Expression of GABAA receptor α3-, θ-, and ε-subunit mRNAs during rat CNS development and immunolocalization of the ε subunit in developing postnatal spinal cord

Ionotropic GABAA receptors are heteromeric structures composed of a combination of five from at least 16 different subunits. Subunit genes are expressed in distinct cell types at specific times during development. The most abundant native GABAA receptors consist of α1-, β2-, and γ2-subunits that are co-expressed in numerous brain areas. α3-, θ-, And ε-subunits are clustered on the X chromosome and show striking overlapping expression patterns throughout the adult rat brain. To establish whether these subunits are temporally and spatially co-expressed, we used in situ hybridization to analyze their expression throughout rat development from embryonic stage E14 to postnatal stage P12. Each transcript exhibited a unique or a shared regional and temporal developmental expression profile. The thalamic expression pattern evolved from a restricted expression of ε and θ transcripts before birth, to a θ and α3 expression at birth, and finally to a grouped ε, θ and α3 expression postpartum. However, strong similarities occurred, such as a grouped expression of the three subunits within the hypothalamus, tegmentum and pontine nuclei throughout the developmental process. At early stages of development (E17), ε and θ appeared to have a greater spatial distribution before the dominance of the α3 subunit transcript around birth. We also revealed expression of α3, θ, and ε in the developing spinal cord and identified neurons that express ε in the postnatal dorsal horn, intermediolateral column and motoneurons. Our findings suggest that various combinations of α3-, θ- and ε-subunits may be assembled at a regional and developmental level in the brain.