Material below summarizes the article Noradrenaline from Locus Coeruleus Neurons Acts on Pedunculo-Pontine Neurons to Prevent REM Sleep and Induces Its Loss-Associated Effects in Rats, published on November 30, 2016, in eNeuro and authored by Mudasir Ahmad Khanday, Bindu I. Somarajan, Rachna Mehta, and Birendra Nath Mallick.
An important characteristic of living systems/organisms is that they undergo cyclical episodes of rest and activity, which in higher species has evolved into sleep and waking. Apparently, rest and activity are defined by physical movement; however, these stages are contiguous and gradually transform from one to another state, which differ in threshold of responsiveness to stimuli, levels of consciousness, cognition, and thinking ability. By and large, these states and their characteristics are subjective in nature.
Technological development has significantly helped in objectively advancing our knowledge in all spheres, including sleep research. The breakthrough came with the discovery of electroencephalography (EEG), which helped to classify sleep and wakefulness using objective criteria. Subsequently, it was possible to record the electrical signals from the muscles, the electromyogram (EMG), and due to the eye movement — the electrooculogram (EOG).
The EEG, EMG, and EOG vary significantly within apparently the same state, leading to our knowledge that sleep is neither a passive phenomenon nor a homogenous state. However, because EEG could be recorded in organisms with well developed and evolved brains, so far sleep and waking has been objectively defined only in higher species. Such recordings showed that within behavioral sleep, a unique state appears when apparently, EEG and EOG show expression as that of waking, while the EMG shows opposite expression than during waking. This state was termed as rapid eye movement sleep (REMS) by Aserinsky and Kleitman, while Jouvet termed it as paradoxical sleep. Interestingly, this state is associated with dreaming when although one is behaviorally asleep, the brain is relatively more active, one’s consciousness, threshold for sensory, psychological and cognitive expressions differ than other states. Although one spends the least amount of time in REMS, it is affected in almost all disorders.
In the mid-1970s, REM-ON and REM-OFF neurons were discovered in pedunculo-pontine tegmentum (PPT) and locus coeruleus (LC), respectively, as neural substrate for REMS regulation. The former remains inactive during waking and non-REMS (NREMS), but increases activity during REMS; the REM-OFF neurons remain active during waking and NREMS, but cease activity during REMS. Findings from isolated and independent studies confirmed that cessation of noradrenaline (NA)-ergic REM-OFF neurons is a pre-requisite for REMS generation. Subsequently, it was confirmed that upon REMS-deprivation (REMSD) non-cessation of NA-ergic REM-OFF neurons elevate NA level in the brain, which is a primary factor for REMS-loss associated acute/chronic effects, including increased Na-K ATPase activity.
Notwithstanding, for REMS regulation cessation of REM-OFF neurons alone was over-simplification and role of other neurons was emphasized by contemporary findings. The latter view was supported by the presence of REM-ON neurons; however, mutual relationship between REM-OFF and REM-ON neurons was unknown. Based on findings from series of sequential neuro-micro-anatomo-pharmaco-physiological studies in freely moving chronically prepared animals we constructed a comprehensive model of neural and neurochemical substrates/circuitry in the brain for REMS regulation. Notwithstanding, to simulate normal in vivo condition multiple brain areas needed simultaneous activation or deactivation, which is extremely difficult to induce in vivo. Therefore, as a compromise, applying our findings from animal studies we constructed an effective mathematical model simulating the neural substrates of REMS regulation. The findings using the mathematical model helped us to infer that withdrawal of wake-active neurons from REM-OFF neurons and simultaneous activation of REM-ON neurons by the NREMS-inducing neurons is necessary for REMS generation.
As our interpretations were based on findings from isolated studies, they did not shed light on the brain structure-function relationship in vivo in the same animal. For confirmation, it needed to be experimentally reproduced in vivo in the same animal that manipulation of the so worked-out neural and neuro-chemical substrates of REMS regulation must reversibly induce REMS-loss and affected associated physiological processes comparable to that obtained upon REMSD in a normal animal. We proposed that if NA from the LC-REM-OFF neurons prevents REMS by not allowing activation of the PPT-REM-ON neurons, then inhibition of synthesis of NA in the LC-neurons should increase REMS, which however, should recover i) once the inhibition of NA synthesis was over, and ii) if NA was injected into PPT. Further, those animals where NA synthesis was inhibited, when subjected to REMSD, should not express REMSD-associated NA-mediated increase in Na-K ATPase activity. In our eNeuro paper we have confirmed all these points in the same chronically prepared freely moving rats, which are summarized below.
Electrophysiological signals identifying sleep-waking-REMS were recorded from separate groups of chronically prepared freely moving rats. In those chronic rats, NA synthesis was down-regulated in vivo by transfecting the LC-neurons (REM-OFF) with si-/sh-RNA of tyrosine hydroxylase (TH), the rate-limiting enzyme for NA synthesis; suitable control experiments were conducted. The TH down-regulation was confirmed by counting TH-immunopositive neurons and by estimation of TH-mRNA by qPCR as well as TH-expression by Western blotting. Indeed, the TH down-regulated rats showed increased REMS, which returned to normal level by local micro-infusion of NA into the PPT (site of REM-ON neurons). Also, the increased REMS returned to normal level 3-weeks post-sh-RNA-injection, when expectedly the effect of sh-RNA was over. Further, upon REMSD although the normal and control rats, which received scrambled si-/sh-RNA into LC, showed increased expression as well as activity of Na-K ATPase in the brain, the rats which received TH si-/sh-RNA into the LC, failed to show such responses even upon REMSD. The results taken together confirmed that NA from the LC-REM-OFF neurons does not allow activation of the PPT-REM-ON neurons and appearance of REMS is prevented e.g. as normally happens during wakefulness and NREMS. Further, if those NA-ergic LC-REM-OFF neurons are not allowed to cease activity, REMS does not appear (or vice versa) causing elevation of brain level of NA, which induces REMS-loss associated effects e.g. increased Na-K ATPase activity and brain excitability.
The findings confirmed our hypothesis at least as a proof-of-principle that REMS maintains level of NA as well as excitability of the brain and thus, serves housekeeping function of the brain. Finally, we propose that REMS might have evolved to maintain the level of NA in the brain.
Noradrenaline from Locus Coeruleus Neurons Acts on Pedunculo-Pontine Neurons to Prevent REM Sleep and Induces Its Loss-Associated Effects in Rats. Mudasir Ahmad Khanday, Bindu I. Somarajan, Rachna Mehta, Birendra Nath Mallick. eNeuro Nov 2016 DOI: 10.1523/ENEURO.0108-16.2016