SUBPOPULATIONS OF GABA NEURONS CONTAINING SOMATOSTATIN, NEUROPEPTIDE-Y, AND PARVALBUMIN IN THE DORSOMEDIAL CORTEX OF THE LIZARD PSAMMODROMUS-ALGIRUS

Different subpopulations of GABA neurons containing the neuropeptides somatostatin and neuropeptide Y, and the calcium binding protein parva lbumin were studied by immunocytochemistry using light and electron mi croscopy in the dorsomedial cortex of the lizard Psammodromus algirus to investigate the connectivity of different subsets of GABA neurons i n the lizard dorsomedial cortical circuitry and to compare cortical re gions of reptiles and mammals. GABA neurons were classified into diffe rent subsets by using the peroxidase anti-peroxidase immunohistochemic al method on adjacent Araldite-embedded semithin sections. GABA neuron s in the dorsomedial cortex fall into three major subsets: 1) neurons with somatostatin (and neuropeptide Y), which accounted for about 44% of the GABA population; 2) neurons with parvalbumin, which accounted f or about 13% of the GABA neurons; and 3) neurons without parvalbumin o r neuropeptides, which represented 40% of all GABA cells. This divisio n of GABA -neurons in non-overlapping subpopulations of neuropeptide- and parvalbumin-containing cells is similar to that found in the mamma lian hippocampal formation. On the basis of the nerve terminal fields, somatostatin- and parvalbumin-immunoreactive neuronal populations app ear to be functionally different, acting on different portions of the projection neurons. Parvalbumin-immunoreactive neurons inhibit the pyr amidal neurons at the cell body level, whereas somatostatin-immunoreac tive neurons inhibit them on distal dendrites. The results of the pres ent study add more similarities between the lizard dorsomedial cortex and parts of the mammalian hippocampus.