Sleep Pathways and General Anaesthesia

Modern surgery would be impossible without general anaesthetics, yet the underlying mechanisms by which they produce unconsciousness and pain relief are only now beginning to be discovered. In parallel with this growing understanding about anaesthetic action, rapid progress has also been made towards understanding the neuronal mechanisms responsible for natural sleep. It now appears that there is a direct connection between sleep and anaesthetics because of mounting evidence that anaesthetics cause their sedative effects by recruiting natural sleep pathways. The central aim of this project is to investigate the link between the sedative actions of general anaesthetics and some of the neuronal pathways underlying natural sleep. Different anaesthetics appear to act on these pathways in different ways, and it is possible that some will provide a state that more closely resembles natural physiological sleep than others. Understanding the molecular actions and neuronal pathways involved presents a major intellectual challenge for basic neuroscience, and research into anaesthetic mechanisms and sleep pathways and how they are related can be expected to provide fundamental information which has broader applicability. It is widely recognised that the anaesthetic drugs presently used in clinical practice are far from satisfactory. Currently used anaesthetics are relatively non-specific, mostly act at high concentrations and affect a variety of different targets. As a consequence, a large number of patients suffer from undesirable side-effects that may be attributed to the anaesthetic and analgesic drugs used for their perioperative care. Serious morbidity can be provoked in already compromised patients, which is an issue of increasing concern in an ageing population. If certain specific neuronal pathways can be selectively targeted, then it may be possible to provide anaesthesia that not only has fewer unwanted side effects, but that is actually restful, and hence beneficial for the patient, particularly in the intensive care setting. Sleep deprivation, which may occur with certain anaesthetic regimens, is known to be harmful, and can, for example, compromise the immune system. In addition, our work should provide fundamental insights into natural sleep pathways, information that is likely to aid in the treatment of a growing number of sleep disorders, which are increasingly prevalent in our society.

The aim of this project is to explore the relationship between the sedative and hypnotic actions of general anaesthetics and the neuronal pathways that induce and regulate natural sleep. Recent work has shown that some hypothalamic brain nuclei that are known to play a key role in the production and maintenance of deep (non-REM) sleep are also involved casually in the sedation and loss of consciousness induced by general anaesthetics. Work that has led up to this project has indicated that anaesthetics such as propofol (PROP - which is known to target specific beta subunit containing GABA-A receptors), and the sedative dexmedetomidine (DEX - an alpha2 adrenergic agonist) act on sleep pathways at different points. We will explore this hypothesis using a combination of in vivo electrophysiology, in vitro brain slice electrophysiology and molecular genetics. We will use in vivo electrophysiology to record activity in the GABAergic ventrolateral preoptic (VLPO) nucleus in free-moving animals. This nucleus is known to promote sleep by increasing GABA release and we will test the idea that anaesthetics also promote GABA release in sleep-active neurons. Using brain-slice electrophysiology, we will record from two key targets that are innervated by VLPO, the histaminergic TMN and orexinergic Pef, whose inhibition is known to promote sleep. This work will be done using tissue from adult animals and a brain-slice preparation that preserves the connections between the TMN and Pef nuclei so that their reciprocal connections can be explored. The involvement of these nuclei in anaesthetic-induced sedation will also be tested using the ?anaesthetic-insensitive? beta3(N265M) knock-in mice together with a novel orexin receptor (OX2R) antagonist. We will also utilise a novel approach using in vivo molecular genetics in which the particular hypothalamus nuclei that we hypothesise are key anaesthetic targets (the TMN and Pef) can be individually inhibited or excited. In preliminary work we are close to developing a novel mouse in which the synaptic GABAergic input to the TMN has been genetically engineered so that it can be rapidly and reversibly enhanced or reduced. This approach minimises the likelihood of compensation which can be a major problem with ?conventional? conditional knock-outs, particularly when studying arousal systems. We will use these animals to test the idea that sedation can be induced simply by enhancement of GABAergic inhibition of the TMN as well as determining if PROP- and DEX-induced sedation can be antagonised by selectively overriding this inhibition.