Professor Tim Hales

Pain was an unavoidable consequence of injury, disease and infection before the advent of clinical anaesthesia. Now, thanks to skilled anesthetists, pain-ameliorating analgesics and general anaesthetics (GAs), millions of people undergo surgery every year and most recover with relatively minor discomfort. While only a small minority of patients experience major negative consequences all anaesthetics have side effects. Most cause respiratory depression and some may cause neurodegeneration, a particular concern in the elderly. Analgesic agents also have severe side effects. Opioids such as morphine and fentanyl are commonly used to treat both perioperative and chronic pain; however their prolonged use leads to physical dependence and a loss of potency due to tolerance. Morphine can also cause hyperalgesia, a paradoxical increase in pain. There is a pressing need to develop better GAs and analgesics.

We are studying the mechanisms of action of opioids and GAs, drugs that influence neuronal excitability by binding to membrane proteins and thereby directly or indirectly regulating the activity of ion channels. By identifying the proteins responsible for their therapeutic and detrimental effects we hope to offer a strategy for improved safety and efficacy.

Opioid receptors (mu, delta and kappa) couple through G proteins to effectors, including K+ and Ca2+ channels. Morphine activates mu receptors thereby inhibiting Ca2+ channels, reducing excitatory transmission within the pain pathway. Prolonged morphine exposure leads to analgesic tolerance. Tolerance is attenuated in mice that lack beta-arrestin2, a protein that interacts with the mu receptor affecting its internalization and coupling it to signaling proteins including the tyrosine kinase, c-Src. We are testing the hypothesis that tolerance requires c-Src activity using electrophysiological recording and measurements of analgesia in mice.

Morphine induced hyperalgesia occurs in opioid receptor knock-out mice and is therefore independent of opioid receptor activation. We are using electrophysiological recording and behavioural assays to test the hypothesis that opioids directly modulate the activity of ion channels (e.g. the 5-HT3 receptor) and that these “off-target” actions contribute to their side effect profiles.

Research that began in the 1980’s in Dundee revealed that GAs, such as the induction agent propofol, enhance neuronal inhibition by the neurotransmitter gamma-aminobutyric acid (GABA) through a direct interaction with the GABAA receptor. GABA activates the GABAA receptor opening the integral Cl- channel and this activity is enhanced by GAs. The GABAA receptor is the primary target for induction agents. Since the 1980’s genes that encode 19 different GABAA receptor subunits have been cloned revealing considerable receptor heterogeneity. We identified the GABAA receptor epsilon subunit which reduces the enhancement of GABAA receptor function by GAs. The epsilon subunit may protect specific brain regions from inhibition by GAs. We are exploring the subtype specificity of GAs. Using chimeric constructs of the epsilon subunit and mutagenesis we are characterizing the nature of the GA interaction with the GABAA receptor.

Mutations in GABAA genes can profoundly affect the GA sensitivity of GABAA receptors. The artificial introduction of mutant receptors that are resistant to GA modulation makes mice resistant to immobilization by propofol validating GABAA receptors as the primary target of induction. We use homology modeling, mutagenesis and electrophysiological techniques to examine the relationship between structure and function of GABAA receptors and other related Cys-loop receptors. We recently demonstrated that mutations in individuals with epilepsy, which reduce GABA efficacy, enhance potentiation by propofol.

Click here to visit our Cys-loop receptor structure-function database:

Peters JA, Cooper MA, Carland JE, Livesey MR, Hales TG, Lambert JJ. (2009). Novel structural determinants of single channel conductance and ion selectivity in 5-hydroxytryptamine-type3 and nicotinic acetylcholine receptors. J Physiol (in press).

Deeb TZ, Sharp D, Hales TG. Direct subunit-dependent multimodal 5-HT3 receptor antagonism by methadone. Mol Pharmacol. (2009) 75:908-17.

Walwyn W., Evans C.J., Hales T.G. beta-Arrestin2 and c-Src regulate the constitutive activity and recycling of m opioid receptors in dorsal root ganglion neurons. J Neurosci. (2007) 27:5092-104.

McCartney, M.R., Deeb, T.Z., Henderson, T.N., Hales, T.G. Tonically active GABAA receptors in hippocampal pyramidal neurons exhibit constitutive GABA-independent gating. Mol. Pharmacol. (2007) 71(2):539-48.

Deeb, T.Z., Carland, J.E., Cooper, M.A., Livesey, M.R., Lambert, J.J., Peters, J.A., Hales, T.G. Dynamic modification of a mutant cytoplasmic cysteine residue modulates the conductance of the human 5-HT3A receptor. J Biol Chem. (2007) 282:6172-82.

Hales, T.G., Deeb, T.Z., Tang, H., Bollan, K.A., King, D.P., Johnson, S.J., and Connolly, C.N. asymmetric contribution to GABAA receptor function of a conserved lysine within TM2-3 of alpha1, beta2 and gamma2 subunits. J Biol Chem. (2006) 281: 17034-43

Davies, P.A., Hanna, M.C., Hales, T.G., and Kirkness, E.F. Insensitivity to anaesthetic agents conferred by a class of GABAA receptor subunit. Nature. (1997), 385: 820-3