The role of GABAB(1) receptor isoforms in anxiety and depression : genetic and pharmacological studies in the mouse

Anxiety and depression disorders represent common, serious and growing health problems world-wide. The neurobiological basis of anxiety and depression, however, remains poorly understood. Further, there is a clear need for the development of better treatments for these disorders. Emerging data with genetic and pharmacological tools supports a role for GABAB receptors in both anxiety and depression. GABAB receptors are metabotropic GABA receptors that are comprised of two subunits, GABAB1 and GABAB2, which form heterodimers. The GABAB(1) gene is transcribed into two predominate isoforms, GABAB(1a) and GABAB(1b) which differ in sequence primarily by the inclusion of a pair of sushi domains (or short consensus repeats) in the GABAB(1a) N-terminus. Both isoforms heterodimerize with GABAB2 subunits to form functional receptors. The two GABAB(1) isoforms and the GABAB(2) subunit constitute the majority of the molecular diversity of the GABAB receptor. However, in the absence of any isoform-selective ligands for research, the behavioural function of mammalian GABAB1 receptor isoforms has been inscrutable. Recently mice deficient in GABAB(1a) and GABAB(1b) isoforms were generated. Aspects of anxiety- and depression-related behaviour may be modelled in mice, by using traditional animal models, and by examining specific biological and behavioural components of the human symptomatology, or ‘endophenotypes’. A preliminary aim of this thesis was to determine the utility of GABAB(1) isoform-deficient mice for the dissection of GABAB(1a) and GABAB(1b) isoform-mediated behaviour. The main aim of this thesis was to test the hypothesis, using a combination of traditional and endophenotype murine models, that GABAB(1) receptor isoforms play an important role in the mediation and anxiety and depression-related behaviour.
GABAB(1) Isoforms in GABAB Receptor Function:
Preparatory work in this thesis examined the influence of genetic background on GABAB receptor-mediated responses. Genetic background, in the form of different mouse strains, had a strong, differential effect on the classic responses to GABAB receptor activation; hypothermia and ataxia. This underlined the necessity of including multiple experimental endpoints in the examination of GABAB receptor function in subsequent work with GABAB(1) isoform-deficient mice. Importantly, this study also demonstrated that the BALB/c mouse strain was an appropriate genetic background for carrying the GABAB(1) isoform mutations.
Initial studies with GABAB(1) isoform-deficient mice demonstrated that they were free of gross sensory-motor deficits that may preclude their application in behavioural tasks. Furthermore, GABAB(1a) and GABAB(1b) diverged in their influences on locomotor responses to novelty and circadian activity, although the GABAB receptor agonists baclofen or γ-hydroxybutyrate (GHB) were not specific for either isoform and were unable to discriminate these differences. These findings demonstrated that the GABAB(1) isoforms had differential influences on behaviour. Together these studies demonstrated that the GABAB(1a)-/- and GABAB(1b)-/- mice were applicable for testing the hypothesis that the GABAB(1) isoforms were differentially implicated in anxiety and depression related behaviour.
GABAB(1) Isoforms in Endophenotypes of Anxiety and Depression:
Deletion of GABAB(1a) and GABAB(1b) isoforms had profound, differential impacts on the acquisition (GABAB(1a)) and extinction (GABAB(1b)) of aversive memories, as determined in a conditioned taste aversion paradigm. These effects, however, were not accompanied by differences in innate anxiety, as assessed in a comprehensive test battery of unconditioned anxiety tests, including autonomic (stress-induced hyperthermia), active (marble burying) and passive exploratory avoidance (staircase, light-dark box, elevated plus maze, elevated zero maze) behavioural readouts. There was no evidence for a specific influence of either isoform in these tests. This indicated that the GABAB(1) isoforms themselves did not have a defining role in innate anxiety.
GABAB(1a)-/- and GABAB(1b)-/- mice diverged in their cognitive phenotypes. GABAB(1a)-/- mice were impaired in tasks of working spatial and recognition memory, but not in passive avoidance. GABAB(1b)-/- mice were also impaired, to a lesser degree, in a working spatial memory task, but showed preservation of working recognition memory and passive avoidance. Long term recognition memory, however, was also impaired in these mice.
The GABAB(1a) isoform was specifically implicated in depression-related behaviour, as indicated by reduced immobility in a classic test of antidepressant-like behaviour – the forced swim test. This was most probably mediated via the striking interactions of the GABAB(1a) isoform with the serotonergic system, as illustrated in particular by the profound desensitisation of presynaptic 5-HT1A receptors in GABAB(1a)-/- mice. A lack of effect on 5-HT1A receptor expression in GABAB(1a)-/- mice, as indicated by normal 5-HT1A autoradiography densities, suggested an intracellular mechanism for this desensitisation.
Together these studies demonstrated that the GABAB(1) isoforms are functionally important variants of the GABAB receptor, with specific relevance in depression and to aversive learning and memory processes that underlie cognitive symptoms in anxiety disorders.