GABAA receptors are implicated in the pathology of psychiatric disorders such as schizophrenia and depression. They are rapidly affected by stress in a sex-dependent fashion, suggesting that GABAA receptors may be relevant to understanding the association between stress and psychiatric disorders. Thus, this thesis examined how GABAA receptors are affected in both male and female mice exposed to stress in adulthood (Chapter 2), early-life (Chapter 3-5) and a combination of both
early-life and adulthood stress (Chapter 6).
2. The effects of acute adulthood stress (3 minute warm swim stress) on GABAA receptor binding in the brains of male and female mice were examined using quantitative receptor autoradiography. The total number of GABAA receptor [3H]GABA binding sites was increased following swim stress in specific forebrain cortical regions of female mice swum individually or in a group, but decreased in male mice when swum in a group only. These findings confirm and extend previous studies, identifying the cortical regions involved in rapid stress-induced changes in GABAA receptors.
3. Post-natal handling models in rodents comparing control (brief handling sessions; EH) with no intervention stress conditions (NH), indicate that the NH condition results in an anxious adulthood phenotype and this was confirmed in the present thesis using the elevated plus-maze behavioural test. Using this model the effects of early-life stress on adulthood GABAA receptors were then examined.
4. Regional densities of GABAA receptor α1 and α2 subunit proteins were observed in the adult brain of male and female mice using immunoperoxidase histochemistry. NH males showed a loss of the α2 subunit from the thalamus and the lower layers (IV-VI) of the primary somatosensory cortex, whilst NH females showed a reduction of α2 but an increase in α1 protein in the lower layers of the primary somatosensory cortex only. These regionally specific alterations in the α1:α2 subunit ratio suggest that early-life stress disrupts the developmental α subunit switch, which occurs in a regionally-dependent fashion over the first two weeks of rodent life.
5. Double-labelling immunofluorescence and confocal microscopy were used to examine the effects of sex and early-life stress on GABAA receptor synaptic clustering. Regardless of sex, mice exposed to early-life stress (NH) showed reduced colocalisation of the GABAA receptor α2 subunit with the synaptic marker protein gephyrin relative to the control condition (EH). This suggests that early-life stress impairs adulthood inhibitory synaptic strength and is consistent with the increased anxiety of the stressed relative to control mice.
6. Finally, the effects of early-life stress on adulthood swim stress-induced changes in GABAA receptor binding were examined using quantitative receptor autoradiography in forebrain cortical regions. Findings showed that the effect of adulthood stress on the total number of GABAA receptor binding sites for [3H]GABA in forebrain cortical regions was altered by early-life stress in both male and female mice, suggesting that the rapid adulthood stress response of GABAA receptors is affected by early-life experience.
7. Together these results show that GABAA receptors are sensitive to subtle changes in the environment in both early-life and adulthood and that these neurochemical responses to stress in adulthood are sex-dependent. The short and long-term stress-sensitivity of the GABAergic system implicates GABAA receptors in the non-genetic aetiology of psychiatric illnesses in which sex and stress are important factors.