The regional development of benzodiazepine-binding sites in fetal brain was studied by autoradiog.Cryostat sections through the brains of rat fetuses were incubated either with [3H]flunitrazepam or with the specific benzodiazepine antagonist [3H]RO15-1788.The specificity of radioligand binding was checked by coincubation with an excess of nonradioactive antagonist or clonazepam, resp.Labeling with either agonist or antagonist revealed a similar developmental pattern of binding sites.Specific binding was first detected on gestational day (GD) 14 in the spinal cord and lower brainstem.Between GD 14 and GD 15, labeling spread throughout the lower brainstem, mesencephalon, and parts of the diencephalon, with higher densities in ventral areas.Matrix zones remained unlabeled.On GD 16, binding was present in the developing caudate putamen, the olfactory bulb, and the frontoventral parts of the neocortex.Binding sites had spread over the remaining neocortex by GD 21 and increased in d. in di- and telenecephalic areas.At earlier stages of development, neocortical binding sites were restricted to the superficial (GD 16 and 18) and deep (GD 18) layers above and below the cortical plate; the latter was unlabeled.By GD 21, binding sites also occurred in the cortical plate.By this time, layer I exhibited the highest d. of Ag grains.Thus, the autoradiog. data reveal a distinct ontogenic pattern of benzodiazepine-binding sites.The general caudorostral gradient, as well as the development in individual brain regions such as the neocortex, appears to be linked with cell differentiation, though not necessarily with synaptogenesis.The capacity of the fetal brain to bind benzodiazepines specifically may help to explain the drug effects described in offspring of benzodiazepine-treated dams.Different brain structures may become targets for the action of this class of drugs at different stages of prenatal development.
