Immunohistochemical distribution of AMPA-type label in the pigeon (C. livia) hippocampus

The avian hippocampal formation (HF) is reported to have a role equivalent to that of the mammalian hippocampus, which may involve the glutamatergic system as well. In the present paper we offer evidence of the occurrence and distribution of the subunits composing AMPA-type glutamate receptors on neurons in the hippocampus region of the pigeon brain. The experiment analyzed the immunolabeling of glutamate receptor (GluR)1, GluR4, and GluR2/3 receptor subunits in adult pigeons and found consistent evidence that neurons located in the five main areas of the avian HF have these AMPA-type subunits, but their incidence varies according to position and neuro-type. About 20%-35% of the irregular and 35%-70% of the triangular neurons on the lateral and medial “V” arms contain GluR1 and GluR2/3, while GluR4 was found only at rounded neurons. The majority of the triangular neurons (over 90%) and about half of the irregular neurons in the medial area contain GluR1 and GluR2/3, whereas the rounded neurons contain primarily GluR4 (95%). Labeling revealed GluR1 (40%-60%) and GluR2/3 (30%) in the dorsomedial and lateral areas but only in irregular neurons, while 60%-80% of the rounded neurons synthesize GluR4. While triangular and irregular neurons appear to match the description of projecting neurons, rounded ones seem to participate in local circuits. A discussion of the functional significance of the avian HF concentrates on a postulation of the “V” arms as equivalent to the dentate gyrus and the dorsomedial area being similar to the Ammon's horn.