Dr Delia Belelli

The physiological and pharmacological significance of GABAA receptor diversity: from synapse to behaviour

GABAA receptor structure and diversity

A fundamental goal of neuroscience is to relate changes in neuronal activity to changes in mood, or behaviour. Our laboratory is specifically interested in understanding the physiological and pharmacological significance of the diversity of the main inhibitory receptor in the CNS, the GABAA receptor. It is apparent that the receptor exists as a number of different subtypes derived from the assembly of 5 receptor subunits drawn from a palette of 19 different types. The expression of the receptor isoforms is both developmentally and regionally regulated thus suggesting the different receptor subtypes to mediate distinct physiological functions and behaviours. To explore the physiological and pharmacological significance of such diversity, we employ a powerful approach based on the combined use of transgenic technologies, in vitro electrophysiological techniques and behavioural studies. In collaboration with behavioural scientists, experts in developing in vivo models and collaborations with industry, we use models where specific GABAA receptor subunit genes (e.g. alpha1) have been either deleted, or mutated at the level of a single amino acid by gene “knock-out” or “knock-in” technology (e.g. the latter engineered to change the pharmacological properties of the receptor while leaving the sensitivity to the neurotransmitter GABA intact).

Some examples of my research using this multidisciplinary approach are given below:

Neurosteroids: The brain’s valium?

“Neurosteroids are compounds which are related to steroid hormones, but do not act on conventional intracellular steroid receptors. Interestingly, they include metabolites of progesterone and androgens which are formed in the nervous system, and may have a physiological role.”

My work and that of others has established that neurosteroids are potent endogenous modulators of the GABAA receptor and may greatly influence our mood and behaviour. I have found the interaction of neurosteroids with the GABAA receptor to be highly neurone dependent and my research is focussed on elucidating the molecular basis of this specificity. This research has revealed that neurosteroid metabolism plays a previously underestimated role by shaping inhibition in a neurone specific manner. Excitingly, these studies revealed that synaptic GABAA receptor function is under the fine control of locally produced neurosteroids, even in an isolated brain slice preparation. These findings have important implications for understanding neuronal signalling in both health and disease.

How do general anaesthetics work:

Although general anaesthetics have been used clinically for over 150 years the molecular mechanism whereby they produce their dramatic behavioural effects has remained elusive. My earlier studies revealed certain i.v. anaesthetics to selectively enhance GABAA receptor function and unexpectedly I found this effect to be governed by the nature of a single amino acid on the GABAA beta subunit . Using this information, in collaboration with Prof. Lambert we have demonstrated in an in vivo model that the sedative effects of the anaesthetic etomidate are blunted upon introduction of a point mutation of a beta subunit and, furthermore, the ability to induce anaesthesia is compromised. This finding has raised the exciting prospect that anaesthetics with reduced sedative (“hangover”) liability might be developed and more fundamentally reveals important information on how such agents induce unconsciousness. By comparing the activity of anaesthetics on neuronal activity in brain regions associated with sleep and consciousness (eg. the thalamus) in both normal and “sedative-resistant” models we are beginning to understand how such agents produce their dramatic effects.

Extrasynaptic GABAA receptors: a new therapeutic target. Recent work has established that for certain neurones extrasynaptic GABAA receptors play a previously unappreciated role in influencing neuronal excitability. We demonstrated the extrasynaptic GABAA receptors in CA1 neurones of the hippocampus (a region associated with learning and memory) to contain the alpha5 subunit. Furthermore, selective inhibitors of alpha5-GABAA receptors act to enhance cognition.

In thalamus (an area associated with consciousness/sleep) we, and others, demonstrated these extrasynaptic receptors to be distinct from those in CA1 and to contain the delta subunit. We are now investigating the actions of delta-GABAA receptor agonists as novel sleep inducing agents. Remarkably we have found the sedative actions of gaboxadol to be abolished following deletion of the delta subunit, and in agreement the dramatic effects of this agent on thalamic neurone excitability are compromised.

Cocaine Addiction and alpha2-GABAA Receptors

My collaborators (Prof. Dai Stephens and Dr Sarah King, University of Sussex), have demonstrated that the addictive properties of cocaine are prevented by deletion of the GABAA receptor alpha2 subunit. Furthermore, in an in vivo model engineered to express alpha2-GABAA receptors that are uniquely sensitive to a benzodiazepine (Ro15-4513) this drug, although not related to cocaine can now substitute for it. In collaboration with Prof Lambert, we have shown that neurones of the nucleus accumbens (a brain region associated with addiction) express synaptic alpha2-GABAA receptors and that in the nucleus accumbens engineered to harbour a mutant form of the αlpha 2 subunit (alpha2H101R), the effects of Ro15-4513 on neuronal signalling are quite distinct from their wild type counterparts. Given the reported association with Single Nucleotides Polomorphisms (SNPs) of GABAA receptor subunits in population study of addicts, these findings may collectively lead to a better understanding and treatment of addictions.

Dixon, C.L., Morris, H.V., Breen, G., Desrivieres, S., Jugurnauth, S., Steiner, R.C., Vallada, R., Guindalini, C., Lavanjeira, R., Messas, G., Rosahl, T.W., Atack, J.R., Peden, D.R., Belelli, D., Lambert, J.J., King, S.L., Schumann, G., Stephens, D.N. (2010). Cocaine effects on mouse incentive-learning and human addiction are linked to alpha2 subunit – containing GABAA receptors. Proc. Natl. Acad. Sci. USA, 107:2289-2294.

Lambert, J.J., Cooper, M.A., Simmons, R.D., Weir, C.J., Belelli, D. (2009). Neurosteroids: Endogenous allosteric modulators of GABA(A) receptors. Psychoneuroendocrinology, 34S: S48-58.

Herd, M.B., Foister,N., Chandra, D., Peden, D.R., Homanics, G.E., Brown,V.J., Balfour, D.J.K., Lambert, J.J. and Belelli, D. (2009) Inhibition of thalamic excitability by 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol: a selective role for delta-GABAA receptors. Eur. J. Neurosci. 29: 1177-1187.

Wafford, K.A., van Niel, M.B., Ma, Q.P., Horridge, E., Herd, M. B.,. Peden, D., Belelli, D. and Lambert, J.J. (2009) Novel compounds selectively enhance delta subunit containing GABAA receptors and increase tonic currents in thalamus. Neuropharmacolgy. 56(1):182-9.

Ahrens J, Leuwer M, Stachura S, Krampfl K, Belelli D, Lambert JJ, Haeseler G. (2008) A transmembrane residue influences the interaction of propofol with the strychnine-sensitive glycine αlpha1 and αlpha1beta receptor. Anesth Analg. 107(6):1875-83.

Mitchell E.A., Herd M.B., Gunn B.G., Lambert J.J. and Belelli D. (2008) Neurosteroid modulation of GABAA receptors: molecular determinants and significance in health and disease. Neurochem. Int. 52(4-5):588-95.

Herd M.B., Haythornthwaite A.R., Rosahl T.W., Wafford K.A., Homanics G.E.,Lambert J.J. and Belelli D. (2008) The expression of GABAA beta subunit isoforms in synaptic and extrasynaptic receptor populations of mouse dentate gyrus granule cells. J. Physiol 586(4):989-1004.

Peden D.R., Petitjean C.M., Herd M.B., Durakoglugil M.S., Rosahl T.W., Wafford K, Homanics G.E., Belelli D., Fritschy J.M. and Lambert J.J. (2008). Developmental maturation of synaptic and extrasynaptic GABAA receptors in mouse thalamic ventrobasal neurones. J. Physiol 586(4):965-987.

Mitchell E.A., Gentet L.J., Dempster J. and Belelli D. (2007) GABAA and glycine receptor-mediated transmission in rat lamina II neurones: relevance to the analgesic actions of neuroactive steroids. J. Physiol. 583(3):1021-40.

Herd M.B., Belelli D. and Lambert J.J. (2007) Neurosteroid modulation of synaptic and extrasynaptic GABAA receptors. Pharmacol. Ther. 116(1):20-34.

Belelli, D., Herd, M.B., Mitchell, E.A., Peden, R.A., Vardy, A.W., Gentet, L. and Lambert, J.J. (2006) Neuroactive steroids and inhibitory neurotransmission: mechanisms of action and physiological relevance. Neuroscience 138 (3):821-29.

Borghese, C.M. Stórustovu, S., Ebert, B., Herd, M.B., Belelli, D. Lambert, J.J. Marshall, G., Wafford, K.A. Harris, R.A. (2006) The delta subunit of gamma-aminobutyric acid type A receptors does not confer sensitivity to low concentrations of ethanol. J. Pharmacol. Exp.Ther. 316 (3): 1360-68.

Belelli, D., Peden, D.A.,. Rosahl, T. W., Wafford, K.A and Lambert J.J. (2005) Extra-synaptic GABAA receptors of thalamo-cortical neurons: a molecular target for hypnotics. J. Neurosci 25(50):11513-20.

Belelli, D and Lambert, J.J. (2005) Neurosteroids: Endogenous regulators of the GABAA receptor. Nature Neurosci.Rev 6(7):565-75.

Weir, C.J., Ling, A.T.Y, Belelli, D., Wildsmith, J.A.W., Peters, J.A., Lambert, J.J. (2004). The interaction of anaesthetic steroids with recombinant glycine and GABAA receptors. Br. J. Anaesthesia 92(5):704-11.

Caraiscos, V.B., Elliott, E.M., Kong, K., You-Ten, E., Cheng, V.Y., Belelli, D., Newell, J.G., Jackson, M.F., Lambert, J.J., Rosahl, T.W., Wafford, K.A., MacDonald, J.F., Orser, B.A. (2004). Tonic inhibition in mouse hippocampal CA1 pyramidal neurons is mediated by alpha5 subunit-containing GABAA receptors. Proc.Nat.Acad. Sci. U.S.A. 101,3662-7.

Lambert, J.J., Belelli, D, Peden, D.R., Vardy AW and Peters J.A. (2003) Neurosteroid modulation of GABAA receptors. Prog. Neurobiol. 71, 67-80

Belelli D. and M.B. Herd (2003) The contraceptive agent Provera enhances GABAA receptor-mediated inhibitory neurotransmission in the rat hippocampus: evidence for endogenous neurosteroids? J. Neurosci.23, 10013-10020.

Reynolds, D.S., Rosahl, T.W., Cirone, J., O’Meara, G.F., Haythornthwaite, A., Newman, R.J., Myers, J., Sur, C., Howell, O., Rutter, R., Atack, J., Macaulay, A.J., Hadingham, K.L., Hutson, P.H., Belelli, D., Lambert, J.J., Dawson, G.R., McKernan, R., Whiting, P.J. & Wafford, K.A. (2003). Sedation and anaesthesia mediated by distinct GABAA receptor isoforms. J. Neurosci., 23, 8608-8617.

D. Pau, Belelli D., Callachan, H., Peden, D.R., Dunlop, J., Peters, J.A., Guitart, X., Guitierrez , B. and Lambert, J.J. (2003) GABAA receptor modulation by the novel intravenous general anaesthetic E6375. Neuropharmacology 45, 1029-1040.

Belelli, D. Muntoni, A.L., Merrywest, S.D, Gentet,L.J. Casula, A. Calllachan, H., Madau, P. Gemmmell, D.K. Hamilton, N.M., McGuire, R., Lambert, J.J., Sillar, K.T. and Peters, J.A. (2003). The in vitro and in vivo enantioselectivity of etomidate implicates the GABAA receptor in general anaesthesia. Neuropharmacology 45: 57-71.

Belelli, D. Casula, A., Ling, A., Lambert, J.J. (2002) The influence of subunit composition on the interaction of neurosteroids with GABAA receptors. Neuropharmacology 43,651-661.

Lambert, J.J., Belelli, D. (2002) Pharmacological characterization of a novel cell line expressing human alpha4beta3delta GABAA recepors. (2002) Br. J. Pharmacol. 136,957-959.

Lambert, J.J., Belelli, D., Harney, S.C., Peters, J.A., Frenguelli, B.G. (2001). Modulation of native and recombinant GABAA receptors by endogenous and synthetic neuroactive steroids. Brain Research Reviews 37, 68-80.

Lambert J.J., Harney S.C., Belelli D., Peters J.A. (2001). Neurosteroid modulation of recombinant & synaptic GABAA receptors. In: Neurosteroids and Brain Function. Int. Rev. Neurobiol. 46, 177-205.

Anderson A., Belelli D., Bennett D.J., Buchanan K.I., Casula A., Cooke A., Feilden H., Gemmell D.K., Hamilton N.M., Hutchinson E.J., Lambert J.J., Maidment M.S., McGuire R., McPhail P., Miller S., Muntoni A., Peters J.A., Sansbury F.H., Stevenson D., Sundaram H. (2001). alpha-Amino acid phenolic ester derivatives: novel water-soluble general anaesthetic agents which allosterically modulate GABAA receptors. J. Med. Chem. 44(22):3582-3591.

Belelli, D., Pistis, M., Peters, J.A. and Lambert, J.J., (1999) The interaction of general anaesthetics with inhibitory amino acid receptors. Trends Pharmacol. Sci. 20(12), 496-502.

Belelli, D., Pau, D., Cabras, G., Peters, J.A. and Lambert, J.J. (1999) A single amino acid confers barbiturate sensitivity upon the GABAA r1 receptor. Br. J. Pharmacol. 127,601-604.

Belelli, D. Pistis, M., Peters, J.A. and Lambert J.J. (1999) The interaction of general anaesthetics and neurosteroids with GABAA and glycine receptors. Neurochem. Int. 34, 447-452.

Hope, A.G., Belelli, D., Mair, I. D., Lambert, J.J. and Peters, J.A. (1999) Molecular determinants of (+)-tubocurarine binding at recombinant 5-hydroxytryptamine3A receptor subunits. Mol. Pharmacol. 55, 1037-1043.