The role of histamine in sleep and arousal

Our understanding of how chemical signals in the brain produce wakefulness or induce sleep is far from complete. Yet unfortunately, sleep disorders and lack of sleep contribute an estimated £13 billion loss to the UK economy each year. Thus for the pharmaceutical industry, developing effective drugs that act in the brain to specifically induce or maintain sleep is an attractive aim (ref. 4). As anyone who has taken a first-generation antihistamine for allergies such as hay fever can testify, a major neurochemical that promotes wakefulness and arousal is histamine. This molecule, which in the brain is uniquely synthesized in neurons of the hypothalamic tuberomammilary nucleus (TMN) by the enzyme histidine decarboxylase (hdc), acts at widespread neuronal receptor sites (refs. 1 & 6). TMN neurons fire during wakefulness partly because of the excitatory effects of the neuropeptide orexin, but are hypothesised to be actively repressed during sleep by the neurotransmitter gamma-ambinobutyric acid (GABA) and the neuropeptide galanin. Drugs that manipulate aspects of the histaminergic system might be very useful for developing new sleep medicines, and histamine is considered a high priority target in the pharmaceutical industry. For this project, Wisden & Franks have constructed a Cre recombinase diver mouse line, hdc-Cre, that allows genetic manipulations of activity specifically in the histaminergic cells of the TMN without affecting other types of neurons (unpublished). The PhD student would use this Cre mouse line to explore if and how GABA inhibitory transmitter pathways inhibit the TMN neurons to aid sleep induction and maintenance. We will examine the two inhibitory GABA components GABA-A (fast, synaptic, ionotropic) and GABA-B (slow, extrasynaptic, metabotropic). For this, the student will analyse a specific deletion of: (i) the GABA-A receptor gamma2 subunit gene (gabrg2) from TMN cells by a hdc-Cre and lox-gabrg2 cross (ref. 5); (ii) the GABA-B receptor R1 gene (gabrb1) from TMN cells using a hdc-Cre and lox-gabrb1 cross (ref. 2). In a third subproject, the student will use AAV vectors and the hdc-Cre mice to engineer the TMN neurons so that they are selectively sensitive to benzodiazepine hypnotics, with all other neurons remaining insensitive (ref. 5). This will enable us to critically assess our hypothesis that sedatives induce sedation by suppressing the histamine system (refs 3 & 7). The student will acquire a thorough training in systems neuroscience. The student will analyse, by in vivo electrophysiological recordings, how the genetic manipulations of the TMN affect the sleep electrophysiological (EEG) profile. Using Lilly's in-house automated behavioural sleep/activity scoring system, the student will examine how the sleep profile changes in response to the genetic manipulations. During the project, the PhD student would acquire a mixture of laboratory skills including molecular biology (recombinant AAV plasmid and virus construction, transgenic mouse breeding strategies); stereotaxic manipulations; neuroanatomical methods such as immunohistochemistry, in vivo electrophysiological recording techniques and analysis, and behavioural analysis. 1. Haas HL et al (2008). Physiol Rev 88, 1183. 2. Haller C et al (2004). Genesis 40, 125. 3. Nelson LE et al (2002). Nat Neurosci 5, 979. 4. Wafford KA & Ebert B (2008). Nat Rev Drug Discov 7, 530. 5. Wulff, P et al. (2007). Nat Neurosci 10, 923-929. 6. Zecharia AY & Franks NP (2009). Anesthesiology 111, 695. 7. Zecharia AY et al., (2009). J Neurosci 29, 2177.