Parvalbumin-immunopositive neurons in rat globus pallidus: a light and electron microscopic study.

To add to our understanding of the anatomical organization of the globus pallidus (GP) of the rat, a light and electron microscopic analysis of parvalbumin (PV, a Ca-binding protein) immunoreactive neurons in the GP was performed. Light microscopic analysis revealed that the GP contains PV-positive and PV-negative neurons. Approximately two-thirds of the GP neurons were PV-positive. The somata of PV-positive neurons were, on average, larger than PV-negative ones. The proximal dendrites of PV-positive neurons were smooth and often lay parallel to the border between the GP and the neostriatum. Distal dendrites of PV-positive neurons were varicose. Thin PV-positive fibers with large boutons (with average diameter of 1.7 microns) were observed in the neuropil of the GP. Some PV-positive boutons formed basket-like aggregates surrounding the somata of PV-positive or negative neurons. Electron microscopic observations revealed that PV-positive neurons were often large and contained deeply indented nuclei and a large volume of cytoplasm. PV-negative neurons had smaller somata that were occupied by deeply indented nuclei and a small volume of cytoplasm. Both PV-positive and negative neurons were contacted by synaptic boutons identical to the known striato-pallidal, subthalamo-pallidal, and local collateral boutons. The PV-positive boutons contained small round or elongated vesicles and often more than one mitochondrion. Most of the boutons (i.e. 86%) formed symmetric synapses with somata and large dendrites and, the other (14%) formed asymmetric synapses with small dendrites. The study indicated that GP projection neurons can be divided into two subgroups according to their PV-immunoreactivity. PV-positive and negative neurons received similar extrinsic synaptic inputs and both types of neurons were connected through their local collateral axons. It is conceivable that the physiology of PV-positive and negative neurons might be different because of a difference in the Ca-buffering mechanisms in these neurons.