleep and Wheel-Running Activity, and Increases the Sensitivity of the Circadian System to Light

Sleep and/or circadian rhythm disruption (SCRD) is seen in up to 80% of schizophrenia patients. The co-morbidity of schizophrenia and SCRD may in part stem from dysfunction in common brain mechanisms, which include the glutamate system, and in particular, the group II metabotropic glutamate receptors mGlu2 and mGlu3 (encoded by the genes Grm2 and Grm3). These receptors are relevant to the pathophysiology and potential treatment of schizophrenia, and have also been implicated in sleep and circadian function. In the present study, we characterised the sleep and circadian rhythms of Grm2/3 double knockout (Grm2/3^-/-) mice, to provide further evidence for the involvement of group II metabotropic glutamate receptors in the regulation of sleep and circadian rhythms. We report several novel findings. Firstly, Grm2/3^-/- mice demonstrated a decrease in immobility-determined sleep time and an increase in immobility-determined sleep fragmentation. Secondly, Grm2/3^-/- mice showed heightened sensitivity to the circadian effects of light, manifested as increased period lengthening in constant light, and greater phase delays in response to nocturnal light pulses. Greater light-induced phase delays were also exhibited by wildtype C57Bl/6J mice following administration of the mGlu2/3 negative allosteric modulator RO4432717. These results confirm the involvement of group II metabotropic glutamate receptors in photic entrainment and sleep regulation pathways. Finally, the diurnal wheel-running rhythms of Grm2/3^-/- mice were perturbed under a standard light/dark cycle, but their diurnal rest-activity rhythms were unaltered in cages lacking running wheels, as determined with passive infrared motion detectors. Hence, when assessing the diurnal rest-activity rhythms of mice, the choice of assay can have a major bearing on the results obtained.

Benzodiazepine site agonists differentially alter acetylcholine release in rat amygdala

BACKGROUND

Agonist binding at the benzodiazepine site of γ-aminobutric acid type A receptors diminishes anxiety and insomnia by actions in the amygdala. The neurochemical effects of benzodiazepine site agonists remain incompletely understood. Cholinergic neurotransmission modulates amygdala function, and this study tested the hypothesis that benzodiazepine site agonists alter acetylcholine (ACh) release in the amygdala.

METHODS

Microdialysis and high-performance liquid chromatography quantified ACh release in the amygdala of Sprague-Dawley rats (n = 33). ACh was measured before and after IV administration (3 mg/kg) of midazolam or eszopiclone, with and without anesthesia. ACh in isoflurane-anesthetized rats during dialysis with Ringer's solution (control) was compared with ACh release during dialysis with Ringer's solution containing (100 μM) midazolam, diazepam, eszopiclone, or zolpidem.

RESULTS

In unanesthetized rats, ACh in the amygdala was decreased by IV midazolam (-51.1%; P = 0.0029; 95% confidence interval [CI], -73.0% to -29.2%) and eszopiclone (-39.6%; P = 0.0222; 95% CI, -69.8% to -9.3%). In anesthetized rats, ACh in the amygdala was decreased by IV administration of midazolam (-46.2%; P = 0.0041; 95% CI, -67.9% to -24.5%) and eszopiclone (-34.0%; P = 0.0009; 95% CI, -44.7% to -23.3%), and increased by amygdala delivery of diazepam (43.2%; P = 0.0434; 95% CI, 2.1% to 84.3%) and eszopiclone (222.2%; P = 0.0159; 95% CI, 68.5% to 375.8%).

CONCLUSIONS

ACh release in the amygdala was decreased by IV delivery of midazolam and eszopiclone. Dialysis delivery directly into the amygdala caused either increased (eszopiclone and diazepam) or likely no significant change (midazolam and zolpidem) in ACh release. These contrasting effects of delivery route on ACh release support the interpretation that systemically administered midazolam and eszopiclone decrease ACh release in the amygdala by acting on neuronal systems outside the amygdala.

Amygdala lesions reduce cataplexy in orexin knock-out mice

Narcolepsy is characterized by excessive sleepiness and cataplexy, sudden episodes of muscle weakness during waking that are thought to be an intrusion of rapid eye movement sleep muscle atonia into wakefulness. One of the most striking aspects of cataplexy is that it is often triggered by strong, generally positive emotions, but little is known about the neural pathways through which positive emotions trigger muscle atonia. We hypothesized that the amygdala is functionally important for cataplexy because the amygdala has a role in processing emotional stimuli and it contains neurons that are active during cataplexy. Using anterograde and retrograde tracing in mice, we found that GABAergic neurons in the central nucleus of the amygdala heavily innervate neurons that maintain waking muscle tone such as those in the ventrolateral periaqueductal gray, lateral pontine tegmentum, locus ceruleus, and dorsal raphe. We then found that bilateral, excitotoxic lesions of the amygdala markedly reduced cataplexy in orexin knock-out mice, a model of narcolepsy. These lesions did not alter basic sleep-wake behavior but substantially reduced the triggering of cataplexy. Lesions also reduced the cataplexy events triggered by conditions associated with high arousal and positive emotions (i.e., wheel running and chocolate). These observations demonstrate that the amygdala is a functionally important part of the circuitry underlying cataplexy and suggest that increased amygdala activity in response to emotional stimuli could directly trigger cataplexy by inhibiting brainstem regions that suppress muscle atonia.

Effects of MPEP, a selective metabotropic glutamate mGlu5 ligand, on sleep and wakefulness in the rat

Metabotropic glutamate receptors (mGlu) have been implicated in the regulation of physiological and behavioral processes. Pharmacological evidence involves group I mGlu receptors in the regulation of emotional states and antagonism of these receptors has been proposed as a novel class of anxiolytic drugs having also antidepressant effects. Here, the effects of mGlu5 receptor selective modulation on sleep and wake states are explored. 32 male Wistar rats were implanted with electrodes for recording sleep and wake states. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP hydrochloride, 5, 10, and 20 mg/kg, i.p.), a potent, selective and systemically active mGlu5 receptor negative allosteric modulator, or vehicle was administered 1 h after the beginning of the light period. Sleep recordings were conducted for 3 h. MPEP (5, 10, and 20 mg/kg) significantly suppressed rapid eye movement (REM) sleep, decreasing the number of episodes and mean episode duration, and increased its latency. A reduction of light and deep slow wave sleep (SWS) latency was observed in the groups receiving 10 or 20 mg/kg, increasing latency to first wakefulness episode. 10 mg/kg of MPEP also increased non rapid eye movement sleep (NREM). The present results suggest that mGlu5 receptors might be involved in sleep regulation, more specifically in REM sleep, and drugs that block these receptors could potentially benefit the treatment of pathologies were REM sleep is enhanced.

Effects of microinjections of Group II metabotropic glutamate agents into the amygdala on sleep

Systemic administration of the Group II metabotropic glutamate (mGlu) receptor agonist, LY379268 (LY37), dose-dependently suppresses rapid eye movement sleep (REM) whereas systemic administration of the mGlu II receptor antagonist, LY341495 (LY34), increases arousal. Group II mGlu receptors are highly expressed in the amygdala, a brain region involved in the regulation of sleep and arousal. To determine whether the amygdala is involved in mediating the effects of Group II mGlu agents on sleep, we microinjected LY37 and LY34 into the basal amygdala (BA) and the central nucleus of the amygdala (CNA) and recorded sleep and wakefulness. Wistar rats were implanted with electrodes for recording sleep and with bilateral cannulae aimed into BA for drug administration. Different groups of rats received bilateral microinjections of LY37 into BA at two dosage ranges (3.2 mM, 5.3 mM or 10.7 mM or 0.1 nM, 2.0 nM or 10.0 nM) or one dosage range of LY34 (1.0 nM, 30.0 nM or 60.0 nM). Microinjections into CNA were conducted at one dosage range for LY37 (0.1 nM, 2.0 nM or 10.0 nM) and for LY34 (1.0 nM, 30.0 nM or 60.0 nM). All drugs or vehicle alone were administered in a counterbalanced order at 5-day intervals. Following microinjection, sleep was recorded for 20 h. Microinjection of LY37 into BA at both nM and mM concentrations significantly decreased REM without significantly altering NREM, total sleep or wakefulness. The high dosage of LY34 in BA significantly suppressed NREM and total sleep. Microinjections of LY37 or LY34 into CNA had no significant impact on sleep. We suggest that Group II mGlu receptors may influence specific cells in BA that control descending output (via the CNA or bed nucleus of the stria terminalis) that in turn regulates pontine REM generator regions.

Neuropeptide B induces slow wave sleep in mice

STUDY OBJECTIVE

Neuropeptide B (NPB) and neuropeptide W (NPW) are two recently identified neuropeptides that act as endogenous ligands to orphan G protein coupled receptors, GPR7 and GPR8. In rodents, the GPR8 ortholog is absent and both NPB and NPW function exclusively through GPR7. Although NPB and NPW are implicated in the regulation of feeding behavior, endocrine function, and pain sensation, their physiological role is incompletely understood.

DESIGN

NPB or saline was administered into the lateral ventricle of mice during both the light and dark periods. In separate experiments, spontaneous locomotor activity or EEG and EMG were recorded after intracerebroventricular (i.c.v). injection. To confirm the involvement of GPR7 in NPB-induced responses, GPR7 knockout mice were also subjected to i.c.v. injections.

MEASUREMENTS AND RESULTS

NPB injections reduced locomotor activity during the dark period, but not during the light period. EEG and EMG recordings in freely moving mice revealed that NPB injection decreased the time spent in the waking state and increased the time spent in slow wave sleep (SWS) during the dark period. The time spent in paradoxical sleep was unaffected. The spectral power of NPB-induced SWS was indistinguishable from that of physiological SWS. The NPB-induced increase in SWS was not observed in GPR7 knockout mice.

CONCLUSION

These results suggest that NPB induced physiological SWS through GPR7 and that NPB and GPR7 may have a role in modulating the occurrence of sleep and wakefulness.

Paradoxical sleep insomnia and decreased cholinergic neurons after myocardial infarction in rats

STUDY OBJECTIVES

Acute myocardial infarction (MI) is followed, within a few hours, by neuronal loss in the central nervous system (CNS), including the limbic system, the hypothalamus, and the brainstem. Sleep before and after MI was investigated in the first experiment. In a parallel experiment, 2 weeks after MI, we quantified brainstem cholinergic neurons known to control paradoxical sleep (PS).

MEASUREMENTS AND RESULTS

Data were obtained from 28 adult male Sprague-Dawley rats weighing 350-375 g and maintained under a 12-12 light-dark cycle in 2 experiments on 16 and 12 rats, respectively. The 16 animals in the first experiment were implanted with chronic electroencephalographic (EEG) and electromyographic (EMG) electrodes. A week after surgery, these animals were habituated for 2 days to the recording equipment, and baseline sleep was charted for 24 h. The next morning, MI was induced in 8 rats by occluding the left anterior descending coronary artery for 40 min. The remaining 8 rats served as sham-operated controls. Sleep was recorded again 2 weeks after MI. The number of choline acetyltransferase (ChAT)-positive neurons was counted in the second, parallel experiment on 6 MI and 6 sham rats. Compared to the sham controls, MI rats displayed longer latency to sleep onset, shorter latency to paradoxical sleep (PS), and curtailed PS duration. The number of ChAT-positive neurons in the pedunculopontine tegmentum (PPT) area of MI rats was significantly decreased compared to the sham controls, while the number of laterodorsal tegmentum (LDT) cholinergic neurons was not different.

CONCLUSION

Acute MI is accompanied, within 2 weeks, by PS-specific insomnia that can be explained, at least partly, by a specific loss of cholinergic neurons in an area known to control PS.

GABAergic and non-GABAergic thalamic, hypothalamic and basal forebrain projections to the ventral oral pontine reticular nucleus: their implication in REM sleep modulation

The ventral part of the oral pontine reticular nucleus (vRPO) is a demonstrated site of brainstem REM-sleep generation and maintenance. The vRPO has reciprocal connections with structures that control other states of the sleep-wakefulness cycle, many situated in the basal forebrain and the diencephalon. Some of these connections utilize the inhibitory neurotransmitter GABA. The aim of the present work is to map the local origin of the basal forebrain and diencephalon projections to the vRPO whether GABAergic or non-GABAergic. A double-labelling technique combining vRPO injections of the neuronal tracer, cholera-toxin (CTB), with GAD-immunohistochemistry, was used for this purpose in adult cats. All of the numerous CTB-positive neurons in the reticular thalamic and dorsocaudal hypothalamic nuclei were double-labelled (CTB/GAD-positive) neurons. Approximately 15%, 14% and 16% of the CTB-positive neurons in the zona incerta and the dorsal and lateral hypothalamic areas are, respectively, CTB/GAD-positive neurons. However, only some double-labelled neurons were found in other hypothalamic nuclei with abundant CTB-positive neurons, such as the paraventricular nucleus, perifornical area and H1 Forel field. In addition, CTB-positive neurons were abundant in the central amygdaline nucleus, terminal stria bed nuclei, median preoptic nucleus, medial and lateral preoptic areas, dorsomedial and ventromedial hypothalamic nuclei, posterior hypothalamic area and periventricular thalamic nucleus. The GABAergic and non-GABAergic connections described here may be the morphological pillar through which these prosencephalic structures modulate, either by inhibiting or by exciting, the vRPO REM-sleep inducing neurons during the different sleep-wakefulness cycle states.