γ-Amminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in adult mammalian brain, mediating its actions
chiefly via a pentameric chloride ion channel, the GABAA receptor. Nineteen different subunits (α1-6, β1-3, γ1-3, δ, δ, π,
θ, ρ1-3) can give rise to multiple receptor subtypes that are the site of action of many clinically important drugs. In the
developing brain, however, GABAA receptors mediate excitatory actions due to an increased chloride concentration within neurons
and seem to control cell proliferation, migration, differentiation, synapse maturation and cell death. Little is known about
the distribution of single subunits in the human brain. Here we describe developmental changes in the immunohistochemical
distribution of four subunits (α1, α2, α3, and γ2) in the human rhombencephalon. The γ2 was the most abundant subunit in all
rhombencephalic stuctures during development and in adults, whereas alpha subunits showed a structure- and age-characteristic
distribution. The α1 was expressed already prenatally in the molecular and Purkinje cell layer, but only postnatally in the
granule cell layer and the dentate nucleus. Expression was completely absent in the inferior olivary nucleus. The α2 gradually
increased during development, showing some layer specificity in the cerebellar cortex. The α3-immunoreactivity in the cerebellar
cortex was relatively weak, but it was abundantly observed in different cell populations in the subcortical cerebellar structures.
Structure- and age-characteristic co-localization between subunits during development suggests differences in GABAA receptor
composition. Interestingly, subunit expression in several instances differed between human and rodent brain, underlining the
importance of immunohistochemical studies in humans.