Striatal efferents preferentially innervate neurons in the ventral pallidum containing GABAA receptor α 1 subunit-like immunoreactivity

The γ‐aminobutyric acid‐A receptor consists of several subunits. In this immunohistochemical study we investigated the regional distribution of the α1 and α2 subunits with subunit‐specific antibodies in the ventral pallidum, and compared the staining patterns to those of substance P (SP). α1 subunit antigenic sites were found to be localized to pallidal neurons, varicosities, and varicose fibers. α1 immunopositive fibers mainly appeared “tubulus‐like” due to the intense staining of the membranes of the long pallidal dendrites. Double labelling of α1 subunit and substance P revealed that α1 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 α1 subunit, whereas only a few axon terminals exhibited α1‐IR. α2‐IR was restricted to a low number of ventral pallidal neurons. The distributional patterns obtained for the α1 and α2 subunits suggest that striatal efferent neurons directly influence pallidal neurons displaying a distinct GABAA subunit composition, which may be of pharmacological importance since the α1βxγ2‐subunits containing receptors have mainly a benzodiazepine type I pharmacology. © 1996 Wiley‐Liss, Inc.