The morphological and chemical characteristics of striatal neurons immunoreactive for the α1-subunit of the GABAA receptor in the rat

The distribution, morphology and chemical characteristics of neurons immunoreactive for the α1-subunit of the GABAA receptor in the striatum of the basal ganglia in the rat brain were investigated at the light, confocal and electron microscope levels using single, double and triple immunohistochemical labelling techniques. The results showed that α1-subunit immunoreactive neurons were sparsely distributed throughout the rat striatum. Double and triple labelling results showed that all the α1-subunit-immunoreactive neurons were positive for glutamate decarboxylase and immunoreactive for the β2,3 and γ2 subunits of the GABAA receptor. Three types of α1-subunit-immunoreactive neurons were identified in the striatum on the basis of cellular morphology and chemical characteristics. The most numerous α1-subunit-immunoreactive neurons were medium-sized, aspiny neurons with a widely branching dendritic tree. They were parvalbumin-negative and were located mainly in the dorsolateral regions of the striatum. Electron microscopy showed that these neurons had an indented nuclear membrane, typical of striatal interneurons, and were surrounded by small numbers of axon terminals which established α1-subunit-immunoreactive synaptic contacts with the soma and dendrites. These cells were classified as type 1 α1-subunit-immunoreactive neurons and comprised 75% of the total population of α1-subunit-immunoreactive neurons in the striatum. The remaining α1-subunit-immunoreactive neurons comprised of a heterogeneous population of large-sized neurons localized in the ventral and medial regions of the striatum. The most numerous large-sized cells were parvalbumin-negative, had two to three relatively short branching dendrites and were designated type 2 α1-subunit-immunoreactive neurons. Electron microscopy showed that the type 2 neurons were characterized by a highly convoluted nuclear membrane and were sparsely covered with small axon terminals. The type 2 neurons comprised 20% of the total population of α1-subunit-immunoreactive neurons. The remaining large-sized α1-immunoreactive cells were designated type 3 cells; they were positive for parvalbumin and were distinguished by long branching dendrites extending dorsally for 600–800μm into the striatum. These neurons comprised 5% of the total population of α1-subunit-immunoreactive neurons and were surrounded by enkephalin-immunoreactive terminals. Electron microscopy showed that the α1-subunit type 3 neurons had an indented nuclear membrane and were densely covered with small axon terminals which established α1-subunit-immunoreactive symmetrical synaptic contacts with the soma and dendrites.