GABA-immunoreactive synaptic boutons in the rat basal forebrain: comparison of neurons that project to the neocortex with pallidosubthalamic neurons.

Although the basal forebrain, including the globus pallidus, contains a high concentration of gamma-aminobutyric acid (GABA), it is not known whether all types of neuron in the globus pallidus receive GABAergic synaptic input. We have studied two types of neuron: typical pallidal neurons that project to the subthalamic nucleus and magnocellular neurons which are found in the medial and ventral borders of the globus and project to the sensorimotor cortex. The postembedding immunogold staining of endogenous GABA revealed many preterminal axons and synaptic boutons that contained GABA immunoreactivity. Neurons that projected to the neocortex were postsynaptic to some of the GABA-immunoreactive boutons, the majority of which formed symmetrical membrane specializations. From a series of random electron micrographs through the perikarya and proximal dendrites of such retrogradely labelled neurons the density of GABA-containing afferent synaptic boutons was estimated to be 0.58 GABA-containing boutons per 100 micron of neuronal membrane. The GABA-containing boutons accounted for 72% of the total afferent input in the proximal regions of the pallidocortical neurons examined. The pallidosubthalamic neurons received many more afferent boutons than did the cortically projecting neurons, a high proportion (80.4%) of which were immunoreactive for GABA. The density of GABA-containing boutons in contact with pallidosubthalamic neurons was 8.9 boutons per 100 micron. It is concluded that cortically projecting basal forebrain neurons, that are probably cholinergic, are innervated by GABA-containing afferent boutons. However, pallidosubthalamic neurons in the same part of the basal forebrain are much more densely innervated by GABA-containing boutons.