DIMORPHIC DISTRIBUTION OF THE 2 MAIN GABA(A) BINDING-SITES IN CORTICAL AND LIMBIC AREAS OF A RODENT LIVING IN NATURAL ENVIRONMENTAL-CONDITIONS

Labeling of the two more important gamma-aminobutyric acid(A) (GABA(A) ) supramolecular sites with [H-3] muscimol (GABA(A)) and [H-3] flunitr azepam (benzodiazepine) provided saturable, stable, and dimorphic bind ing activities in cortical and limbic regions of the wood mouse Apodem us sylvaticus. Of the cortical layers, which contained the highest [H- 3] muscimol binding levels, only the female lamina V supplied a greate r (51%; P <0.01) receptor density than in the male. Areas of the limbi c system instead proved to be the more favorable targets for different ial GABAA binding levels. The highest (P < 0.001) and higher levels we re found in the oriens-pyramidalis CA1 layer of the hippocampus (65%) and in the vertical limb diagonal band-medial septal nucleus (48%), ba solateral amygdala nucleus (45%), and ventromedial hypothalamic nucleu s (43%), respectively, of the female. A similar pattern was obtained f or [H-3] flunitrazepam binding activity, especially in the presence of GABA. The highest and higher binding activities were obtained in the female central amygdala nucleus (78%) and in the ventromedial hypothal amic nucleus (52%), basolateral amygdala nucleus (48%), and oriens-pyr amidalis CA1 layer of the hippocampus (47%), respectively, whereas hig her levels were observed only in the male vertical limb diagonal band- medial septal nucleus (56%). Even in the cortical regions, the female exhibited higher (42%; cortex lamina V) and moderately higher (38%; co rtex lamina VI) levels, with binding differences in the latter site pl us in the basolateral amygdala nucleus occurring in a GABA-nondependen t manner. From the saturation binding analyses it was possible to reve al that both maximal number of binding sites (B-max) and mean dissocia tion constant (K-D) modifications were responsible for receptor differ ences of the two GABAergic sites. These findings tend to suggest that dimorphic variations of the GABA(A) supramolecular sites, in some cort ical and limbic regions, are strongly involved in sex-specific aggress ive and reproductive activities of rodents living in their natural hab itats. (C) 1997 Wiley-Liss, Inc.