Sexual dimorphism of GABAA receptor levels in subcortical brain regions of a woodland rodent (Apodemus sylvaticus)

This is the first report of quantitative autoradiography results showing sex differences of GABA_A receptor levels in brain regions of a wild rodent (wood mouse, Apodemus sylvaticus) living in its natural habitat. The labeling of this GABAergic site with its specific high affinity radioligand [³H] muscimol provided a heterogeneous and dimorphic binding pattern in some of the neural centers. In the female, higher ( ≥ 50 ≥ 65%) to moderately higher (<50%) binding levels than in the male, even after correction of the specific binding values using the calculated quenching coefficients, were observed in the substantia nigra pars reticulata and ventral lateral thalamic nucleus, brain centers that are relays of motor circuits. In the male, on the other hand, a higher level was only obtained in the caudateputamen. Relays of the stria terminalis-hypothalamic-central gray pathway such as the bed nucleus of the stria terminalis, the pontine central gray and the ventromedial hypothalamic nucleus, were among the other female brain areas with an extremely higher (>65%) to higher and moderately higher binding activity than in the male. From the saturation analyses, it appeared that the binding differences were mainly due to B_max variations, although closer examinations revealed that changes in the K_D might have also accounted for [³H] muscimol binding differences, as shown by the high K_D and B_max values in the bed nucleus of the stria terminalis, the substantia nigra pars reticulata and the pontine central gray of the female wood mouse. These findings suggest that the diamorphic binding activity of GABA_A receptors in the above brain regions might be involved in neuronal circuitry mechanisms related to sex-specific social behaviors in rodents living in their natural environmental conditions.