Striatal efferents preferentially innervate neurons in the ventral pallidum containing GABAA receptor alpha 1...

The gamma-aminobutyric acid-A receptor consists of several subunits. In this immunohistochemical study we investigated the regional distribution of the alpha 1 and alpha 2 subunits with subunit-specific antibodies in the ventral pallidum, and compared the staining patterns to those of substance P (SP). alpha 1 subunit antigenic sites were found to be localized to pallidal neurons, varicosities, and varicose fibers. alpha 1 immunopositive fibers mainly appeared "tubulus-like" due to the intense staining of the membranes of the long pallidal dendrites. Double labelling of alpha 1 subunit and substance P revealed that alpha 1 subunit-like immunoreactive (IR) dendrites and somata of the pallidal neurons were often invested by SP-IR striatal efferents. Subcellularly, the dendritic and somatic membranes of pallidal neurons were strongly immunopositive for the alpha 1 subunit, whereas only a few axon terminals exhibited alpha 1-IR. alpha 2-IR was restricted to a low number of ventral pallidal neurons. The distributional patterns obtained for the alpha 1 and alpha 2 subunits suggest that striatal efferent neurons directly influence pallidal neurons displaying a distinct GABAA subunit composition, which may be of pharmacological importance since the alpha 1 beta x gamma 2-subunits containing receptors have mainly a benzodiazepine type I pharmacology.